open bridgedeck catamaran design

Discussion in 'Multihulls' started by BobH, Apr 17, 2015.

  1. BobH
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    BobH Junior Member

    Got it

    Your analysis makes plenty of sense. Hook's Law and all...complicated dynamics. I'll have to think about it....more clearly defined goal. Thanks.
     
  2. BobH
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    BobH Junior Member

    Right, what's needed is an arrangement that results in a cascading process...stiff to a limit with a timed neutral band before release (set just right), then soft, and then reset...hmmm, more thinking, some experiments.
     
  3. John Perry
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    John Perry Senior Member

    Re. drum rudder, if I were making it I think I would prefer to just have a round hole in the bottom of the boat and to make the axis of the drum normal to what would have been the hull skin at the centre of that hole. With the general hull shape you are proposing it looks as though it would then only require slight adjustment to the hull skin to fair it nicely into a slightly domed drum end face, or even a flat drum end face.

    You are only using part of the diameter of the drum to house the rudder blade, presumably to avoid the rudder being overbalanced. I would have thought that if you rake the rudder blade aft relative to the drum axis it could be hydrodynamically balanced with a somewhat smaller drum diameter for a given blade cord - that would make it easier to fair the drum into the hull and allow slightly smaller bearings to be used.

    The FEA software I used in the early stages of my catamaran design was Ansys. I said in my previous post that I did that design three years ago, actually I now realise that I did preliminary FEA analysis with a cruising catamaran in mind more like six years ago - how time flies - I must start some boat building soon, not just drawings! But anyway, summer is coming again and we will probably be off sailing for most of the summer, so no boat drawing or boat building until autumn. I don't have Ansys on my computer any more, so if I do more FEA, which I expect I will do at some point, I would probably use the FEA software in the Solidworks suite, which I understand is based on Cosmos, another program I have used in the past.

    With regard to anchoring, I now see that your proposed method is much the same as I am thinking of except that you plan to stow the anchors and cable at the aft end of the bridgedeck rather than near the main cross beam. For attachment of an anchor bridal, I have drawn a large U bolt on each inner topside a few inches aft of the top of the stem. At the scale I posted the images it is only visible as a bit of a smudge. To me this looks a bit neater than having a projection forward from the top of the stem.

    I have the Gougeon book (for anyone who wants to look at it it is a free download) on my computer so I took a look and, as you say, there are photographs showing a deck built as two layers of thin ply glued to a core, in that case the core is a massive 75 mm thickness of honeycomb, not foam. There is also an example of a cockpit bench made with more closely separated plywood skins and a foam core. So I suppose it must be a viable method. In the long term I would be a bit worried about water intrusion being hard to detect. As far as I can tell, the main advantage of plywood hull construction is speed of building. It also has low material costs although that may be secondary since the cost of material for the structure is a fairly small part of total yacht building cost, let alone the operating cost over a reasonable period of use. I do wonder if using this sandwich ply construction with foam core would largely negate both the speed of building and the cost advantages of plywood, in which case grp foam sandwich is the obvious alternative.

    Dont mean to be over critical - just thoughts

    Elastic elements in rig controls is moving a bit away from the main subject of this thread, but here are a few more thoughts:

    I think the reason you might want this elasticity is different for a multihull than for a monohull. A well designed monohull with deep ballast can be sailed at large angles of heel without being unsafe, so the reason to control heel by changing the angle of attack of the sails is to keep the heel near to the optimum angle for best performance as wind strength changes. I guess that if you could plot the angle of heel for best performance against wind speed for a non-downwind course it would be a smoothly rising curve. Linear elasticty in the rig can possibly track that curve well enough, so no need to deliberately add non linear elastic elements. Just the sail twist and mast bend may provide the required elasticiy, or possibly more than the required elasticity.

    It is different for a multihull since the optimum angle of heel for best performance is within a few degrees of zero for any wind strength (unless you are considering flying hulls to reduce water drag) hence the reason to introduce elasticity in the rig would be to reduce the risk of capsize, not to improve performance. The optimum angle of attack is going to be close to that which produces maximum sail lift up to the point at which there is no longer an adequate factor of safety against capsize. Once that point is reached you ideally dont want any further increase in sail lift regardless of wind strength increase - so maybe it would be worth considering deliberately introducing some non linear elasticity for a multihull. I say maybe, since as with extra gagetry on a boat you need to think whether it is really worth the weight and complexity.

