Front Beam within a beam

Just about to change my old style Stealth beams to the later wider beams and its apparant that we can't get the same diameter tube so I would guess that I will have to go to the smaller diameter tube / dolphin striker arrangement as per the late Stealths.

However doing a bit of thinking I sort of hate the idea of the dolphin stiker so with a bit of lateral thinking I came up with the following idea.

A front beam of say 70mm 1.6mm thick tube with a further 50mm 1.6mm tube down the inside. make a couple of spacing rings say at the centre and end mounting points from Ali and fill the void with expanding foam. Sort of like the honeycomb skins we use for the hulls

My gut feeling is it would be more than strong enough but I need an engineer to do the calcs, any offers and any comments as I'm sure its been done before by one of you guys. <img src="http://www.catsailor.com/forums/images/graemlins/cool.gif" alt="" />

first impressions are it would be too heavy...but it would be stronger...

inital thought on the idea is that the contact faces of the aluminium on the foam would need to have a good bond, shear forces between the foam and aluminium would be high I imagine.

I believe the stiffness of the beam would be only marginally increased. If striker-less, you need to consider the stiffness of the beam because any deflection will reduce your rig tension, which generally affects pointing ability adversely.

70*1.6 tube - I = 201000 mm^4
70*1.6 tube + 50*1.6 tube - I = 272000 mm^4

Assuming material is 6061-T6 with yield stress of 240MPa, E=70GPa.

with a load of 220kg on the first beam (length = 2.5m) the maximum stress will be about 235MPa, and deflection = 50mm.
(With 2% deflection, buckling will probably be a concern!)

with the same load on the second beam the maximum stress will be about 174MPa, and deflection = 37mm (1.5%).

1% deflection is the limit allowed by Australian standards for aluminium boats (AS4132).

My "opinion" is that it is not suitable (I wouldn't do it). 220kg load on the beam is likely (crew weight on trapeze plus initial rig tension could amount to more than this then take into account increased accelerations when sailing in waves!).

Like all engineering, there are multiple solutions/opinions so please make comments.

well lunch is over, better get back to work.

Insert disclaimer here.

Having said all this, it is a racing machine and if you don't mind nursing it around the course in a blow! But it will be a heavy beam so the advantages in light conditions will be lost.

This is assuming the foam is rigid enough to force the internal beam to be parallel to the outer beam when flexing.

Short post with main points :



Beamloading is about 1000 kg to 1500 kg in a static sense already, this means that these numbers are a minimum, real (static + dynamic) stresses are most probably higher !

I seriously doubt whether foam is enough to bend the inner beam into shape. I fully expect the foam to be compressed and the inner beam to work itself some slack in which to stay straight while the outer beam bends.

If you have to do this then do it alu on alu like an 80x2 outer beam and a 75x2 inner beam. That is how unstayed masts are made on landyachts.

I don't think your idea of tube with significantly different diameters and foam to fill the space between them has merit. I'm sorry.

Wouter

sort of figured that there must be a reason that the manufacturers are not doing it this way. Thanks guys for the response.

Any other odd ball solutions like using the tube as a mould for a number of layers of carbon fibre but leaving the ali tube insitu, would you get such uneven stresses between the carbon and ali tube that the epoxy bond between the two would shear ? <img src="http://www.catsailor.com/forums/images/graemlins/cool.gif" alt="" />

Maybe have a look at the Spitfire beam - it does not need a dolphin striker.

It is not difficult to have a dophinstrikerless mainbeam. Just order an alu beam with a much ticker wallthickenss. Actually the first Stealths were build this way and so you can contact John Pierce and ask what he used.

Downsides are indeed added weight and the fact that the mainbeam will never be as stiff as the beam with a dolphinstriker. The latter will impact negatively on your pointing ability especially with a modern squaretop rig as these require quite some leech tension upwind. These are actually the reason, I believe, that the newer stealths were quickle all fitted with dolpinstrikers.

Wouter

Wayne,

What is the exact diameter of tube you are looking for.

Regards,
Hans

Thanks Hans but I'll stick to the standard Stealth beam as its a tried and tested component, I'd only change to a non stricker less beam if I could do it some how without having to take the hit on weight, anyway the striker is good to grab hold of when righting the boat ( which I seem to do all to often )

I'd heard it was because the original section wasn't produced anymore.

Correct, I think Dart 18's might have a similar beam as the original type R Stealth

Carbon and aluminium are not compatible, especially in a marine environment. The aluminium will corrode very quickly.

You can use the aluminium as a mandrel for wrapping carbon around then remove the aluminium. This is quite tricky unless you've done it before, it needs to be done properly or you'll never get the aluminium out.

Coat the aluminium in melted candlewax and it will come out with some boiling water quite easily. Alternatively you can make a single cut the length of the tube nd insert a flat bar the width of the saw cut with a means of pulling it out once the resin has cure. Now yu have a collapsable mandral.
That works just as well nd may be easier than getting a even coating of wax. In the second case you will need mould release while the wax serves this purpose in the first.

However it is better to use the aluminium tube as a female mould and inflate a bladder inside it. The carbon is then compressed between the bladder and aluminium providing a stronger and lighter structure than using the aluminium as a male mould.
In the female form you need an aluminium tube with at least a 3mm wall so all this has to be taken into account when calculating final beam size.

can you use aluiminium as a dolphin striker to replace stainless steel? if not do you no where you can get a black stainless steel to make a striker thanks

you can use aluminium for a dolphin striker, but it will need to be bigger than a stainless one because its strength is less.

the one i have is 40mm wide by 2mm thick so what would aluminium have to be?

You need at about 3 times as much material (=crosssection) when making stuff out of alu then you would out of stainless steel. That is because its max strength is roughly 3 times less.

We can go into details but I think this will suffice for now.

You can also get stainless steel wires and have them act as a dolphinstriker. As Hans for more info on that as he has used this setup frequently on his own boats.

Wouter

Hans what diameter wire would you recommend for a dolphin stiker baring in mind that I sail mainly single handed. Thanks