LAR keels or Boards? The pros and cons

 

When writing about foils (ie boards, keels and rudders) it is easy to baffle people by talking about lift to drag ratios, NACA sections, end plates, twist, stall, Karman vortices and all the rest. So I won't - it's probably best to leave all that to designers and hydrodynamacists Instead I'll try to help those who want to know the basics; like should they have keels or boards on their multihull?

I'm not just a multihull designer; I'm also a (home) boatbuilder and owner. Currently I own two catamarans, one with daggerboards and one with keels. Over the last 30 years I have owned five cruising catamarans with keels, nine with daggerboards and one with one centerboard. Furthermore, I am one of the few designers who has fitted LAR keels and boards to the same hull (on Strider, Sagitta and Banshee) and then sailed them against each other.

As with everything to do with yacht design, leeway prevention is a complex subject. And it is not helped by the fact that boats are not just for sailing. You load them down and treat them as a floating cottage when you live aboard them. You motor them, you dry them out (either when the tide goes out or when stored ashore in a boatyard). And you have to pay for it all.

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First a bit of simple theory

The main factor determining the efficiency of a foil is its "Aspect Ratio" (AR) or the ratio of it's depth to width. I think we all know that a high aspect ratio foil is more "efficient" than one with a low aspect ratio. A 3:1 AR is generally considered ideal for rigs but is usually too high for foils. In fact a 1:1 AR is OK for keels, although 2:1 or a bit more is better. In other words, a 1.5m deep daggerboard on a 10m boat will be about 750mm fore/aft (or more properly called it's "chord"). And it will be about 75mm thick, as the thickness of a foil should be about 8-10% of the chord.

Despite the fact that high AR foils are more efficient, there are a couple of reasons for limiting AR. As the AR increases the chord decreases and so the foil becomes thinner and clearly it is then also weaker. Unfortunately the foil loads have gone up (because the deeper the foil the further the Centre of Lateral Resistance (CLR) is from the bottom of the hull, so the lever has lengthened). So long, narrow foils are more prone to damage (one reason all those monohull keels have snapped off - a J800 lost one just recently).

In addition, the higher the AR the easier it is to stall the foil. When a foil stalls it stops "working", this is most obvious after tacking in a sloppy sea and you find that the boat won't sail properly for a couple of boat lengths. That is because the foil has stalled. In other words, you have to be sailing in flat water and be a very attentive helmsman to avoid stalling a high AR foil.

The lower the AR the bigger the foils area has to be to compensate for the reduced efficiency, however a lower AR is more tolerant of shape, surface roughness and angle of attack (which basically makes it easier to sail)

So how big should a foil be? Roughly 4% of the sail area is a good starting point. Some designers use very small boards. That may be OK in theory, which says that you sail in flat water at maximum speed, but in practice I find you need a bigger board to cope with waves and slow sailing, never mind tacking in a lumpy sea.

Whilst on the dangers of too much theory. Many race boats use sophisticated shapes and low drag sections. Don't use them unless, like top racing boats, you also dry sail your boat. Only the simplest shapes work with a rough surface (even new, clean antifouling is considered rough), these fancy shapes need a smooth, ie a mirror finish, to be of benefit.

I definitely don't like asymmetric foils. True they may give a bit more lift but only one can be used at a time, the other HAS to be raised. So you are carrying around the extra weight of a second board and box plus wasting time adjusting boards after each tack which all makes it not worth the very small efficiency gain.

Many people, especially cruisers, don't mind compromising on performance if it means an easier life, more load carrying and greater protection for the rudders and propellers. And that is why Low Aspect Ratio (LAR) keels are so popular. In much the same way as monohull sailors buy a long keel boat rather than one with a fin keel.

Unfortunately, using LAR keels results in a slower boat and more pitching. That is because to optimize speed you need a hull with buoyancy in the ends, not in the middle (technically you need a high Prismatic Coefficient - Cp). Clearly adding a LAR keel adds buoyancy in exactly the wrong place. Furthermore the midships buoyancy makes the boat pitch more as the hull is more "diamond-like". Having said that the buoyancy in the keel adds to total displacement, so you can carry more gear. And you can use the keel as a water tank/shower sump or even somewhere to fit an engine.

As I have just said, a lower AR means having a bigger foil and so clearly a LAR keel has to have much more area than a daggerboard. Thus the wetted surface area (WSA) is significantly increased, especially when sailing offwind compared to a daggerboarded boat with raised boards. And WSA is the prime source of drag at low speeds. So most designers draw a compromise keel which is smaller than strictly needed to balance the sail forces. But these forces must still balance and this can only be done by having a wider sheeting angle. Which in turn means a LAR boat cannot point as high as one with daggerboards.

So all in all boats with LAR keels are slower on all points of sail and in all conditions when compared to a daggerboarded boat. Say 5deg more leeway and 2deg less pointing. Remember it is hard to tell ones real leeway from looking back at the wake as in a seaway the top 2-3ft of water is blown sideways by the wind. So unless the keel is in deep still water you don't notice that you are drifting sideways until you look at the gps.

