Offset Centerboards

Of late I’ve noticed that some trailer sailors have centerboards offset to one side, I can see where offsetting the centerboard would help foot room, however, at what price in performance? On some boats I’ve seen the centerboard only offset a couple inches, this puzzles me.

 

Of whitehalls in particular, trunks were often offset to clear the keel structure.

On multihulls, where heel angles are low, greater offsets are no problem. My 29' Newick tri's board is 10" off center.

 

There are several advantages and no pit falls. The biggest advantage on V bottom boats and round bilge with some deadrise is it keeps the board slot clear when beached or run aground. Most designs offset the board by 3" or 4", which just isn't enough to get pissy about. Even the Bolger (and others) designs, that place it well off the centerline, don't show significant differences, assuming there is enough bury and area to start with. Naturally, at large angles of heel, these types of designs could be at a disadvantage if on the "unfavored" tack for some time, but in their defense they aren't intended to sail at large heel angles so if you are, you're screwing up anyway.

 

If you are worried about footroom, why have a centerboard at all? The Dutch and others have been using leeboards for centuries. And leeboards, similiar to what PAR pointed out, have little disadvantage when the vessel are sailed flat. Only when pushing them hard on the lee side will they show any significant problems, but they should be to weather anyway.

However, you are correct that if you conduct very careful experiments, there is a slight increase in drag for an off-center board. This is for two reasons, first, there is a slight increase in the root drag due to the asymmetry especially if the foil is in way a large deadrise (see Heorner FDD), and second, if the foil is significantly offcenter of the sailing form, the induced drag of the rudder will be greater due to it's necessary deflection to hold course. Of course, those are lab results with a real sharp pencil. In real practice slot turbulence and seaway effects will totaly mask any detrimental effects.

 

I'm so glad you included you last sentence John. As a much younger and stubbornly difficult fellow at times, I took up the argument that there must be a penalty for an offset centerboard. I butchered a class certified racer and moved it's board and case directly over the centerline. Other then tactical errors and better tacks, there were no discernible differences between the others of the same class I practiced with. They were just as crazy about winning and advantages, maybe sailed me harder then usual to prove a point, but enough practice sessions were arranged that it was clear, no difference was had. Of course they didn't believe me either so they tried in my boat, to the same result. We were all pretty stunned by the outcome.

 

Only the smallest local sailboats are built with a CB or DB on centerline. For those with a keel or keelson, there are just too many advantages to placing the CB off center along the keelson to ignore and no measurable disadvantages in performance.

On an 8' tender I built, the DB fits through the inside edge of the sidetank/seat leaving the entire middle of the boat unobstructed. This frees up lots of room for hauling odd sized stuff to and from the anchored boat. Of even more practical use is that it allows a sliding rowing seat and two sets of rowlocks to give perfect boat balance for different numbers of passengers and gear. From the number of tenders I see being rowed or motored with the bow or stern pointed into the air, this takes my vote as the best idea in a tender I've seen.

 

I have never been bothered by asymmetry in sailboats. How far off center would the board have to be to cause a noticeable effect? 25% of beam for eg?

 

The further off-set from center line, the larger the trim drag from the rudder, there will always be a small amount of trim drag even with a small offset. This is most noticeable when going straight down wind. I notices this when racing a Hobie 18 many years ago, it was common to pull one dagger board up when going down wind, but I noticed it caused the tiller to be off-center. When I had both dagger boards down part way the same amount the tiller was centered and we picked up some speed, and we won that leg of the race.

However, in a monohull the water never "sees" a symmetrical hull under normal sailing conditions anyway since there is always a bit of leeway, even if you can counterbalance so the mast is strait up. So symmetry is way overrated in a sailboat since the hull is not symmetrical to the water in most sailing conditions.

There are now some racing sailboats that have two dagger boards and two rudders, one on each side of the hull. this presumably reduces the need for trimming out the asymmetrical drag, the windward side dagger board and rudder are retracted out of the water.

 

I built a sail rig for one of my kayaks a few years back. It used a single Bruce foil which, by its very nature, had to be offset from the center line, in this case by about 30"/75 cm. The sail was also offset, on one side of the cockpit. The distance from the foil to the rudder measured along the hull axis was less than double the foil offset. It looked strange and did not make much headway into the wind due to its faults (small sail, excess leeway, slow to turn) but it did well on a reach or run and I never was able to decide if it had a good or bad tack. Asymmetry may not rock but it doesn't stink too bad ...

 

So we have an offset cb- should we cant the tip back towards the centerline when fully extended, or not. What would be the greatest cant angle one would want in a boat that heeled 15 degrees? In addition to lessening the rudder problem mentioned above, doing so improves the interior arrangements in a couple of my designs, but I suspect it's a negative from a performance standpoint. But maybe 5 degrees is not bad?

 

Don't cant the board. I will be a much bigger disadvantage then an offset placement.

Again, if the board is offset a few inches to clear a keel, there are advantages and no discernible disadvantages. If it's offset more then 10% of the beam, you will progressively experience a "bad" tack disadvantage, depending on how far off center it is.

If you must use an offset further then 10% of the beam, then you might as well use leeboards, bilge keels or twin boards (dagger or centerboard style). In these twin arrangements then a cant can be advantageous. The amount of can't depends on the hull form, but generally you want the best board "presentation" at the optimum angle of heel, which would typically be in the 10 to 12 degree range.

 

The only time I saw that done was on a trimaran. The board lay close to the cabin side when lifted, to provide more interior space. They may get away with it on a multihull but the cant angle added to the heeling angle on a monohull would increase leeway and the faster you tried to sail the worse it would get. My Bruce foil experiment suffered from leeway bigtime.

Paul: at one time I was considering offsetting the board on Dace. As it turned out, I didn’t because I realized I would not be that far forward except to step the mast. However she so stable I doubt I would have noticed a good/bad tack even with the 25% of beam offset I was considering - that would have tucked the trunk neatly into a side tank. I would say that, when considering how far to offset a board, the characteristics of the hull should be taken into account.

Question: is there a chance the turbulence from the board would blank the rudder on one tack?

 

The rudder could be affected with an offset board on one tack, but you'd have to have a pretty unlucky placement. The rudder and center appendage work in different flow streams when moving before the wind. So, yep you could offset the center appendage, so it's disturbed flow affected the rudder, but a quick course correction would likely fix the situation.