Originally Posted by KJB
I know a longer draft is better for ocean sailing, but I keep running across apparent BW boats with anywhere from 5-6' drafts that seem to be capable in the open water (as in: "crossed the Atlantic twice" - "capable offshore
cruiser" - "bluewater boat" - "take you anywhere you want to go").
Complex question, and you are right that there are many boats with very shallow draft
that have had very successful and safe ocean voyaging records. The flat bottom centerboard
Ovni's come to mind, with only 1m draft with the board up, and have been safely to both the arctic and antarctic.
There are four separate concepts of stability and it helps to examine each to understand the complexity.
(1) Initial righting moment - this is how hard it is to tip the boat the first 1 degree from flat in the water/vertical mast
- usually referred to as how "stiff" the boat is. I deep keel, with the lead on a bulb at the bottom, with minimal weight in the rig, is the way to accomplish this. Its desirable for blue water cruising because (a) it makes the boat able to stand up to gusts better and less need for lots of reefing and unreefing and (b) it makes the boat more powerful and faster.
(2) Limit of positive stability - how far over the boat can be knocked and still come right back up the same side, if you go further the boat will continue and go 180 degrees. Righting moment helps here also but hull
shape (narrow beam) and cabin
trunk shape (big round cabin
trunk) and mast
floatation are as important. This is obviously desirable in blue water because it gives you the ability to bounce back from a knock down.
(3) Energy required to re-right the vessel once upside down - if the boat does get knocked over and goes 180 degrees how big a wave is required to knock it right side up/how fast is it likely to come back up. This factor is very rarely calculated and depends on whether water has gotten inside the boat and if the sails
are up. But a deep keel will help.
(4) Tripping factor - when a boat is pushed sideways how much rotational force is created vs allowing the boat to slide side ways. A deep keel hurts on this factor - creating more tripping/rotation and less sliding.
So, deep keep is almost essential for #1, useful for #2 & #3 but other factors are as important, and negative for #4. So you can see it's a bit of a design trade-off and there is no simple answer.
I know that's not so helpful for you, but I think its realistic. We have sailed round the world
with 4' 3" of draft on one boat and 7' of draft on another and they sailed completely differently and we needed to treat them differently but they were both good blue water boats.