New 2:1 Transmission

[Bakes5]

Ok....here's a question that's been nagging me. Take a boat with 2:1, run a 17" x 14 pitch prop and then run a 15"x14 pitch prop and check out the max speed....no ballast, crew of 6.

I'm curious to see where they both fall out for WOT max speed.

Seems like I read somewhere that if you change the diameter of the prop by an inch, that you need to compensate with an similar pitch change to keep the same holeshot.

So....is a 17x12 same as a 16x13 same as 15x14 when it comes to how much power/torque needed to turn it?

[UNSTUCK]

I agree, you're well within the "industry standard" specs from that article and the increase you're considering isn't big a percentage of what you have now. That said, I suspect the authors of that material were thinking "traditional" boats and not inboards built for surfing, so I have no idea if their standard angles apply to this industry.

So I measured my prop shaft angle today. I was surprised to see it at 17.5*. That totally explains why my boat rides so bow high. It takes a lot of weight to get it down when surfing. But like we said we have no idea what the standard is for these boats. I should have taken my angle finder with me today when I went to the Supra dealer. They had lots of different brands to measure there.

[Germaine Marine]

Ok....here's a question that's been nagging me. Take a boat with 2:1, run a 17" x 14 pitch prop and then run a 15"x14 pitch prop and check out the max speed....no ballast, crew of 6.

I'm curious to see where they both fall out for WOT max speed.

Seems like I read somewhere that if you change the diameter of the prop by an inch, that you need to compensate with an similar pitch change to keep the same holeshot.

So....is a 17x12 same as a 16x13 same as 15x14 when it comes to how much power/torque needed to turn it?

Yes and No. Lots of wacky science that makes the equation not black and white.

I have ran a 2:1 with a 15/14 it was a disaster. I would say 17/14 is closer top end wise to a 15x16. The problem is that with all that bite and less surface area the transfer of power isnt nearly as efficient to the water. You lose a ton on the bottom with that bite and the lack of surface area makes for a big net loss. The 1 inch taller blade although less bite is able to give you similar top speed with way more on the bottom. A 15x15 might run great for stock ballast and for slalom or faster sports though.

The 17/17 runs 39 MPH at sea level empty in the RZX2 and still has a ton to push the boat with full ballast and pnp at surf speed.

So i guess what I am saying is, all the diameter is doing is helping negate the wild shifts when you go from one extreme to the other. Its bridging the gap really from having to choose a stump puller or a high end wheel. Diameter gets you in the middle of both without a gigantic swing.

[Germaine Marine]

The 2:1 is really just giving you an instant bump in torque across the entire power band but its doing it at the expense of RPM obviously. In-order to maximize the reduction, you have to make sure the transfer of power is the most efficient as possible. You do that (within reason) based on Tq or HP in regard to choosing the amount of diameter you can spin without being a detriment. You then adjust pitch based on the diameter selection. The bigger the diameter you can go the bigger the window becomes in regard to a pitch adjustment.

500 Hp going through a 1.5 is going to have a smaller/Tighter window then 500 going through a 2;1.