planning a 30'(ish) flat bottom landing craft

Discussion in 'Boat Design' started by dallas Adams, Feb 23, 2019.

  1. dallas Adams
    Joined: Feb 2019
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    Location: ak

    dallas Adams Junior Member

    hello all,
    this is my first post here. i have a project in mind that i need a ton of help on, so ill start from the beginning.

    i have zero boat building experience. ive got almost 30 year construction experience and am confident i can handle the task so long as i do my research and plan it well. i will not start the project until im back in ak....about 5-6 years from now, so i feel i have enough time to get smarter.

    i have so many questions i barely know where to start, so ill just jump in where my head is right now. i need a large platform to haul at least 2-3 4 wheelers plus people and gear for river based hunts. in addition it can be used on the kenai for dipnetting and even day trips should i feel the need. it would see little to no open ocean time, perhaps in a few bays if they were well sheltered etc. its my considered opinion that a flat bottom suits these needs best, i need a very shallow draft to access river areas that could go as low as1 foot, or even less. in addition the stability gained for moving around on it for rookies is a plus.

    one of my questions is would i benefit by fully welding into the bottom a up to 6" false bottom. if made air tight it could add as much as 6000 lbs of buoyancy, if my calculations are correct. the space would create 98 cubic foot area, at 62 lbs of displacement/cubic foot. if you figure3 wheelers at 800, 4 adult males at 250 each, 100lbs of gear each thats roughly 2200 lbs of extras. 6k would go a long way towards keeping it up higher in the water. now im sure theres so much more to it than what ive presented here, weight of the boat itself, motors, wheel house etc, but im just spitballing here. im guessing the weight of the boat by itself will be close to 2000lbs, and pushing that is gonna require a hefty motor, or pair of motors.

    moving on, should i consider bow thrusters? in addition to rivers it will see some swamp-like area. while jets wouldnt be suitable for main propulsion they might work very well for maneuvering in tight spots at low speeds.

    as for general questions, if anyone has some links for general knowledge on building a boat like this please link it. no matter how trivial i want to read up and learn everything i possibly can about this and any help, criticism or advice is welcome. pop used to tell me what you dont know may be able to fill a library, but fortunately you have the capacity to learn more than that.

    thanks in advance for any help you can give me.
     
  2. dallas Adams
    Joined: Feb 2019
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    Location: ak

    dallas Adams Junior Member

    the link doesnt work, but i appreciate the reply
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    One assumes you want some speed capability, so a planing hull. There would be plenty of plans available, depending what material you want the boat made from. Operating in a foot of water is probably a difficult spec to meet, though, to do that with outboards would require a bottom like a "flats" boat, where the motors can he raised high but still get water inflow.
     
  4. dallas Adams
    Joined: Feb 2019
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    Location: ak

    dallas Adams Junior Member

    yup, for a boat that weighs this much a 1 foot draft is a tough spec, thats why i asked about the"pontoon bottom" idea, trying to add buoyancy. plenty of boats like this in ak, usually not as big, or as heavily built. im looking at 1/8" aluminum for the bottom, sides will probably be thinner to reduce weight, but that decision will be made later.ill look into flats boats and planing hulls. im also looking into a highly complex power trim build that would allow the motor to swing up 90 degrees, almost horizontal, to allow for emergency repair, clearing props etc. thanks for the ideas.

    so after a quick search on planing hulls i found a rather simpe diagram that shows a couple ideas i had been mulling over.
    [​IMG]

    i had wondered about something like the hydroplane style it shows. this is where all my experience means nothing, and i need the advice of those who are better versed in boat building, so any input will help, thanks again.
     
  5. Manfred.pech
    Joined: Apr 2010
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    Manfred.pech Senior Member

  6. dallas Adams
    Joined: Feb 2019
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    Location: ak

    dallas Adams Junior Member

    nice. the sand piper looks to be closer to my untrained eye, mainly due to the much more shallow draft and the fact that a higgins is just way more boat than i need. thanks a ton for the info.
     
