Multihull Structure Thoughts

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

  1. jamez
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    jamez Senior Member

    Just came back from the basement with my Fish n Chip study prints that Ray sent me (all hard copy back then) back in 2002 (thats nearly a third of my life ago - makes ya think). At that time it looked about the roomiest 24' open cat around. I found some pictures from a for sale add a while back. There are some interior pics which I think are useful. Also, it looks like this one has a deeper cockpit floor at the expense of clearance, although having 2 x 45kg 9.9 4 strokes on the back beam wouldn't have helped that. Its definitely squatting in the stern. A 6 hp 4 stroke or 8-10 hp 2 stroke at about 28kg would have been plenty. Under 30 feet I actually think that having the outboard in a central pod like a great barrier express does is worth considering to get the weight more central in the boat.

    -Fish--chips-7.3bunk.jpg Fish--chips-galley.jpg Fish--chips-head.jpg -Fish--chips-side.jpg -Fish--chips-stern.jpg
     
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  2. jamez
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    jamez Senior Member

    I also have a set of Siren 8.4 study prints sent by Ray in Nov 2006. I was in the middle of my Hinemoa rebuild at that time and must have been suffering from why-the-f**k-didn't-I-just-build-something-from-scratch-itis. Attached are screen grabs from his website at the time and a profile scan from the study prints. Its a very graceful design IMO, that uses interior space well (although I like the 9.5 better :)

    siren 8.4.jpeg siren 8.4p2.jpeg.png siren 8.4profile.jpeg.png
     
  3. Burnside Style
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    Burnside Style Junior Member

    Jamez, Thank you for posting those interior pictures, it is nice to see photos to go along with the study plans.
    This design keeps popping up in my head as a good potential coastal cruiser.
    I have been into bi-rigged catamarans recently, but I haven't been able to shake this design for a few years. Every few months, I look at it and think - are the unstayed masts really worth the cost?
    I have seen the exterior photos of that boat, and was concerned with the low deck in the back, good to know that it was built deeper/the two outboards are unnecessary.
     
  4. guzzis3
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    guzzis3 Senior Member

    oldmulti: I'll email RK when I get a chance. I didn't anticipate as much interest in 2 designs that are no longer available.

    Also a small correction. According to RK the siren 9.5 was never finished. The 8.4 was finished but many of the sheets have been lost. Even if he wanted to make them available he would have to finish the designs to issue a complete set of drawings for builders.

    I personally find KH's website chaotic. I understand why he organises it like that, but if you are say looking for something about 30' for example you have to search all over the place to see what he's got.

    I am tempted to get the fish and chips plans just to have a look. Obviously every build method has it's advantages and drawbacks but building multi chin in foam is a nice middle path. I don't know the headroom on F&C but if you stretched it out to 9 - 10 meters built in 12mm foam with 600gsm skins and put a cuddy/pod/sportsdeck (so many names for a detached bridgedeck cabin) on it you'd have a quick boat that's trailerable. Centerboards are appealing here in qld because of all the thin water.
     
    Last edited: Aug 28, 2021
  5. oldmulti
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    oldmulti Senior Member

    Today is about an open source book written by David Lewis. An Australian who was one of the first people to sail a catamaran around the world. Rehu Moana, a 40 foot catamaran designed by Colin Mudie was launched in 1963 and lost in 1982 on Covey island after sailing around the world. Lewis’ studies on the traditional systems of navigation used by the Polynesians was one of his lasting legacies, and are documented in his book, We, the Navigators. He used these methods while sailing around the world on Rehu Moana with this family in the 1960s. This was the first catamaran to complete a circumnavigation of the world.

    After he sold Rehu Moana he purchased a 32 foot steel monohull in which he sailed the Antarctic. His book, Ice Bird, recounts the vessel’s trip. He was an adventurer who educated many about catamaran etc. His greatest contribution though was his research on navigating vessels using no modern instruments. He went to the south seas and was taught by several respected local navigators on how to use the stars, sea patterns, bird movements and simple EG wood instruments to guide boats between islands etc. The old pacific proa's could do 100 to 150 miles per day when sailed by the expert navigators with log canoes and flax sails with rope connections etc. There is also some interesting discussion about proa and transport craft in the pacific in the later part of the book (about page 260 onwards) including assessments of the sailing ability on all points.

