Daggerboards, Centerboards and Keels for multihulls
QUES: I am working on the design for my own boat, but am undecided about the board or keel and then, what factors should I consider in their design re size and proportions etc? Kito-T, Brazil
ANSWER: There is a lot involved here and several reputable, knowledgeable and experienced designers have posted their thoughts on this and are totally worth considering. But as I’d like to touch on some aspects not often addressed, I will wade in and let the reader judge their merit and utility for their own application.
All of the above (DB, CB & keel) are ways to build in enough lateral resistance so that the boat does not drift sideways and can therefore sail efficiently upwind. So in my view, the first thing to decide is, how effective (or not) will the basic boat hulls be in achieving at least a percentage of that? Only then can we know just how large the added foil needs to be. Some suggestions seem to ignore the effect of hull shape yet I personally consider this very significant. For example, a semicircular hull, while ideal for low wetted surface and efficiency at low speed, will show little resistance to drifting sideways, so needs a fairly large foil to do most of the work. By contrast, a chined hull can be very effective at preventing water from slipping sideways under its chine and this will add to directional stability and possibly need only ½ the foil area that a rounded hull will. So while a foil area below the keel of 3 or 4% of the sail area will be required in the more challenging cases (such as for a symmetrical rounded catamaran), only 2% might give an equal result with a chine boat. (Of course, the designer might still elect to go to 3% and use the extra area to perform even better). In the case of trimarans one can with some advantage, design the amas slightly asymmetric and if this is done with experience and backed by tests, the reduction in side slip can be so effective that a foil of only 1% of the sail area is all that may be required!
My own W17 tri design works fine with that … just 2 sqft for 200 sqft of sail! Yes, my board can give the skipper 2.5 sqft when fully down but it’s only needed in really light airs. “She goes upwind like a witch” says a buddy of mine who has sailed often on my boat.
But what about a daggerboard vs centerboard ? Before we even go there, let's discuss the effect of materials as since carbon fiber has become readily available we now see foils becoming longer and slimmer every year. But is this a good and really necessary trend? Personally, with some exceptions, I am not always in favor and I will give my views on why. Sailing with these long and low chord foils (high AR or 'aspect ratio') requires a LOT of attention (other than running aground!), especially when a boat is pushed around in waves; otherwise the short chord quickly stalls and the boat feels jittery and loses its ‘on a rail’ feeling. Yes, we know that a long (vertical) leading edge gives proportionally more lift ... but unless the helmsman is able to constantly sail that ideal line to avoid stalling, the gains can be quickly nullified. So there’s my personal exception …. I go with long foils for outright race boats, sailed with near 100% concentration to keep things working well all the time, but by FAR the majority of boats will rarely be sailed at that level. Even then, I’d not personally design even a HA ratio board with a chord of less than 5% of the waterline length.
Also, as Richard Woods reminds us, these narrow foils are so sensitive that they must be maintained in super top shape without a blemish, or the treasured lift/drag falls away rapidly. Most of us do not want to deal with this level of maintenance. Sure, we’re ready to give our boards an annual coat of paint, but then, to still work effectively we’ll need to bring our aspect ratio down to 2.5 or less .. even down to 1.5 ! (Most LAR keels have an aspect ratio of less than 0.25, but anything less than 1.0 (square) has significantly compromised efficiency).
This proportionally gives a far longer chord and as that holds the directional flow much longer, makes the board less sensitive to stalling … so that the boat can be moved a few degrees off or towards the wind (by waves or loss of attention) without losing the lateral resistance for which the board is there in the first place!
[Just saw this Sept 2018: See what originator Tim Weston did with his ~40ft catamaran in Australia. After being disappointed with his upwind performance, he installed just one ‘mega-board’ with a chord of ~10 % of the waterline, and his performance vastly improved. ( Just don’t hit any rocks with it!) See his very enjoyable YouTube.... https://www.youtube.com/watch?v=7XN_onR9qAc&feature=youtu.be
So let’s say we decide on a board with an AR of just 2, should this be a centerboard or daggerboard?
I’ve already repeated before on my website, about the frightening collision I once had with my 25ft Dragonfly … hitting a submerged, uncharted rock and stopping in barely 50mm, to be thrown off my feet , landing on a winch, breaking ribs and suffering some temporary paralysis. The real scary thing is that I was only going 6 knots ! Since then, I’ve also reported on a 60ft Vendee-Globe monohull that hit a low-floating container while travelling at 17kts and not only destroyed the boat, but would have almost certainly killed the crew if they had not luckily been up against a padded bulkhead at that moment.
So for me personally … the regular vertical daggerboard in a snug case is just not something I want to be always worried about. A light, day racer would not be likely to cause seriously injure or kill the crew, but a strong board in a substantial cruising boat certainly could if the board has no where to go. I see three solutions to this, (excluding the 1960’s crash box that never really worked well and a pain to replace). One is to have the board built to break off easily. But who wants to be cruising with one of these, always wondering if you’re going to lose it …. especially if it’s on a rounded hull boat that totally relies on it to get upwind! (For those with long thin foils, this will likely be the default situation).
