Multihull Structure Thoughts

Discussion in 'Multihulls' started by oldmulti, May 27, 2019.

  1. Russell Brown
    Joined: Jul 2012
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    Location: washington state

    Russell Brown Senior Member

    The sad thing about this race, especially this year, is that lots of boats that could make it to Alaska have a hard time making it the 40 miles to Victoria. This year's race happened in the middle of an extreme tidal exchange that happens only once every 18 years or so. The crossing happened with peak ebb tide flowing toward the NorthWest with about 30 knots of wind from the NorthWest. I've crossed the Straights hundreds of times and have never seen anything like it.
    That extreme tidal exchange also floated lots of big logs off of the beach, which is why all of the faster multihulls were knocked out of the race. Sailing fast at night around here is nuts at the best of times, but people were hitting 5 logs in an hour and finally hitting the "log that broke the boat". "Dragon" was the boat to beat, but they munched an ama bow and the daggerboard the second day. One interesting note is that Eric Pesty, in an F-24, finished second to Victoria and was second overall when he hit a log on the second day. Had he been able to keep going, I'll bet he would have gotten to Alaska second. He was singlehanded.
    We sailed across with the fleet and got a bit of video before things got frightening:
     
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  2. Russell Brown
    Joined: Jul 2012
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    Location: washington state

    Russell Brown Senior Member

    Dragon. I know it's from Down Under, but don't remember the history.

    DSC_1991.jpeg
    _8508149.jpeg
     
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  3. patzefran
    Joined: Feb 2011
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    patzefran patzefran

    Thanks, Oldmulti. I am puzzled by the outstanding maximum velocity "recorded" ! Do you know the conditions, is it a 10 s GPS average, was it on flat water or surfing a wave ?
     
  4. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    Wayne Barrett of trimarankits has again updated his MOJO trimaran web site and plans/images. The original MOJO was conceived in about 2010 and was subsequently superceded by the M60, M80 and M90 designs. But enquiries for the centre cockpit MOJO continued and Wayne decided to revise the plans for a simpler, easier to build design. The MOJO trimaran is 26 x 18.5 foot with about 3000 lbs displacement. The 38 foot aluminium mast (fixed or rotating) carries a 265 square foot mainsail, a 142 square foot jib and a 302 square foot screecher. The mainhull length to beam ratio is 9.2 to 1. The float length to beam is 15 to 1. The daggerboard is in the cockpit with a transom hung kickup rudder.

    The centre cockpit is the main feature of this design. It allows up to 6 people to sit in the best spot for daysailing and performance in the centre of the tri. All mast and daggerboard controls are immediately at hand and you are relatively protected from the elements from all directions. The aft cabin has a double berth, the forward cabin has a single berth, a portapotti and a small galley. This is a tri that a couple could cruise for a few weeks or a family of 5 sail for a weekend if the cockpit seats were used as berths with a tent over.

    The main hull is 7.2 foot wide and the tri can be trailed but MOJO is assembled and disassembled, MOJO is not a folding trimaran. It can be towed as a single unit like a Farrier but it would require several people and some time to assemble for sailing.

    MOJO uses the simple stitch and glue system with 6mm and 9mm plywood as the main components. Epoxy work on the structure is predominantly cove and fillets. All exteriors are covered with glass cloth of 300 gsm with the hull bottom having one extra layer for protection from grounding or running up onto the beach. The cross beams are standard aluminium extrusions without any welding. The beam attachments to the hulls are either metal (for disassembly) or carbon fibre straps (for permeant set ups) to bulkheads as required. This design could easily be converted to flat panel foam glass if that would be cheaper or easier in your area.

    The numbers indicate the MOJO would have good performance with 8 to 10 knots averages possible. This is a very simple easy to build and handle tri that would be an excellent fast short term cruiser. The first 4 jpegs are of the original MOJO. The following jpegs are of the updated design.
     

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    Last edited: Jul 4, 2022
  5. redreuben
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    Location: South Lake Western Australia

    redreuben redreuben

    OM; that trimarankit link just opens on a domains for sale site ?
    Can someone convince me that dory hulls have more value than just “easy to build”because I’m not convinced despite there proliferation !
     
  6. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    Redreuben. Try this link. Trimaran plans https://trimarankits.com/
    I picked the other link up from an old article and it had the "s" left of the end of trimarankit. Sorry.
    As to dory hulls. Yes they are a compromise and cost a small amount of performance. But for many first time builders a flat panel is a lot easier to build than a curved surface. You are skilled at doing rounded hull shapes due to your past experience and probably don't find to much trouble doing round shapes. As Richard Woods said, a good daggerboard makes more difference to performance than a round hull shape over a chine hull shape.
     
