Looking for Prop Recommendations

[dctucker]

I have an Avon Searider 5.4 RIB, 18' 700 pounds with a nice new 90 hp E-TEC. I most commonly run loaded up with me (220#), up to 13 gallons of fuel (70#), battery (40#?), tools & spares (50#), food & water for sailors (20#?). I figure I probably add 400-500+ pounds aboard the boat, plus often another adult and a young kid or two. The boat is used primarily for safety and support for disabled sailors. I spend a lot of time puttering around at low to moderate displacement speeds, next largest chunk of time at low planing speeds and a bit at higher speeds up to WOT, running racecourse marks about, shuttling to and from harbors, etc. I also tow 1150 pound sailboats at displacement and low planing speeds at least 1-2 times when I'm out.

When we rigged the new motor, the dealer tested the boat empty, and installed a BRP aluminum 3 x 13.25 x 17 (p/n 0763463), it's perfect when the boat is empty (just me & 1 fuel tank) pulling 5200-5300 RPM when trimmed out, but that's almost NEVER the way I run the boat. I'm usually seeing about 500-600 RPM low @ WOT (~4600 RPM) with that prop even when properly trimmed out. Motor is mounted 1 hole up, which seems right.

The RIB has a flooding hull for stability, which allows water ballast to enter the hull when you drop off the plane. I have Nauticus Smart Tabs installed, which really help the hole shot while the water ballast drains, at the expense of a bit of top speed.

I'm not overly concerned with top speed. Trim out too far at WOT and the boat can become a chine walking demon. My most important preference is to lower sustainable planing speed and reduce ventilation in tight low to mid planing speed turns. I'm considering trying a Michigan Wheel Apollo XHS 4 blade 14 5/8 x 16 (?).

Any thoughts or other recommendations?

Thanks!

 

[Silvertip]

Re: Looking for Prop Recommendations

The first thing to look at is do you have clearance for the 14-5/8 diameter. Adding 1-1/4 inch of diameter, going four blade and dropping pitch 1 inch would seem to put you in a worse case than you have now. Since towing and hole shot seem to be your priority, the four blade makes sense but the 14-5/8 does not. Stay with the same diameter and perhaps even look at 15P since what you do and the way you operate this motor, it is not generally in its recommended RPM band and when working, you are lugging the engine.

 

[flybuddy]

Re: Looking for Prop Recommendations

I agree with Silvertip. I had the same pitch and diameter that you did and going to a 15p from 17 improved rpm, top end and hole shot. You've only got 90hp and with the loads you're running she's bogging down and probably only giving you 75hp--you need the extra rpm to get your HP (and performance) back.

 

[dctucker]

Re: Looking for Prop Recommendations

I was thinking of dropping to a 15p 3 blade as an alternative, that would likely be more straightforward to get to the right rpm.

What about changing from 3 blade to 4? Assuming I stay with the same diameter, what change in pitch would I likely need to get the proper revs?

 

[Silvertip]

Re: Looking for Prop Recommendations

You might benefit by going with a four blade, but I would think you need to drop pitch even further to a 13P since the 15P 3 blade will only raise RPM about 400. Fully loaded that still leaves you on the short end of the RPM band. Four blade props add blade area, push more water, and therefore drop RPM thus the drop to 13P. With a light load however, you may need to watch the tach to avoid over reving. But with that prop you should have great hole shot, towing ability, and the engine won't be lugging all the time. Check with your prop supplier -- they often let you try a few before buying. But be a sport and buy from them.

 

[walleyehed]

Re: Looking for Prop Recommendations

13-1/4 diameter tells me you have the 20" shaft, so I know it's 13 spline....BRP doesn't have anything in aluminum that will do what you want in 4-blade.
I'd be looking seriously at a Bay Pro I, 24413 Stiletto 13-1/4X13P SS. You really, really need to get the WOT RPM up...4600-5000 is going to eat that engine over time.

 

[dctucker]

Re: Looking for Prop Recommendations

Thanks for the advice! Silvertip, that's the sort of explanation I was looking for. Walleye, Thanks for the specific recommendation I suspect that might be just the ticket. Yes, it's a 20" and yes, I know I need to get the revs up to the right range.

The boat sure does fly with the 17p when it's empty though! Seemed just right when I tested the boat when I picked it up after the repower. When speaking with the dealer tech about props, he initially recommended a 4 blade, but as you said, BRP doesn't offer anything appropriate. They don't seem to sell anything but the BRPs and I'm pretty sure he tested a couple of props before delivery. I think I made the right choice going with the cheaper aluminum 3 blade until I could see some real-world performance.

I can pick up a 15p AL BRP for around $100. I'm wondering if that might be an adequate interim step or a further waste of $$. I don't know of a good prop shop here in the MA coastal area that might let me try stuff till I find the right thing. I'm not keen on dropping $300 on a prop that may or may not be right (nor will my wife understand!). Would getting the rpms right with an AL 3 blade be helpful before dropping the $$ on the more expensive SS 4 blade? Or just pissing more $$ away?