Cat hull advice desired

Discussion in 'Multihulls' started by XHeeler, Apr 1, 2009.

  1. XHeeler
    Joined: Apr 2009
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    XHeeler Junior Member

    Richard,

    Missed your post making mine. Yes to a sailing cat, and the mast is 40% back for what is now a 40 foot length. I was working off of Terho Halme's post on dimensioning a sailing cat. 4 foot hull beams at waterline for a 10:1 ratio of length to beam width. The mast is the larger circle in the forward outdoor cockpit on the above drawing close to the fore part of the starboard seat. Sorry I didn't label things and drawing on a computer is new to me. I've drawn and built a hundred or more homes, but you don't sail a house. My attachment got the bows cut off, which end 16 feet in front of that mast.

    Right now I have a forward deck drawn next to the steps down from the cockpit. But may have to modify for a windlass, unless someone has tried angling the chain over to one of the bows off a block.

    Thanks for response.
     
  2. catsketcher
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    catsketcher Senior Member

    Learn from others

    Hello X

    I had a few really novel things on my 38ft cat when I launched it. It had a convertible pop top over the cockpit, swinging davits, a motor pod that went up and down, large aft storage area, a wishbone boom and a vertical tiller. I had owned multis since I was 16 and had used ideas such as the vertical tiller on my previous tri with great success. Yet I had a 50% failure ratio on my ideas - this has cost me a fair bit of cash and huge amounts of time.

    The pop top, vertical tiller, storage box, swinging davits all are gone. The davits got flung off after a gybe, the tiller stuffed my back, the pop top just was terrible and the storage box haas been cut out and a normal cockpit put in its place. The only good ideas were ones that had been proven before on similar cats - the wishbone (that I still love every time I go sailing) and the raisable motor pod.

    My advice is to stop. Don't pull things out of your head - go and sail cats, talk to owners, walk the boatyards and see the ideas that work. Then go to a designer and show them the features (from similar sized boats to the one you want) you would like. Or you can do what I do and have a heap of new ideas, try them and then spend 3 months putting a normal cockpit (where the storage box was) a normal cabin (where the pop top was) and wheel steering (to replace the tiller) on when you have found your ideas wanting. Go ahead if you like but my advice is to stand on the shoulders of the giants rather than walk your own path.

    cheers

    Phil Thompson
     
  3. XHeeler
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    XHeeler Junior Member

    Bulkheads

    These are the three bulkheads I was anticipating.

    The mast is presently 1 foot aft of the fore bulkhead which is the forward wall of the cockpit. How close should I snug it up to that wall. I placed the mast 1 foot back because there is a 9 inch overhang of the decking to hide ropes and 3 inches clearance for movement of the mast.
     

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  4. XHeeler
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    XHeeler Junior Member

    Phil, I agree and accept your wisdom. Right now I'm having free fun and learning as I bump into walls. I really enjoy the drawing, and when I was young and didn't have a house, I spent years honing my dream house (which never got built- wife's house is home for the kids). Drawing forces you to solve puzzles and compromise between different parameters. Why do Soduku?

    I was thinking of buying a Gemini 105MC which I have sailed on several times, and I like it below deck, but want wider beds aft. The wife likes it, and why spend a fortune on a depreciating asset. But the forward visibility is poor compared to my sailboat, and you hang your head over the side like a train driver. When I saw Chris White's Atlantic series, I fell in love with a forward cockpit, only you have to get a 56 footer to have a rear deck for entertaining.
    So with the present economy, and my background in building, I've been lurking and thinking how I could improve on those two boats especially.

    It can't get expensive until you start making final decisions.
     
  5. masalai
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    masalai masalai

    Looking at your layout sketches, your beam overall, seems to be quite narrow and hull beam overwide - Performance boats rely on being light weight - - This http://boboramdesign.wordpress.com/39-c/ is my build project after researching and annoying people for about 2 years.... Check out some of my threads and look at the "scrumble project" link at the bottom right of Bob Oram's website pages.... Less stuff on a cat is good as weight is BAD.... There is a 44C at Langkawi YC (Malaysia / Thailand based boat) and has a handicap higher than all the other yachts - even the professionally maned racing 60+footers, and in cruising weight still wins its fair share of races....

    I suggest you look at as many designs as possible then get some comments from their owners and the regions they cruise.... Cruising waters do influence design criteria and function.... Mine, I would take on occasional 4 day passages if I was sure I had a good weather window otherwise go the long way round (up the Queensland coast, short hop and follow PNG coast & islands across to Ghizo in Solomon Islands then follow the chains of islands through Vanuatu, New Caledonia then a leap of faith to NZ - - - or the other way round according to the seasonal winds and weather as well as needing to motor sometimes)....

