jon boat question

this may be a lot below most of your abilities but i would appreciate the help. so thanks in advance. i have a crestliner 12 foot jon boat, 38 inches wides, .050 alum thickness. i want to use it for duck hunting this year. having used it for fishing i noticed that the short height is easy to swamp in the rear with the 6hp evinrude on it. what i have seen around the net is a product called flotation pods that bolt or weld to the transom on the outside. they are just a pair of boxes. i would like to build them but i don't know how to figure out what thickness and what size to build them. i would like to know how many cubic inches and what surface area they should each be for about 200lbs of added flotation. if i haven't asked the question properly please let me know and i will try and further explain.
this is what i am after.

http://www.fisherbeavertail.com/floatationpodsintro.html
http://www.jphilarnold.com/pods/firstpage.htm

thanks for any help.

 

200/60.4 = 3.3 cu ft 1.65 feet on either side. I am assuming you will use 2 pound density foam. That means one cubic foot of foam weighs 2 pounds. One cubic foot of water weighs 62.4 pounds. 62.4 minus 2 pounds = 60.4

split it in two and put 1.65 cubic feet on either side. However, I would not put it on the outside. I would fasten it to the inside on either corner near the transom. jon boats were intended for use in calm waters. Unfortunately people use them for a lot of things they were never intended for. I have even seen them out on the ocean. That's just plain crazy.

But, your jon boat should already have built in flotation. It's the law that all mono-hull outboard boats under 20 feet are required by Federal law to have flotation.

 

i lost you on the denisity part. i wasnot planing to use any foam. just a sealed box. i want it on the outside for the added flotation and stability. myabe the links posted above will clear up what im after. so do i need each box to be about 1.65cubic feet? thanks.

 

If these are air chambers then they will support 62.4 lbs per cubic foot.

200/62.4 = 3.2
3.2/2 = 1.6

So each box needs to be about 1.6 cubic feet.

 

Seems it would make more sense to raise the stern maybe 4 inches and taper the sides into it (long wedge shapes on the sides, like low fins maybe 6 ft long).
This could be done with aluminum most easily, by cutting an inch below the gunwale on each side, raising them up, and lapping/gooping/riveting the wedge shapes in.
The transom has to be raised too, similarly, but how to do it is dependant on your boats exact construction.
The advantage is that you are not adding big boxes to your stern, which must allow the motor to swing. Also no matter what is added to the stern, it must be completely submerged to attain full bouyancy. That means sticking out a lot to avoid motor and have volume down low.
Another advantage to the wedge idea is that it costs little. Maybe under fifty bucks if you do the work. Also the additional height keeps the aluminum and the joint out of the water, so the original boat isn't being penetrated, especially into the flotation cavity it probably has at the stern.

Alan

 

I checked out the website for the extension things. Seems they add length in order to increase hull speed and cost as much as any used jonboat two feet longer to begin with.
Remind yourself that the product is tailored to be the only ten-minute add-on that a company could sell for people who don't have as much mechanical ability as they do money to burn.
Copying their response to such a need, especially for different reasons (bouyancy vs speed) doesn't make sense.
If you have a picture of your boat either at the manufacturer's website or one you took showing construction of the stern, that would help.
Boxes without foam, by the way, puzzles me. All the foam can be bought at Home Depot in a 2"x2'x8' single piece for $11.00 and stacked tight inside anything. One of those boxes could fill through a small hole while underway with stern down, and keep the boat down when you stop. Then the boat is MORE likely to swamp than ever before.

A.

 

Alan, thank you.
http://www.crestliner.com/boats/boat_model.asp?TID=304&BID=68
http://www.internetboats.com/boatfiles/85071180.html

i am after the added stability that i understand this will allow. also by adding flotation to the out side of the hull the little boat should be much safer. I'm adding more flotation than the boat ways. double. i just never though about the foam. yes i will add it. i don't have alot of knowledge about hulls and flotation. thats why i joined here. i appreciate your help.

 

lloyd, take it from another duck hunter, unless you are hunting VERY small ponds and creeks that boat is to small. adding the pods to the back will decrease the draft but you would be better off finding a larger used Jon. You are in Canada so you are going to be dealing with cold water and colder air - you do not have room for a boat issue out on the water.

 

I will start a controversy perhaps. Bouyancy tanks, foam or whatever are effective only when they are submerged or partly submerged. Foam inside the boat does not do anything to solve the swamping problem. What it will do is keep the boat from sinking and maybe provide the occupant the opportunity to bail. After partially swamping the foam is now exposed to the water, but the water is inside the boat and if there is no communication between the water in the boat and the water outside the boat then Archimedes has left the building. So what should you do???? Put the flotation pods on the outside and they'll help avoid swamping. Put pods or foam on the inside and they wont help until after the swamping episode.

A short cut for the math is as follows. Multiply the length width and breadth of the pod, foam, balsa, or whatever, in inches to get the number of cubic inches contained. Now multiply that by 0.03611. That will give you the flotation in pounds of fresh water.

Never mind all that. Your boat is too small. Get a bigger one like the other guys say.