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I hope to do quite a bit of winter sailing / camping and need to heat the cabin from time to time. using my cook stove or any other open flame seems unsafe. Any ideas?


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Great stuff for Boxing day amusement....  So a few more numbers for you all.

We need to understand the heating requirement. First we need approximate numbers to see if a solution is viable. As Galway Bay has pointed out 60W is not a lot! And my gut feel is that electrical heating is a non-starter. So what might work:

Here are some guestimates.... 

A: Heat loss from hull/cabin

1. Hull section of a Tiki 21 is about 80cm in an equilateral triangle shape.

2. Cabin length to heat (not sure here but it won't matter a whole lot) is say 3 meters.

3. Surface area available to radiate heat away is therefore 7.2m^2

4. Epoxy conducts heat at least 2 x wood. So I'm ignoring it. So assume plywood only at 6mm

5. Ply conducts heat away at about 0.34 W/m2

6. Let's assume a cool night at 5 deg C and we want to be warm at 14 degs (9 deg difference across the ply)

7. On this basis we are radiating 1.4KW !

So we have a big problem without insulation. But if we add 6mm of polystyrene (or better) this goes down to about 325W! Where can we find 325W per hour for say 4 hours to keep warm during this cold evening?

I think the outboard is the best option. But I need to capture and store the heat rather than run the engine when stationary. Let's start with a basic water based heat store.

B. Heat Store

1. A 25 litre tank. (about 25kg weight plus say 10Kg metal work = 35Kg, so balance with batteries in the other hull?)

2. Heat to 80 degs and allow this to cool over 4 hours to 14 degs.

3. 1 gram of water contains 4.18 Joules per degree C

So if we could heat a water based store to 80 degs C and cool this over 4 hours it would deliver about 480W per hour. This is an average as the heat output is non-linear because it's a function of the temperature difference between the water store and the cabin temperature. But as I said we looking to validate a design at this stage.

C: Heat Source

1. I have a 5HP honda

2. Internal combustion engines work at around 20% efficiency - the rest is waste heat.

3. Assume 50% of the heat is in the exhaust gas - and 50% dissipates in the water cooling system

4. Run the engine at half power (2.5hp at 750W per hp)

5. Assume of the water heating power only 50% transfers to our heat store.

This means that the engine delivers about 300W to the heat store when running at half power.

So now we have some broad numbers and in conclusion with my crude hypothetical design, my engine can keep my cabin at 14 degs C on a cold night when running at half power. But I won't be enjoying that as I'll need to be at the helm if the engine is running. I can dump this heat into my store, but for 4 hour heating I'll need to run my engine for more than 4 hours to charge the store.

It's not looking so good, but there's hope. The numbers (if I've got them right after a few beers) are in the right area. As a starting point this isn't too bad. The trick now is to work on marginal gains throughout the system with good designs.

I think I can carry 22 litres of fuel with me. If the outside temperature was say 8 to 10 degs C, with insulation and a bit of tuning in my design, I might be able to keep warm for 2 to 3 hours before retiring to the sleeping bag.

Here's a few calcs if anyone wants to check for errors (which I've probably made)

So back to the beer and the drawing board ! 

 Ian - Sometimes the important points get lost in the details / figures so I hope you don't mind if I repeat a very good point you make - even 6mm of insulation keeps in 75% of the heat.

Add a little ventilation and you lose all the condensation. So it can be done.

Yes you're right, exactly! 6mm ply and we radiate at 1.4KW, but add 6mm polystyrene and this drops to 325W.

6. Let's assume a cool night at 5 deg C and we want to be warm at 14 degs (9 deg difference across the ply)

7. On this basis we are radiating 1.4KW !

So we have a big problem without insulation. But if we add 6mm of polystyrene (or better) this goes down to about 325W!

I've always lined the cabin sides of my boats with fluffy bathroom carpet.  It makes them much warmer to lay against, as well as more comfortable.  I think the fluffiness tends to hold a layer of air, which slows down the heat transfer.

Sorry Galway Bay, I miss understood your point. You were reiterating the insulating thing, not missing it. And the point is hidden in the detail. Sorry.

