Efficient Hull

Discussion in 'Boat Design' started by fish, Apr 27, 2005.

  1. fish
    Joined: Apr 2005
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    fish Junior Member

    I'm new to this forum and in fact new to boat building. I am currently looking for suitable plans to construct a boat that will do what I require but am slowly resigning myself to the idea that I may have to design and construct myself. I would like a hull that performs very efficiently at planing and displacement speeds for all maner of fishing. The boat will be required to travelling long distances, as much as 500nm in a trip and sleep a couple of fishermen (we don't require a lot of creature comforts). Sailing is not an option as this is a fishing boat. I am thinking of building from plywood for ease of construction and probably a flat bottom dory style. Maybe 7.5m to 8m long and only 1.2m wide at the chines. I would probably flair the gunwales quite a bit to add stability and load carrying ability. I had intended to have a square stern. Such a narrow/long hull should be fairly efficient at both planing and displacement speeds but I am wondering if I could increase the efficiency at displacement speeds by doing something a bit radical.

    Please forgive me for my ignorance here as this may have been done plenty of times before but I have not seen it. The square stern (although narrow) will drag water along with it at displacement speed. Bringing the stern back in to a nice even tapered point will reduce the drag at displacement speeds but will reduce the planing area of the hull aft which is not good. I propose that I may be able to achieve good results at all speeds by bringing the stern back in to a nice even tapered point but adding 50mm - 75mm thick wings that will carry the flat bottom from the widest point of the hull, parallel all the way to the stern and squared off at the stern. I guess they could be refered to as a sort of planing strake. So if you look at the hull from underneath it would be nice and pointy at the bow for a fine point of entry and flair back to a point 1/3 or so along the hull and the chines would run parallel through to the stern which is squared off. However when you look from the stern you would see that the parallel chines from the widest point of the hull to the stern is only 50mm - 75mm thick planing strakes and the main part of the hull tapers back in to the stern. the planing strakes could be faired in such a way that generated additional lift at the stern as the boat was underway and creats minimum turbulance at the aft edge.

    Now I know it is possible to build this but I am wondering if it is worth it. Will the gain in efficiency be significant? Keeping in mind that some days are spent trolling for billfish at displacement speeds and if the weather blows up on a long haul 10kn may be the maximum comfortable speed. Any advice would be very appreciated.
     
  2. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    Fish,

    There is so much here to discuss, I don't know where to begin.

    With a max WL beam of less than 4' with pinched ends on a 24' to 26' dory shape, or even with a square stern, your bottom loading in lbs/sq ft is going to be very high. With enough engine to promote planing, fuel and gear for 400 miles, not to mention fishermen, it's not likely to even begin to plane. The horizontal appendages at the stern are often seen on Calkin Bartenders and Latin American Pangas. They do help support the stern underway but won't save this boat. The 400 miles better be in very sheltered water if you plan to make any real speed with a flat bottom.

    One basic error is that long narrow boats are not efficient planing hulls, just the opposite.

    In my 30 second study, this seems like a bad idea that is in need of some serious study of basic boat design. Read "The Nature of Boats" by Dave Gerr or one of the basic design books before you go any further.

    I don't intend to throw water on your idea, just convince you to do more homework before you go further. Then, maybe I did not understand your proposal.
     
  3. fish
    Joined: Apr 2005
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    fish Junior Member

    Thanks Tom,

    I am anticipating only needing minimal HP and probably 4 stroke outboard. I currently have a 4.55m low deadrise "V" bottom dingy style centre console boat with a 50 Honda that will travel 210km with two fisherman gear eskys etc. on 55lt of fuel in the open sea. Several boat plans I have viewed with similar shape to the one I described recomend 25-40hp for boats of 21ft with cabins and claim speads of 25kn. I am not a spead demon and 10kn in a bit of a sea is all that is really comfortable. That's why I am looking for a boat that is as happy slipping through the waves as riding on top of them. What would you recomend?
     
  4. webbwash
    Joined: Oct 2004
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    webbwash Junior Member

    Another consideration if you want to lose the added stability of a wider hull (check your weights for carrying capability) might be to go with a "high speed narrow boat" similar to the power launches of the turn of the century. Herreshoff (Nat not L Francis) and other designers were able to put a nice turn of speed with low power on some very nice commuter designs. Remember weight is critical -- and is that 500 nm one way or two with how much reserve.

    There are some quick and dirty long and thin plywood boats on the design market as well. Look up "commuter boats" by Phil Bolger. I think with a 30 horse engine he obtains a fair speed. Number in accommodation is also weight increasing with speed or distance reducing effect.
     
  5. Skippy
    Joined: Nov 2004
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    Skippy Senior Member

    I like the stern wing, I was thinking about that too. I call it a "lobster tail". It's a problem on a sailboat, because it's so far aft. But with a motor, that might actually help. It will increase wsa, but I would like to hear more opinions of it.
     
