RPM Question

[FishingCop]

Got a 40 HP Force (no jokes please) on our boat and don't know if the prop was original or replaced at some point.

I think it is a 12 degree, 10 3/4 inch prop. The motor manual says that I should be running 5100 RPM's. When we run at full throttle with four of us in the boat (2-guys, 2-(slim) wives) it gets up to about 56-5700 RPM. With me alone it runs at 50-5100 RPM. A guy said I should select a prop to run at 5100 RPM when we have the most often amount of people (weight) in the boat.

So......... given that the motor manual says 5100 RPM is max, and we run at 56-5700 with all four of us, what should I do? Change prop sizes? Run a slower throttle (to keep it at 5100) and leave it alone? What???

Also, is it normal that the RPM's are higher with more weight in the boat? I assume it is since that's what I experienced when checking this out - but I'm not sure how it all works??

Thanks again for your help....

 

[bassboy1]

While I am by far no prop expert, I am still certain that it is not normal to increase in revs when you increase in weight. Generally, as the motor has to lug more when you add weight, the revs will drop. The only thing I can think of is that you have a trim issue, and the natural riding level of your hull isn't reached until you add more weight in different places when you add friends.

You don't by any chance have an accurate speedo, as in GPS, do you? If so, is your speed increasing when you add people. If the numbers which you posted are correct, and aren't the doings of a dying tach, it would seem as you need a prop with 3 inches more pitch when running with a load, which defys all logic. Basically, you have the ability to go faster with more weight, and you can't go as fast when you are alone.

You have got some anomaly with your setup, or I am just unaware of some odd circumstance that most prop guys would know like the back of your hand.

 

[Zum]

You get higher RPM's with more people(weight)in the boat?
Is it supposed to be the other way around...anyways I use 2 props a 13 and an 10 3/4 pitch,to keep my RPM's where I want them.
Not sure how old your outboard is but it seems to be handling the extra rpm's fine...not sure if there is a plus/minus on the rpm's
I do know some people the go alot over there recommended rpm's and are running some older(80's) Yamaha/Suzuki outboards,whatever thats worth.
They use them for mossing,8 3/4 pitch on 40/50 hp motors.

 

[FishingCop]

While I am by far no prop expert, I am still certain that it is not normal to increase in revs when you increase in weight. Generally, as the motor has to lug more when you add weight, the revs will drop. The only thing I can think of is that you have a trim issue, and the natural riding level of your hull isn't reached until you add more weight in different places when you add friends..

Okay, being the novice that I am, I thought the same thing..... I think I have to get the boat out again and double check these RPM's - with and without 1 or 4 people. The guy in the boat (the resort owner in Arkansas last week) was driving and he's the one that said we need to match the prop with the "most often" number of passengers - which is four (2 guys, 2 wives).
BUT... your "trim" question makes sense... maybe I didn't get it trimmed up all the way when I was alone?? or maybe he didn't when there were 4 of us in the boat??

The boat is a 16.5 deep V spectrum, see here: https://www.tinboats.net/forum/viewtopic.php?f=3&t=139&start=40 Of course, we run without the bimini top....

 

[ben2go]

Have you cycled the trim with the boat out of the water to check that the cylinders are moving their full range?I too believe that it's a trim issue more than a prop issue.

 

[FishingCop]

Have you cycled the trim with the boat out of the water to check that the cylinders are moving their full range?I too believe that it's a trim issue more than a prop issue.

Well, probaly not since I have no idea what "cycling the trim out of water" means ????? As far as the cylinders moving full range, the only thing I can say is that when I move the motor up - it goes all the way out of the water and I put on a transom saver for towing. When I move it down, it goes all the way vertical.....???

Just a reminder - we are new boat owners and don't know much (yet) about most anything

 

[ben2go]

Have you cycled the trim with the boat out of the water to check that the cylinders are moving their full range?I too believe that it's a trim issue more than a prop issue.

Well, probaly not since I have no idea what "cycling the trim out of water" means ????? As far as the cylinders moving full range, the only thing I can say is that when I move the motor up - it goes all the way out of the water and I put on a transom saver for towing. When I move it down, it goes all the way vertical.....???

Just a reminder - we are new boat owners and don't know much (yet) about most anything

That's what I am talking about.I was thinking that your outboard used hydraulic cylinders to trim the it up and down.I understand now that you have an electric unit.Cycling the trim is running the outboard up as far as it can go and bringing it down as far as it can go.Some outboards have external stops and some have them inside the trim motor.Sorry if this is all confusing.