  7. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Not how it works!

    Adding buoyancy foam does not raise the hull up, but lowers it. Not much perhaps, but the hulls displacement here is the critical piece. And you add the weight of the foam; not subtract it. So if you build a craft with available loading of 2000 pounds and it has 300 pounds of foam; the available is 1700, not 2300!

    The buoyancy foam is simply there to provide enough so the vessel cannot sink. It has mass despite being light!

    So the foam in your case would only be beneficial if your foam can offset all others.

    So total weight of non-buoyant stuff is say 8200 pounds. If you can only get 4000 pounds of buoyancy, all you did is added weight for a boat that will still sink and you stole from the available load by the weight of the useless foam.

    Now I know this sounds like I am poo pooing foam. Not at all! But you have to realize the foam is useless if insufficient and the foam reduces your available displacement; it doesn't increase it.

    I'm surprised to be the one to tell you.

    Of course welding is done with mouseholes and at some point, I am out of knowledge on how metal boats deal with this issue other than foam. Chambering with metal adds even more weight. I suspect my aluminum boat manages this by using foam and by using plywood decks and perhaps I would sink anyway now with the big four stroke on the transom.

    Anyhow, foam doesn't add displacement.
     
  8. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    And you also need a modifed flat bottom. You will need some form of wear strakes on the bottom. Perhaps even wood. When you come up on a gravel shore; something has to handle the wear.

    A flat bottom or pontoons are the only solutions, but the toons would need to be massive and modified on the front to make beaching ez.
     
  9. dallas Adams
    Joined: Feb 2019
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    Location: ak

    dallas Adams Junior Member

    ill cover both posts here in this post. not quite sure where you got foam from, never intended to use it. my idea was to turn the bottom of the boat into a pontoon of sorts my adding a second floor plate in and fully sealing it.my displacement figures are based on volume of air alone. i do realize for buoyancy that area does need to be submerged and ill never manage full return on those numbers but its a starting point for calculations. one question about this is does the buoyancy work in a sealed chamber? i think it does, but im not an engineer. obviously there has to be a point where i can attach a gauge to check the seal conditions periodically so any fire from the welding wont consume the oxygen in the chamber. i do have to wonder if a small (5 lbs or less) of air could be compressed to add to the effect.

    as a final point ill discuss the calculations on buoyancy again. 1 cubic foot of air displaces 62 lbs. for volume i figured 28'long 7' wide 6" tall with a final volume of 98 cubic feet. 62x 98 is 6,072 lbs of displacement. 1/8" plate of 5052 aluminum is appr 130lbs/ 4x8 sheet. using 14 to 16 total sheets for the floor alone is no more than 2,080 lbs. i still have motors and wheel house and sides and a ton of other stuff to add, but the boat itself comes in under 3,000lbs for sure, with potential positive buoyancy of 6,000 lbs at under a foot of draft sitting still. now again, im sure physics will waste no time kicking my sorry arse, but these are my rookie calculations.

    the wear strakes are a point of concern i have, and bear serious discussion. since the current idea is pontoon bottom i cant run around drilling holes any time i like. the only two options i have come up with are welding a aluminum strips or 90 metal on and calling it good or welding the strips on and tapping a hole on the strips only and bolting wood onto it. not sure which would would better, leaning toward the wood in my mind though.

    additional concerns are material, is 1/8" strong enough for the bottom? is it too much for the top layer of the bottom?i can just pay someone to run this through a cad program, but i trust people who do the work more than people who draw the work, so ill ask in places like this to start with.
     
  10. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Honestly, sorry about the foam miss.

    Watertights are real tricky in that metal world; I am of no help to you.

    All those metal boat builders on the west coast; someone must make the four wheeler flats boat.
     
  11. dallas Adams
    Joined: Feb 2019
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    Location: ak

    dallas Adams Junior Member

    no worries, foam could have been a great idea that i hadnt even considered though bro. thanks for taking the time to reply. i honestly appreciate it.
     
