Feedback on my first hull design

Discussion in 'Boat Design' started by VinceS, Aug 18, 2005.

  1. VinceS
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    VinceS Junior Member

    I'm posting my first attempt at a hull design. I would like some input as this has been designed on instinct.

    I still have to work on proportions but we are talking 6m length wooden runabout. 100-250hp (Horsepower is open to suggestions - what would you suggest would be appropriate for a boat like this?)

    Design is for chop up to about 2' however mostly designed for a nice flat water run.

    Engine to be mounted in the rear 1/3rd of the hull, cockpit will be quite large, comfortable for 6 persons.

    .Hul file http://hmsv.site.net.au/FINAL2.HUL right click save as.

    I am building a small mockup out of foam core within the next two weeks. if all goes well this one should become the basis for a home built runabout.

    Thanks for your all your.
     
  2. Baldur
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    Baldur Junior Member

    can you post some pics?
     
  3. VinceS
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    VinceS Junior Member

    sure, see below.
     

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  4. VinceS
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    VinceS Junior Member

    I would just like to note that I am an extreme newbie to hull design and any educated advice is very valuable to me and would be greatly appreciated.

    On another note, I am very passionate about getting a boat of my own design built. I will most likely do it myself.

    Please one and all contribute to my dream.
     
  5. KCook
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    KCook Senior Member

    The extreme tumblehome at the stern is rare. Does this serve some purpose?

    Kelly Cook
     
  6. VinceS
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    VinceS Junior Member

    Put Simply, Not really.

    this is the least important part of the hull, it will be more rounded barrel back style (I hope) but at this point I cant add this in hull designer without starting from scratch. I'm concentrating more on the hull shape, chine etc.

    This is a previous attempt at getting the barrel back in hull designer, but the hull is a very strange shape. im sure it would work, but i'm not sure how well.

    Is there any fundamental flaw with having extreme tumblehome? unless going backwards at over 2kn that is.
     

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  7. marshmat
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    marshmat Senior Member

    Tumblehome's harder to build, that's why it's not common. (And the loss of deck area.) Looks absolutely gorgeous though, if you get it right.

    I like the lines.... might sit kind of bow-high at rest with that kind of power in the tail. SOmething like the 3.0L or 4.3L sterndrives found in every other boat this size is average (135 to 190hp). Could go to a 5.0L eight, maybe. Or are you thinking V-drive? Jet? The sharp reverse-sloped transom might cause problems in reverse gear; you'd need a big rear deck or maybe make the transom more conventional.

    What type of construction? (metal? wood? frp?)
     
  8. VinceS
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    VinceS Junior Member

    I have to say I'm a little surprised nobody has ripped it to shreds yet.

    Is the chine too small? is it unacceptable how the chine disapears? should it even have a chine? Should the V in the hull be deeper? or non-existant? (appologies if my terminology is incorrect)

    As for power, I was thinking along the lines of traditional runabout power. I am very concerned about putting in too much power with a boat of my own design, I dont want any nasty tendencies to surface at 40mph+ ... big splash. I was planning to mount the engine aprox 2m from the stern, 25% under the seats, as low as possible in the hull.

    Construction will be in exotic woods (yet to be decided, mahogany would be great)
    I'm still researching how to plank without spoiling the wood look. I am also thinking of glassing the bottom and other random points of perceived weekness. But again, glass is something completely new for me to learn (like hull design.)
     
  9. KCook
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    KCook Senior Member

    Not exactly a "flaw". Just that it makes the back of the boat more cramped. Which would be Ok on a straight inboard with mid engine. But for the typical sterndrive setup its nice to have some "elbow room" in the back. There are ways to finisse this to get a hint of the barrelback look without going to extremes.

    It's common for the chine width to taper a bit in the stern. But I've never seen a design where it completely disappeared before. Mostly chine is to compensate for a deep V bottom. Your V is so shallow a chine may not even be needed. The simpler aluminum skiffs have no chine (as an example).

    As for how much V, there have been numerous discussions on deadrise. What you have now is essentially a tournament ski boat bottom. Good lift for brisk holeshot and low speed control. Not so good for rough water. Problem is, everybody has a different idea of just what "rough" means. And how important it is to deal with it.

    Kelly
     
  10. VinceS
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    VinceS Junior Member

    KCook,

    I love the barrel back look and its one of the things I want on my boat. I intend to mount the engine far enough forward to stop limited space being a problem. I'm hoping to have a smallish sun bed above the engine, and steps built into the transom down to the dive platform.

