Fashioning narrower aluminum CB (with mild ballast)?

[whitejw1967]

My friend and I tackled a sticky centerboard recently. The CB trunk seems a bit warped inside and the 40 year old centerboard also has some mild bulges.

We thus began thinking about fashioning a new centerboard (to the basic size configuration as the original) but out of aluminum (largely hollow with struts and stringers inside for stiffness). It'd be narrower than the original to avoid sticking in the CB trunk. So, a few questions for your giant brains:

First, would there be any disadvantage to a slightly narrower centerboard but with the same length/depth and front-to-back width (and tapered for a 'foil' shape)?

A second, possibly more interesting question/proposition: we've been brainstorming about adding ballast to the lower half of such a new centerboard (inside of our aluminum CB). Doing so would, I hypothesize, help at least a little bit with reducing healing (as I most often sail solo, often in 12-20 knots of wind). 50-100 lbs of ballast on the lower half of the CB would, I would think, add greater secondary stability. The added weight would also be countered by less healing under sail, thus negating or possibly even improving performance. Such a system would of course also require a reworked raising/lowering system (namely raising as added weight on the lower half of the cb would naturally want to fall free) and reinforcing the CB trunk near the pivot point.

Any thoughts? If we were to do either option, I'd keep the original CB as well.

[algonquin]

Additional ballast would help significantly but may not keep your boat race legal. Depending on its use that may not be a problem but could effect resale.

Do some on line research on foil design. This would be a good opportunity to maximize the center boards effectiveness. Most effective leading edges have a slight flat design at the beginning of the foil taper. Brad

[GreenLake]

The modifications (including the change of material) would appear to make for CB that's not class legal. But class-legal reaches only as far as racing within the class goes. It may not be available to or desirable for you. And keeping your old board means keeping your options.

That said, going from the stock board to one that has a nice foil should help getting more lift out of it. I'm less sure about the ballast. The boat would need to be heeled quite far for this ballast to get a good lever-arm. I bet just leaning a few inches further out when hiking would do more.

The biggest effect from adding a bit of weight would seem to be to keep the board down, but there are other ways to do that.

If you do go ahead, don't forget to let us know the details and whether it worked out as expected.

[whitejw1967]

Thanks for that. I do think a better centerboard could make some good differences. The old one is pretty beat up.

One site I found said that a weighted CB makes a noticeable difference. This person talks about Flying Scots--that apparently do have some weight in the lower half of the CB (75 lbs). But you are right--again--in that my friend and I need to do calculations to determine righting arm and whether or not such weight would really do much of anything other than make for complications.

I thought about the racing/class legal stuff, but I am unfortunately far from any Daysailer organizations and, even if I weren't, I wouldn't be racing. I think I may have also made her 'illegal' in that I put on a tabernacle.

I'll keep you posted!

PS I did just add hiking straps so I can get further out now!

[jdubes]

I really like the idea of putting a little weight in the CB. I purchased a used CB last year just for that reason. The DS is designed with assumptions on the amount of weight you should put on the rail in relation to the wind strength. And flying Scots obviously have different sail plans compared to the DS. I agree with GreenLake, it's hard to say if you'll see any value. But keep us in the loop and let us know how the stock DS CB lever can handle another 70lbs.

[whitejw1967]

Will do (if we ever get to fashioning a new CB). The good news--or bad news depending upon how one looks at it--is that the DSII has a different system for lowering and raising the centerboard. We'd attach an "up-pull" probably directly to the back of the CB and let mostly weight pull it down.

Happy sailing (if there is any during these cold months).

John

[jdubes]

Here's a great sailing activity for the winter. DN Class Iceboat. http://theneiya.org/.

[GreenLake]

I thought about the racing/class legal stuff, but I am unfortunately far from any Daysailer organizations and, even if I weren't, I wouldn't be racing. I think I may have also made her 'illegal' in that I put on a tabernacle.

Tabernacle might make you non-competititve, but it shouldn't make the boat 'illegal.'

[Bob Hunkins]

The problem with modifying a boat so not to be class legal is that subsequent owners who might want to use it for racing will have to spend a lot of money to set right the "improvements" that were made.

