Bristol 27

Propeller System

Image Gallery

Questions

Cutlass Bearing

+ What type of cutlass bearing will I purchase?
-> On 3/15 I gave a call to Beta Marine and spoke to Farron. He said that I can buy them them in various sizes and a fair number of cutlass bearings available. He said I could check Check Buck Algonquin’s website for an idea about what is offered. In general, the sizing of these go by a couple of things:
Length of tube (length can be cut down with a hacksaw)
Outer diameter (must fit the prop shaft tube)
Inside diameter (can vary based on the outside diameter of prop shaft)

-> On 3/16, I came back to do some measurements that match the above recommendations:

• Length of Tube = 8 1/2″ (max)
• Outsider Diameter = 1.377952″ (3.5 cm)
• Inside Diameter = 7/8″ (?) (This measurement is based off the propeller shaft I end up buying, however it will likely be 7/8″ size)

I then took these measurements and looked in the 2010 Jamestown catalog (p. 1086) who sells Morse Marine Bearings. The bearing selection guide reads:

Normal design practice is to select a bearing with a length equal to 4 times the shaft diameter. It may be acceptable to deviate from this rule when application load requirements permit the use of shorter bearing lengths. Bearing area = bearing inside diameter (shaft diameter) x bearing length. Loading capacity = bearing area x 35 psi.

It wasn’t quite clear why I needed to know bearing area and loading capacity, but at least I can figure that out if needed. At this point, I need to decide on the final size of the propeller before I can determine the final size of the cutlass bearing to buy.

-> [Continue researching/thinking/considering....]

+ How do I replace the cutlass bearing and check the quality of my work?
-> First lubricate the new bearing with soap – never grease – and slip it onto the shaft to confirm the fit. It should rotate easily. Lightly coat the outside of the shell, this time with a thin grease, and try to slide the bearing into the house. A few love taps with the bearing protected from the hammer by a wood block are OK, but if it doesn’t go easily, don’t just hit it harder or you’ll distort it. Put it in a freezer or pack it in ice for a few hours, then try again. The cold should contract the metal case enough to make the difference….You may also find Cutless bearings around the rudder stick where it exists the hull. These bearings are removed and replaced in the same manner.

-> The prop shaft shouldn’t wiggle, that’s how you know it’s good.

Propeller

+ What type of propeller do I want?
-> I think a two bladed folded prop for long voyages under sail, and a feathered prop (that doesn’t require modification of the prop shaft) for under power. Purchase the feathered first and purchase the folded prop when getting ready for final voyage.

-> Rare is the cruiser who does not hit the start button when boat speed drops to 2 knots. (This Old Boat, p. 138)

-> On 3/15 I spoke with Farron from Beta Marine and he suggested I go with a bronze, Dynajet 3 bladed conventional propeller. He says he wouldn’t go for a sailor type propeller. Here’s his reasons why:
Pro – 3 blade prop gives better propulsion due to increased blade area. Also, since the keel will have water coming down both sides, it will grab into the water in every position, unlike a 2 blade which loses water flow @ the 12 and 6 positions. It also has better functionality in reverse. It’s not expensive, easier to handle and easier overall.
Con – A 3 blade propeller adds a little more drag on my boat going forward. Most the drag will be at 4 knots or less. If, for example, the sailboat was running at 4 knots, there may be a .5 knot speed loss. A folding prop could work and a 2 blade prop will give less drag, however the blade area is less.

-> So, with that all said (3/16), I believe the best arrangement is to go with a 2 bladed fixed propeller that can lock in place when under sail. There are two parts to this decision, the first is feathered vs. non-feathered and I have compared the two below:

• Pro - Feathered is better at backing up and performs just about as well as a fixed blade in forward.
• Con - Non-feathered are much more simple. Feathering may require more disassembly to get on and off (not so important, now that I will have 1 full time propeller). Also, fast algae growth or “stuff” can clog up feathered propellers feathering mechanism.

The next decision was to compare a propeller with 3 blades or 2 blades:

• Pro - Very little drag while sailing if it can be locked into the keel’s slipstream. Two bladed give 5 – 10% more speed when sailing than a 3-bladed.
• Con - Two bladed are less fuel efficient and vibrate more than 3-bladed. They are also not as good in reverse as a 3-bladed propeller.

