15.7 ft trihull with 85 evinrude prop help

[zachfarmall]

I have a 1984 thundercraft 160 magnum 15.7 ft trihull with a 75 evinrude 85 on it.the boat weighs 700lbs dry I carry 15 gallons of gas and about 500-700lbs in gear max. Looking for ideas on a prop for max speed.it has no power trim or tilt.I will be pulling tubers with 2 200 lb people on board and that's it.any ideas would be great.and I can't afford anything stainless so about 150$ is my max.thanks in advance

 

[steelespike]

While we could run this through the prop selector It may be more accurate if you do a lightly loaded wot test run get us the prop size The rpm and gps speed.
This will tell us how your setup works.A calculator can't take that into consideration.
Your prop size should be near the base of the blades or under the prop nut.You may have to figure out your gear ratio.

 

[eavega]

As Steelspike states, you really need to do a lightly loaded test because the real determination of the right prop for your setup is your RPM at WOT.

THAT HAVING BEEN SAID...I had a similar setup to yours (15' tri-hull with a 1976 70 HP Johnson, usually ran with 4 of us - two adults and two smaller kids), and I had a 17" Pitch Stainless Steel OMC 3-blade prop, and a 19" pitch aluminum OMC prop. Both spun up into the recommended range (about 5600 RPM), and was enough to get tubers and/or kneeboards in, but didn't have enough holeshot to get skiers up reliably. Knowing what I know today, I would've tried a 15" 4-blade prop if I really wanted to do watersports. A decent "starting point" if you do need a prop is a 19" three-blade aluminum. I found the Solas Amita props to be very good all-around (I had one on a previous motor I had on the 15' tri-hull and am currently running a 4-blade Solas Amita aluminum prop on my Mercruiser/Alpha 1). Price is between $100-$125 here on iBoats.

Rgds

 

[zachfarmall]

Would cupping help slippage because I have no trim?

 

[southkogs]

Welcome aboard zach.

Have you already run this boat? What kinda' results are you getting? What prop does it currently have on it?

You need power more than speed. That's typically going to call for an easier pitch ... cupping would help your top end. Kinda' opposing goals.

 

[steelespike]

Would cupping help slippage because I have no trim?

Adding cup can help get the boat up better and improve slip numbers. You do have manual trim This can be fine tuned
with weight placement and you can make or buy wedges to use to add or remove some trim.

 

[zachfarmall]

The motor isn't on boat yet just looking for starting point but now I'm being told the switch from 60hp merc to 85 evinrude v4 might not gain much