Big engine in small sailboat but not much forward motion

[Hugo B]

Hi

I recently bought a sailboat, which I am very happy with except one thing. When I use the engine there is very little forward motion, never more than 3.5 knots. This is a problem when I have to deal with currents, as she sometimes hardly makes headway in them.

The engine is a Yanmar 2GM20 and the propeller is a 3 blade propeller. The (monohull) boat is only 27 feet long and weights about 4 metric tonnes. I used to have a boat of similar size and weight with a 2YM15 Yanmar and she did 6 knots! Everyone keeps telling me in my new boat it must be the size of the propeller, but until I lift the boat I can not really measure it and look it up in relation to the engine size.

In any case, she is a full-keeler with attached rudder with the prop sitting in the cut-out space between the keel and the rudder. At the top, that space is the hull itself so there is no room to cut a bigger hole to fit a larger propeller.

I was thinking of perhaps fitting a 4 blade propeller which seems to give better performance at lower speeds, but not as good at high speeds. I am wondering, say at 2000 - 2500 RPM whether this would help or perhaps even make it worse. The other possible solution is to change the shape of the propeller to give better pitch, but the one that is there seems quite standard to me in shape when I look at other props on the web.

One more thing I can think of is that the engine is positioned "backwards" in the boat, with the fan belt aft and the gearbox in the front when you access the engine from under the stairs. Perhaps this changes the transfer of power to the shaft drive in some way?

Any ideas or suggestions how to improve speed would be most welcome.

Thanks

Hugo

[Chris31415]

You don’t say the make & model of the boat. Do other people with the same boat get the same results from their engine?

[Hugo B]

Hi

The boat is a Top Hat 27. Although the Top Hat 25 model is very common (and regarded as very seaworthy), only 4 Top Hat 27 yachts were ever made. I know of one other but it has a different engine, so that does not really provide a good comparison. I would say the Top Hat 27 is very similar in specs to a Cape Dory 27. But it is not a standard boat with a standard engine so it is hard to find out how fast it should go by comparison. I go by engine size, boat length and weight, which at a certain RPM i.e. 2500 I would think should give a beter speed than 3.5 knots.

[Jim Cate]

Are the hull and prop clean and free of growth? Fouling on either or both can severely degrade motoring performance.

Jim

[Hugo B]

I do not think it is a fouled prop as when I discovered this I spoke to the previous owner and also the owners before that. They say under engine the boat has always been slow. I assume the prop must have been clean at least some of the time. The hull itself is quite clean.

Of course I cannot rule out growth on the prop (I will dive and have a look when the weather gets better), but let's assume for now that is not the problem. What alse could it be and what are the options (such as a 4 blade prop) to resolve it?

[Wheelguns]

How is the bottom? Any growth? When motoring, does the engine reach the specified max rpm?

[Jim Cate]

Well, a prop with the same diameter and more pitch might help, and a prop shop can usually alter the pitch up an inch or two.

Knowing the pitch, diameter and transmission reduction ratio will be useful data bits. The power should easily be adequate.

Jim

[Ballsnall]

If you have a clean bottom and the props not fouled then I would suggest you go back to basics before making any drastic changes.

What is the WOT revs?
What is the gearbox reduction gearing?
What is the prop diameter & pitch?

Without this basic information your just guessing.

[Ballsnall]

Originally Posted by Hugo B

IWhat alse could it be and what are the options (such as a 4 blade prop) to resolve it?

Either under prop'ed or wrong transmission ratio. A 4 bladed prop will be only a marginal improvement in this case if the same pitch. You need to go back to basics and collect all the relevant information in order to make an informed decision.

[nuku34]

You say the engine is "backwards' with transmission facing forwards. In that case, there must be some sort of "V" drive to connect the transmission with the propshaft. What type of unit is that? If that unit is in bad shape, it may be passing all the engine hp to the propshaft. My boat has a "V" drive with a 3 row chain running in an oil bath.

[skipperpete]

I recently read about forum member “Sailorchic’s 3GM30 rebuild and she mentioned having a Vee drive on the back of that Yanmar.

[SY Harmony]

@2500 rpm with a 2GM20 you are only delivering about 10-12hp to the shaft. Run it up to 3000-3200 and see what happens.

[Benz]

I have a small plywood schooner with a 1GM and a V-drive. Same problem-no speed even at higher RPM. Changed the prop to 4-blade and still no improvement...I think it's a transmission gear-ratio problem.
I'm selling the boat (I have too many boats), but if I were keeping it, I'd look into a differently-ratio'd transmission.

[Greatest Lakes]

I have a 2gm 20f and I'm good for about 4- 4.5 knots with a 24' waterline. With a 20' waterline, you are probably about where you can expect to be.

[JimsCAL]

I would expect much more speed than you are getting. Totally agree with the suggestions to check the bottom and prop for fouling, and to determine the transmission ratio and prop pitch. Then you can think about a new prop. I doubt a 4 bladed prop would make any difference. If anything a change in prop pitch might be helpful.