25' trailer sailer

Discussion in 'Sailboats' started by LP, Jul 25, 2005.

  1. gonzo
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    gonzo Senior Member

    A common mistake is to expect the boat to float as drawn. You need to do the displacement/weight distribution calculation and it will show how the boat will float. If it doesn't float on her lines the ballast needs to be shifted. With water, in that design, it won't be easy. You may put bulkheads to compartmentilize the water, but it is the opposite of what you are trying to do. For a 'water ballasted boat" you need a design that is appropiate. The one you are showing would only work with a denser ballast that can be localized.
     
  2. PAR
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    PAR Yacht Designer/Builder

    Your idea of water, free flowing into the bilge will require it be sectioned into rather small compartments, each with it's own vent and valve. You'll still be subject to the effect of free surface liquids and have a poor density ballast material to boot.

    After looking at your lines, I too do not think your boat will float as you've got it drawn. You need to perform a time honored routines center of masses and displaced areas before you can have any idea of a target to shoot at. I'm not sure of the software you're using, but the centers and area calculations should be available. If your "winging it" using a drawing program, then you'll have to resort to the old fashion way with a calculator, pen and pad (you'll love the center of masses by hand)

    For what it's worth, that waterline shape will not help you with speed, nor those buttocks.
     
  3. LP
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    LP Flying Boatman

    Bilge water

    Actually, the center of buoyancy in this design is quite tolerably in place at 52% aft along the lwl. My current boat is very bluff in the bow and it drags its transom. Problems that are remedied in this design. Actually, I don't see a problem with the buttocks. It's a smooth transition to a gradual slope rising to the waterline. Waterline shape has less of an effect on speed that do the diagonals, which would reveal any humps or hard spots for the water to flow around. I'm not looking to build a minitransat. I am looking to built a traditional "in spirit" design that incorporates many of the style/design aspects of an earlier era (raking mid-section) while applying modern materials and construction techniques.

    You keep talking about "free surface liquids." What free surface liquids? All of the water is fully contained. The best example I can give you would be to take a Coke bottle and immerse it in water, top up, cap off, until it is full. Invert it and slosh it around to your hearts content, but keep the open submerged. Slosh is obviously a bad word since the is no sloshing taking place at all. The liquid is fully contained and the center of mass is constant. Having free openings in this configuation is the same as having no openings at all. If the openings were exposed to free air, then there would be a problem. Thus the reason for having the ports at the lowest part of the keel. There will be no free air in the ballast tank, no free surface liquid, no shifting ballast and no change in the center of mass of the ballast. I agree that water is not the best material for ballast, but I have a very specific design criteria I'd like to meet. 1000 lbs. times 1000 miles equals a lot of work expended. If I am completely missing the mark, in regard to why my ballast is going to be unstable, then let's digress to single syllable words that will help me understand the situation. Straight and simple, I see one shortcoming to water ballast. Density. The center of gravity issue just has to be dealt with, but it's a known. The volume has been dealt with. The distribution is dealt with through additional conventioal ballast as mentioned earlier. There is one issue that I'm surprised that no one has brought up. The moment of inertial of such a large distribution of mass.

    Let's go ahead and digress right now. I guess I need to know exactly what a free surface liquid is. Perhaps the name is misleading. I just had a little brain-storm that left some free surface liquid on my head. Actually, I started thinking about eggs and electrons. :idea: How do you tell if an egg is hard-boiled? Spin it. If it spins, it's hard-boiled. If it doesn't, it's not. Would that be your free surface liquid? Electrons->eddy currents?

    Sorry if I'm hard-boiled --- I mean hard headed. Some design elements are going to stay and others may go. Either way, the scrutiny is welcome, even though it may hurt. :(

    LP

    P.S. Here's my current boat. Eve-N-Temper
     

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  4. gonzo
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    gonzo Senior Member

    The original design has hollow bilges to allow placing the ballast low. You took that away and replace it with wood- a bouyant material. Then replace a dense ballast with water, again less density. If you want traditional, why don't you stay with the original design. 1000lbs are not a great difference if they will make the boat perform properly. On the other hand, design a hull that has inherent form stability and doesn't need a dense ballast.
     
