40 ft cruising cat design. First steps.

Discussion in 'Multihulls' started by bscatam, Mar 28, 2016.

  1. bscatam
    Joined: Mar 2016
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    bscatam Junior Member

    BD front.
     

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  2. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    I agree with Richard regarding a softer front to bridgedeck, also some aft trim/some extra clearance forward will be good too as she trims forward while sailing- same for sheer.
    I feel Catsketchers comments on volume to be valid also, some volume aft, looks a bit slack(although slippery) to carry saildrives, dinghy etc.
    Some extra thickness to cores(foam?) in bridgedeck and cabin top might yield some labour savings with cleaner and simpler structure over all those stringers.

    Jeff.
     
  3. Alik
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    Alik Senior Member

    1. New ISO12217-2:2013 imposes quite tough requirements to flotation of catamarans once inverted. The boat should float, should have certain trim and escape hatches should be accessible - they regulate the depth of hatch submersion. Thus, catamaran design should include some play with buoyancy volumes, at initial design stages. In these sketches I don't see any attempt to provide inverted buoyancy.

    2. Buoyancy aft does not seem sufficient to me. Weights tend to accumulate at stern, say a tender or larger genset, and I don't see how those are catered for.
     
  4. bscatam
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    bscatam Junior Member

    Thank you guys for remarks. Thank you Alik (спасибо;)) This is all work in progress.

    Oleg
     
  5. Gus7119
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    Gus7119 Senior Member

    Just a quick question. Do you reqire over 20 bulkheads with strip plank build? Would have thought that was over the top unless Im missing something. Also are you going for forward double births or just lockers forward of cabin? As was mentioned a diagonal line rather ten curved may assist with slaming. Rod Denney offers great advice if you can find him on forum.
    Also there are many people here that have agendas and will shoot down your efforts. Luckily you haven't had any of these guys pop up on your page yet all these users gave great knowledge to offer. But after extensive discussions while going through my design stage I was lucky to quickly work out those that build and design professionally from the couch critic which you'll need to do as you continue with your build so as you take advice from people with the practical knowledge an experience.
    As said above you have some good users helping out in your new forum. So can't wait to follow your progress.
    Cheers
    Gus
     
  6. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    As extra on the weight aft thing. Many Australian designs get an extra metre added onto their sterns because the owners have put more weight there than the designer reckoned.

    If you work out the weight of saildrives, a RIB with an 15hp motor, strong davits, 400 watts of solar panels, wind generator, BBQ, and fuel for the motors it could come out to close to 800kg near the stern of the boat. This pulls the CG and CB aft.

    As to the inverted flotation aspect - I am happy to see this part of the picture. Most cats have pretty terrible inverted flotation. This is easy to fix up forward. Don't have walk in accommodations in the bow.

    When I built my cat I was very worried about capsize. I previously had a Twiggy, a design known for flipping at the time. So I asked the designer to design in inverted flotation. He was very surprised and said that no onme had ever asked him for that before. How has it affected my cat? Some aspects worked well and others have been redone.

    One important aspect is to include a totally watertight bulkhead at about 3 metres aft of the bow. You can't get any further forward than the end of the bedroom. Has this been a problem? No only an advantage. The area is a large locker with access from on deck and a floor at half freeboard height. Together with another bulkhead that divides this area in two we have a great collision bulkhead and inverted flotation up front. As for usability it is great. I have ample storage for fenders, reachers, sailboard equipment anything light and the water never penetrates the interior. A row of top opening hatches in the nosecone also provide inverted flotation.

    I do not understand why most catamarans allow you to walk up the front of the hull from inside - it is less safe and allows water to get into the interior from the hatches. You can't use this volume easily anyway.

    Getting inverted flotation aft is more problematic. Dividing the accomodation with bulkheads is one answer but not a good one. If you used foam for the deck as well you would get about 12m x 7m area with 18mm foam gives 1.68 metres cubed which gives you about 1680 litres of flotation. Coupled with the bulkheads up front this should get her to float high and be nice and easy to live in.

    Also paint your underwings with non skid. It means you don't have to fair the large area and so saves heaps of time. As to escape hatches - I have an axe tied onto the outboard nacelle when cruising. Hatches can be a pain near the water.

    Gus - they aren't bulkheads they are mould frames. They are for laying the hull material over when building - often made from MDF and thrown away after use.

    cheers

    Phil
     
  7. bscatam
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    bscatam Junior Member

    Gus, there are frames for stripplanking not bulkheads.
     
  8. bscatam
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    bscatam Junior Member

    Flotation forward (B)
     

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  9. catsketcher
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    catsketcher Senior Member

    You need much more

    I think you are falling into the same thinking as many other cat builders with the amount of divided flotation your boat will have. The amount of volume behind these bulkheads is very low. This has three effects.

    One - the chance of the bulkheads containing all of the damage is lowered. As the volume is small it is likely that a big hit with a container, log or rock will damage more than the area sealed off and water will still enter the rest of the hull.

    Two - a likely event is that the bulkheads will be needed to reduce the flooded volume of the rest of the hull if the centre of the hull is damaged. This part of what the bulkheads do is vital.

    Three - the inverted volume is negligible. If you want to have a habitable catamaran when inverted you must include an area (the bows are easier) in which to include substantial inverted volume. Your boat will be very low in the water if inverted.

    As a cat builder and sailor I still can't understand why people want to get to the front of their boats from inside. Do you really want a toilet next to the bed? I like the toilet a long way away from my wife so my ablutions don't spoil the image she has of me (at not least straightaway). What are you going to put up there? A single berth. Why? No one will every use it and you would be much better off using the bridgdeck cabin for extra berths.