    Considering I would like my boat to handle 2 foot chop without too much problem, would you suggest a deeper V (More deadrise? is my interpretation of terminology correct?) or should this amount be sufficient?

    In terms of power, should I worry about nasty tendencies when adding power? I would like a speedy boat, but I would rather have a nice stable comfortable ride at WOT.

    What are the common problems with hulls of this general design?

    Thanks so much for all the ideas so far. This is my first serious try and I take the advice of more experienced designers very seriously.
     
  11. Tim B
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    Tim B Senior Member

    Firstly, I would say don't go too extreme with the barrel-back, then mount the engine over the center of bouyancy. you may be able to get seating in behind it. This was a classic trick with run-abouts to keep the bow down. What are the actual deadrise angles at the stern, amidships and L/4 aft of the bow? Also, what speed would you like to go (maximum) as I would suggest building a model from wood, as you'll build the full-size, with the same propulsion systems (ie. props and props), then run the model on a lake (with some radio control gear) at the same Froude number as the full-size ( Fn=V/SQRT(g*L) ) and see how it handles.

    You will find that greater deadrise smooths out the ride, and increases turning accuracy. Generally, boats with little or no deadrise slide about like mad (take a hovercraft as an extreme example) and are not nice to helm, obviously this gets worse with increasing power. Typically, anything over about 20 degrees is termed "Deep V" and anything below 10 degrees is considered "Shallow V". The deadrise is measured between the line between keel and chine and a horizontal line out from the keel.

    For materials, I'd be tempted to use Birch-ply on the hull bottom and sides (but I like the colour), a mahogany ply should do just as well (more expensive). Certainly all bulkheads can be Birch-ply. All longitudinals should be Spruce of some description.

    In terms of glass-fibre, I'd reinforce every joint (to the hull surface) inside with glass-fibre tape and epoxy (which you will have used for all the joints), then use glass-cloth and expoxy outside, right up to the deck. Properly wetted out and finished it will look great.

    Feel free to E-mail me at Tim@MarineDesign.tk ,

    Tim B.
     
  12. VinceS
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    VinceS Junior Member

    Thanks Tim.

    As you can see in the image attached, the deadrise decreases a lot towards the rear. At the front its a very very deep V, at the rear its close to non-existant.

    My assumptions are: With a rudder, the back would be stablised substantialy. Because the deadrise is greater at the fore-most point of contact with the water (or where I imagine such a point might be), handling shouldn't be too shabby.

    I imagine on a full plane, the fore-most point of contact with the water will be at about the 6th line (I was going to say rib, but that wouldnt be accurate) from the bottom. I guestimate the angle here to be atleast 10 degrees.

    In terms of contruction, I want the classic runabout wooden look. I'm not real sure about woods and glass yet, I just want the design out of the way first.

    Would people recommend a greater deadrise considering the 2 foot chop I mentioned previously? and for general handling purposes?
     

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  13. VinceS
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    VinceS Junior Member

    I forgot to ask, whats the easiest way to get a rough idea of COB - at the moment I would guestimate it to be around the rear 1/3rd
     
  14. tom28571
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    tom28571 Senior Member

    First, don't mistake this forum for an accredited class in hull design. Designing a planing boat like you show with the power and speed you want is difficult to get right. The shallow deadrise and extreme warp with a large engine aft is going to run bow high. This means that the hull is going to meet the water well aft of the sharper bow sections and will POUND at speed in any chop large enough to warant the name. So, like the gentlemen's runabouts of old, it is a lake boat.

    There is no reason to take the chine flat so far forward and I assume it is only for spray control anyway. I think that this much warp can create instability problems at high speed. It will need a skid fin to keep it in line in high speed turns, It will also need non tripping chines aft or it may go flip flop with you under it in high speed turns.

    Whether the hull is "developable" in plywood is doubtful and the barrelback side panels are most certainly not and small planks will be needed

    Altogether, some serious book study is needed to attempt a job like this. Ripping the "design" apart is not an issue since it is not really a design at all but some computer lines drawn on paper. A design means that it is backed up by reams of data and I don't see any here. The question on how to find the COB points to that conclusion.

    Don't mean to be mean since I am only an amateur "designer" myself but I do know that a lot of work preceeds drawing the lines of a boat and if that is missing, then it is only a picture, not a design.
     

  15. asathor
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    asathor Senior Member

    We get a lot of short steep chop and confused seas among the Apostle islands where I sail. Too many of the powerboaters have miserable runs back getting airborne whenever they hit the larger waves.

    Depending on where you sail you might want a deeper V and the ability to sail the boat at hull speed without the bow rising too far out of the water. A center engine with the seats on top would be help that.
     
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