Centerboard suggestionsSeeing as I have used only Daggerboards and Leeboards

Seeing as I have used only Daggerboards and Leeboards in my small amount of designing, building and sailing... I would appreciate some advice on Centerboards. I am working on a 17' Faering style boat, rigged with a balanced lug and yawl mizzen and would like to use a Centerboard on it, however I am not conversant in the pros and cons of the various styles of board. My inclination is to use a board that is foil shaped with a high(er) aspect ratio but I would consider Pie shaped or any other styles if I hear enough good things about them. I expect the foil would be built with a flat piece of metal enclosed in a shaped outer shell, so I can get some weight in it, along with strength. Other building methods will also be considered if they are suggested. I would like to hear from those who have had experience with one or preferably more styles of centerboard, comparing the merits of each kind so I can make a decision and add it to my pretty picture... which isn't done yet.





Oops... sorry about the title... ended up pasting some of the text in there.

 

Steve,

I have not built one but the Tornado Catamaran has a centerboard with a "pseudo" airfoil shape (I mean it is generally shaped like an airfoil but I don't think it has a specific shape like a 0012, etc).
The board is not weighted, but it always went down easy, even though it was made as a foam sandwich - earlier boards were laminated wood with a glass coating.
The pivot is not captured thru the centerboard cases (no leak path). There was a slot on both sides of the case (inside) leading down to the pin area. You just inserted the pin in the board and the board down the case (open top) with the pin in the slots. Once in the case, the board was hauled down via a rope from the top of the board to a cleat on the aft end of the case. When released the board would float up, but had to be pushed by hand to fully retract. An uphaul could be easily fitted, but that is one more line. If the board is weighted, the uphaul would be required.

A side view showed this to be a pie shaped board below the keel line, so the case slot was "mostly" filled when fully down. This was a "low aspect ratio" board.

Very easy to adjust for area on different points of sail, but when the board was not fully down there was nothing to fill the centerboard slot at the back. Die hards (racers - the majority of the sailors) would use a gasket of mylar to close off the slot when partially or fully retracted.
The gasket was rather delicate - not something you wanted to drive the boat up onto the beach on.

I liked this example of the centerboard.
Note that metal in the centerline of the board will do nothing for stiffness. Of course it will add to weight of the board, but that would be better done by adding lead in the tip of the board.
Do you want weight for positive depolyment or righting moment?

 

On that style of boat, you really don't need a NACA foil shaped board, though it will not hurt anything. A "slab sided" foil will do just as well.

Construction is best if strip planked, rather then alloy plate. I've just built a centerboard, in fact casting a 30 pound weight for it a few days ago. This weight is just enough to sink the board with authority, not to act as ballast. The lead weight was cast to form the lower part of the leading edge and bottom of the board. This places it as low as practical and offers considerable protection to these edges too.

If you do want effective ballast, in the board, the case and the pivot have to be well thought out, because the loads rise exponentially.

The plan form is up to you, but a pie wedge will do fine for this type of boat, as you're not going to reach speeds where a well shaped, higher aspect foil is going to offer anything. The "elliptical" tip thing isn't as import as one might think, compared to the area of lift you loose with this shape. Simply put, yeah you save some tip vortic generation with an elliptical shape, but you lose more in lift from the lose of area, compared to the slightly more draggy squared off tip. Everyone sights the WWII SpitFire as an example, but what they don't site is the fact, they squared off the SpitFires wing tip in the last generation, for better high altitude performance, suggesting, they figured it out too.

This said, I prefer to use higher aspect, when the design permits. A slice of pie style of board needs at least 1/3rd of the board area, to remain in the case, preferably 1/2, so it doesn't "twist out" and has enough purchase on the case sides to stay put. This style of board can be lower in the boat and less intrusive.

A higher aspect board will require more boat interior intrusion and a more complex hoisting arrangement. The point about a "captive" pivot is a good one. I use a set of brackets that reach down the sides of the case (usually in a slot) that attach at the top of the case. The bracket has a barrel bolt arrangement to act as the pivot bolt. The nice thing about this setup is the board and it's pivot, both drop into the case from above and no hull penetrations to deal with. There are other ways to handle this, but leaking cases are common, unless bonded and stoutly built.

 

I have owned both dagger board and centerboard boats. Dagger boards are much simpler and light, performes better (less drag, more effective). but most important of all it is much easier to build and since it does not weaken the hull by cutting through as many ribs it can be a lot lighter.

I think there is a false perception that it is 'safer' in a strike on a submerged object, but I have not found that to be the case. the pivot puts a lot of impact loads in the center board case in a strike, and usually results in damage to the hull, particularly since the hull is already weaker in the area. Only if the strike is straight aft AND you have some breakaway down haul gear, will it pivot up into the case. In a sailboat the impact almost always has some angle to it.

