Multihull Structure Thoughts

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

  1. manuahi
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    manuahi Junior Member

    Hi, The firebird "Manuahi" in the post above is mine. Unfortunately the design was flawed. Basically the main hull was a GBE catamaran hull and did not have sufficient buoyancy to support the weight by itself. So the floats were in the water at rest and there was too much transom immersed on the main hull. A GBE weighs about 1200kg sitting on two hulls and the tri about 800kg but on one hull......otherwise it was very fast, with no accommodation and fragile. Tb
     
  2. oldmulti
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    oldmulti Senior Member

    Occasionally this thread surprises me as to its reach as Manuahi has just shown. Thank you for your contribution, just one question you said the boat was fragile, in what area crossbeams, hull structure or ?

    The next boat for discussion is a design that had high interest but became a political toy. The Seawind 950 was a 31 x 19 full bridge cat that weighed 7200 lbs and displaced 10000 lbs. The sail area is 530 square foot. It was designed to be a performance cruising cat capable of extended coastal sailing. It was a foam glass design and build. But the real interest was its initial design ability to be transported anywhere in 2 containers. This was due to the manufacturer being Korean and they were aiming at a world market. Now the fun begins. The Korean company who are a separate entity from Seawind got a development loan from the Korean government. The loan was to be repaid progressively or if production “ceased”. After the initial design, obtaining of the Seawind naming rights for a fee, mold build and first 2 boats were built but the advertised price of the boat was found to be to low to be commercially viable. The Korean company “suspended” production but keep the design rights and naming rights to the boat. Result a great boat that no one can buy new because someone does not want to admit to the Korean government the project has failed. Seawind has had to much success with the Seawind 1000 and XL version to care about doing a shorter version.

    Back to the Seawind 950. How do you fit a 31 x 19 foot wide full bridge deck cat into 2 containers. By making it in 3 component parts, Hulls are 2 discreate units each about 7.5 foot wide. There is a third central unit 4 foot wide is slotted in between the 2 hull units. The 3 rd central unit is basically the sail and anchor locker between the 2 double bunk cabins in the hulls and the central cockpit floor. How is this held together? By 2 sets of 2 x 25 mm bolts at the top and bottom of the wall diving the double bunk and sail/chain locker. At the forward end of the double bunk sail locker wall is another set of 2 x 25 mm bolts. The rear beam is an aluminium beam that holds up the cockpit end.

    The jpegs will give an idea of the boat, how it can be transported and what bits are used to assemble it. Nice idea.
     

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  3. manuahi
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    manuahi Junior Member

    Oldmulti,
    First let me say thank you for this thread, it is awesome, I am continuously amazed by the amount of information you have put across.

    Fragile, the beams were mast section with the long dimension horizontal. they were braced with wire below. All good except the design had them in three parts with internal sleeves. under the compression loads the ends of the tubes worked and deformed. I also had one rear beam fold under, it folded at the point the bracing wire and the mast stay attached. The boat was very flexible, no torsional resistance at all so the windward float would take on a bow down attitude cause it was holding the mast up, while the leeward shroud slopped all over the place.

    By the way the floats are 4mm klinki ply , 10 oz glass both sides gougeon stress ply construction which worked very well and they are very light and strong. I dropped the mast on on and it just bounced off. the main hull was a Pilkington reject I think. It's at the rubbish tip now. I still have the rest of the bits though ...... one day.

    Cheers
     
  4. oldmulti
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    oldmulti Senior Member

