 |
| OCB in forward, sailing position (Photo from Pete Frost) |
 |
| OCB in aft, stowed position Windows clear! (Photo from John Herschenrider) |
Truly, she doth block a staircase!
-- From
by Wallace Tripp
Travelling OffCenterBoards (aka LeeBoards)
Let's start with a leeboard.
As we sail into the wind with our leeboard down (on the lee side), the leeway we make presses it against the hull. It likes to slip forward but not sideways (LR for Lateral Resistance), so we don't make as much leeway as we would without it.
Ready about!
Helm down, we turn into the wind, sails flogging as the bow and boom cross the wind. We drop the leeboard on the new leeward side and raise the first one clear of the water. Sails fill, we heel over and off we go on the new tack.
If it sounds like allemande left and dosido, it is. A little choreography on what may be a heaving, wet deck and your partner is a big, heavy leeboard.
Lazy sailors that we be, our interest was aroused by Ida Little as she clamped a 2x2 outboard of her Bolger DUGONG's leeboards to form a slot, instantly converting them to OCBs (Off Center Boards).
No need to tend 'em between tacks! Just put 'em down and leave 'em. If you want to reduce drag sailing off the wind (no need for lateral resistance), you can raise 'em if you want to.
Years went by, and we transitioned to big view side windows. Problem is, with the boards raised (always the case at anchor), they block our view.
We wanted to haul and store them out of the way. We wanted a simple installation. We wanted it quickly and easily handled by one person.
So we scratched our heads and came up with travelling OCBs.
*****
Walk-through of set and stow
Please watch first for overview
The video above walks through from stowed aft to set forward and back. Notice that the stowed position is in the 'blind spot' below the pilot house.
Tour of system components:
 |
| Upper cable, eve guard and stop |
This is one of our early prototype stops for the board. It's two purposes are to act as a stop at one of the board's two positions, and to shorten the span of the cable.
The traveling block (description to follow) travels along the upper cable until it hits the forward stop (set) or aft stop (stow).
Both functions will eventually be met by a shorter upper cable (shorter span) and crimp on swage sleeves (stops).
In the meantime, we currently tie a double sided rolling hitch around the cable, with the ends led up to a handrail post (i.e., the vertical part, and not the grip), rather than the arrangement shown.
That aft tail is just 'cuz I hate cutting line.
 |
| Aft lower guard, mounting plate, lower cable and slot |
 |
| Forward lower guard, sheathed turnbuckle, mounting plate and slot Note the guard struts |
The lower guard and lower cable - offset by mounting plates - form the slot.
The lower guard doubles as bearing for the OCBs and mounting steps fore and aft. Handy for boarding from the ground or a dock.
A rule of thumb for leeboards is to toe them in 2 degrees. This helps the vessel 'climb' to windward. We haven't done this on these lower guards, but plan to at the first opportunity.
On the lee side, the OCB acts like an ordinary leeboard. It bears up against the lower guard and pulls at the upper cable.
On the windward side, the lower cable prevents the board from winging out, and the upper board bears up on the eave guard.
We adjust cable tension to let them angle inboard, also at 2 degrees. Our thinking is that, when heeled, this produces a downward force at their Center of Effort, equivalent to someone hiking out, acting against heel. I mean, since we're leaving it down, why not?
Incidentally, since both boards remain down, we use only a little more than half the area one might in a leeboard (where one board handles all LR). This is cheaper, faster to build, lighter, and induces less stress.
There are big stresses on OCBs. I would like bigger and thicker mounting plates down the road, though these (1/4in aluminum) seem to be doing fine. I'm guessing the cables help to absorb and disperse some of the stress.
 |
| Bottom of OCB from aft |
Here we see the retrieval line, its block and cleat. It is used to raise, lower and angle the board with 2 to 1 purchase (we're trying it out... may go to 3:1).
For stowing, the board is moved to the aft stop and raised clear of the water, but with its lower edge still in the slot. This prevents waves from clunking the board when stowed.