    Something like a gas spring could possibly provide the non-linear elasticity. At low force the piston would be up against a stop so the device would be effectively rigid. When the force rises above a threshold the piston lifts off the stop and the device can then elongate with little increase in force, depending on the swept volume relative to the total gas volume. I dont see why such a device would need the preload (gas pressure) to be released then reapplied when tacking. If, for example, it were incorporated in a mainsheet, it would work the same way on either tack with no adjustment needed between tacks. However, the gas pressure would probably need adjustment when reefing and possibly for different apparent wind angles, although don't they say that fast boats are always close hauled. Conventional sheeting arrangements intended for manual operation may have too much friction and require force input over too long a distance for such an automatic system to work well but one can imagine a double ended sheet with two main tails, one connected to the automatic system and the other to a multiple pulley purchase for manual trimming.

    I can see that if a multihull is subjected to a short term wind gust and a longer term wind gust, both of the same peak wind strength, the long duration gust might cause a capsize whereas the short one might not. This is because to cause a capsise, not only does a certain sail force need to be exceeded but it needs to be exceeded for long enough to overcome the roll inertia of the boat, i.e. there needs to be a certain impulse (force integrated w.r.t. time). Wave action may also play a part of course. I am sceptical about the feasibility of an automatic system being able to guess how long a gust will last before taking appropriate action - when a gust starts, how can the system know what the duration of that gust will be. Perhaps a human helmsman can do so to a limited extent by looking at the effect of the wind on the water surface. And maybe a sophisticated electronic system might do so, perhaps using a scanning laser anemometer to determine the wind velocity distribution over an area covering perhaps a few hundred meters to windward, but that is getting a bit far fetched. I cant see a practical mechanical system being able to do more than limit further increase in sail lift once a certain threashold is exceeded and even that may prove to be more trouble than it is worth.

    Brian mentions the 'culprit sail' that appeared to cause the recent capsize of the Gunboat. From the video link posted on this forum it would indeed appear that failure to release the mainsail in addition to the genoa caused the capsize, but had the mainsail been released and the genoa left sheeted in, the capsizse might still have occurred, in which case we would have to describe the genoa as the 'culprit sail'!
     
  4. catsketcher
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    catsketcher Senior Member

    Nice looking boat Bob

    As everyone else says I think there is no need to go ketch in this size, I have no trouble sailing a 38ft sloop cat by myself. I am a big fan of wishbones and have a few comments about experimentation.

    I had more than a few innovations on my 38 ft cat when launched - only half of these were any good and I have been removing and replacing them over Kankama's 15 years- the vertical tiller, the folding davits, the large aft storage box, the partial cockpit dodger, the spectra mainsheet bridle, the no tiller bar steering system are all gone. In the end Kankama looks pretty normal apart from her wishbone mainsail.

    I would be very critical of the need for any innovation you will actually build. In the end you will either love it and keep it or hate it and have to pull it out and rebuild. I love my boat and there are very few cats that would suit me but I should have been tougher on myself when building her.

    A few comments

    - the bows - I don't understand why any cruising boat has reverse bows. Bow shape should come from hull flare and it seems hard for you knuckle hull shape to resolve itself into reverse bows - they will be wetter and date your boat when they become unfashionable.
    - the rig - go for ease of sailing for sure - just don't throw babies out with the bathwater. A good divided wishbone cutter rig is easy to build, cheap and super easy to sail with. I can throw away my winch handles and go sailing in 20 knots (staysail up - genoa furled). You don't have to use an untried rig full size to get easy to use.
    - Diesels - I would recommend petrol outboards - they are fabulously reliable and well engineered. If you really do need to go down the diesel route don't make your own bevel gears - get a sail drive, mount it sideways and make a round housing to hold the gear box onto the motor and lift it this way. Fastback 43 cats used this method successfully.
    - That mainsail track - I really don't like mainsheet track. If you have two wishbones on the boat put another one on the bottom of the main and save lots of dollars in deck gear.
    - No dodger - The drawings of my cat started out the same as yours - no cockpit protection - I launched with a small dodger and after 15 years I now have the third version which is here to stay - it looks like a normal cabin. How will you get from one side to the other? What about the rain? What about spray to windward? It gets old without protection really quickly. A year after cruising the small dodger got enlarged and 5 years ago replaced with a foam cabin with removable doors and windows - much nicer. fish trap buoys offshore
    - Rudders - you will run aground - how will these rudders cope? Don't say you won't hit things. My rudders have popped up after hitting fish trap buoys offshore I didn't see until I was looking at them out the back. Get them to kick up.
    - Mizzen - why go freestanding? It looks like a world of pain. Chainplates are easy to make in composite and spreaderless is fine if you want to build the mast yourself.

