Prop/performance question

[rjolley]

I have a 8? model yamaha 90 HP with a 17 pitch prop, 15.5 ft sea raider fiberglass bass boat. I rev 5200rpm at WOT top speed 36 mph. I have little to no trim capability without porpoising. Question is do I need a prop change, should I be able to get more speed? I have a dolfin. Any help would be appreciated, I am kinda dumb to this stuff.

 

[LubeDude]

Re: Prop/performance question

Welcome to Iboats rjolley:<br /><br />My last two Bass boats were 15' boats, one was a ranger, the other a Skeeter. The skeeter was just like yours as far as performance. It only had a 75 Evenrude, but had the same caracteristics. I put a hydofoil on it and it made it much better, but having said that, the reason you are having trouble trimming without porposing is all the weight in the back of the boat. Fuel, batteries, etc. I now having learned more about boats in the last few years between then and now wouldnt recomend the hydrofoil unless price is the only decision. Smart Tabs will make you think you have an all together different boat. Your boat will jump on plane and you will be able to plane at about 15 MPH if you want to pull tubes or just run slow through channels and such. The smart tabs let your boat do what it is ment to do and your engine do what it was ment to do, (propell your boat).<br /><br />I also think your engine may be a little low, Ventilation plate should be at least 1" above the bottom of the boat. Remove the foil and I bet your porposing will be less. I think because your engine is low that when you trim, the foil is catching and causing the problem.<br />------------------------ <br />

 

[rjolley]

Re: Prop/performance question

Thanks for the reply, I'll try it this week and see what happens. I have trim tabs (fixed position type) I took them off once and the boat chine walked like crazy, needless to say I put them right back on. I lowered the motor one hole trying to fix the porpoising problem, gave me a better hole shot, but didn't do anything for the porpoise. I'll take of the fin and see what happens.

 

[Mercathode]

Re: Prop/performance question

rjolly<br />Some boats don't respond to trim as others. If the hull does not have a surface such as a pad to ride they may porpoise and chine walk when trimmed out after a certain point. Shorter hulls seem to porpoise more than longer ones. Many times with smaller outboards and prop diameters they do not have enough bite to hold the bow high. I have found that a jack plate will give the motor tons of hull leverage and actually make the motor think it has a longer hull.

 

[LubeDude]

Re: Prop/performance question

Yes, your engine is too low, also having a fin and trim tabs isnt a good Idea. Lose the fin, Raise the engine.<br />----------------------<br />

 

[rjolley]

Re: Prop/performance question

I'll try some of this stuff if it will ever quit raining down here in Alabama. Thanks again guys.

 

[rjolley]

Re: Prop/performance question

Hey lube, took off the foil, what a difference. Didn't raise the motor but I intend to. Removed the foil, jumped out of the hole, went from 36mph previous to 40 at 5400 rmp trimmed all the way down. As I trimmed RPM increased to 5800 and speed to 42 at 3/4 trim and began a slight porpoise continued to trim and RPM went to 6K and 45mph with a porpoise. I backed off because I don't know the max RPM of this motor. Any Ideas, for the Max RPM and what will raising the motor do for me?