Need help on naval project!!

[depthofit]

Im currently doing my final year project which is roughly about building and testing a scale down catamaran model in order to estimate values for the actual one. I realise that there are quite a few type of test tat my team needs 2 carry out like towing test and wind tunnel. Chances are we wont have access to any towing basin and so we need to simulate the tests. Why do ship model, primarily catamaran need to go through a wind tunnel? What other type of test can i carry out to test the model? Is it ok if i only focus on the weight and geometry of the model? thnks alot.

[depthofit]

Any other useful tips regarding how to go abt building the model will be greatly appreciated. I estimate the actual 2-man catamaran to be 6m and i intend to build a 1:20 model. I live in Singapore. The proposed catamaran will only be sailing in fresh water reservoir or river as a sea going one will be too complex for us. Im sorry i couldn provide sufficient specs and details as the project is only at an early stage although the date of submission is only 3months away. Thus, any help will be appreciated. Thnks!

[Amgine]

I cannot answer all of your questions, or even very many.

Why do ship model, primarily catamaran need to go through a wind tunnel?

Catamaran designs, generally, are non-displacement vessels which may experience high apparent windspeeds. They also tend to have high surface area to mass ratios. Which is another way of saying they are known to become successful airfoils, and this is not a desirable design characteristic.

Ship models should be tested to experience the ultimate storm environment they might face in a normal working life. This would include a class 3 tropical cyclone, defined as having sustained winds of 178-209km/h (111-130mph, Beaufort 12). Effects of winds such as these are very hard to model virtually.

What other type of test can i carry out to test the model?

Static stability test, inverted stability test. roll inertia. These determine how resistant the design (form) is to tipping over, either from it's proper orientation or from the unfortunate capsized orientation, and the vessel's design resistance rotational forces, respectively. I would predict the two stability tests will have high numbers relative to monohulls (indicating the boat is able to resist tipping over from either orientation), and has very low roll inertia (indicating it can carry a large sail plan relative to a monohull) though on a small model that may be difficult to measure.

Depending on the hull plan, you can determine the center of lateral resistance with daggerboards and rudders in raised and lowered positions, and their affect on navigation. Especially if your model can be modified to change the position of the blades, so you can measure the effects of changes. Same with rig elements, such as a moving bowsprit or shifting the mast step fore and aft, but I think your project is focused on the hull.

A towing tank can be simulated with an electric winch and an open expanse of water. With imagination I'm sure you can come up with many different tests and ideas to explore in this project. A 30cm model gives many opportunities for experimentation.

Is it ok if i only focus on the weight and geometry of the model?

I don't know.That is a question you should ask your instructor. I would assume you would receive more recognition if you do more.

[Roy M]

How long would it take you to build an actual prototype? You mention it is a 2-man cat. Whack it out and see the actual results. Some things which would not be apparent in test conditions will become obvious in actual use. It doesn't have to look nice, just fair and strong enough (and if it isn't, you know what needs to be built up!). And as far as the testing parameters, you don't actually expect to take a craft of this size out in cyclones, or do you?

[depthofit]

Ship models should be tested to experience the ultimate storm environment they might face in a normal working life.

Since i am living in Singapore where there wont be any cyclone or hurricane, and i wish to focus the catamaran for enclosed water for the time being, my intended catamaran will not have a mast or sail instead it will be powered by an electric motor. Correct me if i am wrong, the primary purpose of the wind tunnel test is to obtain values of lift coefficient and angle of attack and i know that they are vital to a craft from safety to fuel efficiency. Can my wind tunnel be just a cylinder with a fan at the inlet and the model mounted on a force balance? (This what i understand from the Wikipedia article on wind tunnel)

Does it mean that the wind tunnel test would mean little use for my project since there wont be any outrageous wind speed occurring in my country? (Probably the worst is only during a thunder storm)
If no, how can i best simulate a wind tunnel test and from there?
What is a force balance?
How can i vary the angle of attack?

Thank you guys alot.

[Pblais]

I think you need to focus on the realistic aspects of this. You won't be building a full sized boat and you won't be expected to deliver a boat design on a budget that people will buy. What seems to me to be expected is to demonstrate an understanding about ship building. In that aspect it is less about what you can actually do but more about what you can demonstrate about yourself. Anything you can't really do yourself is not important nor expected. Some things are about how well you can play the game as a final project that is the task at hand. You need to focus on the game because you are not going to build a real boat. There is merit in that attempt. You are not constrained. If it can show your skills then it is a requirement not an option.

