Props for flat bottom boats

[Todd Humphreys]

Currently I have a SS OMC 13x19 on my 1982 75hp Johnny on a fiberglass flat turning about 6000 rpms. I looking into purchasing a new SS prop from iboats, there are a few different types, some with high rake, some with vent holes, and others with cup. Good overall performace is preferred. Any suggestions is appreciated ie. express, rapture, .

 

[Dhadley]

Re: Props for flat bottom boats

We kind of assume youre looking at 3 blade props. We have had excellent results with Rakers, Stilettos and Turbos. If you dont have a jackplate of any kind I'd suggest an 18/20 Raker or a 19 Stiletto.

 

[Todd Humphreys]

Re: Props for flat bottom boats

3 blade yes. No jackplate. For what its worth, my X demension is 1/2" above the boat bottom. The problem I'm having is the prop ventalates on full throttle hole shots. Initial full throttle the bow rises and when the boat start to plane (bow starts to drop) ventalation occurs in which rpms rise to 5200 to 5400 and continues there until the WOT is reached. The prop is not slipping (put marks on hub and prop). Using a prop calculator my slippage is 15%. GPS 38 mph @ 5800 rpm. Should I get a new prop, rework the OMC, or lower the motor one hole to even with the bottom of boat?

 

[mjbrueck]

Re: Props for flat bottom boats

Flat bottoms like to ventilate, especially in turns, a heavy chop or changes in speed. A similar-sized, cupped prop would probably solve you're ventilating. If you don't mind losing a little top end, lowering the motor a little would also likely solve your problem without a prop change.

 

[Todd Humphreys]

Re: Props for flat bottom boats

The OMC 13X19 is definetly cupped, both on the pitch and rake lines (verified by reading the prop technology on mercury web site). The prop has some minor nicks on the leading edge. Also has home made vent holes about 1/4" on all three blades. There is no ventalation on turns, only if trying to accelerate with motor positively trimmed and during full throttle hole shot full negative trim. This motor is recently rebuilt 30 over pistons. Also has a hydrofoil.

 

[Salmonseeker]

Re: Props for flat bottom boats

There are vent holes on the actual blade or on the hub? I have heard of them on the hub, but not the blade.

 

[Todd Humphreys]

Re: Props for flat bottom boats

I meant there on the hub not the blade.

 

[Dhadley]

Re: Props for flat bottom boats

With positive trim on acceleration we would expect some venting. Thats an old trick to get the r's up quicker. Helps sometimes when trying to get up in a hole in shallow water.<br /><br />Slight venting with full negative trim means the vent holes are working. If its more venting than you want, plug the holes completely or redrill the plug smaller.

 

[Todd Humphreys]

Re: Props for flat bottom boats

I'm leaning toward a 13 1/4 X 19 Rapture here on iboats, simply because the money I'll spend getting my OMC prop serviced is about the same price as the rapture prop. Thanks for all your input.

 

[Todd Humphreys]

Re: Props for flat bottom boats

Ok, since my last reply I still didn't buy a new prop (wife, kids, etc, etc.) I'm now ready to purchase a new prop. After months of deliberating and learning from these forums, I believe a higher thrust prop would be my best bet. Which would you choose, the SS Express(131/4x19) or the Michigan Match(13x19)?

 

[Dhadley]

Re: Props for flat bottom boats

Which would I choose?<br /><br />A Turbo Lightning Jr.