  12. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    DA from AK,

    Perhaps you should look into a trailerable, flat bottom, twin jet-drive, landing barge.
    Have you ever seen one in your neck of the woods?
    Build one if you must but it will be WAAAAAAAAY easier and close to 5X cheaper than building one.
    Good luck.
     
  13. dallas Adams
    Joined: Feb 2019
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    Location: ak

    dallas Adams Junior Member

    jet wont work. in open river they are fine, most of the time, once you get into shallow stuff they suck up every weed within a half mile radius. they are finicky and if trouble happens on the river its way harder to service a jet system than a outboard. my first choice is jet for this, but it just doesnt make sense. as for cheaper, the closest boat ive found to this had close to a 3 foot draft and was over 70k boats come at a premium in alaska. granted, i havent been looking that long, so it is an option, but from the prices im seeing currently this boat will cost around 10k depending on me finding used motors or buying brand new. also doesnt include any forming of aluminum sheets that would be needed if i use a tunneled hull design.
     
  14. fallguy
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    fallguy Senior Member

    A barge hull is what you need.
     

  15. messabout
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    messabout Senior Member

    Dallas; one of your requirements is shallow draft. If you are to use that as a primary need then only a wide flat bottom will meet the requirement. At planing speeds the flat bottomed boat will be far more likely to pound your guts out than a shape such as a deep vee. Deep vee is out of the question because it will need lots more depth of water in which to operate.

    The flat bottom will plane with less horsepower than other bottom shapes, especially deep vee. The problem with flatties, aside from the pounding tendency, is that there are structural complications that must be addressed. ....Try this simple homespun experiment............Place a piece of corrugated cardboard, better yet a piece of thin plywood or even a piece of aluminum or steel sheet metal, between two supports..... two sawhorses or chair backs or whatever you have at hand. The space between the supports is the critical matter. If you support a flat plate say 24 inches apart and place a one pound weight in the middle, there will be a sag in the middle of the span. Now if you try that with a 48 inch space the sag will be far far more....exponentially more. I suspect that you know that intuitively.

    The point is that if the flat bottom is wide, which it will need to support all that weight, then it will need a lot of reinforcement in the form of ribs running across the boat. You will need some engineering help to calculate the number and strengths of the ribs. Having a floor above the enclosed the bottom will have no affect on the stiffness of the bottom unless there are bulkheads that are attached both to the bottom and also the floor. The hollow space between the two surfaces will have no influence on buoyancy ......... Permit me to expand on that just to be sure we are on the same page. If you understand this then please pardon me for taking the conversation to its basics...............

    Take a five gallon bucket and push it down into some water. The bucket had to move some water out of the way. The amount of water that it moved out of the way depends on the weight of the bucket and whatever heavy object that might be in the bucket. Lets say that you have put 15 p0unds of weight in the bucket and the bucket itself weighs one pound. The bucket will push 15 + 1 = 16 pounds of water out of the way. That is Archimedes Principle in a nut shell ,or in this case a bucket.

    Lets say that your boat and all its' contents weighs 8000 pounds. We have to push 8000 pounds of water out of the way. You already know that fresh water weighs about 62 pounds per cubic foot. Divide 8000 pounds by 62 to get about 129 cubic feet. You want to float in 6 inches of water. That is one half foot so here we go.....We need to have a bottom and side box that will displace (push out of the way) 129 cubic feet and so we need an area of 129 x 2 = 258 square feet. fiddle with the numbers........say the bottom is 8 feet wide......then the length must be 32.25 feet of immersed area. Or let the bottom be 10 feet wide and we only need the immersed part to be 259/10 = 25 feet 9.6 inches of wet area. You can adjust the dimensions to suit yourself as long as it occupies the 129 cubic feet of volume.

    Once again I respectfully apologize if I have assumed that you might not know how this deal actually works. Now on with the conversation.
     
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