Building a 12' jon boat

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

  1. Raptor88
    Joined: Apr 2021
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    Location: Hawaii

    Raptor88 Junior Member

    Planning to build a Jon boat and need advice. Long post with lots of questions so please just answer what you can.

    BACKGROUND INFO:

    Planned Usage:
    Will only use it for close to shore fishing. Not more than a half mile off shore max on the ocean in Hawaii. Relatively calm waters with no breaking waves. Just 2 people, my wife and me. Will car top the boat on our 2019 Honda Odyssey van.

    Length, width and height:
    ... About 11' 6" long (12' plywood with bottom bent upward at nose)
    ... 47" wide
    ... 16" high sides. Sides will be vertical and not sloping outward to make it easy to store the boat on a side, next to a hollow tile wall with concrete sidewalk.

    Plywood:
    Will use marine plywood. Only comes in 4x8 feet sheets locally so will have to join sheets for the length.


    QUESTIONS: (Assume the boat is 47" wide at the bottom and 12 feet long)

    1. Weight:
    Since the boat will not have a trailer and will be car topped, it needs to be as light as possible.
    Would using 3/8" ply for the bottom and 1/4" ply for the sides be strong enough? Or should I go with 3/8" sides?

    2. Width tapered at front:
    Why do commercial Jon boats have the width tapered in at the front? Just for the looks? For weight savings?
    For instance, if the width of my boat is a constant 47" wide from the transom to 8 feet forward, and then the sides taper in to 3 feet at the front. What's the difference performance wise if I were to just make the width a constant 47" wide for the entire length of the boat?

    3. Bending the bottom plywood at the front:
    What should the minimum amount of bend upward of the bottom be at the front to not have the flat nose piece plowing through the water?
    For instance, the bottom will be flat from the transom to 8 feet forward and then slope upward the remaining 4 feet to the nose piece. Would 4" upward slope in the front 4 feet to the nose be enough? I'm willing to have more upward slope but I'm not sure how bendable marine plywood is at 3/8" thick x 47" wide.

    4. Joining sheets of plywood to get 12 feet length:
    If I use a butt joint of the 3/8" ply, using a 1/2" ply that is 6" wide, will that be as strong as the original 3/8" ply? Just epoxy glue enough or glue and screw?

    5. Joining bottom to sides:
    I want to use the glue and screw method of joining the bottom to the sides. I could use a 1-1/4" square x 8 feet long strip of wood to glue and screw the bottom to the sides in the aft 8 feet before the bottom plywood starts bending upward. But I can't figure out how to manage the front 4 feet that slopes upward. Stitch and glue the front 4 feet?

    6. Transom:
    Would gluing together two pieces of 1/2" marine plywood be recommended for the transom, assuming a 6 hp outboard or smaller?

    7. Sides:
    Will 16" high sides be good for off shore ocean fishing, not more than .5 miles out.

    8. Fiberglass:
    I will fiberglass only the outside of the hull, bottom and sides.
    ... What is the minimum weight of fiberglass cloth that I should use?
    ... Apply with laminating resin and one coat of finishing resin?

    9. Motor:
    What is the minimum hp outboard that you would recommend for the boat above? I don't need planing speed but adequate speed to get to destination.

    10. Oars:
    Will two oars be good enough to get back to shore if the outboard fails even in strong wind? Assuming a maximum of .5 miles off shore.

    Done... thanks!
     
    Last edited: Apr 26, 2021
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Welcome to the Forum Raptor.

    Re most of your questions above, I think that you will probably find reasonably good answers in the link below -

    Spira International Inc - Crawdad Jon Boat https://spirainternational.com/hp_craw.php

    Click on the study plan link as well - here they mention 3/8" ply bottom AND sides, and give some scantlings for framing.

    And then decide if you would like to part with $60 for a set of plans - even if you only use the plans as guidance for designing your own boat, I think it would be a bargain.
     
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  3. Raptor88
    Joined: Apr 2021
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    Location: Hawaii

    Raptor88 Junior Member

    Thanks for your response and link.
    If other folks here can answer some of my questions, it would be very helpful for me and maybe other potential builders.
     
  4. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    Welcome to the forum.

    I'll try to answer some of your questions.

    Banyonsailer gave amazing great advice. I recommend you follow it and spend the $60.
    Stick with the 3/8 it will simplify things with minimal weight difference.

    Curved panels are usually stiffer and stronger than flat ones of the same construction.

    3/8 plywood will easily bend to any radius your likely to desire. Weight fore and aft distribution will affect

    Scarf joins are the preferred method. 8-1 ratio, so three inch wide joint on 3/8 plywood.

    Steam bent framing, or laminate thinner (more flexible) stock together. I.E. five layers of 1/4 inch to achieve the total of 1-1/4

    Four layers of 3/8

    6oz woven not mat
    Polyester is not recumended for wood substrates. USE EPOXY

    Probably
     
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  5. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Here are a few random thoughts.

