Thread: centreboard weights

I would think if you use the lighter boards you would need to ballast
the boat to design weight with internal ballast. I would go ahead and
build the weighted board. I would not use steel, rust will cause it to
swell. I would go with lead shot. You need to weigh the parts of the
board and add the needed amount of lead.

I would like to see how the board is made?

Did some figuring, that is a lot of lead.

Thanks for your thoughts on this. This is, in US measure, an 18 footer with a pivoting CB. (drawings aren't here so can't scan design for you) The designer, I think, is happy about an unweighted board- the weight is an option so that you can drive her harder I think, and still stay 'dry'.
He does suggest a lead tip (cast or fabricated) to achieve the max weight- and this makes sense because weight at the end of the board has more relative leverage. He also has a lot to say about CB sections and airfoils, but that is another story.

I did a search and found the site for the boat, It gives a ballast weight,
I am sure if it is not in the center board, it will have to be inside the
boat. I think it gave a total of 90kg.

The different shapes will give different performance,

The all up weight is not the same as the trailer weight,
does it have water ballast tanks?

nope. no tanks..and no mention of any required. At 90kg I think I will be the ballast!
When I asked him about this he said 'just build as per plan and she will sail fast and well'- mentioning that only very serious racing types need be concerned...and that certainly isn't me, although my son will want to give it a nudge

If you haven't done a lot of building or sailing I would probably recommend sticking pretty close to Mike's options. After all ... they work! And they take everything into consideration.

MIK

Thanks Mik, yes I will keep to his suggestions,
but he does offer these alternatives and I was just interested in exploring the implications/benefits of them. I want to build 'as per plan' in deference to the designer's experience.

Now ... it depends where you sail and the type of sailing you are wanting to do.

If you have a lot of shallow water where you plan to sail then a lightly weighted board will make skimming over shallows a lot easier. You can pull it up with little effort but you would have to put more weight than that in the bilge or under the seats to compensate if necessary. Still need enough lead in the keel to sink it.

But if mostly deeper water then it won't hurt if you put it in the keel as you won't need to haul it up often and you will get better performance.

Adding a substantial amount of weight to a centerboard can have unexpected results. Some of the things that most novices might encounter could be: disproportionately increased loads on the rig, increased side loads on the case, pivot pin distortion, board binding and a few other things.

I've know more then one that has looked to increase "righting leverage" by adding weight to their boards, just to have the mast fold up like a twig in a gust that previously wouldn't have caused concern. This is actually a common issue on both shoal draft and fixed fin boats.

The weight does add considerable leverage, which it does across the full spectrum and cases that remained tight prior to the modification can begin to leak after. The side loads on a case can be surprisingly high as the board tries it's best to twist it way out of the boat on a hard slosh to windward. The additional weight adds to these loads and can split cases, cause the board to bend or bind, etc.

Trying to keep a pivot pin from leaking is always a chore. Designers have attempted several clever engineering "solutions" over the years, including raising the pin above the LWL. Each has their own personal favorites (I use low mounted pins), but all have good and bad points to consider. Most use a hefty pin to ease the possibility of it bending, but the weak links are the bushings and the end plates (if designed as such), which can distort and buckle with increased loading.

I'm not suggesting you not ballast the board, just offering some elements that are quite often overlooked or not recognized by builders.

On boats under a ton in displacement, I design "neutrally" buoyant boards. By this, the board is the same weight as the volume of water it displaces. I then add just enough weight to sink her firmly against her lanyard (so it'll stay down underway) or install a down haul. I don't like the added complication of a down haul (usually a bungee cord), so I'll weight the board slightly.

On your boat, it's getting into the range where I'll consider adding weight to help the righting moment, but it's usually not much because of the dramatic increases in loads it imposes. These additional loads force me to design stiffer and beefier structures to support the board, which adds weight and more complication, so it's a dog chasing it's tail sort of thing. I tend to error on the side of a short tailed dog, rather then a long one that he might be able to catch. In this vain, weight the board enough to sink it firmly against the stops, but enough so it can bounce over obstructions and can be hauled up reasonably easily. The hoisting tackle is lighter, the case is lighter, the structure is lighter and the board less prone to damage in a strike.

On the other hand, if you intend to race other boats of similar class, then weight the crap out of the board. You'll be able to keep her on her lines longer then the competition, with the understanding that racers often lose their rigs and rip up gear, all in their efforts to gain a 1/10th of a knot more speed. I know a guy that blows out sails every three or four races, but he's fast.

Thanks for your thoughtful responses, you both tease out a range of costs/benefits for different solutions to the problem. Corio Bay has plenty of shallow water, Mik, and I can't afford to rip the rig for speed, Par!
Attached jpeg shows incomplete CB case/spine (training skid on ATV!)

Nice work, but she's a bit narrow of beam!

I suppose I could make it a bit wider...

To my eye it does look fast already. I guess it depends what you do to make it wider!

MIK!

Some outriggers and a fat roached main, she'd be sprightly. Lots of foot holds to grab.

Of course I'm relying on those fat tyres for buoyancy, but I'm thinking the whole rig might be more stable upside down. These design decisions are an absolute killer aren't they?
then there is the problem of where to put a mast...