side by side centerboards?

Discussion in 'Boat Design' started by Ian, Apr 8, 2004.

  1. Ian
    Joined: Apr 2004
    Posts: 22
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Camden, Maine, USA

    Ian Junior Member

    I am working on a 16 foot cruising dinghy. Twin side by side center-boards fascinate me. Tandem (fore and aft ) boards and keels get a lot of press but not the alongside arrangement. It's extra work to build, but it would allow me to get the case out of the way. I could put the cases on either side of the well and built into the seat fronts. I would be planning on using both boards at once rather than alternately, as in lee-boards. It seems that I could get shallower draft for a given aspect ratio. Also, I suspect the heeling moment would be reduced as the center the combined lift would not be so far below the heeling axis. Is this true? It seems obvious, but what is obvious is not always what is true.
    Side by side boards would not interfere with each others hydrodynamics like fore and aft boards. I can see that ventilation at the root of the windward keel might be a problem in a keel boat (I suspect that bilge keel boats use a little extra area and the more vertical position of the lee-ward keel, due the splay, to make up for this) but I do not believe that a dinghy, which would be sailed more upright, would encounter this problem. I would not consider splaying the boards for practical reasons.
    I do, however, worry about not having a nice long single center-board to use as a righting lever should I turn turtle. My other plan is to have a single offset board.
    I was planning on having a considerable amount of water-tight storage fore and aft and in the side benches. I have since decided to make the starboard bench mostly free-flooding. It seemed to me that I might be designing a boat that was too stable up-side-down. The starboard bench would have wet storage. I feel that, if I can use my weight to easily submerge one side of the inverted boat, it would be possible to roll her back up. I noticed that the Drascombe boats have very slim, low bouancy side benches, perhaps for this reason.
    I would like to right this boat very quickly in adverse conditions as I intend fairly extensive cruises in very cold water. She will not capsize easily--probably never (except for testing purposes) but it would be unseamanlike to be unprepared.
    I would appreciate any comments whatsoever. I am not very experienced with sailing dinghys though I have had a fair amount of sea time in on very large yachts. It is not at all the same thing. I have had no experience righting dinghys any larger and heavier than a laser. Go ahead and pile on the suggestions! Thanks.
     
  2. tspeer
    Joined: Feb 2002
    Posts: 2,319
    Likes: 303, Points: 83, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    What you call side-by-side boards are known as bilge boards. They've been used for over a century on the Inland Lakes Scows. The two bilge board trunks are located at the sides of the cockpit, as you describe. They are also angled in at 15 degrees so the board is vertical when the boat is heeled. And they are toed in a few degrees to act like jibing centerboards - effectively rotating the hull to reduce leeway and altering the angle of the foretriangle to the wind.

    The boats are sailed with the windward board retracted and the leeward board down. On a scow, you don't say, "Ready about," you say, "Board down." The crew lowers the windward board, the boat tacks with both boards extended, and the crew raises the windward board as they come up onto the rail. Here's a photo of a scow in which you can clearly see the slot for the windward board: http://www.inland20.org/images/DSC_7011.JPG (The small dark rectangle next to it is a bailer.)

    Before the days of hiking straps, the windward board was left part way down, and the crew would climb out on it to balance the boat: http://www.quested.com/A/boats/J7-old-1.jpg.

    Bilge boards do result in a wonderfully unobstructed cockpit (http://www.m20scow.com/ScowBuilders/M20Specs.pdf). The deck supports the upper end of the trunk and the trunk acts as a shear web for the hull. The result is two "D" shaped tubes running fore and aft, making for a stiff, strong hull.

    I also recommend the twin rudders and tillers. They put the tiller right beside you within easy reach at all times, and only a short hiking stick is needed. When tacking, jibing, or raising a spinnaker, you can crouch with your knee in the crook of the connecting link to steer while you use both hands to handle the lines.

    Aspect ratio does not really mean "skinny-ness". It's really a nondimensional measure of span (depth). Aspect ratio is the depth-squared divided by the total board area, not the area of one board. So for good windward performance, you don't want to skimp on the length of the boards.

    The ability to put both boards down part way does provide a way to get adequate lateral area when sailing in shallow water, at the expense of additional drag and poorer windward performance. But the performance with two boards almost touching the bottom is far better than one board stick in the mud!

    When you capsize a scow with bilge boards, the side decks are wide enough that the cockpit floats out of the water. The leeward board (extended) is just about at water level. You can climb up onto the board, then lean over the gunwale to uncleat the upper board and fish it out of the trunk. The scow boards are thin aluminum plate, and I was always leery of bending the 1/8" thick board on my M-16 by standing on the tip. So I would stand on the base of the bottom board, hold onto the tip of the upper board, and lean out. That was always effective in bringing the boat right up.
    On the bigger scows, you'd get well out onto the bottom board to get more leverage.

