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Old 03-01-2009, 03:13   #1
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Need help on catamaran model!

I am currently doing a project for my diploma in marine engineering on designing and constructing a model catamaran, conducting tests on it and propose the specs for the actual one.( actual one will not be built).

Can anyone tell me should there be another arm at the bow connecting the hulls? Or is there anything else wrong with my model. I have also received comments like the length of my hulls forward of the cockpit is too long and might create a fairly long lever arm. I just finish constructing it, it has not gone through any tests yet. I hope to remedy the problems before towing and heeling the boat to investigate its drag and range of stability. Thanks a lot. I welcome any comments or criticism.
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Old 03-01-2009, 03:47   #2
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I would suspect the bows would need some transverse support - ie a beam. This design is not dissimilar to team phillips - a large cat, which sufferred a break off one bow, not presuming to suggest that the same would happen to your design , just an observation.
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Old 03-01-2009, 06:13   #3
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Is it a powercat or a sailing cat??

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Old 03-01-2009, 08:26   #4
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Its a power cat. Thanks.
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Old 03-01-2009, 08:30   #5
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Thanks factor. In that way, an additional arm would just to act as a support to prevent anything from happening at the bow of the hulls? Or are there actually more additional effects? Sorry i am very new to this.
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Old 03-01-2009, 13:27   #6
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The model appears to have a significant curve upward toward the stern. Don't be surprised if it squats when you tow it toward planing speed. Be careful where you attach the towline. Your towline angle needs to simulate the thrust line of your motor to get true behavior.
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Old 03-01-2009, 14:25   #7
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Crossing waves and wakes at any angle will create a tortion between the hulls, fatiguing the cockpit supports. The bow waves will meet between the bows piling up on each other and filling the cockpit.
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Old 03-01-2009, 14:45   #8
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Thanks guys. Calculator could you please elaborate on your comment about the point of tow in simpler words because it has been bothering me also. Thanks.
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Old 03-01-2009, 16:24   #9
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You haven't said whether it will be outboard or inboard powered. Draw a side view of the boat with the propulsion unit in place. Note the direction the propeller will be pushing. Arrange your towing apparatus to duplicate this. For example, with an outboard, the propeller will be X inches below the surface, pointing essentially straight forward. With an inboard driving the prop through an angled driveshaft, the propeller will be pointing up the shaft at the shaft angle and position.
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Old 03-01-2009, 23:25   #10
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The boat will most probably use an outboard motor mounted on the middle structure with the shaft extending down into the water. Does it means my towing point has to be at the same level as the propeller, with the propeller pushing straight, the towing should also be pulling straight. That would also means the towing line will be submerged?
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Old 04-01-2009, 14:37   #11
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Yes, and you will need a way to control directional stability. You do this with the tiller on an outboard, but the towed model will need some sort of strut toward the bow that will allow the towline to pivot up and down but control the sideways movement.
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Old 05-01-2009, 08:44   #12
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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.

Please feel free to drop any comments.

For the towing test, if the towing line is submerged, where is the spring gauge going to be placed? And there will be no outboard or rudder or propeller involved in the towing, there will only be the boat, spring gauge, motor and any other simple accessible instrument to facilitate the experiment which i have no idea on what they might be. Thanks.
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Old 05-01-2009, 11:35   #13
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Depthofit - I would like to know more about your course of study and your group.

Capsize means to turn over. It is hard to imagine a situation where this boat would capsize unless you were to turn it too hard at a high speed. With a sailboat, wind pressure on the sail can cause a capsize. Being sidewise in a large, breaking wave could also cause it.

I would have to see a sketch of your towing appratus to answer the question on location of a spring gauge.
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Old 05-01-2009, 11:47   #14
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From your pictures it appears to be a hard chine hull. Why a hard chine? You only want a hard chine if you want the hull to plane. Otherwise you want a parabolic transverse cross sectional area which would mean less wetted surface per amount of volume. Less wetted surface area of course means less drag.

Wetted surface area at lower speeds is the significant amount of resistance. As you start going faster, your wave making resistance increases as a factor in your overall drag.

Another factor will be waves. Its rare when there are no waves. You must tank test with waves for your tests to reflect reality.

Given you have waves, your boat will need a realistic moment of inertia. Moment of inertia comes from the amount of mass you have away or towards your center of buoyancy. Placing a big weight at the center of buoyancy (LCB) to simulate true displacement is not going to be realistic. On real boats you typically have engines aft of the center of buoyancy and anchor chain and rode well forward of the boats center of buoyancy. The higher the moment of inertia that you have the deeper your bow will pitch into waves. You will need a realistic moment of inertia to reflect reality. You will need two weights, one placed aft and the other placed forward of the center of buoyancy to reflect the true moment of inertia of your model. The distance of the weights from the LCB is something that you are going to have to calculate.

I hate to say this, but I think your model will go faster backwards. If you look at a symmetrical foil and even most fish, the maximum cord width is about 1/3 the distance from the leading edge. I think you have it backwards where your maximum cord with appears to be at 2/3 the distance from the leading edge. You don't want your laminar flow breaking up with a foil that closes up too fast. I would reverse this and see what happens. You will probably want your vertical cross section to go by this 1/3 rule as well. Water does not compress so it must accelerate around the hull....one reason we get wake...it wants to go vertical some in order to equalize pressure as well when it accelerates around backwards around the hull.
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Old 05-01-2009, 22:36   #15
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David - You missed where he stated that is an outboard. And from the looks of it, I suspect he intends for it to be a planing boat. I think most of your comments infer a displacement boat. My concern is that the stern should be flatter, not curved up.
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