speed ideas?

[niss_det]

well im putting a 40hp outboard on a 12' aluminum boat that weighs prob around 150 lbs alone.. so add 140 for the motor plus bat and gas and me say 300lbs so about 600ish lbs total.. the prop os a 10.5 diameter 13 pitch any ideas of what kinda speed this thing will have? i would go try but everything is frozen right now lol. i just cant see getting much top speed with such a small pitch prop and i think 13.5 is the biggest avaliable for the motor

 

[cobra 3.0]

Re: speed ideas?

Somewhere around 30+ mph is my guess.

 

[Scaaty]

Re: speed ideas?

It will probably spin ya in circles from the torque, and then rip the transom off...don't worry about it

 

[niss_det]

Re: speed ideas?

It will probably spin ya in circles from the torque, and then rip the transom off...don't worry about it

yea i knew that the boat was only rated for 6.5hp so i went ahead and made everything stronger.. the transom use to flex alot but not any more.. i just hope the rest of the boat holds up lol.. i might end up just keeping a lifevest on the motor incase something happens i can deal with loosing the boat.. id like to keep the motor

 

[itstippy]

Re: speed ideas?

Scaaty is telling you that such a rig will be horrendously overpowered and dangerous. I put a 18HP on my shallow V aluminum 12' and it was scary. A 40HP? Those boats are rated for about 10HP max. A 12' 150# aluminum is not designed to be a speedboat.

EDIT: Your last post beat mine - I was still typing.

 

[tashasdaddy]

Re: speed ideas?

very fast to the bottom of the lake. it's going to stand up on the transom and sink backwards.

 

[niss_det]

Re: speed ideas?

very fast to the bottom of the lake. it's going to stand up on the transom and sink backwards.

wow isnt there any gear heads here?? all i get is negative results lol i was planning on putting the battery and maybe the fuel tank up front to try and hold it down.. and beleve me if i had the cash for something bigger and nicer id get it but this is all i have right now...

 

[Silvertip]

Re: speed ideas?

You asked for advice and got it. Further, you did not tell us what make, year, and model of engine you are using. There are a bunch of prop calculators that you can use to get a rough idea what speed will be be. But you need to know maximum wide open throttle rpm for your engine, the lower unit gear ratio, use approximately 10 - 12% slip, and plug in any prop pitch and it will calculate speed. You said 13.5 inch is the largest prop available. That is very likely the diameter of the prop -- not its pitch. 40 HP motors (Johnson/Evinrudes of later years) run 15 and 17P. On that light a boat you could probably run a 19 or even a 21P. IF you can control this thing, it will be well in excess of 30 MPH and you best not even think about running that fast except on the glass-smooth lake and preferably not when anyone else is around. If anyone else runs this boat with or without your permission, I sincerely hope you either own nothing of value or that you and your insurance agent have had a serious discussion about this project. An accident involving personal injury will result in some else owning your stuff. It's called personal liability.

 

[Silvertip]

Re: speed ideas?

I just ran some numbers and assuming you have a Johnson/Evinrude 40 which can be propped to run at 6000 RPM, a 2.43 gear ratio, 12% slip, and a 19P prop will put you at about 39+ MPH. If this boat has rounded chines, you are asking for trouble.

 

[JB]

Re: speed ideas?

No pro is going to recommend or condone such a dangerous practice, Niss Det. It is a matter of professional ethics not to cooperate in dangerous adventures.

Suppose you killed yourself, which is a distinct possibility, or destroyed your boat, which is a distinct probability. What is the accountability of anyone who helped you do it?

There ia a big difference between a gearhead and an airhead.

 

[roscoe]

Re: speed ideas?

Take some photos of it when it is all set up and ready to go, post them here, along with your float plan.

We will forward them to the recovery team.

 

[deejaycee_2000]

Re: speed ideas?

What is the accountability of anyone who helped you do it?

There ia a big difference between a gearhead and an airhead.

I had nothing to do with this ... lol ...... even for me that is a bit hectic, maybe on an inflatable but on a aluminium I would not suggest that, you can maybe still do it with a 25hp but not a 40hp ...... my 8,5 foot inflatable with the 30hp modified on is hectic enough and dangerous and would not suggest it to anyone who hasn't been crazy from the day they were born ........ but here is my advice to you ...... you can get a custom 16 pitch stainless steel cleaver prop will push you at about 40mph (on an inflatable) I doubt you will be able to hold the boat stable without a hydraulic assisted arm fitting ....... I say go for it .... just make sure you get someone to take photos and a vid they can post here after your funeral I would love to see it .... lol I believe ALANR tried it (from this forum) and came back saying there is no way ...... just a thought .....

