Centerboard size mismatch?

[pandelume]

I've been working on a little day-sailer I bought last September; perhaps because of the unpleasant weather, this is the first time I've really taken a good look at the construction drawing in the plans I was given with the boat. I found what seems like an obvious mismatch between the size of the centerboard and the size of the trunk, as can be seen in the following pictures.

http://i231.photobucket.com/albums/ee70/pandelume/CB_Trunk.jpg
Centerboard Trunk

http://i231.photobucket.com/albums/ee70/pandelume/CB.jpg
Centerboard

Please excuse the distortion in the images - as near as I can tell these plans were xeroxed from a book of plans somewhere.

As I read it, the top of the trunk is 22" from the keel, and the CB is ~30.5" wide at the widest. Anyone have ideas on how this is supposed to work? Perhaps the trunk is supposed to be completely open at the top, and the CB was intended to stick out when retracted?

I can say that the trunk as built is not open, and the CB retracts fully inside the trunk. It must not have been built to the size indicated on the drawings, but I haven't had a chance to take it off for measurement. Any thoughts on how the original CB was supposed to work or whether I should replace the existing one with one made per spec?

[mmd]

Sure looks to me like it was designed to be an open-top C/B trunk, with the retracted centerboard sticking up above the trunk. Not an uncommon arrangement, by the way, but prone to squirting water out at unsuspecting passengers (sailors aboard know to sit to windward aft!)

[Thorne]

I'm a real fan of closed CB cases, both for the reason MMD cites above, and for the ability to bail the boat out when swamped.

One option to consider is just to rebuild the CB and case as they currently are built -- if indeed you need to fix either of them. Try the boat out on the water and see how the CB works at that time.

You can always pull the CB out and the top of the CB case off -- I like mine removable anyway to fix potential jams with gravel or whatnot jammed around the CB. I use the old traditional "champagne cork" in the hole in the CB case top, but shoving the boathook handle down that doesn't always allow you to fix what's needed with CB woes.

[Bob Triggs]

In John Gardner's book "Classic Small Craft You Can Build" he goes into balancing centerboard and mast locations, sizes and shapes etc in good detail in the chapter on "balancing sailing rigs". He provides a simple technique for modeling the locations etc, and some suggestons for working it out on the water.

[merlinron]

looks to me that the size of the centerboard may have been revised due to maybee being placed in a slightly wrong location . being discovered after initial trials, it was easier to simply increase the size of the board and leave it hang slightly out of the bottom when fully up..... not the best solution, but maybe workable where the boat was sailed, then never changed on the prints for some reason(?)....

are you sure, by the prints that the double line across the top of the case indicates a cap. it is shown in the posted print as so, at the back of the case above the wider curved member..... that might be just to cover the end grain of that part and the double line represents a stiffener along each side of the case's top edge, with the case open, as MMD suggests. is there an athwartship section of the case construction that would clarify that drawing?..... what i wouldn't give for two views on all details at work.......

what ever the case..... i would think the dimensions of the board would be more important and i would build the case to accomidate the board, with your decision to leave it hang out the bottom or make the top open to fully retract the board.

many boats have a removable cap that is placed when the board is down to stop the slpashing as well.....that might be an option.

[Hwyl]

I echo what everyone else says about open top centreboard cases. I'd also add they have a propensity for causing bodily harm, in a most painful way. I'd say (shooting from the hip) that the chord of your board as built is probably O.K. Try sailing the boat, usually a high aspect ratio is considered desirable, less wetted surface for almost the same amount of lift, you do pay for t with less directional stability and an easier stall, but all in all I'd take the trade off.


Disclaimer, the aforementioned are the ramblings of a rank amateur, any NA has the right to render the words useless.

