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Old 06-11-2008, 07:35   #46
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Determining the actual center of gravity and center of buoyancy is pretty necessary in most vessel designing. However, determining CG requires knowing all the equipment planned for the finished full-sized craft and where it will be placed, which may be beyond the scope of your project. Perhaps you can make approximations for how much batteries, engine, crew, and hull would displace and their distances from the center of the design.

Unfortunately, I don't have an answer for determining either the CG or the center of bouyancy. There are many good textbooks describing this, and I suggest you check a good reference. You probably already know how to determine the center of a solid: the center of buoyancy for your hulls is the center of the portion which will be submerged - so at rest everything from the load waterline down.

However, I do have a shortcut to determine the approximate center of buoyancy of one hull: draw the load waterline plane on a piece of thick paper and cut it out (or on regular paper, cut it out, then fold it up accordion-style - /\/\/\/\ - so it is stiff enough not to droop too much.) Now balance the cut-out on the tip of a pin. When you figure out where it will balance, that is pretty close to where the hull would balance.

Mark that point on each hull on your main drawing. Draw a line between the two points. The center of this line will be approximately the center of buoyancy for the model when it is at rest and equal trim.
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Old 18-12-2008, 07:08   #47
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Hi amgine, haha i haven post for more den a month already. I was working on the model and i think i am almost done except for a small bump. From the pictures that i have attached, u can notice tat the bow of the ama is not joined together, there is a gap between the port and starboard 'skin' due to some miscalculations, any good suggestions on how i can solve it so that my bow can have the usual sharp edge. (The last 3 pictures illustrates my problems, the rest are just pictures of the construction progress)
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Old 18-12-2008, 10:43   #48
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Several construction solutions

This is minor problem for construction, but might be more important in design. (It is also exactly the kind of problem engineers in the field are faced with all the time - how to solve a real problem *now*.)

If this were a real-life boat, I would suggest creating a stem timber shaped to accept ends of the hull planks, use fasteners and epoxy to secure it in place and seal it for a water-tight bow. I would fair the wooden timber to the right profile, and use epoxy with microballoons or similar material to get the smooth surface you want at the bow.

For a model your best solution *may* be to cut new topside planks, taking into account the design error. This will be true if you have plenty of material and time to do so.

If this is not an option, and looking at the gap it appears to be approximately the same width as the thickness of your hull planks, I suggest trying to slide a small piece of wood into place. If it fits reasonably tightly, glue it in place and shape the end to the bow profile. Use glue thickened with sawdust or other light, sandable filler to fair the bow, filling in the gaps on either side of the filler wood piece. Use a *file*, not sandpaper, to shape the glue after it has set. (The balsa wood is much softer than the glue, so you need to be very careful in shaping.)

Another option would be to glue a piece of wood into the gap leaving a flat surface forward. For your application - an inland pond with very minimal wave action - the flat surface will not greatly increase drag. This would be a much simpler solution, much less likely to have problems in implementation.
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Old 18-12-2008, 10:46   #49
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By the way...

The shape of your hulls seems very nice! There's more rocker to the bottom of the hulls than I expected, so the boat should have good tracking (ability to maintain a course). Good width to the hulls - that suggests a reasonably large load-carrying capacity.

Thanks for all the images!
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Old 30-12-2008, 05:06   #50
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Hey amgine, remember previously u suggested the frame and pontoon method for my towing test? Right now that my boat is almost complete and de dateline is very near also, i would like to ask you tat the reason for the pontoon and frame is to provide a platform and secure the spring gauge only? What would be the difference without the frame and pontoon?
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Old 30-12-2008, 16:20   #51
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The primary reason is to give you the opportunity to improve the accuracy of the observations, in part by having a platform on which many instruments may be mounted (including the camera.) For example, having a log measuring actual speed through the water of the platform is more accurate than measuring how quickly the rope is being reeled in because there may also be current affecting the model. The platform is usually more stable, and because the instruments are not in the model you avoid accuracy questions about adding weight or drag to the model.

Also with the platform you may make several different kinds of measurements, in addition to just straight-line drag. You can also position the craft at different angles of heel - not an issue for your design but might be important for a single-hulled vessel - to measure weather helm. The ability to measure side forces from the rudders, daggerboards, or other hull appendages is more difficult with straight-line force instruments.
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Old 02-01-2009, 07:43   #52
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Hi amgine. How can i determine a maximum safe draft that my boat should have? I have uploaded the latest pictures of the model. Feel free to drop any comments.
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Old 02-01-2009, 12:30   #53
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Hoi....