    Know your cruising grounds and where you can hide if bad weather arises... Know the sheltered bays, creeks and rivers that you can venture up to escape the tumult outside - have then all marked on your charts, with googleearth images and a measurement of clearance widths - remember the images may be several years old so there could be sand-bars and fallen trees etc... and with boards up, cats are shallow draft often less than 700mm (27.6inches).... I like bridgedeck clearance of about 7% of loa and around 5% seems common...

    Mine will have a forward cockpit for sail handling and steering/nav inside near the midship access doors forward (one of the innovations tested in this design when Bob built the first one - "Mango") - - On a cat you must use a bridle to stop "swinging" whilst at anchor as they seem to want to sail into the breeze or something like that tacking then drifting back to go again... I have a feeling your mast will burst through the bridgdeck seeking the sandy bottom under your boat before stopping its downward momentum.... On a mono that is planted on the keel - on a cat a structural bulkhead must go from both hull bases and up & across to support and hold the mast base as one of 3 or more critical cross beam loaded bulkheads/beams... some cats also rely on monocoque design principles where the skin adds to the structural integrity - The engineering is complex and beyond my capacity except to know the principles used and defer to the experts....
     
  6. XHeeler
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    XHeeler Junior Member

    Thanks masalai,
    I have drawn 40 foot hulls with 5 foot beam each from the bird's eye view. However at waterline the beam of the hulls are 4 feet and to about 2 feet above the waterline it is very close to 4 feet, then flares to 5 feet by adding a half foot on each side of each hull.

    I would have said that in metric for you, but I'm really not that smart.

    90 - 99% of the time would be spent between San Francisco and San Diego. We are not world sailors. That's what charters and airplanes are for.
     
  7. masalai
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    masalai masalai

    Go narrower, 3 ft at w/l as beam:length ratio influences performance at an optimised example, look for some images of John Hitch design called "XIT" hull beam =2 ft loa = ? 60? cant remember, a big w/l beam will behave like a bath-tub - defeating the purpose.... One reason for the plumb/vertical bows is to extract every inch of lwl relative to a given wl hull beam - - Reduce wetted surface and remember to add buoyancy potential bow and stern above the waterline (a complex design exercise) to minimise "purposing" - stick to the basics - where you would like what, then find a design that meets most of those desires and build the STANDARD design in the name of resale-ability.... Many of the best cruising designs come from Australia / NZ....

    OK so you get some no-wind days and some pretty wild weather, check out if there are any shallow marina bays that you could rent cheap and handle a boat less than 21 ft wide and 39ft loa and needs about 3 ft of water.... or haul it out on a trailer as it weighs 4862kg MAX = 10719pounds MAXIMUM in cruise ready state full water, fuel & provisions etc...
     
  8. Pericles
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    Pericles Senior Member

  9. masalai
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    masalai masalai

    Where is that video? - sorted the deposit yet? :D:D:D
     
  10. Dusan
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    Dusan Junior Member

    Does anybody know where I can find some more detail information about the power catamaran stabillity and resistance?
     
  11. XHeeler
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    XHeeler Junior Member

    Thank you Pericles,
    I will try to incorporate as many ideas as possible. I am measuring off the screen for the 48 foot Gunboat a L/BW of 11:1. Do you know if there are significant advantages for above my goal of 10:1, or is it the weight advantage for a carbon fiber boat?
     
  12. XHeeler
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    XHeeler Junior Member

    masalai,
    I have the fever, but saving the funds. I am designing more for comfort than speed, as long as I can beat my friend's Gemini. But not by too much. I will certainly try to get the hulls as narrow as possible, but frankly that is going to be the designer's area once the final weight is known and final compromises made.
     
  13. XHeeler
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    XHeeler Junior Member

    I have been thinking of negatives for my plan which are:

    Weight and windage- I have a large pilothouse for a 40 foot cat since 4-6 people will be very common. Maybe this will be a motorsailer. I sail my Nacra cat when I feel the need for speed. I think the sweet spot for production catamarans is slower and more amenities, because wives want showers and even a washer and dryer more than 22 knots of speed.

    Docking- I probably can't see the aft corners for docking with the inside and forward cockpit steering. I was thinking of putting engine controls in each aft corner and using remote control autopilot for steering while docking. Stupid?

    Expense- this is going to cost more than a production cat probably.

    Anything else?
     

  14. masalai
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    masalai masalai

    I think you will find the performance range of "gunboat" is nearer the 1:18 end of the spectrum... (WL measurements) - Speed is not the objective but a by-product of efficient hull shape and light weight construction using regular stuff and avoiding super hi-tech such as carbon fiber & kevlar cloth which have issues when using epoxy and the other glues necessary and if you need to repair the kevlar sections big problems with fairing and grinding back - read up on stitch & glue construction for plywood construction and other books suggested earlier and remember KISS and use a proven design that you can sell later when you seek to upgrade or change over...

    If your budget is tight remember - KISS (Keep It Simple, Stupid) as a catch phrase firmly etched in mind as it is my mantra too... Spend lots of time looking at what is around - If it is for sale - Why???
     
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