  7. redreuben
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    redreuben redreuben

    That works.
     
  8. cavalier mk2
    Joined: Mar 2010
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    cavalier mk2 Senior Member

    I'm pretty unimpressed with the "breezy" organizer of this event, the posted prose is too cringeworthy to read.

    This year's start sort of predicts the eventual tragedy that will put a cork in "Breezy's" bottle.

    Reading between the lines I wonder at the timing of the event with the long forecast tides and complete disregard for the actual weather in going ahead with the start on schedule. Man made timetables have no place in the interface with nature, most of the worst weather I've been in has been the result of keeping to some shore set timeline.

    Equally silly are predictable results like the Weta "wetter?"inversion or log strikes. Plumb or reverse bows are the worst for debris damage. I should reactivate the inflatable safety bows thread for this group so they could have a chance to keep sailing.

    Silly stuff and too bad really, the tech bucks sort of detract from the pioneer spirit of the concept. If you are in a rowboat, don't worry about winning, pick your weather windows, enjoy the scenery and camp to Alaska.
     
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  9. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    Robert. I am still looking for the magazine article on pyramid rigs but to get you started look at page 29 onwards of AYRS 81 booklet. https://www.ayrs.org/repository/AYRS081.all_A5.pdf
    This Prout cat CHEFREN originally had a pyramid rig on it but some parts broke and it was converted back to a normal Prout rig.

    Patzenfran. I do not know the circumstances of the Weta 21.4 knot speed beyond the quote from the web page. Try this link for a Weta doing 19.2 knots. Wētā Trimarans - 19.2 Knots - New Australian Wētā Speed Record by Paul White. | Facebook https://www.facebook.com/191861036168/videos/691487658580726/?__so__=permalink
     
  10. luckystrike
    Joined: Feb 2010
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    luckystrike Power Kraut

    Hello Redreuben,

    Iam working with Dory hulls quite a long time now, monohull and multihull alike. From my experience I can say that they perform very well with light weight multihulls and slim waterlines. The waterline length to waterline beam ratio should be better than 10 and bottom panel width better than 12 in a hull not deeper than half of waterline beam. They are no good load carriers. When I sketch a Tri I end up with a boat 1 or 2 foot longer than the round bottom equivalent. So, if you want to have a fast Trimaran with a dory hull, forget heavy batteries, fridge and electronics, a folding mechanism or a big outboard motor. Keep everything light and the crew small. Then the dory hull will work well. Daysailers as the W17, Strike 16, 18 or Husky 6 have proven it. For Tri's with a cabin I go up to 8 metres in the moment, but with round bottom ama's (beachcat or custom tortured ply). Crew 2 - 3 persons and Dick Newick style interior (smile).

    Have Fun, Michel
     
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  11. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    We have spoken about Jzerro the 36 foot pacific proa designed and built by Russell Brown previously, but I would like to congratulate Ryan Finn for his magnificent effort of sailing singled handed from New York to San Francisco singlehanded around Cape Horn. What I did not realise was how hard Ryan pushed Jzerro and some of the sailing conditions he went through. Ryan has serious courage and seamanship skills.

    New York to San Francisco around Cape Horn is a 13,225-mile course that was completed in 93 days overall but there was 74 days of actual sailing. He had 2 stops in Brazil and Argentina for autopilot “repairs” which in one case had him sitting around for 10 days without anything arriving, he then moved on to a second location for assistance. 74 actual sailing days means a 7.5 knot average speed overall, not bad for a single hander, but he also had to go through the doldrums twice with wind speeds of 0 to 5 knots. So how did he go during the higher wind speed days.

    This is where a 36 foot proa with a displacement of 3200 lbs and a 36 foot high mast helps. Quotes like “The North Atlantic was pretty violent. It's a rough-*** place to sail in the wintertime on a reach. I was averaging 10.5 knots for days.” and after rounding Cape Horn “I was doing over 300 miles a day at that point to get out of there.” At the end of the trip his summary on speed was “The lighweight proa hit top speeds in the low 20s. It's not a planing hull shape. No foils or anything fancy. It'll sit at 19, but it won't go a lot faster, which I'm glad about. Going 19 knots in flat water is great and fun. Going 19 knots in big waves on a boat that weighs as much as a J/24 is not as much fun."

