Big engine in small sailboat but not much forward motion

[rslifkin]

Run it at WOT and see what happens. If you reach rated RPM with plenty of throttle left to go, either something is slipping or it's underpropped. Now that you know what the gear reduction is, you'll want to find out what size the prop is to determine if they're a reasonable match, or if it just has totally the wrong prop on it.

[Talbot]

One area nobody has mentioned but is worth a check. Take the exhaust elbow off and ensure that it is not choked with carbon. This is a common problem and will significantly reduce power.

[sinnerman]

My boat would run at max rpms/wide open throttle (WOT) and go no where.

Paddleboats would pass me & the boat could barely make headway against an opposing current.

I pulled the prop (3 blade) when doing the bottom job and brought it to a prop shop to clean/measure and the pitch was HALF of what it was supposed to be for the boat/engine/Transmission gear ratio setup.

It was basically spinning without driving the boat forward

I am guessing that a previous owner wanted a 3 blade and installed what ever he could find that would fit.

I installed a new prop with the correct pitch and now have plenty of power and less prop walk in reverse.

I bought a Campbell prop as these are more efficient with "cupped" blades.

https://westbynorth.com/campbell-sailer/


cheers

[wyb2]

Originally Posted by Hugo B

…

If there is no solution, I am considering mounting secondary propulsion to the boat, in the form of a 6 or so HP outboard, like the Tohatsu Pro Sail with a high thrust propeller. I know that 8 HP would be better, but then the outboard gets quite heavy to mount.

By itself it will not make the boat go faster of course, but together with the Yanmar, it might make the boat fast enough against a heavy current for example. It also is a good backup of course. The question is whether an inboard and outboard prop will work together well or whether the wash from the Yanmar prop will make the outboard prop useless. Any thoughts on that? Anyone ever done this?

…

This is the equivalent of planning to add a second mast to your boat because it came with an incorrectly sized mainsail.

You have a working inboard. Adding an outboard will be more work and cost more money for a significantly worse result than simply troubleshooting and fixing the problem at hand. The day your Yanmar stops with a loud clunk and won’t turn over, then you can start shopping for outboards. This is coming from someone who has a 6 hp outboard on a bracket on my 25 ft boat.

Listen to Ballsnall and others, gather the info requested and work the problem. It’s sounding like an under pitched prop, but who knows until you gather the necessary info.

[sailorladd]

Well, I had a similar issue with my Yanmar YSM 10 HP engine, until I hauled out and enlarged the aperture where the prop goes, and installed a rather larger prop. The extra speed was very noticeable, but always bear in mind there is a formula with sailboats whose hulls are not designed to plane. Without looking it up, as I recall, the square root of your waterline times 1.2 if you have a bolt on keel or times 1.4 of your square rooted waterline for long or full keeled yachts as was mine.. you won't exceed that speed...

[RedneckRedcoat]

So been recently talking to yanmar about prop sizes for our boat. Here is what we got told

First rev the engine in neutral to maximum revs , that will tell you if the engine is working to the specified engine max rpm’s

Second whilst underway with a clean prop and hull rev the engine forward to max rpm’s . There should be a difference.

On ours, our rpm was 4300 in neutral and 3800 in forward the yanmar is rated at 3600 which means we could go slightly bigger or change our pitch.

If you did this then least you would know if it was the engine or the prop.

[Bowdrie]

A friend of mine moved-up from a light weight 25' to a Westsail 32.
The W32 had an almost new Yanmar of ~30>35hp and a new MaxProp.
Whilst the Yanmar would happily scoot right-up against the governor, the boat would barely make ~4Kts.
At first he thought that that was all he could expect from a "tubby boat".
Hauled-out and found that the Maxprop was set for ~6" pitch.
Re-set the prop and all is well, now it easily makes hull speed with some RPM to spare.

[capt jgw]

Originally Posted by sailorladd

Well, I had a similar issue with my Yanmar YSM 10 HP engine, until I hauled out and enlarged the aperture where the prop goes, and installed a rather larger prop. The extra speed was very noticeable, but always bear in mind there is a formula with sailboats whose hulls are not designed to plane. Without looking it up, as I recall, the square root of your waterline times 1.2 if you have a bolt on keel or times 1.4 of your square rooted waterline for long or full keeled yachts as was mine.. you won't exceed that speed...

