keel science

[qwerty]

When considering stock monohull sailboat designs with fin keels, you are almost always given a choice of keels: the deepest, various compromises and perhaps a centreboard alternative.

http://www.stadtdesign.com/images/products/459-7-0.jpg

It has always been my belief that deeper the better, performance-wise. I sail and anchor in relatively deep water, so whether the keel is one metre or two metres makes little difference to my convenience.

But it might if I sail to the Bahamas.

So I find myself tooking at alternatives, such as in the link above. The shorter ones are usually longer fore and aft, effectively about the same surface area.

For a non-designer such as myself, is there a way to tell -- assuming I am giving up performance by choosing a shorter alternative -- what it roughly amounts to?

[FAST FRED]

For a long keel as found on most cruising boats (for whom a grounding is a concern) is 1/7 the LWL for the keel depth gives good windward service , and does not need a centerboard.

N.G. Herrishoff "common Sense of Yacht Design

FAST FRED

[qwerty]

Hi Fast Fred,
So depth is the only criteria? Keel fore and aft length (and total surface area) has no relevance?

[FAST FRED]

The cruisers from the H era were "full keel", cutaway forward for better steering , and the rudder supported at the aft end of the keel.

The wetted surface is higher than a modern boat , but the sail area was also MUCH higher so ghosting could be done.

Today in 3Kor 4K of wind most folks motor , as they and todays boats lack the skills and ability to Ghost.

FAST FRED

[SailDesign]

Don't forget, Fred, that weight (displacement) is a HUGE factor in ghosting. That is the real reason that today's boats find it hard to ghost, IMNSHO
Steve

[qwerty]

Given that we are not in the H era (though I currently have a full keel with cutaway forefoot), is there a way to quantify what I would be giving up in performance by opting for a slightly shallower yet deeper fin keel?

I am probably asking for something that doesn't exist, that I will only be able to seek out people willing to give educated guesses. Even finding someone who has chosen to build a version with a shallower keel, he is not going to know unless he has also extensively sailed a deep keel version to compare.

cheers

[Crag Cay]

for a slightly shallower yet deeper fin keel?

Shallower but longer?

You can derive at the dimensions of a keel in two ways. You either compute the lift you require to counteract the side loads or you use an empirical 'rule of thumb' which correlates keel area to some other dimension such as sail area. So you can see that if keels become shorter they should become longer. The area should generally stay the same, or even increase on a shallower draft.

HOWEVER: There are plenty of other considerations. A good treatese of all this is given in Piérre Gutelle book 'Design of Sailing Yachts' as the shape of today's cruising boat keels were regarded as cutting edge when that book was written, and he draws heavily on the work that was done around then at MIT, Stevens, Southampton and Delph. Although lots of variables are investigated, one conclusion is inescapable: Depth helps both windward performance and stability. Extra length can only compensate to some degree, even if adorned with wings.

With a cruising boat you also have to factor in the other desirable characteristics of a cruising keel, such as robustness on grounding, self clearing of flotsom and jetsom, resistance to stalling, provision of a bilge pump sump, space for tankage, ease of blocking her up on dry land, cost of construction, and even space low down for the ballast without the complexity of a bulb etc.

So in the Pacific Northwest the 'best' keel on something like 40ft would be a 6ft deep, medium aspect fin. If you were prioritising a boat for the area from the Chesapeake to the Keys and Bahamas, you might want to trade some draft, but in my experience, you can still enjoy these areas with 6ft. Real advantages in cruising options don't come until you can get down to 2ft of draft. But then you have made a desision that will effect every aspect of your boat, and you will have to consider the whole concept of a boat like a Southerly or a French lifting keel boat against say a multihull in looking for your dream.

It's horses for courses, but in general: deep keels sail better and are more stable. There may be exceptions to this rule, but they will cost you. Big time.

[FAST FRED]

The real question is weather the boat is to sail getting most of the lateral resistance from the keel, or weather the hull will be giving most of the lateral resistance with the keel acting as an end plate or fence , and to carry the ballast.

Racing boats choose the first , many cruisers prefer the second.

Grounding WILL happen in cruising , so it should be painless .

The offshore boat needs HP to climb waves , so the fin keelers have to head off to get power , and seldom will be pointing higher than the full keeled boat going to windward in over 20K .

The somewhat slower roll , with an easier check. is more seakindly .

You going inshore club racing or cruising?

FAST FRED