extruded rudders and centreboards

[grob]

I am having a die made to extrude aluminium foil shapes, the idea is that a common nose extrusion can be made to produce a variety of sizes of foils between 15mm-30mm thick see attached picture.

This is a similar concept to the extruded aluminium rudders used on the 29er dinghy.

My question is would people want to have the rudder filled with foam and if so can anyone recommend a foam to fill the rudder with.

Gareth

[daiquiri]

Aluminum is very sensible to pitting and crevice corrosion, so it is of vital importance that you impede the water penetration under the foam surface, where it can stagnate in the form of moisture.
Therefore, a closed-cell foam is a must, and it has to adhere perfectly to the walls.

A good choice might be an elastomeric nitrile foam. It is used for Armaflex insulation sheets, has 100% closed-cell structure which makes it very resistant to water and moisture penetration. Another one might be a neoprene, another elastomeric foam similar to Armaflex.
They both have the same disadvantage, though... Their formation requires a thermal curing, so if you don't have tools for that, you'll have to buy them in form of sheets. Then you would need to cut them into a correct form and fill the rudder, sealing carefuly any gap between the foam and the aluminum walls, before welding (or glueing?) the two aluminum pieces together.

An alternative could be a polyurethane foam, which is also closed-cell, though 90% only. In contact with other surfaces (like aluminum walls) it forms a silicone-like layer which stops the spreading of the moisture along the outer walls.

[Ad Hoc]

Concur with daiquiri above.
Moisture is a killer in these situations. Also you need to be careful what grade of alloy you are using too, since some are not very marine friendly!

[grob]

Originally Posted by daiquiri

Aluminum is very sensible to pitting corrosion, so it is of vital importance that you impede the water penetration under the foam surface, where it can stagnate in the form of moisture.
Therefore, a closed-cell foam is a must.

Thanks for the replies, this is one of the reasons I wonder if it is better to have no foam at all. The aluminium will be 6082-t6 anodised.

[Ad Hoc]

6082-T6 is ideal...no need to anodise either.

One question..where are you putting the stock and how?

[grob]

Originally Posted by Ad Hoc

One question..where are you putting the stock and how?

I am still undecided as to whether to use a propietary rudder stock or make my own.

Gareth

[Ad Hoc]

well, best thing would be to ascertain the max torque for the rudder, then design the stock to suit, this would then be applicable across the board. Ok, heaver and 'over engineered' for some applications..but mass produced!

But where to put it within the section and how to attach it...have you decided that yet too?

[grob]

From your description we may mean different things when talking about rudder stock, what do you mean by rudder stock?

Gareth

[Ad Hoc]

the solid bar, or hollow (depending upon the calculated stress values), that extends from the tillar/yoke inside the boat, through the hull, via a bearing, to the rudder and down the inside the rudder.
As the rudder turns, the force on the rudder is trying to rotate it and it is resisted from rotating by the stock, the solid bar.

[grob]

This is designed mostly for dinghies, rudder stocks as you described them are not usually used on these boats, the rudders are usually sandwiched between cheeks in a rudder block.

Gareth

[HJS]

Keep the trailing edge sharp or flat
absolutely not rounded
otherwise you can get vibrations at some speeds
attachment from 1979

good luck

js