    There are various philosophies on the proper distribution of internal buoyancy in dinghies. The American solution tends to use side tanks that are comfortable for hiking and keep the cockpit out of the water when capsized so the boat comes up dry (eg, Vanguard 470, FJ, etc.) But the mast is angled down somewhat when capsized, and since the hull is "over the top", there may be a greater tendency to turtle.

    The British philosophy decries side tanks as allowing the hull to ride too high, have too much windage, and possibly drift away from the crew. Consequently, British dinghies are typically designed to swamp when capsized so the centerboard is near the water surface. A double bottom that is normally above the waterline will have half the buoyancy in the capsized condition, allowing the boat to float low, but will rapidly drain the water through transom flaps when the boat is righted.

    Personally, I've never had a problem with a capsized boat that rides high, and it always seemed to take forever to empty out my Merlin Rocket after a capsize. But the double-bottom may be the way to go for your boat, since it will drain any water that gets in the boat.

    You can prevent turtling by putting a flotation panel into the head of the sail: http://www.mcscow.org/rigging/float.htm. It's easy to provide a pocket that can hold a piece of closed-cell foam, and it doesn't take a lot of volume due to the leverage of the mast. The foam can be left out for light air performance, and put in when conditions dictate. It's not much weight aloft, and the performance penalty is minimal. Especially compared to those dorky-looking mast-head floats some boats have.

    When I taught sailing, the first thing we would do is take a Laser into the pool and have everyone get proficient at righting it. Then, the students had already encountered the worst that would happen before they ever got to the lake. They learned that it's no big deal to capsize a small boat - it's just a lot slower sailing that way! It was a huge confidence builder.

    I recommend you rent a Laser or other small dinghy to get experience sailing a small boat single-handed, where you have to coordinate your weight, the sheet, and the tiller. Capsize the boat in warm water close to shore and practice bringing it up. Dinghy-racing skippers are always looking for crew, so join your local sailing club and offer your services. You'll learn a lot that way. Sail in lots of different types of boats to find out what you like and don't like about them. After a while, I think your cruising dinghy will design itself, because you'll have learned what your real requirements are.
     
  3. Ian
    Joined: Apr 2004
    Posts: 22
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Camden, Maine, USA

    Ian Junior Member

    reply to Tom Speer

    Thanks a lot for your reply. The comparison of the contrasting American and British aproaches to dinghy floatation is invaluable. You stated that scows incline their boards fifteen degrees, so I assume that that must be the most usual dinghy sailing angle--something that confirms my best quess. I was planning on having the foot-well two feet wide, or a little under, so the windward board (if I use both boards at once) will be in little danger of ventilating at fifteen degrees of heel.
    I have been using the method of determining AR that I was taught--the same one that is in Principles Of Yacht Design i.e. Divide the depth of the foil by the product of the the cord at the root and the cord at the tip divided by two (mean cord). This seems to give me the same answer as your method. Are there situations in which the two methods would differ? I don't quite get why one would apply the area of both boards to the depth of one board, even when using both together--surely they are treated separately?
    I did notice, when I calculated the size of two boards that would provide the same area as a single board, that due to scaling effects, halving the depth and mean cord did not provide equal area--so using twin boards only reduces the total draft by about a third (which in my case works out to about a foot). A foot is pretty significant however when gunkholing. It has occurred to me that having two tip-vortexes rather than one may increase the total drag, even if both vortexes are smaller. Would this be significant? I have pretty much decided to use the method of determining lateral area used in Principles Of Yacht Design. This does produce a lateral area estimate that is lower than most so I intend to use a fairly sophisticated foil shape. I wonder if having two tip vortexes should be compensated for with a little extra area? I would rather not have two full size boards as this yawl rig (with loose-footed main) will give me quite enough to do when tacking as it is, without having to tack the boards. I may also have to deal with running backstays (at least in stronger winds) if it turns out that my jib is a little large for a free-standing mast
    I will be using a hollow spruce spar with no openings at the top, so it will not flood. The mast will however not be very tall as the rig is a yawl with a standing lug main. I quess I will have to just do a capsizing test when she is built and then look into a floatation panel in the sail (a excellent idea). I will start building this summer. Part of my concern about making the boat easy to right is that I will frequently be single-handing, also I would like her to be easily rightable by two light-weight grandchildren.
     
