Building a 12' jon boat

Discussion in 'Boat Design' started by Raptor88, Apr 26, 2021.

  1. fredrosse
    Joined: Jan 2005
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    Location: Philadelphia PA

    fredrosse USACE Steam

    An eleven ft boat built with 3/16 Luan, 10 OZ fiberglass cloth & Epoxy, plenty strong enough, except standing on the bottom would not be a good idea. Made a removable floor with 1 x 3 furring strips, works well.
     

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  2. fredrosse
    Joined: Jan 2005
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    Location: Philadelphia PA

    fredrosse USACE Steam

    "If the bottom butt joint is 4 feet from the transom, can the block on the butt joint be under the rear seat? I need to figure out how far the rear seat should be from the transom to be able to comfortably steer a 6hp outboard."

    I was just suggesting that the forward part of the bottom should not have a joint. Thus the 8 ft length of plywood forms the forward section, and the joint in the bottom has to be there, about 4 ft from the transom.
     
  3. Raptor88
    Joined: Apr 2021
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    Location: Hawaii

    Raptor88 Junior Member

    Thanks for the pics of your repaired transom. Yes, polyester fiberglass works. The outside of the flat bottom boat's hull that we built over 30 years ago was fiberglassed using polyester and it lasted for many years just fine.

    I will go with two pieces of 1/2" marine plywood glued together for the transom and add a 3rd 1/2" piece later for the engine mount if necessary for a 1.5" thick transom. But for a 6 hp outboard, it looks like a 1" thick transom will work fine. Thanks.
     
  4. Raptor88
    Joined: Apr 2021
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    Location: Hawaii

    Raptor88 Junior Member

    I found the dimensions for a Tohatsu 6hp outboard. The tiller handle is 13.7 inches from the transom so I'll place the rear seat so I can steer the tiller. That being the case, and since I prefer not to have the butt joint's joining block on the floor where I will be walking, I'll place the butt joint forward 8 feet from the transom. Assuming the joining block has 4" overlap, the bend of the bottom ply can start 8' 4" from the transom. That will give about a 3.5 foot area for the upward bend of the bottom ply at the front. My current plan anyway subject to change ;).
     
  5. Raptor88
    Joined: Apr 2021
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    Location: Hawaii

    Raptor88 Junior Member

    Regarding positive buoyancy, how's the following calculation?

    A 6hp Tohatsu outboard weighs 55 lbs. Assuming the gas tank holds 3 gallons, that will weigh 25 lbs. Motor + gas tank = 80 lbs.

    I'm estimating that the rear seat in a Jon boat is about 13" high and 13" wide. Since my Jon boat will be 48" wide that will be 8112 cu inches under the rear seat. 8112 cu inches of water weighs 292.5 lbs. I will fill the underside of the rear seat with foam so that will provide more than adequate floatation for the motor and gas tank. Assuming the amount of foam does not fill every cubic inch of space, say it displaces 250 lbs of water.

    The 250 lbs of floatation of the rear seat will be enough to keep the boat from sinking even if the boat is filled with water. I could add some more foam in the bow compartment to help floatation in the bow but since my boat will be made out of plywood that in itself will provide some floatation.

    Is my method of figuring out floatation correct?
     
  6. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    To a certain extent, yes - but the poor boat is not going to be of much use to you with the stern (possibly) out of the water and the rest of the boat with the gunwhales almost awash.
    And where are you and your crew and all the fishing gear, safety equipment, etc?
    Swimming in the water next to the boat, assuming that you have your lifejackets on.
    And the equipment has probably headed for Davy Jones' Locker.
    You are half a mile offshore, and there is a current taking you further out, towards Polynesia - what happens then?
    I think that you will need positive buoyancy under a middle seat as well as in the bow.
    And I still think that you will be better off with a more 'seaworthy' design in the first place for going half a mile offshore, rather than a Jon Boat.

    A rental jet ski went missing here a couple of years ago - a couple visiting for the day on a cruise liner had rented it for half an hour.
    Something happened, and they didn't return to the beach when the time was up.
    The jet ski owner started a search, initially with other jet ski operators, and later involving the Coastguard.
    The jet ski was eventually spotted by a ship about 4 or 5 days later, partially submerged (they have positive foam buoyancy built in), off the coast of Guadeloupe, about 150 miles to the north-west. The two crew were never found. They would have had buoyancy aids on, but that was about it.
    I doubt that during their half hour of usage that they went more than half a mile offshore. But something happened, maybe they broke down and panicked, and tried to swim back to shore, and didn't make it?
    The moral of this story is that you need positive buoyancy, not just 'adequate' buoyancy, and you need to have appropriate safety equipment for the conditions and usage.
    The same thing could have happened to them if they had been in a Jon Boat or similar.
     
  7. Raptor88
    Joined: Apr 2021
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    Location: Hawaii

    Raptor88 Junior Member

    Frankly, I just wanted confirmation of buoyancy calculations. I did not come here for a lecture on safety. I thought this was a "boat design" forum.

    As far as buoyancy is concerned, I just want to keep the boat afloat to not have it sink and lose the boat and the outboard motor. Which is why I mentioned offsetting the weight of the motor and gas tank. With buoyancy in the rear seat and bow, that will be enough to accomplish my goal while not filling the underside of the middle seat with foam to keep it open for tackle boxes, and other gear.

    Any help with boat design is appreciated .... lectures - nada.
     
  8. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    It is. And 'Boat design' covers a lot more than simply drawing pictures of boats and estimating weights - it also includes all the elements to make the boat safe(r).

    And to achieve this, you will need more buoyancy than your proposed 250 lbs of floatation in the aft seat.

    You WILL need floatation in the bow as well - it is not a case of just 'adding some more foam' - you need enough so that combined with the aft buoyancy, the boat is floating reasonably level, with you and your crew (and gear) on board, and with sufficient freeboard such that you can bail out the water faster than the waves try to slosh it back in.

    Re using the middle seat for stowage, if you make it a watertight box, with a watertight access hatch on top, then you will have some reserve buoyancy there as well (which will be useful), assuming that it stays intact in the event that something happens to cause an ingress of water into the boat.
    Although I think that the USCG would probably prefer for this compartment to be foam filled as well - if @Ike is reading this, maybe he could offer an opinion?
     

  9. fredrosse
    Joined: Jan 2005
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    Location: Philadelphia PA

    fredrosse USACE Steam

    To be really safe, you could never go outside and just watch TV for the rest of your life (Not). I have had just 50 motorcycles, (mostly 4 cylinder rice rockets) and statistically that is about 10,000 times more dangerous than any boating endeavor. Still here.
     
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