    The modern interpretation of the float bow forward design is represented by Time Machine, a partial Tony Grainger design, tri of 26.25 (main hull LOA) x 23 foot, the floats are 30 foot long. It displaces ready to race excluding crew 2880 lbs and carries a cloud of carbon fibre sails. It has a canting rig that after some refinement could maintain full tension as it shifted the mast head up to 15 degrees from side to side in the time it takes to tack the boat. The canting mechanism is all electric. The mast is by Marstrom (Sweden) autoclaved pre-preg carbon rotating wing mast custom engineered for this design (specifically for the boat weight and righting moment by Torbjorn Lindersen who is now with Southern Spars). The mast has a single spreader / diamond and is two piece (joined at the spreader) for ease of transport / shipping. The rigging (side stays and forestay) is 9mm dynex dux side stays and forestay with spectra chafe cover custom made by Sydney Rigging. (12 ton BL). Diamond wires on mast are -6 nitronic rod supplied by Marstrom. Boom is custom carbon fibre and bow spirt specifically engineered by Simon Flitcroft (ex Sparcraft and Formula Spars engineer) utilising FEA analysis for this boat. Professionally built by Mark Mathews Marine. Halyards are all 6mm dynex dux and all are 2:1. All halyards are polyprop hollowcore to reduce weight.

    Why is it a partial Grainger design? The main hull started as a Grainger O75 trimaran main hull designed in 1985 but the main modification to the mainhull was to have the cabin cut down. Then Essential Eight (another Grainger tri design) floats were attached. Initial sailing indicated it could not power up as required so the owner with the help of Gordon Myers (Essential Eight builder) extended the floats “by eye” with a 3.6 feet reverse bow that added about 300 lbs of additional buoyancy in each float bow. The boat could now power up in higher winds and reaches. According to Grainger “Gordon Myers was one of the first people to recognise the importance of high buoyancy floats if you want good performance (and safety) and has been a strong advocate of them. They have replaced strongly rockered floats with the fine sterns quickly became outmoded as we realized that powerful buoyant floats got the main hull out of the water and delivered a lot more power.”

    The 075 main hull is strip plank foam glass, the Essential Eight floats are foam glass. All decks are Kevlar and the custom beams are carbon and glass. Now you want better rudders foils so you get Oracle foil designer Paul Beiker to design them. Next you need a new centreboard so its designed by DSS foil designer Hugh Welbourne with the latest design hollow trailing edge. Add 2 curved asymmetric foils in outer hulls as per ORMA 60. All foils are built by Mark Mathews Marine in female moulds. The foils are carbon over foam cores. All foils have shear webs through the centre joining the laminates on either side of the foil.

    Time Machine is very competitive in racing but is variable, under appropriate condition it can beat Diamond 24’s and keep up with a Formula 40 cat Boatworks. In other conditions such as a light air run it acts like a 26 foot tri. In one 7 race series it won one race overall, but came in as low as eight in other races. Final comments from the owner “The floats are original Grainger Essential 8 floats. Both Bare Essentials and Purple Haze (an Essential Eight) have since cut their's down the centreline and added width / volume. We felt that with the addition of the curved foil the skinny floats were ok. They certainly don’t hurt in the light to medium airs. We added 1100mm to the bow of the float purely to make the boat safer. With the original bows if you drove them in the boat would stop and want to pitchpole. With this design you can drive through and out without stopping which is much much safer.”

    This tri is technology plus and it could be yours second hand for under $100,000. This is not a KISS boat but if you want a trailable race to win boat it would be an interesting start.
     

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  5. oldmulti
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    oldmulti Senior Member

    A short one on a 40 foot Crowther racing cat John West which raced in the around Australia race. The 40 x 26.25 x 4500 lbs cat had a rig of 1100 square foot. This boat was partially mentioned on Page 27 number 401. The reason for it now, is detail I have about the rig. The rotating aluminium mast was 56 foot long with a section of 225 x 159 mm. The forestay as 8 mm, the side stays and diamonds were 6.5 mm 1 x 19 stainless steel wire. The forward diamond spreaders and jumper struts were 40 mm outside diameter with 3 mm walls. The rear diamond spreaders were 45 x 3 mm aluminium tubing. The boom was 180 x 120 mm aluminium. This boat was reasonably fast for its day and was eventually converted into a cruiser with a bigger central cabin.
     

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  6. oldmulti
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    oldmulti Senior Member

    Jim Brown (Searunner tri designer) spoke/ presented a paper at a William and Mary scholarwork series in 1982 about “New working watercraft and a return to former capabilities”. The top level entry point is the first web address which guides you to a PDF. The second web address is the actual PDF. Or just Google the above.

