Prop Sizing

[Wild and Free]

I realize that, but what am i going to use fuel wise with another 500rpm? Yamaha says I should use 5-6gph at 5500. I'd love the extra speed and power but at $4+/gal I'm kind of afraid to spend the money on a new prop. If I knew that my fuel rates would stay within a gal/hr of what I was getting now I'd switch in a heartbeat.

Anybody else with a 90 yamaha got some real #'s to compare to?

You have to realize that your engine being over proped will use more fuel if its being worked real hard at lower rpms than being worked easy at higher rpms. I read on some other forums where there had been extensive experiments on boats with larger engines getting almost twice the fuel economy than a smaller over propped engine on the exact same boat.

Think of a bigger prop on a small engine like putting 35 inch super swamper tires on a geo metro because it would lower the rpms considerably to go the same speed, it would use considerable more gas than the properly sized tires.

 

[seven twenty]

At 5100 rpm's at WOT, you're motor isn't happy at all. You are putting a constant overload on the engine throughout the entire rpm range by having it over-propped. The engine's ECU (computer) senses this and compensates by adding more fuel, thus your decreased fuel economy. The 5500-5600 rpm range is better, but ideally, you should be shooting for the 6000 rpm range. The motor was designed to run in that range and will be much happier......and more fuel efficient.

The motor doesn't seem to be "lugging" at all. It plains and hits top end pretty quickly. I'll see if I can't try a different prop and then check the economy. From what I've read, I'm getting "exceptional" economy at this point. Approx 2.5 gph actual versus 5-6 gph as predicted by Yamaha. Those numbers also indicate that the engine is not being overworked. It would be great if I could do better though. THX for the info.

I think it has a 13.75X15 on it now. So I should try a 13.75X14???

 

[ericscher]

There's a concept known as YMMV, or "Your Mileage May Vary". It came from the automotive world but spread everywhere because it embodies a useful reality.

YOUR usage will differ in it's particulars from pretty much everyone else's usage.

It's not crazy to think; "Hmmm... I'm happy. Maybe I'll just leave well enough alone."

And honestly...? At 5000+ rpm, you aren't lugging your engine. Is your prop the best match? Technically no, because the best match has max throttle and max rpm occurring simultaneously.

But does that mean your prop is BAD? Nope, not at all.

Relatively low powered pontoon boats, meaning anything under 150 generally do well with something in the 14" x 9-12 pitch range.

But you're already running at 22mph. If that's GPS confirmed, then you're not going to a see a huge improvement no matter WHAT you do.

But something else is bugging me...

You say you're getting 5100rpm registered at WOT both alone and with a passenger and two teens on a tube? That SHOULD lug you down.

Makes me wonder if your tachometer is registering correctly.

Your mechanic can test that for you, OR you can buy a cheap electronic tach at your local

autoparts store, hook it up temporarily and see what you get. That assumes you're comfortable playing with your own motor/electronics and such.

I also wonder...

Did you get your prop specs off the prop or were you told them by someone else?

Fortunately, you have another option. other than sorting through all this stuff I mean...

Enjoy the rest of the season as is... you're not going to hurt anything.

This fall, when it time to pull the boat out, have the marina check these things for you and you can personally verify what your prop specs are by looking at it yourself.

Boats are supposed to RELIEVE worry, not cause it. B)

 

[Ben & Suzy]

deleted.

 

[seven twenty]

OK, verified the prop visually, it is a 13.75X15. Gps says 24.3mph at between 5-5100 WOT. It dit hit a max speed of 25.7mph somewhere in the trip. Boat was loaded with myself, my wife, daughter, and a full load of fuel (24gal). It is an 08 2050 Fsi. I didn't check the toon size but assume they are 23'. (capacity placard 8 person, 1095 lbs max 90 hp) Bennington lists this as an optional setup that year with the F90. Something that concerns me is that I played with a couple of different "calculators" online and they both come back with a 20%+ slip figure. They also show a decrease in speed with a decrease in pitch. From reading everything on here I realize the speed part probably isn't right but should i worry about the slip part? Slip % is related alot to prop design isn't it?

Has anyone tried Yamaha's Pontoon Performance Series of props. I'm thinking of trying a 14X13 or 14X12. I don't have access to a variety so I'm trying to get it right or as close as possible on the first try. Thx for all of the great info so far.

 

[ericscher]

Actually, I would guess you have 25" logs on that boat. Could be wrong, but more likely than not that's what size they are.

You're also doing fairly well for speed.

Most of us end up experimenting with props and in all likelihood you will too. Aluminum props are cheap enough to do that with, and honestly... it's ALWAYS nice to have a spare.

But if I were you the FIRST thing I'd do would be to verify the tachometer. I have a sneaking suspicion that it's not registering correctly; but is instead simply topping out at 5100.

 

[seven twenty]

Thx Eric, never considered that the tach might be off. I'll try to check it tomorrow. They are 23" toons.