Why does multihulls have pointy bows?

Discussion in 'Multihulls' started by bjn, Jan 22, 2017.

  1. Konstanty
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    Konstanty Junior Member

    No pointy bow

    That multihull proa "Sailrocket" got no pointy bow, but Paul Larsen got world speed sail record on this.
     

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  2. UpOnStands
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    UpOnStands Senior Member

    you do know that that at a practical level all yacht bows have some radius, can be several cm. The reasons are strength and impact resistance. A knife-edge of fiberglass is extremely weak and very expensive to build.
    We sail and walk around marinas and we have touched many stems. Have you?
    This is not a criticism, its an acknowledgement that our terminologies may be clashing.
    Here is a good example of a modern full bow. The radius is generous at the top of the stem to make fabrication simple and provide more deck space for mounting stays etc. The large radius stem is more draggy on wave impact but the designer feels that the top of the stem hitting water is an extreme event and if that happens the increase in drag will not be the main concern of the sailor. As the stem approaches the water, the radius drops rapidly as water contact becomes more frequent so the extra drag becomes a more important factor determining average boat speed. Designers would sacrifice their mothers to get an extra knot of speed. :D
     

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  3. bjn
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    bjn Junior Member

    I have been around boats all my life. Mostly smaller ones, and not many sailboats. As a young child, I used to sit in the stern with my grandfather in his small flat bottomed dory (powered by an 18hp Evinrude Fastwin, which he had bought new) on our way to his house on a rocky island in the shallow waters of the northern Baltic sea.

    I built an Optimist with my dad as a child. I sailed it once and really liked it, but we never used the boat again, so he gave it to charity.

    I've had a few motorboats. Right know I own an open wooden boat with an old single cylinder engine which is started by pouring a shot of gas into a cup on the cylinder head, and then turning a handle on the flywheel. The boat is steered with a tiller in the stern and used to have a tar finish, but is now varnished. It's the kind of boat my grandfather used to use when he was hunting seals, catching perch, and transporting buckets of herring to the marketplace.

    A few years back, I tried sailing again, and I really enjoy it. I bought a sailboat, which I day/weekend sailed in the Stockholm Archipelago. I've since sold it and recently moved to another part of Sweden. Now I'm on the west coast, next to the "real ocean" (Atlantic).

    I have only sailed on the "real ocean" once. It was in 35-40 knots of wind, and I was amazed by the size of the waves. But I always feel at home on a boat. So even though it was tough with that wind, I was never scared. I actually enjoyed, and really felt alive.

    I tried a catamaran once, but there was no wind. So for the most part I don't know much about these things. But I've been building that 6m proa for the fun of it. And I enjoy thinking about boats.

    I know that most boats have a quite large radius on the bow. But many high-tech catamarans and trimarans I've seen in the harbors have sharp bows. For example the DNA a-cat with foils I was amazed to stumble upon in Helsingör, Denmark. And the carbon masted Dragonfly trimaran I was studying in a Harbor in Porto, Portugal. And then my plywood proa in progress of course, which sparked this thread.
     
  4. Rastapop
    Joined: Mar 2014
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    Rastapop Naval Architect

    What is best for foils is not (necessarily) what is best for hulls (btw note also that there are foil shapes with sharp leading edges) - they have different goals and operate under different circumstances.
    A foil will generally be moving through fluid at an angle of attack not equal to 0, which means a sharp leading edge will cause separation there, which (in most cases) isn't desirable.
    A hull will (theoretically) be moving at an angle of attack of 0, so doesn't need to give up its sharp leading edge to avoid flow separation there.

    Of course if a hull is subject to leeway it is moving at an angle of attack other than 0, but the difference then is that, unlike a foil, leeway is undesirable, and rounding the bow will only make leeway worse.

    Sharpness of the bow is a compromise between the two umbrella categories of resistance: viscous and pressure. A sharp bow will typically have more wetted surface area (i.e. viscous resistance) than if it were rounded off early, but lower pressure resistance. The relative difference between the two will determine which is "better" (from a perspective looking only at resistance). This is why you aren't likely to see a high speed semi-dsplacememt ferry with a round bow, but you may see very large slow displacement vessels with rounded bows.

    No, a couple of cm probably won't make any discernible difference. Any medium sized metal hulled vessel you've seen with a sharp bow will usually at the leading edge actually be a piece of plate with its edge forward, say 20mm thick.
     
    Last edited: Feb 22, 2017
  5. oldsailor7
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    oldsailor7 Senior Member

    A flat plate has a stall angle of 4deg.
    So a sharp entry point of a foiled shape is going to be better than that.
    Remember water,as a fluid, is heavy, and of course for our use, non compressable.
    Thus it can't be compared to the performance of a foil in air, which is light and compressable. Air becomes compressed and starts to act like water at the speed of sound.
    Note the term "Supersonic" is just a reference term. It has nothing to do with the speed of sound in Water.
    So a shaped body moving in water at Any Speed is, for our purpose, already "Supersonic", in aeronautical terms.
    Thus a NACA 0006 or 0008 section is good for a leeway resisting board.
    A boat hull however is much fatter than that, and a sharp front edge is going to act like a flat plate and start making turbulence at 4-5 deg. AFAIK there is no instrument manufacturer supplyng a "Leeway Angle Instrument". But I wish there was. Then a skipper would only have to arrange his sails and wind angle to make the instrumant read 4 deg ON ANY Heading, except dead downwind. Food for thought.
    Of course a sharp entry must have SOME small roundness, if only to protect it from damage.
     
