Boat only going 4800 rpm's

[Zeus911]

I have a new 20' ft. Century with a 150 -4stroke Yamaha. With the throttle burried I am only running 35mph and turing 4800 rpm's. I checked the prop and have the correct size and pitch on that is recommended for the boat. I should be getting at least 48 mph at 5800 rpm's. What is my next step to look at.

 

[QC]

Re: Boat only going 4800 rpm's

New? Brand new? Where you getting the expected results? How are you getting your speed? GPS?

Oh, BTW, welcome aboard!!

 

[Zeus911]

Re: Boat only going 4800 rpm's

It is brand new with only about 33 hours right now the engine. I had the issue last year, dealer said they corrected, but they never did. I am verifying the speed from the gauges and it is identical to my GPS.

 

[QC]

Re: Boat only going 4800 rpm's

Where'd you get your expectation?

 

[QC]

Re: Boat only going 4800 rpm's

What model Century? Load? Stuff?

 

[steelespike]

Re: Boat only going 4800 rpm's

Don't be so secretive let us know what you have.
What size prop? What is boat weight?What is the weight as tested?
New boat or not; Is the throttle opening all the way running on all cylinders?
Don't assume it is.How is the performance other than wot speed/rpm?
Antivent plate (just above the prop) even with ther bottom is considered the starting point.could be raised an inch or more above the bottom.

 

[Zeus911]

Re: Boat only going 4800 rpm's

Prop is 18 stainless steel. It only had M18 stamped on it.
2001SV Center console, roughly weight with load is 3400 boat weight minus gear and engine is 2300 lbs. The performance is alright not missing, gets on plane, and sounds fine, but just does not go any faster when the throttle is buried. When looking at the performance ratings they have it at 5600 rpms with a speed of 45mph.

 

[QC]

Re: Boat only going 4800 rpm's

When looking at the performance ratings they have it at 5600 rpms with a speed of 45mph.

Can we see this?

steelespike is heading where I was, but wanted to get some of this other info as well. Just because you have the control mashed does not mean that the engine's throttle is mashed . . .

Oh, although your speed sounds low, don't ever expect her to run at test speeds, they never do. Never. You said that you thought "at least 48", and then posted that the performance data showed 45. Why that discrepancy? Please don't take any of this as negative. The more info the more merrier (and accurate) our responses . . .

What'd the dealer say they did? Have you ever seen higher speeds?

 

[steelespike]

Re: Boat only going 4800 rpm's

Is it doing 35 at 4,800 at 3,400 lbs?Is that a 4 blade prop?
You may be slightly over propped.A 2" drop might get you to 5,200 Where is the antivent plate at?Raising the motor could gain a couple of hundred rpm and a couple of mph at no cost if you do it yourself.
You could have a prop shop measure the prop.Guessing is like guessing about all the rest.

 

[hwsiii]

Re: Boat only going 4800 rpm's

Zeus, they ran that performance test with a Reliance model 17" Pitch the prop number is 68F-45972-00-00. And they almost got 44 MPH at 6,000 RPM with the motor on the second hole. You need to have them exchange the prop you have with the exact one they used in the test. You may be running their performance series prop and I have seen that prop eat RPM like it was candy with NO performance to justify it. There is a 5 MPH speed difference between the Performance 16" and the Painted Steel 17".

Here are actual numbers from my friends boat comparing the Performance series 16" and 18" against the Painted Stainless 17".

H