Getting prop confused here.

[bernieb]

The boat is a 82' "mark-twain" 21 footer with a add on bracket with a 250 hp yamaha engine and with four fisherman 3500 /4000 lbs.My question is with a three bladed prop 48 16318 a4 21p 13 diam?(quick silver)4 1/2inch hub, run 5400 rpm.I bought a 4 bladed 20 inch pitch a 14 diam.4 inch hub and i got 5100 rpm.The hole shot seemed the same ,the speed about the same,but it seems that i use a lot more gas with 4 blades and can it hurt the engine with the hub on the prop being a little smaller than the lower unit gearcase? I ask another boater that had a 26 foot parker and he said his prop was a 15 diam. and a 15 pitch a three blader.My 21 3 blader did cavitate with engine trimmed up and in slowing down,then hammer down the cavitation would take place.What should i do ? Thanks

 

[Dhadley]

Re: Getting prop confused here.

Generally speaking a 4 blade will run mounted higher than a 3 blade. If you raise the motor on the bracket you'll probably get some rpm's back. better holeshot and top speed too.<br /><br />What brand is the 4 blade?<br /><br />Your info suggests that the 4 blade is biting much better than the 3 blade Merc prop.

 

[Dhadley]

Re: Getting prop confused here.

The smaller hub is to allow exhaust to vent the blades for a better holeshot. At planing speeds it should make no difference.<br /><br />Didnt mean to leave out that part of the answer....

 

[bernieb]

Re: Getting prop confused here.

Thanks Dhadley for your answer,and the 4 blader is a yamaha prop .

 

[Dhadley]

Re: Getting prop confused here.

The 4 blade Yamaha prop is a decent prop. Try raising the motor to gain performance. <br /><br />Keep us posted on what you find. Good luck!