Placing Center of Effort ???

Discussion in 'Multihulls' started by mcm, Oct 16, 2015.

  1. mcm
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    mcm Senior Member

    Are there other important factors besides Center of Lateral Resistance that would influence where to place the Center of Effort ?

    Are the Centers of Flotation, Buoyancy, and Gravity important factors in regards to where the Center of Effort should be ?

    What is the most efficient process; deciding the location of the Center of Effort first, and then designing the hull around it, or designing the hull first and then placing the Center of Effort to balance it ?
     
  2. Doug Lord
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    Doug Lord Flight Ready

    What kind of boat are you talking about?
     
  3. PAR
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    PAR Yacht Designer/Builder

    Yes, there are several factors to consider when placing a rig over a hull. This would include the rig type, appendages, the usual "centers", volume distribution, hull entry/exit features, etc. Can you be more specific about what you're trying to do?
     
  4. mcm
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    mcm Senior Member

    It's the pitch and trim balance of multihulls in general, but i am specifically interested in 18m long(20/1, lwl/bwl), light, fast, spartan cruisers whether cat or tri.

    I've read opinions about the relationship between COE and CLR in regards weather helm vs. lee helm, but i haven't seen any discussions on the relationship between COE and other centers that help reduce pitch.
     
  5. mcm
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    mcm Senior Member

    Rig type that i'm looking for is a sloop. Probably 3/4.

    Dagger boards and rudders for close upwind.

    Extended long bow for buoyancy, but with sharp, low profile, wave piercing characteristics.

    Volume distribution is part of the question i'm seeking answers for.

    Intention is: I want to be an informed consumer as i seek designs for a fast, light, spartan, open deck cruiser that can cruise up to 25knts with little pitching and good diagonal stability.
     
  6. DCockey
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    DCockey Senior Member

    Why do you think CoE influences pitching?
     
  7. mcm
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    mcm Senior Member

    DCocky, what are your thoughts on the COE of rig placement and its relation to COF, COB, & COG ?

    The rig is well forward in the sloop i currently sail, and if it didn't have a lot of reserve buoyancy forward i get the definite impression it would pitch a lot if for no other reason than the weight of the rig.

    Most of the fast multihulls i see out there have their rigs stepped pretty far aft, and pitch reduction is what i usually hear mentioned as a reason.
     
    Last edited: Oct 19, 2015
  8. mcm
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    mcm Senior Member

    Is this the wrong forum to seek info about the influence that the centers of flotation, buoyancy, and gravity have on choosing where to place the center of effort ?
     
  9. catsketcher
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    catsketcher Senior Member

    Just the wrong way around

    Not the wrong forum but maybe the wrong way around.

    You really should start designing by working out what the boat will carry (for a cruising boat). The accommodation will determine the layout. This will determine the centre of gravity. The CG of the hull will determine the longitudinal centre of buoyancy (probably about 55-60 LOA aft).

    Okay so then you have your hull and accommodation design and THEN you do the rig. get the CE of the rig and the CLR of the underwater profile to talk to each other. As for CE and CLR - this is a dark art. My 38ft cat has a tiny amount of lee helm even though when I draw the CE and CLR is behind the CLR (minus rudder). My little cat with similar relative positioning has a little weather helm. The only reason I can think of is that CE of the rig is less important than we think and has to be taken into account along with the windage of the above water profile. The high bows of the 38 footer help to generate lee helm. ( I don't mind some lee helm upwind in light winds because the boat is an Aussie cruiser and usually sails downwind where weather helm is not wanted or upwind in a bit of a breeze where she goes neutral - especially when I go to the staysail.

    So don't worry about CE yet. Some boats - Irens tris and Gunboats have rigs and CE aft. Other equally valid boats - F22 seems to have the CE further forward. It is hard to say because the addition of a sugar scoop totally alters the equations. Draw the accommodation and locate the longitudinal CG. That is absolutely vital. Get this wrong and the boat will be a dud.

    cheers

    Phil
     
  10. Jetboy
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    Jetboy Senior Member

    I think it's a little more complex question than just center of effort but also the relative angle of the forces. fast multihulls with high aspect main sails tend to generate more percentage of the force in a forward direction than laterally. I would intuitively design the center of buoyancy to be forward of the mast by about the average force on a broad reach. By that I mean if the typical vector of the force on the sail is 30* forward, I would follow that line to the beam of the boat and use that as a rough starting point for center of buoyancy. I would guess that generally puts the COB about 30-40% of the hull length from the bow. That would be the best place to resist heeling force.

    That's just kinda my off hand thoughts - but I'm not a boat designer. It does seem to generally match up about where I see most current boats. I would think that moving the mast aft is a function of ever increasing efficiency in the sail plan. The boards would then be based off of mast location and sail plan to balance the helm.
     
  11. mcm
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    mcm Senior Member

    Thanks Guys,
    That's the kind of info that's really useful to the consumer.
    I'm in the market to buy plans not design myself, but i want to be as informed a consumer as possible.

    The more i know how things balance the more i know what to look for, and what questions to ask the potential designer.
     
  12. catsketcher
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    catsketcher Senior Member

    not 30-40% LOA aft on a cat

    Jetboy

    You can't put the CB at 30-40% on a cat. It will sink its bum as soon as you start to heel. What you are saying to do is pretty much what some designers did in the 70s. If you load up the boat on a reach and work out the vector from the rig then it may be that you want the deepest part of a trimaran float 40% aft.

    The problem is that this is only ONE load case. Are there more? What about reversing backwards after a bad tack in heavy seas and high winds? So we need some flotation here. Getting hit by a bullet under an island abeam? So meat needed sideways.

    In the end you will need a broad range of flotation to cater for a broad range of issues. To speak in general terms CB has been going aft for hundreds of years because it is faster. Cods head and mackerel tail is now the other way around - look at the 49er, Open 60s and AC cats. Reduce pitchpoling by reducing drag - hence the wave piercing deck profile.

    Follow the weight and keep things broadly distributed. Most cats drag their bums because their CG is further aft than the designer thought it would be. A common problem. If you make the sterns too big then the boat becomes bow down when under a full press of sail - CB too far aft. Keep the davits and dinghy light!

    As in all things boating - the design is simpler when you make the hulls longer and keep the rest the same.
     

  13. Jetboy
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    Jetboy Senior Member

    It's certainly a fair observation that reserve buoyancy is necessary all around in every direction. The only boat I've actually built myself is a trimaran - and it was based on plans from another. On the trimaran like most the floats primary purpose is to resist healing. I've never heard anyone argue that a trimaran floats had too much buoyancy or that it was too far forward. The center hull would ideally be a bit more balanced.

    My general thought that wasn't included in my comment would be that I imagine one would start with buoyancy balanced for the loaded CG of the boat and then add more reserve buoyancy to resists the heeling moment of the sail plan.

    For racing cats it seems as though more buoyancy forward is also trending to be favorable. But trends change. Foils are going to change a lot of what we think about buoyancy, and not necessarily for flying, but even just for stabilizing pitching and things of that nature.

    [​IMG]
     
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