    [PDF] New working watercraft : a return to former capabilities | Semantic Scholar https://www.semanticscholar.org/paper/New-working-watercraft-%3A-a-return-to-former-Brown/d0f03807c4b0a584db3b9134972c0956f6bd6868

    https://pdfs.semanticscholar.org/ba47/ffaa461b360e192173326070c8e4f52ff3e3.pdf

    The PDF describes the problem of island people needing cheap non engine powered boats and the application of Constant Camber approaches to build appropriate boats. It describes several versions of Constant Camber building (including a CC mould that has a 3rd dimension curve in it) and shows many designs that can be achieved from 15 feet to 80 feet. The detail is good, the designs are interesting and a few CC boats may surprise you (eg a monohull). The PDF is 33 meg and is 107 pages. It is a fast read.
     

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  7. oldmulti
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    oldmulti Senior Member

    The TS 42 is an example of a modern cruising catamaran. The TS 42 is designed by Christophe Barreau and is 42.5 x 24.3 foot weighing 13000 lbs displacing 17000 lbs carrying on a 57 foot aluminium mast with a 600 square foot main, a 370 square foot solent jib and a 860 square foot gennaker. The hulls length to beam is 12.1:1. Sounds fairly normal and comparable to other designers work until you look at the position of the mast. Older cruising catamarans had their sloop rig masts back about 40 % from the bow waterline. Some designers have shifted their masts on their latest cruising designs to 45 to 48% back from the bow in their performance designs. Barreau has the TS 42 mast 54% back from the bow waterline. Several things occur when you do this.

    One, the centre of gravity of the cat is moved aft by moving the rig aft especially if you also have no substantial decking forward of the mast. Second, the rig is closer to the pitch centre of the boat resulting in reduced pitching and reduced total rig movement allowing with less disturbed airflow over the sails especially upwind. Third, the rigs sail drive centre of effort is closer to the centre of the boat allowing the cat to be driven harder in reaching and running conditions as there is an increase in diagonal stability which reduces the chance of eg pitch poling.

    So, compared to an older performance designs the TS 42 has about the same displacement and sail area but it can be sailed harder in stronger winds because it has greater diagonal stability and reduced airflow disturbance over the rig especially upwind due to reduced pitching. The hulls also have reverse bows to maximise waterline length and allow the bows to have more buoyancy down low to reduce pitching but allow the top of the bow to drive through waves and not provide too much buoyancy up high that would create a pitching moment.

    The TS 42 is built from PVC foam glass with polyester and vinylester resins. The boat is built in female moulds using resin infusion processes. There are fixed mini keels for good upwind sailing or a daggerboard option.

    Please do not assume you can just shift a rig aft and you will have magic happen. It requires a total rethink of your multi’s hull shape and buoyancy distribution to gain the most advantage of a mast further aft. As with EG Crowthers Twiggy trimaran design 32 x 29 foot with a mast 60% aft of the bow. The initial design had cut back bows, after 2 capsizes he released a MK 3 version with vertical bows and increased buoyancy forward in the floats. The later Twiggy’s could be driven harder and faster.

    This is one version of a cruising catamaran; some people want more accommodation for their length and will put up with more displacement and fatter hulls and not care where the mast is. The owner of the slow cruiser will wonder why a boat the same length will always sail faster, be able to more comfortable sailing at sea and spend less time exposed to storms etc. The jpegs and PDF’s give an idea of the boat.
     

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    Lucainbarca and bajansailor like this.
  8. oldmulti
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    oldmulti Senior Member

    The TC 601 is a Tim Clissold trimaran design. It’s a simple tri that you built a central hull then use a EG Hobie 18 hulls as floats and the 18 rig as the motive force. Clissold has tried to tap into the really modern well developed rig and using the hulls as amas saves considerable build time. The tri is 19.7 x 16.3 foot able to fold to 8.2 foot. The boats weight is 830 lbs all up. The mast is a 29.5 Hobie 18 mast with 188 square foot main and 80 square foot jib. To trail you remove 4 pins whilst on the trailer, the amas will fold fully to 2.5m for the road.

