Now that the building and re-working season is approaching, I am considering a centerboard. I was wondering about theoretical placement, downward angle (as oppposed to mast rake) if it makes a difference, and how long (or deep) should it be. On the weekender, it would be possible to not have an intruding centerboard trunk, I believe. But I am also working on a trimaran, which would have an intrusive centerboard trunk.
Basically, what I want to know is how to figure out where to place the d@mn thing, and how to know what size it should be.
The placement of a centerboard or any appendage is one of the black arts of yacht design. Some math, some calculations a few geometric drawings and you can still get it way wrong (I've first hand experience at just this).
There is no cut and dry method for exact placement, just some rules of thumb, which can be altered by many factors, including; depth of forefoot, hull type, beam/length ratio, type of rig, hull efficiency, etc.
Sizing the board also falls into the black magic thingie, but there are some reasonably set rules you can follow. The rudder size is determined in a similar fashion.
The nice thing about a centerboard on a Weekender (or Vacationer) is it would have to be a low aspect board. This is what we'd call a "slice 'o pie" style board. This is forced on us for a few reasons. First is the wish of no board case intruding into the cabin, which forces a small amount of "purchase" within the boat. This purchase means it doesn't have a lot of boat (in this point keel) to hold onto, so it doesn't get twisted out of the boat when under strain. The loads on a board can be substantial, so you want as much board "in" the boat as you can afford, to resist the twisting motion it will experience. A low aspect board just peeks out from under the keel, which looks like a slice of pie from the side. These can usually have at least half of the board remaining in the boat, which provides good purchase.
Now you can have a high aspect board, which is one that drops well down and would be typical of most modern craft with centerboards. The extreme majority of these types of boards have an internal case, that will intrude into the cabin space, but on Weekender's keel this can be enclosed all outside, with no intrusion. The keel as designed isn't strong enough to keep this type of board in the boat. It will twist it's way out, likely splitting the keel laminates in the process, or worse, breaking the keel to bottom joint open. An athwartship member or two will need be installed to transmit the new loads to the hull shell. Through bolts or the less effective lag bolts can be driven down into the keel, through these new structural members, which would be called floors. The floors would be bonded and screwed to the hull, like other elements of the boat, such as the cabin bulkheads. Epoxy fillets would be strongly recommended on the floor to hull connections.
The Weekender keel profile will not benefit a lot from a centerboard, without revisions. The deep forefoot, will fight the board for control in turns and general maneuvering. This deep forefoot provides the bulk of her lateral area, so she can sail without a board, but isn't particularly effective at preventing excessive leeway or generating lift for windward work. Most folks building Weekender are relatively new to sailing, maybe this being their first sailboat, so they don't know what they're missing in windward ability or leeward slippage. The very aspects that permit her to sail without a board, can cause balance issues if one is installed without keel modifications.
What to do? The forefoot has to go away, so the boat will pivot on the board in maneuvers, not the forefoot. The forefoot area removed is replaced with the board area for upwind work and retracted for down wind work, where it's not needed. Since the keel is getting a reshaping, you might as well take advantage of it and install some additional area aft, to completely enclose the board and provide more lateral plane area (which she sorely needs, like most small boats do). Removing a portion of the forefoot will not cause strength issues with the boat, nor affect the looks of the boat above the waterline. She'll still have her cutwater and clipper knee.
Brit, if you send me an email, I'll send you some drawings to show what's possible and how to get it done with a minimum of fuss.
I sailed on an original design Weekender that had been built with the center board. It made little difference whether the board was up or down in the ability to handle the Weekender in moderately heavy winds and wave conditions. In light air it was totally ineffective. This builder never used the centerboard as it really didn't contribute significantly to the windward ability of the Weekender.
If you were interested in cruising or following set course tracks then you will find that there is quite a bit of slipage. Tim Diebert had modified his Weekender's forefoot, keel and installed a centerboard and centerboard trunk. He was entirely satisfied with the end result in the improvements and ability to maneuver as he wanted. .
Here is a link to his site and a whole lot of photos.
These may help with your thinking.
http://www.timtone.com/tt/ttphotos/SailingSitemap.htm
Personally, I would go out and sail the begeebers out of it the way you built it and get used to what it will do and how to do it before making any dramatic changes to it. Once you get the rig tuned and get used to what it will do under varying conditions, you may still wish to make some changes. But the first one I would make would be a larger headsail to help balance the sail plan and handle better in light to moderate wind.
Thanks for the replies, and your concerns. First, I'm more concerned about the trimaran centerboard than modifying the Weekender. After sailing mostly in unfavorable conditions, I'm certain that reducing the forefoot and squaring the rear of the keel would dramatically help.
IF I were to do that modification, the centerboard would be a simple addition without hurting anything. More so, however, it would do a person well to have total (or as much as possible) single-hand control over the sail area, shape, and such. A working jib downhaul would be a tremendous help, also some sort of "quick" reef system for the main. Once those issues are put to bed, then I'll get back to the centerboard
.
(I have a copy of the original plans for the centerboard)
The main issue, which I didn't make entirely clear, is the trimaran I am working on. I believe I have found enough similar designs to make an accurate assessment of where to put the centerboard (in relation to the mast), and how wide and deep it should go. That was my real concern.