  6. valery gaulin
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    valery gaulin Senior Member

    What matter is a t what Reynold number a foil operate. Reynold number include the density of the fluid. Reynold =viscous force / friction force.

    @oldsailor7: Good idea to get a leeway instruments. I think if some one is handy with electronic it could probably be made easilly with an Aduino, compas chip and gps chip.

    Basically the compass chip would be used to calculate were the sailboat is pointing and the gps the actual track and then the angle would be calculated. I am suprise no imstriment on the market can do this!

    Of curse no sailboat manufacturer wants you to know that their sailboat they just sold you has alot of leeway even if itnpojnt high!!!!
     
  7. hump101
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    hump101 Senior Member

    Many can, e.g. the airmar range, plus any decent pilot system.
     
  8. Turnpoint
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    Turnpoint Junior Member

    I am definately a fan of a knife edge bow on high speed multihulls... especially if you are going to be going over 15 knots and have a plumb or wave piercing bow shape. With a thicker bow at these speeds you get a constant spray as water gets pulled up and over the top of the bow. With a sharp enough bow you do not have this. On a powercat I built a while back I went with a 3/8" bow and it was just barely thin enough to minimize spray. The smallest little defect up there would be the difference between a dry windshield or having to use the windshield wipers. Just recently I built a racing catamaran sailboat with an extreme knife edge bow...came down to about 1/8" of solid carbon. There is no spray at all from those bows at speed and not a hint of a disturbance as it slices through waves at speed... really cool to watch. I dont necisarilly think it is faster because of this but definately makes the boat nicer to sail. I think if you were limited to length from a class rule the blunt bow would be faster in that it mimics a longer waterline length ...but you would have to put up with the spray. On another tangent, the acats were favoring lots of volume in the bows before they were foiling to help keep the bows up while reaching because they would pitch pole very easily....but the t foil rudders and foils now have lessened the tendancy to pitch pole and they are going back to sharper bows. Just my two cents and Hopefully some of this is applicable.
    All the best,
    Brandon
     
  9. UpOnStands
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    UpOnStands Senior Member

    would love some detail as to how you created the stem. Formed it on the bow in place or infused a flat profile and inset it into the bow? Photos much appreciated.
     
  10. Turnpoint
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    Turnpoint Junior Member

    We built the hulls in one off female molds from Carbon prepreg and Nomex---the hulls split down the centerline. We included a tapping rebate along the seam and terminated the molded bow shape as a rounded 1/2" radius. Once the two hull halves were bonded together and the seam was taped, we then started laminating the bow edge by hand. The outside layers of the laminate tied into the rebate that was molded into the bow, while we clamped on a piece of Melamine to act as a temporary mold surface. I think I would have to sketch something up to explain this more clearly.... but here are some pictures of the boat and in one you can see how fine the bow shape is.
     

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  11. UpOnStands
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    UpOnStands Senior Member

    Thanks for info and the photos. That is one wicked-fast looking boat. The melamine temp mold makes sense as does the centerline split. Bows that fine would almost impossible to infuse reliably w/o the split. This is more for another thread, but what did you use for foils? I assume daggerboards but you have hidden them well. edit: found port board up in racing photo.
    Zero powered winches, you have foot power for winching. Lots of good stuff.
     
  12. Turnpoint
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    Turnpoint Junior Member

    Thanks UpOnStands,
    We put in all the bulkheads while it was still in the mold and then lowered the other hull half onto that to insure everything stayed true... with such sharp bows it is very easy for them to wander-- both pulling to one side and or having a ripple across the centerline. This makes glueing them up in a precise jig extremely important

    (The boat is only 24' and 750 lbs ready to sail...so just need two tiny winches for halyards and 10:1 on main sheet. We designed and built her for the race to Alaska...so the pedal drive is for propulsion which is allowed in the race. The boat has high aspect ratio foils all around. The first year we had straight foils, last year we had curved foils with AOA control, and right now we are building some z-foils that will get her completely flying. Rudders have always had t-foils on them for pitch dampening which can be a killer in a small cat like this in big seas...but this year the t-foils get a little bigger to help stabilize the boat in flight. Sorry about the drift)
     
  13. UpOnStands
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    UpOnStands Senior Member

    all good design stuff for sure but flying that hull completely clear is going to be a whole new world of speed. Good luck with the balance. Hope you nail it the first time.
     
  14. Turnpoint
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    Turnpoint Junior Member

    Thanks, lots of calcs and background....enough that the only thing left is to take it to the water and learn what I can.
     

  15. valery gaulin
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    valery gaulin Senior Member

    Amazing catamaran for the best race around right now!!! No big ******** nonsense sponsorship for this race anybody can do it and risk to succeed or fail. I am a follower and wish you good luck! Good design...
     
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