Prop to pad vs prop pitch question

[Frank Downey]

I have a 15' starcraft bass boat with a 100 hp evinrude, 6 inch jack plate. Currently running a 13 3/8"×19 pitch 3 blade prop. My prop to pad is 4 1/4" below pad. I'm blowing the prop on straight runs and porpoises badly. If I trim or raise my jack plate any it porpoises bad and blows out. Is 4 1/4" ptp to much. I want to get the most out of the boat for fuel economy reasons. I dont need to run 60 or 70 mph

 

[jimmbo]

You only have 100hp, I doubt there is much chance of 60mph. When you are saying "prop to pad is 4 1/2" below pad", are you saying the propshaft centerline is 4 1/2" inches below the hull bottom? If so, I would be very worried about starving the water pump. How high is the Anti-ventilation plate above the hull bottom. What kind of Prop are you running? If you are trying to use a standard Aluminum prop in conjunction with a jack plate, expect lots of the prop losing its grip on the water

 

[QBhoy]

Hi. Are you sure it is blowing out ? That’s a whole other serious situation. Usually only happens over the high side of 80+ mph really.

 

[Frank Downey]

Yes prop centerline is 4 1/4" below pad. Its cavating(sp)

 

[jimmbo]

Try lowering the engine height, BTW, it is ventilating, not cavitatiing. You haven't mentioned what propeller, other than Dia. and Pitch, that you are using

 

[Frank Downey]

I'm sorry bout that. It's just a regular OMC 3 blade Aluminum 13 spline 13 3/8"×19 pitch prop. I lowered it 3/8 tonight and taking it out at first light to try it out. I was worried because everyone says go 3 1/2" ptp and mine was already way lower than that

 

[Frank Downey]

I got it sorted out this morning. Thanks gang for your helpful advice

 

[jimmbo]

If you want to run with that much of the prop out of the water, I suggest you get a prop that is designed for that, the standard AL BRP and 3rd Party props are not.

 

[Texasmark]

If you want to run with that much of the prop out of the water, I suggest you get a prop that is designed for that, the standard AL BRP and 3rd Party props are not.

Agree, at least a "cupped" prop which is made to hold tighter near the surface. Turning point Hustler has cupping as standard equipment in their aluminum lineup, sold herein for about 100 bucks. Don't forget the insert, same brand made especially for their props. I don't work for them nor have anything to do with them other than I am currently running their props (have 3) and like what they do for the price!

 

[jimmbo]

BRP AL props have lots of cup, at least the V6 models do. TP props have major QC issues, Last yr I found several that had one of the three blades with a different contour. For Surfacing apps, a SS is the way to go, much thinner blades and stronger to handle hitting water at high rpm