Deeper Keel = New Rig? Seaworthy?

First I want to thank thank Steve for the prompt reply on the Beneteau - Shaft/Prop exit = ugly thread.

Here is another delima... On a Beneteau with a shoal draft keel. I was told that to go deeper witha thinner section with bulb (not exceeding existing keel weight, just distribution) would stress the rig. (44.5' to 6.5') The goal is not to improve windward ability but to increase the stiffness. Is this do-able? Or, are these beamy big butts boats not suitable? In fact, there was a big fury over these type of boats being seaworthy at all after one was lost off the coast of england for failing to right itself after capsizing. . (the long and narrow crowd like to point this out)

Thanks for any imput?

Michael Puig tranmkp@neosoft.com
2000 Beneteau 321

 

Michael:

Consider two extreme cases: In the first you have a circular bottomed boat with very little ballast. Even with a large rig the rigging itself wouldn't have to be very strong because even the slightest gust would heel the boat, dissipating the energy. Put the same rig into a very wide, flat bottomed boat with a large amount of ballast and it wouldn't be strong enough. The rig would come tumbling down, unable to dissipate the energy of the first strong gust it faced.

Designers engineer the rigs of their boats based on how tender or stiff the boat is (with a safety margin). By making your boat stiffer you may exceed those margins. It would probably be worthwile consulting with a designer before making changes. Considering the cost of the new keel and the potential for a lost rig his fees would probably be reasonable.

 

James_r is correct about the dimensioning of the rig - it is done based on the yacht's stiffness. If you want my opinion, take in account the following:
-some production yachts come with an option between shallow and deep draught. Find out if your beneteau also comes with a deeper keel (which will make the yacht stiffer). If that option uses the same rig dimensions, then you're probably ok if your conversion is similar (keel weight and VCG) to the deep keel production version.
-It will not be big or expensive for you to increase some of the rig dimensions (for example, the shroud diameter). However, unfortunately, you will most probably have to increase chainplate scantlings, spreader dimensions and even mast section if you discover that you need a considerably stronger rig. What you can do quite easily, at this point, is calculate the rig strength required for your proposed keel. Then you can compare it to your existing rig dimensions and decide if a change is needed or not. The popular yacht design books contain the required equations, and you might also find Matthew Sheahan's 'Sailing Rigs and Spars' very useful reference. If you need any help, let me know.

 

Thanks for your insite on this keel issue. Beneteau did offer a deeper keel, however the shole keel is heaver than the fin. In both cases the rig is the same. I mght just go on and throw some lead shot bags in the bilge to see what happens...

 

once again - don't give up, check the numbers first!
as I said, it's quite easy to calculate the rig dimensions. it's also feasible to calculate the present vertical centre of gravity of the yacht, from the boatyards sources or with a simple stability experiment.
if you have the numbers in front of you, you'll be able to decide on what to do more easily. again, if you need any help with these notify me.

 

I think a deeper keel will add little to the stiffness of your hull at normal sailing angles, but would make your boat sail to windward better, and be safer in a capsize situation. So saith Douglas Phillips Birt, yacht designer, in his book "Sailing Yacht Design."

Tim Dunn
Arlington WA USA

 

Another thing to consider is that the deeper keel will stress the hull more. The same mass on a longer arm produces more torque.

 

see the thread in "sailboats, sail are vs ballast."

i'm trying to work out this problem on a racer hull. i want to reduce draft and don't know what type of keel to use that will retain the pointing ability and low wetted area of the wing that is on it, while offering enough weight to carry a good amount of sail.

 

make that, in the "sailboats" thread.