    The first builder has done a build blog (there are 8 build blog pages not 5 indicated on some pages) at Home http://lwr600.co.uk/TC601/index.html

    The builder made these comments “Using more exotic materials such as carbon has been the right decision for us, the central hull is a little over weight than planned at 115kgs but still quite good for its rather large surface area. All up we should be in around that 300 - 310kg with the rig and such as a bare boat. Yes you may argue that glass fibre may be cheaper but if you take into account we used 200gsm carbon fibre and 200gsm glass rather than the 600 - 700 gsm glass equivalent, you save a very large amount of epoxy which is actually now the most expensive component.

    The total weight will be about 370kgs if you want to include anchors and other boat needs such as outboards and cookers, but then no other manufacturer or designer does that. Its looking like 280kgs estimated for the bare boat won't be far off, F18 is 180kgs inc of all sails masts etc, we can loose a few bits like 1 daggerboard and the very heavy beams and other odds and ends, hull should be in that 90 - 100kgs + about 10kgs for the folding arms and a few ancillaries. I guess the first one being built will tell but we are learning on that one so any future builds will be probably lighter as already we have seen ways to advise on other builders to shave weight if needed.”

    The build of the main hull is internally 200 gsm 45/45 carbon fibre cloth with an additional 300 gsm unidirectional cloth around the cockpit area. The hull and bulkhead foam is 10 mm PVC foam. The external layup is 200 gsm 45/45 carbon fibre covered with 200 gsm e glass cloth. The half hull joint material includes a Kevlar strip. This builder’s aluminium cross beams are 75 mm aluminium tubes with water stays. The bulkhead that supports the water stays has 60 layers of CF glass to handle the loads.
     

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    Last edited: Dec 21, 2019
  9. oldmulti
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    oldmulti Senior Member

    Today a short piece with a bit of reading for those interested on multihull capsize. These are the factual information documents that have some viable analysis. The summary document of capsize of sailing multihulls contains only a subset of the real recommendations of the full 8.5 and 14 meg papers. Chris Whites analysis of leopards capsize is insightful. If I could afford it, I would own one of these cats tomorrow, this is a good Chris White design and many other cat designs in the same situation would have ended in the same result.

    Shuttleworth paper on seaworthiness. Considerations for Seaworthiness http://www.shuttleworthdesign.com/NESTalk.html

    http://www.wumtia.soton.ac.uk/sites/default/files/1238_BD.pdf 1994 Wolfson Unit initial study of Capsize of sailing multihulls. 8.5 meg PDF.

    http://www.wumtia.soton.ac.uk/sites/default/files/1441_merged.pdf 1999 phase 2 study of Capsize of sailing multihulls. 14 meg PDF

    http://www.wumtia.soton.ac.uk/sites/default/files/uploads/pages/STAB2000BD.pdf Summary document of Capsize of sailing multihulls. Under a meg PDF.

    Leopard Capsize https://www.chriswhitedesigns.com/leopard-capsize Chris White/ owners analysis of a 57 foot cat capsize

    Kurt Hughes also has an entry on his blog relating to wave action catamaran capsizes Feb 18 2013.

    Multihull Design Blog | Kurt Hughes on Catamarans, Trimarans, and Boat Design | Page 64 http://multihullblog.com/page/64/

    A summary of all the above is wide beams, low centre of gravity of the multi, minimise windage, full buoyancy cat hulls or floats for trimarans (at least 150% of all up displacement), not to aggressive on rig size and finally if the wave/sea size is bigger than you boats beam, take down all sails, put a parachute anchor out the front and wait for calmer conditions.
     
    Last edited: Dec 21, 2019
  10. catsketcher
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    catsketcher Senior Member

    I credit the designer and builder of this boat for giving their ideas a go, but would caution anyone from thinking this is a great cruising design. There are a few facets I think are contradictory.

