Design feedback wanted!

Discussion in 'Boat Design' started by markoc85, Jun 18, 2012.

  1. markoc85
    Joined: Jun 2012
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    markoc85 Junior Member

    Hi team,

    I'm working on a design for a little boat to take 2 or 3 fishing, I want it to take 15hp, be light weight, and quick and easy to build. It needs to be able to get me home safely and efficiently in a reasonable chop. My plan is to CAD 3d into 2d then laser cut 4mm or 6mm ply to the right size, scarf or puzzle joint, stitch and glue, glass both sides. Minimal fluffing around fairing when I could be fishing!

    Here is what I've come up with so far, I'd be really pleased to hear your feedback on the design you see, and how it is likely to perform.

    I've developed it in Pro-e with conical surfaces so the plywood can bend naturally without torturing.

    8° stern deadrise, which carries forward in a triangle (you can make out in plan view) all the way to the start of the sweep up to the stem. Between the triangle and the chines the Vee gets steeper in a convex manner.
    1.2m waterline beam
    ~4.3m waterline length

    https://www.dropbox.com/s/rh5wu2svg7nci4v/boat.jpg
     
  2. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Your boat will work as drawn. The design is satisfactory but not inspired. It is pretty much the same as millions of other small boats built for the same general purpose.

    Fifteen horsepower and 4mm ply may not be a good idea except where the 4mm is used as decking or other non load bearing parts. Six millimeter ply is alright but it is about the minimum to be used on a boat of this type. The bottom will need to be stiffened with stringers and frames where the bottom panels become flat or nearly flat.

    I can not determine, from the drawing, whether the transom is sloped. Let it slope at about a 1 to 4 ratio which works out at a bit less than 15 degrees.

    Good luck with your build.
     
  3. markoc85
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    markoc85 Junior Member

    Thanks messabout,

    So do you think 15hp is too small for this? And even if I glassed the 4mm you reckon 6mm would be more appropriate?

    Yeah I wondered about the bottom panels flexing, but each panel is only 600mm wide so I thought maybe I could get away without any.

    What is the reason for needing a sloped transom?

    Also, is this classed as a warped plane hull and will it bow steer badly?

    Kind Regards, Mark
     
  4. WestVanHan
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    WestVanHan Not a Senior Member

    The 15 is too big and you need to go thicker esp. on the transom.

    I witnessed a proud owner with a similar boat he built, gave the too big outboard on the too thin transom a real handful of throttle and he nearly lost the outboard and almost sank...
     
  5. markoc85
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    markoc85 Junior Member

    Oh ok I see. Yeah no matter what thickness I make it from I'll thicken the transom in the middle and add gusset from the centre to the floor
     
  6. messabout
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    messabout Senior Member

    The 15 HP motor is more than enough for a boat of this size. You might get 35 to 40 KPH from this combination. That will exert a considerable pressure on the bottom panels especially somewhere near midships where the planing impact point is located. Your design has that area nearly flat and the bottom will flex dangerously. Consider that 600mm of unsupported width will have a tendency to bend inward. It will also have to resist impact loads from wave and wake encounters. You will be wise to install stringers in those panels so as to reduce the effective panel widths by half or thirds.

    A better option is to use a thicker bottom. An 8mm bottom will be more than two times as stiff as a 6mm bottom. You would not be able to twist the 8mm forward sections to the degree that you have drawn them. The solution is to laminate two plies of 4mm making the total thickness 8mm. That would be an aggrevation and might just lead you to modifying the design in such a way as to use the thicker ply in the first place. You will not be able to twist the thick ply into the conic shape that you have drawn. Less twist would make it more practical.

    You will need a very robust transom at least 25mm thick and framed with another 25mm of wood all around the perimeter of the transom. Your engine clamps will bite into the 50mm thickness. The outboard clamp mechaism is designed so that it requires a tilted (sloped) transom in order to have the prop shaft parallel, or nearly so, to the longitudinal keel line. Even with the thickness of the transom it needs to be braced with gussets as you mentioned.

    I strongly urge you to look carefully at some conventional small boats and build your boat pretty much like the ones you see.

    The absolute best advice that I and others will give you is this: Buy a set of plans from an acknowledged designer. You will absolutely save time and money and disappointment.
     
  7. markoc85
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    markoc85 Junior Member

    Ok thanks. I'm pretty keen to stick to 6mm, I plan to have three bench seats which will act as frames, and on your advice will put a couple of stringers down, maybe even externally is that legit?

    I'll stick with 6mm for the transom too, no need to go overkill if it is properly gusseted and thickened below the motor, will also gusset the transom corners to the gunwhale (crafted as handles for lifting/dragging). My Fyran 12 is 1.6mm Aluminium all round and carries a 15hp, and it is tough. Its just reinforced properly.

    Structural talk aside, any feedback on the lines? Will it ride softly, and is bow steer going to be a thing with the fine entry and shallow transom deadrise?

