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Old 05-10-2008, 22:11   #31
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Waterline length will determine at what speed the boat *must* begin to plane or be limited in its speed.

Assuming the hulls are nearly perpendicular at both bow and stern, a 6m boat would begin to get close to hull speed at 5.94-6.03 kt (11-11.17 km/h). This is not a very accurate number, due to the probably narrow shape of the hulls, but is a reasonable base to work from. I believe it is unlikely your catamaran could go faster than 7.5 kt (14 km/h) without some semi-planing or planing.

In my opinion, no, it's not very feasible to go for a planing hull design given the small size of the waterways.

Planing hulls tend to be flat-bottomed, and wider and flatter at the aft end. There are additional techniques, such as stepped hulls, which might be used to reduce the wetted surface of the hull once planing. The amount of energy necessary to get the vessel up over its wave is considerable, and higher speeds may require specialized steering and propeller designs and, in an electric drive, considerably more storage capacity.


Keeping in mind everything I said, there are other ways to get planing such as hydrofoils. For example, there are hydrofoil small sailboats such as the moth.


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Old 06-10-2008, 20:09   #32
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Amgine, thnks for the telling me about hydrofoils, although i am still not very clear about its construction and design on a multihull because i cant seems to find any information on it, but from what i have read about it, this design feature seems to be a must in my model.

Guys, heres a link from Wikipedia with information on the reservoirs in singapore.
Reservoirs in Singapore - Wikipedia, the free encyclopedia

Can anyone tell me tat base on the given sizes of the reservoirs in my country, is it feasible for me to incorporate hydroplaning designs into my project or should i jus stick to displacement hull.

The thing is that i am intending to have my boat operate at a cruising speed of 5-7 knots, with a cap at 10-12 knots because its primary purpose is just for recreational. Can normal recreational boat have hydrofoil to boost its efficiency also, specifically in my case or are they employed only when high speed are concerned. Thnks.
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Old 07-10-2008, 09:53   #33
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Not too surprisingly, there is a lot of information about hydrofoil catamarans. Do a google search for those two terms.

Here are a couple interesting ones:
* The Predicted and Measured Performance of a 16' Hydrofoil Catamaran
* Prototype of a Single-handed Hydrofoil Sailing Catamaran

Both of these appear to be student projects similar in some respects to your own, though I think at the graduate student level.
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Old 12-10-2008, 08:16   #34
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Hydrofoil vs no hydrofoil

What do you guys think if i conduct the experiment with and without the hydrofoil to compare the performance of both scenario? I have a feeling that at cruising speed of 5-7 knots and unless i maintain certain speed over certain distance, else the hydrofoil would not be able to exhibit its full potential, am i right?
Plus if the boat is unable to travel fast enough for the hydrofoil to come into effect and push the bow out of the water, the boat will actually be travellling with an increased drag with the hydrofoil as some sort of burden as compared to one without the foil, am i right?
Issit that regardless of the speed the boat is travelling at, the hydrofoil will be aiding with the boat's performance, or there are specific measurements and types of hydrofoil that are suited for different sizes and types of boats cruising at different speed. (Sorry because i haven really seen a boat that is cruising at a speed like mine that is fitted with hydrofoil, all the boats that i have come acrossed online that are fitted with hydrofoils are travelling at rather high)
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Old 12-10-2008, 19:15   #35
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I don't know anything about hydrofoils worth mentioning. I do not think you would be able to operate the vessel on hydrofoils at 5-7 kts; that is, it probably won't have enough lift to get the boat out of the water at that speed.

The only benefit of a hydrofoil is to allow speeds greater than displacement speeds without increasing the hull waterline length. The options are either a planing hull, or hydrofoils. I do not know if your electric drive is suitable for higher than displacement speeds.
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Old 15-10-2008, 08:04   #36
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Guys i will be settling with a simple displacement hull and hopefully the plans will be done before the week ends. Next thing is the tests that are going to be conducted on the boat. Right now there is only the drag test, what other tests can i do to assess the stability and comfort and how exactly do i go about executing it. I will be posting the plans of the boat once i am done. Feel free to drop any comments. Thanks alot amgine.
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Old 15-10-2008, 09:29   #37
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Most of the comfort screens and tests I know of are designed for monohulls. This thread
has a number of links to numeric tests on the design. Induced heel, in which mass is applied at a measured distance from the centerline to achieve a specific angle of heel, can be useful in monohulls, and I expect it might be relevant for catamarans as well.

I suggest you ask your instructor for suggestions on what tests you can do, or for literature suggestions. Read what other researchers have tested on similar models, and see if you can adapt their methods to your project.
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Old 17-10-2008, 04:34   #38
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What type of displacement hullform is best suited for my model? I am looking at rounded bottom or flat bottom. Can anyone point me in the right direction?
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Old 29-10-2008, 09:14   #39
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Hi, amgine i have a few questions i hope u can fill me in.