  5. PAR
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    PAR Yacht Designer/Builder

    The few manufactured trailer craft that employ water ballast use a hollow keel, to contain the additional ballast in a narrow compartment, below the CG. As I previously pointed out, is an acceptable use, because the free surface (of the liquid) is small and narrow (relative to beam) and helpful (weight below the CG) developing additional righting leverage because of it's location in the keel. Another method employed is to pump to a windward tank, again as an assist to existing ballast. The successes of canting appendages will quickly displace the water pumpers in the next several years and fad following trailer sailors will sport canting foils shortly there after.

    Free surface of liquids has a bad effect on dynamic stability because the volume and the free surfaces are separate from the hull. Only in compartments of relatively small area, can these effects be managed and not done so at the expense of the main portion of ballast. Because the volume and free surfaces are separate from the hull, their roll period differs from that of the boat. When the roll periods are out of phase with the general motion of the boat, a beneficial effect is gained. When they're in phase with the craft's motion, their heeling moment is added to the work effected by the general heeling moment and the critical dynamic stability angle is reduced.

    In theory, your idea sounds good, fill the bilge, cork it (and hope it doesn't pop the cork or leak) and bingo, instant ballast. In use, you'll never get the bilge completely full (very necessary for safety reasons) without pumps and valves. The act of walking to the foredeck or sitting over, where ever you decide on placing this cork (likely a valve) will shift the center of buoyancy just enough to insure a reasonable air bubble remains in the bilge. Even with baffles, which do help control sudden center shifts as the boat pitches about, you'll find yourself in a world of "frequency phase". Of course this will not occur on a nice calm Sunday morn's sail, but just when you really don't need to broach, which it typical of most things I figure. The thought of a completely filled vessel of water not having air is a false one. Water has a lot of air (oxygen) in it, but it's "captured" not free like the stuff we breath. The oxygen can be 'freed" with agitation, which will occur with the normal motions of the craft, as the water is compressed all over the internal structure of the yacht.

    Water as part of a trailer born vessel's ballast is a good idea and can be well executed, with simple, off the shelf pieces. I figure your vessel is in the area of a ton and a half displacement. This will require a tandem axle trailer and reasonable sized car or truck to pull it. This is within the realm of a mid size, V6 powered SUV, big six powered mid size car or light duty truck. Towing with something smaller then this isn't fun, because you'll be towing something heavier then you. Towing a boat that's 800 pounds lees, still means you'll be towing very near your cars weight, so you'll want a reasonable tow car/truck. I know gas isn't cheap, but towing a 2,200 pound boat on a 800 pound trailer with a Celica is going to chew up more gas then towing a 3,000 pound boat on the same trailer with a mid size pickup or SUV, plus kill the brakes and the engine pretty quickly on the Celica. Think about trying to live with half of your ballast requirements in water (a third would be better) you save weight and maybe a smaller car. A general rule about towing is to pull with more then you're dragging if you want it to live for a while.
     
  6. LP
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    LP Flying Boatman

    Delta COB

    PAR,

    Care to talk about trim change with heel. I'm curious what is acceptable.

    To regress though, the center of buoyancy is purposefully aft. I may not be up on all of the reasons, but since the center of gravity is aft (due to crew, engines ,etc..), the center of buoyancy needs to coincide with the C.G. The waterline is also taken from theories for speed at the turn of the century and are thus in corporated here. It's the "spirit of" yacht thing.

    So, to continue, I've got the displacement of my design at 4100 lbs. I'd like to keep the hull weight under 2000 lbs. (sans ballast) so that I have about 1000 lbs. of useful load. Enough for two people to cruise. (?)

    Earlier, you were talking about trim changes with a Center of Buoyancy so far aft. As the design stands right now, the CB is 16.5" aft of midships. At 15 deg. of heel, it shifts 4" aft. That equates to the bow sinking 1.25" and stern raising by the same amount. Unless my trig is failing me, that is a 0.6 deg. trim change, bow down. That doesn't seem like a lot, but then maybe I'm not being critical enough. I'm aware of the increase in weather helm in this situation. If this trim angle is unacceptable, what kind of trim changes are acceptable?

