Designing a fast open deck catamaran

Discussion in 'Multihulls' started by Mulkari, Nov 26, 2024.

  1. montero
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    montero Senior Member

    Beside struggling upwind , shallow water board handling is important. Did you start your build already ? Board and boardbox materials ?
     
  2. Mulkari
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    Mulkari Senior Member

    Shallow water capability definitely is important. My design currently have draft of 0.55m at 8.8 ton displacement. I suppose with boards partially pivoted up I could still somewhat sail upwind in as little as 1m water although using engine probably makes more sense in such conditions because with rudders also partially pivoted up there would be huge weather helm force. I haven't started building anything yet, it is brainstorming phase currently. Hangar where I will build also needs to be cleared from junk. Boards would be wood core covered with fiberglass like most homebuilt boats have. Board boxes would be plywood covered with fiberglass.
     
  3. waterbear
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    waterbear Senior Member

    Why would you look to Mumby as an example? A guy with no credentials and no website designing and building catamarans in a Philippine jungle? Sure, they might be fine boats with a good track record, but maybe not the best optimized.

    Someone in the cruising forum said their Mumby 48 does 5.37 knots VMG in 15 knots true. Compare that to the Shuttleworth 35 cruising cat which does 7.3 knots VMG in 15 knots true. Mumby claims the Mumby 48 has seen a top speed of 22 knots, while the Shuttleworth 35 is claimed to have seen a top speed of 24 knots with a crew of 9 people onboard. One would think the much larger Mumby would fare much better against the smaller Shuttleworth design.

    Here are the polars for the Shuttleworth:

    Screenshot_20250112-224453.png

    Shuttleworth, by the way, has this to say about daggerboards:

    "There has been a tendency in multihulls to reduce the size of the daggerboards on the basis that they sail faster and, hence, need less area to provide the same lift. I have never agreed with this philosophy and in my designs I am making the daggerboards as deep as the structure will allow. I would consider 11 foot draft as normal for a 60-footer, with the board down. Bruce Banks, sailmaker for Livery Dole 111 said "The only boat that could come near to this for pointing ability is a 12 Meter." That is high praise from one of the sailmakers to the Victory syndicate. Therefore the boards on this catamaran are larger than usual, with the intention that both should be used in light airs, retracting the downwind board as the wind increases."

    Now lets compare the daggerboard of the Shuttleworth to your design:

    Screenshot_20250112-083356.png
    Screenshot_20250112-081754.png
     
    Last edited: Jan 13, 2025
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  4. Jacobra
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    Jacobra New Member

    This is an interesting comparison, but again the scale is significantly different.
    It's my understanding that fast catamarans with daggerboards generally raises them part way as speed increases so there is little penalty in specifying large boards optimized for low speeds. Centerboards don't have that option and can only be optimized for a single design condition. I'd expect them to be a bit smaller than a daggerboard all else being equal.
     
  5. waterbear
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    waterbear Senior Member


    There is also the Shuttleworth Tektron 50 (see below) which has two daggerboards. I estimate the boards to be 2.05M^2 each. The length is 1.78M and the width is about 1.15M (estimated from a photo). This is about 2.5x the 0.8M area Mulkari has drawn and also what you would expect if you scaled up the 35.

    Screenshot_20250113-204115.png
     
  6. Mulkari
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    Mulkari Senior Member

    Interesting, does it mean nearly every performance cruising cat comes with severely undersized boards even Gunboats and Dazcats that claim no compromises.

    I find them interesting design. Aluminium is generally thought too heavy for performance cats yet Mumbys manage an empty weight around 6 tons which are among lightest in 14 - 15 m performance cat size range. Maybe they sacrifice long term fatigue resistance? Of all materials suitable for performance cats aluminium probably has best abuse resistance. A grounding that may end up with major repair to foam fiberglass or carbon boat may just cause some dents in aluminum. A very desirable property for world cruising boat.

