Draft - Opinions please - SailNet Community

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  #1  
Old 12-22-2009
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Draft - Opinions please

When I was a young bloke 5'6" was considered about right for the draft of a cruising boat.

Racers of course would question that and quite a few so called "cruiser racers" have over 7'.

The current Womboat draws just on 6'.

I find myself now looking at boats with 6'6" draft and thinking that is OK and I'm unsure about another with only 5'6".

Maybe it is location specific this draft thing. In Sydney 6'6" v 5'6" is not considered at all crucial but I guess that anyone thinking of cruising the Bahamas would think differently.

Any thoughts ? Just how much difference in performance (on different points of sail) would you get with an extra 12' of fin ?
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Old 12-22-2009
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A deeper keel (and rudder), indicates that the boat can carry more sail and point better, all things else being equal. But if the 12' will make a big difference on two different boats is really hard to tell.
My personal choice is very much a compromise, as we have a lot of beautiful places you can go with shallower draft, but I wanted a keel.boat that also performed ok, so ended up with 2m (~7') draft. I was seriously considering a Tri: Dragonfly Trimarans by Quorning Boats of Denmark | welcome but the 920 or 1200's are a tad expensive
So I ended up with the Contessa 35 (which we love)
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Last edited by JomsViking; 12-22-2009 at 03:01 AM.
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Old 12-22-2009
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I guess my point is: If deeper draft is not an issue for you in the majority of places where you want to cruise, go for deeper draft....
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Old 12-22-2009
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I love my 3 ft draft , im able to snuggle up in places others only wish they could!
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Old 12-22-2009
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I'd agree it's a regional thing... deep draft here in the PNW is rarely an issue.. finding shallow enough areas to easily anchor can be...

I think in most cases the deep draft version of any particular boat will likely perform better.. comparing different boats' drafts gets more complicated.
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Old 12-22-2009
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I see two basic issues that would push for the deepest draft that works acceptably within your sailing venue. The first of course is the performance (by which I do not mean speed) balance that can be achieved with a deeper keel. For a given displacement and ballast ratio, the deeper keel will will result in more stability and typically less drag. This allows a particular sail inventory to have a wider wind range. The lower drag allows you to carry a smaller sail into the lighter air range and still have reasonable performance, and the greater stability and smaller headsail will allow you to carry the same sail plan into a higher wind range before reefing or making a sail change. For cruisers this is a very serious consideration.

But the second factor is an often neglected one. The deeper draft typically offers very superior motion comfort as well. The greater roll moment of inertia, superior dampening, and higher levels of stability, typically result a slower roll through a narrower roll angle.

It is also important to think of the impact of a boat with a 30 cm (1 foot) less draft in terms of the relative depth between the two keel. If you consider that the instanteous static roll axis (of a boat like the one that you are considering) is roughly 15-20 cm (6-8 inches) below the water line, you can see that the the deeper keel would have a 20-25% longer span and I would suggest that is very significant in terms of righting morment, or pointing ability.

Respectfully,
Jeff
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Old 12-22-2009
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Quote:
Originally Posted by JomsViking View Post
A deeper keel (and rudder), indicates that the boat can carry more sail and point better..
I agree with the second point and not with the first.

Usually when mass production cruising boats are concerned, for a given model there is a choice of diferent keels with diferent drafts. If you take a look at their weight you will see that the lower one is lighter and the shorter heavier.

If you ask to the manufacturer the stability curves for each case you are going to see that they are so similar that is almost impossible to detect any difference. That means that the boat stability is the same and therefore the boat can carry the same sail.

Regards

Paulo
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Old 12-22-2009
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I can't see how there could be an absolute answer to this question. In my mind, it's a question of relative draft for the length of the boat, but you didn't mention boat length.

If you are talking about a 30' boat, a 5'6" draft is fairly deep (in the cruising realm). If you are talking about a 50' boat, I'd call 5'6" fairly shallow bordering on shoal draft.

So I'd suggest you approach this with a sort of "sliding scale". On a 40 some odd foot boat, something in the realm of six to seven feet of draft would seem desirable to achieve decent sailing performance. But shallower drafts may be preferable for certain cruising grounds.

Another factor often overlooked is "draft" vs. "keel span". Keel span being the length of the keel below the underside of the boat's hull (rather than the draft measurement taken from the static waterline to the deepest part of the keel.)

While "keel span" can be viewed proportionally, as in the sliding scale when comparing boats of different sizes, it can also be looked at in absolute terms when comparing boats of similar length. In other words, depending on the hull form, two boats of identical draft may have markedly different keel spans, resulting in noticeably different performance behavior.

By way of example, our 5' draft boat has a keel span only slightly greater than my sister's 4' draft boat. Hers is a flat bottomed Beneteau, ours is a wine-glass shaped hull form. There is no bilge on her boat, while ours is quite voluminous. The point where you'd begin to measure the "span" of our keel is almost a foot deeper on our boat than hers. So their hull form yields the benefit of a similar keel span to ours but with less draft (of course, there are penalties, but that's another subject.)

I guess what I'm saying is that you really need to look at the entire package -- draft, keel span, boat length, hull form, etc -- and asses any candidates with all these factors in mind as well as the intended use of the boat and primary cruising grounds.
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Old 12-22-2009
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Personally, I'd go with a deeper draft. Is there anything in your range that has a lifting keel? A lot of sport boats have come a long way in the lifting systems and engineering for lifting keels.

My next boat will be a lifting keel, although I'm not sure of any offered in your size range.
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Old 12-22-2009
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I think all people in the know generally agree that deeper draft will yield better performance in just about all areas (pointing, stability, roll, etc.), but that assumes you are floating. To me, for cruisers, the answer to the question is dependent entirely on the area in which you sail. That is, all else being equal, get the deepest draft you can that still allows the boat to float. If you are in the PNW, for instance, draft is not really a concern. If you are on the US east coast planning to cruise the Bahamas, however, going much above 6' is going to limit you in many ways, regardless of how large your boat may be. Can't really give you an answer about Down Under, as I just don't have the smarts for that area (but I'm reasonably confident you do!).
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