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#1 21 May 2023 5:27 pm

New Member
Registered: 21 May 2023
Posts: 1

XPS foam for Flotation Blocks - Tenderly Dinghy

I am having a happy time as a first time boat builder making lots of wire twisting and epoxy mistakes on a Tenderly build in my back garden. Having so far made something boat shaped that has some fibreglass attached, I am looking ahead in the instructions to making the flotation blocks for under the seats.
Our American friends seem to be able to cheaply and easily get hold of "Pink foam" or "Blue foam", both of which are styrofoam or closed cell extruded polystyrene, at any D.I.Y. outlet, but all my local B&Q, Wickes and Jewson stores can offer is expanded polystyrene or PIR board.
Searching online has not been fruitful, and the very best I can find is a site selling a pack of Ravatherm XPS 1200x600x50mm blocks for £175 delivered.
Before I take this option, does anyone have any better options or advice?


#2 22 May 2023 10:10 am

Registered: 24 Nov 2014
Posts: 97

Re: XPS foam for Flotation Blocks - Tenderly Dinghy

I have used PIR board.  The foil peels off easily and then you can cut it and shape it.  The foam itself is often scored with v-shaped grooves across it, hidden under the foil but visible from the edges.  Get some without the scores, if you can, otherwise you'll have to fill them, which makes it harder to get a flat surface.

I haven't tried expanded polystyrene, but I imagine it could be quite crumbly when you are trying to shape it.

I don't remember what it cost, but I only needed one full sheet for my Nesting Eastport Pram.  Full sheets were cheaper than smaller blocks, but won't fit in a car without cutting in half (very easy, but will make dust in the car).  I was lucky enough to have a Travis Perkins half a mile from home, so I just carried it down the street.


#3 25 May 2023 2:33 pm

Registered: 8 Jun 2011
Posts: 68

Re: XPS foam for Flotation Blocks - Tenderly Dinghy

Another option would be floatation or air bags.


#4 30 May 2023 10:22 am

Paul Green
Registered: 8 Nov 2020
Posts: 6

Re: XPS foam for Flotation Blocks - Tenderly Dinghy

I had same issue.  I could not find anywhere selling the equivalent of the Pink or Blue foam in small quantities at a reasonable price.     

I used PIR board on my Peapod build: Recticel Eurothan GP from the local Jewsons.  They were prepared to deliver a single 1200 x 2400 sheet as it would not fit in the car.  A friend doing the same build resorted to carving the sheet up in the builders merchant's car park. 

Other makes are available (e.g. Kingspan) but I went for the cheapest option I could find (£30 a sheet plus £10 delivery in 2022 - somewhat pricier now).

I would not recommend expanded polystyrene - it crumbles and does not carve or sand well.  As it is the PIR foam generates a blizzard of dust when you saw into it.

Beware the slight difference in size between US imperial sheets and UK metric (ours are slightly smaller).  The Peapod block components fit a US sheet but on a UK sheet I had to adjust the template patterns to fit.

On the subject of air bags.  I know of a peapod builder who used inflatable beach rollers under the side benches instead of the buoyancy under the centre thwart (they wanted a clear space for sleeping when camping) - they seem to be happy with the result and it probably provides more bouyancy than the original design.


#5 28 Jun 2023 3:13 pm

Registered: 24 Sep 2019
Posts: 2

Re: XPS foam for Flotation Blocks - Tenderly Dinghy

Extruded polystyrene [EPS] as you have found is not cheap or easy to get hold of in the UK. You could try a local builders merchant which might be a bit cheaper than internet suppliers though you'd still be looking at a full pack or swimming pool installers might be worth a try as they use it as insulation. It has plenty of advantages in a boat as its strong, easily shaped with hand or power tools, easy to glue and will not absorb water as its closed cell .


#6 18 Jul 2023 7:09 pm

Registered: 4 Sep 2022
Posts: 2

Re: XPS foam for Flotation Blocks - Tenderly Dinghy

I’ve just about finished my Tenderly build, floor boards going in at present. The next job are the flotation blocks. I had considered using floatation bags but then I couldn’t find ideal sizes. My stepson runs a building company so he’s providing me with the foil backed insulation board…I assume it’s PIR foam going by the comments so far.

Last edited by Jays (18 Jul 2023 7:10 pm)


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