Engine too small

[Capn Mud]

I bought a 25 foot boat with with a Yanmar 10GM (8 hp) sail drive motor.

On its delivery trip in calm conditions (indeed almost millpond) it would only do 3.5 to 3.7 knots.

Delivery skipper (an industry professional - not just another grotty yachty) things that it could be the prop is incorrectly pitched or the motor undersized. Yanmar rep seems to advise that it is in fact undersized for the boat and that correct motor size and prop should give about 6+ knots estimated.

Problem is that the previous owner did an excellent job of the fitout (part of the reason I bought the boat) and a larger engine just will not fit in the engine well. A major strip out and refit would be required.

Are there any other options for a fix?

Data as follows:
Data collected to data.
Vessel Swabrick
Loa 9 m
Beam 3 m
Draft 2 m
Disp 2.5 tonnes (guesstimate only no data available)
LWL (will need to measure assume 7 m)
Engine 1 GM 10CE inc SD 20 leg
Prop - Gori 2 blade folding prop
Current speed at 2500 rpm (max rpm) is approx 3.5 knots, engine works hard and emits black smoke, burns oil.
Have been cruising at 2250 rpm, speed approx 3.1 knots
Vendor cruised at 2000 rpm and stated on survey report
Mike estimates this engine is lacking 900 rpm, (8 hp rated cont at 3400 rpm)

[Rick Willoughby]

I had a very similar yacht that displaced 3 tonne. It had an 8HP Bukh sail drive and could get almost 7kts at full throttle. The fuel economy at 5 to 6kts was impressive.

I would be looking for something that is wrong. Motor is not stuffed. Prop is in good shape, clean and properly selected. Hull does not have excessive growth or some other drag inducing incumbrance.

I would be surprised if you need more than 5HP to get the boat near hull speed - something like 6.5kts.

Rick W.

[Capn Mud]

Rick,

Thanks for the feedback....

Yes she has a bit of a dirty bum - cleaned in March by a diver but then sat still in Marina in Cairns for 3 months. But I would expect that would only reduce speed by 0.5 to 1 knot at outside wouldn't it?

So something else is wrong. Motor seems in very good condition..... That leaves a prop problem I guess.

Cheers,
Andrew

[FAST FRED]

That leaves a prop problem I guess.

First see if the warm engine will reach the rated 3400 RPM in neutral.

If it does I would look for either a dirty prop or a damaged prop.

With the hull clean , the prop clean and still not getting rated rpm, its time to reduce either the prop diameter or pitch.

Could you borrow a prop more suited to the boat , even if not a folding prop?

FF

[Rick Willoughby]

Rick,

Thanks for the feedback....

Yes she has a bit of a dirty bum - cleaned in March by a diver but then sat still in Marina in Cairns for 3 months. But I would expect that would only reduce speed by 0.5 to 1 knot at outside wouldn't it?

So something else is wrong. Motor seems in very good condition..... That leaves a prop problem I guess.

Cheers,
Andrew

Sitting for 3 months in Cairns could result in a lot of fouling. The prop is unlikely to be antifouled so it could be encrusted and just not working well. If it is a folding prop it may not be getting fully open.

Worth a look underneath as soon as you get a chance.

Rick W.

[Capn Mud]

Thanks guys for you're assistance.