canting keel opinions

Hi,
I just re-designed a canting keel for a Mini, the bulb weighs about 230 kg, the fin is made of solid steel and weighs about 110 kg.
I decided to insert the shaft (diameter 4 cm) in a Vesconite bearing inside the support that you can see in the picture. The red thing under the support is supposed to be the boat's tranverse stiffener's wood core, I used it to make finite element analysys. what do you think?

 

The lever on top of the axis, is that welded on? Or is it fraised out of a block solid steel? I mean is the whole keel unit with the exception of the bulbweight one solid piece?

 

exactly, except for the bulb, the keel is a one piece of steel fraised. We have a contact with an industry that has the cnc machine that could do that.
The real lever will be longer, but it won't be all made of steel, I am thinking about carbon for the top part of it.

 

Than you have a very solid designed canting keel construction that may last very long. By doing fraising out of a solid block of steel you have one homogeneous construcion with no failure lines.
My compliments.

Note: Carbon bonds bad to steel. And may come off. In my opinion you should leave your design as it is. By replacing the lever for a carbon one, you have an increased chance of failure due to the combination of the two. No, the single solid stell unit will provide you with the strongest possible design.

 

Thank you!
I think a canting keel has to be as reliable as possible..
The thing that concerns me is a possible keel support failure.. do you think the one I designed could be ok? the plate is 20 cm wide and it is bolted with 6 M10 bolts to the transverse stiffener with solid wood core.

 

The keel support is absolutely of vital importance - look at the Bavaria disaster!

Try to think of a something like a stainless steel webframe that distributes the forces of the moving keel evenly over the bottom and sides of the hull, preventing flex in the hullskin and cater for solid framework on each end of the keelconstruction.
It 's not just the weight-force that is worrysome, it's the lateral arm of the keel.
If you have only two frames to support the axis, I think that might be a bit too meager. I think more of a construction like an engine bed - where also the forces are evenly distrubuted over the section where the engine is mounted; where the forces can easily flow away, you understand what I mean.

 

A stainless steel webframe would be a very good solution, but it's absolutely too much heavy for a minitransat boat.
I think I would go for 2 transverse stiffener and two longitudinal stringers.
Here I post the FEM analysis I made: the first with the keel at rest, the other with the keel canted, with a dynamic acceleration too. The touching surfaces are modeled so that they can move, but they can't penetrate into each other. Here is shown the safety factor distributions based on the von mises stress (i don't know if you're into structural stuff..) and, even if it looks all red, the average FOS is quite high. But I don't trust FEM so much, that's why I'm here asking opinions..

 

Do you have any idea what the torsional forces are? A guy in such a boat is absolutely helpless when something happens to the keel construction, I have been reading about some failures in that respect on their forum, caused by too light constructions. Therefore, your decision of milling the whole unit out of solid steel is absolutely the best possible option to avoid eventual metal failure.
The only thing that bothers me - speaking of the canting keel issue - is that you sail with a large hole in the bottom!

On the other hand, those boats are light and the forces they generate are hopefully modest. I understand therefore your concern but that's the problem with "on edge engineering" - specifically in the mini-transat, where every ounce exess weight tends to count.

 

Nemo,

My opinion from a construction point of view is to make a "port" connecting the two angular profiles bij a "bridge" of two flat inox strips. This way you keep room for
canting your keel and still can pick up deformations.
One thing not calculated in your model is the point stress in the hull, generated by torsion because of relative movement between hull and keel. The wood core is less stiff as the hull. So the hull will build op stess localy en it will wear out in the corners.
If you make a steel bridge, this will be prevented largely

 

Do you have a very thin profile since you use steel?
Would aluminium be an advantage?
The construction looks strong.

 

Sander, I understood your suggestions and it makes sense, but I think it's always a matter of weights. Two flat inox strips should not weigh too much, I will think about it.

Raggi, the profile maximum thickness is 45 mm.. I didn't even think about aluminium for the keel fin, it could be good because it's lighter and the saved weight could be put in the bulb. But it's much more expensive too, I don't know, but I guess it's not so easy to find a block of alluminium to be milled.
Costs and weights are the things that you have to deal with when designing a minitransat boat. Anyway I will run a FEM analysis with alluminium, just for curiosity.

 

Have you seen the Backman 21?
It uses an extruded aluminium fin.
I have 200, 300 and 400mm profiles (cord length).
The thickness is 12% (24, 36 and 48mm), it's a naca0012 profile.
The cost is around 200 to 300 Euro per meter.

 

http://www.backmanboats.com/Index.html

 

hey that's very interesting!!
Have you got pictures that can be posted?
Since the backman 21 has canting keel as well, I'm wondering how did they put a shaft on the keel fin, is it welded?
And, you say the profile is extruded, so what is the wall thickness of the profile?

 

Ragnar,

This sounds strange to me. Unless you use a steel beam to stiffen this construction,
you need three times the aluminum to get the same deflection You can solve that bij using an extruded profile and bring more material to the outside, but you will raise the hydro risistance?