canting keel opinions

[Raggi_Thor]

I think i's just glued with epoxy, but I can check.
The wall thickness is approx 5mm, with some inside stiffeners across.
I can post some pictures on monday.

[Raggi_Thor]

Sander, I didn't see your reply at first.
A 45mm thick solid steel profile will certainly be stiffer than a 48mm hollow alu profile...
But how stiff does it need to be?
The B21 with 225kg ballast works fine...

[nemo]

My FEM analysis with the same keel but made of Aluminium failed!
Stresses reaches yeld strenght in various points, and deflection at the end of the keel is 5 cm!
My bulb weight is 230 kg, so it's not so different from Backman's.. I can't believe it's just glued with epoxy..

[D'ARTOIS]

Nemo, you could use a titanium unit instead - if that is allowed. Shifting up and down rules makes one to forget them. If they are allowed I will most podssibly fin a good source for you.

The spec weight of titanium is around 4 - 4.1 but the strength is 4 times that of stainless steel at same dimensions.
The form memory one can only dream of and the corrosion aspect one may forget about it.

[nemo]

D'Artois, unfortunately, titanium is not allowed by the rules! Carbon is a pretty new material in Minitransat, too, it was prohibited until 2003 I think.
I guess most of the racing classes do not allow titanium to be used, so far.

[Raggi_Thor]

I don't know if the bulb is just glued. We have a 100kg bulb on the B18 and we glue it and then run two bolts through the bulb and keel.

Nemo, I suppose your keel is longer (deeper)?
Isn't defelection at the end FL^3 / EI?
So if you suppose the boat is heeld to 90 degrees and the keel is horisontal and out of the water, and the bulb is weighing 230kg or 2300N, then you can calculate a deflection by hand.
What sort of forces do you calculate with?

[nemo]

Originally Posted by Raggi_Thor

Nemo, I suppose your keel is longer (deeper)?
Isn't defelection at the end FL^3 / EI?
So if you suppose the boat is heeld to 90 degrees and the keel is horisontal and out of the water, and the bulb is weighing 230kg or 2300N, then you can calculate a deflection by hand.
What sort of forces do you calculate with?

The bulb weigh 230 kg, and the fin wieghs about 110 kg itself..
I think the correct formula you were thinking of is FL^3/3EI.. but I don't think it fits good in case of a canting keel: rotation is not fixed at the connection with the hull, because of course keel can rotate. I also added the dynamic vertical acceleration given by a 10 metres wave.
Also, a little part of deflection is also due to torsion.

[Tim B]

Nemo,
It seems to me (though it's a long time since I last did the analysis) that for an initial estimate, EBT is good enough.
There shouldn't be any question of torsion what the keel is not immersed, as the CG of the bulb and the torsional center of the fin should be in the same plane.

The last time I considered the problem, one solution seemed to be a Carbon I-beam spar which can take the main loads, then a foil section laminated fore/aft of it (shell structure). This is an extremely light way to build the keel, and it then allows you to get the weight where you really want it, which is as low as possible... ie. the bulb.

It's worth a look, I think.

Tim B.