Originally Posted by jbe
We just had the bottom cleaned and painted, all oils changed, belts done etc.
We are only sailboats and the 31's have smaller engines. Today at 2950 we are doing 6 mph with little current. The prop is a 16 x 11. Just checked with dealer and that is what all e33's are equipped with. If we were dragging something wouldn't we feel it? Also, I have not heard our cruising RPM's should be over 2750. Where can I find that confirmation?
I have a 2GM20
and it goes well. Your prop sounds about right. I used to have a folding prop of the recommended pitch / diameter. I think it was finally 16"X10. I did have it adjusted an inch or two finer than original to achieve 3400 rpm flat out. Now I have a Kiwi Feathering prop which is still 16" but the pitch is not relevant as they are in degrees and adjustable. That didn't change the flat out speed, but is better sailing (in my opinion) and adjustable pre set and reverse is much better.
You should theoretically be able to achieve full revs 3600 at full throttle in calm conditions. I've chosen 3400 myself. The prop shop suggested 1" of pitch is a change of 100 rpm maximum rpm as an approximation.
The only problem I've had that sounds similar to your problem was the rubber fairing under the hull
around the sail drive leg coming loose and forming an effective water
brake. It's possible that's happened to you since you were hauled out. Mine is coming loose at the front again. I'm going to make a fibreglass replacement with a small gap screwed and Sikaflexed on next haul out
The other thing that greatly affects motoring efficiency is barnacles
on the prop. They can appear fairly quickly. I use Prop-Speed which it seems can only be applied by a licensed agent. It does work very well. Herreshoff wrote that a prop is driving a boat forward with the same force that the hull
is holding it back. He said that because the prop is 1/1000 the area of the hull underwater, 1 barnacle on a prop is like 1000 barnacles
on the hull ( or similar).
Flat out, we motor
faster than most similar sized yachts but I usually cruise around 2900 / 3000 at just over 6 knots. We can get up to 7.2 knots @ 3400 with ideal conditions. That is correctly measured at slack tide no wind
and running 2 ways. Our waterline is 27.5' / 28'. ( you are quoting MPH which isn't knots)
Another thing that can greatly affect engine efficiency is your exhaust
system. It should be the same internal diameter as the exhaust
bend all the way including into and out of your water
lock. I think that is 1,1/2". Often people stick a plastic hose extension in a cast skin fitting at the transom outlet and that reduces it to around 1", making a huge restriction, as it also carries the cooling
water. I once found that my water lock connections were apparently 1,1/2" from the outside but inside, the thick wall made it only 1". I replaced that.
If you can only achieve 2750 rpm and your hull is clean and clear you could also check that your throttle cable is correctly adjusted. You can see that just by the injector pump
. Disconnect the cable to make sure the lever on the motor is traveling the whole way to its stop inside the engine when the cable pulls it. The full cable travel should be limited by the stop at that engine lever, not back at the controls.
I hope that all helps