    In the end the pointy end gets used fore sails, fenders, swim fins, wetsuits, sailboard gear, spinnakers, reachers - pretty light stuff. All stuff that can be wet. So in a normal cat this wet stuff gets thrown into the interior - when wet. Why? I don't get it. We never get water into our interior on Kankama but used to all the time in our previous boat with a forehatch that was not sealed off.

    On Kankama I could give you an underwater drill and you could go and put a hole anywhere under the waterline you like. It has a 40% chance of being in a sealed void. If you don't put it in a void then I can still sail home on the flotation the voids provide. It is a such a good feeling to have and easy to design in. I really don't get why designers don't put more voids in.

    I have drawn what I would do. Kankama has a bulkhead a A. In front of that is storage. You could have a full bulkhead at B if you wanted to move the bulkhead forward. Also I have a floor at C. This means when I throw in the fenders, reacher, kite I can reach them from on deck. I love my flotation system - it makes the boat drier, safer and easier to use. And I have peace of mind knowing that if we are inverted we have somewhere (On the bottom of the floor- hence the axe) to stay for a couple of days until the EPIRB lets someone know we could do with a lift from a passing ship.

    cheers

    Phil
     

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  10. catsketcher
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    catsketcher Senior Member

    Reasons for big voids

    Read these if you don't think cats should have larger sealed voids

    http://www.maritime-executive.com/features/a-tale-of-survival-after-whale-hits-catamaran

    I mean - Come on. What is the point of having an unsinkable boat. Sitting around the beach campfire "Well cats can capsize but they can't sink" when they half sink and capsize because of a large hole in their side. It is so easy to design in large voids that would have allowed the hole to settle the boat but allow it to make port under its own steam.

    https://www.youtube.com/watch?v=8CLlwKtdW94

    Look at these guys on a great boat with a bulkhead up front. She floats high up front doesn't she. That is because the crew have somewhere dry and warm to wait for help in front of a big fore bulkhead. Read here.

    http://www.chriswhitedesigns.com/news/anna_capsize/post_capsize.shtml

    Then we have a cat that again hits something and capsizes. Why? There is no lead in the boat.

    http://www.adelaidenow.com.au/news/...-kangaroo-island/story-e6frea83-1225972175636

    It does not take much research to learn that cats are flipping and capsizing because they are half sinking. If it was not for EPIRBS and helicopters then we would be having the large numbers of deaths we had in the 60s. Still the case below illustrates the need for better capsize habitation.

    http://www.yachtingmonthly.com/news/missing-sunsail-yacht-found-after-a-year-34308

    In the 70s and 80s designers tried to get people to design in inverted habitation - somewhere safe to sleep, working out ways to keep water running out from tanks, how to keep food dry etc. Tris are inherently great at capsize habitation and cats are pretty lousy unless you design it in. There was a book published by Multihulls magazine called the Capsize Bugaboo which dealt with many issues. Jim Brown writes about the issue in the Case for the cruising trimaran.

    What I don't get is the head in the sand mentality from designers and builders. Why are cats not designed to cater for accidents? It reminds me of the hassle Ralph Nader had getting the car industry to think of safety. Car makers wanted romance and style. Educating the public about safety would counter the message and scare the consumers. Cat designers don't sell safety - they sell space and speed - the romance and style of the cat world.

    Sad that if we don't remember history we are doomed to repeat its mistakes.
     
  11. Gus7119
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    Gus7119 Senior Member

    I have seen that vid before. I was under the impression that most modern off shore cats floated inverted sounds liike thats not the case. Out of the 150 or so crossings of oceans by friends and I on cat I have only known of 5 rescues all cause by striking something and all 5 stayed afloat. Theses were not coastal cats.
     
  12. Gus7119
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    Gus7119 Senior Member

    Couldnt edit last post so to add;

    I find it strange that the same arguments are being had. A cat is a boat and some new sailors zee them as a house or floating caravan as they used to be called. Now that they are foam not just glass they have some onverted buoyancy built in and its not to hard to add inverted buoyancy to even cats 25 to 35 foot. But if your doing a build itd by unlikely your a new sailor. And after been a baboon that has been rescued I can honestly say Id prefer to stay on a boat rather than a raft or in the water.
    I love the idea of the righting of a cat thats something Id love to see, the bow pulled over the stern. When you see vids of coastal rescue trying it they alwayz pull across the boat from one hull.
    But if you can't include this sort of thing on a boat the size of a cat theres a problem. I understand the rental boats not doing it as they rarely leave a harbour and in alot of cases in Australia are not allowed to go outside headlands especially in NSW where Ive worked. But for cruising boats its something I'll be making sure my buid does and its only 30 foot.
     
  13. bscatam
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    bscatam Junior Member

    After some corrections.

    I have made some corrections aft and forward.
     

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  14. Gus7119
    Joined: Dec 2015
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    Gus7119 Senior Member

    Quick questio to all. Is there much diffrence if you dont have anything forward of the mast beam, ie births. And just net that area on slaming.
    Is my thinking wrong or would it reduce the weight allow the boat to raise after going off a wave before it get to slap that area. Or does the removal of say 1400mm which is an approx width of a double bunk, have no real real effect.
    Full disclosure. Am wanting to start build that has been delayed due to a family death, and have decided I have enough space in the hulls and bridge deck for everything I need such as births, 2x doubles stern 1x single front of one hull and 1xhead front of other hull.
    Cheers
     

  15. DennisRB
    Joined: Sep 2004
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    DennisRB Senior Member

    A friend of mine with a few posts here built a vacuum infused 40 footer. He only has small storage areas where the forward bridgedeck births would be. The main births are aft.

    Nice boat.
     
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