    What sort of weather and waves was Ryan talking about? "When I was surfing with no sails in the Straits of Le Maire (around Cape Horn), those were probably the worst waves I saw on the trip, 12-15 feet but they'd be 3 seconds, 4 seconds apart. They were really close and steep and breaking at the top. If I had to sail against those, I don't know how much progress I'd be able to make. If you tried to reach across it, I think it would be full-on dangerous. It's the kind of thing that would roll a monohull and flip a multihull. Going down with it was definitely helpful.” And after Cape Horn "I had a storm that I had to deal with after rounding Cape Horn, maybe 24 hours into it or so. It wasn't bad, like 30, gusts to 40 knots. It's really cold down there.” and “They were forecasting an average sea state of 28 feet. That's not good. "I had no choice. I had to sail through the first one, through the cold front, into the northerly-flowing circulation of that depression to get as far north as possible to get out of the way of the second one. "I made it out right as that door shut”

    OK, 30 knots gusting to 40 knots is “wasn’t bad” and 12 to 15 foot waves closely set together. Hmm. As I said Ryan is a courageous man with good seamanship skills. The other part I had not understood previously is how hard Ryan sails Jzerro. Ryan said “The leeward side of Jzerro has a flotation pod on it so you can really tip the boat over pretty far”. In the first jpeg you get the idea of how far over he is willing to push Jzerro. The pod is a significant safety valve for fast single handed sailing.

    Ryan Finn an excellent achievement. Russell Brown a very good vessel. Well done to all.
     

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  12. Adrian Baker
    Joined: May 2022
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    Location: tasmania

    Adrian Baker New Member

    Congratulations to Ryan and Russell a great boat,I've been tempted by the proa concept a few times it might get me yet ,I'd certainly enjoy building one .
     
  13. Adrian Baker
    Joined: May 2022
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    Location: tasmania

    Adrian Baker New Member

    Gary, it would be good to hear some more info and your experience re. these masts,
    I'm hoping Oldmulti will post the manual /building notes but I understand he's got a back log after his break
     
  14. patzefran
    Joined: Feb 2011
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    Location: france

    patzefran patzefran

    Thank you, oldmulti. I have some doubt about the validity of the recorded number 19.2 .On most GPS (except Velocitek and few others which use Doppler effect), Velocity is derived by differenciating of location versus time. GPS location measurement are affected by a random error which can induce a non negligible velocity error . You can check this by plotting the locations when at rest. If you observe carefully the youtube movie, you can see that the weta is moving steadily downwind along an approximate straight course, wereas the location plots on the under right corner are going back and forth ! I have even seen some Weta people claiming 36 kt !
    Serious GPS builders, as Velocitek, use Doppler derived velocities and don't give intantaneous maximum velocity but 10 s maximum velocity average which are much more accurate and avoid irrealistic claims. I have no doubt that Weta, under canvassed with planing dinghy mainhull, is able to achieve high velocity planing downwind in strong winds, but record claims should be more seriously checked !
     

  15. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    The following proa is called “Herbie” after a cartoon Volkswagen. The base of the design appeared as the “Volkscruiser” in Proa web pages. Herbie is 34.5 x 21.9 foot with a displacement of about 4500 lbs. The schooner rig that have 26 foot (above deck) freestanding carbon masts. The sails are lugs of about 230 square foot each. The length to beam of the main hull is 10 to 1. The 24 foot long float has a length to beam of 15 to 1. The draft is 5.2 foot over the moveable (fore and aft) daggerboard. The daggerboard is moveable to help balance the rig under various sail combinations and course selections. The rudders are attached to each end of the hull. The hull draft is 1.2 foot.

    The accommodation is functional basic. 2 single/narrow doubles berths are available with a simple galley and seating. A portapotti can be used in 1 area. The cockpit is small and effective with a tent like cover at one side for protection. The design is intended to be a simple to build practical cruiser not a high speed fast cruiser or racer.

    The simple to build requirement has been meet with simple dory hull shapes built in plywood encapsulated in epoxy. Shelving, seats and bunk bottoms provide additional stiffening to the hull panels. There is no specifications of the ply or lumber but if you were prepared to do stringers the hull panels could be 6 mm but an easier solution may be 9 mm ply with minimal stringers using internal furniture as major reinforcement.

    This would be an interesting design and build with reasonable accommodation for a relatively cheap price. The design could also be done in flat panel foam glass. The jpegs are of the design. The one grey jpeg of the hull cross section is another person’s interpretation of the shape.
     

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