Boyancy. thinking about puting a 20 hp Motor on a 14ft flatbottom aluminum Jon boat.

[Carpentrator]

I have a 14ft aluminum flatbottom john boat. I want to put a 1970s 20 hp Johnson with electric starter on it. Im going to rebuild the transom (of course). The boat is pretty old (70s) and its not very wide. But it is deeper than most of those type boats that I have seen in the past. Im wondering if I put plywood in the front of the boat (to help weight it down) and if I fill the seat wells with foam(Great Stuff) if this will make the boat more boyant to allow it plane out and work.

If Im about to kill myself someone please let me know.

 

[ezmobee]

Re: Boyancy. thinking about puting a 20 hp Motor on a 14ft flatbottom aluminum Jon bo

Foam will not make the boat more buoyant. Only the boat's water displacement makes it buoyant. I would not exceed the hulls max rated horsepower.

 

[Jack Shellac]

Re: Boyancy. thinking about puting a 20 hp Motor on a 14ft flatbottom aluminum Jon bo

With a flat bottom and aluminum's light weight, it should be very fast. But they do put throttles on the motors, so you can just dial it down a bit. Weight distribution can be tinkered with until you get it right. I would just be very careful until you get used to how it handles. They can get squirrely at high speeds.

 

[fishrdan]

Re: Boyancy. thinking about puting a 20 hp Motor on a 14ft flatbottom aluminum Jon bo

How wide is the bottom of the hull, transom height? I have a 7.5 on my 1436 jon boat and it's fast. 20HP would be scary fast, if the transom could handle it. My friend has a 25HP on an ol' 12' Lund and it scared me to open it up, fast, and not stable in wavy conditions.

To balance the boat you will need to move weight forward, gas tank, battery, gear. I have my gas tank and batteries places mid-ship which works well, even with a passenger.

And yeah, adding anything to the inside of the boat (foam) will not make it more buoyant (unless it's filled with water ). I like the addition of low decks to my boat, makes it feel more stable while standing.

 

[Numlaar]

Re: Boyancy. thinking about puting a 20 hp Motor on a 14ft flatbottom aluminum Jon bo

My 14' Flat bottom Jon boat, is max rated for a 15HP... so it really depends on what you think the max rating for this boat was/is.

I run a 15HP 4-stroke (Honda), so its a bit heavier than a 2-stroke, and it stress cracked the Gunnel at the rear of the boat where the corner plate welds are... had to have it re-welded, but it's been fine since.

just work on distributing your load, and you will probably be ok... and make sure your transom can handle the 20HP. (I put my trolling motor/battery/tackle all up front to help with the weight of me and the motor in the rear).

Hope that helps!

 

[Thad]

Re: Boyancy. thinking about puting a 20 hp Motor on a 14ft flatbottom aluminum Jon bo

I have a 14' xtra wide xtra deep Alumacraft flat bootom. It is rated for a 25. I have a 15 Evinrude on it and is great with two people.

I was going fishing on a rather large lake one time, so I borrowed my dads 25 electric start Johnson. WOW, that thing was fast! I am a speed freak but that was nuts. About 3/4 trottle all day and was alright.

If the hull is rated for a 20, then you should be ok. Just be mindfull of where you place the battery, fuel tank, tackle, and of course...you.

It is more than just the transom that factors the ratings given to any boat.
Even "beefing up" the transom will not allow you to safely add a larger motor than recommended.
Where is the transom and what is it attached to? The gunwales. How thick is the aluminum, how and where are the cross braces and support arms as well as how many? Those gunwales would, after beefing the transom, become considerably weaker due to added stress. That leads to this, hp is hp, engine weight contibutes little at this point, although still a factor it is different. There is torque associated with that hp as well as a constant forward stress being applied to the hull. And reverse adds a negetive stress, or pulling on the hull. These two things alone are enough to cause irreversable damage.
And then, there are the handling characteristics involved. Most noticable of a small boat with too large a motor will be coming off of plane. When you trottle down, you could swamp the boat over the transom from your own wake catching you.

Just my opinion, I never give advise, only opinions.