If you can answer some questions I can give you a very close location for your board and it's size, on your tri.
How much sail area and the percentage of jib to main?
The sail area relationship with the LWL of the main hull (CE is @ what % of the LWL)?
Aspect ratio of mainsail?
Type of rig (fractional [what %], masthead, etc.)
Net displacement of the main hull, the outer hulls?
Location of the CLP on the main hull?
Location of the CB (Center of Buoyancy) on the main hull?
Point of max LWL beam of the main hull (% aft of the bow on the LWL)?
General hull form used on the main hull?
Main hull rudder or twins?
If you're carrying ballast, the % of displacement?
This is enough information for me to locate your appendages.
Not askin' fer much, are ya.....
Gimme a day or two to figure those out, I think I can get most of it.
Thanks again
Paul,
Most of the info you want.
Beam 24 1/2", Waterline beam will be 23 1/2"
LWL 13' 8"
Hull shape is symetrical, C/B should be at CLP, 7' 1 1/4" from bow. I am assuming the LWL beam to be at C/B
Mast steps 3' 6" from bow, requiring small bow sprit.
Rig: Sliding gunter/batwing, main mast 68", topmast measures 48" from top of main mast.
Sail area - Main approx 36 sq ft, jib is small, about 8-9 sq ft. No idea on C/E at this time. Aspect ratios elude me. I can email an approximation of the rig, if you need it. I would expect the C/E to be about 5 1/2' to 6' back from the bow.
Main hull displacement should be approx 270 lbs, making draft about 4"
Outer hull displacement approx 100-110 lbs each, -2" draft at level
Rudder will be on main hull, size to be determined.
Ballast would only be in the form of lightweight camping gear, approx 50 lbs, if any.
This is an experiment with a hull I already have 80% built. The hull cannot be changed (easily :lol: ), but the rest of the design is pretty flexible. It may be on hold for a while longer now, due to an ever-extending recovery time from surgery.
(09-27-2006, 01:49 PM)Paul Riccelli PE,NA link Wrote: [ -> ]The placement of a centerboard or any appendage is one of the black arts of yacht design. Some math, some calculations a few geometric drawings and you can still get it way wrong (I've first hand experience at just this).
There is no cut and dry method for exact placement, just some rules of thumb, which can be altered by many factors, including; depth of forefoot, hull type, beam/length ratio, type of rig, hull efficiency, etc.
Sizing the board also falls into the black magic thingie, but there are some reasonably set rules you can follow. The rudder size is determined in a similar fashion.
Is it possible for me to see these drawings?
The nice thing about a centerboard on a Weekender (or Vacationer) is it would have to be a low aspect board. This is what we'd call a "slice 'o pie" style board. This is forced on us for a few reasons. First is the wish of no board case intruding into the cabin, which forces a small amount of "purchase" within the boat. This purchase means it doesn't have a lot of boat (in this point keel) to hold onto, so it doesn't get twisted out of the boat when under strain. The loads on a board can be substantial, so you want as much board "in" the boat as you can afford, to resist the twisting motion it will experience. A low aspect board just peeks out from under the keel, which looks like a slice of pie from the side. These can usually have at least half of the board remaining in the boat, which provides good purchase.
Now you can have a high aspect board, which is one that drops well down and would be typical of most modern craft with centerboards. The extreme majority of these types of boards have an internal case, that will intrude into the cabin space, but on Weekender's keel this can be enclosed all outside, with no intrusion. The keel as designed isn't strong enough to keep this type of board in the boat. It will twist it's way out, likely splitting the keel laminates in the process, or worse, breaking the keel to bottom joint open. An athwartship member or two will need be installed to transmit the new loads to the hull shell. Through bolts or the less effective lag bolts can be driven down into the keel, through these new structural members, which would be called floors. The floors would be bonded and screwed to the hull, like other elements of the boat, such as the cabin bulkheads. Epoxy fillets would be strongly recommended on the floor to hull connections.
The Weekender keel profile will not benefit a lot from a centerboard, without revisions. The deep forefoot, will fight the board for control in turns and general maneuvering. This deep forefoot provides the bulk of her lateral area, so she can sail without a board, but isn't particularly effective at preventing excessive leeway or generating lift for windward work. Most folks building Weekender are relatively new to sailing, maybe this being their first sailboat, so they don't know what they're missing in windward ability or leeward slippage. The very aspects that permit her to sail without a board, can cause balance issues if one is installed without keel modifications.
What to do? The forefoot has to go away, so the boat will pivot on the board in maneuvers, not the forefoot. The forefoot area removed is replaced with the board area for upwind work and retracted for down wind work, where it's not needed. Since the keel is getting a reshaping, you might as well take advantage of it and install some additional area aft, to completely enclose the board and provide more lateral plane area (which she sorely needs, like most small boats do). Removing a portion of the forefoot will not cause strength issues with the boat, nor affect the looks of the boat above the waterline. She'll still have her cutwater and clipper knee.
Brit, if you send me an email, I'll send you some drawings to show what's possible and how to get it done with a minimum of fuss.