|Topic Review (Newest First)|
|07-15-2010 08:51 AM|
It's like I said in both posts, there are a whole lot of factors that determine the safety of a boat in in breaking waves. It is all about the individual boat as a whole and not any one specific trait. The generic keel type really isn't inherently close to being one of the most significant factors.
In all cases and all vessels, there are trade offs and compromises, factors and counter indicators. You need to look at the vessel as a whole. While there are clearly full keeled vessels which make great offshore cruising vessels, there are also full-keeled vessels which are dangerously under-ballasted and whose motions will wear out any crew. By the same token, the same can be said about fin keeled boats. The reality is in the specifics of the boat in question.
If you asked me, I would tell you that full keels are an anachronism. Full keels worked well in the days when the materials and methods of building boats limited the stability and sailing efficiency of the boat, and when small offshore cruising boats were closely based on working watercraft; but somewhat ignored the reality of a typical working watercraft's need to carry a whole lot of load in an inexpensively constructed vessel.
Today, our materials and methods of construction allow us to build boats which are more efficient and easier to handle, and which have tremendous stability relative to their drag. But this is not to say that all modern designs are inherently equally seaworthy or equally capable of providing a safe and comfortable offshore vessel. As I have said to you before, it is all dependent on the specifics of the boat in question.
With regards to the comment from my esteemed colleague Cormeum, I would agree that "forereaching at 5 knots isn't going to leave you in your "slick" for any length of time. (too bad there isn't multiquote here). If you're not behind the slick it'll do no good" but my own GPS observations have been that fin keel boats when hove-to make between a knot and a knot and a half of leeway and that only a small portion of that is forereaching with the slick leaving the boat at 120 to 135 degrees from dead ahead; in other words, approximately varying between 30 to 45 degrees abaft abeam. The angle is not all that different than I have experienced on my previously owned traditional full keel designs (like my folkboat or 1939 Stadel Cutter) but my sense is that the leeway speed is greater and so the slick is larger.
|07-15-2010 03:51 AM|
you said that "... when you have a deep draft keel that is short fore and aft, there is a tendency of that keel to stall as the water is moving closer to perpendicular to the keel. When the keel stalls, it generates smaller sideward resistance relative to its area .... the tendency of modern deep fin keels to stall reduces the impact of surface sheer and so reduces the tendency for surface sheer to rapidly heel the boat. As a result, even if shallower than a modern fin with bulb, a full keel or lower aspect ratio keel could actually have greater tendency towards a surface sheer induced capsize."
As I understand, while in breaking waves there are to cases in which the shape of the keel is to be considered:
1 - when you are caught unexpectedly abeam to breaking waves. In this case it is better to be with fin keel since it produces less sheer forces and hence has less tendency to capsize.
2 - when you prepare for the breaking waves by heaving-to with or without a para anchor. In this case the question is whether a fin keel (with low aspect ratio) can properly heave-to and whether it produces the same "magical" slick that protects from the breaking waves.
If the conclusion is that fin keels are positive in case 2 then one can conclude that fin keel is safer in breaking waves.
If the conclusion is that fin keels behave less positive in case 2 then the question is what to prefer, full keel which better in case 2 but worse in case 1 or fin keel which is good in case 1 and worse in case 2.
|07-14-2010 11:15 PM|
|CalebD||Try heaving to. You will like it.|
|07-14-2010 10:17 PM|
Well forereaching at 5 knots isn't going to leave you in your "slick" for any length of time. (to bad there isn't multiquote here). If you're not behind the slick it'll do no good.
I'd be curious as to how well fin keeled boats of various keel chord ratios behave under wave impact- intuitively, you'd think that having a small longitudinal plane would make the boat more suseptable to wave motion when heaved-to.
|07-14-2010 02:56 PM|
I am not sure how to answer that because if a boat has a fin keel "the other parameters" usually would not be "identical" (to a boat with a long keel). But more to the point in my experience the slick left by a boat has some impact on motion comfort and less of an impact on safety in breaking waves, so that in my mind simply making a larger slick is not directly translatable into saying the boat is more safe in breaking waves.
It probably goes without saying that boats behave as a system of a whole. Keel type is but one aspect of that whole and since the term fin keel really describes a broad spectrum of keel shapes and sizes and each behave differently, I would not say that generically fin keel boats are either safer or more dangerous than longer keel boats when lying abeam to large waves.
|07-14-2010 08:56 AM|
|07-14-2010 08:19 AM|
In my experience, most fin keel boats that I have owned will heave-to about as easily as the long keeled boat that I have owned. This is only to be expected since heaving-to is more dependent on sail balance than the keel shape.
Where keel area and shape comes into play is that fin keels will tend to stall sooner than a long keel and so produce less side force. This means that the fin keeled boat makes more rapid leeway than a longer, more traditional keel. This results in a bigger slick (rather than a smaller one).
|07-14-2010 07:04 AM|
|sailingdog||Yes, a fin keel can heave too. Yes, it will generate a protective slick, but not as large as a full keeled boat would. More of an issue is that most fin keel boats will forereach a bit, and not stay in the protective slick they create.|
|07-14-2010 03:19 AM|
SD, I am talking about the traditional, low aspect ration fin keel.
So fin keeled can heave-to but do they create such a protective slick that forces breaking waves to dissipate before reaching the boat ?
Somehow I feel (assume) that the slick created by a fin keel may be less powerfull and might not have the magical forces that calm the breaking waves
|07-13-2010 11:09 PM|
Here is some info. It's part of the story of :
Atom Voyages | Voyages Aboard the Sailboat Atom - Sailing a Catalina 320 to the Virgin Islands
in a Catalina 320 wing keel
By sunset I was worn out and chilled through. I decided to heave-to, letting her drift where she will as I got some rest to prepare for another day at the wheel. I rolled up the genoa to the size of a storm jib, sheeted it to windward, and locked the wheel a few degrees to leeward. I didnít expect this skittish wing-keeler to heave-to in the manner of a more traditional hull shape, which would fore-reach at 1-2 knots or drift to leeward leaving a protective slick of calmer water to windward. Still, I was unprepared for the surprising result of this experiment. With backed jib in 30 knot southwest winds she sailed off at 5 knots to the southeast. Without a hand on the wheel she held steady all night, galloping away with ease on the same course I hand steered all day with such drudgery. From despair to ecstasy in one sail maneuver! I dropped into my bunk for a few hours of blissful sleep as Mei kept a weary watch from the companionway.
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