Overpowered???

[metriccrescentwrench]

I recently got a heck of a deal on a 12 ft jon boat for my son, wich he uses on our private lake with a trolling moter. Along with everything else the seller threw in a 7.5 hp outboard that needs some work. Once I get the outboard running I'm thinking about letting my son use it on the jon boat if he ever wants to us it at one of the bigger lakes in our area.

Here's the problem - data plate on the jon boat says it's rated for 5 hp. I'm hesitant to overpower any boat, but in this case will it be a big deal? On one hand it's "only" 2.5 hp over, on the other hand it's 50% over.

 

[chiefalen]

Re: Overpowered???

I have a 3hp on a jon boat and it flies. If the plate says 5 it's there for a reason.

 

[tx1961whaler]

Re: Overpowered???

The consensus here (there have been many many posts on this) seems to be not to do it, if only for liability issues. It most likely won't hurt the boat any, and probably isn't illegal (check your state rules),but if something bad happens, the insurance company either won't cover you or it will be used as evidence of unsafe operation against you.

 

[chiefalen]

Re: Overpowered???

O yea alot of things can happen, to mush power on a flat bottom boat, he hits a wave will flip him right over.

Can and will pull that transom right off and sink the boat.

Whats he using it for fishing ?

Give him 2 oars. I use them on my jon to get in quiet to the fish.

Give him a 2-3 hp like i posted in my first post. He will go fast enough.

My brother uses a 5 hp to push his sail boat. To mush for a flat bottom boat.

And 7 no way.

 

[crashnburn63]

Re: Overpowered???

Definitely illegal here in WA state, and as you pointed out 50% over.

Mitigating factor might be if it is an older motor the power quoted is at the powerhead, not the prop, and will be less at the prop. But don't ask me how much less or what year it changed.

If it is not illegal where you are, and you still wanted to give it a go, you might try swapping out the prop. It probably has a general purpose prop on it, and if you could find a compatible thrust prop (larger, low pitch blades) you would effectively make it so that the top speed of the boat was limited by the pitch speed of the prop, rather than the horsepower. Outboards sold as backup motors for sailboats usually have thrust props on them.

I have a Merc 9.9 Bigfoot on a 14 foot al. boat. The motor was designed to be a kicker for a much larger boat, and with the stock 4 blade, 7 degree pitch prop it tops out at 14 mph with 1, 2, or 3 people in the boat. Put a 3 blade, 12 degree prop on it and it does 19mph with one person, 17 with two, and 14 again with 3. With 4 in the boat it would probably be faster with the thrust prop, but won't plane either way.

My kids are both pretty responsible (and my daughter has completed the now required WA state boating class) but I still don't let them out in the boat with the speed prop on it.

I'm probably going to buy a used 25hp motor soon to use in the summer for tubing. That will definitely get the thrust prop.

Good Luck,

-dm

 

[chiefalen]

Re: Overpowered???

Thats a flat bottom jon boat maybe 3-4 foot wide?

 

[mphy98]

Re: Overpowered???

no such thing as an overpowered boat. Just an undersized hull!!!

 

[INJUN]

Re: Overpowered???

IMHO

I have to side with being safe as the correct way to go.

But, it couldn't hurt tring it out and see just how safe it is by using your good judgement. Is it waay to fast? Unstable? You decide. Take into account the age/responsibility level of your son, (boys will be boys), and the water it's used on.

If not, be safe and go buy the correct motor.

 

[Silvertip]

Re: Overpowered???

Using a prop with less pitch does not automatically slow the boat down. In fact in some cases it can actually speed it up. While it is true that at the same rpm a prop with less pitch will have the boat running slower. However, less pitch also means the engine can run more freely and can actually over rev by a considerable amount so speed increases. You can carry this theory to extremes but at some point the engine will exceed its ability to keeps its inards inside the block at which point there is a bang followed by silence and small bits and pieces collect under the cowl.

 

[crashnburn63]

Re: Overpowered???

Using a prop with less pitch does not automatically slow the boat down. In fact in some cases it can actually speed it up. While it is true that at the same rpm a prop with less pitch will have the boat running slower. However, less pitch also means the engine can run more freely and can actually over rev by a considerable amount so speed increases. You can carry this theory to extremes but at some point the engine will exceed its ability to keeps its inards inside the block at which point there is a bang followed by silence and small bits and pieces collect under the cowl.

