I have a 16 Chaos flats boat with the Yamaha 90 LTR 2 stroke motor. I’m trying to figure out what is the correct propeller for my motor. I’m getting only 5000-5100 rpms at full throttle and 41-42 mph on the gauge. No, I don’t have a gps on the boat yet. I’m not sure what redline is and I don’t really need the boat to go faster; but, feel like the motor should operate thru it’s full range of RPMs. Any ideas?
oh yea, the hull weighs 595 ibs. and carries 22 gals of fuel. When I run for topspeed the river has small chop, tabs are retracted, and motor is reasonably trimmed with no cavitation.
PropMD has a website with a calculator for this. If you email them, they can give you a recommendation. We also have local folks who can help:
Skip’s Iii Propeller Service
7169 Cross County Rd
North Charleston, SC 29418
(843) 552-4000
To choose correctly, find out what the manual says is your redline. Basically you want to be able to go from zero to redline, and not higher, if you want a good overall prop. A prop for acceleration (like to pull skiers) will usually take you over redline if you stay at full throttle. For extreme economy, your prop might never reach redline. Just depends on what you want to do. Does your manufacturer lists prop recommendations for your boat/motor combo?
RPM range is 4500-5500 on that engine so you are right in the middle of the range which is good, but it wouldn’t hurt to be at 5200-5300 rpm. Given a 2.0 ratio, 5100 rpm and a pitch of 16 (I’m guessing the prop is cupped so I’ll give it a pitch of 16.5) you should be running 36-37 mph with estimated 8% slip. I have heard that the Yamaha 90 is not the strongest engine, but I would expect a little more out of that boat. I would expect to be able to run a good 17" pitch without a problem which will put your speed in the 39-40 mph range.
What is the make and model of the prop?
Iain Pelto
Sea Hunt Triton 160
Native Manta Ray 14
The prop is a Mercury Quicksilver Silverado. Basically the same design as the Vengeance. It is a good all around prop. You could play the prop game and get 1-2 mph more speed, but like I said it is a game. The prop you have now is a good mid-rpm prop and performs well at a variety of speeds, another faster prop may not. A limiting factor is there are not a lot of choices for a 4.25" gear case in that HP range. Something you may want to consider is a jack plate. It will give you some more speed, allow the boat to be trimmed better, and decrease running draft. If you do get a jack plate I recommend installing a water pressure gauge to know when you are trimmed too high and starving the engine of cooling water.
So this weekend I got my hands on a 13 1/8" x 15p SS prop that is ventalated. The boat jumps out of the water from a dead stop and picked up a couple mph on the topend. My gps says 42mph at 5300 rpms and my crusing speed and rpms seemed to be the same. I guess the biggest change is the hole shot with a fully loaded boat. Thanks for everyones help. Now I just need to sell my old prop.