So the next step would be to turn this theoretical design into a real one. I think my design my have a problem with the temperature of the cooling water from the motor. I've no idea what it would be. 

I'm wondering if it would be possible to use sodium acetate as a heat store. It might be possible to charge this from the motor and release the heat on demand. This has the big advantage that you charge it when you need the motor and can release the heat weeks later!

Yes Ian thanks for all the detail posted it is all valuable. However it seems to be a feature of 'net posts that they are read with little attention or retention. It seems to be a medium that is not well suited to long explanations or making multiple points.

So I was repeating to emphasise your point.

In house insulation there is a saying " The first 10% does 90% of the work " Your figures make this point for our boats.

Robert this is an effective and comfortable and probably very decorative idea. Do you do anything on the cabintop ? I have a total of 18mm of timber [good] plus 25mm polystyrene. Just as in a house the roof is the primary source of heat loss. This also keeps the cabin cool in hot sunshine - now I can catch a comfortable snooze off watch.

On my Frances 26 I had a GRP / balsa sandwich deck, then screwed up 3mm plywood with a foam-backed vinyl glued and wrapped around it, and about 8mm polystyrene behind it.  Quite effective overall.  We had a big brass Taylors 030 paraffin cooker with an oven which got the cabin toasty warm, and of course it took ages for the oven to cool down once it was off.  All that was ideal on a deep keel yacht weighing 3.5 tons at 26 ft long.

The Tiki is a very different matter.  Zest is also 26 feet long but only a fraction of the weight, and every pound makes a difference to the performance, so a big brass oven is out of the question...  She is one of the Imagine Multihulls glass Tikis though, and the deck is a sandwich construction with about 8mm of foam in the middle of two very thin GRP skins.  I suspect it has about the same insulation value as 6mm ply plus 6mm polystyrene, but that's just a guess.  At the moment it's a case of "summer only" cruising in England, but hopefully in a few years time I'll be able to convert to using latitude heating...

Hi. Seems an old forum, but I still wanted to add my experience. I isolated the hull with extruded stirophome, cut and fitted (not glued) between the battens and some aluminium buble foil tackered to the seiling and as a curtain. Then I heated with a GAS LAMP - you get the light, too. I lived on board for one winter and brittany and it was nice and cozy, even once with snow and frost outside.

Hi, the biggest problem on these Tikis is that your boats walls are a bit underwater at the level of the bunk, so they are really cold when the water is cold. In May this year in Greece the water had only 16°C so it got cold down there... I did put all our sitting cushions against the walls and had a much more agreable cabin... But I needed to dry the matrasse much more when the water was that cold. I have always a couple of these rubber "Wärmflaschen" on board now, they are also nice when sailing at night just fill them with hot water and heat your feet or put them under the jacket ... :-) instant heating..

cheers Matthias

And to think - I was just going to post that as the sea water in Ireland remains at about 11c in the winter the boat afloat never gets very cold ! I spent one or two nights afloat every week through the winter of 12 / 13 with only a gas mantle lamp to take the edge off the chill... that and Guinness and great music !

Very true point. The berth of my Pahi - in the main cabins, is above the Waterline. Still, with isolation, having not much more than a coffin to heat, even a petroleum storm lantern could do it, tucked in my sleeping bag.

Anyone know of sailing to Spitzberg or cap Horn on a Wharram? It's not what it's made for, but loves makes us do crazy things.

I've sailed Ushuaia to Antartica on a tall ship and would definitely not recommend doing it on a Wharram... :-(

There's crazy and there's crazy (no offense meant if somebody's done it though).  Call me a wus if you like, but even with six layers of clothing I was getting cold after half an hour on watch.  Yachts do go down there but they tend to be fairly big and heavy displacement, well insulated with good heating inside.  You need something designed for spending lots of time down below.  Wharrams are the opposite, ideal for the outdoor lifestyle, enjoying the sun, partying, drinking rum punch, generally relaxing and having a good time.  England is as cold a place as I want to be on my Wharram!  ;-)

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