For sailing into the wind, the board is moved to the forward stop and tied, then the retrieval line is hauled in a bit to angle it back a skosh. This encourages it to kick back if we bounce it on the bottom in a swell, relieving stress all round.
For sailing off the wind, we haul the board to as near horizontal as possible. This reduces drag from the now useless boards (lateral resistance is not required off the wind).
Due to the relatively poor lead for the retrieval line, while deployed, we can't quite clear the water with the OCBs' lower edges. We're considering a snatch hook under the eave directly above the load... this would improve the lead, but requires awkward handling with low payoff. Still mulling that one over.
Note the upper mounting plates which were cut from aluminum angle.
 |
| Top of OCB from Inboard |
These two pics show the hanging / travelling arrangement for the OCBs.
As seen above, an inverted block runs its sheave above and along the upper cable. A lanyard is fixed to the block, and passed through the board to the outboard face. The board pivots from the block when raising and lowering.
[NOTE: We've found Stanley™ Pulleys to be perfect for the job... cheap, strong enough, and the sheave pin is R-sprung for easy dis/mounting.]
Below, we see the lanyard emerge and tie off to a cleat. We pull the block as snug as possible, inboard to set the board as high as possible. This gives good landing on the eave guard. The eave has the dual function of bearing against the upper board, and keeping deck runoff from the windows.
The tail of the lanyard - stopper knotted at its end - is used to haul the boards fore and aft. In the forward position, we tie off to the handrail with a clove hitch to keep the board from pulling aft when raised in place for off-wind sailing.
 |
| Top of OCB from Outboard |
A little miscellany...
There are two main criticisms of OCB/leeboards; clunking and picking up 'scultch' (seaweed Klingons).
We've only had board clunking issues in light winds and riptide conditions. Otherwise, the boards either immediately bear up and go silent or 'shuffle' along in no wind. In LUNA, we added firehose along the lower guard, which eliminated clunking, but it's seems only vaguely worthwhile.
We do pick up weed now an then, but it's easily dislodged by raising the board briefly.
As a frequent criticism of OCB/leeboards, I don't get it. Any LR device picks weed, and in most it's a LOT harder to ditch. I see plenty of fixed keelers dragging a garland.
OCBs are easy to construct and maintain, requiring no hull apertures or complex structures. Ventilation is most excellent!
Maintenance is a relative snap. To dismount, we uncleat the lanyard, which can then lower the board with 2:1 purchase. We temporarily hang the board by its cleat's lower horn hooked over the lower cable. We can leave it like that until ready to remount or take it ashore.
We shoot for a ballast load that floats them vertically, about a foot or so proud of the water when dismounted. We can easily tow them ashore on a 'leash' for whatever. Propped up flat, they make a great work bench, ashore.
We don't always sail with both boards down. Each significantly reduces leeway, but there are days we just don't need to be sailing at our best to windward (e.g., a short leg after a long run). In case we change our mind, the second is easy enough to set up, underway. If not, that much less to put away at the end of a pleasant sail.
Retractable LR is a given for ultra-shoal draft. It opens up a hundred sheltered spots for every reasonably deep anchorage. When the wind come on, it can save hours to the next hidey-hole.
*****
We've been sailing with OCBs, now, for 20 years, and travelling OCBs for about half of that. We sail year round in SE Alaska most years, across a considerable range of wind, weather and water.
I'd say it's working!
[NOTE: In SLACKTIDE we had windows along the entire cabin. To stow, we had to lift the boards clear of the slot and slide them dead aft. This complicated retrieval to brute force. Lighter boards are recommended.
There's a primitive ancestor of this post on SLACKTIDE's system, here.]
ReplyDeleteI LIKE OCBs, a lot, even with the little experience I've had with them.
If I have a concern, here, its moving parts...
