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-   -   Newbie: Centerboard / Swing keel distinction (http://www.sailnet.com/forums/learning-sail/72435-newbie-centerboard-swing-keel-distinction.html)

ExperiencedInTheory 02-28-2011 01:10 PM

Newbie: Centerboard / Swing keel distinction
 
As a newbie to both sailing and this forum, I apologize if I am duplicating a thread here, but a brief search through the forums didn't answer my questions.

I am teaching myself to sail on a 1969 Newport 16, which displaces 800-900 lbs empty with a 4 ft 200-300 lb lead swing keel. (Finding the specifics for this particular year are difficult).

Most of the literature I have been reading suggests partially lifting the "centerboard" in many situations such as when sailing downwind, or to quickly relief excessive heel at the expense of leeway, but I am never sure if the term is meant to refer to a non-ballasted hydrofoil or a ballasted swing keel. In either case, It makes sense to retract it heading downwind, but what if you jibe(accidentally or otherwise)? I have read that the centerboard should be at no more than 1/3 of its full extent when jibing. That makes sense for a non-ballasted hydrofoil...I can imagine a boat yawing significantly when the boom snaps against the mainsheet to leeward, and the resulting angle of attack on the board might cause some extreme heeling. With a heavy swing keel though, retracting it also reduces your righting moment, and I am not sure which effect is more significant.

I am probably just overthinking these things. When I first got the boat, I just naively assumed the keel was only raised to trailer or motor over shallows. It just seems to me that "centerboard" is often used to mean two things, and I am not sure how much of that would be relevant to my boat.

catamount 02-28-2011 01:23 PM

As I've always understood it, the reason for pulling up a centerboard (or swing keel, or daggerboard for that matter) when sailing off the wind is so as to reduce your wetted surface area and thus drag, with the result being that you go faster. You can get away with this off the wind because you don't have as much leeway to contend with, nor are you in need of righting moment.

I am not aware of any particular harm to come from leaving your board/keel down off the wind -- just more wetted surface area.

That said, I know of one swing-keel class (SJ21) where the class rules require that the keel be pinned in the down position when racing, even off the wind. I think the concern here is that if the boat does broach, besides the righting-moment issue, the momentum of the un-restrained swinging ballast could damage the trunk.

MarkCK 02-28-2011 02:57 PM

At this stage in the game you are probably over thinking things. I would just leave the board down while you are in the learning stages. It will be one less thing to think about. Unless you are racing, trying squeeze that extra couple of tenths of a knot out of the boat is a lot of hassle.

sailingdog 02-28-2011 04:31 PM

I'd first point out that a centerboard is usually an unballasted foil which is designed to allow the boat to resist leeway and point higher, where a swing keel is ballasted and designed to help the boat point higher but also to help keep it upright.

While you are learning, leave the swing keel down all the way. After you get some experience, you can start experimenting with raising it. Raising a swing keel or centerboard will also affect the boat's Center Of Lateral Resistance, and can change the amount of weather/lee helm the boat experiences. Retracting it completely can reduce wetted surface area and drag.

ExperiencedInTheory 02-28-2011 04:49 PM

Thank you for the replies. One good reason to winch up the keel when reaching is that is keeps quite a bit of weight aft, but my serious concerns are jibing; I see a lot of warnings about not having the centerboard down when jibing a dinghy, and I am not sure there is much distinction between a dinghy and a 16' pocket cruiser. I guess I will just try retracting it when heading downwind.

I am stuffing enough foam under the cockpit so that having anything worse than a learning experience is unlikely.

catamount 02-28-2011 05:29 PM

Quote:

Originally Posted by ExperiencedInTheory (Post 703421)
... but my serious concerns are jibing; I see a lot of warnings about not having the centerboard down when jibing a dinghy, and I am not sure there is much distinction between a dinghy and a 16' pocket cruiser. ....

I'm not sure where these warnings you are seeing come from, but I would point out that there probably IS quite a bit of distinction between a planing dinghy with an un-ballasted centerboard that relies on crew weight for righting moment vs. a pocket cruiser with a ballasted swing keel.

ExperiencedInTheory 03-01-2011 08:03 PM

With 3 crew, for ballast I would have 200 lbs of lead and ~500 lbs of meat. I was worried that, with a deep skinny keel and a 14' waterline, the boat could pivot on the keel during a jibe and the high angle of attack on the keel might capsize the boat to leeward. As a learner, I would prefer safety over efficiency, and was just wondering what the safest option would be when running, i.e. if running with the keel down is actually hazardous for this sort of boat.

(Though on second thought, my rudder has ~50% the surface area of the keel, and a stiff grip on the tiller is probably sufficient to prevent the scenario I was describing.)

Anyway, thanks for all the input so far.

StartedSailing 03-03-2011 03:58 PM

The act of bringing up the centreboard during the run is to reduce the drag on the boat, thereby reducing speed. However, this can compromise speed for stability and so if you are hit by a gust on a run, becareful of an accidental gybe, because you can broach and then capsize.

Depending on the wind strength, I would have it down a portion of the way in strong weather and all the way up in light weather.

MarkSF 03-07-2011 12:01 AM

Well I've read a bit on the subject of jibing and broaching in my Wayfarer 16ft centreboard sloop (no keel).

The consensus seems to be that keeping the centreboard down during a run enhances directional stability, making a broach and death roll less likely.

I had problems with it coming up of it's own accord on a run, so installed some shockcord to hold it down.

The other tips for not broaching were :

Never overtake waves on a run. The boat makes the waves break, then the rudder ends up in aerated water, losing it's effect. Slow down if you seem to be overtaking waves.

If it's really windy, jib only for the run can work well.

If it's at all windy, just don't jibe. Do a chicken jibe instead.

ExperiencedInTheory 03-22-2011 09:55 PM

Sorry for missing some replies here; I thought the thread had died. I had a wonderful three day trip to lake pleasant two weeks ago (my third sail), did not turtle my boat, and with some frayed nylon tied to the stays I did not repeat any mistakes with accidental jibes, but left the keel down all the time anyway as our runs were often short. Aside from a slipped anchor at 3 am and an instance of "what the hell is that?" as I pulled past a boat 30x the worth of mine in crowded cove, it was a fun extended weekend without incident.


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