my 8hp johnson only goes 7mph......

Alright, My friend got hold of a 8hp johnson outboard for his MFG. I have a 12 foot aluminum deep V (row boat) that i don't have a motor for, but I want to get it one. I would like to get something that would get it about 12-15 MPH on the water. I figured, lets put that motor on my boat to see what it'll do. Well we went out today. We had 2 gallons of gas, a trolling motor (since it was the first time he had this motor in the water) a batter, he weighs 250lbs, I weight 190 lbs.

I was so surprised it only got that boat and us going 7mph on the water. it was lake and not much current on it, since I was using my GPS on my phone to clock us, which is real acurate, and I would agree with what it was reading. we weren't going to fast at all. I was real surprised. 7mph?? Does that sound about right?? Is there some things else I'm not taking into consideration?? I have heard of guys getting there simliar boat 20 mph on a 15 hp motor, so i thought an 8 hp motor woulf get it going atleast 10 mph minimum, this was a 7 mph MAX. Whats the deal??

Insight would be appreciated...

Thanks so much....

Re: my 8hp johnson only goes 7mph......

Well, with you guys and all your gear, that's about 500 pounds. I wouldn't be too dissapppointed. What's the total weight that boat is rated for?

Re: my 8hp johnson only goes 7mph......

Are you sure that the motor is running correctly, on both cylinders?

Re: my 8hp johnson only goes 7mph......

Two big guys, gas & gear - I think you're about where you're going to get. Lighten up the load and I bet it gets over the hump and takes off....

Re: my 8hp johnson only goes 7mph......

Another thing you can do is a cyl drop test on the mtr to see if it's running on both cyl's.

Re: my 8hp johnson only goes 7mph......

a complete tune up may help, but your hp to weight ratio is not so good! 500lbs+ , for an 8 is making the motor work hard. ( 8hp/500lbs=0.016hp per pound...hope my math isnt flawed, but thats not a good number, and its only you , yer buddy and the battery for the trolling motor.....add in the weight of the johnson, and the hull, the numbers go south a bit more)

Re: my 8hp johnson only goes 7mph......

With that much load in a 12 footer, I am not sure you want a lot of speed...stability would then be a concern.
You are probably getting reasonable performance from the setup, but may be undersized on boat AND motor.

Re: my 8hp johnson only goes 7mph......

Yep, I'm guessing with the two boys, all the gear and the boat itself, there's at least 600 pounds there. That little 8hp motor is gettin it's work out.

Re: my 8hp johnson only goes 7mph......

For reference, I have a Gheenoe with a 9.9 'Rude. I weigh 140 pounds, and carry a trolling motor and battery and some basic fishing gear when I run solo. I can hit maybe 22 MPH if I had to guess, pretty fast for that boat. But I weigh very little and have a faster and narrower hull than you do. When I add a passenger (about 150 lbs) it brings me down to say 16-18 MPH, and if I run with another passenger it brings me down to what seems like a crawl. You've got a lot of weight and not a lot of power, and not a super efficient/fast hull design either.

Re: my 8hp johnson only goes 7mph......

I have an 8HP longshaft pushing a well loaded 16' Starcraft aluminum boat with two people, while dragging a 55hp with it. It is a kicker motor but since our local lakes are <10hp rated this is what we use 98% of the time. My speedo does not work but my best guess is 5-6MPH.

Re: my 8hp johnson only goes 7mph......

aaahhhh hhhhaaaa, sounds like the consensus is.......That's what I'm gonna get. I'm getting it now. the motor sounds so smooth and seems like it's running great. way to go to be firing on one cylinder. Sounds like this is what I should be getting with this boat. appreciate the responses all. Sounds like I gotta get over it. I think the boat may be rated for about 700lbs. The placard is missing.

Thanks everyone..

Re: my 8hp johnson only goes 7mph......

Empty the boat and try it with the 190 pounder. You may have to lean forward to get it to plane.Trim in (prop closer to the boat to help it plane. I'd guess about 15-18 mph empty.