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Wood Burner on GRP Cruiser


Sharkness

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Hi,

 

I'm in the process of installing a wood burner on a GRP cruiser. The top of the burner sits roughly level with the gunwale and about 120mm from the side of the boat at that point, most of the burner is much further away from the hull because it is next to the part of the hull which is under the gunwale. The flue goes through a wooden wall and is wrapped in 1200°c-proof furnace insulation at the point where it goes through the wall, and then it goes through the roof of the boat through a dektite roof flashing which is filled with the furnace insulation too. The flue is wrapped in exhaust wrap at the point it goes through the flashing and then sealed around with kapton tape. I'm certain that the flue is adequately insulated and will cause me no problems, the technique used for the flue is the same as my brother has used in a bell tent for years with no damage to the canvas.

 

My concern though, is how to protect the hull at the side of the stove. My furnace insulation is bits left over from a pizza oven I built earlier this year. The insulation goes over the top of the oven and the bottom of the oven sits on a wooden table protected by vermiculite. Pizzas are cooked with the stone temperature at 500°c and there are no signs of damage whatsoever on the wooden table it sits on with a vermiculite board underneath it so I'm guessing in an ideal world, that vermiculite would be the option to go for, but I'm trying to do this on the cheap and it'd cost me more in vermiculite than I've spent on the stove! I can get Hardiebacker boards for next to nothing from a friend who uses them for bathroom installations and he reckons that it'll do the job just glued on to the wall above the gunwale and that will create a cavity area between the board and the hull below the gunwale. Does this sound reasonable? I have read that at 80°, damage can be done to the resin in the hull, 80° doesn't sound a lot to me! 

IMG_20181212_134810422.jpg

Edited by Sharkness
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I would guess, that if you have good airflow at the back of the backerboard, should stop it transmitting any heat to the fiberglass. I've had a rocket stove in action next to a wood wall and found it stayed cool, with a thin metal sheet acting as a deflector and good airflow behind.

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47 minutes ago, Bugbugs said:

I would guess, that if you have good airflow at the back of the backerboard, should stop it transmitting any heat to the fiberglass. I've had a rocket stove in action next to a wood wall and found it stayed cool, with a thin metal sheet acting as a deflector and good airflow behind.

Cheers. The board will be flush to the hull with no cavity in the section above the gunwale but with a significant cavity below the gunwale with things as they are at the moment. I presume a gap behind the backerboard makes more difference than one in front of it? If that is the case, it might be that I can put some timber on the wall above the gunwale to bring the board a bit closer to the stove and achieve a gap behind it. The gap in the area below the gunwale is more than enough I would think

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2 hours ago, Sharkness said:

I'm guessing in an ideal world, that vermiculite would be the option to go for, but I'm trying to do this on the cheap and it'd cost me more in vermiculite than I've spent on the stove!

The question may be more along the lines of is your boat worth more than the cost of using the correct board ?

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Thinking about it, you could use the same system, as I did to make the burn chamber of the rocket stove - just get vermiculite granulit (I got a big bag of it for £5 from a garden centre), mix this with firecement (a tub from B&Q) was about £5 and you can build anything (including a burn chamber) .... including, in this case case a board, if that is more of use. I'm a ceramicist in my spare time and built a few kilns, my current one sits in a wooden garden shed ...... which is a bit more susceptible then fiberglass ;)

 

 

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13 minutes ago, Alan de Enfield said:

The question may be more along the lines of is your boat worth more than the cost of using the correct board ?

Nope, the question is definitely along the lines of "is there a product that provides an adequate amount of protection for less money than vermiculite?".

 

I'm not asking whether I can make a protective board out of sawdust and tissue paper, I'm researching a cost-effective and viable alternative. I don't even know what the 'correct board' is, hence me asking the question. Vermiculite is probably overkill seeing as it is heat resistant to over 1100 centigrade which is considerably more than I will ever kick out of my stove. I'm looking for something that will protect from temperatures of up to 300 degrees.