When comparing the pro’s and con’s (above), it was clear to me that it’s a bit of a competition between sail and engine power. Since my plan is to sail as much as possible and I also prefer the gains to be had in the sailing department. The other part is that during my voyages I will allow myself to move slow and never let time force me into run the engine. I can also add a 3 bladed propeller to my inventory when required (perhaps for an engine heavy portions of a cruise like up river). Backing up with a 3-bladed propeller is supposedly easier, but my question is if I never learn to use a 3-bladed propeller, wouldn’t I learn to master 2-blades as well as if I had 3 blades? Also, considering the feathering, I don’t believe it’s that big of a deal to deal with disassembly and therefore feathered was the option for me.

-> I spoke to Larry Thompson from General propeller Company. He said that the 2 blades they sell is the “cruisers sailor’ 2 blade. It will, under power, maneuver the boat better than the older ones would without a lot of the sacrifice of the 3 blade with additional drag. Those are fixed propeller. The common one now is the maxi-prop and are out of PYI in Washington State. Maxi-prop (and any propeller w/ moveable blades) is going to be very expensive. If I go with a feathering propeller, I wouldn’t have to make it stationary, because the blades would be flat in the water, but the cost of those is rough – well over $1,000. A lot of that stuff is now sold and distributed by Martec from California. They make their own brand of folding 2 blades. Down in Florida, he isn’t a proponent of propellers with moving parts – sailboat owners are lax in maintenance and you get some algae and growth and the propeller won’t be able to operate (especially with boats in deep coves, in the heat increases the need to clean the props every 2 weeks). He said he is considered the “sailboat specialist”, but he’s never stepped on a sailboat (but he’s been doing this for 30 years). He said he has a book that wrote “Question: On a sailboat under sail, should prop be rotated or locked? Answer: there is less drag when prop rotates”

-> On 4/14 Ben Thomas mentioned “If budget were not a concern I would get the feathering prop from Variprop. German engineering at its best. You will get more thrust with the three blade. There is an ongoing debate about the two versus three blade and loss of speed etc. etc. If you were building a haul ass racing vessel and concerned about every possible tenths of knots the feathering or folding two blade would be the way to go. But if you are cruising and want performance under power use the three blade, it will have a smaller diameter than the two blade.” Plus, he also described how a shaft break wouldn’t be such a good idea nor do I think I have the room. So, I’m a bit back to the drawing board on this question. I think I want a 3 bladed propeller, but what about feathering versus non?

-> As of 10/11, and considering all the research above, I’ve decided to go with a 3 bladed, no feathering propeller. The reason for this is that it will be cheaper initially and I will be using the engine a lot while learning to sail here in the Columba River. Later, when I want to focus more on very long sailing journeys, I might switch to a folding propeller.

+ How can you lock a 2-bladed propeller in line with the keel while sailing?
-> I spoke to Larry Thompson from General propeller Company. He said that they make a shaft break (he doesn’t know where to get them). His understanding is that it’s almost like a disc break set-up. You have to somehow line the propeller up with the keel and mark the shaft inside. You can bring that mark up inside the boat and then line it up with the break. He think it’s called a “Shaft Break”.

-> I think I’ve actually decided against the two bladed propeller due to the shaft break requirement taking up too much space in the engine room. Plus, I had this answer from my query about using shaft breaks from Ben Thomas from Beta Marine: “Setting a shaft brake takes up room. after shutting the engine off, trimming sail you will need to go below, be able to see the prop shaft and set brake when you see your mark that it is in line with keel. In the meantime you are running blind. Doesn’t sound like a very relaxed procedure. Is it really worth the effort? for a couple tenths of a knot in speed maybe.”

+ What size of propeller?
-> See pages 199 – 202 of Cruising Handbook for a full walk through (images also available)

-> Here’s a site that seems worthwhile for finding this: http://www.electricboats.co.uk/surfprop/

-> I asked Stanley from Beta Marine about this (3/10). He said I should prop the engine as 13.5 horsepower – all the propeller calcs done by Beta have been done by a computer and he has seen a lot of people do a lot of calculations based on boat weight and so on and get it totally wrong. He suggested that I use a 12X7 3 blade right hand with 50% blade ratio. Think of the propeller as a circle with 100% of the circle. 50% of blade area ratio (BAR) the blade area only occupies ½ of the circle.

-> I called Beta Marine again on 3/16 and spoke with Farron. He said to remember that since I’m operating in an aperture, I need to measure that so I can have 10% clearance on top and bottom. Don’t want to get it too tight in that area. Want to maintain tip clearance while still getting the largest diameter. He also sent me contact information for a company that sells propellers and shafts and that I can get a discount from them if I am a Beta customer. I will have to give them a call and made it an item on my to-do list.

