School project - Designing a fast cargo boat

Discussion in 'Boat Design' started by Krautern, Dec 2, 2014.

1. Joined: Dec 2014
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Location: Norway

KrauternJunior Member

Hello, boat designers.

We are supposed to design and build a cargo boat made out of polysterene. It's going to be 90cm long and must be able to carry a total cargo weight of 3kg in addition to a 3kg metal plate at the bottom of the boat. To win this, our boat has to be fast, stable and easy to steer in a pool. So we're just wondering if you have any ideas on how we can make this boat superior. We are using the program Freeship to design it.

So we are curious, how should the hull be formed? And should we use a bulb at the bow? Thanks for any and all answers.

2. Joined: May 2008
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Location: Beaufort, SC and H'ville, NC

philSweetSenior Member

The three criteria are in competition with one another. Deciding on a good trade-off requires detailed knowledge of the course, the propulsion system, and the rules and scoring.

If you are mathematically inclined, you should begin by studying metacentric heights and how they are calculated and how they relate to stability. Then look at the displacement to length ratio of the craft, estimate the speed based on your power and efficiency, and choose a curve of displacement (the fore/aft volume distribution) that is appropriate for that speed.

Probably the best way to estimate speed is to get something in the water, however crude, which has a reasonable curve of displacement, and test it at the correct weight. You can make a better model based on the results.

What tools do you have to shape the foam? Foam in one block, or lifts of some thickness?

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Leo LazauskasSenior Member

Phil has given you great advice to start you off.

I would add that you should avoid a transom stern if possible. I doubt that
your small hull will travel at a Froude number where it is beneficial.

Find a hydrophobic surface coating that is allowed within the rules. Your
hull is very small and so surface tension effects will be important.
A hydrophobic coating will act to reduce (or completely eliminate) the meniscus
at the waterline which contributes to the resistance. Usually a meniscus is about
1.5mm high (but I'm happy to be corrected!). That's significant on a very small hull.

4. Joined: Jan 2006
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Location: Lakeland Fl USA

Use the internet to access various RC boat sites. There you will find the best of hulls and rigs and descriptions that may be useful. I suggest the International One Meter class boat for study. (IOM) That class of model boat is among the most developed classes in the world. I have never seen one with a bulb bow so the answer to your question is probably; No.

Your craftsmanship will be a major determinant in whether the boat is fast or easily propelled. Surface finish is a critical matter on such a boat. That means exceptional smooth lines and surfaces with absolutely no bumps or ripples. Final paint finish should be equivalent to at least 1000 grit wet or dry sandpaper with final sanding wet. You will find that type of sandpaper at automotive refinishing/paint and body stores, not likely in the hardware store. Avoid waxing the surface below the waterline.

Respondent Leo Laszauskas is a professor at the University of Adelaide, Phil Sweet is an acknowledged authority. You have gotten valuable counsel from two of the best of minds out there. I am merely an old model maker that has learned from experience.

Good luck with your project. We will be interested in the outcome. Keep us informed.

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TANSLSenior Member

Krautern has been asked to "design and build a cargo boat". Is it true that the rules for International One Meter class boat can help him?. How ?. I´m curious.

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Leo LazauskasSenior Member

No I'm not.
I was involved in research there, but I was not, nor am I an academic, e.g. I
have never given a lecture.

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philSweetSenior Member

... And I'm just a fellow boater. I'm not and never have been in the business. I have a technical background, but boats are just a hobby.

It would help if the OP would identify what class this is for and what classes he has behind him. If this is in pursuit of an NA degree or some such, then I should probably bow out.

8. Joined: Sep 2011
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TANSLSenior Member

It would be necessary to have more data to give valid answers. For example, the total weight at full load, should be 6 kg plus the hull weight ?.
To be a fast boat is necessary that the ratio L / B is greater than normal. A small breadth reduces the stability but this should not be a problem since the 3 kg of iron at the bottom will give you more than enough stability. So you can go to a relatively small breadth.
The block coefficient should also be small, say 0.65.
With all this, assuming a beam of 0.1 m and a waterline length of 0.9 m, we get a draft of 0.11 m.
Very thin bow, like a warship, and transom at the stern.
Although the above may be wrong, it can be a starting point to begin talking.