[Amgine]

::grin:: This statement is very useful! I didn't know if you were talkling about all ships, or just your specific project.

So, your catamaran will be electric drive. This is an important thing in determining what you want to test.

Your primary concern will be drag. For your application, drag will likely be a question of u-shaped bottoms to the hulls, or flat surfaces. In part the storage for elelctricity will influence this: if you will be carrying a lot of batteries your hull shape will need to be more efficient than if you're going to be a solar-only system. I'm not sure how you could do drag tests without building some way to do tank-style testing.

(You could build a frame to drag the hull through the water with an electric winch, as I mentioned before. The model woud be attached to instruments on the frame to measure and record actual drag and actual speed through the water.)

I've experienced 60 kt winds from directly overhead during a summer thunderstorm. Rare, yes, but quite frightening in an 18' boat. Winds like that could turn a catamaran into a wing, lifting and then flipping it over. This is why a designer might do a wind tunnel test on a hull, but no I don't think you need to build one unless you also try to figure out at what wind speed the hull builds enough lift to overturn it - not easy to do with a fan and a cardboard tube.

I've been assuming this senior project is an engineering project for school. Is this an accurate assumption?

[Amgine]

If I understand the project correctly, Depthofit will be building a 1:20 scale mode - 30cm - on which to perform tests. Then they'll build a full scale model.

Since the question of materials hasn't come up, I assume that's set in stone.

But I have this sudden image of the Dads at a cub scout's pinewood derby event... where the kids have pretty much been left out of the picture.

[Roy M]

I'm still confused. As I understand it, the design is for a projected 18' catamaran. That is a very light craft. Subjecting a very light craft, without a mast or sails, to a wind tunnel test of the vessel headed directly into the wind, gives results, but what is the objective? As wind speed increases, the vessel will eventually simply become airborne and flip backwards, won't it? What does that show about the seaworthiness or efficiency of the design. If the construction of the model is reflective of the relative mass of the actual boat, and if the waves generated by the wind reach significant height, assuming realistic fetch, still the question arises, why are you doing this demonstration?

I can understand using smoke and slow motion video taping to show vortices and other effects, but we're talking about testing for what?

[Amgine]

More a case of "don't go out in thunderstorms, and here's why". And it's for the 30cm (12") *model* .

[depthofit]

Ya, this is a engineering project. I am currently a 3rd year diploma student in marine engineering and i have very little knowledge in naval architecture.

No, we are not intending to build the actual one. Our project will only revolve around the model.

Firstly in response to Roy M, i am not very familiar with a wind tunnel but i assume tat if i were to put my model through a wind tunnel test the purpose would be to find out the threshold of the hullform, at what wind speed will the craft flip and from there we can sort of like do something abt it. Lets look at it this way, this project is like a demonstration of us understanding various engineering concepts, the wind tunnel experiment will show that my team wouldn not just build a model and claim that it is good and will overcome anything, we are willing to carry out experiments to back that up. I thought tats the difference between somebody who have went through an engineering course and somebody who didnt.

Please correct me if i said anything stupid.

[depthofit]

Ya, this is a engineering project. I am currently a 3rd year diploma student in marine engineering and i have very little knowledge in naval architecture.

No, we are not intending to build the actual one. Our project will only revolve around the model.

Firstly in response to Roy M, i am not very familiar with a wind tunnel but i assume tat if i were to put my model through a wind tunnel test the purpose would be to find out the threshold of the hullform, at what wind speed will the craft flip and from there we can sort of like do something abt it. Lets look at it this way, this project is like a demonstration of us understanding the engineering concepts that we have been taught and are able to apply it, the wind tunnel experiment will show that my team wouldn just build a model and claim that it is good, we are willing to carry out experiments to back that up with values.

In response to Amgine, the intended model will be build using balsa wood and coated with epoxy. I know that in order for the whole project to be convincing, the model has to be as similar as possible to the "actual".(But we are nv going to build the actual)The best we can do is to stick to a logically proportional weight, the weight distribution should be as close as possible, a similar coating(epoxy) so that the drag we obtain will be as convincing as possible. I will be reading up on ship building and hullform of catamaran asap but for the time being any suggestions regarding this will be very helpful.