-> I spoke to Larry Thompson from General propeller Company (3/17). Now that I’m going with a 2 bladed propeller, he suggested to use a 12 x 9 on a 2 blade which is a Michigan-M “Sailor 2” series. These propellers are made in the Philippines than made in Grand Rapids so are considerably cheaper and that’s basically just labor costs – the material is the same, they are good propellers and run @ a cost of $337.69 (retailed @ $397.28). The regular Sailor 2 made is Grand Rapids $483.60. This particular blade is a disc ratio of .36 – disc area ratio is lay propeller down, how much percentage does blade area fill – 36% of circle. From the center of the bore on the propeller and double it, that give the diameter of the propeller. A 3 blade is 55% of the circle – they deal with some high performance power boats that have a 105% ratio which are propellers $30,000 a set, custom made (for sports fisherman with huge diesel engines). It’s almost a compromise between a narrow blade 3 blade and a standard 2 blade. You still have pretty good use of power compared to 3 blade, but there is less drag. No one has really told him what the true drag of a 3 bladed propeller has on a sailboat – but in light wind conditions there will be more affects. He’s heard everything from 1/4 -> 1.5 knot(s) can be lost, but think it’s really quite low. I need to think about where I’m going to motor and in what conditions, that should be a part of my consideration. In Florida, they have to go through passes and draw bridges, it can take everything the boat has to move ahead and so local guys use 3 blades. If you’re just in and out of the marina out of a slip and into the channel, it might not be such of an issue

-> As of 10/11, I’ve changed my decision to go with a 3 bladed fixed propeller. I may purchase a 2 bladed or feathering prop in the future, but not for now. I will determine what size I need during a future call with the General Propeller Company.

+  What is the size of my propeller aperature?
->  13″ absolute max. 12″ better, but still near max.

Propeller Shaft

+ What length propeller shaft do I need?
-> On 3/16 I measured this and found it would be around 66 – 69cm (26 – 27″). I’ll ask the company in Florida as to what they suggest for the length.

-> I spoke to Larry Thompson from General propeller Company (3/17).  He said that what I need to know is the measurement from the output coupling on the transmission to the exit point of a stern tube (or strut). Once you get to the exit point, then add (by theory) 7/8” (which puts us to the start of the taper, where the prop sits). Beyond that 7/8”, there is an extension beyond the taper (the thread body, where the nuts go and the little cotter pin) is 3 7/8”. Total protrusion aft is 4 3/4” of shaft. Now inside…you have to make allowances for the thickness of the R & D coupling. You can take 3 shaft measurements: Overall (complete total length of shaft). OC length (assembled coupling to end of shaft), Set Length (includes toward small end of taper, right before the threads for the end nut. This is common for Chris Craft and a few other builders who make sure you don’t build that too short of a shaft.) A standard, double-nut shaft that hold the prop in place are for normal props (besides special maxi-prop, because of the way they mount. They have a special mount, a button where the normal cotter pin has to be cut off and the propeller has to be fit to the shaft. Tighten everything up, drill the hole for the cotter pin through the nut and shaft because there’s a cap that covers the nuts in a recess of the aft end of the maxi-prop which is where the nut goes into. There’s a cap cover and gears that can be accessed (for feathering and folding capabilities)). They build shafts to ASEJ755 standard (I should be able to find this diagram on the internet to see the tapered end). The price for a 1”, Aquamet 19 is $152 for the length of my shaft (up to 3′). They break everything into full foot increments, if you go 2 – 3” longer than foot.

At this point, I need to find the measurement Larry mentioned above (underlined)

-> [Continue researching/thinking/considering....]

+ What diameter propeller shaft do I need?
-> I called Farron from Beta Marine on 3/15 and he said that the size of the propeller has to do with the shaft tube – do not get a prop shaft that is too big for the shaft tube. The cutlass bearing also must be sized for a particular diameter which is dependent on the propeller size. The Beta 14 diesel engine can take 3/4″, 7/8″ and 1″. The conventional size of 7/8” or 3/4” will work fine and there isn’t a real reason to go past 7/8”. 1” is heavier duty, but is heavier and more expensive. Beta deals with a company in Breigthton, Florida and he will send me information on them. I’ve made it a point to give them a call and get some more information from them.