A 2nd solution is to design and build it with a shear-off-tip … as most contacts will initially be at the bottom of the foil. This is something I adopted on my Dragonfly after my collision injury. I cut the board off at about 30 degrees, scooped out the core for about 15mm, and then epoxied in a diaphragm into both parts. Before closing I bonded in a nylon chord of 60% boat length with the ends strongly attached to each part for 'tip-recovery'. I then bonded on the tip with a ‘not-too strong’ filler … estimating it would take about 300lbs to shear it free …. something a human on board could more reasonably cope with. I sailed with peace after that.
A 3rd solution is more for a small open boat say under 20ft ... and that is to open up the case a bit so that the board can move back about 30 degrees. This will raise the bottom of the board about 300 mm, absorbing a good deal of the shock, giving a much greater distance to stop in. This is the solution I developed for the W17 and it works well. After the first touch, I generally lift up the board or remove it completely. I’ve only needed to do repairs on this board once in 5 years when I hit an anchor …. and it’s easily removable anyway.
One very interesting and related factor that’s introduced by the Tim Weston solution noted above of one large board instead of two, is that any collision will now be significantly off-center ! This means that the CG or center of mass will still be able to move forward and lower the overall shock, and in fact, shock in the hull without the board would be a small fraction of what the other will experience. So you know which berth I will want ;-). In practice, it's unlikely any catamaran would collide with both boards at the same time, so there's one safety collision argument for the catamaran. So if there's a choice, always occupy the hull without a keel down when risking collision.
Of course, the ultimate solution to such collision is to incorporate a pivoting centerboard. But again there are issues. When the board is down, the long slot is typically all open, so needs a seal to stop the water surging and dragging on the boats speed. Such seals are easily damaged if not well designed. Also, poorly designed pivots can leak, and small rocks can become jammed in the case, preventing the board from being lowered or raised.
To reduce the risks of rocks jamming, it can be advantageous to locate the case bottom somewhere off-center, just above the deepest part of the section where it is located, so that if the boat sits on the seabed, small rocks are not being directly forced into the slot.
On a light boat that can readily be lifted out on a trailer, dealing with seals is not such a big job. (I have found seals made with 1 layer of fiberglass to have just the right degree of flex and stiffness to always stay down, but some experimentation is commonly needed to find the ideal seal width). But another design of centerboard can avoid the seal issue entirely, as one is then not required! This is with the ‘quadrant board’ that always fills the slot. Naturally, this will have less draft and aspect ratio than a regular centerboard, but is often quite adequate. And if fitted on a chine-design boat that does not need a super-effective foil for lateral resistance, the overall result can be excellent.
Sections through a daggerboard or foil should be fair and consistent and as an aerofoil, have the maximum thickness about 30% of the way back, with a nose radius of approximately 1/4 the max. board width.
In terms of material, I am personally not in favor of plywood as they ultimately do breakdown and also, nearly ½ the wood has grain going across the board, so weakening the end result even when new. Cedar or mahogany make a better core, with filled-epoxy serving to complete and fair the section before sealing with fiberglass. Prior to this exterior glass, adding UNI carbon fiber will greatly add to its stiffness, but plan ahead for this or your board will end up too thick to operate easily inside the case.
The final option is a fixed keel. Although never as efficient as a daggerboard or centerboard, fixed keels do avoid the problems noted above so are typically fitted to cruising boats that accept to take longer to get somewhere. Such keels can be built strong so that the boat can sit on them during a low tide, but then, they should not be too short or the boat can nose-dive into an ‘uncomfortable posture’. Some designs have fitted a centerboard inside the shallow keel, but that just adds the above issues back in.
Readers may be interested in a study undertaken by the Wolfson Unit for Marine Technology and Industrial Aerodynamics at the University of Southampton, UK. for the Balance Design team in South Africa. Now commendably available on line, it compares the performance of a fixed keel with a daggerboard, each mounted under the same 52ft hull, also developed by the Balance Team for their Model 526.
http://balancecatamarans.com/project/daggerboards-versus-keels/
Keep in mind that these results only apply to this specific hull or one very close to it in cross-section and length-width-depth proportions. The keel design chosen for this was considered more hydrodynamically efficient than the average production boat keel … giving a slimmer and deeper keel than is typical, in a commendable effort to minimize the difference between the two types. These keels also added about 6% to the total buoyancy, lifting the boat a little higher. So, as the report notes, the difference between these two options could be measurably greater for the average (non-Balance) production boat. Also note that the daggerboard was assumed to be down all the time, when in fact, it would likely be raised quite often with more positive results. All things to factor-in when interpreting the given results.
But overall, for a cruising boat – even a fast one, I personally favor the quadrant board as the best compromise, and unless something better comes along, you will find this option on my own designs over 20ft. as well as on some of the larger cruising Dragonfly trimarans. However, it’s not likely that out-and-out racers will give up on their high aspect ratio daggerboards …. but expect to devote near 100% attention when sailing, to get the best from them.
Mike Waters
"See the Copyright Information & Legal Disclaimer page for copyright info and use of ANY part of this text or article"