 

Smart-assery isn't helpful

12 minutes ago, Bugbugs said:

Thinking about it, you could use the same system, as I did to make the burn chamber of the rocket stove - just get vermiculite granulit (I got a big bag of it for £5 from a garden centre), mix this with firecement (a tub from B&Q) was about £5 and you can build anything (including a burn chamber) .... including, in this case case a board, if that is more of use. I'm a ceramicist in my spare time and built a few kilns, my current one sits in a wooden garden shed ...... which is a bit more susceptible then fiberglass ;)

 

 

Cheers, would you suggest making a wooden frame and making the board in it before removing, maybe? Doesn't fire cement need quite a bit of heat to go off? I have a tub at home that suggests applying it and gradually increasing the stove temperature to set it

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5 minutes ago, Sharkness said:

Nope, the question is definitely along the lines of "is there a product that provides an adequate amount of protection for less money than vermiculite?".

 

I'm not asking whether I can make a protective board out of sawdust and tissue paper, I'm researching a cost-effective and viable alternative. I don't even know what the 'correct board' is, hence me asking the question. Vermiculite is probably overkill seeing as it is heat resistant to over 1100 centigrade which is considerably more than I will ever kick out of my stove. I'm looking for something that will protect from temperatures of up to 300 degrees.

 

Smart-assery isn't helpful

I think there is an optimum minimum distance between a stove and a timber structure (wall lard etc). Someone helpful will be along to give you exact amounts, but I do know that heat can transfer through the air a bit further than one thinks. A neighbour set fire to his boat this way and woke up to a boatful of smoke. The worst bit was cleaning up the powder from the extinguisher afterwards. As his neighbour I had to help, and will never forget the mess.

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3 minutes ago, Stilllearning said:

I think there is an optimum minimum distance between a stove and a timber structure 

The document above suggests about 80mm as a minimum. So OP’s 120mm would be fine for wood. Dunno about GRP though. 

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29 minutes ago, Sharkness said:

Doesn't fire cement need quite a bit of heat to go off? I have a tub at home that suggests applying it and gradually increasing the stove temperature to set it

It will dry and act as a cement for the mix, even with a little heat or no heat but if you wanted it to be less brittle or actually turn very hard, then make small tiles and bake, grill or fire them in an oven or fire pit. The advantage of tiles, is, that you could make them modular and silicone (high temp gasket stuff would be good in hot spots) but actually most normal stuff will handle what manages to get through.

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17 minutes ago, WotEver said:

The document above suggests about 80mm as a minimum. So OP’s 120mm would be fine for wood. Dunno about GRP though. 

I saw a Reliant 3-wheeler catch fire, the brakes overheated and from the 1st signs of smoke to total destruction and a heap of 'black ash' was less than 1 minute. GRP is "not particularly resistant to fire."

 

Using wood is much, much better - it is going off at a tangent, but when we had our house built they used Steel "RSJ's" and then boarded them in with fire resistant plaster board, I asked why, to be told that steel RSJ do not react well in fire, they sag allowing the walls to fall inwards and block doors,  crush anyone in the room etc - wooden beams on the other hand 'char' and the 'char' then protects the structure of the beam and it will function many, many times longer than an RSJ.

 

Maybe the OP could look at using fireproof plaster board - but check with your insurance company that they will accept 'non-recommended' materials and still provide 'cover'.

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29 minutes ago, Stilllearning said:

heat can transfer through the air a bit further than one thinks.

Well, it's radiated heat (infra red), which does it, so that's why you use stuff which does not allow this heat transmission, ie you block it with vermiculite, metal & air gap, or any other material which acts as an insulator .... or make sure, depending on the heat you run your stove at - coke, diesel, charcoal or wood are all different outputs, that the distance to stuff which you could ignite through pyrolysis, is fare enough away 

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29 minutes ago, Alan de Enfield said:

I saw a Reliant 3-wheeler catch fire, the brakes overheated and from the 1st signs of smoke to total destruction and a heap of 'black ash' was less than 1 minute.

