new prop, set-up help

[mpsyamaha]

im hoping some of you experts can help me pick a new prop and get my boat set up as best possible. 16ft fiberglass semi-v with 06 70hp yamaha 2 stroke, weighs a little over 700lbs dry without engine. currently has a yamaha 13 1/4" X 17 aluminum prop with "cleaver" shaped blades. ventilation plate is 1" above bottom of the V. wot rpm listed at 5000-6000.

with only me and light load of fishing gear i can only get 5600 out of it at 36-37mph (gps) with it trimmed up to the point right before it starts porpoising. and when i let off the throttle and slow down to a slow plane, then hit it again, sometimes it will ventilate if i dont trim down first. at high trim levels it immediately ventilates when turning.

so what i am looking for is a prop that will hold better, and provide the most top end speed. after looking at the amount of available props here on iboats, and other places, i need some recommendations. also considering a jack plate... if so, how much set back? thanks

 

[walleyehed]

Re: new prop, set-up help

I think we could get to 5800 with the prop you have now, but with it venting as easily as you say, going up on the transom may not work with this prop. I don't like to hear that it porpoises though...which "could" mean a couple of things...at higher trim angles, that prop may not have enough holding ability to keep the bow up and steady.
The boat may be balanced in a manner that is causing the porpoise at top-speed, or high-trim.
Normally, we don't like to drop pitch to gain RPM...usually it's best to work the set-up to gain the RPM back with the same prop, but I think we may have problems getting that prop to hook-up with a higher engine mounting.
How high off the transom is the engine mounted now?
By the way, the venting in turns and under hard acceleration is 100% normal with much positive trim.

 

[mpsyamaha]

Re: new prop, set-up help

when i got the boat the vent plate was about even with the bottom of the hull, and i moved it up one notch and now it sits 1" above the bottom. it is in the highest mounting hole now so i cant go any higher.

i would like to get a better prop for it, i was thinking that switching to a good ss prop may get me some rpms. i think the 13 1/4X17 is a good size prop for the boat, it has a real nice hole shot. i just would like to add some top end, and the old aluminum prop is not in brand new condition either. any recommendations? also about a jack plate, think i could benefit from one? thanks!

 

[walleyehed]

Re: new prop, set-up help

On that boat I'm not sure about the jackplate....I haven't played with that style much.
Looking at your prop, I can see why it vents....it has no "traditional" cup.
If we go to a good, cupped 17P, you will lose some rpm.
If you want to go SS, I would recommend a Stiletto in a 15P. It May come up a tad rpm-wise, but the holding and lifting ability should add some speed...maybe enough to loosen it up and gain some rpm too.

 

[mpsyamaha]

Re: new prop, set-up help

thank you for the help! the stilletto is actually the one i was looking at already. im thinking that with a 15p it is going to be a nasty hole shot!

im still debating on picking up a manual jackplate or not. should i try that first with the prop i have and see what i can get, or should i just buy the 15p stilletto and be done with it, or go all out and get the jp AND a new prop? i would really like to turn 6000 or close to it, with a 17p but im not sure its possible even with some set back and some more height.

the the back half or so of the hull is a pretty shallow v...

 

[Turbo1]

Re: new prop, set-up help

I'd say wally has ya pointin in the right direction there hoss.
Yer hull ain't far off a bass boat hull but ya got a wider front and no pad.
They tell me some guy goes by texmark or somthin like that hangs out here. maybe he knows how a plate would work on that boat.