Bigger diameter?

[Chaparralboater]

18 foot chap, open bow, 3.0 mercruiser, alpha 1 gen 2, no known weight. Went from a 14.5 x 15 aluminum to a 15.5 x19 aluminum. Runs great and rpms get to 4400. Speed went from 32 to 38..but what is the bigger diameter doing for me? Good or bad?

 

[Bayliner175xt]

Re: Bigger diameter?

It will drop your WOT RPM. We have the same setup on an Bayliner 175 and we run a 13" - 20 P 4blade prop, your 15 is a bit big from what I have seen on here,
We ski, wakeboard and cruise with ours and it works really well.
The are allot of posts on here with very similar setups to yours.

 

[Chaparralboater]

Re: Bigger diameter?

My 15.5 diameter on the 19 p prop is big or the 14.x 15 p is big? The 15.5x19 is the only size the shop had. I would have loved to get a smaller diameter 19p prop but didn't know it lowers my rpm..so if I got a 14.5x 19 it would raise my rpm's ? I would like that as I am at 4400 now and would love to get close to 4800 as possible without going to a 17 pitch prop

 

[steelespike]

Re: Bigger diameter?

What year is the boat and what hp is the 3.0.
Usually diameter is not considered in prop selection except for actual fit.
Typicaly as pitch size goes up diameter goes down.15.5 does seem extreme.
In your setup smaller diameter could help rpm without reducing pitch.
Obviously a 1" decrease seems that it would increase rpm and I believe is a more typical demension. The Solas props are usually very predictable though I have seen some with the 15.3/10 diameter.There are bunch of Solas props listed Solas Amita a Solas Rubex
I can't tell you the difference at this point.I have seen very good results here with the Amita props. They are under $100 and as I said have been very predictable.

 

[Chaparralboater]

Re: Bigger diameter?

Sorry, 1991, 135 hp...I have the solas prop..if I go down to a 14.5 diameter I will see increase in rpm?

 

[steelespike]

Re: Bigger diameter?

Can't really say for sure except that 15.5 seems extreme and again about an 1" change in diameter should gain some rpm.

 

[fishrdan]

Re: Bigger diameter?

if I go down to a 14.5 diameter I will see increase in rpm?

I think so and will let you know next week. I have a 15.5 x 19P prop on my 140HP Mercruiser and have the same problem, WOT of 4200RPM. I bought a used 14 x 19P prop to see how my boat reacts and get a good base line for which new prop to buy. I have a feeling I'll be a bit under propped with the 14" instead of 14.5", but it's just a test with the used prop.

Here's my thread from a week ago;
http://forums.iboats.com/showthread.php?t=424657

Seeing that you are only looking to gain 400RPM, the 15.5 to 14.5 diameter change might be too much, maybe not, don't know While reducing diameter, you may have to also raise pitch. Reducing diameter might overshoot your WOT RPM, but raising pitch will bring it back down, like a 14.25 x 21P prop.

Have you run through Mercury's prop selector yet? Go to your boat MFG's website to see how much the boat weights if you don't know, it will get you close.

http://www.mercurymarine.com/propellers/prop-selector/#

 

[Chaparralboater]

Re: Bigger diameter?

Well I would like to get to 4800 but don't want a 17p ..so if I stay at 4200 that's fine. But if I can return the 15.5x 19 knowing that a 14.5x19 will get me some more rpms I will..thanks guys

 

[Triton II]

Re: Bigger diameter?

Well I would like to get to 4800 but don't want a 17p ..so if I stay at 4200 that's fine. But if I can return the 15.5x 19 knowing that a 14.5x19 will get me some more rpms I will..thanks guys

You should get about 4700rpm with the 14.5x19 as there's less change in the amount of slip when changing diameter compared to changing pitch. With a smaller diameter prop you'll also get less ventilation so you can trim out a little more for any set rpm.

TII