Heat Shield

[lighthousekeeper]

Hi Folks, just a question my boiler will be about 5 inchs from the bottom of my wood boat, cedar strip epoxied inside and out. The base of the boiler is 24 by 24 inchs should I install a heat shield underneath the boiler? thanks!

[froya66]

Hi

What is the bottom of you boiler - an uninsulated ash pan - taking siftings/ash from a grate, or something else?

I have an uninsulated ash pan Ø200 - 1" above an MDF plate. That is for sure too close. It is too hot and catches fire if my grate gets worn out and let too much coal down the ash pan.

But 5" seems very safe to me - regardless of the boiler bottom temperature.

If you want to be 500% sure, you could install a 1/2" layer of ceramic fiber at the wooden bottom. This is what I intend to do with my new boiler as I do not want to have that much air gap beneath the ash pan.

Best regards
Jørgen

[fredrosse]

Need more details about the boiler.

If a boiling water bottom, such as a scotch type boiler, then one inch of insulation on the boiler itself, with an air gap ventilation of an inch or more to the hull would be OK.

If it is a hot ash pan as the boiler bottom, then a wet pan below the ash pan can keep 100C (212F) max temp, I would suggest one inch of insulation on the bottom of the wet pan, with an air gap of an inch or more to the hull would be OK.

If it is a hot ash pan as the boiler bottom, then a 2 to 3 inch layer of hi temp insulation below the ash pan can work, provided there is some air gap for cooling air flow, at least 2 inches of open space between the hull and the bottom of the insulation.

A ventilating space under the insulation is required. If you put 5 inches of insulation with no air ventilation between the insulation and hull, the hull will overheat.

[barts]

Note that a sheet of stainless on standoffs will work well so long as there's airspace on either side. This can be formed in the manner of a tray w/ folded up edges. This is pretty standard in marine stoves, for example. Insulation is problematic unless enclosed to prevent it from getting wet in rain, etc.

- Bart

[fredrosse]

I second Barts input here, one or more layers of sheet metal, the closer to poilshed the beter, forms reasonably good insulation for high temperatures. A sandwich with 20 gauge sheet metal, stainless or aluminum, or brass for that matter, with about 1/2 inch between the sheets, and several layers of ordinary aluminum foil will do a good job as a heat shield. Air gaps are required. A real advantage in that the sheet metal and foil sandwich is not injured by water in the bilge.

Maybe sea (salt) water is a problem for the aluminum, but that is easily replaced with low cost if stolen from the kitchen at the right moment. However if your "better half" catches you, there may be consequences.

[Lopez Mike]

The original builder installed an assembly that seems to be sheet metal over maybe a half inch of some sort of solid insulation. I believe it came from Beckman. I fire with wood and the ashes just fall on it from the grate maybe four inches above it. The first layer of ashes act as a very effective insulating layer. No smoke coming from the wood platform under the insulation yet.

[Jack Innes]

Not that it would have an application in our type of steamboats but many years ago I discovered the remains of about a 90' steamboat that spent its working life on Ahmic Lake in Northern Ontario. It had been stripped of its machinery & run aground in a bay & left to rot. By the time I found it the elements had reduced it to the waterline but the bottom was intact. The boiler supports were there as well as a single layer of red, clay bricks laid flat, without mortar, on top of the planking under the boiler. The bricks had obviously insulated the hull from the many years of wood fire in the boiler.
The supports held the boiler about 10" above the sleepers.

Jack

[DetroiTug]

I have no insulation under the boiler and I have not observed any excessive high temps underneath the boiler i.e. painted steel keel directly below and no damage from heat. There is so much cold air being pulled in to the ash area below the grate, it probably keeps the bottom cool. However, with draft closed and the fire banked, it could get warm. I've just never observed that and there is no evidence of it. Better to be safe than sorry with a flammable hull though. And as others point out above - shields on standoffs work very well to block radiant heat. That is what is used on modern home wood heating stoves. Usually two shields stacked with airspace atop one another.

-Ron

[lighthousekeeper]

here is a pic of my boiler had a guy in Seattle build it for me

https://scontent-ord.xx.fbcdn.net/hphot ... e=558218E8

[Lopez Mike]

Enjoy your hearing while it lasts!!

I think you might end up moving that whistle up away from your ears a bit.