# Foldable catamaran

Discussion in 'Boat Design' started by skatun, Nov 20, 2018.

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?

## Feasible?

3 vote(s)
60.0%
2. ### No

2 vote(s)
40.0%
1. Joined: Sep 2010
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### skatunJunior Member

The critical point is the air thickness between the windows.. and that they are 100% air tight...

10mm gap:

30mm gap

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### JamesG123Senior Member

Attack of the Maths! lol

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### HeimfriedSenior Member

The way you are calculating the thermal resistance of the air gap between the acrylic panels is not appropriate, because the convective flow of heat is not included. In engineering tables for heat energy calculations on buildings an vertical oriented air gap with a thickness of 10 mm to 20 mm is set to the value of 0.14 (m² K)/W (regardless if it is 10 mm or 20 mm or inbetween). Likewise an air gap of >20 mm to 500 mm is set to 0.17 (m² K)/W .

In addition you should consider the heat transmission resistance between air and surface. The values are depending on wind velocity and the used value of (about) 40 W/(m² K) is for buildings in average climatic conditions in Germany. A boat is potentially prone to considerably more wind. And the heat transition resistance will decrease. Depending on how your heating will work it may be you will get different layers of air temperature, e. g. 16 °C on the floor and 28 °C and more at the ceiling, so your Delta T goes up and so does the heat loss. If you use a ventilator, this would be probably better but the same time you are increasing the thermal loss because of lowering the transmission resistance at the inner surface.

So most probably your calculations will give you a too optimistic result.

Last edited: Jan 25, 2019
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### skatunJunior Member

Excelent, can you share these?

Yes, I have included the nusselt number yet, I will get around to it...

Was thinking about using a heatpump with saltwater and circulate heating coolant in the floor, any suggestion around this, links to where to get heatpump?

At this stage I am just trying to figure out roughly if I can have windows all around, or if I should only have partial windows, lets say from 90cm and up to ceiling. Also if I can use windows with aluminium frame, because then i can weld them straight in and dont have to worry about leakages.(changing windows might be hard though). Glass windows does have better R values, but are heavier and I am not sure how well they are in heavy seas. So any tips around the window topic will be highly appreciated.

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### skatunJunior Member

Updated numbers:

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### HeimfriedSenior Member

Sorry, no, the tables are part of printed books.

Searching the web "books.google.de" provided some pages of "Lehrbuch der Bauphysik", Jenisch, Storer et.al.
A table there shows the resistance (exterior surface) decreases from 0.04 (m² K)/W for wind speed 4 m/s to 0.02 (m² K)/W for 10 m/s.

Heatpumps I know, are electric driven an that is potentially a problem on board, because you need AFAIK about a quarter of the generated thermal energy as electric energy input.
I'm building a kind of houseboat, but only for weekends and holidays, not to live on it. So heating is not a point for me (may be later for use in spring or autumn).
This thread could give you some ideas about a liveaboard katamaran planning in many aspects (e.g. windows):
boote-forum.de - Das Forum rund um Boote https://www.boote-forum.de/showthread.php?t=270807

Last edited: Jan 25, 2019
7. Joined: Sep 2010
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### skatunJunior Member

So this weekend I ran into a guy from the car racing society, and he told me how he would make the glass panels:

1 layer of thin glass(<1mm) laminated to 3mm acrylic and then darkening film, then air gap, then electrical film(for curtain), 3mm acrylic and final layer of thin glass for scratch resistance.

Now I only need to figure out how expensive such a glass is and who can produce it...

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### skatunJunior Member

Sorry for not updating lately,

So I have done some work lately, mainly I have finished in CAD the mechanism. I am now gonna lasercut beams in 3mm plywood for the 1/10 scale mockup. The bushing and 3d print material for the mockup arrived.

I started to do some CFD with OpenFoam, but this will take some time to finish, works with stationary flow, working on stokes waves now.

So here is the problem that has been bugging me lately, I do have 1m x 1.2m as maximum envelope for the hull X section shape. So how to shape the hull? If I want a draft of 500mm the hull needs to look like below, this gives me 700mm from the waterline to underneeth the cabin on top of the hulls. I could reduce the width to decrease the resistance but then the distance to the cabin would be reduced, is this a problem. How little clearance to the waterline can you have?

So what is the recommended distance from the waterline to underneeth the cabin? Where can I find literature on how to design catamaran hulls? The DNV standard does not specify this distance. Any tips and hints on this will be highly appreciated.