-> I spoke to Larry Thompson from General propeller Company (3/17). He said that he couldn’t open the program to size it exactly, but ABYC specifies a safety factor of 5 or higher. He enters in the horsepower, grade of material (which pulls up torsional strength), engine RPM and shaft diameter. He’s run it enough that 7/8” is more than adequate for this application. If I’m going to be doing a lot of intense sailing, it might not hurt to go up to 1” for the size of the shaft as not only is it stronger, but 1” is a lot easier to get parts like nuts, keys, cutlass bearings for. So, with that said I have decided to go with 1”.  That being said, I still need to determine the actual measurements of my shaft tube to determine what will fit best

->  Awhile later, I measured my shaft tube.  Here are the key measurements:

• Length of Tube = 8 1/2″ (max)
• Outsider Diameter = 1.377952″ (3.5 cm)
• Inside Diameter = 7/8″

These measurements understood, I will not be able to purchase a 1″ propeller shaft due to the inside measurement of my shaft tube.  Instead, I will need to determine which size cutlass bearing will fit, then size the prop shaft to this bearing.  I’ll have to come back around to this question when I have more time.

-> [Continue researching/thinking/considering....]

+ What metal will I use for the prop shaft?
-> Bronze corrodes a lot less than stainless steel, but I don’t think it’s as strong. The corrosion problem can be essentially solved with zincs, though they must be replaced once a year (another thing to have to keep on hand). So the question probably comes down to the cheaper option for the moment which is a stainless steel shaft with zincs. Well, speaking of corrosion :

The shaft and the propeller are both protected by the installation and renewal of a zinc collar clamps around the shaft. If the shaft bolts directly to the output flange on the transmission, your prop is electrically connected to the shaft, the engine and every other metal item on the boat that is grounded to the engine. A bronze prop is the least noble of the submerged parts of this chain and can be ravaged by corrosion unless protected by a less noble zinc. However, when the shaft is isolated with a flexible coupling, the only electrical interaction is between the shaft and prop. The corrosion potential depends on the type of alloys used for the shaft and the prop and on their relative submerged surface areas (This Old Boat, p. 172)

So…that quote just makes me realize I need to understand exactly how the prop shaft connects (does my style of flexible coupling truly isolate the engine?)

-> I called Farron from Beta Marine on 3/15 and he suggested I go with stainless steel. I could price out a bronze shaft, however not many props are done that way and it will be more expensive.

-> I spoke to Larry Thompson from General propeller Company (3/17). He said that of the 2 grades available, Aquamet 19 & 22, 98% of market uses 19. 22 is best combination of strength and corrosion resistance, but it’s ultra expensive. Aquamet 19 is a trade name it is somewhat compatible to 304 stainless steel. It’s non-magnetic and has some additives for corrosion. There are several different names in the market depending on who is selling what. Their main supplier is Western Branch metals in Chesapeake, Virginia. Their in house material is Aqua-loy, but aqua-loy and Aquamet are the same thing. There are some other companies using things from overseas. He wasn’t sure if Aquamet gets their materials internally, but they do the finishing and polishing in the states.

Stuffing Box & Shaft Seal

+ What type of shaft seal do I want to install?
-> I believe the drip-free, mechanical seals are best. Only issue is that propeller shaft has to be withdrawn to install a new seal. But they are supposed to work quite well.

-> Though I just read today that if you use a certain kind of stuffing called Gore that it’s also quite good. It’s also nice that they don’t fail catastrophically like drip-less, PYI style shaft seals do. I think it’s probably going to come down to cost.

-> Giving this some more thought (3/16) and based on the research I’ve done about shaft seals, I do see the benefit of a packing type seals as they don’t fail catastrophically and they also go with my whole idea of wanting to be as simple as possible. That being said, the technology for dripless seals has gotten a lot better and since I’ve gone to such great lengths to make my boat a “dry” boat (e.g. hull-to-deck joint repair, epoxying all fasteners, etc. etc.), it would be a shame to have a drip-drop all the time. So, my decision is to go with the drip-less shaft seal and to keep a PYI shaft seal on hand in case of emergency.

At this point, I need to research which drip-less shaft seal I will go with exactly so I can insure that I have enough room between my engine transmission, shaft log and cockpit bulkhead to have good access to the drip-less.

-> I spoke to Larry Thompson from General propeller Company (3/17). He said he likes the PSS seals. He hasn’t had any problems out of them. They used to make a slow speed for sailboats, but they have dropped that and they are water injected. If some problem should arise, this will fail catastrophically. The old school way with the flax packing, has 5-7 drips a minute and makes a mess in the bilge. However, 20 miles offshore and something happens, that starts pouring water in, you can grab a wrench and tighten that down tighter. If nothing else, remove old packing and replace in a pinch. Here’s information about both PSS and packing gland shaft seals:

