6.5 to 7.5 metre performance/cruise multihulls

Discussion in 'Multihulls' started by Gary Baigent, Feb 27, 2015.

  1. Gary Baigent
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    Gary Baigent Senior Member

    Hydroman, you might be interested in this model that Jim Young made; it is similar in some ways to your boat in that it too has clever stability - but it also has foils for high speed.

    The mono-foiler
    THE SOLE OBJECT of hydrofoils is to reduce drag and provide a smoother ride. Recently I designed a five-metre monohull with foil assistance and made a quarter model to prove a theory in search of that smooth, economical ride. I tested it in the Milford creek.
    It is just a normal monohull, with an extremely fine, narrow 70-degree steeply veed pod approximately 30 centimetres deep added to the bottom. The after end of the pod is open so it can immediately flood with water the moment it is launched allowing it to settle to normal stable static flotation.
    When it reaches a speed of approximately eight knots, the pod quickly drains out. The added buoyancy then lifts the hull clear of the surface so it is running on the extremely narrow pod. This is where the foils come into play. They lift the pod 300mm further and in doing so automatically provide the stability equal to what the hull provided when at rest. Slow the boat down and it settles down to its static flotation. The model has foils with a three degree angle of attack. The outboard leg also has a foil attached so that power trim can be used to control angle of attack for another two degrees to the nose-up attitude, for a total of approximately five degrees, depending upon speed.
    My quarter scale model was loaded with five kilos of ballast to bring it down to its static waterline and tested on the creek using a battery-powered drill turning a plywood drum of the correct circumference to simulate a speed of 22 knots – more or less. Of course there was no accurate data, but the visible result was all I needed; the detail could wait. The model had a small foil mounted at the stern to represent the trim provided by the adjustable power trim. The model behaved exactly as expected and represents a vast improvement in the efficiency and ride of a monohull at speed. In a proper purpose-designed boat the foil could be retractable so as to protect it on a trailer. All that's needed now is the full-sized model.
    The same concept would be ideal for double-ended catamaran ferries running to areas where there are no wharves. The foils could be retractable and have trim control, depending on loading.
    While I was testing my model, I got into conversation with a chap who suddenly asked,
    "Hey, what are you doing, you putting out a setline?"
    "No, I'm testing this model."
    "How old are you?"
    "How old are you?"
    "52. How old are you, then?" I was caught, so I told him.
    "Oh," he says. "That's oooold – not long now, eh?"
    I asked for it didn't I?

    Attached Files:

  2. hump101
    Joined: Oct 2004
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    hump101 Senior Member

    The Jim Young hull is very similar to our tri powerboat hulls, which run as air-cushioned monohulls at speed (the outer hulls only provide stability at rest, avoiding the need to flood, and help trap the air cushion at speed). The improvement in ride over a deep V monohull is huge, with similar top speeds but much higher speeds in rough water. We also tried hydrofoils at model and full scale, but didn't see the same improvement at full scale as the model showed. There are some photo's in other posts of mine, I'll hunt some more out if anyone is interested (it's a long time ago, so I'll have to go back a couple of computers!).
  3. Hydromann7
    Joined: Sep 2015
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    Location: Perth WA

    Hydromann7 Junior Member


    Hi Gary and thanks for the reply.

    I Like Jim Youngs design especially how he has integrated a number of concepts for stability at low speed / rest and reduced drag at speed.

    As for foils I am a big fan, only issue with many current designs is that they are add on appendages and need to be adjusted or removed or set and tinkered with for trim or cavitate or vent or need to be located exactly X distance from CE etc etc...

    This is way beyond the expertise of many average sailors, and goodness knows that trying to balance on a beach ball is something that I gave up on 25 years ago.

    I also developed a foiling model Tri with a completely different approach to most. I used 4 x twisted washout foils similar in concept to a wind turbine vane. Calculated them for a specific lift at a different velocities and in testing they worked a treat. But instead of mounting them as add on items to the amas they are the mounting connectors to the amas, so imagine a W shaped craft with each of the tops of the W having floatation and the legs being the fixed foils. The outer tops (amas) being carbon over foam core with the foils made from carbon fibre over paulownia and have sufficient floatation to give stability at rest. As the craft accelerates it lifts on the foils from a low speed and according to my calculations for higher speed should remain stable over 40knts.

    The model worked well on a drag test for the foils, for this one I did not use a strain gauge but still used a fishing reel but driven by a drill to get the speeds I needed. It reminded me of a water strider the way it skated across the surface.

    As much as I liked it I still prefer to follow the mono design as I think in the real world it would offer a much more user friendly package for a family.

    The skater would be more of a thrill machine like a Rave or a Trifoiler.

    All good fun this conceptual model designing, once I find the right mix of form and function for the way we want to sail I will build a full scale version.

  4. Gary Baigent
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    Gary Baigent Senior Member

    Getting the bunks in, using up the scraps - also painted bottom 3 coats of 2 pot white.

    Attached Files:

  5. Gary Baigent
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    Gary Baigent Senior Member

    Latest thinking, as can be seen from this highly accurate shed wall graffiti, is to shorten the main beam to around 6 metres, not the 7 of my original idea, and angle the small floats outwards slightly to have the (dare I mention) uptip foils also angle outwards - as can be seen in the fore and aft swung beam for transporting artwork. Advantage is less beam material required and it will pivot fore and aft to fit neatly with the reverse bow and retrousse transom - and maybe the foils could be left in, lifted that is, on the trailer.
    I played around a decade or so ago with outfacing foils on Groucho so I know what they can do; it is really, with the 6.5m, the same as the recent outfacing foils on the IMOCA 60s (which obviously work too well and break the boats), well 4 of them anyway; the one that survived won. They will get this extra righting moment power sorted but on a multihull it is easier, because it is same principle we've always had, meaning with trimarans and floats.

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  6. Marmoset
    Joined: Aug 2014
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    Marmoset Senior Member

    now while its shiney and upside down, make a mold! haha

  7. Gary Baigent
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    Gary Baigent Senior Member

    Our spoiled chook has gone clucky and taken over a bench corner of my boatbuilding shed, screams at me when I enter. Ruthlessly I chuck her out the door so I can continue with Sid 650 - but she goes squawking bonkers, tries to get through window ... which has maybe a three fingers thick gap. In the end I wimpishly give in.

    Attached Files:

  8. saltdragon
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    saltdragon Junior Member

    There must be a chicken related boat name there :)........ Rooster ?
  9. Gary Baigent
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    Gary Baigent Senior Member

    Some rudder assembly. Also lifted the forward central area deckline - to allow gracious living and comfort below?

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  10. oldsailor7
    Joined: May 2008
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    oldsailor7 Senior Member

    Nice going Gary. Superb workmanship. :cool:
  11. tspeer
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    tspeer Senior Member

  12. Gary Baigent
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    Gary Baigent Senior Member

    Considered humorous suggestions - but too many letters; that is why I like Sid.
    An earlier boat was named Misguided Angel, laboriously hand painted in large format both sides of main hull - won't do that again.
  13. redreuben
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    redreuben redreuben

    Clutch Cargo
  14. Corley
    Joined: Oct 2009
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    Corley epoxy coated

    Strippergram, I've always thought that was a good name for a lightweight boat :)

  15. ALL AT SEA
    Joined: Nov 2013
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    ALL AT SEA Junior Member

    Perhaps a bit crude, but I like the name "Shits and Giggles"...
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