9. Joined: Jan 2006
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Location: Lakeland Fl USA

I've been around the forum for long enough to sort out who is who.. We have some exceptional contributors here. We have engineers, physicists, aerodynamicists, mathematicians NAs, veteran boat builders and venerable old salts who know a thing or two.

You two can put away your gentlemanly modesty.

Lets see if we can offer some good advice to the student OP as has been in so many other cases of the past.

I'm still pleased and proud that we gave so much encouragement to the young fellow in Lithuania who has, for two seasons, desperately tried to make a hopeless boat into a sailing dinghy.

Please accept my respect and admiration for the good guys here, one and all. We have a lot of them on this forum.

10. Joined: Dec 2014
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Location: Norway

KrauternJunior Member

Hey guys! Sorry for the late response and thanks for all the answers
I can see that I have not given you enough information about the project.

The model shall (rules):
Be built in a 1:100 scale of a ship that is 90m long (Total length), and must be formed with one hull of hard polystyrene. (We will glue together slices of polystyrene and use screws as well. Then form it by using knifes and sandpaper)
Be able to carry a cargo of total 3.0 kg water, divided on six juice boxes. These will simulate containers and must be places on deck. The deck must be over the waterline.
Be equipped with a 3kg metal plate some place at the bottom of the ship. (Most likely inside the polystyrene)
Be powered by an electric battery, motor and propel. (What kind has already been decided by our teacher)
Have some kind of a structure on the deck. (For example where the captain is)
And we must design the ship with the data program “freeship”. We will also use “freeship” to calculate the water displacement, the metacenter and the resistance. (We must find the center of mass by ourselves)
Other stuff:
We can choose the width of the ship, and the design and shape of the hull.
I do not know the weight of the hull, the motor etc….. But I will try to find that out.
We are 17 years old high-school students from Norway with little knowledge about boats, but we have had some lessons on how the metacenter, the center of mass and water displacement works. We can handle some math and physics, so don’t hold it back!

11. Joined: Jun 2010
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NoEyeDeerSenior Member

Couple of quick observations.

1/ Don't use screws. They're just extra weight for no benefit. The glue is more than adequate by itself.

2/ Put the steel plate underneath the foam, and recessed flush with the foam. It can be secured by glue (again, more than adequate). This will give you a slighter lower CG and therefore better stability.

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NoEyeDeerSenior Member

Oh and:

3/ If allowed, make sure the juice boxes are entirely full of water, and capped, to eliminate any free surface effect. If necessary (and allowed) fill any excess volume with chunks of foam.

4/ Work it out so that the weight of the juice boxes is supported by the sides of the hull, enabling you to minimise deck weight.

13. Joined: Jun 2010
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NoEyeDeerSenior Member

Oh and here's another idea: if you can arrange the weight of the deck cargo to be supported by the hull sides, use something ridiculously light, like tissue paper or cellophane, for the deck.

The rules you quoted say "one hull of hard polystyrene" but say nothing about materials for the deck.

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gonzoSenior Member

How can you choose the width if the rules say it has to built to scale? By the way, where in Norway are you? I can hook you up with a shipwright friend of mine. We have worked together and he is really good at it.

15. Joined: Dec 2014
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Location: Norway

KrauternJunior Member

NoEyeDeer:
I think we have to use screws to be sure that the polystyrene frames will stay 100% together. So maybe we could use plastic or wooden screws instead, so as to minimize the weight?

When it comes to the juice boxes, they will be 0,5l each, so we will have to fill them completely.

Gonzo:
It is only the length that has to be to scale. The width is completely up to us. We are living close to Oslo

Btw, i have posted out two photos of earlier winner of the contest on my profile. Thought it could give you guys a better picture of the project.

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