    That seem to agree well with 4% of upwind sail area rule. Probably have to find a way to increase boards to at least 1.5 m2 each. Definitely want to get this right since upgrading to bigger board later would be very hard because case also would have to be rebuilt while fitting longer daggerboard uses same old case.

    There is always a possibility to retract one board. I think it is generally recommended to use only windward board when sailing upwind in strong winds to reduce chance of flipping the boat. Some adjustment is possible, but definitely not as good as daggerboards have. On the other hand daggerboard cat have little to no interrior penalty for having big boards since they only stick out higher on deck when raised.
     
  7. montero
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    montero Senior Member

    You want to change plywood to aluminum ? That's always good idea . Durability , no roting . Made of flats elements just like plywood .
     
  8. Mulkari
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    Mulkari Senior Member

    I most likely will use epoxy plywood, it is just so much more friendly to low tech DIY project. Aluminium would make basic structure about 2 - 3X more expensive, besides I wouldn't trust my rudimentary welding skills to do large scale flawless welds so would need to pay a welder to do it which would be another cost increase. But yeah, if no other considerations aluminium would be my material of choice.
     
  9. waterbear
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    waterbear Senior Member

    Again, I don't know. To be fair, some Gunboats do have very large boards, like the Gunboat 80 pictured below. That boat came in 1st out of a field of 600 in last year's round the island race, so no slouch.

    yysw421228.jpg

    As for Dazcat, they fit triangular keels and boards. As you can see, there is a fair amount of lateral area between the boards and the keels

    Screenshot_20250114-103534.png



    I would take those weight claims with a healthy dose of skepticism. The 4mm plating on the hull of the Mumby, for example, is almost as heavy as 25mm okoume plywood.

    1.5M^2 seems reasonable.

    I think the idea is to use two boards when the wind is light, one when the wind is up.

    And it might be 2x heavier too! Aluminum also loses more than half it's strength when welded and it's very prone to stress corrosion cracking. And that's when the welder knows what they are doing! I personally wouldn't want to use it in catamaran.
     
    Last edited: Jan 14, 2025
  10. Mulkari
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    Mulkari Senior Member

    Maybe, but would his customers just accept overweight boats without asking serious questions given the money involved? When searching pictures and videos of Mumby 48s they all seem to sit nicely in water without looking overweight.

    Aren't there ways to mitigate that? There are plenty of aluminum work boats and they are not falling apart after few years despite often being abused far more than your average fiberglass recreational boat.

    That's interesting design although it adds non removable wet surface area which would add drag in light wind downwind sailing when boards are normally retracted. Would be nice to know reasoning behind that. It is definitely unusual design.
     
  11. montero
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    montero Senior Member

    Propeller protection and additional lateral drag surface.
     
  12. waterbear
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    waterbear Senior Member

    You're assuming a boat that weighs more than claimed can't sit on its lines. I could very easily be wrong, but I'm not the only one who has expressed skepticism. In his post about the Mumby, oldmulti said:

    Sure, but in those applications the stresses are not as bad as a cat and the weight is not as critical. Not saying it's a bad idea, but I wouldn't design/build my own. I think marine plywood is much better from a weight/performance perspective.
     
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  13. waterbear
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    waterbear Senior Member

    Also, RW said this about aluminum cats

     
  14. montero
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    montero Senior Member

    Insulation also add weight especially if you sail to Norway or further.
     

  15. Mulkari
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    Mulkari Senior Member

    Yes definitely for DIY home builder aluminum construction also would be fare more challenging than plywood with more potential for hidden defects. What I like the most about aluminum is impact resistance, small logs and chunks of ice or minor groundings at slow speed would cause much less if any damage. Maybe on a plywood boat you could add a some kewlar in areas likely to be impacted to somewhat mitigate it?


    That's exactly where I want to sail. I will need insulation anyway since 9 mm plywood while better than aluminum is still not good enough. I plan on about 50 mm insulation in hull used for living.
     
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