Boat Speed

[fishndirk]

I was recently debating with a friend of mine over an ad we saw of a fiberglass 16ft flatbottom with a mid-80s johnson motor who's owner claimed it would do 55mph. I said he was full of BS, but my friend believed to accept my argument. I said, there is no way that that motor with any prop it can run is capable of running at 55mph. He based his argument on HP and weight. I based my argument on prop and rpm speed. Am I wrong here? I cant foresee any 50hp motor on any boat running at 55mph with any prop that it is designed to turn.

 

[F_R]

Re: Boat Speed

Everybody thinks they are going faster than they really are. A GPS is a great de-liar. I dunno, I guess we'd have to know a lot more about the boat and setup.

 

[fishndirk]

Re: Boat Speed

My point was that even if you ran that motor through the water by itself (if it was possible) you still would'nt achieve 55mph. No propeller that that motor is designed to operate can achieve 55mph because the rpm x prop pitch doesnt add up. The closest figures ive found to that speed are theoretical speed and do not account for prop slip. The ones that do account for that show 35-40mph which was my original estimate.

 

[5150abf]

Re: Boat Speed

I have a 16 with a 50 and you are correct, unless he has a hurricane force tail wind pushing him there is no way a 50 is going to go that fast, the best i have ever gotten from mine is 31 and that was with a decent tail wind, normally I get 26-29 depending on current and wind.

 

[John_S]

Re: Boat Speed

I thinks he ment to say "Feels like 55".

I do think there are stock engine class records, and wouldn't be surprised if the 50HP was over 55mph.

http://www.glen-l.com/how-fast-chart.html indicates that a motor by itself could optain those speeds.

 

[Faztbullet]

Re: Boat Speed

In 1974 Johnson set a speed record with stock 70hp on 14ft Allison of 70mph and was broken at Eufaula, Alabama with a record at 70.560 mph in 1980 on a 14 ft Allison. So it is possible to run 55mph with that motor but not with any stock production prop as previously posted. Owner would need a solid hub racing prop and a very light boat.....

 

[laotou]

Re: Boat Speed

The boat can do 55 mph on trailer in an interstate highway.

 

[Frank Acampora]

Re: Boat Speed



This 10 footer Cub Cadet does 45-47 with a 50 hp Chrysler and a bronze Michigan racing prop running 5,000 rpm at the flywheel. SUPPOSEDLY, it has done 47 with a 25 hp MERC in class racing. Lots of weight for a small boat though--156 for the engine, 48-50 for 6 gallons fuel, 50 for a full sized battery, and 180 for the driver.

 

[hungupthespikes]

Re: Boat Speed

If you work the numbers then yes it can be done on paper, but paper is one thing, real numbers another.

http://www.rbbi.com/folders/prop/propcalc.htm

So the debate with your buddy is never going to end. LOL sorry :facepalm:
huts

 

[jkust]

Re: Boat Speed

Easy to figure out theoretical speed and an 85hp isn't going to do it. I have a two stroke 85hp on a 1000 lb boat and there is just no way it is going to go 55. Suprisingly fast but not even close to 55.

 

[erwinner]

Re: Boat Speed

So the debate with your buddy is never going to end. LOL sorry :facepalm:
huts

The obvious answer to an end for the debate would be a friendly wager.
Each put up 1/2 on the boat, loser pays winner his 1/2, winner gets to keep the boat.

 

[steelespike]

Re: Boat Speed

Your right my calculator doesn't even consider hull design and::
5500, 2.42 gear ratio ,19" 7% slip= 38 mph 21" = 42.
And this assumes it can turn those props to 5500 I don't think its possible on a 16' fiberglass boat.
A 50 hp is more than capable of going 55mph but not without some serious modification.
circa 1953 a neighbor had a 15 or 16 ft Raveau he ran in outboard marathons like the one in Albany NY.
It had a beautiful late 40s 60 cu.in 50 hp Evinrude loaded with chrome and polished aluminum
I think it had a special gearcase and of course a special prop.I know it ran over 50 way back then.
And in the 60s I saw it run even in a casual competition with a 100 hp Merc on a slightly smaller Raveau.
The 100 was basically stock.

 

[V153]

Re: Boat Speed

this assumes it can turn those props to 5500 I don't think its possible on a 16' fiberglass boat.

Ya, highly unlikely. 'Less the boat's made out of carbon fiber ...?

 

[dave11]

Re: Boat Speed

I have a 16ft Alumaweld with a 60HP Yamaha. Under ideal conditions I MIGHT get 37 or 38 MPH. Besides with a flat bottom boat, you will get the crap beat out of you in any kind of chop.

 

[smallboatlover]

Re: Boat Speed

you did say that it was a flat bottom boat. those boats glide on the water rather then put through it. so you never know he might be going that fast.

 

[dave11]

Re: Boat Speed

you did say that it was a flat bottom boat. those boats glide on the water rather then put through it. so you never know he might be going that fast.

Mine isn't a flat bottom, but the deadrise is onlt 10 degrees.

 

[Handyman1]

Re: Boat Speed

I had a 1994 30hp Evinrude on a 14ft open fiberglass boat 32 miles max speed with a 15 pitch prop and that was measured with a gps if you convert it to metric that is close to 55 kph I sold the motor too fast for the boat now I'm running a 15hp on it 15-17 miles. Perfect.

 

[V153]

Re: Boat Speed

55mph from a 50hp on a 16' flat bottom boat? Sorry just cain't see it happenin'. Gonna need a li more hp?

Tell ya this: Trying to keep a 16' flat bottom under control at 55mph is not for the faint of heart. Whatever you do, do NOT turn!

 

[2ndtry]

Re: Boat Speed

An "80's" Johnson 50 has a gear ratio of 2:1, so in order to get into the mid 50's he would need to swing a 23" prop over 5K which just isn't going to happen. He likely has a 13" (recommended for that motor), which puts him at about 30 MPH with reasonable slip.

 

[southkogs]

Re: Boat Speed

Tell ya this: Trying to keep a 16' flat bottom under control at 55mph is not for the faint of heart. Whatever you do, do NOT turn!

No kiddin'. My dad and I have played with overpowering a 14' fishing boat (rowboat type) a couple of times just 'cause we had the outboards available to monkey with. You can gain some speed with the extra power, but handling becomes pretty squirrely. Go to a flat bottom with that kind of thrust, you're asking for trouble.