New engine still no power

K

KAM

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I made a few posts about trouble I was having last season getting anything like full power from my new engine. I now suspect it is overpropped. 28hp trying to drive a 16 x 10 at 1600 shaft speed. It's a 33 ft fin keeler and used to do 7.5 knots on 23 HP saildrive. I now only get 5 knots. Research indicates the prop should be about 14 X 9 or possibly 8 which is starting to look very small. Problem is I have no black smoke at all which I am led to believe is a feature of overpropping. Would appreciate any comments preferably from someone with similar engine and boat.
 
Max revs should be 3600 but I can only get 2200. I have checked all the usual points, linkage,fuel,governor.
 
Yes its very free. Its a saildrive. I checked it before launching 3 weeks ago. I also double checked the ratio 2.14 and the rotation direction. Left hand same as prop.
 
I forgot to mention it's a folding prop. I swam under the boat with the engine running to check it was opening OK. (getting desperate!)
 
the 16 x 10 was supplied with the new saildrive package. The supplier said it would match the boat. They are now offering a selection of props to try but before shelling out for a lift I could do with a second opinion.
 
Sorry I don't know I changed the engine when I bought the boat. I was quoting book speed. I don't have the md11 data to hand at the moment.
 
i would imagine 3000 RPM or a little less. you need to spin the biggest Dia prop you can as it will be more efficient

with props a rule of thumb
remove 25m/m from the dia = add 25 m/m to the pitch

you need to swing 16 inch for optimum Dia
my guess for the pitch ( and only a guess ) would be 7 > 8 inch .
the package supplier ought to be able to work out what is needed
 
When you talk of speed is it STW or SOG?

In otherwords, have you ruled out the log misreading?

Ducking for cover!
 
Yes thats what I was thinking the blades on a 14 inch folding prop start to get small with the massive hub. Just found the data. The original engine was 2500 rpm. Gearbox ratio 1.91:1.
 
The engine should get up to full revs or almost when going flat out. I have a Moody33 and I re-engined and only get up to 3300 RPM(max. is 3600 RPM). I fitted a 13x10 three bladed prop. I believe I'm also overpropped but am living with it at the moment. No black smoke at all. The M33 is difficult as it originally had a tiny 11" prop with a 1:1 g/box and in theory there is not sufficient room to swing a much larger prop and still have min. 15% tip clearance to the hull. I took a chance and fitted a 13" prop which only gives me a 10% clearance. I was initially advised a 15" prop but had to reduce to 13" due to the tip clearance problem. I'm assuming you have a 2:1 reduction g/box so you have a 3200 engine RPM.(assuming a 3600 max engine RPM) Try having the prop re/pitched to 9" pitch which is much cheaper than a new prop. What size prop did you have before?
 
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