70 HP / 20 Sl Power Question

Poppy21

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I just bought my 2012 Bennington 20sl with a 70 HP Yamaha. I took it out the other day for the first time with 7 passengers and it only ran 13 knots.Today I took it out by myself and it ran 16 knots with adjusted trim. The only problem I could see it that it only got to 4400 rpm at full throttle. I'm thinking I should at least see 5500 Rpm safely. Could it be possible that I need to get the throttle adjusted? Is the normal for a 70 hp? Any suggestions?
 

goldnrod24

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Probably propped wrong. In order to know for sure, get a GPS verified speed and determine the prop diameter and pitch. Post it here and maybe we can help. You should also look up the max rpms for your motor. I'll bet it's higher than 5500.

I am moving this post to the "Props and Power" sub-section.
 

Steve21slx

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Check your intake. On my 2016 90 HP yamaha the part that surrounds the intake warped and caused the motor to starve for air. Couldn't get it to run above 4000 RPM. Not sure if that is your issue but it is one you can easily check off.
 

Poppy21

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Probably propped wrong. In order to know for sure, get a GPS verified speed and determine the prop diameter and pitch. Post it here and maybe we can help. You should also look up the max rpms for your motor. I'll bet it's higher than 5500.

I am moving this post to the "Props and Power" sub-section.
Thanks. Never thought about the prop size.
 

SEMPERFI8387

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2young2retire

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I have the same boat with a t50 motor and had the exact same problem. Approx 4400 RPM with just the wife and I at 18 MPH. When pulling the grandkids in a tube the RPM dropped to 4000. Went to The Hull Truth forum and asked for recommendations. I was running a 14
x11 prop and what was recommended to me was a 14X9. Made the change and now very happy. WOT is now around 5500 RPM and the speed increased to 19/20 MPH. Only about 200 RPM drop when pulling the kids. Very happy now.

 
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JohnL

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I’m interested to see what you find out. I have the 2014 20 SFX with the F70 Yamaha. My top speed has been 17 mph by GPS at 6100 rpm. I have the K-11 aluminum prop. I asked my dealer and his mechanic what I could do to get more speed and was told that I was getting as much speed as I was going to get. I didn’t quite follow their explanation of the K prop, but they both told me that prop was the best match for the F70. Does anyone know anything different?
 

Armycopter

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Following.
I have a 2013 20SL with a Suzuki 70HP motor. I just bought it yesterday and took it on its’ maiden voyage with my wife and son. I checked WOT and got 20MPH on my GPS at about 4800 RPM.
 

Armycopter

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Took it out today by myself and clocked 23 MPH WOT at 4900 RPM. Supposed to max out 5000-6000 so probably have it propped a little wrong.
 

Dgj

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I vote for the dual axle trailer. I just don’t like single axles for anything Never seem to pull as well.
You have plenty of truck to pull it down the road with out the trailer under it but that doesn’t mean you wont have the need to use something else in the future that will need the breaks.
My brother just bought a used tritoon with out a trailer and had one heck of a time finding a trailer to buy. All manufacturers are 4 to 14 weeks out and the dealerships that have trailers on the ground have new boats coming in for the trailers and wouldn’t let go of them. He got lucky one of the local dealers sold him there used shop trailer reluctantly
 

Potomacbassin’

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I’m interested to see what you find out. I have the 2014 20 SFX with the F70 Yamaha. My top speed has been 17 mph by GPS at 6100 rpm. I have the K-11 aluminum prop. I asked my dealer and his mechanic what I could do to get more speed and was told that I was getting as much speed as I was going to get. I didn’t quite follow their explanation of the K prop, but they both told me that prop was the best match for the F70. Does anyone know anything different?
Letters correspond to your gear case diameter. So they may be saying the K is the correct prop based on barrel diameter, but that doesn’t explain the blade diameter or pitch. Sounds like they’re not trying real hard to help...

 
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