    Last edited: Apr 21, 2004
  4. brian eiland
    Joined: Jun 2002
    Posts: 5,067
    Likes: 216, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Side By Side CB's

    Sorry I didn't have time to read these threads yet but you might have a look at another thread I wrote on a similar subject, "asymmetrical CB's, nacelle mounted".

    <http://www.boatdesign.net/forums/showthread.php?t=2225>
    Big Cat, Alt CB's....
     
  5. SeaDrive
    Joined: Feb 2004
    Posts: 223
    Likes: 1, Points: 0, Legacy Rep: 15
    Location: Connecticut

    SeaDrive Senior Member

    When you have two boards and the boat is heeled, the asymmetry pretty much guarrantees that one board will be operating at a higher angle of attack than the other. To make sure this angle is not too high (and generating too much drag), the combined area is going to be greater than would be enough for a single board. To minimize this problem in the sort of boat you describe, keep the boards parallel, and use a profile that's not too sensitive.
     
  6. Ian
    Joined: Apr 2004
    Posts: 22
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Camden, Maine, USA

    Ian Junior Member

    side by side cb's

    Thanks, Sea Drive--good point. That had not ocurred to me. Now that I think of it, the difference in angle of attack must be even greater between tandem boards that are separated fore and aft--up by the root of the board, anyway. I am going to have to think about that!
     
  7. SeaDrive
    Joined: Feb 2004
    Posts: 223
    Likes: 1, Points: 0, Legacy Rep: 15
    Location: Connecticut

    SeaDrive Senior Member

    In a racing design, this may be an important point, but don't overthink the problem for a cruiser/daysailor.

    The general point was made in an article many years ago about keel/centerboard yachts. The author was a designer, but I don't remember who it was. He pointed out that most of the time, either stub keel or the centerboard would be operating at an inefficient angle of attack, so the design could could not be as a straight keel design. I've notice that over the years, centerboards in k/cb designs have gotten bigger, and I assume that's because the cb is more efficient than the stub keel, so it has to take the greater share of the load.
     
  8. gggGUest

    gggGUest Guest

    Heeling angles.
    Most dinghies are best sailed bolt upright, or at least as upright as practical. the US Scows are an exception, very wide and flat there's too much hull in the water if you do.

    Turtling
    If you will sail the boat away from safety cover a boat that inverts is a definite safety feature. It means if you lose contact with the boat in a capsize it will be iun swimming distance. A boat floating on it side will be blown downwind faster than a sailor can swim. This has happened to me, and the sea is a very large place if you're just two heads half a mile upwind from your boat. After that I very quickly took the foam out of my mast that stopped the boat inverting. You stay alive a lot longer and are a lot easier to spot sitting on a capsized boat than you do in the water.

    Capsize recovery is best eased by things like righting lines and perhaps for a cruising boat having things like rubbing strips on the bottom of the hull to give toe holds. Oh, also have a wide enough gunwhale that you can stand on it to start getting the boat back to horizontal. I'd also suggest that you make sure that there is a part of the boat that is easy to climb over when the boat is upright, an open transom or one with transom flaps so that you can swamp the boat to climb in easily is a big help. Bilge boards would probably be great to help pull a boat back up from inverted and also in making it less far to climb over the gunwhale as it comes up.
     

  9. brian eiland
    Joined: Jun 2002
    Posts: 5,067
    Likes: 216, Points: 73, Legacy Rep: 1903
    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Side-by-Side CB's

    Now on either side of this flat plate I propose to mount a centerboard, not a single, symmetrical one, but rather two asymmetrical ones; sort of like a single board split in half. The flat sides of these asymmetric boards would fit up against the flat plate nacelle, and rotate on oversize diameter bearings. The flat fit & big bearings would together supply a great big surface for the large bending moments to bear against. Only one board at a time would be lowered.

    I would further suggest that surplus helicopter blades are prime candidate sources for both CB blades ....high tech, extremely strong carbon fiber fabrications that have a prescribed limited life span aboard aircraft, but are perfectly happy for our use
     
Loading...
Similar Threads
  1. Squidly-Diddly
    Replies:
    9
    Views:
    261
  2. Boatyboatboat
    Replies:
    13
    Views:
    1,328
  3. serdarbas
    Replies:
    6
    Views:
    905
  4. GersonPerezbr
    Replies:
    10
    Views:
    1,525
  5. Coracular
    Replies:
    14
    Views:
    1,624
  6. Titu
    Replies:
    4
    Views:
    1,005
  7. Squidly-Diddly
    Replies:
    13
    Views:
    1,071
  8. rony2014
    Replies:
    6
    Views:
    1,042
  9. schakel
    Replies:
    6
    Views:
    1,290
  10. Squidly-Diddly
    Replies:
    6
    Views:
    1,009
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.