Piranha prop selection help

[Edan7]

Hi, I've been trying a few different blades for my boat and wanted input on where I should be heading from some of you with more experience in prop selection.

My boat is a 15ft silverline aruba 15v. I have a 1986 evinrude 70hp on it that's running strong.

The motor came of a larger boat and had a 15 inch pitch aluminum prop. With that prop it would easily run past 6000 with two in the boat and full fuel tank. It would top out at 35 mph. I then got a piranha prop hub with 13.25" Dia. by 17 pitch blades and my new max rpm (same running scenario) was 5200 rpm and max speed of 32 mph). I then tried their semi-cleaver 1317xb blades and that got me faster planning speeds

 

[jimmbo]

I would say the 15" was too small for the Boat, but an 800+ rpm drop is about double the change expected. So something else is involved. Perhaps the 15 was also slipping from air injection. I can't see how high the engine is mounted on the Transom...

This what a Cleaver Blade Looks like
I never saw a Piranha Blade that looked like these, but maybe they exist

They are good at providing Stern Lift, and were used on Tunnel Hull Racing boats, as they help keep the Bow Down to reduce the chance of it getting too high, catching Air and flipping the boat over backwards

 

[brodmann]

I'm guessing you went from a 15 pitch aluminum to a 17 pitch Stainless prop. That's the only thing I can think of that would make that much difference. I'd try a 19 pitch aluminum prop.

 

[Edan7]

Thanks for the input. I think the larger than expected change came from changing from the aluminum to the Piranha prop. I just tried the 19 inch semi cleaver piranha prop this weekend and that brought my max rpm to around 5900 at 34 mph with a full fuel tank and two in the boat. It seamed to perform better at 3/4 throttle with a higher cruising speed at around 4500 rpm than the others. Now I'm just wondering if I should try up to 21 inch pitch or if I'm likely gonna be taxing the motor too much with that. I'm okay loosing on some bottom end if I can get more top speed out of it.

 

[Texasmark]

21P on a 70 HP I think is a bit much, but the truth lies in the testing. I ran a 21P 3 blade aluminum on a 3/4"jacked up 70 on a 15' kingfisher bass boat, very light weight, basically flat hull and with just me and minimal gear and fuel and the engine was loaded.

 

[flyingscott]

A pretty common pitch for that motor is a 17" pitch aluminum. You want to prop to 6000rpm

 

[Edan7]

Right now with this 19 pitch I'm maxing out at 6000 rpm +/- 100 depending on weight in the boat. Don't think I want to create more load with a higher pitch so I'm gonna stick with this for now at least. Thanks for the advice

 

[jimmbo]

What is the WOT Rpm Range for your engine?

 

[flyingscott]

5000-6000 rpm for that motor