Performance questions

[patrickcudd]

Please forgive my ignorance about Pontoon performance, but after reading several threads, I think I can get better performance from my boat. Here are the boat specs:
2014 20' Silverwave
2 round logs
2014 90 hp Suzuki
Top speed 26 (gps) with a half tank of gas and me alone.
The prop is stamped 3x14x17, So I'm assuming 17 degree…….

My question is, is there somewhat to improve the top speed by something like a prop change?

[mpilot]

The prop specifications are stamped on the prop. The two boat tests done by Suzuki both have the boat running about the same as yours. It's a displacement hull so that's probably pretty close to all you are going to get out of it. Even if you added lifting strakes I would expect a 90 hp to get you into the low 30's.

[Bamaman]

What kind of RPM's are you pulling at full throttle? What's the posted redline on that engine?

[patrickcudd]

[quote="Bamaman"]What kind of RPM's are you pulling at full throttle? What's the posted redline on that engine?[/quote]

I'm pulling right at 5000 RPM and, according to the manual, the full throttle RPM should be between 5500 and 6300 RPM.

[mpilot]

Here's the closest test I could find. I'd make sure you're pulling the same prop. Your speed is hull limited and you'll most likely get the same speed as now but not lug the engine. Propping a two toon boat isn't the same as a v hull since it's a pure displacement hull. Even a tritoon is different than a v hull as sometimes the numbers say changing the prop will do one thing but it does something totally different.

These tests they do they play around with motor height, props, etc. They don't just do one run and that's the end of it, so usually the setup they are running is the best overall setup.

http://www.suzukimarine.com/Product%20L ... CL%20DF90A

[Bamaman]

Suzuki outboards have a planetary gear in their driveshaft, and your motor has a 2.59:1 gear ratio. That means most Sizilo powered boats require large diameter, high pitch propellers. They're just another animal when it comes to setting them up on all kinds of boats--especially pontoons.

5000 rpm's with a 17 pitch prop. is pretty surprisingly low. The above boat test using a 15 pitch prop was getting 5900 rpm's.

Obviously you need a lower pitch propeller, however you're already on the lower end of Suzuki propellers' availability. You might want to switch to the 15 pitch propeller and also consider raising your motor a notch or two to see if it ventilates in a tight turn.

Unfortunately, twin pontoon boats are displacement hulls and they have their limitations. You're not far off on mph, but most owners try to get closer to the redline so the engines don't lug--making them last longer.

[Jake_Daddy]

I agree you don't have the right prop based on your RPM being too low and for the health of your engine it needs to be swapped. That being said your getting good bite out of that prop you have on there right now, for a two logger at those speeds and RPM's your getting about 15-16% slip and I don't think your gonna improve on that significantly (If I could get my 20' Sweetwater with a 90hp to 20% slip I would be ecstatic). If I were you I would try raising the motor before going out for a new prop which may get you into the lower end of the RPM range also verify that RPM's are correct just to be sure your baselines numbers are good. You definitely wanna be in the upper part of your WOT range when its just you that way when your out fully loaded it should still keep you in the safe zone.

As a comparison for you as a similar sized boat and motor combo when I purchased boat used it had a 16P on it which keep me about 200rpm below my range of 5000-5500 rpm I dropped it to a 13P prop (different brand and went from SS to Aluminum) which puts me about 200rpm above my range and in a couple of weeks I am planning to try a 15P which I am hoping puts me right where I want to be RPM wise. Our dang battery died last weekend and the wife didnt approve on buying a prop and battery at the same time so in order to hit the water I had to go for the battery, looking back I should have gone with the old saying "sometimes its better to ask for forgiveness than permission"!

[rancherlee]

for a good cheap prop I'd suggest the Solas Amita 4 in a 13 or 15 pitch. That's one of the few Suzuki specific props on the market designed to work well with the low gearing in the case. I run the 17p on my 140 and it gives me some pretty good slip numbers.

[taylorjm]

So first. When the title of the post is "performance questions", I figured it was going to be someone selling viagra. But no, it's about props.

Question though. What is the slip percentage and how do you calculate it? What's good and what's bad?

[BobG]

[quote="taylorjm"]What is the slip percentage and how do you calculate it? What's good and what's bad?[/quote]

It starts with the "Pitch" of your prop.
A 17 isn't 17 DEGREES, it's 17 INCHES.
So for every revolution of the prop, with zero slip, your boat would move forward 17 inches.

So you take the RPMs of your motor, divided by the total gear ratio of your lower unit, to get your actual PROP RPM.
Let's say it's turning 2,000 RPM, when your motor is turning 5,500.

That's means with that 17" prop, zero slip would be 17 * 2,000 inches per minute.
You convert inches per minute into miles per hour - it's fairly straightforward math.

If zero slip speed was 32.2 MPH (that's what you would get with a 17 at 2,000 rpm), and your GPS speed is 28 MPH, your slip would be 100 - (28 / 32.2 X 100) percent, or 13% slip in this example.

[BobG]

Here's the simple math:

17" X 2,000 RPM = 34,000 Inches per Minute
34,000 / 12 = 2,833.33 Feet per minute
2,833.33 / 5,280 = 0.536616 Miles per minute
0.536616 * 60 = 32.2 Miles per Hour

[BobG]

But what is my final gear ratio???

When I look up the specs on my Merc 115, it says my gear ratio is something like 2.33:1

So at 6,000 RPM, my prop is turning:

6,000 / 2.33 = 2,575 RPM.