 

[niss_det]

Re: speed ideas?

ok the motor is a 1975 evinrude 40hp 2 cylinder. i have no idea of the gear ratio but i thought i saw somewhere saying it was only 4500rpm and the prop is micro its the cone design not the through hub exhaust.. and beleve me im not gonna just toss it in the water and hold it at full throttle till something happens.. if i think it feels stable i will keep pushing it till it gets shaky then ill back off..

 

[niss_det]

Re: speed ideas?



ok this is what i found online
model # 40555
evinrude 1975 40hp
4500 rpm
10 1/2 x 13 opt.: 10 3/8 x 11 1/2 , 10 3/8 x 14 , 10 1/2 x 1
gear ratio 12:21

http://www.old-omc.de/index2.html this is where i got the information

 

[cobra 3.0]

Re: speed ideas?

Wowa! The boat is only rated for a 10? I missed that part!!! A battery and tank and...

a couple of 60lb sand bags up front to keep it somewhat stable!!!

Still these boats have a poor design for speed and stability. I'm thinking back to a buddy's 12 footer. Yikes! It wasn't that stable with a 10!

I have a fairly stable 12 foot fiberglass and had to reinforce the whole transom just to run my 15 on it! The transom was somewhat rotted, but never noticed with the 6 hp originally on it. It almost literally ripped right off when I first put on the 15! I can't even imagine a 40. 40's are usually run on newer wide 14 footers and many 16 foot aluminum utility type row boats that can take the torque and weight on the transom.


Hey, are you doing this to be on those JACKASS movies?

http://www.jackassmovie.com/

 

[deejaycee_2000]

Re: speed ideas?

ok the motor is a 1975 evinrude 40hp 2 cylinder. i have no idea of the gear ratio but i thought i saw somewhere saying it was only 4500rpm and the prop is micro its the cone design not the through hub exhaust.. and beleve me im not gonna just toss it in the water and hold it at full throttle till something happens.. if i think it feels stable i will keep pushing it till it gets shaky then ill back off..

Ok now this is way different, with such an old heavy motor, forget what I said above ...... lol it sounds to me like you rather need to buy yourself a new rig .....

 

[WillyBWright]

Re: speed ideas?

When I was a teen, my father rented boats out. We had 2 12' and 2 14' aluminum cartop rowboats. We also had a 40 Johnson like yours. The boat that it went on was appropriate for the motor. The rentals were not. I was stripping and painting that boat and got antsy to go for a boat ride, So I hose-clamped a broomstick to the motor to steer with and hung it on a 14. I survived and so did the boat and motor, but a few short runs convinced me to putt home. I could see the boat twist as I swiveled the motor back and forth.

Boating is unique. It's the only sport where they regulate maximum horsepower. That's because boats are made to take just so much both structurally and hydrodynamically. A beefy transom doesn't insure the hull it's attached to. Exceeding the max is illegal in many states and easy money for ambulance-chasers in all of them.

Now, if we were talking about a racing hull...

 

[niss_det]

Re: speed ideas?

well im gonna give it a try as soon as the river thaws out.. if its to out of controll maybe ill build a pontoon boat.. and if its not fast enough for me ill put the merc 65 on it lol

 

[Chinewalker]

Re: speed ideas?

Believe it or not, it is quite possible you will simply "run out of boat". I've run 20hp on a 12-foot Dura-Nautic tinnie. It was more than a handful and downright scary. The motor wanted to swing right out of my hand from the torque steer. Even with the gas tank in the bow it was squirrely. It is possible to have too much power...

"High Performance Boating" is much more than clamping more horsepower on the transom and adding some braces. A boat relies on air and water to provide stability at speed. Get too much boat out of the water, particularly a boat that is designed to remain partially in the water, and you have problems. True high performance boats take that into account and add in aerodynamics to counteract the instabilities incurred when you lose your hydrodynamics. Your average tinnie has virtually no designed in aerodynamics, and precious little hydrodynamics.

We're not telling you not to do it out of hand. We are telling you that it is not a good idea - from our own experiences. Been there, done that, got the scars to show for it...

- Scott

 

[BF]

Re: speed ideas?

maybe this is all troll-bait, but hmmm, 40 hp on a boat rated for a 6.5 hp.... I have a 12' shallow tinny cartopper like that. It usually has a 6 hp on it... I put a 15 hp on it (to test run the new to me motor), and I'm sure it did ~30 mph (felt like 90). It was on calm like glass water, and once I got close to warp 3, I noticed the whole bottom of the boat dishing up.. .only about the last foot was touching. That's when I figured I'd been lucky until that point and idled it back in.

There's a big difference between 12' boats rated for 10 or 15 hp, than those rated for 6.5 hp.... a 40 hp on the more sturdy boats would be madness, a 40 hp on your boat... well that's a whole 'nother level of madness.

If you are serious about running it (which I doubt), make sure you rig a kill swich thingy that attaches to you, so you don't let this thing fly into someone else once it dumps you out. (oh and take video)