Interesting little metal ‘T’ left when that happens ;)

 

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1 minute ago, WotEver said:

Ahh, I saw one go up and it had a T-shaped steel chassis. 

 

You sure it wasn’t a Morgan you saw?

No - I had a Reliant 'Robin' Van at the time and it was a van I was passing as it burst inti flame, by the time I had passed to and pulled into help it was 'gone'.

 

Mine certainly had a wooden chassis as I drilled into it to mount a tow-bar.

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16 minutes ago, Alan de Enfield said:

No - I had a Reliant 'Robin' Van at the time and it was a van I was passing as it burst inti flame, by the time I had passed to and pulled into help it was 'gone'.

 

Mine certainly had a wooden chassis as I drilled into it to mount a tow-bar.

Wooden frame perhaps but they all had steel chassis going right back to 1935. My memory is wrong about it being T shaped though. 

 

Here’s a guy welding and bashing one about:

 

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3 minutes ago, WotEver said:

Wooden frame perhaps but they all had steel chassis going right back to 1935. My memory is wrong about it being T shaped though. 

Then I must be wrong as well - it must have been lost in the heap of carbonised Ex-GRP Van.

 

I certainly drilled into wood to fit the tow-bar.

 

One of my worst trips was in North Yorkshire going up Sutton-Bank (a 1 in 4 hill with hairpins) near Thirsk,  I couldn't make it and had to be towed up.

Found out the problem was a crack in the distributor cap and only running on 2 - 3 cylinders. I had 'happily come up the A1 at 70mph.

 

I did 'roll it' once, coming back from College late at night in the snow - being a smart arse I overtook a car creeping along in 2 inches of snow. As I overtook him the front wheel got a mind of its own as soon as it left the 'clear bit' of road and 'hit' the snow banks and over it went and into a ditch. We went and towed it out the following morning.

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4 hours ago, Alan de Enfield said:

Using wood is much, much better - it is going off at a tangent, but when we had our house built they used Steel "RSJ's" and then boarded them in with fire resistant plaster board, I asked why, to be told that steel RSJ do not react well in fire, they sag allowing the walls to fall inwards and block doors,  crush anyone in the room etc - wooden beams on the other hand 'char' and the 'char' then protects the structure of the beam and it will function many, many times longer than an RSJ.

 

Maybe the OP could look at using fireproof plaster board - but check with your insurance company that they will accept 'non-recommended' materials and still provide 'cover'.

There is some truth in that .

The rate of char for spruce is about 0.7mm per minute  on each face exposed to the fire . In structural design of timber the section remaining after the deduction of the charred material may be used to calculate the residual load capacity of the timber. If the timber is charring it is time to exit the building (or boat).

 

In residential (and other) building construction plasterboard is used for fire protection . However plasterboard  does not offer indefinite fire protection buy buys time while people escape. But I would say plasterboard  is not suitable for the purpose under discussion here .

 

Steel has relatively little inherent fire resistance which is why it usually needs to be fire protected.

RSJ is  a term no longer in use in structural engineering but lets not get too picky .?

 

6 hours ago, WotEver said:

I presume you’ve already seen this document?

http://www.soliftec.com/boat stoves 1-page.pdf

That seems to be providing the solution and product thickness in this case.

 

 

 

 

 

 

 

 

 

 

 

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Insulate the wall with an air gap behind and then put a shiny stainless steel sheet around the stove to reflect radiant heat back into the boat. Convected heat is not the problem, neither is conducted heat as the stove is not touching the wall.

Its the radiant heat that causes the fires and if you reflect it the problem is  solved.

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22 minutes ago, WotEver said:

But only for wood, not GRP. I suspect that GRP is less tolerant of heat than wood?

I had not spotted the drawing was specific to the hull material. GRP certainly burns very well once it gets going. I would imagine the need to protect from heat is the same for GRP and wood and steel  but that's just a guess. 

 

 

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