50hp 4stroke not reaching top speed

[csotherland]

Hello - I have a 16ft '05 smokercraft with a 50hp yamaha 4 stroke. With 3 people on board (big - totalling about 700lbs) we are unable to get to top speed. Only goes about 15 mph and only acheives 2800 rpm. If I drop a person, the boat goes 32 mph at about 4200 rpm. When I go at higher altitudes, the problem is worse. No problems planing the boat, it just doesn't seem to sit on the water. Any ideas? I was told to go to a lower pitch prop, does this sound right?

Thanks for your help.

Carl

 

[JB]

Re: 50hp 4stroke not reaching top speed

Sounds badly overpropped to me.

I will move this to the Prop Forum.

 

[K.P.]

Re: 50hp 4stroke not reaching top speed

What prop do you have on the rig now? What Max RPM's are you getting with just you in the boat? With every person you add people to the boat, your RPM will decrease. Higher altitudes also reduce RPM because of the thinner air, less oxygen mixing with the fuel to make power.

In all likelyhood you need to reduce prop pitch or switch to a reduced pitch prop when you have a heavy load of people and/or boating at higher altitudes.

Your Yami 50 Four Stroke has a Wide Open Throttle RPM Range (WOT) of 5000 to 6000 RPM. You want to select a prop(s) that will not let you exceed 6000 RPM but also not operate lower than 5000 RPM when running at full throttle. So it's best to select a prop that will turn 5900 to 6000 RPM at full throttle with just you in the boat and a light load of fuel, and turn no less than 5000 RPM with a full load of fuel an people.

Answer the above questions and we can help you.