Below some screenshots of progress.

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### DejayJunior Member

I keep finding interesting threads in this forum I'm also interested in solar electric power boats but as a liveaboard option to travel slowly around Europe. Kite sails also seem to have potential. I seem to spiral a bit from crazy idea to crazy idea.

My opinion on the folding mechanism as a novice is that it could work but probably isn't worth it. I've seen the videos before but there are a lot of steps after folding and a lot of possible failure points. You could consider a "slide out" like they have for RVs. An inner box that slides out of the outer box. But even that creates complexity, sealing issues, reduces strength and stiffness. And it adds weight, which is bad for propulsion energy efficiency and build costs. The large glass windows would also add lots of weight and would cause insulation problems and cost a fortune.

Shipping in containers I'd look at the demountable designs from Richard Woods (demountable 36) or James Wharram who uses lashings to tie the amas together. Folding connection between the amas probably won't save you that much time but require more compromises. Maybe you can have your bridge deck build as separate wall pieces to fit in a container and you only glue and screw them together. That is the only way I see a full bridge deck fitting in a single container with the amas. You seem to want a wide bridgedeck saloon and 2.4m isn't enough?

About the bridgedeck clearance, there are some threads and articles on that with rules of thumb. It has to get longer dependent on length, how long / full your bridgedeck is and how wide your beam is. Also increases windage. A larger sailing vessel can compensate by having more sail. But the low power density of electric doesn't scale as well.

I found your other thread first about the foiling for energy conservation. I too am interested in this. A spreadsheet (hull calc) tells me that 6 knots with a lightweight 40' 3.5t catamaran should be possible with 2kW input power assuming a 50% efficient motor+prop. You don't need much to go slow in displacement mode. But in less than ideal conditions with wind and high windage of a full bridgedeck it might look pretty bad. At speeds of 8kts or 10kts you need more length and multitudes of power. I have yet to experiment with michlet or maybe openFoam to see what is realistically possible.

I'm currently working through the Hydrofoils book by Ray Vellinga and want to figure out if that would enable faster speeds on low electric power. Say 20 kts with less than 20 kW. The induced drag from the lifting action of the foil is relatively low and the parasitic drag from less wetted area is reduced too, so it would stand to reason that it would be more energy efficient than planing or displacement. The power curve seems to be almost linear with increasing the target cruising speed because you need less and less surface area and create less drag. Lots of complications with this of course. I haven't found much discussions on this though, only on racing or super big ferry hydrofoils. The sensible compromise is probably adding the foils to provide partial lift. But if you really want to make use of this you probably need to optimize the design from the start. I do think there is quite some potential.

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### skatunJunior Member

A quick input on the numbers given in the article...

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### DejayJunior Member

Ok I just looked at the picture you posted for inspiration and just had an idea that might be of interest for you. Or maybe for me. They call it the design spiral but really I'm just spiraling haha. But it's not a folding design, more like a container tetris design.

With amas in "L" profile you could have two hulls manufactured and shipped in one container, plus the beams connecting them (possibly more than 2) plus a floor that goes between the amas. Height of the hull would be 2m and the inset would be 1m x 1m for example. You can put bunks or seating in there or kitchen stuff. You'd have a long corridor where you can access everything and already have all the stuff you need for sleeping cooking maybe even showers and toilets at the ends.

Now you can imagine you could build a glass palace / lounge on top of that big platform without having to accommodate the accommodations. No idea if this helps you but I wanted to make a quick render to how it would look like:

The hulls are symmetrical front/back up/down so you could use one mold 8 times. It's not typical catamaran but with long slender hulls and low speeds I think it's irrelevant. The idea is from the "intelligent infusion" of the harry proa designs.

PS: Good question would be if you can allow for less bridgedeck clearance near the hulls or if you'd have to make the berths higher.

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### skatunJunior Member

Nice work, which CAD software do you use @Dejay

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### DejayJunior Member

Fusion 360. But I just realized that this would of course NOT fit into a container if it's a 2m x 2m "L" shape... I used to be good at Tetris!

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### skatunJunior Member

So I have now started also with some first estimates of structural analysis, however more to come.

Does anyone know how much weight for appliances, cables, pipes etc I should give as an estimate for the total weight for a 40 feet cat?

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### skatunJunior Member

Just my feedback on this....

or with your estimate of 3500kg

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