Originally Posted by catty
I disagree. The ability to heave to is a reverse function of a boat's ability to point. The more poorly a boat goes to weather
, regardless of the number of hulls, the more narrow the "slot" in which it will heave to.
Any boat that tacks through 100 degrees will heave to more readily than a boat that tacks through 70 degrees. This is why the older full-keelers are so easy to heave to when compared to fin-keeled monos, and why some cruising cats are easier to heave to than racing
cats. The less a boat wants to point, the more it wants to heave to.
Given that we agree upon the general design categories of vessels which heave-to "well" versus not (full-keel vs fin-keel respecively), I expect that our primary difference of opinion relates to our respective usage of terminology (specifically "lateral resistance" versus "lift").
So, with that in mind, I am going to expand on the terminology and concepts a bit below (sorry for the long response). Even if we don't ultimately agree it should be an interesting discussion.
First lets define what "heaves-to well" means. I use it to mean: easily assumes a heave-to attitude and maintains approximately a close-reach angle to wind
with out excessive mucking about with the sail plan.
When I used the term "lateral resistance characteristics" previously it was in the context of a vessel in a "static" state, e.g., "hove-to". The forces
generated by the keel
are different for a vessel in a static state versus a "dynamic" state, e.g., "underway and making headway". When in a hove-to attitude the keel is being pushed laterally through the water
. To push a full keel laterally through the water
requires an enourmous amount of force. To push a narrow fin keel laterally through the water requires proportionally much less force. Thus a full keel boat heaves-to better than a fin keel boat. In both cases the keels are generating lateral resistance, but, since there is effectively no fore-to-aft flow over the keel, essentially no "lift".
In contrast, when a vessel is in a dynamic state (as defined above), the keel is moving forward through through the water. In this mode it provides not only lateral resistance, but it also generates "lift" which helps drive the vessel to windward. In terms of forces generated by the keel (ignoring for the purpose of this dicussion forces generated by other components of the boat), it is this lift
force which primary contributes to the windard and tacking abilities of the vessel -- not lateral resistance per se.
Take for example a Hobie 33. It has a very narrow fin keel which is optimized to produce lift
. As a result, the Hobie 33 will sail amazingly close to the wind
(polar diagram plots end at just less than 30 degrees). In a static state the H33 keel provides relatively little lateral resistance, but as boat speed, and thus water flow over the keel, increases the lift force increases dramatically as does windward performance. As a result of these characteristics, the H33 also does not heave-to very well (she tends to ocilate through a wide range of wind angles and defintely requires a lot of mucking about with the sail plan to keep her reasonably stable...I own a Hobie 33 so I know this from experience).
In contrast, take a full keeled boat (or at least relatively full keeled) and the behavior changes significantly. Most full keels are relatively flat sided
and are not optimized to produce lift. As a result, windward sailing angles and tacking angles are increased. They do however produce huge amounts of lateral resistance in either a static (hove-to) or dynamic mode of operation. This gives them great tracking stability and excellent heave-to behaivior, but not stellar windward or tacking performance.
Therefore, in terms of keel type and the resulting forces in a static state: it is lateral resistance which primarily determines heave-to behavior. Windward performance and tacking angle are a function of the lift characteristics of the keel operating in a dynamic state.
To round us back up to the subject of this thread "catamarans heaving-to" lets compare two cats in a similar size class that I know: A Moorings 3800
(Robertson & Caine) and a Wildcat 3500 (Charter Cats). Both of these boats have similar effective tacking angles (100+ degrees -- similar to many other production cats), but different heave-to characteristics. The Wildcat has an unusally full keel for a cat -- thus it tracks well and heaves-to quite well. The R&C designs have much less full keels so they don't heave-to quite as well (but they do). Aboard either of these boats, as boat speed increases so does lift from the keel. This allows you to bring the boat up, from it's 100+ tacking angle, to a more respectable windward sailing angle of about 45 degrees as you gain boat speed (the Wildcat in fact will pinch up to just under 30 degrees, but with significant loss of boat speed). If tacking angle were the primary determinant of heave-to behavior then these two vessels would have essentially the same heave-to behavior.
One other example: Cats by Gemini have tacking angles of about 100 degrees also and do not heave-to very well. Thus the statement that "Any boat that tacks through 100 degrees will heave to more readily than a boat that tacks through 70 degrees..." is clearly not true.
Bottom-line (IMHO): In a static state, lateral resistance is the primary characteristic which determines heave-to behavior. Windward performance and tacking angle are more significantly determined by lift in a dynamic state and are much more difficult to clearly map to heave-to behavior.
Caveat: I am a professional sailor not a naval architect -- maybe someone with these credentials could chime in on this discussion?