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Old 03-10-2008, 22:49   #16
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Think of it like this:

Code:
                winch
                  |
                  |
     [--]-------frame------[--]
     [P ]------------------[P ]
     [o ]         |        [o ]
     [n ]       model      [n ]
     [t ]                  [t ]
     [o ]                  [o ]
     [o ]------------------[o ]
     [n ]------frame-------[n ]
With the frame on top of the pontoons, and the pontoons parallel with with the direction of travel, the model should experience very little outside turbulence. Remember that you *must* measure the water speed, not the speed over the ground.
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Old 03-10-2008, 22:59   #17
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Oh..so i place the strain gauge on the frame attached to the model.

What do you mean by water speed and not speed over the ground?

Since i am using a winch and the winch will tow the boat in constant speed, can i just tow the model over a known distance and divide the time taken to get the speed?
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Old 03-10-2008, 23:34   #18
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Speed through the water measures the speed of the boat relative to the water it is traveling through, usually by means of a paddle-wheel style speed log.

Speed over the ground is the speed of the boat relative to the earth's surface.

To give an example of the difference, I was motoring in a channel against a tidal current. The current was 5.1 knots, and my boat's speed through the water was approximately the same, so my movement through the channel was zero - that is, my speed over the ground was 0. When the tidal current slowed, my speed over the ground increased at the same rate as the current subsided, but my speed through the water was a constant 5.1.

Water is a liquid, and may be moving when your model travels through it. So by measuring the actual speed through the water you will know that you are measuring the drag relative to the actual speed, and not an assumed speed.

This will also, generally, take into account the turbulence from the pontoon frame's movement through the water.

If you cannot measure this, you will need to make a much larger set of observations of drag at each speed to average out the variability if you want accurate data to work with.

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Old 04-10-2008, 01:48   #19
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What if the test is conducted in a calm pool, will the speed of the boat relative to the ground and water varies alot?
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Old 04-10-2008, 03:46   #20
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I am not an engineer (as I am about to prove )......

6 Metre / 20 foot 2 man electric Catamaran? My first question is: "Why?" - by that I mean what is it's intended use - every boat is built for a purpose(s). The reason I ask is that this will affect what you design. For example, do you simply want a boat that will potter around at 6 knots carrying 2 people and their lunch for a couple of hours on a sunny afternoon or a boat that can carry 2 people at 20 knots with a range of 12 hours - whilst carrying provisions for a week, and full scuba diving gear for each person.........On an 20 foot Cat the latter may be a "bit" of a challenge - but the answer will probably affect the hull shape / boat design you want, given that weight and load carrying capacity on a Cat can be more of an issue than on Monohulls - especially the smaller they are.

And of course just because it is lake use does not mean the wind does not blow a lot or the water get rough! (indeed can be worse and more sudden than at sea!)......but boats are generally not designed to be capable of both rounding Cape Horn in a gale and being perfect for pottering around the bay / a lake for a couple of hours on a sunny afternoon.

Remember that all boats are a compromise, so their is no one answer - it is a matter of choosing which things are the most important for the intended use, and these can conflict......I am guessing that you are being required to not only demonstrate why you have come up with a design (with the testing to support) but also why you have ruled out certain design aspects.

Be interesting to follow how things go on the project.........
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Old 04-10-2008, 06:13   #21
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depthofit,

Sorry I can't help directly, but you might be able to pick the minds of the experts. You might try contacting someone at the Ocean Engineering Centre at the University of British Columbia - they do tank testing so should be able to answer your questions. You might also try corresponding with a catamaran designer - Richard Woods (Woods Design) is on this forum and might have some insights; you could PM him. John Shuttleworth is a designer who, I believe, has done extensive testing with scale models - you should be able to reach him through his site: John Shuttleworth Yacht Designs Ltd.

Good luck.