    First is the concept of moving accommodation aft on a cruising cat. It makes me worry that the designer hasn't been down to the dockyard and looked at almost every cat - down on its lines - especially at the bum. This is why lots of cats get transom extensions - because people like putting things and weight aft - dinghies, solar panels, BBQs etc. You really need the accomodation/cockpit centred over the CB. If you put weight in this accomodation, even with the CB being slightly aft, the bum will go down quickly.

    Putting the cabin further aft exacerbates this tendency. I get that this is a product of racing but we should be careful of using racing design facets in our cruising boats. Take the heavily immersed stern.

    In light winds you should never have a heavily immersed stern - it produces huge amounts of drag. In a racing boat you can get the crew forward and get up to speed. When moving fast the release angle being shallower is better for higher top speeds.

    But why do it here? When cruising it is really hard to get it on and go fast. The waves and offshore conditions mean that almost every performance cruiser is throttled back in winds over 18 knots. Otherwise your family wants to get off and you could start breaking gear. So don't include ANY design compromises that would allow you to go a little faster in 20 when they make you much slower in 5-7 knots. You want to be gliding along in the lightest of breezes under the screecher whilst everyone else is motoring. The heavily immersed stern is pretty dumb design - ensuring you will be motoring in 7 knots to get you a bit faster when the kids shout at you to slow down in 20.

    Same with the rudder volume. Rudder volume is the area of the rudder times the action arm of the rudder CLR to the boat CLR. Having a large rudder volume makes boats steer really well - think Farrier tris. They can tack and steer beautifully with their large distance between rudder and CLR. On this cat, getting everything aft reduces rudder volume. Then the designer has put small rudders on the thing too. I would hazard a guess the boat would not tack terribly well in a chop.

    Then there is the lack of cantilivered doubles in front of the mast. I get that you may want to get rid of these, but when you have reduced cabin size it seems dumb to reduce the interior even more by getting rid of the cantilever bunks. They provide excellent accomodation on a performance cat and pound incredibly rarely - pretty much never. Why get rid of the hard deck, and extra room for the non-usable performance premium?

    For me, this design is an example of racing features having a very negative effect on a design. If you really wanted to make the cat go faster, make it more normal - Schionning, Pescott etc and then get rid of the crosscut dacron sails this one has and put tri radial laminates on it. Thats's it and you will go faster in all conditions than this one. It seems like someone has picked all of the easy to spot design features of an Ultime tri and plonked them on a cat. It is counter to what the boat is supposed to do and impacts a lot on its utility.

    I don't know why I am a little cranky about this design. Maybe it is because it has things like a seat hanging off the edge of the gunwale - like really? A cruiser is under autopilot most of the time and even if it isn't anyone cruising for more than 3 hours wants to get out of the sun/rain/wind. Then there is the slight problem of getting into a dock and whacking the seat with the marina piling - the whole thing just makes me go - Why? Why would anyone build a boat with features so anti cruiser on a cruising cat?
     
  11. oldmulti
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    oldmulti Senior Member

    Catsketcher has brought up some very relevant points about the TS 42 design for a cruising boat. But the problem here is what is cruising? For some people it full time on the water for a year, for others it’s a 2 week vacation. I have done both forms. In cruising mode it was the number 2 jib, a reef in the main and dial in 7 to 8 knots in anything above about 12 knots of wind. No need to worry because the wind would have to hit 30 knots before you would have to change the sail configuration. The motion was comfortable, people were relaxed. But occasionally the winds were really favourable and on one coastal hop we averaged 16 knots for 100 miles. 6 hours trip time versus 14 hours at 7 to 8 knots. Yes, we had to use the heavy number one and full main and have a good person hand steer all the way, but once it was set up, we reached along happily with no more discomfort than going up wind at 7 knots.

    What I am saying is some people’s version of a cruising boat is a cruiser/racer with many mod cons other people have a racer cruiser emphasis. I want a boat that can go fast but I am willing to dial the speed back for comfort when conditions are not favourable. The number of people you have with you is personal preference but a few separate cabins help and double on wing decks aft of the mast are the best spot for me. Catsketcher, I fully agree about the steering seat. Anyone who has done cruising in Australia soon learns about the need for sun protection and hand steering for long distances is a pain.