• They sell PYI made PSS seals, but doesn’t have them in 7/8” (General Propeller sells PSS seals for $221). The PSS shaft seal has body, bellows (with a carbon seal attached) and a stainless steel rotor. It slides up the shaft – has 2 o-rings, 4 set-screws and 2 holes. Once the bellows are in place and know the compressed length, push the stainless steel rotor back against the carbon until the 6 1/4” is squished down. The carbon piece (the seal) doesn’t rotate, only the stainless steel rotor. The PSS can be available from for stern tubes/shaft logs of1 1/4” -> 2 1/4”. Another thing on the PSS, the compressed length of that is 6 1/4” and so will require at least that much room between the transmission and the shaft log will require around 8”; this 8” cannot be shortened because the carbon seal has to sit up against the stainless steel rotor to have enough to seal.
• They sell the packing gland seals as well ( General Propeller sells sells them for $78.40 ). The only other thing he wanted to mention is to watch the hose that is placed around the shaft log for the conventional (inside diameter) is only available in 3/4” (smallest) The conventional set-up should require somewhere around 5 – 6”, but the hose can be shortened by cutting the hose and moving it further towards the stern. Nothing rotates on the conventional. There is just the backing and the nut – can tighten the nut to compress the packing.

After this conversation, I gave some more thought about the seal to use and decided that a the packing gland type is best for me. I don’t want to have anything that can “catastrophically” fail on the boat (as a PSS seal has the ability to do) and a couple drips of water while the engine is running really isn’t too-too big of a deal. I can just kind of thing that it’s cleaning the bilge. That being said, the little bit of water that sits in the bilge is what causes smells. Boy, this is a hard one, but I also think that I won’t have enough room for the PSS type so the decision is essentially already made for me.

Flexible Coupling

+ How does a flexible coupling attach to a prop shaft and engine gearbox exactly?
-> A flexible coupling should be mounted on the gearbox output flange and is strongly recommended in almost every case. I believe this is the coupling I have from R & D Marine - link. Here’s some information about it

• Bolts between existing shaft flanges.
• Requires no machining
• Simple to install
• Simple to periodically check alignment
• Reduces engine noise and vibration.
• Fail Safe
• The contoured disc gives clearance for the bolt heads, and is able to flex freely to take up any temporary misalignment of the engine and shaft, due to flexing of the boat structure or the engine moving on its rubber vibration isolation mountings. Forward thrust is taken in compression on the disc between the two half couplings and reverse thrust is taken again in compression on the disc between the two fail straps. In the unlikely event of a disc failure the steel straps make the coupling fail safe and ensure drive is maintained in both forward and reverse.

Here are the instructions from the site as to how to align it:

1. Roughly align engine and stern gear without flexible coupling i.e. only two rigid half couplings pushed together.
2. Bolt ‘R&D Marine’ coupling between the two rigid couplings. Tightening details as below.
3. Check alignment of engine by placing feeler gauges between the RED CONE HEADED BOLT and the rigid half coupling. Repeat for the SAME bolt at 90° intervals by rotating the shaft.
4. If the gap is the same in all four positions, the engine is accurately aligned. Recommended minimum to maximum gap difference: 0.25mm 0.010″
5. Run installation to bring engine compartment to working temperature. Re-check torque settings.

A heads-up about flexible couplings, here is a quote out of Dave Gerr’s “Boat Mechanical Systems Handbook”:

It’s … vital that the shaft coupling match the engine mounts. Soft mounts should have very flexible couplings, slightly flexible mounts should have slightly flexible couplings, and rigid mounts should have rigid couplings.”Also, for vibration reasons he recommends to not have more than one shaft diameter between the prop hub and bearing housing, however that’s probably more a rule for power boats, and 1.5 to 2 diameters is probably ok for sailboats. (http://www.cruisersforum.com/forums/f114/prop-shaft-flexible-coupling-question-52187.html)

So, I’m pretty sure it goes like this: Gearbox -> Flexible Coupling -> [weird metal object thing that connects to prop shaft] -> prop shaft. I kept looking around for more information on what the “weird metal object” was (1/31), but I couldn’t figure it out. I’ll have to keep researching…

-> I haven’t made much progress on this topic (3/16) so I’ve added a note to give Ben from Beta Marine an e-mail and see if he has any photos of installations so I can get a better idea how this comes together. I will mention that I took some up-close images of my coupling tonight and also took a look at the instructions that were included which mirror what I’d already written above. One thing they say in all red is the following:
Please Note: All couplings will give electrical isolation. If electrical conductivity is required an earthing connector can be supplied.

I’m still researching the bonding/grounding bit, but I do think that I will need this earthing connector, so maybe I can ask Ben about this as well.