    Thanks for the input.
     
  7. gonzo
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    gonzo Senior Member

    Is that the CB with or without water?
     
  8. LP
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    LP Flying Boatman

    Cb

    The CB doesn't change based on the type of ballast I use. The CB is based on the distribution of displacement when the vessel is brought down to it's lines in water. The water ballast tank will always be full, except for trailering.

    I guess the answer is with water. However, if I dump the waterballast idea and go with lead ballast mounted to the keel, the CB will still be in the same location.
     
  9. gonzo
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    gonzo Senior Member

    Yes it changes. Observe the ballast on older boats with long keels. It is usually lead or iron in the forward part of the keel. That is to change the CB and make the boat float on its lines. Water ballast won't do that unless you have narrow very high tanks with atwarthship dividers.
     
  10. LP
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    LP Flying Boatman

    CG vs CB

    Gonzo,

    You're confusing a couple of concepts. The Center of Gravity (CG) is what changes with the movement of ballast. The Center of Buoyancy (CB) or Center of Flotation (Xf) is fixed with the design of the lines. Granted, if a ship is not floating on her lines, then it's possible for the CB to be in a different location and yes the center of buoyancy shifts as heel angle changes, but the trim changes until the CB is aligned with CG. Obviously, that is the trick. Too align the CG with the CB, or vise-versa, and that's where additional trim ballast comes in to play.
     
  11. gonzo
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    gonzo Senior Member

    CB and CG have to be aligned vertically for the boat to float on its designed lines. With water ballast evenly spread, it won't. This means the CB will shift. CB and CG will align themselves naturally. The trick is to make them do it where you want them to.
     
  12. LP
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    LP Flying Boatman

    Gonzo,

    Sounds like we are argueing the same point.

    My statement: "but the trim changes until the CB is aligned with CG."

    Your statement: "CB and CG will align themselves naturally."

    My statement: "Obviously, that is the trick."

    Your statement: "The trick is to make them do it where you want them to."

    My Statement: "To align the CG with the CB, or vise-versa, and that's where additional trim ballast comes in to play."

    Your statement: "CB and CG have to be aligned vertically for the boat to float on its designed lines. With water ballast evenly spread, it won't."

    My statement: "Granted, if a ship is not floating on her lines, then it's possible for the CB to be in a different location"

    Your statement: "This means the CB will shift."

    I certainly appreciate the input. My statement though is that, if a vessel is floating on her lines, then the CB is in a fixed location and the goal is to put the CG at that location. It doesn't matter how spread out the ballast is. That ballast will have it's own CG, and all computations can be made as if it were a point mass at that location. The complexities arise when you start introducing accelerations to the mass(ballast). A long, spread out mass will have a different moment of inertia than a small concentrated one.

    As far as shifting CB goes, every time the vessel pitches or rolls, the CB changes. That why a properly designed boat floats upright and returns to that position when it is moved from that position. To reiterate, If the CG is in the wrong location, the vessel will trim until the CB is aligned with the CG. In which case, the CB does shift, but in designing a watercraft, the CB becomes a fixed location, A target or "bullseye" so to speak.

    Sorry to go "basic" on you, but I feel that my understanding of CB vs. CG are adequate. What I am looking for though, is an understanding of an acceptable pitch change with regard to heel. The CB in my design is 16.5" aft of midship at no heel. At 15 deg. of heel, it shifts aft by 4". This cause a pitch change of 1.25" down by the bow and up by the stern by the same amount. This equates less than a degree of pitch change.
     
  13. PAR
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    PAR Yacht Designer/Builder

    You (LP) and Gonzo seem to be having difficulty over each others syntax, basically saying the same thing. Any object that floats will adopt a natural position that will place the CB in alignment with the CG. This may not be where you'd like it to be, but it will occur, like it or not. Neither are fixed (though we'd like to think so) but move around continuously. The true trick, is to design your immersed areas to permit the craft to float on her intended lines. Any mass changes, for any reason, will force the two centers to find a new common axis to live on. An old Greek worked this out some time back and it's well understood.

