Correct prop sizing

[maynardrupp]

We have a Crusader 350 powered 1986 36 Convertable. The props supplied from Hatteras are 26 x 25. The Crusader book says that the full throttle RPM should be between 4000 and 4400. The best we get downwind in flat sees, (fresh water), is 3800. We have digital tachs, so I guess I can believe them.I can get a couple inches of pitch taken out of the props but I can't figure how Hatteras got it that wrong. We bought the boat this spring and it has a total of 760 hours. I am an engine man and these two are just fine in all respects. I have timed them at 8 degrees instead of 10 to cover for the increased load from the lower RPM. Will anything improve if I get the RPM up? Is it worth the bother. The boat does 26.8 knots at full throttle,(3800).

[jim rosenthal]

That is the fastest gasoline-powered Hatteras 36 I have ever heard of. You must mean 350hp 454s...right? If those are SMALL-BLOCK engines I think all the laws of physics just went out the window.....
REgarding your lost rpms....is the bottom clean? are the throttles opening all the way? and is the boat loaded up with excess weight which is slowing it down? all those things could cause you to lose revs the way you have. Also haul the boat and make sure there isn't excess friction in the cutlass bearings and that your shaft alignment is good,and the engine alignment also. All these things can contribute to the drivetrain not working as well as it ought to.

[Genesis]

My 45C had 26x28 3-blades from the factory.

I have 26x26 wheels on there now, and she's right on the line - some would say overwheeled, but I never run over 2000 (WOT is 2300), and as long as the idiot on the throttle is nice to her, its ok.

But not ideal.

They didn't always get it right from the factory.....

[Jackman]

That sounds kinda fast to me as well. Even if its a 454 that RPM and speed with the weight of that boat is a suprise, but then again, I was running "tired" engines and his sound like their fairly new.

Make sure you're head gasket doesn't have issues. My starboard engine head gasket was blown, which is what contributed to me not being able to get over 3000rpms with it. At 3100 it would bog down as though it was going to stall because its compression was low. Maybe you have too much prop for those engines as stated above.

[maynardrupp]

They are Crusader 350 hp, 454 cu.in. engines with 750 hours each. I am an aircraft mechanic. We bought this boat last winter and I did a differential compression check on each cylinder. Not one leaked more than 7 lbs. I have reset the timing, (they have pointless distributers), checked all other engine stuff. I think they are both quite healthy original engines. We have two sets of props and they are all original as supplied by Hatteras. The engines turn 3800 at full throttle and both our GPS and transducer boat speed reads 23.8. The speed is good, but I am worried about the RPM. I don't want to detonate these engines. The Crusader book asks for between 4000 and 4400 RPM. I have set the timing 2 degrees low of the 10 recomended to ward off the detonation. Should I remove 2" of pitch. The boats bottom isn't up to a racing sailboat standard, but has fresh paint and is only in fresh water..

[Hatt36]

I have a 1984 36C that was orginailly gas powered with 350 Crusaders. It came with 26x25 3-blade props. The owners manual indicates in a marginal note that the propellers were repitched in 1993 to 26x23 to reach 3900 rpm. Obviously, loading has an effect, but it also seems that Hatteras may have provided props with a little to much pitch.