-> I spoke to Larry Thompson from General propeller Company. He said that a solid coupling is on the transmission and there is a solid coupling on the shaft. With the R & D, it goes between the two rigid coupling on both the transmission and the shaft. You have to do engine alignment with engine coupling to shaft coupling, once you get that down to 4/1000th or lest, then you break the two couplings apart and insert flexible to help for that last little bit (at least that’s how the drive saver works). The drive saver will accept a small amount of misalignment and serves as a possible isolator and will absorb some vibration. The coupling is an electronic isolator as well. Ductol Iron for the shaft coupling because the shaft coupling is cast. Instead of taking a solid round bar and doing all the machine work it’s a lot cheaper as well. The engine transmission (as it has a bearing race) are some sort of hardened steel.

+ How will I secure the set screws for the shaft coupling?
-> Calder suggest drilling through and using a Nylok nut.

-> I also hope to clarify this question a bit from the question I asked above

-> I asked Ben Thomas about this (4/14) and he wrote “…there are two set screws on the coupling itself that help keep the coupling in place. although it is bolted to the coupling of transmission. Drilling stainless is a bitch, especially something round. I would use what was provided by Beta, along with the flexible coupling.”

+ Is there a coupling on the Beta 14? If not, do I need to get one?
-> Yes, a coupling came with my Beta 14 purchase.

Research

Alignment

• With one person in the cabin adjusting the mounting bolts, another should position [themselves] under the cockpit where [they] can see the coupling and call out instructions to raise or lower.  By rotating the shaft with your hand, you can see where the two coupling halves begin to kiss one another.  When all four coupling halves begin to kiss one another.  When all four couping flanges barely touch is a near-perfect alignment.  Hunker down on the engine mounting bolts so that nothing can vibrate out of place.  While a feeler gauge is the traditional method, an easier way is to insert a piece of paper between the two couplings and study the imprints and grease markings for uniformity.  (Upgrading the Cruising Sailboat, p. 147)

Cutlass Bearing

• The cutlass bearing is therubber-surfaced insert through which your shaft runs…be warned that these eventually wear, especially with a lot of use. Wear can cause vibration severe enough to do damage. We usually check ours by diving, grabbing the shaft, and shaking it to see if there is any play. There shouldn’t be any lateral movement at all. If there is, you will probably have to get hauled to replace it. A spare should be part of your inventory. If you keep the old one, you can use it as a tool to force the bad one out.  (All in the Same Boat, p. 150)
• ….gear that are[is] all too often ignored are the cutlass bearing…The cutlass bearing is a bronze tube with a grooved rubber liner that fits inside the fiberglass tube through which the prop shaft exists the hull. Replacing it is relatively simple. While the shaft is out, pound out the cutlass bearing with a hammer using a section of wooden dowel of the right diameter. There may be a few set screws or allen nuts to remove first, but that’s it.  (Upgrading the Cruising Sailboat, p. 150)
• A cutlass bearing is nothing more than a short length of naval brass or composite tubing with a splined hard-rubber liner. The rubber supports the propeller shaft where it exits the hull, and channels allow water into the bearing to serve as a lubricant and to wash out sand and other abrasives. The bearing’s inner diameter matches the diameter of the shaft, and the outer diameter is a snug fit for the stern tube or bearing housing in the strut. A Cutlass bearing can last 10 years or longer when the shaft is properly aligned. No maintenance can be done on a Cutlass bearing; either is is OK or you replace it.  (This Old Boat, p. 175)
• Compared to removal, replacement is a piece of cake. First lubricate the new bearing with soap – never grease – and slip it onto the shaft to confirm the fit. It should rotate easily. Lightly coat the outside of the shell, this time with a thin grease, and try to slide the bearing into the house. A few love taps with the bearing protected from the hammer by a wood block are OK, but if it doesn’t go easily, don’t just hit it harder or you’ll distort it. Put it in a freezer or pack it in ice for a few hours, then try again. The cold should contract the metal case enough to make the difference….You may also find Cutless bearings around the rudder stick where it exists the hull. These bearings are removed and replaced in the same manner.  (This Old Boat, p. 175)
• DIY Removal of Cutlass Bearing
• Removal of cutlass bearing of Triton boats
• Replacing the Cutlass Bearing

