Lifting Bulb Keel Experiences

Hello all, I am looking for people with experience of larger yachts with lifting keels. I am currently designing a 12 ton yacht in aluminium, with a lifting fin keel and a 4 ton lead bulb.

Obviously there are many ways of dealing with the lifting machinery and the internal structural support of the keel.

So if anyone has experiences or pointers they could share about installations that worked well or bad, that would be very helpful. Especially if you also have information about how the fixtures that keep the keel in place while sailing worked.

Cheers,
Jesper

 

What you need is a trunk like a big centreboard case. The first problem is how to support it at the top end. You have to tie it into the deck or coach roof. In the down position the top of the keel fin should be at saloon sole level because you will need lots of structure here. If the top is above sole level this structure will intrude into the cabin.

To keep the keel in position and stop rattel it's best to have a hydraulic locking pin that wedges the keel fin to the trunk.

The loads are quite massive. Assuming a draft of 2m with keel down and that the keel fin extends 50cm into the trunk with a knock down factor of 3 you will end up with a side load of 60000kg in your hull. And that is without the weight of the fin. Grounding loads are probably similar and in this case the trailing edge of your fin wants to slice through the hull.

 

For much, much smaller drop keels we use aluminium keels and simply wind many layers of glass+epoxy around the keel to make the case. This makes a perfect fit and no rattling. I don't know if this will work for your size though, we have a few hundred kg of ballast.

 

Thank you Karsten for your reply. Yes, I have a box that extends through the entire hull, structurally well connected to the deck and hull in both ends. I am calculating with 700mm of the length of the fin being above the hull bottom when the keel is fully extended.

I will be a bit more specific, maybe I should have been that from the beginning :

The design of the fin itself is not a problem to me, I have this designed and FEM-analysed.

The biggest question I have is how to design the bearings that the top of the fin slides up and down on. Assuming that the top of the fin (the part that is always inside the hull) expands to a rectangular profile that provides a larger surface for the bearings, they still have to be very strong, not require any extreme precision in the manufacturing, and not risk jamming. Any suggestions here? Materials? Design?

Raggi_Thor, I did not quite follow your design there. Are you saying you make the "box" (that the keel slides up and down in) by using the actual fin as a mould? It seems to me that would fit rather too tight! (i e not move at all ) Or do you put a fibreglass top on the fin and then let that slide through some other box?

It seems to me the most straight forward lifting mechanism is a hydraulic cylinder hanging from deck level and connecting to the keel just above the bulb. More elaborate mechanisms with chains or wires from a cylinder at the bottom are more expensive and have a higher risk of failing.

One issue here that I am curious as to how people deal with is how to keep water out of the fin blade, it needs to have a hole for the cylinder at the top of the fin, and that top will certainly come under the water level in the keel box at times. This is especially a concern since the fin is aluminium and the cylinder most likely stainless or bronse. One could of course put the fixed part of the cylinder in the fin and have a sealed fin, but then the hydraulic cables have to ride with the fin up and down, which seems like trouble too.

There might be other ways in doing this that I didn't think of, too!

Karsten, I am very interested in what you call "hydralic locking pin", would you care to describe this further? How to secure the keel in the up and down positions with the minimum amount of complicated machinery that can fail is also on my list of unsolved questions....

Greetings,
/jesper

 

We put a thinlayer of wax on the aluminium fin.
Then we wrap it in glass and epoxy.
When it has cured we can add a little heat if needed, and remove the fin.
Then we cut the glass case to fit between the hull bottom and (in our case) the cockpit sole.

The fit is tight, but we have no problems lifting the keel.

 

Maybe you should fill the fin with foam?

 

Aha! I just checked your homepage I see you are using extruded profiles for the fin, that makes more sense. Tooling for such a profile is not withing my budget I'm afraid. With a welded fin I don't think this approach would work, since the profile would have significantly more dimensional variation than an extruded. But it's a great idea with an extruded profile, I must say!

Polyurethane foam has two problems: It absorbs water (slowly) with time, and it makes service problematic. I would prefer to avoid it...
/j

 

Yes, our keel proofiles are very nice, made by SAPA, who also makes the masts for Selden...

We use expanding foam, but our boats are most of the time on the trailer

You will maybe weld a top to the keel and make it watertight that way, just an idea.

 

JesperW Have you considered a vertical screw drive system, you could then use either hydraulics’ or electric motor. This system may be lighter too, plus the screw could slide into a sealed pipe, so you could seal the whole fin.

 

Jesper

We used teflon or PTFE (would need to look up the final spec) vertical strips embedded in the case as the load bearing surfaces, you need side to side and fore-aft surfaces .

Our case was made with the wax method, sheets of wax were stuck to the fin the teflon/PTFE was fixed against the fin with a much thinner layer of wax and we installed the fixing screws then glassed the whole assembly.
Works very well.
The moments you need to design for are very high, 700mm sounds a bit on the short side.... we worked on 1500mm and the board projects above deck level when up.
Remember to allow for a certain amount grounding forces in the fore-aft direction.
we used a simple pin to lock in place and a deck winch with wire, since the fin projects above deck we chopped away the top aft portion of the fin for an attachment eye so it is always below the deck level, grounding forces will be on the fwd top side anyway (unless going astern).

The fin itself was a steel box girder carrying the lead bulb then shaped with foam and epoxy glassed over to the profile with a very accurate travelling jig carrying a profile board. At the upper end the steel ends below the glass surface with no foam and the epoxy transfers the force.

Hope this helps.