Keel bumper systems

[citlab]

In the April issue of YM Alan Burns submitted a question about shock absorbing systems for yacht keels. This was replied with some speculation on the technical feasibility of such systems.

To date, I have been made aware of two systems that are technically fully able to absorb the shock loads during a grounding.

The first one is based on a swing keel system, where the lower ballast part of the keel is attached to the upper keel structure with a hinge. This system has a several years of track record of survival from full speed groundings. The system also demonstrates advantages in the headwind performance due to its low point of gravity when lowered.

Another system is based on an integrated keel appendix. A water filled compartment, that is located in the front part of the keel is swiveled from the upper end of the keel. This compartment is filled with water. During a grounding, the water inside the compartment is acting as the shock absorber as it flows out through holes. After the collision, a spring will return the front end of the compartment back to its regular position - ready for a new grounding. This system could be easily implemented into any production cruiser.

Maybe a keel bumper is something everyone wish they had. How many of us would be willing to pay 1k£ for one?

 

[earlybird]

April issue eh?

 

[Searush]

Ever bin 'ad?

 

[Blueboatman]

I think that the Stanley retractable/snap off keel has some merit and is , AFAIK, good for 10 groundings.
After 10 such groundings you would of course need to order a new keel, or consider changing sports.

 

[Searush]

I think that the Stanley retractable/snap off keel has some merit and is , AFAIK, good for 10 groundings.
After 10 such groundings you would of course need to order a new keel, or consider changing sports.

But regular & frequent grounding is a requirement when you sail around drying harbours.

 

[sighmoon]

Some Dehlers have a rubber boot on the front of the lower half of the keel. Guessing from a diagram, there's probably 4 - 6 inches of rubber bumper before you get to the cast iron.

I don't know how effective it is, but I do know that the surveyor reported no grounding damage, whereas keel matrix repairs of uncertain provenance were a big problem on some other boats I'd looked at. Could be that previous owners were just careful.

The keel grounding is at 5:30 of their famous crash test video:
http://www.youtube.com/watch?v=rvxhQO4pw2E

The way the boat bounces when hitting the rocks suggests that maybe the bumper doesn't do that much. OTOH, no damage to the keel or fixings

 

[citlab]

linkto a keel bumper developers site

After some web-digging, I found a link with a picture to the innovators site. Some other published articles report on 20+ successful full speed groundings w/o structural damages. The front part of the keel is giving in more than 10cm, water absorbing the energy. Apparently the market interest did not support further implementations as there has not been any published activity during the past 10 years.

http://www.ailmet.fi/proj_testeri.html

 

[dt4134]

Some Dehlers have a rubber boot on the front of the lower half of the keel. Guessing from a diagram, there's probably 4 - 6 inches of rubber bumper before you get to the cast iron.

Never seen that on a Dehler. Certainly not on a Dehler 31 of YouTube fame. Where did you get your info?

 

[Tranona]

After some web-digging, I found a link with a picture to the innovators site. Some other published articles report on 20+ successful full speed groundings w/o structural damages. The front part of the keel is giving in more than 10cm, water absorbing the energy. Apparently the market interest did not support further implementations as there has not been any published activity during the past 10 years.

http://www.ailmet.fi/proj_testeri.html

History is littered with "good" ideas to solve problems that don't really exist. Not suggesting that boats don't run aground or hit underwater obstructions, nor that such incidents can't cause significant damage. However, in real life the number of incidents is tiny in relation to the number of boats in service. So owners/buyers accept the risk rather than pay for something they may or may not use, and may or may not work. There are other ways of reducing the risk and you can see that in designs coming from areas where grounding, particularly hard rocks is common.

 

[sighmoon]

Never seen that on a Dehler. Certainly not on a Dehler 31 of YouTube fame. Where did you get your info?

It's mentioned in the Dehler 38 brochure. I assumed the other boats in the range had it too. It's faired in, so I think you'd have to look closely to see it in real life

 

[snooks]

Arcona yachts have a "bumper" option for their keels, being Swedish, there are a lot of rocks to bump into.

It's a D shaped bit of black rubber on the bottom 8-10 inches of the leading edge of the lowest part of the keel

 

[Neil_Y]

Any bumber system will still trsnafer huge load to the keel fixings and a large upwards force of the rear of the keel into the hull.

Bavaria had a very nice design of keel/hull flange (on older boats) that incororated a large step with an angle forward edge, this took any impact load away from th efixings bolts and prevented the upwards thrust of the rear of the keel into the hull. It needs drawing!

 

[prv]

I solve it like this - instead of CLANG-CRUNCH, I just gently slide up over the obstruction.

Admittedly, your ten-foot deep fin does go to windward rather better

Pete

 

[Seajet]

The Anderson 22 lift keel is tapered to a 2-3mm trailing edge; there are 2 built in design points to avoid - or minimise - the effect of hitting something nasty with the keel down.

The upper section of the keel casting, which does not protrude below the hull profile, is kept to a rectagular, untapered section so as not to 'knife' into the aft edge of the keel case on impact; also the aft keel case inside the boat has a tufnol shock absorber.

Not much effort to design or fit, but may well save the day.

As for general shock absorbers, I'd tend to agree with the idea of a hard rubber insert on the lower leading edge of fin keels, ( and for seperate fin keels, maybe some sort of shock absorbing - replaceable ? - seating at the keel upper aft edge ) if one could attach it well; it's a fact of life that at some stage in her career a deep keel boat will be driven hard into something on if not the actual seabed, so preparing for it seems sensible...

 

[snooks]

Any bumber system will still trsnafer huge load to the keel fixings and a large upwards force of the rear of the keel into the hull.

Arcona have a big galvanized steel frame inside their boats to take these forces into account.

The Arcona 430 we managed to drive into a rock, had no damage except the dented lead on the foot of the keel, no trace of any damage to the trailing edge area around the hull, or to the frame or the root of the keel. This boat didn't have the bumper system, but using a rock as a brake I can see why they have them!

 

[sighmoon]

Any bumber system will still trsnafer huge load to the keel fixings and a large upwards force of the rear of the keel into the hull.

But even if it only deforms 1cm, it probably reduces the load on the hull/ keel joint by a factor of ten or so (assuming the alternative with no bumper, there is about 1mm deformation of keel /rock interface).

Bavaria had a very nice design of keel/hull flange (on older boats) that incororated a large step with an angle forward edge, this took any impact load away from th efixings bolts and prevented the upwards thrust of the rear of the keel into the hull. It needs drawing!

Still massive forces somewhere though, unless something is flexing.