Easy prop/rpm question

[FlyinGuy1017]

I have a '92 Sea Ray 200, 4.3L Mercruiser, Alpha I, cheapo 14.5 x 17 alumnium prop with some dings/scrapes. Last weekend I was out with an average load: 5ppl, coolers, full fuel. After launch while running, I had to pull back because I was over my WOT by 200+.

As the day progressed, we dropped a person off, drank much of the cooler weight (and expelled it from ourselves so it's not loading the boat), and burnt 1/2 tank of fuel. While coming in, at WOT I could only hold around 4500 rpm (range is 4400-4800).

We weren't running on a river, and lake conditions didn't change much while out. Why the drop in WOT while getting lighter?!? Shouldn't it get higher as the boat gets lighter?

It was trimmed properly while running.

Any thoughts are appreciated.

 

[steelespike]

Re: Easy prop/rpm question

My first guess is temperature Affecting the tach components.I also find a 4.3 20 footer with 5 people and gear overeving so easily a surprise.
Again I would suspect the tach.I think I would start by verifying the tach is accurate.Perhaps you could borrow a shop tach.Another possibility running with or against the wind.

 

[Silvertip]

Re: Easy prop/rpm question

Picking up some weeds or debris on the outdrive can ventilate the prop causing the over rev problem. When you throttle back or stop, the debris comes off.

 

[boat1010]

Re: Easy prop/rpm question

Was the motor trim in the same position? The trim effects the RPM Quite a bit. Are ya sure the cooler contents weren't effecting anything??? Smiling

 

[boat1010]

Re: Easy prop/rpm question

Are you sure your trim was in the same position. That will effect the RPM quite a bit. And are you sure the contents of the cooler aren't effecting anything?? Smiling..

 

[FlyinGuy1017]

Re: Easy prop/rpm question

Not weeds, wind, or cooler contents...I tend to lean toward a fluxuating tach. I've seen it jump a few times w/o any changes in control or sound (real jumps, large and irratic). Dang...just another thing to put on the list