Months ago i have already attempt building a model using balsa wood and coating with lacquer. We conducted the drag test in a pool by towing our model manually at various speed, using nylon thread attached to a strain guage and to the model and plotted a curve of resistance vs speed. When we show our model to a lecturer who is a naval architect, he pointed out several flaw in our experiment and model.

Our model clearly shows that we have poor knowledge in naval architecture and we are clearly morons because previously my model was based very little on existing concept and it was more of a my-own-design.(i still have my amas and akas)

I didnt know the weight distribution, righting moment, metacentric height,...... Evrything that i have been taught was not utilised. The coating should also be the same as the "actual" by we used lacquer.

When he asked, "what type of block coefficient are we looking at?"
Only then, i was like "why didnt i include all that?".

During our experiment, we towed our model across a fixed distance at various speed manually resulting in many inconsistency, making our curve highly unreliable.(thats what he said)

So this time we decided to redo everything and include more experiments.


Sorry for the long and unorganise post, please correct me if i said anything stupid.

[Roy M]

You are clearly not stupid. Stupid would have been looking at your professor with a confused, glazed expression when he started tossing out terms such as "block coefficient", and other technical expressions that are meaningless to me. What I am referring to is simply that, for an experiment in naval design to have significance to learning, there ought to be some degree of relativity and connection to an actual condition one might encounter on the water in a vessel of the size and characteristics of the design in question.

As you observed, substituting a coat of lacquer for a coat of epoxy, is probably more appropriate when designing a model catamaran, as opposed to a model monohull or oil tanker. What I was questioning is the reason for a wind tunnel experiment AT ALL. A power boat, heading into stiff winds, and bucking short square waves, might exhibit some of the design flaws or benefits of the engineer's experiment. A sailboat (which, I must admit, you may not be considering) doesn't sail directly into the wind, unless at anchor or when tacking. And, for a wind tunnel experiment with a sailboat, you would have to explore several points of sail (close reach, beam reach, downwind) to understand the effects, more fully, on the hull design.

It is good you are enthusiastic about this process of learning and testing your ideas. I am only suggesting that you attempt to make your designs reflect more accurately the real environments that a vessel might experience. This is an exciting time in marine design. Some people have reexamined the anchor and brought new performance and safety to our lives. Others have explored the limits of exotic materials or alternative hull forms. Such experiments often result in failure, but they may also reveal design secrets to the observant investigator. Remember Edison's failures with incandescent lamps. What this world needs are more curious students, as yourself, who search for cost-effective, environmentally supportive ways to transport people and cargo across the seas, and to do so while protecting their crews from the hazards that exist in that very real world. When you complete your testing, would you show us some photos of the results?

[Amgine]

It sounds to me like your original experiments are a good start, and your teacher is interested in how you will overcome the weaknesses of your original method.

To determine the drag you will need a form of logging strain gauge and an instrument which measures actual speed through the water. This will allow you determine what the actual drag is at real speeds.

One way to do this would be to build a frame mounted on pontoons. Mount the strain gauge on the frame, and attach the model. Mount the speed log to the frame as well. Then drag the frame across the water. Using a winch should allow you to do repeated tests at controlled rates of speed.

Using the frame you can also mount model at different angles of heel, and test the drag, which would be important for a sailing model but not too important for your electric drive. You can also use the frame and two or more strain guages to measure the force developed by your steering mechanism.

For your construction method... if this will be a solid balsa carved hulls model it may be easier to test the difference between u-bottomed hulls and flat surfaces. If your hulls will be constructed of thin panels of balsa you will have a much lighter design, but it may be somewhat more difficult to get the rounded bottoms.

You might do some searching online for model-building of catamarans in balsa: I suspect there are people who have done so.

[depthofit]

First of all, thnks alot guys.

Roy M, i guess i understand what u have driving to me all along, it is meaningless for my model catamaran to go through a wind tunnel test since the test is not simulating anything that is going to be experienced by the 'actual' boat and the results will have little use. You suggested that we can investigate real life problems and issues and explore the unexplored areas. Am i right? Ya sure i will definitely post the photos of my project after it is completed.



Amgine, if i have a pontoon in front of the model during testing, would that affect the wave the model is experiencing?