Flexible Coupling

Propeller

• Engine performance is only as good as the propeller. The first consideration should be to deal with the mistmatch that can occur between efficient engine speed and efficient propeller speed. Most engines for cruising boats run efficiently in the 1800 – 3000 rpm range, while propeller tip speed and blade areas work best at about ½ those speeds. The solution to this problem is to install a reduction gear on the engine to cut the shaft speed. But…the compromise is that reducing shaft speed (and, with it, propeller rpm’s) means that a larger propeller must be installed so that thrust is not lost, and a larger propeller adds drag and reduces sailing speed.  (Desirable and Undesirable Characteristics of Offshore Yachts, p. 238)
• If the yacht has a full-length keel with the propeller in an aperture, a good compromise between good thrust and low drag is to use a two-bladed solid, nonfolding or nonfeathering propeller, whose blades can be aligned with the keel when the boat is under sail,  (Desirable and Undesirable Characteristics of Offshore Yachts, p. 238)
• Cruising people have benefited by the development of the low-drag folding or feathering propellers for racing yachts, and modern folding and feathering propellers are nearly as efficient as solid ones. However, some owners elect to have both kinds of propellers – the feathering or folding type to be installed before a cruise that will made mainly under sail, and a solid type for a long passage that will require a lot of powering. If the two propellers have matched machining, they can be changed in a quiet harbor without the need of…haulout.  (Desirable and Undesirable Characteristics of Offshore Yachts, p. 238 – 9)
• Choosing the right propeller for a given boat and engine makes all the difference in performance and fuel efficiency. There are so many variables affecting correct pitch and size that scientific selection is difficult for the average person. Propeller sizes are given in pair of numbers, such as 13×9 or 12×8. The first number refers to the diameter of the blades in inches. The second refers to the pitch, or angle of the blades and is the number of inches the prop theoretically should move forward with one revolution. The greater the pitch, the bigger the “bite” the prop takes.  (Upgrading the Cruising Sailboat, p. 148)
• Many engines have reduction gears to the propeller shaft. A 2:1 reduction gear means that the engine is alays turning twice as fast as the prop shaft. An engine with 2:1 reduction gear would use a larger prop than one with direct drive. The smaller size for the direct drive is necessary because through the prop turns faster, there is less torque to keep it turning under load.  (Upgrading the Cruising Sailboat, p. 148)
• …fixed two blade props locked in the vertical position offer about 15 – 20% more resistance than feathering two-blade props. While this evidence is compelling sometimes these props don’t like to open correctly. The cruising sailor who wishes to avoid as many repairs as possible may find the fixed blade prop the wisest choice. For the cruising sailor, it is less a decision and more a matter of personal inclination whether to power to windward or accept that Neptune delivers and do the best under sail alone. To effectively motorsail in heavy seas, a large propeller with high pitch coupled with reduction gear is the best combination. But the increased drag acts a price in sailing performance.  (Upgrading the Cruising Sailboat, p. 148 – 9)
• There is a choice to be made between two and three bladed props as well. The two bladed prop will give about 5 – 10% more speed than the three-bladed prop under sail, though the latter is slightly more fuel efficient under power and vibrates less. While converting from three blades to two, it is unusual to increase the diameter.  (Upgrading the Cruising Sailboat, p. 149)
• Variable pitch propellers have much to recommend them in that the pitch can be adjust for different speeds and sea conditions, depending on the torque requied.
• At full speed the tach should reach the engine’s rated maximum rpm. If the engine fails to achieve the rated rpm, the prop may be too large or the pitch too great.
• …the folding propellers, especially the two-bladed type, offer the least drag under sail but have the worst performance under power, especially when in reverse. The feathering propellers (e.g., Max Prop, Martec, and Luke) create a little more drag under sail, but perform about as well as a fixed bladed propeller in forward, and almost always perform considerably better than the fixed propeller in reverse….my only comment is that at some point, the propeller may get damaged….Some of the feathering propellers require more disassembly to get on and off that others. Some also require the propeller shaft to be modified (which makes it difficult to retrofit a fixed-blade propeller); others fit onto a standard propeller shaft.  (Cruising Handbook, p. 202)
• An MIT velocity prediciton program suggests that the average impact of a three-blade prop compared to no prop is about 5% of boat speed, with the impact being greater in light air and less as sails develop sufficient power to overcome additional drag. Rare is the cruiser who does not hit the start button when boat speed drops to 2 knots.  (This Old Boat, p. 138)
• …I tend to come down on the side of less complexity. Fixed props are simple and foolproof. Feathering and folding props are neither, so be sure the benefit for you will justify the added complexity.  (This Old Boat, p. 174)

Propeller Shaft

• The distance between the engine-half coupling and the end of the cutlass bearing in the stern should be measured. Most prop shops use dimensions calculated to the SET or small end of taper.
• A measure of security [if a shaft were to fall off] can be provided by putting a host clamp around the propeller shaft just forward of the shaft seal. If the shaft comes loose, the hose clamp keeps the shaft in the boat. However, with certain high speed shafts, the clamp may cause sufficient imbalance to create vibration. It may be possible to remedy this by using two hose clamps with counter-posed screws. If not,t he clamps will have to be left off.  (Cruising Handbook, p. 203)
• The installation manual will recommend shaft diameter, but the rule of thumb is 1/14 of prop diameter. This assumes bronze or stainless steel. An Aquamet shaft can be up to 20% smaller….be sure to buy a cutlass bearing to fit it, but with the shell diameter of your existing bearing. Otherwise you will have to replace the stern tube, a bigger job than you might want to take on.