    The following information was reported on page 35 number 520. A French guy paid for some serious tank testing work done in 1980 to understand the effect of stern shape, trim and displacement on a 27 foot catamaran hull for hull speeds up to 14 knots. Up to 3 knots an immersed transom had no effect on boat speed. Almost every catamaran I know, when sailing fast, drags an immersed transom on one hull. So what is the real effect of an immersed transom? The best guide I know came from 12 meter America’s cup racing yachts. Britton Chance designed a 12 meter called Mariner that had a chopped of section aft under water (see jpegs) to maximise fullness in the ends and minimise the mid section size. A bit of a rule beater. Mariner in its original version was 2% slower than Intrepid around a race course. 2% in America’s Cup racing is death and instant modification to the stern happened. PS Mariner was tank tested in the same tank within a few weeks of Intrepid one of the most successful 12 meters of all time. As Olin Stevens said its not the data from tank tests, it’s how the data is interpreted.

    If the same logic applies (big assumption) for immersed transom multihull cruiser racers 2% would not be noticed. The water looks disturbed therefore it must be slow logic applies here. We need some rich guy to fund some real research to help find the truth.
     

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  12. oldmulti
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    oldmulti Senior Member

    Dick Newicks tris are legendary but he was very evolutionary in his approach. During the early 80’s his boats were adopting many features to gain additional speed. Some were later changed, other features were taken up by others and refined. The 1986 design was intended to be a racer cruiser aimed at things like the OSTAR. The tri is 40 x 29 foot displaced 5500 lbs and on a 51 foot Stressform wing mast carried 985 square foot of sail including the mast. The main hull length to beam of 13:1 and the Bruce number of 1.77 indicates performance plus. The evolutionary features on this boat were the Newicks “half moon” floats and the dagger foils angled at 45 degrees in the float. The half moon floats were conceived to provide sufficient buoyancy at low speed to support the boat and at higher speed the half moon shape provided dynamic lift for additional righting moment. As time went by Newick reverted to more conventional float shapes as it provided better all round performance.

    The angled dagger foils were a different story. They were the fore runner of the curved float foils seen on many of todays high performance trimarans. It took time for designers/builders to work out how to locate and strengthen the dagger cases in floats. Then it took time to develop foils shapes and construction techniques to get the foils to work across a wind range and stay in one piece. The final problem was maximising the total platform torsional rigidity to help maintain a consistent foil angle of attack. Also it took some time before centreboards were removed from the main hulls and rudders to appear on floats.

    The structure of similar 40 foot Newick racer cruiser tris were 3 layers of 3 mm western red cedar on spruce stringers and frames with Honduras mahogany keels and stems. Decks are 6 mm ply 9 mm Verticel 3 mm ply inside. All covered with 4 oz polypropylene. Bulkheads are 9 mm ply with top and bottom timber flanges on the mast and rear cross beam bulk heads. I do not know if this specific design was built. The first jpeg is the design we are talking about, the next jpegs are of a 36 foot tri with half moon floats and dagger foils, then some other 40 footers.
     

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  13. waynemarlow
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    waynemarlow Senior Member

    Sadly the boat was never completed but still alas sitting in my garage waiting for that moment in time when finances and time allow it to be finished. A few updates are that all up weight will be in that 320kg bracket when using a F18 as a donor.

    In doing a lot of the development of the folding and water stays, its really apparent that the F18 hulls are heavy heavy duty beasts at about 55kgs each. They may have a lot of volume but with the penalty of ground handling is a ***** on your own. Its probably the main reason why the boat has never been finished as for my use, it has to be able to be folded relatively easily and with the F18 Amas at 55kgs, its a handful. Do we bite the bullet and go down the Farrier route as most seem to end up doing or do as the very original idea had been, use old A Class hulls with C boards which there are plenty about at the moment. The downside is you then loose all the nice sails incl of spinny and jib, masts and hardware that the F18 brings.