Formula I've always used is 1.34 x square root of waterline length, for all non planing hulls. The physics don't change if you have masts and sails. The physics do change for long skinny hulls like a catamaran, they can go faster.

[cutlass27]

Check your exhaust elbow. I have seen these so reduced in size by soot etc that the engine just can’t function properly.
Good luck hope that works.

[capt jgw]

Originally Posted by Bowdrie

A friend of mine moved-up from a light weight 25' to a Westsail 32.
The W32 had an almost new Yanmar of ~30>35hp and a new MaxProp.
Whilst the Yanmar would happily scoot right-up against the governor, the boat would barely make ~4Kts.
At first he thought that that was all he could expect from a "tubby boat".
Hauled-out and found that the Maxprop was set for ~6" pitch.
Re-set the prop and all is well, now it easily makes hull speed with some RPM to spare.

Which sounds like what the OP is experiencing, though I haven't seen him say what RPM he can get in gear. Most diesels hit max HP around 80-90% of max rpm, that's where you should hit max boat speed. You do that by having the correct propeller pitch.

[Algee]

I have a 29ft Trintella, about 4t and also has a full keel, skeg hung rudder with rudder cut out. The similarity continues, mine also has the 2gm20 engine, I get 5,5kts without pushing the engine revs, can get 6kts.

If the boat was re-engined, what was the gearbox ratio and rotation out, the compatability of of the prop, LH, RH.

From memory, the max revs for this engine is 3,600, continuous would be around 3,200. If you have a way of checking the maximum revs that your set up allows, this is a simple way of checking the pitch and dia of your prop. If max revs, whilst in gear, only reach, say 2,800, your prop dia or pitch is too great. I say a simple way to check, it does not suggest that you have the ideal setup even if the revs are within limits.

Hope you get it worked out, I can imagine your frustration.

[GrowleyMonster]

Remove your prop. Yes, you can do it with the boat in the water. I have done it many times. Once, twice in one day on the same boat. Make sure you have spare washers, cotter pins, key stock in various sizes, and Jesus nut on hand. You will probably drop them. Working over shallow sandy bottom helps. Sometimes you can drop something and recover it easily. Tie off your prop and have your helper pull it up. You might need a puller.

Scuba or hookah helps a LOT!!! But you can do it just free diving. Again, I have done it.

Your first time, clean everything up with a wire brush and remove the nut, being careful not to drop it. Tie the prop in place. You may be able to secure the prop with a hose clamp on the bit of shaft sticking through the prop. Very likely it will take an act of congress to get it free so it probably isn't going anywhere. Go to your local chandler and get a couple more nuts, and washers. And cotter key. And a puller. This is an assembly with three hooked legs and a central screw that you tighten to pop a gear or flywheel or prop off a shaft. You'll figure it out. You may be able to borrow one from a marina neighbor. Try to rotate the prop so that the keyway is facing up. Have some 3/16", 1/4", and maybe 5/16" square brass key stock is a very good precaution against loss of the key. It cuts dead easy with a hacksaw. Chamfer the cut piece at both ends. It will fit better. But try not to drop the old key. Tie off the prop and puller ahead of time and have your helper pull them up on the boat. Tie a line to them BEFORE they are free especially if you are free diving with no bouyancy compensation and are in deep water. I know a guy tried this, prop was too heavy, and he had a choice to let go of the prop or go down to the bottom with it. He kinda did both, almost drowned, and still lost his prop. Think out every step ahead of time. It is do-able. A diver could do it in less than an hour, easy, and you may find that hiring a pro for the difficult part, pulling the prop for the first time in years, is better than DIY and cheaper than a haulout.

Anyway, so yeah, get your prop off and up in the dry. Measure diameter. For a 3 blade prop measure from the center of the shaft hole to the tip of a blade, and double it. Pitch is basically the distance forward that the prop will travel when revolved one complete revolution with zero slip. If you take the diameter and multiply by π to get the circumference. Measure the angle from vertical, that the blade is twisted. Solve the triangle. The circumference is the ADJACENT side. You have the angle. Solve for the OPPOSITE side with the tangent formula. T=O/A. Tangent of the angle = OPPOSITE side divided by ADJACENT side, so multiply ADJACENT side by the Tangent of the angle to get the OPPOSITE side, which is basically your pitch. You can also jsut do this graphically on paper with no math, just measuring and marking.

