Ballast vs. weighted centerboard

[sixball]

I’m green and apologize because I’m sure this has all been worked out before. I just can’t find the answer with a search.

I’m wondering if ballast in the centerboard is more effective than ballast in the boat. Wouldn’t fifty pounds of lead three feet down (when the centerboard is extended) be comparable to 150 pounds bolted to the floor next to the centerboard trunk? My figures are arbitrary but I’m sure there’s a ratio. If this were the case, why do plans so often call for ballast in the boat and not on the centerboard?

Forgive me if I’ve exposed my ignorance, but as they say “the only dumb question…”

[JimD]

The only reason I know of for inside ballast on a centerboard design is to keep the centerboard light so it requires a less substantial trunk and for easier raising and lowering, ie no winch required. My daysailer has both. Inside v outside can also affect the motion of the boat but I doubt that's really a concern for smaller boats.

[Tom Brady]

Hello Sixball,
Your questions can't possibly be any dumber than some of mine so don't worry about it. I had a 22' with a 500# "centerboard" or whatever you want to call it and I could tell a BIG difference between board up and board down. Of course it took a winch to crank it up but it was worth it. I did have to repack the pivot pin on occasion to keep it from leaking and the case was strongly braced at both ends. If you find anyone that thinks weight down doesn't make a difference tell them to talk to a West Wright Potter 19 owner that has tried to sail with the board up.
Fair Winds! Tom in SW Florida.

[Keith Wilson]

For a robust structure, it's a lot nicer to have the ballast attached to the hull, and outside is better then inside. A heavy centerboard puts a lot of stress on the lifting mechaism, the pivot, the case, and the board itself - nothing that can't be dealt with through good design, but not nearly as clean as some lead bolted to the bottom of the hull. Weight lower down certainly gives a greater righting moment, but that force has to be transmitted through all those components. I particularly like what Phil Bolger has done on ome his later shoal-draft designs, where there is a plate-steel shoe bolted to the bottom of the boat - both good ballast and armor plating for the bottom.

[Bruce Hooke]

In short, I think the answer is that it clearly does help the stability and can be done, but a lot of the time it's not worth the extra stress, engineering and work for the user. This is especially true on a very small boat where moving the people around can have such a huge impact on stability anyway. I'd see it as a last resort for a boat that is large enough so that moving people cannot make up for the lower stability, where a centerboard is a must to keep the draft shallow, and where the hull cannot, for whatever reason, be shaped so that it provides enough form stability to make up for the lower weight stability.

[Venchka]

...and

I've wondered about this a lot. The other bad thing, to me at least, is that when the board is up the C.G. of the ballast rises. Or, said another way, with fixed ballast the effect of the ballast remains constant. I don't know if that is good or bad, but a whole lot of centerboard dinghies have fixed ballast and light boards.

[rbgarr]

In most small boats with (swinging) centerboards that have weight in the board the purpose is to get them to have negative bouyancy so they don't float back up into the centerboard box. This weight isn't necessary on a boat with a daggerboard, for example, which is a tighter fit, usually, although Sunfish daggerboards are known to 'creep' up out of the hull.

You usually raise the board when sailing beam to or downwind, so raising whatever weight there is in the board is not so consequential, IMO.

[ 04-08-2004, 07:05 PM: Message edited by: rbgarr ]

[mattb]

There is not an easy "ratio" you can use to calculate the affect of lowering the center of gravity by ballasting the centerboard.
Lowering the CG increases the righting moment, but that is determined by where the transverse center of bouyancy moves to as the boat rolls. Short answer is that the mathmatical solution is not linear.
If I had to choose between inside ballast and a heavy centerboard I think I would go with the inside ballast. Less headaches in both the short and long term. An interesting example is the dutch Botter, a traditional leeboard fishing vessel. With the combination of hull form and inside ballast you won't find a stiffer boat anywhere.

[Pernicious Atavist]

as said, raising a weighted cb can be a chore, especially if you have only one hand free to do it. a point in case: my thistle carries a 75#, fiberglass cb. when it's down, it's a great piece of ballast, but raising it single-handed requires tackle, which i have, and significant strength, ditto. my mate on the other hand has to use both her hands to raise it and in a grounding situation just can't muster the strength and speed to lift the weight rapidly. additionally, the trunk is very stout and well braced and carries a lot of strain when heeled, rail-down.
i'd opt for hull ballast, as said, externally if possible, and only weight the cb to keep it down under speed.
g'luck! :cool:

[JimD]

A drum winch will effortlessly raise a heavy steel centerboard

[TimothyB]

Using 'ballast bags' on a smaller CB design really is so that you can trim her out for various loads and conditions. Makes it easier to bring her out of the surf as well.

If you had a 6 Meter open boat, and only put 2 people + camping supplies in her you would have lots of xtra buoyancy you could eat up with ballast bags tucked in by the CB to give you a more easy motion. Sand is a favorite. Just make sure the bags don't break!

[sixball]

Thanks for the enjoyable discussion and information. A very nice welcome to the forum.

[Pernicious Atavist]

yeah, sixball, this is a very informative and helpful group! joing was about the best thing i've done for myself in a long time.
welcome!