centre of buoyancy question

Discussion in 'Multihulls' started by pilotdude, Jul 6, 2009.

  1. pilotdude
    Joined: Jul 2009
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    pilotdude Junior Member

    Hello everyone,

    This is my first post, although I have been lurking on this site for more than a year...

    When it comes to boating, I am only interested in motor-sailing cruising catamarans. So, there is no chance that I will enter into a conversation about the difference between the monos and multis or which one might win the race :D

    Currently I am busy designing and building my dream boat. I have been doing so for almost a year - I did not want to rush it and have been making small steps, systematically building a good and detailed understanding of the boat-building and nautical in general world.

    Now to my question - I've stumbled upon an obstacle that I can not overcome on my own, I believe there are many people on this site that might be able to help.

    What is a good ratio of reserve buoyancy to displacement to have on the bow of an ocean going class catamaran?

    I will try to qualify this bit better.

    I know that the purpose of the reserve buoyancy is to shift the center of the buoyancy forward when a hull is pressed into the water. That and also applicable in near-pitch pole situation - to resist submerging the fine hull when the boat surf down a wave and riches the trough.

    So, I am looking for two figures - one is ratio of reserve buoyancy to hull displacement and the other one is how much should a center of buoyancy move forward at certain change in displacement - that will affect certain moment arms that I am interested in.

    I will appreciate your honest advises.
     
  2. gonzo
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    gonzo Senior Member

    It depends on the design, rig and how and where you intend to sail. You may be better off hiring an arquitect. What you are asking is for someone to do a major part of the design for you.
     
  3. TeddyDiver
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    TeddyDiver Gollywobbler

    It's not so simple.. depends also of the longitudinal moment of inertia, location of the center of effort etc..
     
  4. yipster
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    yipster designer

    checking for inertia i was today reading this relevant doc
    today also read "project cheers" note the added anti dive plates at the bows
     
  5. pilotdude
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    pilotdude Junior Member

    Stunning!

    That's exactly what I was missing - so the moment created with the changed reserve buoyancy should equal the total moment of inertia, therefore cancelling it.

    Nice!

    This site, besides the odd nuts, rocks!
     
  6. catsketcher
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    catsketcher Senior Member

    Be careful

    The Cheers anti-diving plates worked okay on her but exacerbated the problem on Twiggy - a 31ft tri from the early 80s. If the bow got depressed at a greater angle than the diving plates angle of attack they went negative in lift and increased the risk of pitchpole.

    This also happened on a Kelsall tri - Kawasaki - that had foils up front. Be very scared of forward mounted foils.

    I asked this question to Nigel Irens - the best racing designer there is - and he said that it was not easy. John Shuttleworth has a figure as does Britton Chance (helped design Stars and Stripes) - they both figure on having about twice the stability diagonally as sideways.

    The problem with this is that if you design a nice flat foredeck to increase your reserve buoyancy up front you will increase drag if the bow goes under. It is actually the increase in drag that will cause a pitchpole. Drag is necessary to cause a moment. Like tripping over a crack in the concrete.

    So the answer is - no one knows. Here is my normal reply. If you have to ask this question then you should be getting a designer to help you. This is pretty basic principle. If you want to go offshore on this boat spend the money on a set of plans from someone who has experimented on someone else. You will get the plan money back 5 times over when you sell the boat.

    cheers

    Phil
     
  7. TeddyDiver
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    TeddyDiver Gollywobbler

    Or, becouse you have no interest in racing, just build conservative and reef earlier :)
     
  8. pilotdude
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    pilotdude Junior Member

    catsketcher, you will not believe how much I appreciate your comments!

    The question I asked was not related to racing or performance, it was rather an issue with the dynamic longitudinal stability in quartering seas, more of an oscillation damping issue instead of an amplitude issue.

    But I've got my answer now. Thanks to yipster!

    On the other comment - it is very easy to suggest "go and find someone to design you what you are looking for", but can you point me to someone who actually uses science in his designs and also targets the DIY builders? I can't think of anyone.
     
  9. catsketcher
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    catsketcher Senior Member

    Most Aussie designers do

    Gday all

    As to pitching, it is not really a problem downwind with quartering seas. Nosediving may be.

    The best way to get some numbers is to reverse engineer a good boat or series of boats. This is the science that yacht designers do. They have a series of boats that they sail and get feedback on. The numbers from these boats get used in later designs. Subsequent clients get the benefit of this knowledge. There will be no magic number for any feature of a catamaran.

    So this refinement is done by most of the designers I see. Grainger, Schionning, Chamberlin, Easy by Snell, Oram. All of them do this. If you mean are these designers slave to certain numbers I would hazard a guess as to them not being so.

    I still maintain that by designing yourself you will lose a large amount of money and waste a huge amount of time. You really have to have an awfully good reason to do so. If you can't find a designer to do what you want it probably can't be done well. Make sure your wife hears contrary views and you can make her see that you really have to design your cat yourself. Do what most designers do if you must and start small. A big cat is an awfully expensive prototype.

    cheers

    Phil
     
  10. Richard Woods
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    Richard Woods Woods Designs

    As always, Catsketcher gives sensible advice. Not least that probably you will sell your boat in a few years. And it will be hard to sell if it is an "amateur" design - even if it is a good boat.

    I was working for Derek Kelsall when the foils on Kawasaki were designed. The boat capsized when surfing under spinnaker in big waves and at high speed. We thought that the problem was that the foil on the aft rudder was too big and lifted the stern thus causing a pitchpole. I don't think it was due to the front foil.

    I also got to know Ian and Cathy on Twiggy well when they were in the UK for the Round Britain race.

    What I find interesting is that the radio controlled trimarans I "sailed" a few years ago needed a rudder foil to hold the stern down. Otherwise they would pitchpole immediately.

    Shows you cannot assume models can scale up to full size.

    To answer the question:

    "So, I am looking for two figures - one is ratio of reserve buoyancy to hull displacement and the other one is how much should a center of buoyancy move forward at certain change in displacement - that will affect certain moment arms that I am interested in."

    Unless your boat is radical or extreme (and I hope it isn't) then you don't need to worry about this too much. Sounds a cop out, but really there is so much else that determines motion in a seaway I don't think you can have any hard and fast ratios about reserve buoyancy.

    Hope this helps

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     

  11. pilotdude
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    pilotdude Junior Member

    Now this is sad! Although I had almost given up trying to find an engineer type designer, one that only deals with facts and leaves the hearsay and old-wifes tells for the pub, I had kept some hopes. Pity.

    Richard Woods,

    Thank you for your advise. I am not obsessed with the reserve buoyancy, I just wondered if there is going to be any surprise attitude to be discovered later hence decided to see if I am missing a calculation or two. The point you make is logical, with exception of the contest where you described a near-pitch-pole situation in your Cape to Rio trip on a poorly (too fine bow) designed catamaran. So, which one is it then?

    Shifting slightly the subject, more of a better perspective really.

    Looking at the hull of the famous boat "A", switching my hi-imagination mode on, I can not miss the similarities between the hull of the "A" and the ones a catamaran has.

    Long slim hull, with one distinct feature though - did you notice that the hull bow is not as fine as your will find in a typical typical catamaran?

    I see huge reserve buoyancy on it . Now, everyone says that "A" has the most advanced designed hull. I mean every single boat authority (even the self-professed ones) say that. Makes me wonder if this particular bow profile has anything to do with that statement?
     
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