88 mach 1 prop question

[passngass]

ok here goes, another prop question from another newbie.

88 mach 1 challenger 2700lbs empty
newly rebuilt gm 4.6l v6
newly rebuilt alpha 1 with michigan wheel 14.5 x 17p and 1;81{i think still waiting for confirmation from builder} gearing.

the prop listed is what came with the boat. full gas and 2 adults WOT was 3800 35mph i feel i can do a bit better...
after reading a ton of material im thinking a 16 x 16 3 blade aluminum would put me in the good. all prop calculators point to this prop more so then not but i like real people input and thoughts.

thank you in advance.

 

[Bob_VT]

Re: 88 mach 1 prop question

Nope. Going down in pitch is one thing but up in diameter can negate the rpm gain. You should be running closer to 4800 rpms.

Is this new to you? Was the outdrive changed?

 

[jestor68]

Re: 88 mach 1 prop question

I think that you need to verify the accuracy of your tachometer if you have a 1.81 gear ratio.

A 1.81 ratio with a 17 pitch at 3800 rpm has a theoretical speed of 34 mph ( not accounting for any slip).

A 10% slip with actual speed of 35 mph equals a theoretical speed of 39 mph; which comes out to almost 4400 (4385) rpm.

If you have something like a 1.62 gear ratio, your numbers are in line with a 10% slip.

 

[passngass]

Re: 88 mach 1 prop question

Re: bob vt yes this boat is new to me ive always had a out board
The builder of both the engine and the drive is a big race engine builder of water and road sports
I agree about the diameter it makes me nervous

I feel fairly confident about the tach being accurate as the owner/builder of this boat was very meticulous as to having new properly working equipment throughout the boat however i will verify just so i can be certian
I think what im going to do is keep the original 14.5 diameter and go with the 16p. That way i know im really close to what i should running at and the experiment later on with other diameters.
The reason im looking at a 16 diameter is,i,keep reading heavier boats need bigger diameter props. Any thoughts on this?