RPMs a bit higher at WOT than last year

cwiert

Active Member
May 7, 2008
560
Chester, CT
Boat Info
1998 400 Sedan Bridge
Engines
CAT 3116 - 350HP
Last year at WOT, the most I could ever max out at was 2750 RPMs on my 350HP CAT 3116s. Yesterday I did my first real WOT test since last year and both engines went a tick past the 2800 mark. Probably a hair less than 2850. This was with 3/4 fuel and 1/2 water and a dinghy. I'd imagine if I had no dinghy, less fuel and no water, the RPMs would be even higher.

So what changed since last year?
1. I had the bottom media blasted and I barrier coated and repainted in the spring.
2. I swapped the Racors, going from 2 micron to 10 micron.

That's all I can think of. My $ is on the fresh bottom, but it's not like the bottom was dirty last year (in fact the boat was hauled and the bottom was power washed in June of last year).
Could the less restrictive filters be a factor?

Lastly, when we go on a 1-2 hour trip, I like to go WOT for a minute or two during the trip to make sure the engines can still reach the spec max RPM (which is 2800) and to "clean" stuff out of the engines. So when I do this, should I stop at 2800 RPM even if it could go a shade higher?

Props are OEM spec. 22x24

Thanks.
 
What happened with your speed in relation to the rpm?
Were the props touched up when the bottom was done?
 
I think I gained a couple knots. Yesterday at WOT I was doing 28-29 knots at WOT via GPS, which I don't remember ever going that fast before. But I was going with a the outgoing tide which certainly helped push me along.
No, on the props. They were sanded and painted last spring and this spring as well. No difference there.
 
The answer is #1.

The fuel filters won't increase rpms because the Cat secondary filter is limiting the filtration fuel flow.

If your boat has never had the bottom stripped, you would be surprised how much paint they retain, even though we use an ablative bottom paint. Besides the weight of the old bottom paint.......ever pick up a gallon can of the stuff? - then you use 3-4 gal every 3 years and it adds up, the paint roller leaves a fairly rough surface. Every new coat of bottom paint increases the texture on the bottom and texture adds drag. When you soda blasted and barrier coated the bottom, you essentially brought the bottom back to new condition. What you are experiencing is the way the boat performed when she was new.

Caterpillar powered boats are so predictable that you really only need to run the engines at WOT when you suspect something has changed that will affect speed or rpms. I do the WOT thing 2-3 times a year, and never leave the engines at 2800 any longer than it takes the boat to settle down at WOT trim to see a consistent rpm and speed.....2-3 minutes. Caterpillar says E performance ratings engines....pleasure use......can run at WOT for up to 8% of the time, but it doesn't take me that long to see what I need to see, so i figure why stress the engines, run the cylinder temps up, and waste fuel when I don't need to. I also think the rpms question is going to answer itself when you get a coating of algae or slime on the bottom. 2850 rpm isn't going to hurt anything and an occasional venture slightly above that isn't going to overload the engines significantly, but I think you will be back to 2800-2850 before very long, anyway.
 
Colder water is less dense than the warmer water at the end of the season. These changing water dynamics have to be considered as a factor. I always see increased speed/Rpms in the beginning of the season versus the end.


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Colder water is less dense than the warmer water at the end of the season. These changing water dynamics have to be considered as a factor. I always see increased speed/Rpms in the beginning of the season versus the end.


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Cold water is denser than warm water. Because of that a boat would sit slightly higher in the water and there'd be less drag when moving.
 
Yea that's what I meant to say! :)


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Makes sense to me. A clean freshly painted bottom and colder dense water = about 75ish RPM. Thanks for the explanation, guys.
 

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