[pandelume]

Here's the top view of the CB from the construction drawing.

http://i231.photobucket.com/albums/ee70/pandelume/CB_Trunk_Top.jpg

It shows the Trunk as open, but it doesn't show stiffening strips or a cap. As I look at it, it seems to show the walls of the trunk as a line with no thickness. In the picture below, I've drawn in the CB and trunk in various positions - from this it seems like it was intended to be open-topped. I suspect that the board as built was just like the intended board, but with the part that sticks out chopped off.

http://i231.photobucket.com/albums/ee70/pandelume/CB_Retracted.jpg

I'm not familiar with open CB Cases, although dagger board cases are open, so I guess I shouldn't be surprised. Anyhow, I suppose the best way to evaluate it would be to take her for a sail, as some of you suggested. I'm not that experienced, so any advice on what to look for would be appreciated. In other words, how should I isolate behavior while sailing as being caused by the CB size or position?

[rbgarr]

Do you know what class this boat is? I wouldn't be worried about the open top cb trunk construction unless you will be towing the boat at higher than normal sailing speeds. Not that much water will come in during normal use.

[Hwyl]

I'm not familiar with open CB Cases, although dagger board cases are open, so I guess I shouldn't be surprised. Anyhow, I suppose the best way to evaluate it would be to take her for a sail, as some of you suggested. I'm not that experienced, so any advice on what to look for would be appreciated. In other words, how should I isolate behavior while sailing as being caused by the CB size or position?

Enterprises, one of the most popular dinghies in the world have open CB cases.

Excessive leeway, or weather helm. My (uneducated, but fairly experienced) thinking is, the board as drawn is hugely oversized and you're good the way you are.

[Thorne]

I'm certainly no NA, but in my limited experience these older CB's were often not designed to be used fully down -- look at some of the Chapelle plans and Gardner's books to see what I mean.

They often look like the middle of the three drawings above -=- "board retracted fully while still in trunk case". When I asked last year on this forum if the older plans showed the CB's fully down, the consensus was that they did, and the old CB's were used with a lot of the top of the CB still in the case to give more lateral support via the head of the CB touching the case walls.

From personal experience, moving from a wide to narrow aspect CB/foil will create balance issues -- moving the CLR forward and often causing excessive weather helm. Same problem would probably happen if you took a CB designed to be partially-lowered and used it fully down.

I took these plans and made the CB thinner and a bit longer (blue lines), plus had it swing down 90 degrees. Result is savage weather helm that is corrected by a larger jib on a bowsprit -
http://www.luckhardt.com/cb-design1.jpg

http://www.luckhardt.com/newcb7.jpg

We still don't have much info on this daysailor you are restoring. What is the condition, what needs to be fixed, do you have the original spars, sails, rudder, etc?

If you have all of the original stuff, try sailing in both light and strong winds to see what the balance is like. Try the CB fully down and partially down on different points of sail. If your arm or hands hurt from the tiller -- you've got balance issues...

;0 )

[pandelume]

I'm not an expert, but overall I'd say she's in decent shape. Most of the work that needs to be done at this point is finishing and reassembly. The exceptions are a number of bottom frames that need to be replaced - rotten due to standing water in the bilge, near as I can tell - and a section of plywood deck right up in the bow that should be replaced due to water damage where some fairleads were mounted but not bedded properly.

However, she came into my hands already altered: a new compartment was added onto the stern, and a large skeg was apparently added and then removed. The extended lines can be seen in the picture below.

http://i231.photobucket.com/albums/ee70/pandelume/ExtendedLines.jpg

I do have all the original spars and hardware, but the hardware has all been removed from the hull for sanding and repainting. I also have the material to change the rig from the original one:

http://i231.photobucket.com/albums/ee70/pandelume/Old_Sail_Plan.jpg

To this:

http://i231.photobucket.com/albums/ee70/pandelume/New_Sail_Plan.jpg

I have to admit that I don't have a very good reason to change this except that it seemed like fun and I like the new one better. The plan I drew has the same geometric center as the old one, according to my calculations. I am planning on keeping the old rig intact so that I can change back if the one I drew doesn't work out.

Anyway, I can't take her out on the water right away, but I probably could in the next week or two with the old rig and without replacing any frames. This doesn't seem like that wisest course to me, but if I'm going to alter the CB trunk, I suppose it would be better do do so at the same time I replace the frames. Construction is 1x2 frames with 3/8" ply planking. Thoughts?

[Steve Lansdowne]

One local source of advice you might consider is to contact someone at the Center for Wooden Boats on Lake Union in Seattle. http://www.cwb.org/