Max. Speed of 15 MPH

[MJRuddy]

My 2010 20’ Bennington SLi has a 2010 50 hp Mercury Big Foot 4-stroke. I can only get 4,800 rpm and a max of 15 mph out of it. The prop is what is speced for it. An independent marine shop serviced the outboard and tested it on the water. $3,000 later they represent that I should not expect more power or speed out of this combination (to be fair they also replaced injectors, pumps, plugs, wires, etc.). This SLi model was only designed for a max of a 60 hp motor. Does the rpm and speed sound right? The motor is designed for WOT of 6,000 rpm. Thank you for any insight you may have.

 

[Bill N]

My 2010 20’ Bennington SLi has a 2010 50 hp Mercury Big Foot 4-stroke. I can only get 4,800 rpm and a max of 15 mph out of it. The prop is what is speced for it.

What led you to believe that the current prop is "spec'd" for it? I've never seen such a chart? What is current prop? 23" or 25" tubes? Are you playing with trim at all?
You are correct that max RPM with lightest load should be 6000 RPM. Achieving this does not mean that you will net much speed improvement though. I would guess 17-18 mph with no load & clean tubes is all you can get.

 

[Jack M]

I have a Bennington Prop recommendation chart from a few years back , on a 20 ft with a 50 HP Big Foot a 13.8 X 11 Spitfire , on a regular 50 a 12.25 X 9 Black Max.
You can also call Mercury customer service . I have called them a few times ,they were very helpful
1 (920) 929-5040

Have a Question? Contact Us!

www.mercurymarine.com

 

[DVW]

Your engine speed and boat speed indicates you have too great of a pitch on your current prop. Seek council and you will probably end up with 2” less pitch. Test with full throttle while slowly increasing trim until speed no longer increases. Engine speed should be about 6000 rpm in your case. You should be able to trim up a little further before aeration. If aeration does not happen you can raise your engine on the transom for a little more speed. Expect 10 to 15 percent more speed. Your max speed will be with top down, light load, clean hull on a cold day. That condition is 1% of the time. Do not pitch for that 1% of the time. Chose a pitch that better suits your use. Hope this helps.

 

[Maynard G. Krebs]

My boat's specs are similar to yours, 2006 20' (2075 FSi), 23" tubes with the 50 hp Yamaha Thrust. A Quicksilver 14 x 11 prop gets me 21mph on flat water. My normal passenger load is 310 lbs. I've found that adding an additional 450 pounds of passengers cuts my speed nearly in half. The tubes are clean and polished. I ran alongside a friend's 18' pontoon with a 60hp Yamaha and easily outran him. The difference was his boat sits in the water year-round and mine gets trailered. Assuming your motor is running properly, there are only three things that can affect your boat's speed: dirty toons, wrong prop, or simply too much weight onboard.

 

[mng8r]

My 2013 20SF with Yamaha T50 and a Yamaha 14x11 turns approx 6000 RPM and will hit 18 mph under ideal conditions.
Note that I've removed the restrictor plate from the T50 (only difference from the T60)