propeller pitch

[foilled]

anti ventilation plate was broken from last owner with a hydro foil trying to patch it. air could still be sucked down into prop. i feel like it was allowing higher rpms. recently i fixed that and my WOT rpm are maxing at 4100. im looking the see if i need to fix other issues or it was the anti ventilation plate. about what size prop should be used on a omc 5.7 with a cobra. boat is a Larson dc-250 sport cruiser. a guess would be boat wieghs about 4200lbs. currently there is a 15 dia 17 pitch 3 blade prop on there. does that prop seem to big and previous owner couldve been compensating for cavitation or something? i need to get upwards of 4600 rpm

 

[Bondo]

Ayuh,..... Try a lower pitched prop,.....

 

[foilled]

Ayuh,..... Try a lower pitched prop,.....

do you think a 15 x 15 would work good for 4200 - 4600 rpm range. i want to be at the top of the range and with a decently heavy load i was at about i was at 4100 with a 15 x 17 pitch prob. the marinas say once i buy its its mine so i cant test which wouldve been ideal

 

[Bondo]

Ayuh,..... You should see 'bout 200 or slightly more increase in rpms,....

 

[PITBoat]

So will the extra rpm translate directly into more speed, or not? I've got a similar deal - only up to about 4,200rpm with a 14.5x19 3-blade.

I am contenting myself by usually keeping it down around 3,500 or maybe 3,750 for the high cruise setting, hoping that will cut back on any extra stress the engine may be getting from having too much prop.

I want to try the 15.5 x 15 I believe it is, when I can spring for that.

 

[Howard Sterndrive]

If you don't want to risk owning a prop, you can compression test the motor first and check spark plugs etc to get an idea of the engine's health - and/or poke around under the floor etc to see if boats holding any water weight. Although that is kind of a big boat for a 5.7, I woulda guessed 17P as the original pitch it came with.

 

[jimmbo]

You might have to drop it down to a 15, that should give you about 400 which will put you near the top of the range. But check to see if the engine is up to snuff

 

[PITBoat]

So will the extra rpm translate directly into more speed, or not? I've got a similar deal - only up to about 4,200rpm with a 14.5x19 3-blade.

I actually got 4,350rpm this morning at just over 45mph GPS, on calm water and with only 2 people in the boat. That was with part 89 and part 87 octane gas. I need to check the points and the timing before I decide for sure my prop isn't quite the right one.

I find I'm actually running at more like 3,000rpm if I'm dealing much with other wakes and that.

 

[foilled]

If you don't want to risk owning a prop, you can compression test the motor first and check spark plugs etc to get an idea of the engine's health - and/or poke around under the floor etc to see if boats holding any water weight. Although that is kind of a big boat for a 5.7, I woulda guessed 17P as the original pitch it came with.

i was considering the previous owner was compensating for ventilation to the prop or something. he didnt no much about boats it seemed anyways idk how aware of rpm range he was to begin with but the anti ventilation plate on the outdrive was broken and previous owner had put a whale tail thing on it yet there was still a nice sized hole that was where the anti ventilation plate was. i have now fixed that and my rpm are at about 4050 to 4100. i was thinking maybe the the prop was spinning faster to sucking in air but i dont know to much about it. plus at link below i have me 87 omc and the prop they sell as oem that you can see on the link is 15.5 x 14. so im thinking the 17pitch i have could be to high pitched? i need to be at 4600 rpm. i dont think anything is wrong with engine but i dont want to be stuck with a prop. last year i was in rpm range but like i said ive done some changes to boat and the only one was thinking that couldve changed anything over 1 winter was the broken part in the anti ventilation plate.

 

[Howard Sterndrive]

i need to be at 4600 rpm.

You need to be at 260HP first, then get the prop that gives you 4600 rpm.

 

[foilled]

You need to be at 260HP first, then get the prop that gives you 4600 rpm.

compression tested fine and initial timing is set. where else could i be losing hp or anything else i should look into before experimenting with different prop sizes?

 

[jimmbo]

There are lots of ways an engine can fail to make its rated power. Just because the Timing is set to spec doesn't mean it is advancing correctly at higher engine speeds. A cam with worn lobes can still have fair low speed power but fall flat at higher speeds, a stretched Timing chain will affect power out put. If the engine has a Four Barrel and the secondaries aren't opening, If the exhaust is restricted. A HIgh Speed miss in the Ignition system.

 

[foilled]

engine was rebuilt by jasper in 2012/13 the internals seem early to fail and that would be unfortunate. exhaust should be ok, just got new manifolds risers and flappers looked perfect. although there could be other restrictions im assuming not. secondaries not opening is a possiblity aswell. ill have to take a peak but i can almost get 4k rpm would i be able to still if secondaries werent open? and advanced timing idk much about. where would u start ?

 

[Lou C]

The next size you could try is a 15.5X15, that should put you where you want to be..and the biggest prop you can put on a Cobra drive is a 15.5 I believe. Michigan Wheel makes it in aluminum.

 

[foilled]

The next size you could try is a 15.5X15, that should put you where you want to be..and the biggest prop you can put on a Cobra drive is a 15.5 I believe. Michigan Wheel makes it in aluminum.

how much rpm should i expect to gain switch to the 15.5 x 15. also should i go up from 15 to 15.5 or should i do a 15 x 15?