Question on high RPM's at WOT

runtpacket

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Boat is a 20' SLM SPS with a Suzuki 115. With full tank of gas and 2 passengers. Tach shows 6400 RPM, the over rev light on the engine monitoring gauge flashes.

During my 1st long hard WOT run (about 20 minutes) in chop I got beeping and the engine dropped power and seemed to sputter until I went to neutral. Rev limiter light was on. (could have been over trimmed, or prop coming out of water)

I don't have prop specs at the moment. I will get them next time I am at the boat.

So my question is should I leave the prop as is and expect more weight, growth on toons etc to slow the boat down and get RPM's into range (6000) or should I go up 4" in pitch to drop it 400 RPM's?

Speed is very good. as configured I can hit 35.5 Mph (gps).
 

lakeliving

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What prop is on there now? Banging off the rev limiter isn't going to be good long term. How will you normally boat? You should be able to go full throttle with just you on it and not hit the rev limiter.
 

runtpacket

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IMG_20190719_081744.jpg Untitled.png

My eyes arent the greatest and taking a picture while on the water is a fun challenge. Anyone want to help figure out what it says only thing I can make out for sure is that it's a 17p
 

DaveyJ

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My guess is 3x13.75x17.....
 

runtpacket

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Now that the dealer is open I called them. They "think" it has a 13.785 X17P, they have to check with the other store that rigged it.
 

2fast4u

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That sounds like the correct prop pitch, I bet you over trimmed it causing the prop to slip. most pontoons boats do not require that much trim. I only trim my boat to no more than 25% out on trim gauge, re test your boat using very little trim and read your rpms at full throttle. 17p should be a enough prop with 115hp. I also noticed you were testing in choppy waters, this may have un hooked the prop causing you to hit the rev limiter. Prop testing needs to be done in calm waters and light winds to get accurate rpm data.
 
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Vikingstaff

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My guess would have been you are way over propped at 17p on a 115hp motor, so this does surprise me. I’d do what 2fast4u suggests, and see what happens. Is it more of a diameter of proper issue than a prop pitch issue? Is the engine mount TOO HIGH and maybe the propping isn’t down in there enough to bit and catch water as it should? Is there any issue with how the prop is on the motor causing slippage? Things to consider if after following the guidelines above you are still having the same issue.
 

runtpacket

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That sounds like the correct prop pitch, I bet you over trimmed it causing the prop to slip. most pontoons boats do not require that much trim. I only trim my boat to no more than 25% out on trim gauge, re test your boat using very little trim and read your rpms at full throttle. 17p should be a enough prop with 115hp. I also noticed you were testing in choppy waters, this may have un hooked the prop causing you to hit the rev limiter. Prop testing needs to be done in calm waters and light winds to get accurate rpm data.
I also tested in calm (glass waters) this is a trioon. It doesn't hit rev limiter or cut spark but over rev light flashes. I trim up just until bow raises and then the gauge shows 1/4 on trim gauge
 
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2fast4u

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I also tested in calm (glass waters) this is a trioon. It doesn't hit rev limiter or cut spark but over rev light flashes. I trim up just until bow raises shows 1/4 on trim gauge
Based on the information you have posted and my experience with boats (now retired, 3 time apba/Superboat national champion and 2 time world champion in apba / Superboat offshore racing) you motor mounting height needs to be checked to be sure the center toon is not blocking some water flow to the prop. It might need to be lowered one mounting hole or two. a 17pitch prop is enough on a 115 hp motor, My 22' bennington twin toon with a 115 hp Yamaha runs 29 to 30 mph with a 15P at 5700 rpms and 6100 rpms with a 13P two people 1/2 tank of fuel.
 

runtpacket

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Ok guys I think it's trim. and I think my trim gauge is not accurate. I usually run at 1/4 trim (on the gauge). Last night I noticed alot of "slack" between when I trim and the gauge reading. So I tried a run at 0, 6000 RPMS. So now I will just trim up until the bow lifts, not to 1/4 on the gauge.

Also had a full Load on deck last night. 6 adults with estimated at 1450 lbs. Trimmed to 0, I got 5800 RPMs and 30 MPH on GPS
 

2fast4u

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Ok guys I think it's trim. and I think my trim gauge is not accurate. I usually run at 1/4 trim (on the gauge). Last night I noticed alot of "slack" between when I trim and the gauge reading. So I tried a run at 0, 6000 RPMS. So now I will just trim up until the bow lifts, not to 1/4 on the gauge.

Also had a full Load on deck last night. 6 adults with estimated at 1450 lbs. Trimmed to 0, I got 5800 RPMs and 30 MPH on GPS
Try this please while at the dock set the motor in the vertical position to the water. Not tucked in or tilted out. do not change this setting. go out and run the boat and read your max rpms without touching the trim setting. most pontoon boats require very little trim especially ones with small HP motors on them. My boat will run with in 1 to 2 mph in this vertical position and if I trim it out it only goes 1 mph faster.
 

2fast4u

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I took my GPS out and found the trim setting where my boat ran the fastest. if I tucked it the speed would drop and if I over trimmed it the speed would drop also and rpms would rise . I noted the reading on the trim gauge and put a small piece of white tape where the best setting on the gauge was and when I use my boat I run it at this mark for best speed and fuel consumption also. Do this test with two people in the boat and 1/2 fuel in tank.
 

Chris21

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I have a 2012 Bennington 2275 GFS and at 5000 rpm I cannot get over 21mph. Do I need to change out the prop? A friend told me to put a SS prop on along with something called a dolphin fin. Have you guys ever heard of a dolphin fin? I took my mom out for a ride today and with just the two of us the boat would not go over 23 mph at 5400 rpm with a half tank of gas. What do you guys think? I guess the prop I have is the normal factory white prop that comes with the boat. I have the motor trimmed all the way down but on the gauge it shows it is still trimmed a quarter of the way. Any info is greatly appreciated.
 

2fast4u

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I have a 2012 Bennington 2275 GFS and at 5000 rpm I cannot get over 21mph. Do I need to change out the prop? A friend told me to put a SS prop on along with something called a dolphin fin. Have you guys ever heard of a dolphin fin? I took my mom out for a ride today and with just the two of us the boat would not go over 23 mph at 5400 rpm with a half tank of gas. What do you guys think? I guess the prop I have is the normal factory white prop that comes with the boat. I have the motor trimmed all the way down but on the gauge it shows it is still trimmed a quarter of the way. Any info is greatly appreciated.
Are your pontoons clean? Dirty pontoons with lake scum on them can cost you 5 mph. also you need to post up what size motor and prop dia. and pitch you have so the guys on the forum can help you.
 
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