    As far as pitch, I'm not sure what your referring to. Your suggesting the CB slides aft 4" at 15 degrees, which effects the trim, but it will also move to leeward. I'm not sure what you're asking frankly. Are you talking about trim changes, because of the distortion of the hull's waterlines when heeling?

    My view of your hull shape suggests you've got the waterlines down reasonably close to the model you're after, but you've removed considerable area in the bilge, where the original form was "built down". I can understand the need for the volume reduction, given modern construction methods and materials, but that area of the hull probably provided most of the sailing qualities of the hull. Bill Atkins was rather famous for his shapes in this area and it attributed to the fine sailing qualities of his yachts as a result.
     
  14. LP
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    LP Flying Boatman

    Cb

    PAR,

    Thanks foe the reply. I was looking forward to your input.

    The CB change I was mentioning is when the vessel is heeled to 15 deg. while maintaining pitch and displacement. The fuller after body creates more buoyancy there. By my calculations, the bow will sink by 1.25 inches and stern raises by the same amount. This puts the CB back at it's original location. So in this case, the bow trims down by the bow when heeled. My understanding is that this is common and is one of those many compromises that are part of the design process. My question is, "How much trim with heel is acceptable?" This will cause a forward shift in CLR and cause a certain amount of weather helm because the bow is more deeply immersed and the stern is less immersed. Guess I'll have to run numbers from some other designs

    I'm curious what you mean by "built down." Are you talking about at the turn of the bilge or down at the keel. I was looking to reduce displacement a bit and have read that a deep V with little keel does not have the best sailing qualities. With a draft restricted hull, I felt that having keel exposure was important and so have compromised beween displacement and keel area.

    I think I've the Bill Atkins name before, but have studied any of his works. I'll have to track down some of his work. Do you have any leads.

    Thanks again.
     

  15. asathor
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    asathor Senior Member

    Fresh thinking.

    I am not an expert on anything including traditional boats but I just sailed a brand new "old" workboat of the size you want (it sailed very well considering the antiquated rigging) so I have a few ideas to add. I have also been trailering a shoal draft Freedom 21 for 18 years and that I do know something about.

    Bear with me as I state some design considerations and assumptions:



    • Since you want to use fiber glass you really don't need your keel to be one continous "log" for strengt (like the old boats).
    • You are expecting to give up hull dept to accomodate voluminous water ballast - so it doesn't really matter to you how that space is used, right?
    • when you trailer you need low draft in the front of the boat to get on the trailer - the back of the boat floats a lot higher.
    • If the "keel" does not keep the boat from getting off the trailer or back on the trailer it is NOT in the way.
    • You are willing to build you boat strong enough to be on a trailer with rollers.
    • You are satisfied with the way small full kealed boat sail and are only hoping for incremental improvements
    And where does it take us?

    A "Full Swing Keel" or should we say "Jack Knife Keel"

    Let me explain. At some point near the front of the boat you put a giant (strong) hinge in yor keel so that it can drop down further like the blade on a jackknife.

    When at rest on the trailer the keel fits nicely in its trunk in the bottom of your boat.

    When you launch the keel it is latched in the up position - after you float/roll off the trailer you release it and it drops down to make a lower keel.

    When you go on the trailer you simply winch the boat up - the keel will fold up and out of the way - off you go.

    If you want to beach it you can add a mechanism for raising it if you feel you need to.

    You can add as much lead as you can stomack in your keel - in the botton where it actually helps.

    You still have most of the storage under the floor for growing mildew on old life jackets or cooling beer.

    Build properly you can race across reefs instead of riding the Bronco at the local Cowboy Bar without fear of fallling off (it might spill your beer), because it will autoretract when pushed up.

    Your boat will sail exactly like the old deep full keel type it looks like - just better.

    The "new" boat types will have to trim their sails to keep up.
     
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