    Ps buying plans is not an option, I'm an engineer I like designing things :)
     
  8. messabout
    Joined: Jan 2006
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    messabout Senior Member

    When the boat is in planeing mode, the bow will be slightly above the water surface. It will not bow steer while in that attitude, It may very well steer or twitch alarmingly if you quarter some waves or wakes. You must take some care in balancing the boat in a fore and aft direction when moving at speed. If the loaded CG is too far forward the bow will not come up far enough to avoid bow influences. Thus, you must put those seats in the right places and guess how much the occupant will weigh, etc.

    As an engineer you will have no problem with doing plate design analysis. Those calculations will apply to bottom loading as well as torsional resistance of the whole body. You will also be able to calculate the cantilever load applied by the outboard lever arm. You will then be able to build the transom to adequate strength.

    One of your countrymen, John wellsford is an experienced small boat designer. Better to pay him a few dollars than to risk failure of your own design. That is not to question your ability, but successful boat design is heavily loaded in favor of experience. Your call, of course.
     
  9. markoc85
    Joined: Jun 2012
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    markoc85 Junior Member

    Ok thanks for that.

    The challenge with those calculations is simplifying the problem so that the equations can be useful, i.e. knowing which assumptions to make and what loads to apply and so on. Have you done this before, can you recommend any books that talk this through?

    Yep I know of him, I nearly bought the deal huff boat he built when it came up for sale. Glad to hear his name has travelled. Experience has to start somewhere :)
     
  10. tunnels

    tunnels Previous Member

    Divide the hull into 3rds the back two thirds is for load carrying the front third is water entry . this is the really important part of the whole boat !!. fine will cut through the water and you will get wet sitting in the back but theres little lift or abouyncy .
    full in the the bow and it could be a dog!, thumps ,bangs and hard riding could want to bow steer or even broach!!.
    so like has been suggested go find some boats and take a camera and pad and tape measure and pen .
    a little rocker closer to the bow area can help get the boat up and on the go reall quickly . 4mm ply wood you are wasteing you time ! when you look at sheets of ply they come in differant numbers of veneers , not all sheets of ply are the same weight even out of the same pallet stack they can be surprisingly differant weights , so bending and ridgedity !! play the number of veneers thing and see which is best suited for what you could be making .
    6 mm in the sides and 8 mm in the bottom . stitch and tape is worth a look at .
    The transome need to be strong the leverage in all directions is a lot specially if you run agound at speed, 25 mm is a little scary!!!, the attachment to the hull sides and bottom are absolutly critical . or you could be looking at the transome and out board laying on the sand bank and wondering why it all went quiet !!
    Would look at a small keel strip running from near the bow fore foot and stopping about 1.500 from the stern , that way the keel wont cause the out board to get airated in a turn ,will help greatly in keeping the fron of the boat ina straight line and not wating to dive off one way or another !, also a coule of rubbing strips each side where the hull will sit and get draged on the beach of mud or what ever , they will slso help to stiffen the side panels of the hull bottom have you ever walked round in a thin bottomed boat ? its terrible and over 6 knots you dont want to see what the hull bottom is doing !! .
    take lots a care when making the transom specially the motor height . Hardly no one with small boats takes any notice !!,just drop it on , screw up the clamps plug in the fuel and go !!
    Motor set up can make a horrible old clanker get up and go faster ,easyer to steer and at the end the motor uses less fuel because it runner easyer !!:confused:.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Haha tunnels, an outboard without power trim will kick up when it hits the sandbank, not rip the transom off ! Anyways, I think the boat shown in the OP's drawing would at least need some kind of external keel or it will broach pretty readily.
     
  12. tunnels

    tunnels Previous Member

    Haha tunnels, an outboard without power trim will kick up when it hits the sandbank, not rip the transom off

    HAHA got news for you . it can and it will and has been know to tear a transom completely out of a rubber boat because i had to repair a couple and they both had 15 hp on each of them and no lock down !! the prop wants blades want to jack the motor up off the transom .
    Even surf resue boats have a stainless steel safety wire rope crimped round the motor and fixed into the inside bottom of the hull because when they hit the sand they fly up in the air and they loose motors and usually a part of the transome goes with it . they have either 25 or 30 hp motors and may motors gone never to be seen again hence the ss wire and bolted to the hull inside !!:eek:
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Not going to do it any good, for sure, and in the surf there are bigger forces in play as well.
     
  14. tunnels

    tunnels Previous Member

    when you hit a sand bar at speed you dont have a choice ! be its kids playing at the beach or riding waves !! a hit is just that a hit , the downward force of the propeller digging sand forces the out board motor up for a spit second just before it stalls,in that spit second is when the damage is done .:(
     

  15. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Isn't the rubber bushing in the hub on purpose to prevent this from happening? It should absorb the shock from blades hitting the ground, flexing and allowing the prop to slip relative to the shaft?
     
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