constant deadrise v varied or warped

Discussion in 'Powerboats' started by gtflash, Jan 1, 2010.

  1. gtflash
    Joined: Jun 2006
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    gtflash Senior Member

    I have been offered the chance to purchase a powerboat bare hull, shes 28ft long and 8ft wide. Constant deadrise 24deg. Sure footed and fast but struggles in a good head sea. Would it be possible to modify the hull to add a warp, increasing the deadrise forward, to cut through the constant wind over tide situations in my cruising area. If so. My questions are.

    How much additional length would have to be added to get a smooth transition from a sharp bow to the set 24 deg aft??

    On normal warped hulled boats of equal length, how much of the hulls surface would normally be parallel from the transom

    Is there any other better alternatives, would it be better to start a fresh with a new hull design??

    Thanks
     
  2. narwhal
    Joined: Aug 2009
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    narwhal Junior Member

    How is she constructed, wood or fiberglass or...? I don't think it will be easy either way, but the methods to achieve your goal will certainly differ with the material.
     
  3. apex1

    apex1 Guest

    Redesigning, rebuilding a hull shape is probably the worst task in boatbuilding in general.
    A restoration is a different pair of shoes though.

    But much more info would be needed for a sufficient advice.

    Regards
    Richard
     
  4. Guest62110524

    Guest62110524 Previous Member

    constant deadrise(monohydon) is best suited to jets

    your 24, is that the deadrise at the bow too, if so it will pound you to death in even a slight sea

    Perhaps the best names in offshore power, game fishers etc, Don Senior NZ Don Shead UK, Don NZ uses 7 \or nine aft and 50 at entry
    I do too, very steep deadrise entry and 16 aft. the reason for flattening aft is , one to get the boat up on the plane, and two, stability in following seas
    If you wish to take your boat onto a lake when the wind has been blowing awhile, you will be at walking speed
    Still maybe yours is a slow boat? pics?
     
  5. gtflash
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    gtflash Senior Member

    Thanks for the replys. The boat in question is desgined by a well known british designer, it is infact fibreglass and does have a constant 24deg from bow to stern. They were very popular hull and handle well in average seas, predominantly designed for high speed and were quite popular in the 80s racing scene. Three full length strakes.

    The hull tended to throw its head in anything of an oncoming sea, but the stability at speed in lesser conditions is very good.

    Any additional info??
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    24 degrees of deadrise is pushing the limit and part of the extremes seen of that era (80's). No one knew how much deadrise was too much, so the angle went up and up and into the "oh my God this boat sucks" category.

    It sounds like you have one of these boats. Not enough entry to keep her manners in a sea and likely tender under foot when fishing with your buddies.

    Making the modifications you're thinking of would be exceedingly difficult, not to mention I don't know a single designer, that would want their name attached to such a bastardization. Consider another design.
     
  7. gtflash
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    gtflash Senior Member

    Maybe my fault for playing this hull down. I understand your opinion on bastardising a hull but without knowing the design it is difficult for ay other to critisise. It is a very popuar hull in the UK, or derivatives of it are. Made by a very well known designer, not a back street buddy. He may be better known to you as the man that designed the steps on the Formula range.

    Ive personally seen these boats run at over 80mph very well and set many records. But in real heavy seas is where they fail. My thinking was if a hull can run fast in average seas, could it not be tamed to run slightly slower , but cope more in rougher seas

    Thanks anyway
     
  8. marshmat
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    marshmat Senior Member

    gtflash,

    Building a hull is a relatively small component of a boat's total build time and cost- anywhere from 10% on an elegant yacht, to maybe 30-40% on a really stripped-down workboat. Most of the money and effort goes into the hardware, the systems, the interior, the powertrain.....

    Is it really worth putting all that money and effort into outfitting a hull that isn't quite what you need for your application? The general rule is no, it's not.

    There's also a general rule about modifying existing hulls. Don't. A lot of money, time and materials get sucked up to produce something that may or may not perform as expected, and will almost certainly draw raised eyebrows from surveyors and brokers when it comes time to sell it. If someone says "I want to modify my hull form", the most common answer is "Sell it and buy the one you actually want".

    You will be far ahead in terms of finances, time and the suitability of the finished product if you start with a hull design that is appropriate for your needs- whether built from scratch or built on a good used hull- instead of trying to modify an existing hull into something it wasn't meant to be.
     
  9. Tim B
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    Tim B Senior Member

    I think marshmat is right... either accept the limitations of constant deadrise, or re-build the hull.

    Typically, when you design a variable deadrise hullform, you set the forward deadrise (generally 40 to 50 degrees) and the aft deadrise (from 6 to 20 degrees). You then draw the keel and chine curves, so that you keep the change in deadrise smooth.

    How much deadrise you use, where you put the changes, whether you have constant deadrise aft are all related to the power, handling and seakeeping considerations, and there is no "one-size fits all". On a lake you could get away with low deadrise, which will allow planing at lower power, at the expense of "grip" in turns. In the sea, you need the seakeeping abilities that high deadrise gives, particularly at the bow.

    One thing is certain though. You want some deadrise aft, otherwise it'll be like trying to drive a rear-wheel drive car with no grip on the tyres! It'll also slam very badly in very small waves.

    Hope this helps a bit,

    Tim B.
     
  10. Easy Rider
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    Easy Rider Senior Member

    24 degrees. Wow. Thats a lot. I thought production boats maxed out at 20. The chines would be out of the water. To have reasonable stability it would need to be very heavy and consume gobs of power so how'd it get popular? Don't worry about the bow .. it won't pound as it will not be in contact with the water. The boat will ride from midships aft .. not the bow. Plan on two 500hp engines. Seriously .. if it's so popular go talk to some owners.

    Easy Rider
     
  11. Grant Nelson
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    Grant Nelson Senior Member

    what makes me curious is why you do no mention the boat model, or the designer, as this information is always helpful, espcially if someone has experience with these design...
     

  12. gtflash
    Joined: Jun 2006
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    gtflash Senior Member

    not in the uk, infact most good fast v planning models sport atleast 20deg or more. My current ride for example is 24deg aft and 60deg bow. It is a much loved design and considered one of the best in the uk.

    http://www.scorpion-ribs.co.uk/

    Thanks for advice on constant deadrise model. Reason I do not mention make or model is due to the nature of the sale. But for info 2x250hp mercs see the old hull around 80mph. I am happy it would be too much work though, thanks for advise.

    This thread does make me consider "what" hull charecteristics are considered for a good hull here in uk as againt your side of the pond. Most of our sportsboat/rib market is dominated by deep deadrise aft hulls.. only inshore speedboats and lake boats are commonly less than 20-22deg deadrise aft obviously for speed and efficiency
     
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