Once you have diameter and pitch, you can more effectively get advice, or try other props. Some prop shops will loan you a prop to try, if they have an old one in the desired diameter, hand, and pitch. You can also take your prop to a prop shop for measurement, nick and ding repair, balancing, polishing, etc.

Higher speeds through the water call for higher pitch. Lower speeds through the water call for lower pitch, relative to diameter. "Square" pitch is where pitch equals diameter. This can be about right for some motorboat applications. Sailboats typically rate a pitch about 70% of diameter, very small boats with low hull speeds as low as 50%. Too high a pitch is inefficient and creates cavitation, and increases load on the engine. Too low a pitch requires much higher revs (more cavitation. Go figure!) and reduces the load on the engine, and that isn't necessarily a good thing, either. Your diesel wants to run within a certain load range and a certain RPM range. Your prop wants to turn at a certain prop RPM for a certain speed through the water. Your hull doesn't want to exceed hull speed and is more efficient at a certain fractional amount of hull speed, generally estimated in Knots as 1.34 x √L, where L is waterline length in feet.

Like everyone else I doubt if this is an engine issue, although could be your RPM is a bit low for what you are expecting in speed.

Another thing you can do that might help you out is take some reflective life jacket tape, just a little square, and stick it on your prop shaft or flange. Get a cheap laser tachometer and zap the shaft for your shaft RPM. Compare it to your engine RPM for a gear ratio and to get an idea of whether or not you are overpropped or underpitched or whatever.

Some gearboxes have a different ratio in Ahead vs Reverse. If someone put the wrong hand prop on the boat he could have reversed the controls, basically turning a backwards prop in the backwards direction to go forward, but using what is supposed to be the reverse gear for going ahead. Just thinking out loud. Anyway knowing your ACTUAL gear ratio and not just nameplate data could be a good clue.

A dirty or dinged or bent prop IMHO is actually the most likely cause, with a dirty hull contributing to it. Your local prop shop guy can recondition your sad and tired prop in no time and probably for less than $100. $300 for "while you wait" service. Consider that before you start randomly trying other props.

If it is the original prop, then it is probably the right prop for the boat and engine. Still, good to know exactly what you got.

[gs41escapade]

Prop needs to be examined. If it ever had any damage to the prop tips, then performance would be terrible. (speaking from experience after contact with a submerged object - never found out what bent my tips and prop had to be replaced). Any vibration when under way?

[capt jgw]

Originally Posted by GrowleyMonster

Remove your prop. Yes, you can do it with the boat in the water. I have done it many times. Once, twice in one day on the same boat. Make sure you have spare washers, cotter pins, key stock in various sizes, and Jesus nut on hand

I changed a prop once by backing the boat into a launch ramp. Shallow water with a nice concrete bottom to stand on and you can pick up your dropped parts and tools with your toes.

[GrowleyMonster]

Originally Posted by capt jgw

I changed a prop once by backing the boat into a launch ramp. Shallow water with a nice concrete bottom to stand on and you can pick up your dropped parts and tools with your toes.

A very good idea! I have a spot where I take smaller boats, with clear sandy bottom and neck deep water, where I can do prop changes and stuff, bottom and prop cleaning, zinc replacement, through hull clearing, stuff like that, without gearing up, but a nice concrete boat ramp is even better I'm sure.


And OP, I forgot to mention, a lot of props have the hand, diameter, and pitch stamped on the hub. A good cleaning and polishing will make it stand out nice, where you can read it. It will probably be in a format like, "RH 13-10" or something like that, and might even give shaft diameter too, though that is easy enough to measure. Shaft is tapered for the prop hub, and there are a couple of different tapers in use. Reboring requires a lathe and is not trivial for the average wannabe machinist.

Also, replace your zincs. I guarantee you, they are gone by now. Measure your shaft diameter and get the right size. If there is a piece of the old left, and you can't get the screws out, don't worry too much about it, just mount the new one on the shaft right by the old one.

while scrubbing and scraping your hull, look for blisters. Minor - no biggie. Lots of big ones mean a haulout and spending a few boat bucks down the line some time. You will be wanting new bottom paint at some point, too.