Stuffing Box & Shaft Seal

• Conventional packing depends on water to lubricate it, and if you have the box well adjusted, it drips only when the shaft is turning, but there is a way for sailors to make a conventional packing dripless  (This Old Boat, p. 164)
• Replace the stuffing box with a mechanical seal. However, stuffing boxes lose their seal graudually and never catastrophically. Shaft seal failure is more likely to be sudden and dramatic. Be sure to learn what care your particular shaft seal requires and see that it gets it.  (This Old Boat, p. 164 – 5)
• Face seals operate on the principle of a sealing ring rotating against a machined flange, one turning with the shaft, the other held stationary by clamping it to the stern tube. The two are pressed together by some type of “spring” pressure, usually a rubber bellows but sometimes water pressure. Unvented face seals need to be “burped” after launch by compressing the bellows at the top until water pours out. Otherwise trapped air will cause the seal to run dry, which it is not designed to do. Burping the seal each time you service is probably not a bad idea. Bellows “set” combined with weakening engine mounts can allow enough forward shaft movement when the prop is engaged to open a gap in a face seal. Poor engine alignment or excessive drivetrain vibration will stress any mechanical shaft seal.  (This Old Boat, p. 164 – 5)
• Lip seals are simply an adaption of a common oil seal. They are installed in a bearing housing that clamps inside the shaft tube hose. The thin rubber lip of the seal runs against the surface of the shaft, usually held in contact with an internal elastic or spring collar. Lip seals are nearly always plumbed for lubrication by either water injection or gravity-fed oil from a reservoir mounted above the seal.  (This Old Boat, p. 164 – 5)
• Shaft seals are not supposed to require any maintenance, but don’t believe it. All of them use hose clamps that need to be checked at least annually, some face seal designs depend on setscrews to secure half of the seal to the shaft. Make sure the set screws are not tightened against the hard, smooth surface of the shaft. The shaft should have been dimpled with a drill when the seal was installed. To do this, buy a new cobalt bit smaller than the hole, shrink an inch of electrical shrink tubing onto the bit to avoid damaging the threads, and dimple the shaft through the threaded hoe. Turn the shaft to roll the hole down and blow or flush out the drill shavings with an ear syringe. Coast the setcrew with Loctite and reinstall it. Give the remaining setscrews the same treatment, one at a time. For belt-and-suspenders security, tighten a hose clamp or a new zinc collar around the shaft hard against the seal collar. (This Old Boat, p. 164 – 5)
• …I am a fan of the “drip-free” mechanical seals, of which of my two favorites are the PSS and the John Crane (Halyard) seal. Given a well-aligned engine, they really are drip-free…If a mechanical seal is fitted and the boat is hauled, it is important to remember to pull back the seal until water squires into the boat when the boat is re-launced. If the seal is not lubricated in this way, it may burn up – which is when you discover the real drawback of these seals: the propeller shaft has to be withdrawn from its coupling to install a new seal.  (Cruising Handbook, p. 203)
• I have had dripless or PSS seals on many boats as well as traditional stuffing boxes. I worked on a 55 foot sport fisher that had over 5k hours on two PSS seals with zero issues. My buddy Jerry owns a 100+ foot “yacht” (how the other half lives) and it has two huge 16 cyl Cats and two PSS shaft seals. Boat has perhaps 70k nm on her and not a single issue. The one on our own boat went for 2782 hours before I replaced the bellows. I shipped the bellows to PSS and they tested it within like new specs at 6 years old and 2782 hours. Lots of reputable builders such as Sabre, Valiant, Hinckley, Little Harbor, Pacific Seacraft, Swan and the USCG use them too. Both types work and as Tim said with new packings, such as Johnson Duramax Ultra X, Gore GFO or Western Pacific Trading GTU the traditional boxes can be made to drip very, very minimally.  (http://www.plasticclassicforum.com/forum/)

Purchase List

• 3/4” Aquamet 19 (~$150)
• 3 Bladed Propeller
• Cutlass Bearing
• Shaft Seal (packing gland type) ($78.40)

Page Menu

• Image Gallery
• Questions
• Research
• Comments