  6. cyclops
    Joined: Feb 2005
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    cyclops Senior Member

    I love old skinney woodies. Very eff. with hp. But I don't think they are safe in offshore waves. They cut thru a wave. That means to me, it is going to stick it's bow into a + 5' wave and submarine it out the other side. That is not a good way to boat in heavy seas. Long and lean is for smoother or long easy ground swells. You would need a very strong deck and cabin to stop taking in water. Depth of the hull would be a real issue if we use normal- pounds per cubic foot factors. A ball in heavy seas rides up and down. That same volume made long and skinney, is a, stick one end in the head on waves and hope you are lucky. My concerns.------------------------------------------------------------------There is another thread here about "wave piercers". No body seems to care about being stuck in a squall or a storm. Stop engines? Drop sea anchor? What good lifesaving storm qualites do these skinney boats have?
     
  7. fish
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    fish Junior Member

    I was intending to build a self bailing deck and have quite a high bow with reasonable amount of flair to stop waves coming over the bow. Of course it will not be as safe as a well ballested boat in a really big storm but you can't have everything. Most of my travel will be along the coast anyway so there is always ample options for shelter. 500nm would be total maximum fuel capacity. That's roughly four times the range of my little boat with 55lt of fuel. Provided I can get similar efficiencies, surely it is not unrealistic to ask an 8m long boat to carry 220lt of fuel. As far as sea keeping ability of the long narrow boats goes, I live in the far north of Australia and we have 5-10 illegal Indonesian fishing boats being detained in the bay here at any one time. These boats are all long, skinny, have very little freeboard and come from Indonesia across the Timor sea in the time of year when the trade winds are howling and most sensible Aussies won't even look at their boat. Most of these boats run 2 or 3, 5hp air cooled industrial diesels. These are not planing hulls but in Indonesia, as with most developing contries, the work boats are long narrow wooden planing boats that require minimum hp and will carry a couple tons of nets and fish.
     
  8. cyclops
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    cyclops Senior Member

    What stops them from filling and sinking? I am really interested. Rich--------Are there any AU sites which show the boats well?
     
  9. fish
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    fish Junior Member

    I had the pleasure of helping to sink one of the illegal fishing vesels confiscated by the Customs Dept. to add to an artificial reef. What kept this boat from sinking was a bilge pump made from PVC pipe, bamboo pole, and home made flapper valve plunged up and down continually by the youngest crew member. Mind, these boats are generally made from rough sawn timber (cut with a chain saw) and no such employment of modern technology like fiberglass cloth and epoxy resins.
     
  10. cyclops
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    cyclops Senior Member

    Thanks for the old time reliable marine boat design. WOOD.-------------------------------- Plastic, Aluminum or steel does NOT belong in a boat that must float for the crew to survive. I knew there had to be safe boats somewhere. Many thanks, Rich.
     
  11. fish
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    fish Junior Member

    Thank you all for the input. After talking to a friend about the planing strake concept mentioned above I have decided not to do it. His very valid point is that, even if it worked wonderfully, it will look very different to what people are used to seeing on a boat hull and I may never be able to sell the thing if I ever wanted to.
     
  12. cyclops
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    cyclops Senior Member

    A piece of good truthfull advice. Build what boat you REALLY want. As long as it is not dangerous in bad weather. You will NEVER finish what other people tell you to build as you own boat, because it is no longer YOURS. Build what YOU want!!! Otherwise, pick a plan that is close enough.
     
  13. Cary
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    Cary Junior Member

    Keep it simple

    Sounds like it might develop a porpoising problem. One picture is worth a thousand words. I drew up plans for a 46 foot sort of catamaran. You can look at my Dad's patented hull design on my website at caryart.com , click on Cary Marine in the directory on the left side of the page. He is quite a fisherman, and it is quite a bit different than my sort of catamaran design, and anything else I've ever seen or drafted up.
     
    Last edited: Apr 28, 2005
  14. sharpii2
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    sharpii2 Senior Member


    Here! Here!

    Most marine inovations happen this way.

    Bob
     

  15. sal's Dad
    Joined: Apr 2005
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    sal's Dad Atkin/Bolger fan

    I am thinking about building an Atkin Tunnel Stern skiff - like this - http://www.boat-links.com/Atkinco/Utilities/ShoalsRunner.html

    The earlier (WWII), slightly smaller design, "Rescue Minor" was built by Robb White. He reports economy of about .5 gph at his preferred 12 knots and top speed of about 18 knots (with even more economical fuel consumption), running an 18hp Kubota diesel, IIRC.

    If a gallon will take you 24 nm, you'd need about 20+ (80 liters) for your proposed range

    These are reported to be a good coastal boats, but their chief claim to fame is economy and shallow draft (6" at speed) rather than off-shore capabilities. But they're reported to behave very well in every conditions they've been out in - who knows?
     
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