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Hulls - Wet or Dry?

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Hulls - Wet or Dry?

Postby Lash151 » Fri Mar 08, 2013 4:18 am

I imagine this has been discussed a thousand times but I can't find any debates on it... My obvious question is... what are the pros and cons of wet or dry hulls.
I am planning to build a 1/80th scale model of Surcouf using wood... I like the idea of a dry hull because it feels more natural to me but that isn't based on any science... what are your thoughts on this? Thanks in advance for your help guys.
Lee
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Re: Hulls - Wet or Dry?

Postby Sub culture » Fri Mar 08, 2013 7:03 am

I think Jeff answered this pretty well in your other thread.

Dry hulls are really a throw back to an era when batteries and radio equipment were physically much bulkier, and in order to get sufficient space to house them, the hull was made completely buoyant.

These days unless it's a small boat, a dry hull doesn't offer any advantage, and you're just building something that will keep your chiropractor well employed.

It's also worth bearing in mind that most full size submarines are in fact wet hulls, especially boats built between the wars like Surcouf, where the outer hull was optimised for surface travel, and the inner hull was a cylinder, required to take the crushing pressure at depth, with a myriad of slots cut into the outer hull to permit flooding of the space inbetween.

Also bear in mind that fullsize subs and miniature subs have rather different requirments. For the former, the designer has to take into account fuel, crew, housing of machinery , weapons etc. For a miniature version, we only need to take into account the equipment needs for control, batteries and ballast tank, and the latter should always be as small as you can get away with, as moving ballast about takes time and saps power.

Therefore we can build our cylinders physically smaller than the 1:1 counterparts.
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Re: Hulls - Wet or Dry?

Postby Lash151 » Fri Mar 08, 2013 7:35 am

Thanks... that settles that then... wet hull it is!
Lee
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Re: Hulls - Wet or Dry?

Postby Lash151 » Fri Mar 08, 2013 7:46 am

Hi... I was just watching a video where the guy was building a U boat using what I would call 'lost foam' technique... where he glassed over a polystyrene shape and then poured petrol in to dissolve the polystyrene.... I know you guys are probably very familiar with this technique... I hadn't seen it before, although I have used lost wax for casting smaller items.... this is worth thinking about.
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Re: Hulls - Wet or Dry?

Postby Sub culture » Fri Mar 08, 2013 9:22 am

The lost foam method is quite popular- a lot of members on here have used that technique to build hulls. It's quite economical, because you use very little material in construction, and foam is light and easy to work with, and inexpensive compared with most woods.

It's best to use the higher density blue or pink coloured foam, the polystyrene foam that you get in packaging is too soft and doesn't work very cleanly. Polyester resin dissolves polystyrene foam, epoxy resin doesn't. If you want to stick with polyester resin, then you can paint the foam with oil or emulsion paint or polyurethane varnish, which acts as a barrier coat.

Another alternative is to use polyurethane foam, which is the yellow stuff used for insulating lofts and also comes in aerosol tins for filling gaps in walls around pipes. You can use polyester or epoxy resin straight over this foam.

The sheets used to insulate lofts tend to be filled with glass, so wear gloves when sanding otherwise you'll be itching until Christmas.

I have used polyurethane and polystyrene foam for making hull masters, I skin the foam with a layer of resin and matting for strength, then finish with polyester filler. However, I then make a reverse mould from this, and cast my hulls conventionally. It's more work, but you can produce more hulls if you want quite easily and spares if required.

If you haven't praticed any of these techniques before, I advise you to learn on something small first. Perhaps you could try carving the turret for the Surcouf, being a relatively simple shape and small at the scale you will be working at. That way if you make a hash of things, you haven't lost too much.
'Why are you staring at an empty pond?'

Want to dive your boat in crystal clear water? Then you had better Dive-in- http://www.diveintomodelsubmarines.co.uk
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Re: Hulls - Wet or Dry?

Postby Lash151 » Fri Mar 08, 2013 10:19 am

Thanks for that advice Andy... I am talking to you on several different threads... sorry about that. I'm also sorry to bore you with my questioning... i promise i will settle on a method soon!
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