2nd trip, RPM question

[trout74]

Today we took the crew to Lake Red Rocks, near Pella Ia. Its the largest lake in iowa ( reservoir ) but very pretty. Anyways, today according to my manual I could go WOT for short bursts. So when we would cruz across the big coves I would open it up for about 1-2 minute spurts, then go back to 2500-4000 RPM for the majority of the time.
When I was going full throttle I could fiddle with the trim and on flat water I could get it to go 26-27MPH. I am very happy with that speed, BUT my RPM's were 5000.
My load was two adults and three kids, and coolers etc. My Prop is a 15 pitch. The speed seems right on for a 20 ft. pontoon and 75 HP motor, but are my RPM's correct? and if I did go to a 13 pitch, I assume my RPM's would go up....but...would I gain or loose top speed?

Thanks, for your time.
trout

 

[EGlideRider]

Re: 2nd trip, RPM question

Today we took the crew to Lake Red Rocks, near Pella Ia. Its the largest lake in iowa ( reservoir ) but very pretty. Anyways, today according to my manual I could go WOT for short bursts. So when we would cruz across the big coves I would open it up for about 1-2 minute spurts, then go back to 2500-4000 RPM for the majority of the time.
When I was going full throttle I could fiddle with the trim and on flat water I could get it to go 26-27MPH. I am very happy with that speed, BUT my RPM's were 5000.
My load was two adults and three kids, and coolers etc. My Prop is a 15 pitch. The speed seems right on for a 20 ft. pontoon and 75 HP motor, but are my RPM's correct? and if I did go to a 13 pitch, I assume my RPM's would go up....but...would I gain or loose top speed?

Thanks, for your time.
trout

You will need a 13 prop. The R's will go up a little as the motor breaks in but I don't think it make it to the mid 5's that your manual probably calls for.

 

[trout74]

Re: 2nd trip, RPM question

My manual says 5000-6000 rpm max. So 5000 is on the low end. So then if I go to a 13 pitch and my rpm's go to 5500-6000 will my top speed increase or will the smaller pitch prop just spin faster pushing the same amount of water?
Thanks,
Trout

 

[rrhodes]

Re: 2nd trip, RPM question

I would guess that you will pickup 1 mph or so. Was that speed with gps? Numbers sound high for a 75hp rolling 5000 rpm. I would expect that speed at 6000 rpm. Of course I am not sure how much your boat weighs.

 

[EGlideRider]

Re: 2nd trip, RPM question

I would guess that you will pickup 1 mph or so. Was that speed with gps? Numbers sound high for a 75hp rolling 5000 rpm. I would expect that speed at 6000 rpm. Of course I am not sure how much your boat weighs.

I agree with rrhodes. Speed should not be your primary concern for a pontoon but rather the life of your engine. 5000 RPM is lugging the engine at all RPMs. Think of driving your pickup it's entire life while loaded to its max..

 

[Silvertip]

Re: 2nd trip, RPM question

What you need to be careful of though, is to make prop changes based on what you consider an AVERAGE LOAD. If you had the boat heavily loaded, then the prop you have may be ok since going out with a lighter load may then cause the engine to rev into the proper range. If you changed to a 13P prop based on a heavy load, and then went WOT with a light load you would likely seriously over rev the engine. Make this choice carefully -- not after just the first or second time out with the boat. Get it well broken in first.

 

[trout74]

Re: 2nd trip, RPM question

It was not with a GPS. but I do have one, so next time I go out ill test it with that. My boat is not overly heavy, as its not a " luxury " toon. It weighs 1700 pds empty. Motor is about 400 pds, 500-600pds of people, 80pds of fuel, 50pds battery, and ill guess 100pds of gear. Im guessing 2800-3000 pds would be toward the heavy end.

Ill give it a few more trip and GPS the speed, and report then.
Thanks,

 

[rrhodes]

Re: 2nd trip, RPM question

Just remember to baseline your speed with what you consider to be a normal load. With the Wife and Kids and a half tank of fuel I average about 5900 RPM. When I have 10 people on board I can only get about 5600 RPM. If I dropped down in pitch when I was loaded with 10 people I would over -rev when it was just the wife and kids. Always try to prop your Toon to get as close to max rpms as you can with an average load. This will allow you to remain in the safe WOT range while you are loaded. In your case if you took another family with you I will bet you would end up 400 rpm under the recommended range.

 

[trout74]

Re: 2nd trip, RPM question

Talked with dealer, told him my approximate weight, and that I was getting 5000 RPM's with the 15 pitch and he said the 13 would only give me 300 more RPM,s and that he is going to send me an 11.

Sound correct?

Thanks

 

[lncoop]

Re: 2nd trip, RPM question

I don't know troutster, 11 sounds a bit extreme to me. Generally every 1" decrease yields an additional 200 RPMs or so. In your case that would mean a 13 would put you right in that sweet spot, which is often the case with barges. It sounds like your dealer is willing to hold your hand while you experiment, which is nice, so if you really want to give the 11 a shot it won't hurt anything, but my money is on the 13. Also, I'll echo others and say don't get in a hurry. Good luck.

 

[EGlideRider]

Re: 2nd trip, RPM question

I agree that the 11 is a bit extreme. Your load of "My load was two adults and three kids, and coolers etc." is a medium load for that boat. I would go with the 13.

 

[lmuss53]

Re: 2nd trip, RPM question

I would go with the dealer's recommendation. I think you would rather see the motor in the top of the recommended range rather than the lower side. You won't hurt the motor with an occasional burst into the 6100 range, and you can always back off the throttle a bit to keep it in the range when you are loaded lighter. I think it probably has a rev limiter to keep it from getting into dangerous range, but check on that with the dealer or in your manual. If you consistantly see the revs going over the 6000 mark you can always ask him to give you the 13. Be careful with these props, if you bang one up I'm sure you will be done swapping them out. By the way kudos to the dealer again, he really seems to want to help you get this right. Good luck and enjoy the testing. Where else can you have that much fun getting things straight?