From the demo and video, the application looks fairly simple and straight forward, and you do have the redundancy built in with two. Still, moving parts... Your experience suggests a history of long and effective robustness, with little or no failure, yet, moving parts...
When you sailed before without the traveling OCBs, how did you attach them, and how happy were you with them? (I take it you adjusted F/A balance by trimming main and mizzen?)
Thank you for the demo and clear explanation.
Hi Peter,
DeleteOur NON-travelling boards on LUNA pivoted via a rope grommet that would twist tighter on lowering. Bolger calls it "Flemish Geometry". I loved them for their simplicity and economy.
The only moving part in the travelling board is the block. So far it seems totally robust, and is easily dismounted and inspected.
Its weak point would be the sheave pin, but that's as big as any of the shackle pins in the works. My guess is that the real weak point in the system (as in LUNAs) would be line chafe. But that's also easily inspected and so far hasn't given any sign of trouble.
I have heard that stainless steel cable can develop micro-fissures over time that ARE hard to detect. We'll want to change it out on some as-yet-to-be-determined schedule... luckily, it's pretty cheap. For now we carry backups.
Dave Z
I've long been idly curious, so finally I'll ask: What's the distance between the waterline and the bottom of the window on this craft? From the photo, it looks to be less than 4'. Am I looking right?
ReplyDeleteHi Peter,
DeleteYes... less than 4ft.
They're a split 2x4 above the top of the inside furniture which is 3ft off the bottom. Draft is 12 to 16in, so window height-above-WL is about 26 to 22in.
Another way to look at it... the sides are 5ft tall (1 1/4 sheets) and the windows are shy of 2ft.
Square, shoal boats sit a lot higher than most shapely, high ballast boats... we rarely dip the OCB guards, and have never gotten any more than a little splash on the windows.
Dave Z
I’ve been very happy reading about and learning from your Triloboat Talk pages. Yet, with all I am learning, I seem to have more and more questions.
ReplyDeleteThe simplest questions are sometimes the easiest to overlook. For example, it’s surprisingly hard to find direct information on how to size off-center boards.
So I’m asking you: for the Wayward—about 30’ LOA and with and 8’ beam is it?—what are the dimensions of your off-center boards?
Also, I see in the photos what looks like a screw-on access port in your boards? Because they are hollow, is that there for maintenance? (Also, again: because the boards are hollow, how did you frame them internally?
One more question, too: how did you locate the forward sailing position of your boards? Was it resorting to complex math, or was it more rule-of-thumb based upon experience and basic principles?
Hi Sail4th,
DeleteGlad you're getting something from the blog. Please keep in mind that I'm just a fellow bumbler out here attempting to reinvent the wheel!
For sizing, what I do is try to find similar boats and pretty much do it by eye onto graph paper (for scale). Then round to materials as seems sensible.
WWs boards are built within a 4ft x 8ft rectangle for ply construction without joins. Individually, they are a little scant, but since we can use them together, I figgered together they're on the generous side.
Another Bolger comment, if they are flat against the hull, their area counts from the hull down; if wung out a bit, it's from the WL down. We wing 'em out so gain extra effective area.
In practice, we do much of our sailing with one board. If things get wild or time is urgent, we'll deploy both.
RE the Inspection Plate. You're right, that's to get back into the board. It sits on a 1 1/2in shelf all round, and has an internal 2x4 running down the middle to divide up the large open space.
Figuring out how much lead is an arcane science. We spend a day with a blackboard and sacrifice fitbits. Or you could build 'em, then clamp on weights until satisfied and convert that to lead.
The fwd position was chosen for LUNA from Bolger's AS29, but that felt a little far aft for best performance. Since, we've put them a little further fwd. One can always swing 'em a bit aft, but now that they're traveling, we can put 'em anywhere. Still, they seem to like it way fwd.
In general, fwd increases weather helm; aft increases lee helm. Moderate weather helm helps you on the wind and brings the boat up to stall if no one's at the helm.