I don't actually know how you'd determine the maximum safe displacement for your design. (For boats, draft and displacement are exactly the same.) You might be able to figure out the volume per cm of hull immersion from your designs in m^3, multiply by 1000kg (for freshwater, 1026 for saltwater). I think the maximum safe displacement will be the safe working load limit of the ammas (minus a safety factor for dynamic shock loading,) because the design of a catamaran is extremely stable so you don't need to be as concerned with heeling moments.

I do wonder about the arrangement of the ammas on your design. The model looks wrong to me without another arm between the bows, but keep in mind that I do not know much about multihulls. It would seem to me the length of the hulls forward of the cockpit might create a fairly long lever arm.

Maybe you could ask this question in the multi-hull forum?
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Old 05-01-2009, 07:39   #54
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Hey amgine, this is the test that my group conducted today. Please comment on it. We hang known weights along the CG of the boat and measure the change in draft to calculate how much the boat heeled, however our draft reading was taken at mid ship. What i wan to know is what about the forward and aft draft?

Another thing is that as we increase the weight slowly, the boat began to heel over a larger angle but it reaches a point where it refuses to capsize even though one hull is already submerged and the other hull is barely submerged anymore, the catamaran will only sink when the boat heel enough for the water to get into the cockpit, in that case we can just raise the wall of the cockpit by i believe the wall of the cockpit should not have such direct effect on the stability of the boat. How do we differentiate whether the boat is considered in equilibrium or in another word, how do we consider the boat capsized? By the cat flipping over completely etc.

Is the test that we conduct even correct or serve any purpose in the first place? Sorry for the long question.
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Old 05-01-2009, 15:38   #55
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Complexities

There are some things which you need to remember when recording the results of this test.

Remember that the weights, because they are suspended in water, do not 'weigh' as much. You need to subtract the volume of water they displaced. For example, a cm^3 of iron has a mass of 7.874 g, but suspended in fresh water it has an effective mass of 6.874 g.

Now, the moment of effort is the mass times the length of the lever arm, in this case the distance from the centerline (because you took care to calculate the fore-aft centerline there should be no other distance in the lever.) If you know how much mass was necessary to submerge one hull, then you know both the righting moment (the amount of force due to buoyancy plus the counter-acting weight of the other hull) of the hulls. This isn't directly important to your electric-drive vessel, but it might be important if you later explore how large a sail plan could be mounted on your design.

It's interesting that the hull will not capsize further until the buoyancy of the hull is overcome. Remember that this is with static forces - wave action or wind might cause the boat to capsize more readily by changing the angles and adding other forces. But I don't know what other conclusions may be drawn from this test. My guess is the total mass minus the water displacement of the weights used will be very close to volume of the hull times the lever arm plus a small percentage, indicating to me that your hull design is very stable and your boat is nearly unsinkable. If the hulls were partially filled with closed cell foam and sealed during construction the boat would be unsinkable - a good selling point and important to marine insurance companies.


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Old 11-01-2009, 11:59   #56
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Hi amgine, do you know how i can calculate the total resistance of a real size boat if i have the total resistance of its model at 1:20 ratio.

I understand that if i were to propose a suitable horsepower, i would need to know some kind of information regarding efficiency of the outrigger and everything. Anybody know roughly the coefficient of efficiency for them or how much should i be expecting?
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Old 15-01-2009, 22:04   #57
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Sorry about the delayed response - I've been home ill.

Okay, I'm looking up a bit about tank testing in Skene's Elements of Yacht Design, and here are the three scaling laws:

1. The speed of model and ship are proportional to the square roots of their length.
2. The wave-making resistance of model and ship are proportional to the cubes of their lengths.
3. The displacement of model and ship vary as cubes of their lengths.

Remember the whole book (I have an historical copy, not a current edition) is solely in imperial units, so you will have to convert your measurements from metric to imperial to use these formulae.

So, to get the appropriate towing speed for your model to measure the resistance, it should be approximately (square root(DWL) * 1.35) / square root(20). DWL would be expressed in feet for this formula; there is a similar formula to get the theoretical hull speed (displacement speed) in metric, but I don't remember what it is. And the 20 is from the ratio of model to full-sized boat.

To get the estimated total resistance of the full-sized boat it would be resistance in pounds times the cube of of the ratio. So Rr = model resistance * 8 000.

Now to determine the horsepower, HP = Rr * (square root(DWL) * 1.35) * 0.003071. There will be inefficiencies, friction and slip, so you may need to add a multiplier up to 2 for actual horsepower requirements.
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Old 16-01-2009, 16:05   #58
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Probably the next big step in commercial cats is the assymetric hulls. not as extreme as a proa but a big hull and a small hull. Chaps like you will bring it sooner.
Why not build a radio controlled version? Cheap radio, big pond, speed controller for boat speed. Floats on a long string for a 'measured course' and nature will provide the head and tail winds. Prop in each hullwired in series. Hydrofoils? Air wing (downforce/upforce).
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