    Be rigorous with why you want to change the status quo. I wish I could go back in time and slap myself in the face to stop myself from innovating too far - hundreds of hours and heaps of dollars could have been saved and put to better use elsewhere in my life. Be very careful about why you want to change.
     
  5. BobH
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    BobH Junior Member

    Thanks for sharing your experience. Seems to me you sincerely want to save me from climbing the same learning curve.

    Do you have any photos of your rig?

    Also, did the Fastback 43 use a lifting leg like the Sillette Sonic Cat drive with a universal at the pivot (with engine fore/aft) like the Gemini in the US? The few photos I can find show a drive leg like the Sillette type.

    http://www.boatdesign.net/forums/archive/t-37597.html ...find Fastback...there's an entry with links to this photo:

    http://www.boatdesign.net/forums/at...t-wharram-catamaran-yanmar-diesel-leg-006.jpg
     
  6. catsketcher
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    catsketcher Senior Member

    Attached is a photo of Kankama in the Whitsundays off the Queensland coast. She has eventually ended up looking pretty normal. The view from inside the cabin is great and we can sit inside and see all around when sailing. The wishbone system makes taming the main very easy. I love the spreaderless rig as it cheaper, allows the use of a staysail (we furl the genoa above 18 knots and sail with the storm jib to windward), has the same amount of wire as a double diamond rig, allows me to reef and pull the mainsail up on a square run in 25 knot tradewind conditions (no spreaders to poke into the sail), wishbone means you have nothing to brain yourself on, saves thousands on deck gear (traveller, blocks, winches) vangs downwind beautifully, prevents mainsail chafe (no mainsail oscillating up and down with the waves), and cleans up the cockpit no end. For me the lack of wishbones on other cats and the design of the cabin was one reason I never thought of selling Kankama when we weren't cruising for 12 years.

    As to the Fastback type system - I have no photos.

    Mount the saildrive (with saildrive unit) motor ACROSS the boat, not fore and aft. Then at the junction where the gearbox meets the motor make a ROUND housing (your engineering friend will have to do this) to hold the gearbox onto the motor itself. This allows the gearbox to rotate. Make an extension for the saildrive leg out of aluminium or fibreglass (I would do it in glass) and turn the bottom of the leg 90 degrees so I goes fore and aft. Reroute the water inlet through the hull.

    Seems like a good system. Sonic drives can be awfully noisy and heavy. I just gave away one that was under my house that was discarded off a friends boat. He went twin outboard legs with hydraulic drives mounted where the powerheads were - fed by a centrally mounted diesel.

    You can hear Sonic drives miles away and all of the ones I know of are locked into a central position.

    cheers

    Phil
     

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  7. BobH
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    BobH Junior Member

    Phil,
    Can you give some particulars on the rig...mainsail area, mast height, mast section measurements. That would be appreciated. You called it a cutter rig, so it's masthead not fractional? Any idea on how far back (degrees from athartship) the shrouds are? Also, what are what looks like two small chainplates on the side of the deckhouse for? Thanks in advance for any answers.
     
  8. redreuben
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    redreuben redreuben

    Phil,
    First thing I thought of when I saw your rig was the Chamberlain Cirrostratus trimaran, lol, same heritage, I checked out your youtube vid, very nice. I would like to play with a wishbone rig, looks like the ideal cruising rig.
    Cheers, RR
     
  9. waikikin
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    waikikin Senior Member

    Same here, looks to eliminate some exxy & potentially dangerous traveller track/cars & control. I'm wondering if the trade off is running angles & interferance to shrouds but maybe the "vang" control it gives makes that irrelevent, Phil references less chafe, plenty cats not ideal with shrouds led well aft maybe a little extra windage also? The simplicity of Phil's rig with no diamonds is cool, I'm imagining two or three very similar sets of hounds along the height, - lowers & inters inside edge of side deck?

