Thunderbird 4

[Banshee Ambulance]

The time has come for me to order some foam. What grade do I buy and who do I buy it from? I am Southampton based. Any advice appreciated.

[phil_kirk]

impag in dursley, gloucestershire are the main supplier of structural/core foams. i'm sure they have a website.

Arex or divinicell 80kg/m^3 is commonly used for shells and bulkheads.  Green in colour.  Bulkheads and hull pannels are built from 8mm thick foam where as hull sides can be reduced to 6mm.  This option really depends on what you have allowed for when making your plug.  8mm 2.4m x 1.2m sheet costs in the region of £40.
1 sheet does the spine and bulkheads of an elway 5.  you should need 1 &1/2 sheets for the deck but should have some off cuts. Probably 3 more sheets to do the hull shell.

final no. of sheets depends on the dimensions of your hull. You will be fine if you buy 6 but if you're really carefull and you have a skinny hull you may do it with 5.

order thicker foam of the same or a higher density for the foils.  A sheet of 20mm thick will do your rudder blade, t foil blade and centreboard. if you bond a blank of 8mm thick to a 20mm thick sheet.

[phil_kirk]

And to finish my last sentence:  to get a centreboard blank thicker than 20mm  you can bond an 8mm thick peice to a 20mm thick peice. I know that foils have been successfully made from 80 kg/m^3 foam however you can go to higher densities if you want it to be a little more rugged.

a 20mm thick sheet of 80kg/m^3 will cost about £100. 
Since I made the cross beams and C/b case pads out of high density foam £180 per sheet (6' by 2') i bought 2 and made all the foils as well.

If i did it again I would make beams from carbon tubes with no foam core and make the foils from a medium density possibly 120kg/m^3.

You can collect. or pay postage.  I would pick up fo you but wouldn't easily get more than 1 of 8' x 4' sheet in the focus.

[Banshee Ambulance]

Airex comes in a confusing array of letters and numbers. What sort do I need?

[john_hamilton]

rob could i order a sheet with you if there is discount for bulk? high density for foils,etc...

[phil_kirk]

I think it is the Airex C70 foam that you want. appears right from the data sheet on IMPAG's website.

Please confirm when you enquire and ask what they supply to Ardvark or bloodaxe. They should be using the same stuff as you need.
I previously found a product sheet which showed the different sheet sizes and densities.  might be worth googling Airex C70 and working out how much you need.

john - check the sheet sizes and that you can get all the foils out of 1 sheet. Don't forget that the thickness is proportional to the cord and the naca section. Our board was tapered from a cord of 0.3m at the top so needed to be 30mm thick. I had to cut out two blanks glue them together before routing all the contors.  This produced a lot of wastage. It may be more efficient to get 1 sheet of one thickness and another of a second thickness.

My Point is work it so you have enough and save the hasel of a second order and postage.  I'm not sure if there is a discount until you buy a whole box. that's a lot of foam! but you would save on postage.

[Banshee Ambulance]

John, My first set of foils will be done in Cedar I think. Its easier to work than foam and the core will be more structual so I can use less of the black stuff.

[phil_kirk]

Paul Croote may be able to help you with the cedar if you talk very nicely to him.

Cedar is close in density to the expensive high density foam so a good alternative.

We re-sheathed Slippery's  cedar foils with 1 strip of uni 2/3rds the way down the centre of each side and 1 layer of weave over the whole lot.  I would suggest adding an epoxy trailing edge to the cedar and fairng this before putting the weave on because the grain in the wood makes it very easy for the trailing edge to get damaged. Because foam dosn't have a grain it's better than wood in this respect.

I agree that it's a pleaseure to work with wood. Cedar has a nice smell to it.

Wood sniffers of the world  unite!

[roland_trim]

I would also suggest leaving a bit of thickness to your trailing edge.

The tinner trailing edge increases the bending arm and causing cracks that would not have been possible with a thicker edge (i.e. it breaks at a thickness that would have been ok if the edge had ended nearer it, not tapered to nothing).

Have learnt this the hard way and am still picking parts of what was left of "knife" trailing edge out of my hands - not to mention the mess it made of the inside leg of my dry suit.

[Will_Lee]

I would suggest shaping your core right down to a feather edge (that way the two layups touch each other fairly), but shaping the final foil it so that there is thickness of the edge you go sailing with to avoid damage (see above).

 A bit less than a mm seems to be the norm.

[john_hamilton]

whats the best way to shape a foam core....router???

[Phil Alderson]

I normally just mark the thickest point of the section 1/3 back from the LE and use a belt sander to remove material judging by eye and template if it is the right shape.

Some people use a router to cut grooves to the appropriate depth, and then sand the rest down to the grooves.

[phil_kirk]

Phil you are a god when it comes to foils if you only need to do that.

For the lesser mortals:

you can draw the foil in cad or crunch the same numbers in a spreadsheet.

First you need a profile.
then at equal intervals down the foil mark lines. measure the cord of each line and scale your chosen foil section to the cord.  this will give you a 3d shape which you can cut with planes at each thickness. 1mm intervals work well.  you may need some 1/2 mm intervals near the center cord line.

Do we need a sticky weekend on this?


Remember to convert each of these thicknesses into depths from the surface of the foam.

I did it in a spreadsheet and drew the contors onto draughting film.  I then (or rather sarah then) transfered the lines  to the foam by dotting along the lines through the film with a fine point pen.  join the lines up. route each contor working from the outside to the inside.

Then the easy bit is sand the foam down until the grooves just dissapear. Use a long board for the flatter bits to ensure fairness.  wood or high density foam requires more sanding but is less delicate.

A paralell section is much quicker to do because all the contors are straight lines.
Some of our photos will show the practical steps.

[john_hamilton]

Do we need a sticky weekend on this?

i think we do, well me anyway, i just dont quite understand how to get the section diagrams,etc. people give me the NACA sections but i have no clue where to find them

[Phil Alderson]

Phil you are a god when it comes to foils if you only need to do that.

The final shape is more approximation than perfection, besides the router I have has over 1.5mm of end float so is useless for doing any sort of accurate work