Volvo Duo Props Size and Gearing

[Schnikies]

I have a 19 ft open bow larson with a dp 290 drive with a 4.3 mildly built intake,carb & cam .030 over ~250-275 hp it started with b5's on it which the motor was over reving. I switched to b6's (aftermarket props) the motor is still over reving i get to about 3/4 throttle secondarys haven't even opened @5000rpm where i lift off the throttle, will i beable to stay with the same gear ratio? and just change props i dont really want to buy several sets of props and trial and error?? any suggestions??

 

[Maclin]

Re: Volvo Duo Props Size and Gearing

What ratio do you have now? 1.98 or 2.3 ? Is this the first time you have run this boat with that engine? What speed are you at when 5000rpm is reached? Also what size boat is this in.

It would be good to know what it was propped with before the engine mods, and what rpm and top speed so we could have some kind of a baseline to compare to.

My SWAG is.....I figure you must have the 2.3 to be at the place you are now. The B series goes to B8, with 1.75-2 inch pitch change for each step. If the engine is as free to rev as you state, and it has the 2.3 ratio, then I think a gear swap to the 1.95 is in order. Need to get a feeling for the torque curve, though the smaller the boat the less it will be a factor. Does it jump out of the hole now? Or does the engine need to work at it just for a second or so? If the power band is up higher in the RPM range and it is a "big" boat the 2.3 ratio and higher pitch props may be better, to keep from lugging out of the hole.

 

[Schnikies]

Re: Volvo Duo Props Size and Gearing

What ratio do you have now? 1.98 or 2.3 ? Is this the first time you have run this boat with that engine? What speed are you at when 5000rpm is reached? Also what size boat is this in.

It would be good to know what it was propped with before the engine mods, and what rpm and top speed so we could have some kind of a baseline to compare to.

My SWAG is.....I figure you must have the 2.3 to be at the place you are now. The B series goes to B8, with 1.75-2 inch pitch change for each step. If the engine is as free to rev as you state, and it has the 2.3 ratio, then I think a gear swap to the 1.95 is in order. Need to get a feeling for the torque curve, though the smaller the boat the less it will be a factor. Does it jump out of the hole now? Or does the engine need to work at it just for a second or so? If the power band is up higher in the RPM range and it is a "big" boat the 2.3 ratio and higher pitch props may be better, to keep from lugging out of the hole.

I dont know the ratio i have is there any way besides counting revolutions prop to crankshaft? Its a small 19 ft boat runabout at 5000 rpm i am getting low 40's mph out of it. I did run the boat before when it was stock with the b5's it seemed actually seemed faster on the top end. It jumps out of the hole really good but as soon as it does i have to let off the throttle so it doenst over rev with the b6's is it possible that they are aftermarket and flexing?

 

[Maclin]

Re: Volvo Duo Props Size and Gearing

Those numbers support the ratio is almost certainly 2.3.

It does sound like your boat thinks you went down in pitch! The aftermarket B6's may be labeled wrong or not well made, maybe even flexing as you suggest. I would triple check all the numbers, and call the manufacturer with some questions.

From a myriad of personal and webb-based data I believe that B5's are about 22-23 pitch and B6 should be closer to 24-25 pitch.

Again, just my guess, but it seems like your boat (assuming the 2.3 ratio) would have come with B7's from the factory.