Further Design Considerations

Boards can be daggerboards (ie adjusted vertically) or centerboards (ie adjusted by rotating). Even though centre boards sometimes fit better into the accommodation layout and can pivot aft without damage when hitting the ground, daggerboards have proven far more popular. Probably because the centerboard pivot bolt often leaks, while it is very difficult to stop the water surging in the case with the board down, and it is easier for "Stuff" to get stuck in the gap between slot and board, thus jamming the board. A centerboard box/centerboard is probably heavier than using daggerboards. And finally it is harder to remove a centerboard for painting/maintenance.

Angled boards actually take up little interior room and can always be incorporated into accommodation dividers - the nav table bulkhead on a Banshee, the hanging locker on a Sagitta, against the hull side on a Merlin for example. More important is that they then don't bang around in a sloppy sea. Providing the board is angled at under 15deg to the vertical I haven't found any disadvantage in using angled boards. Nor have I noticed any difference between boards placed on the inside of the hull and those on the outside. Certainly there is no advantage in having vertical boards fitted on the hull centerline.

Don't use central boards though the bridgedeck. They simply don't work well. The Prout brothers discovered that in 1953 when they fitted a central board on the prototype Shearwater. By 1954 they had fitted boards in the hulls. The Stiletto catamaran started with a central daggerboard but owners quickly found that converting to ones in the hulls improved performance significantly.

The bottom of a LAR keel should be horizontal, otherwise you'll dry out at angle (OK I know that in many areas boats never dry out, but you want your boat level when in a boatyard) Even worse is a keel that is too short as then the boat can fall forward or aft when people move to bow/stern. Clearly dangerous and damaging to crew and boat.

Building Considerations

Boards are heavy, and more expensive than LAR keels, there's nothing one can do about that. To save weight you can make a short board, so that when down the top of the board is below deck level. Daggerboards can be profiled over their full length. But in that case you need to make the board first, and then make the box round the board. So you need to make expensive boards early on during the build.

The alternative is to profile only the below water portion, leaving the rest rectangular. This means the box is also rectangular, so the board can be made after the box. True, there is some extra turbulence when the board is half raised, but you can negate much of that by fitting a profiled cover plate on the outer side. Also I've found that there are less chance of leaks with a rectangular box and more important, weed, twigs etc are less likely to get between board and case and jam the board.

Many people suggest building a "crash box" to absorb any grounding shock. But be warned! I made one once years ago as an experiment on a dinghy. I ran aground and the board moved back into the crash box, exactly as planned. But then jammed - not part of the plan!! I had to capsize to free the board so I could raise it. Hardly practical on a cruising catamaran!

I haven't used a crash box since, relying instead on a very heavy laminate at the back of the box. My reasoning is that the tapered trailing edge of a board is far weaker than the box and so will always fail first. I proved that when hitting an unmarked reef off Nicaragua and we lost about 300mm x 100 mm of board, yet the box itself was undamaged.

Boards are generally made in timber or foam/glass. I prefer plywood over laminated timber as it is easier to profile. Foam boards are attractive in theory, but it is very hard to stop them warping and getting the correct shape without making a mould first is almost impossible. If you do make a mould you have to join the two halves, and remember boards don't just bend they also twist.

If you can, fit LAR keels as late as possible, this keeps the boat nearer the ground during building and saves a lot of ladder climbing.

What about in use?

Unlike boards, LAR keels are very much fit and forget. So there isn't much to say about keels, except to make sure you have a good sacrificial beaching strip and keep it maintained.

It is sensible to lift the lee daggerboard when reaching, as I have found that most breakages are of the lee board as it gets very loaded at speed. However lifting both boards in a big cross sea is not a good idea as that loads the rudders and that, apart from anything else, makes it heavier to steer, so have both boards half down.

Not being able to lift the lee board for half the time is one reason why I don't like using only one board in one hull. Doing so also means the board is much bigger so there are more local loads and of course it is harder to adjust. And it doesn't seem right to have the CLR so far offline. However if you do fit one board don't fit it in the galley hull as that is always heavier than the other hull.

Lifting the lee board to allow the boat to slip sideways is commonly thought a good idea. I'm not convinced. Tank test work has shown that having a deep lee hull actually helps prevent capsizing. That is because the windward hull lifts to a breaking sea, the sea goes under the bridgedeck and hits the lee hull. Because it is deep it is pushed sideways, which dissipates any overturning energy.

People often think you need a long keel (monohull or multihull) to steer straight "hands off". That simply isn't true. Good directional stability depends on the hull balance. A properly designed daggerboarded boat will sail as straight as a LAR keel one, and have the additional benefit of being quicker to tack and maneuver when necessary.

Finally, if you run aground, you can lift the boards to get off, but remain stuck with keels.

In short

If you don't ever expect to race then keels are OK

If you think crosscut Dacron sails are good enough then keels are OK

If you trail your boat regularly fit dagger boards

If you have wheel steering and inboard engines and expect to dry out often fit keels

If you want the best performance then fit dagger boards, but be prepared to use them