    You mention that you will use the boat on the ocean in Hawaii - it might be calm on the lee side, but what are the typical wind strengths like where you will be using the boat?
    If they can get up to 15 - 20 knots, you will probably be hard pressed to row a boat like this half a mile to windward with a passenger as well (re your question #10 about oars).
    You could consider carrying a small auxiliary outboard motor, say 2 hp, in addition to the main engine, but this then starts getting complicated.
    Jeff Spira recommends 10 hp, with a maximum of 20 hp for his Jon boat. I would think that 6 - 8 hp would be fine generally for a boat like this, unless you have a passion for speed.
    Be aware also that a 20 hp 4 stroke engine will be much heavier to carry than a 6 or 8 hp version.

    The Spira link does specifically mention that Jon boats are for use on 'protected waters' - I think this typically means bayous and creeks et al.
    Even if your ocean is generally calm, there will be times when it can kick up fairly quickly. It would be a good idea to have positive buoyancy (eg under the seating, with perhaps a bow compartment built in as well) in case you do get swamped.
    And make sure that you carry some safety equipment - apart from lifejackets and oars, you could include a suitable anchor and cable, bottles of water, a first aid kit, maybe a hand held VHF radio (I am sure that you will always carry your cell phones anyway) and perhaps a Personal Locator Beacon.
    Here is a typical example of a PLB - the cost is very reasonable.
    Ocean Signal rescueME PLB1 https://www.defender.com/product.jsp?id=4576649
    Even if you only venture half a mile offshore, if something happens and you get blown further offshore, the Pacific is a big ocean.

    The Spira page mentions a typical weight of a Jon boat as being around 180 lbs, and I am thinking it might be difficult for you and your wife to lift one of these on to the roof of your van?
     
  6. Raptor88
    Joined: Apr 2021
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    Location: Hawaii

    Raptor88 Junior Member

    After thinking about it, I agree. Since I know that a 5/8" sheet of 4'x8' plywood weighs about 50 lbs, that would be 10 lbs per 1/8 thickness. So a 3/8" sheet of 4'x8' would weigh about 30 lbs and a 1/4" sheet would weigh about 20 lbs. A difference of 10 lbs added weight for more sturdy side panels. I'll go with 3/8" ply for the sides.

    Makes sense. I'll try to figure out how to taper the front 4 feet of the sides inward from 47" to 36".

    OK. Will take examples from other jon boats and bend the bottom 3/8" ply at the nose similarly.

    I don't think my skills will make a pro level scarf joint 4 feet wide with all of the wood touching together. So I want to use a butt joint with a wood patch over it. After more googling, I found that the block over a butt joint needs to be a minimum of 16 times the thickness of the plywood. So 3/8" ply would need a minimum width patch of 6". I'll use 1/2" marine ply as the patch piece since I have to buy 1/2" ply for the seats anyway. I'll glue and then screw through the 3/8" ply into the 1/2" patch with the tips of the screws protruding out. Then cut the tips of the screws off with an angle grinder. Will save having to clamp the pieces together. .... If anyone sees a flaw in my plan, please let me know. ..... PS: The patches will be in the straight parts of the boat so they won't be in a bend portion.

    I was afraid that would be the answer. The bend would be a compound curve since it needs to be bent upward for the bottom ply and inward for the side ply. Sounds complex. Will have to research how to do it.

    I won't have spare pieces of 3/8" ply but I have to buy 1/2" ply for the seats so will have spare 1/2" marine plywood. Would gluing 2 pieces together for a 1" thick transom be sufficient for a 6 hp outboard?

    No answer yet

    I'll try to find epoxy for glassing the hull. But if I can't, wouldn't polyester fiberglass work for water proofing the hull since the boat will only be in the water for about 8 hours once a week maximum?

    No answer yet.

    Thanks a lot Blueknarr for taking the time to help.
     
  7. Raptor88
    Joined: Apr 2021
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    Raptor88 Junior Member

    About 30 years ago my brothers and I built a flat bottom boat that worked fine for the waters we will be fishing on. The problem is that being 30 years ago I don't remember the specs of that boat like how high the sides were. We only went out during relatively calm days.

    Precisely! I'm investigating using foam for buoyancy to overcome the weight of the motor. Without the motor, the boat would float even if it is filled with water since it will be made out of wood (unlike an aluminum jon boat).

    My research so far indicates that the length of rope for an anchor needs to be 8 times the depth of water with chain on the anchor equal to the length of the boat. I'm fine with doing that for safety. We'll be in relatively shallow water. I need to learn how to store the anchor and rope so that the rope does not get tangled when deployed. We'll have two cell phones and will only be a max of 1/2 mile from shore so that should be fine.