    The book is "We The Navigators" and is available as an open source book at
    https://openresearch-repository.anu.edu.au/bitstream/1885/114975/2/b13540300.pdf

    The book is a 20 meg download and is 330 pages. It is an interesting read for the day your GPS, electronics fail. David Lewis died in Queensland in October 2002. PS there were several front covers on this book but the content is the same. The jpegs are of David Lewis, Rehu Moana cat with 2 different rigs and Ice Bird 32 foot mono.
     

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    Last edited: Aug 29, 2021
  6. guzzis3
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    guzzis3 Senior Member

  7. guzzis3
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    guzzis3 Senior Member

    Mr Kendrick got back to me. Here is what I have. Sorry the formatting is a mess.
     
  8. peterAustralia
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    peterAustralia Senior Member

    david lewis wrote another book, in this case an autobiography, a good read.

    There is one story in his book, he was a doctor in the UK, and he was dating a girl of african descent who happened to be an identical twin. One day a girl turns up, David Lewis kisses her, the girl is suprised but does not object, and they end up in bed. In David Lewis's words, he realises his error when it was far too late; in other words, it was the other twin. I found that story funny (sigh; this issue/problem/situation/mix-up/experience has never happened to me)

    If I may digress, oltmulti;
    A few months ago you wrote an article comparing the Wharram Tiki 21 and a small Wharram Pahi. It turns out the small Pahi design was discontinued because the Tiki sailed so well. Could you go on as to why the Tiki sails better than the Pahi? Is it because of the deeper rocker of the Tiki hull provides more draught, and thus a defacto deeper keel, or is it that the rounded keel on the Tiki, versus the straight keel of the Pahi, provides a similar shape in the water as the boat pitches, whereas the Pahi design will have a completely different shape in the water as the boat pitches. By different shape I mean that at one time the keel is flat, when going over a wave the keel may be inclined ten degrees or so, when going past a wave the keel may be inclined stern down by ten degrees or so
     
  9. redreuben
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    redreuben redreuben

    I’m a bit gobsmacked. I had no idea those larger RK designs existed !
    That 8.4 with the flared Shuttleworth type hulls is just beautiful !
     
  10. oldmulti
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    oldmulti Senior Member

    Peteraustralia. The quote came from a very experienced Wharram sailor in Britain who has sailed on both Tiki's and Pahi designs of several sizes. He was comparing in general Pahi's but had sailed a 26 foot Tiki and a 26 foot Pahi. He basically said the Tiki hull shape was better. Please reference Pahi20 http://wharrambuilders.ning.com/profiles/blogs/pahi20-1
    the Wharram sailing group and his comments will be in the discussion.

    My thoughts now. The Tiki hull shape is continues fore and aft curve and a rounded bottom deep V. The Pahi hull shapes are a pointy bottom V. The fore and aft curves in the middle are more straight lines with a sharper curve at the bow and stern where the plywood panels have a dart cut in them to allow the fore and aft sections pinch in to the stems and sterns. I suggest that Pahis have slightly more wetted surface and relatively "blunter" ends which may make a difference.

    Has anyone else experience of both types of design? All input welcome.
     
  11. Scuff
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    Scuff Senior Member

  12. oldmulti
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    oldmulti Senior Member

    Scuff. Type "we the navigators david lewis pdf" into "Google", then search in images for the book cover you saw in the original post with openresearch-repository.anu under the image. It should download the pdf. The only other problem may be a geographic block put on the download, as the PDF is an open source uploaded by an Australian University. Hope this helps.
     
  13. oldmulti
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    oldmulti Senior Member

    The Pacific Cat was one of the first fiberglass day sailor catamaran designed and built by Carter Pyle. The cat is 18.8 x 7.95 foot and weighs 540 lbs. The fixed 26 foot aluminium mast carries a 267 square foot rig with a roller furling jib. A spinnaker was optional. The rigging is stainless steel wire. The length to beam on the hulls is about 13 to 1. The draft draws from 0.6 to 3 foot draft depending on the dagger board depth. The rudders are kickup on the sterns.