Regarding the towing test, where should the towing point be on the boat?
What should be the boat's condition during the test? Specifically shld the prop be there, weight distribution during the test? Should there be a trim?
I will be posting the plans in a few hrs. They are all hand drawn amateur plans. Thnks!
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Old 29-10-2008, 10:42   #40
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Towing point:

To begin with, I think you should use a bridle of some form - a flexible line between the two bows. Your primary towing test will be from the center point of this bridle, which should distribute the forces from towing.

You will probably want to tow the model without propellers, with propellers in fixed position, and with propellers in free-wheel position (able to turn with the current.) You should find there are different levels of drag for each condition, and the important one for your electric drive is the one without propellers.

You will also wish to measure the force generated with the rudders at varying angles. (Keeping in mind that rudders are more powerful with motors than in a drag test.)

I am looking forward to seeing your designs! I hope you are prepared to ignore a lot of comments from people here about them. We are all very obssessed boaters, and we know a lot, but we don't always remember that you might not have access to materials, knowledge, or skills that we think every boat "must have". So be polite, but don't assume we are right.
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Old 29-10-2008, 11:09   #41
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Haha dun worry i will be more den ready to treat every comment good or bad as a piece of advise.

Can u elaborate on the rudder test tat you mention?
Is the purpose to assess the drag created by the rudder? Varying the angle of rudder will has something to do with turning moment of the boat i presume?
And why issit tat the important one for my electric drive is without propellers?

Another thing i dun understand is tat why do people test boat models in lightship condition when most of the time the actual boat is not traveling at lightship condition or do they actually load the ship with weights(I am not very sure of the practice), are the results still practically useful if it is conducted at lightship condition? I am just curious about the realistic aspect of the experiement and its been bothering me for quite sometime already.

Thnks alot.
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Old 29-10-2008, 12:41   #42
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Yes, the purpose of the test using the rudders is to test the amount of turning moment developed by the rudders. You can also do this, using the towing frame, by mounting the drag force measurement at right angles to the towing direction, with the attachment point at the bow.

Code:
                winch
                  |
                  |
     [--]-------frame------[--]
     [  ]------------------[P ]
     [  ]         |        [o ]
    [drag]------model      [n ]
     [  ]       model      [t ]
     [  ]                  [o ]
     [  ]------------------[o ]
     [  ]------frame-------[n ]
Which should measure more specefically the force developed from the rudders.

When the propellers are driving the boat they will present zero drag on the hull. This is why, when measuring drag of the hull shape, you don't need to account for the propellers, so measuring the drag without them is the important one.

Usually a model is measusered at the Design Water Line, which is not lightship. For a pleasure craft it is usually 3/4 of the design maximum displacement - that is, with 3/4 of the fuel, water, stores & gear, and live load (passengers). For fishing craft it would be measured with a full hold of salt water, 3/4 fuel and other stores. For other craft I don't know.

To get this load for testing usually water and weights are used. I know of one case where this test accidentally capsized the full-size vessel being tested (not a model) when the incline test found the boat was much less stable than predicted, and the weights (which were not secured in place) slide down the decks in a free-surface effect.

For commercial shipping, yes the results would still be useful in determining lightload stability and efficiency, but I would assume design water line tests would also be required.
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Old 30-10-2008, 06:46   #43
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Hey here is the plan. Anyway i manage to acquire an electric motor but the problem is that it only give one speed, the motor will be used to tow the model and so any idea on how i can vary the speed of the motor?

Haha sorry i couldnt find my curve rule while i was drawing so i left out the curves. The first drawing is the side view of the hull, it is flat-bottom and 2 hard chine, has moderate rocker and i am still not very sure about the transom. I cant wait for your comments, good or bad.
Second drawing is the accommodation plan and side view. Lastly, the hull at plan view. Model will be twice the dimensions of the drawing and the actual one to be around 25 to 30 times.
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Old 30-10-2008, 08:34   #44
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Varying the speed

Use pullies of different diameters.

The design appears to to be straight-forward, the shapes are reasonable for the time and materials you have available for this project. The transoms may be influenced by how the rudders are attached and how the electric drives will be mounted, keeping in mind the mass of the battery storage and the need for short power-cable wiring (to reduce voltage drop.)

The curve on the hull plan is especially nice.


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Old 06-11-2008, 06:50   #45
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Hey amgine, i haf already got all the thing needed for the construction of the model. Jig saw, balsa wood, epoxy, fiber glass cloth and epoxy resin are the main players basically. However, there are still some questions in my mind, iu could am hoping that you could fill me in. Thank you so much.

Is it all right for my model to have a trim during equilibrium?
Is it necessary to find out the exact position of the cg and centre of buoyancy of my model? If yes, how can i do so? (i was thinking about subjecting the model to different loads in order to determine the amount of moment required for it to heel over a certain degree and the load should be placed in line with the cg in order to facilitate calculations)
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