    Certainly plenty of pluses there by the looks, the inverted V has me intrigued?

    Jeff.
     
  10. ThomD
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    ThomD Senior Member

    "- the bows - I don't understand why any cruising boat has reverse bows. Bow shape should come from hull flare and it seems hard for you knuckle hull shape to resolve itself into reverse bows - they will be wetter and date your boat when they become unfashionable"

    That is sorta what I thought about the whole thing.... Except. The theory on the reverse bows is that they give less drag and spray. When I first saw them I just thought it was the usual nonsense about saving a few pounds but there are videos where that kind of bow at some scale is running cleaner than older designs. Makes one think, though whether it makes sense on boats that may be wave piercing at times, or with different scale factors, I haven't figured out.

    The criticisms of these reverse bows are similar to criticisms of plum bows, but those do not date boats as much as overhanging bows do. It is true that one somehow has to fair the bow with the rest of the boat, particularly in ply.

    Overhanging bows made use of the idea that the shape of the boat could help with it ridding in a seaway, with keeping dry or generating lift from how the shape delivered lift from the vectors acting on it. Plumb bows seemed more a mater of where the displacement is in the boat (more forward for the same buck probably better). The new reverse bows are another trip down the memory lane of how the shape interacts with the seaway. Shape and displacement being issues in any design, along with other factors, but suffice it to say the trade offs are not always the apparent ones. I do think with ply the toothpaste tube, as nearly as it can be recreated, is pretty enticing.

    https://www.youtube.com/watch?v=GJsogw9fHE0
     
  11. catsketcher
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    catsketcher Senior Member

    Thanks for the kind comments on Kankama - proud father I am.

    As to the rig, here are some particulars

    Mast height - 14.1m
    7/8ths cap shroud - 12.3m
    intermediates - 8.2
    lowers - no inner forestay - 4.1m

    Mast section is Allyacht 200mm fore and aft section IM20 I think

    Wishbone attaches with block and tack to chainplate at front of mast in at same height as lowers - provides forward thrust on lower section of mast obviating need for baby stay. Lowers go tight when wishbone is pulled tight.

    Caps go to outside of hulls. Lower and intermediates go to base of cabin. (Probably would go to cabin top outer edge if I was to do the whole thing again - nothing to bang toes on)

    Sweepback is 30 degrees for shrouds - pretty common I find.

    As for chafe and the wishbone. I ease the main out till the wishbone touches the intermediates. If there was no wishbone I could maybe ease another 5 cm BUT and this is a big but I could never do this as the chafe would be terrible. Even vanged down and tied off with a preventer your average mainsail will move up and down causing chafe so the main gets pulled on more. I can ease out the wishbone main at least as far as the same rig with a boom and get no chafe - my 15 year old main gets a bit of work every 4 years, the photo was taken 6 months ago and the main looks good. The wishbione protects it well.

    As to the Vee the main is sheeted to - it is an old idea from 505s and skiffs. As thew wishbone does not need a traveller you can use a bridle. This is what vang sheeting dinghies do to raise the mainsheet position up high. The higher you can raise the mainsheeting position the more it is like pulling the traveller up to windward. So I made a composite bridle (properly called a Loveday Loop after the 505 sailor who did it in the 70s) and sheet the main to it. It is like permanently having the traveller to windward meaning I need even less tension to get the main to where a normal main gets to. One handed pull in 15knots, it gets hard (need to heave a bit) just when I have to reef in a tradewind 22-25 knots. Then the staysail goes up too and the rig is bulletproof. we use this rig of reefed main and staysail when it blows a heap and we feel awfully snug and safe.

    cheers

    Phil
     
  12. BobH
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    BobH Junior Member

    Good information. I will try out a similar rig on my design and see what results.

    The genesis of the rig I designed was twofold: first, a desire to minimize the tendency to pitch (by reducing the moment of inertia of the rig), and second, to reduce the risk of capsize by lowering the CE of the sails.

    The section of the two smaller masts results in less mass per foot, while doubling the number of masts increases the total mass. Likely a single 42 foot mast alone of suitable section would weigh about 190 pounds. Two smaller masts (33 ft and 28 ft) would probably together weigh on the order of 230 pounds. However, that mass would end up closer to the axis of rotation (I understand that to be the center of buoyancy...that changes with motion of the boat), hence we get a smaller moment. At the same time the center of effort of the sails is lower and the roller reefed main quick to reduce, all of which should yield a gentler motion and reduced danger of capsize.