    In 1959 Carter Pyle built by eye, a wooden version of the Pacific Cat and found it worked well. Carter then explored the possibility of productionising the cat. His initial investigations showed that the loads on a rigid semi bridge deck catamaran was 6 to 8 times that of an equivalent sized monohull. Carter then took detailed measurements of the wooden version of the Pacific Cat to build female moulds for the fiberglass version. The weight of the cat would indicate it is mainly csm solid glass in the hulls. The bridge deck area may have some version of reinforcement in the core. There were 700 plus Pacific Cats produced with 4 different versions (same hulls, improved rig) over its near 30 year production life.

    This type of cat was enjoyed by many as a racer but also a good day sailor or camp cruiser. As an example, there is jpegs of Mike Lenenman’s Beach cruiser 22. It was 22 x 8.5 foot but I do not know if the design was completed. Also the final jpegs are from Mike Schacht, again 22 x 8.5 foot.

    All these boats are arrive at a ramp pull up the mast and launch. Easy to do and then you have a fast fun day cat. Jpegs give the idea.
     

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  14. guzzis3
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    guzzis3 Senior Member

    Neither did I until a few weeks ago. RK said the plans for the 8.4 had been lost to computer changes. They still might be somewhere. Maybe if there was more than one potential buyer he might have a dig, or if you asked really nicely.

    Shuttleworth's boats are pretty and by all accounts very fast. He has an 8 meter cat but it carries ludicrous beam. The plans aren't cheap but not unfeasably expensive either if that's what you want.

    Fish and Chips seems to have about 5' headroom. If I stretched it I'd need to raise the height also. A significant redesign.
     
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  15. oldmulti
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    oldmulti Senior Member

    A short comparison between the Scarab 7.3 meter “Fish and Chips” and the Siren 8.4 catamaran.

    Fish and Chips is 24 x 15 foot with a weight of 2,128 lbs and a displacement of 3.580 lbs. The hull length to beam at the waterline is 8.3 to 1. The hull draft is 375 mm. The hulls are 4 foot wide at the gunnel and it has 5 foot of headroom. The hulls are built from 6 mm plywood with 400 gsm glass outside and 300 gsm inside.

    The Siren 8.4 is 27.5 x 19.5 foot with a unknown weight and a displacement of 3,580 lbs The hull length to beam ratio is 11 to 1. The hull draft is 400 mm. The hulls are 5.5 foot at the gunnel with 5.5 foot of headroom. The hulls are 9 mm plywood with I assume 400 gsm glass outside and 300 gsm inside. When I did a quick model of the hull in my Hullform program the maximum displacement at the 11 to 1 waterline with a PC of 0.59 for the total boat is 4100 lbs. After that point the flare of hull above the waterline picks up displacement quickly but the length to beam drops to about 9 to 1. Sailing in heavy airs means one hull is carrying to eg 3,000 lbs, with a 9 to 1 length to beam, the other hull is carrying only 500 lbs. with a length to beam of about 12 to 1.

    Now we come to my dilemma. I will assume a plywood build in both cats. The deck and hull surface area of the Siren 8.4 is about 20% more than Fish and Chips. The hull plywood in the Siren 8.4 is 9 mm versus Fish and Chips hull plywood of 6 mm. Fish and Chips crossbeam structure may be similar to the Siren 8.4 but the Siren 8.4 has more righting moment. Combine the increased righting moment with longer crossbeams means that the crossbeams are likely to stronger and heavier. The Siren 8.4 also has a larger rig. My guess, repeat guess, is the Siren 8.4 is at least a 20% heavier build than Fish and Chips. My guess is that the Siren 8.4 would weigh about 2,600 lbs if well built. If the Siren 8.4 was built in foam glass my guess, is it could be made lighter by maybe 300 lbs.

    Summary of the above. I have made a lot of assumptions but basically I am saying is the Siren 8.4 would need to be carefully built to minimise weight. The reason to minimise weight is the payload capacity. Fish and Chips has a payload capacity of 1400 lbs, realistic. The Siren 8.4 with more berths has a guess payload capacity of 980 lbs if built in plywood.

    I am not trying to be a designer, I was a little confused by a smaller 24 foot cat has the same total displacement of a larger 27.5 foot cat. The 27.5 foot cat is built from heavier materials with more surface area and larger, heavier cross beams etc, the only thing that can give is the payload capability.

    PS no criticism of the concept or designs. Both will perform and should be good cruisers. I to would like a set of Siren 8.4 plans.
     

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    Last edited: Aug 30, 2021
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