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Adding lead to SK for 40% B/D ratio

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SeFairMarin
Deckhand

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USA
3 Posts

Initially Posted - 04/21/2006 : 05:37:50

Hello,
Given 4150# Displacement and 1500# Ballast the SK has a 36.7% Ballast/Displacement ratio. Adding 200# of lead would give the SK a B/D ratio over 40%. Has anyone done this?

Thank you in Advance.
Bill

Bill Williston

Edited by - on

ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 04/21/2006 : 09:29:08

True enough, you can easily get 'bolt on' wings that should stiffen the boat up quite a bit. However, there are other design considerations.

Will the rig take the additional load? Heeling is how a sailboat 'self-regulates' the forces on the rig and hull. A change in one area may mandate changes in other areas too. Standing rigging, Chainplates, Keel attachments would likely need to be looked at.

Why are you looking for a magic number?

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Steve Milby
Past Commodore

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USA
5902 Posts

Response Posted - 04/21/2006 : 10:46:57

Lowering the vertical center of gravity (VCG) generally improves motion comfort and stability of a boat, and that's a good thing, but it also has some tradeoffs.

Adding 200# of lead to a swing keel boat is loaded with problems. First, the swing keel mechanism is strong enough for a 1500# keel, but it's certainly not overbuilt. If any part of the swing keel mechanism fails, the result can be catastrophic. Adding 200# would greatly increase the potential for failure. Instead of having a 1500# keel hanging from a pin, you'd have a 1700# keel with a considerably longer lever arm hanging from a pin that was only designed to support a 1500# keel of a certain length. Secondly, where would you add the lead? Adding it to the bottom of the swing keel would give it maximum effectiveness, but that would increase the already very deep draft of the boat significantly when the keel is down, and it would add stress to the mechanism. When the keel is up, that 200# would swing up and aft, which would cause the stern to squat in the water.

Also, as Bruce mentioned, it would increase the stress on the rig, and especially on the chainplates, which are strong enough for the boat as designed and built, but are not overbuilt.

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JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 04/21/2006 : 11:15:37

my fin keel boat is perfect. No need for extra ballast. You can always hike out and use a tiller extender.

When I am fully loaded for weeks of cruising, the ride is noticably nicer and the boat is slower. But I am talking 2000 lbs of gear all loaded low.

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ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 04/21/2006 : 21:09:42

For some silly reason I read the orginal post as wanting to add weight to a fin keel. Kinda befuddled this week from having the flu.

I'd absolutely not add weight to a swinger... unless you did some very serious engineering/reinforcing of the entire swing keel area. Likely it would cost more than the boat is worth.

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sweetcraft
Admiral

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USA
816 Posts

Response Posted - 04/21/2006 : 21:42:19

There was discussion of adding weight before but to adjust the healing at the dock. Isn't moving weight lower and forward good for speed? I added a third battery forward and to the opposite side of the outboard rather than just weight inside the hull after the discussion. No one has ever noticed a difference and I reef sooner now too.

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Steve Milby
Past Commodore

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USA
5902 Posts

Response Posted - 04/22/2006 : 07:47:05

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by sweetcraft
 There was discussion of adding weight before but to adjust the healing at the dock. Isn't moving weight lower and forward good for speed? I added a third battery forward and to the opposite side of the outboard rather than just weight inside the hull after the discussion. No one has ever noticed a difference and I reef sooner now too.<hr height="1" noshade id="quote"></blockquote id="quote">

Moving weight lower in the boat is usually, but not always, a good thing, and moving weight forward or to one side or the other can also help.

Ordinarily, a boat is designed so that it sails best upright and on its lines, but there are exceptions to the rule. Occasionally, the boat designer makes a mistake that can be corrected by changing the weight distribution. For example, the weight distribution on the pre-1981 C25 was off very slightly, and shifting a little weight to the starboard side will cure a slight list to port. Also C250 owners have found that the C250 sails faster if a little ballast is added forward. I'm not sure whether that's a design mistake or simply a natural trait of the modern hull shape. I tend to think it's the latter.

But, even if your boat doesn't have any design eccentricities, there are times when you can improve the boat's performance by shifting your "movable ballast." (Your crew members are your movable ballast.) In strong winds, you should position your movable ballast on the windward rail, to reduce the boat's heeling. In extremely light wind, you should position your movable ballast forward and on the leeward side of the boat, to increase the boat's heeling. When you put crew on the rail and forward in light air, the boat doesn't heel very much, so the unequal underwater shape of the boat doesn't induce much drag. But, putting them there helps the sails hold their shape better, and it lifts the stern slightly, which reduces the boat's wetted surface, and thereby reduces drag. Most boats also sail better downwind if you position crew weight forward and to windward, because it lifts the stern slightly, and reduces the boat's wetted surface.

Shifting your movable ballast doesn't increase boatspeed so much that you can feel it. If you have good instruments, sometimes the instrument will register the speed increase. When your sails start holding their shape better, instead of collapsing from their own weight in light air, and when you see the boat start to move again, when it had previously been sitting motionless, and when you start overtaking and passing boats ahead of you, then you'll know the shift in weight is working. For the most part, you just have to take it on faith that, if you shift your ballast in a way that improves the orientation of the boat, it will also improve the boat's performance.

Edited by - Steve Milby on 04/22/2006 09:23:16

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The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.