    I get that the split rig will not be as fast as a sloop, but the wishbone mizzen and the main with a circular track should both lend themselves to quick and easy tacking. Ease of use was big on the list of desirables. I realize there is a trade off of greater complexity (given two instead of one), but the individual systems are no more complex nor are the two systems linked. So there is redundancy and inherent safety. The sail area as designed is 650 sq ft...a lot for what really is a 30 foot boat with bow and stern stretched to 34 feet (all of the accommodations and equipment are in the middle 20 feet..the bridgedeck is relatively short). With a sloop or cutter and a 42 ft mast the area would likely be closer to 600.

    So much for theory...I do greatly value experience.
     
  13. BobH
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    BobH Junior Member

    nonlinear spring sail control

    Idea for sail control to prevent rig damage/capsize: electromagnet/spring arrangement with electromagnet electronically controlled via strain gauge embedded in mast. Allows for implementation of timed delay to prevent nuisance triggering by dynamics below set threshold. The release is by electronic control or by massive overload breaking the magnetic grip. Perhaps it would be possible to implement a preset device (no strain gauge/electronic control) with neodymium magnet of predetermined strength...would not have timed delay feature yet still would produce the nonlinear response to overload. The advantage being simplicity and reliability. The magnet provides the high initial resistance...the spring (metal, gas, or elastic) provides added resistance and reset. The magnetic and spring forces keep the sail in play producing lift, over load of the magnetic force provides the highly nonlinear response allowing release when a gust significantly above average wind speed hits. The arrangement could be tuned to a particular rig by varying the elastic element and the magnet strength. First image shows the non-linearity of magnet pull vs. distance (lb. vs in.), the second is the arrangement. Not shown is a damper/bumper needed to protect magnet against impact on reset.

    I'm thinking mostly about control of a wishbone boom and gaff allowing the sail to dump wind. I really like the benefits of a wishbone boom.
     

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  14. catsketcher
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    catsketcher Senior Member

    Gday Bob

    A couple of more points

    Sheeting to the wishbone gaff may seem warranted to reduce twist but it will exacerbate a thing called leech stall or hooking leech.

    In the old days of crosscut sails and roachy mains you had to be careful you didn't sheet the main too tightly in light winds. If you did the top of the main might get very full and the leech actually hook to windward. It caused the whole head to stall and was very slow in races. You really don't want to trim the sail up top - the head always has to be the most twisted part of the sail.

    Due to the changes in apparent wind up the rig the upper parts of the sail need to be twisted off more than the lower portions. (The wind blows faster away from the water)This means the sail needs to be eased off more as the apparent hasn't been pulled as far forward as lower down. This does not fit in with a sheeting arrangement to the upper parts of the sail.

    What you will end up with is an oversheeted head with the middle of the sail trimmed well or a well trimmed head and undersheeted middle. There is no need to innovate here - sails with reasonable leech tension work really well.

    As to sheet release systems. Some of the highest loadings I get are when I am in beam sea (I try and avoid them but often can't - around an island or headland this can get very bad) and the windward hull drops into a trough and rips the rig to windward. Very shaky and massive acceleration and therefore force but hard to engineer a release system for.

    If you sail like me just get scared of high loads. When the ropes start to twang you reef. Kankama has never thought about lifting a hull. Sail with a coward or become one and install an easy to reef rig. That should do it.

    cheers

    Phil
     

  15. brian eiland
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    brian eiland Senior Member

    Sonic Drives vs Chain Drives

    I've long been a fan of some alternatives to these 90 degree drives, particularly with hi-torque diesel engines. I spoke about the subject on several forums if you care to look:
    http://www.yachtforums.com/threads/new-drive-system-from-volvo-penta.2232/

    http://www.boatdesign.net/forums/inboards/belt-drive-37290.html

    http://www.runningtideyachts.com/dynarig/Tennant_Hull_V_ChainDrive.php

    BTW, it might not be that difficult to fabricate one's own chain drive propulsion unit for low-hp units. The manufactures of these industrial chains are readily available.,....one such
    http://www.ramseychain.com/prod_sc.asp
    http://www.ramseychain.com/index.asp
     
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