Correct. My reply was based upon the assumption that the motor has a rev limiter, which the Bigfoot does. With the stock prop on the boat, it tries to push it faster than 14mph, but when the RPMs hit 5k, the rev limiter stops the engine from reving any faster. It is very noticable on that motor because it causes the engine to miss.

If no rev limiter, it is up to the operator to hear that the motor is turning to fast and back off... or maybe put a restrictor plate on the carb? Nah, at that point you are better of trading for a smaller motor.

-dm

 

[Frank Acampora]

Re: Overpowered???

Well, Bubba, When I was young, I had a 6 or 8 foot jon and I powered it with a 20 Johnson. It did a flying mile in 60 seconds = 60 MPH. On one run, It got a little loose and before you could say snot, it flipped--360. Threw me into the water and landed upright but full of water--not quite swamped. The tank was floating outside it, and the engine was still puttering in circles. Had to grab it and hit the kill switch then dog paddle about 200 feet to shore (fully clothed--it was November in the north.) to bail it. Then drove it home a bit slower.

At 16 or 18 it was a lot of fun. At that age I was "indestructable". Never told my parents why I was soaked though. Said I tripped off the dock. Let them laugh and call me clumsy, not stupid and crazy.

I think the reason it flipped was because it DID NOT HAVE ENOUGH HORSEPOWER!

 

[coolguy147]

Re: Overpowered???

well i got a 12 foot john rated to 15hp pretty wide i guess in comparison and stable. well i havent tried it with my ten horse but i would expect 25 mphs tops. i mean when i went with my cousin with his fishing boat probably 16-18ft with a 50hp. did about 30mph i mean at first i felt scared. like the first time i went on water that fast

 

[coolguy147]

Re: Overpowered???

well if that 7.5 is in really good u could repair it in the forum which the manufacter is. and get a new john boat or used thats rated to 10-15hp or even more if u pleaase

 

[pvanv]

Re: Overpowered???

Legalities aside, this combo would work, but you have to be conservative on the throttle. I wouldn't advocate keeping it that way, for all of the previous reasons. However, for testing [only], you can just block the throttle to only go about half or so, possibly by adjusting the linkage, and it won't overpower the boat. Then if you like the boat, you can get (or swap for) a more appropriately-sized motor that is correctly rated, and that motor can be run at WOT without overpowering.

 

[Wee Hooker]

Re: Overpowered???

It really depends on the specs and age of boat. I've had 10 different 12' aluminum boats. 6 Jon type. Some of the lighter "cartop" versions were less tolerant of overpowering (read: buckled transom) than their wider/heavier cousins. Having said that, any jon with a 32" or less bottom I've owned will do wll over 20mph and chine walk n the turns with a 6hp. ( From your 5 hp rating, I'm guessing yours falls into this class.) I've found a 4 or 5 hp is PLENTY and 3.5 will usually plane it. In your case, I'd survey the condition of the boat and compare the weight of your OB with that of a typical 2 stroke 5hp. If everything looks reasonable, I would try the 7.5 but turn the throttle adjustment down so it won't produce more RPMs/HP/speed than your comfortable letting the boy get hold of. Not hard to do (Scott-Atwater actually made a 7-12hp that had a key switched mechanical throttle limiter . Very cool) In any case, make sure you have and enforce use of a "dead man" kill switch cord. A 12' john @ high speed is an easy boat to get unhorsed from for a novice boater.

 

[metriccrescentwrench]

Re: Overpowered???

Legalities aside, this combo would work, but you have to be conservative on the throttle.......Then if you like the boat, you can get (or swap for) a more appropriately-sized motor that is correctly rated, and that motor can be run at WOT without overpowering.

OR.... leave the jon boat alone (with the electric trolling motor) and get another slightly larger boat that's appropriate for that size engine. (I knew this was going to end up costing me more money.)

My son's a great kid, he's done the boater safety class, and does a good job when I let him drive when we're fishing in our 'big' boat. However, I can see already that he's too much like me - when he's driving my bass boat the throttle keeps moving forward all by itself.

 

[jasper60103]

Re: Overpowered???

Yea, I would keep the 12' jon for now.

In the mean time, get the 7.5 working then you'll have some options.
1) Sell the 7.5
2) Trade it for a smaller motor.
3) Get a larger boat.