Motor Question

[jconstab]

A few months ago I bought a 17 ft with a 2000 50 hp Yamaha 4 stroke. The boat tops out around 30 mph at 4500 rps. It seems to me that the boat is underpowered. Not that I would run it higher than this, but I would think that WOT is more than 4500 rps for this motor. Any thoughts???

[cagrove]

It could be that you need a different pitch prop in order to get the RPMs up a little. You're not far off, but somewhere around 5200 to 5500 should be your goal. As for speed, mid to high 30s should be obtainable. Of course, how much weight you have in your boat will affect this also. I hope this helps.

[jconstab]

Those stats are with a full fuel tank, not much gear, and me by myself on the boat. With people on board the performance decreases.

[cagrove]

That being said, I'd first start with a new prop then evaluate to see if you need to change anything esle. I hope this helps.

[Troll]

If you have the owner's manual for the 50 hp Yamaha 4 stroke, see what it says about WOT rpm's. If you do not have the manual, check with a Yamaha dealer. My motor is a 70 hp Suzuki 4 stroke and it says that the correct pitched prop will deliver between 5200 and 5800 rpm's at WOT. However the 60 HP version of this same engine has a range of 4700 to 5300. Until you know what Yamaha specifies for your engine, you won't know if you are at the correct rpm's at WOT.

I have two props, a 15 pitch which I use at elevations over 3000 feet and a 17 pitch for lower elevations. At their respective elevations, I usually get 5600 rpm's at WOT with the water relatively smooth.

Once you know what your ideal rpm range is, then you can determie if you need a prop with a lower pitch. A good dealer should be able to tell you an approximate rpm change with every drop in pitch.

My manual has the following warning: "Installing a properller with pitch either too high or too low will cause incorrect maximum enginge speed, which may result in severe damage to the motor".

Good luck, Troll

[PileDriver]

copied from a webpage, but here it shows your WOT rpms. I know it is for a 2006 but the top rpms never really change. By bet is you need to decrease the pitch of your prop. 2" inches in prop pitch (generally speaking) changes the RPMs by 500.

GENERAL INFORMATION
Manufacturer Yamaha
Model Year 2006
Model T50 hp High Thrust
Engine Type 4-cylinder Horsepower 50 hp
Displacement 60.8 ci (996 cc)
RPM Range 5,000-6,000 rpm
Fuel Induction System SOHC Fuel Injection
Lubrication System Wet sump
Ignition System TCI Micro Computer
Starting System Electric
Alternator Output 17A @ W.O.T.
Gear Ratio 12:28 (2.33)
Shaft Length 20 in.
Weight 248 lbs. (113 kg)

[woodNfish]

Your boat will take a 75hp engine, so you are 25hp under the maximum. Your boat should have plaque on it that gives you the maximum weight load, number of occupants and horsepower. If you exceed any of those and marine patrol pulls you over, you may get a fine.

woodNfish