[persephone]

Hello,

If you are still thinking about this here is a link discussing how to get the lead into wood centerboards.

http://forum.woodenboat.com/showthread.php?125553-Putting-the-lead-in-my-centerboard

Personally I don't see a problem with it from a resale standpoint. First off the 2s aren't as popular for racing as the 1. Also if you keep the original board intact to go with the boat if sold it's not that big a deal to change.

[GreenLake]

Reading the link, I still don't see it. These guys are talking about a 3x7 piece of lead, presumably about 1" thick. That's half a lead brick (don't ask me how I know about lead bricks...). The righting moment of even a full lead brick is minuscule compared to crew weight sitting out - except possibly in the scenario after a capsize.

For comparison, the Flying Scot has 100lbs in its centerboard. That would be about 5-10 times as much as these people are pouring into their wooden centerboards.

But when you sail your boat upright, even 100lbs doesn't add any righting moment, while crew weight does.

Just got my latest DSA quarterly and learned that there's now a new class board, with a nice foil shape, to be available through Cape Cod. I had been thinking of building my own, but this news is making me think a lot harder...

[Moose]

I've often thought about Getting a spare C.B. and cutting off the bottom foot or so and replacing it with a casting that matches the original C.B. profile and Chord.
I think that there are some inherent problems to consider when doing this though. For one The extra weight on a boat with such a low displacement will change the boats W.L. when the weighted C.B. is in the down position and create a different W.L. when up. This might not be a big problem if you do not keep your boat in the water year round though. I've noticed in my boat that having 3 on-board (about 135# ea.) can really pitch the aft section of the boat and take up a fair amount of the relatively low free-board. So
Greenlake does have a good point. Adding about 80# of ballast to the C.B. at 30° of heel would create about the same amount of righting moment as a 120# person sitting on the rail ((roughly, working from memory here.) also said person would not be hiking over the side.) So if it is available to you crew weight would be slightly more effective.
If the C.B. was longer a 100 or so pounds would make a big difference but the boat isn't really designed to carry that extra weight down low. Structural problems could become a serious issue considering the C.B. is only held in with one 1/2" bolt (through fiberglass) and not much else.
I can imagine a modernized DS with a retractable bulb keel, more freeboard, and a slightly redesigned hull shape and rig. Something like that would be great for the class. But it's not really what the majority of the D.S. association is about. The class is currently one that's not very expensive to race competitively in and the addition a modernized boat would push out all of the current D.S. 1,2, and 3 owners out. Sort of like the modernization of the moth class with the addition of foils and now wing sails.
I think that the addition of ballast to the keel could be helpful but you would really have to do your math. You would notice better heeling resistance when sailing alone, but reefing might be a better option due to the possible complications involved in changing the C.G. of the boat and its meta-center in relation to the C.E. You would also have to consider the resistance to capsize recovery would also be increased.
Ultimately without a full set of lines plans it would be impossible to say whether or not the modification would actually slow the boat considerably more than reefing would and rally just make your efforts futile.
I'm not knocking the idea, don't get me wrong, but I think it would require some heavily thought out and calculated decisions.

Moose.

[jdoorly]

Most centerboarders have a bit of lead in their CB just to keep the board down, i.e. from floating back up into the CB box due to bouyancy and water pressure. DS's use other techniques to hold the board down. But when I had my CB out last autumn I found the board was splitting so I pried it open a little more and was able to see there was a round cavity in the interior of the foot of the board the shape of a free weight. My immediate thought was that the board was originally designed to have a bit of lead there but they decided that the board would stay down with the lever system and didn't need to buy and form lead improving their profit margin. In the case of the Flying Scot's 100 pound CB seems like they've gone beyond holding the board down by about double or more.

I continue to search for economical ways to make a centerboard into a keel. This week I checked into using sheet lead available from your local plumbing supply; don't have to melt it, can glue up thin sheets and then cover with epoxy and fair. (But it still costs almost $2 a pound.) If you just replaced the bottom half of your CB with lead that would be 120 pounds centered at 34" below waterline or a moment of 340 ft lbs.

Moose is right about checking this out using the lines drawings (or offsets), does anybody have some or know somebody who does?

[Moose]

Cape cod shipbuilding owns the rights to the plans and they are extremely protective of their plans (even suing companies who produce the h12-1/2 which they dont technically own rights too).

Honestly if your looking to do this type of modification for comfort the DS might not be the right boat for you. It's not, nor will it be a cruiser without serious modification.