Kevin
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Old 04-10-2008, 08:22   #22
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The boat will most probably be for recreational, spending hours out in the lake, fishing, picnic or even reading a book. I didnt want the boat to be too long because the enclosed waters in my country are not so big, and i want to keep the maneuvering simple. Can anybody advise me on an experiment that can test the boat's responds to wind, say how much wind is needed to make the ship roll or heel really bad and cause discomfort to the occupants.
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Old 04-10-2008, 10:00   #23
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You are going to do well in your new career. Just keep asking questions of yourself and others, and keep trying to answer them. Have you explored the ability of hydrofoil daggerboards to achieve lift to windward and reduce the effects of surface waves? Who knows? Maybe the wind tunnel could allow you to use wings to have a vessel that flies at the wavetops and is anchored below the surface by the foil. We are going to see new ideas in the future that, to steal a phrase from Shakespeare, are undreamed of in our knowledge base today.
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Old 04-10-2008, 10:14   #24
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What do you guys think if i were to utilise a software to help me better present my project, the software would help me prove to the panel of judges that decisions were made based on the data analysed by the software.
Anybody know of any software that can help me analyse the behavior of the model?
Would the software be too complex or out of reach for an amateur like me?
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Old 04-10-2008, 11:17   #25
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I am sure you have done plenty of Googles already

But maybe these guys are worth a bit of "research" seems to be plenty of thinking material on their website - maybe even drop them a line?

M2 Motor Yachts | 800.547.1247 | High Speed Center Console Power Catamarans



"M2Motoryachts has introduced a 21 foot power catamaran that uses a revolutionary foil system to increase performance on several fronts. These include: up to a 40% increase in speed; cost savings that result from using sm.............."
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Old 04-10-2008, 12:20   #26
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Depthofit: There will still be some turbulence from the pontoon platform, but it will be fairly easy to do a series of measurements at each speed to create an average.

I think the advice to contact one or more catamaran designers is a very good one. They can advise you much better than I as to what you should test on a limited budget.


Everyone: This is a final year project to build a model, and show that the engineering team has taken into account as many of the situation variables as they can as though they were going to build the project in full scale. The design criteria - small 2-person electric catamaran for recreational use on lakes/ponds - are entirely imaginary, but might reasonably be considered marketable.
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Old 04-10-2008, 13:56   #27
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I wonder if the purpose of this exercise is more related to designing large ships, if only at scale sizes. I'm trying to look at this from an instructor's point of view. I would expect my students to address big issues, such as metacenter, and wind effect on tall superstructures. One question might be "If a ship is healing 12 degrees as a result of heavy winds on the beam, how much of the vessel's righting moment is compromised in a given sea state?

So a wind simulation is called for. Fortunately, a lot of information can be derived from an inexpensive handheld video camera, which records at a consistant enough rate and can be back calibrated with a watch in the scene.

I think there are more points to be gained in the design of the project than in the execution of it.
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Old 04-10-2008, 19:57   #28
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The m2motoryatchs concept is indeed very useful, however if i were to incorporate some of its design features in my own model, would i be infringing any copyright laws?

In respond to sandy daugherty, can you elaborate on your hand held camcorder experiment? Yes what you mention is exactly part of what my project wants to cover, i would like to find out at what wind speed in what direction would have what type of effects on the boat, particularly the 2 occupants inside.
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Old 05-10-2008, 01:50   #29
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Quote:
Originally Posted by depthofit View Post
The m2motoryatchs concept is indeed very useful, however if i were to incorporate some of its design features in my own model, would i be infringing any copyright laws?
I dunno - I guess their is a line between copying and learning from.........

I appreciate that this project is more about the principles and methods used rather than to come up with a boat to be made as a commercial venture , but one thought that occurred to me is that the testing methods proposed seem rather homemade (no criticism meant by that) - so would it be worthwhile making the model larger than 1:20 (which my calculator says is 1 foot. and my ruler translates as 30cms )....I am thinking that the larger the model the easier to get better measurements as inconsistencies from the measuring equipment may have less impact (even if still present), plus maybe a bit less fiddly to build? What about 1:10 - 2 foot........

Weight of the Cat will also affect your design (apparently ) - I am thinking that with Electric drive that the battery weight alone may be significant, especially if you are thinking of going for a fast planing hull like the M2 instead of a slow displacement hull. More power generally equals more weight.

I know it is only a test boat, but should really have a name . and for a name you need a naming ceremony (to appease the sea gods - who also live in lakes!) - the good news is you get to create your own naming ceremony, often involving beer
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Old 05-10-2008, 21:07   #30
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Assuming the displacement of the 'actual' boat is around 500kg.

And how fast does it need to go before it can climb on its own wave?
Is it feasible to for my boat to adopt a planing hull if the lakes and reservoir in my country are not very big?
Basically, what is the difference between a displacement hull and planing hull in terms of its hull form.
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