Sound fudgy? For us it is. So much of nautical lore seems to be weighing up a hundred approaches b'guess and b'gawd using what common sense we got. Seems like most work reasonably well, and it's all upward from there.
I find that comforting!
Dave Z
Looking at the photos, the Wayward's OCB appear to be set too much 'way forward' for the CLR to be aft of the boat's CG. Obviously, because the boat sails and handles so well, this isn't so. Can you explain?
ReplyDeleteHi Sail4th,
ReplyDeleteI'm thinking you mean "...aft of the boat's CE"?
I agree that it does seem pretty far forward, violating a number of rules-of-thumb for placing the 'lead'. However, I note that Thames and other barges are similar...
Hypothesis: A barge (especially with very hard chines) spreads its hull's LR along a good chunk of mid-body, but remains mostly centered. Barges typically have very large rudders, and often aft skegs, which somewhat pull the CLR aft. When adding boards, then they go unusually far forward to balance?
Dunno, really. But you're right... not only does it seem to work that far forward, but putting the further aft develops lee helm in a hurry, and that's not a good thing.
Dave Z
Can you say something about how you build the boards themselves? I'm interested in keeping weight down, so the flooding notion is very attractive. I've thought that with a good stress-skin design, 1/4" (6mm-8mm) plywood might be sufficient for the walls, with care toward epoxy encapsulation of the interior (flooded) faces in the course of construction (likely glued with epoxy as well).
ReplyDeleteThe boards you have seem to be quite specialized, and even in a design that doesn't call for moving boards half the hull, having some movement for adjustment seems a good idea, as does arranging the boards like you do in Wayward, so that they can "kick up" in shallows.
SO, I'm really interested in knowing more about how you actually build the boards.
S'il vous plaît.
What forces are the guard struts resisting?
ReplyDeleteWhoops... sorry, I missed this one and am late in replying.
DeleteRE Struts... One force is middle-age spread. 8) Seriously, boarding with my weight plus a heavy pack (firewood? groceries?) puts a lot of strain at the edge of the board.
From below, there's a LOT of force when a green water wave slaps up along its whole length.
In either case, the lag screws from inside the hull into the guards can get seriously worked. I don't really know if the struts are necessary, but they're cheap, easy and I feel better with them!
RE Board Construction... This should really be a post at abargeinthemaking.blogspot.com... maybe I can get to it this winter. For now, the 'brief' version:
We use two layers of 1/2in ply separated by 2x lumber for edges and middle, and 2x3/4in ply top and bottom for a total of ~2.5in total thickness. If at all possible, we try to stay within the 4ft x 8ft limitation of a single sheet of ply (as viewed in profile).
An inspection plate is fitted inboard to access the 'hollows' between lumber planks and ends. Holes in the upper forward and lower aft of the plate let air and water in and out respectively.
In the bottom end, a hollow is cut to take enough lead to sink that end when deployed, but not so much as to sink the board when detached and afloat. A second 'inspection' plate is screwed over this.
Edges are shaped from just above the guard level downward to approximate a crude wing cross-section (mid-body is flat, so not super efficient).
The upper end is to a circular edge with a midpoint at a radius large enough to keep the upper edge above the eave of the deck (which serves as the upper land for each board while to windward).
As you might imagine, the math is tricky. It takes us a day or two with a chalk-board and many takes to balance the volume, density (these drawn from the Pocket Reference booklet), leverage and materials in a best guesstimate. The only further guidance I can offer you is to remember to factor in water volume/density for the immersed part of the board vs the full weight of the upper board.
In theory this is a tractable problem; in practice, we've gotten into the ballpark each of the two times we've attempted it.
We haven't tried this in our new age of AI abetted everything... I'd be very curious to work with one on the problem. If you try, do be intelligently skeptical and check its work; they are at present extremely helpful but not 100% reliable. I've found it best to work both of us along, step by step, rather than tackling the whole mess in one go.
Good luck!
Dave Z