    The other issue has been a relatively unforeseen one, that of the F18 masts are heavy and the boats shape and mast location is just such that you can't really easily lift the base into position without clashing the cabin roof nor can you clamber over the front of the cabin carrying the mast base to the ball for the rotating mast. That and in combination that the dagger board goes fully through the cabin area which means it badly restricts the cabin area, has meant that I've taken the decision to move the mast back about 350mm to push the dagger board out into the cockpit the same as the Pulse 600 ( Tim will be tut tutting but I've long thought that practicality may have to ensue and that the mast was too far forward ). That has the repercussion that the ring frame ( its not a bulkhead ) for the crossbeams is now not in the right place for the mast. But we need the Amas as far forward as we can ( they presently are matched with the bow of the central hull ). I've changed the design of mine to now have a small carbon beam from the ring frame back to the rear of the cabin wall and to put a simple Ali tube beneath the mast foot directly to the floor which can be removed once the sail is down. The F18 dagger boards can now be used as they are long enough to fit nicely through the cockpit floor.

    The central bulkheads that take the ama crossbeams are ring beams to maximise the cabin and gobsmackingly heavy duty but the calculated loads of both the inward crossbeam ends and the pull of the water stays was in the tonnes. With the removal of the dagger board case through the cabin, with a simple infill floor doubling up as the chart table when sailing, its now probably wide enough for a cozy two sleeping arrangement. The cabin is bit like a tardis and is big inside for its length.

    Perhaps this winter I may get to finish it.
     
  14. catsketcher
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    catsketcher Senior Member

    I built a stretched Echo that has only recently started sailing. It was started in 1997. We talked with Dick about half moon amas but in reality using float shape for dynamic lift is a high drag method of producing lift. If you think of the hull as an airfoil then the long thin ama is a very low aspect foil and hence has low efficiency.

    What you really want is high efficiency foil. So make hull short fat and stubby (higher aspect for planing surface) or don't go for dynamic lift. Look at Formula boards and other sailboards. If all you want is speed go wide and short, this produces lift more efficiently than a long thin hull.

    So I think that the new moon ama was an evolutionary dead end. When it came time to build the Echo amas in 1997 Dick was playing around with a new computer design package and sent us new amas every week or so. None were new moon.
     

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  15. catsketcher
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    catsketcher Senior Member

    I don't agree with the need for tank testing - it has been done hundreds of times in Australia with real world cats and tris.

    In one instance my mum's Crowther 10 was an absolute dog, she had installed diesels aft and the transom was immersed about 25cm. She sailed terribly. Then she got new props and a transom extension and was turned into a totally new boat.

    Many owners have installed transom extensions on many styles of cats. A mate even has a mould to put extensions on Seawind 1000s. Every person I have talked to about them say that speed is increased and pitch resistance is improved. So with lots of real world tank testing done I think the verdict is in. Most cruisers find their cats sail better with only minimal transom immersion.

    In reality, transom immersion is tricky. The tank tests noted may have been completed on a canoe sterned hull, where transom immersion may not make much of a difference. My take on the mentioned cat design is that the transom is wide and heavily immersed. No boats I have ever sailed - cats, tris, monos, sailboards, or dinghies sail fast in light winds with a heavily immersed transom so I would view the tank test results with healthy skepticism until I saw the plans of the tested hulls.

    As a follow on from the fast trips I have done two of 160 miles in 16 hours, which is about as fast as my family and I can take. The boat had more potential in her but it was the accelerations and a regard from not breaking stuff that slowed us down. My pretty normal, transom kissing 38 footer could take more than me. That is almost always the case when the wind blows up. Where I really want more speed is when the wind sighs. But a better speed in light winds doesn't look cool because the features are not those shared by the big ultimate multis - that is why our cruising boats should not look like modern fast racing multis.

    As an aside - I remember watching John West get launched in the 80s. We all looked at her immersed transoms and wondered whether Lock got it wrong. Upon asking Lock said it was because the boat was going to be fast enough that speeds lower than about 6-7 knots were not important design considerations, so the transoms were immersed to better handle higher top speeds.
     
    Last edited: Dec 21, 2019
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