    That won't be a problem. My estimate is that my 12 foot boat will not weigh more than 150 lbs. I'll have two 2x4's at about a 42 degree angle to two cross braces on the roof rack. The 2x4's will have pegs to be able to "walk" the boat up until the high side of the boat reaches the roof rack. Then the 2x4's will be lifted up and be supported by two 1x4's so that the 2x4's are level. Then the boat can be walked from the 2x4's to the roof rack. Here's a link to a youtube video on how that will work. In the video when the guy places the boat on the angled 2x4's, the high side of the boat is already even with the roof rack. In our case we will have to "walk" the boat up the 2x4's using pegs since our van's roof rack is 6 feet high and the boat will be 4 feet wide.

    Thanks bajansailer for your help.
     
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  8. Blueknarr
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    Blueknarr Senior Member

     
  9. kapnD
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    kapnD Senior Member

    I’d be looking for a good used fiberglass or aluminum boat with at least a moderate vee bottom.
    You’d save yourself a lot of time and money, be safer, and have a boat you could pass on to your grandkids.
    Hawaii’s waters, even what limited protected waters that there are should be approached with the utmost regard for safety.
    Offshore breezes make for calm inshore conditions, but can be quite strong at less than 1/2 mile out, and blowing offshore, will easily take a boat beyond reasonable anchorage depth in short order.
    Tide changes and swell action can create incredibly strong rip currents, some that a pair of oars and six hp outboard would not be able to overcome.
    A slab sided Jon boat will not be a nice boat to row, does not track well, and will be very wet in any chop.
    Flat bottomed boats become very unstable when they get a few gallons of water sloshing about in them, and are hard to pump or bail, as there is no sump to collect the water.
     
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  10. Raptor88
    Joined: Apr 2021
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    Location: Hawaii

    Raptor88 Junior Member

    All duly noted. Thanks.
    As previously mentioned, my brothers and I built a flat bottom boat about 30 years ago and it worked well for the waters we fish in. This will not be an untested adventure but a do it again 30 years later. I just don't remember the spec's for that boat so here I am.
    Keep it warm :) .
     
  11. messabout
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    messabout Senior Member

    Raptor: The distance from shore does not determine the weather or water conditions. You can get into deep trouble at less than 50 yards from the shore. Sea conditions are not a linear function that involves distance from shore. As you suggest, it is prudent to stay within a reasonable distance from dry land if your boat is small.

    My take is in agreement with Kapn D's good council.
     
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  12. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    And I would agree with the advice given above by Messabout and KapnD - just because you you built a flat bottom jon boat type 30 years ago with your brothers (I guess while teenagers probably), and lived to tell the tale, doesn't mean that have to build the same type again now.
    There are many much better and more seaworthy designs available for use on the ocean - even close to shore, this is a very different type of weather and sea conditions compared to the usual haunts where Jon boats excel, like in calm backwaters and bayous.
    And KapnD is in Hawaii, so he knows first hand what conditions you might encounter - and his suggestion to look for an existing used fibreglass or aluminium boat is is very sound advice.
     
  13. fredrosse
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    fredrosse USACE Steam

    "I don't think my skills will make a pro level scarf joint 4 feet wide with all of the wood touching together. So I want to use a butt joint with a wood patch over it. "

    I built my 19ft-3inch plywood hull with 1/2 inch sides, 5/8 bottom. At about 1 ton displacement, if I had it to do again I would have used 3/8 sides and 1/2 bottom. So 3/8 is plenty strong for your project, even 1/4 would be fine, with interior furring strips for a floor where you will drop sharp, heavy objects.

    Butt joints in the plywood, joined with epoxy, and a layer of 10 OZ cloth between, are plenty adequate for your build. Have the bottom butt joint aboit 4 ft from the stern, making the curve at the bow without a butt joint.
     

    Attached Files:

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  14. fredrosse
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    fredrosse USACE Steam

    "I won't have spare pieces of 3/8" ply but I have to buy 1/2" ply for the seats so will have spare 1/2" marine plywood. Would gluing 2 pieces together for a 1" thick transom be sufficient for a 6 hp outboard? "

    My O'Day fiberglass Javelin was bought well used, and had a rotten transom, 3/4 plywood. I made a new transom with two layers of 3/8 plywood glued together. This is plenty strong for a moderate outboard. If you are going for something like 15 horsepower, a 1 x 4 glued inside the transom will be plenty strong, but for a reasonable small motor this is not necessary.

    PS This was all dome with polyester rather than Epoxy. Yes, Epoxy is much better, but after 10 years, this transom rebuild is holding up well.
     

    Attached Files:

    Last edited: Apr 28, 2021

  15. Raptor88
    Joined: Apr 2021
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    Raptor88 Junior Member

    Thanks fred for confirming use of 3/8" ply. I will use 3/8" ply for the bottom and two sides.

    First I've heard of putting fiberglass fabric between glued joints for strength. I've only seen folks on youtube glue boards together using epoxy between the boards.

    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.

    Thanks for you input.
     
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