A high speed strike with either a centerboard or a dagger board can result in hull damage. And a dagger board can be design to pop up in the event of a strike, minimizing the risk of damage to the hull.

 

Interesting beast, though she would sail better to windward with a jib as well, like the old Royal Navy 27' whalers. Those have a centreboard with a massive pull up handle which pivots, and they are galvanised steel I believe. Just a while since I've sailed one though I do have a charge certificate for them!.

PAR has sugested a mode of assembly which we call a 'Stirrup' (this side of the Pond) ie dropped down into grooves from above and fastened so no through the case bolt. The other mode is to do like the OK, Scorpion and others and use a U slot in the board and a permanent rocking pin or point in the case construction. The latter is not my personal favourite - too many boards break around the U part even if good timber and glassed....

My least favourite thing with daggerboards is the interference with the kicking strap (vang) though of course a gnav would obviate this. Not really had problems with well built conventional cases with a bolt through, or leaks. A friction rubber is still a reasonable compromise if you line the box with a melamine laminate such as the products from Formica and Perstorp Warite. Unless you add a lot of lead you won't get that much extra stability. Check out the 2.4 Meter keel boats, probably 220Kg of lead in their keels and the daggerboard Access 303 Class. Different boats but both using the lead for sufficient righting moment. Note the latter has glass moulded foils and the lead is inserted as shot with a binding paste into the hollow moulding. Up to 43 Kg, pity the mouldings are so poor (they often split from impacts) but that is a different story...

I would also consider the lever arm to raise a heavily weighted board. It will be significant if you place a lot of weight at the tip. So a very long 'handle' will be required, and this could be a pain inside the boat.

PAR as usual has it right, much easier to strip laminate and glass a wooden foil than try a metal centre insert. Stronger, better integrity etc. If you really want lead in there, I would be tempted to splice it in at the end and glass it over fairly heavily. Casting it is not too hard if you can create a mould. My gut feel would be that you might be looking at 1.3 to 1.5 meter depth if you go for a high AR board, but as already pointed out area is key for reducing leeway.

Upchurchmr is right about the Mylar gaskets requiring frequent replacement if abused, but a good compromise is a sailcloth covered Mylar one. It seems to give longer life as the tough sailcloth 'protects' the more brittle stiffer Mylar. There is a whole other discussion about arranging gaskets etc.

 

I've used rubber gasket material for slot covers, which I find work better than sail cloth or Mylar. It's available in sheet form of various thickness and can be set in a shallow rabbit, inside the slot, so they're flush when the board is retracted.

 

Not everyone tries to build the lightest structure possible. I agree with all said above if that is important, but in home building, durability is often a motivator, and the cost of saving one pound (as opposed to a ton in a production setting) not much of a motivator.

I prefer centerboards on all craft that don't race. I use the centerboard case as the primary structural unit and integrate it with one bombproof frame to distribute the loads to the skin of small craft. It servers and a trailer bunk land, hoisting location, and a good place to clamp the outboard when trailering.

My preference is for boards that pivot about 100 degrees, and leave a good bit of board sticking down when fully retracted, say three inches or so. This centers the boat on the trailer bunks, gives you beach protection (it is very easy to shove a throwable square pfd under the boat and have it ride on the cushion and cb on the hard).

I just use a throughbolt for a pin, but I go big. My 16'er had twin boards. Neither ever leaked in 25 years. The case and board were drilled to take a sleeve of 1.25 stainless boiler tube that was epoxied in place, and a 1" stainless bolt ran through. Fender washers were screwed to the case and retained butylrubber "dookie tape", creating a seal that I never replaced, although I removed the boards often enough. The case frame was built from 6/4 mahogany top and bottom and sided with 17mm ply. A metal bar stuck up from the top of the slot and provided the handle (or footle, as was more often the case) I suppose each case with board weighed about 40 pounds.

If you build heavy, you don't need much board sticking in the case. That is more a function of the thickness of the board than anything else, and slot clearances, to some extent. Just imagine a couple big guys jumping up and down on the tip of the board with it all the way down and the boat on its side. That is sufficient, and good one inch ply, a skin of glass and a couple inches of board in the slot can handle that. I slam the pin as low forward as I can in the case. That's what that 6/4 mahogany is for, to carry a couple tons of prying pressure for 50 years. The Cb housing only needs to be 9 or ten inches high inside the boat, depending on how you work the mechanism (it may need to be higher up front for the mechanism to not spew water).

I've taken a couple of photos of the boards from my 16'er, which I still carry around with me (the boat was converted to motor skiff, so I pulled the boards). In a couple days, my camera will get it into the computer, and I can post it. (Why it has to start by loading the other 5000 pictures each time I just want the last one, I'll never know). In the mean time - a sketch.

<photo has arrived>

 

That looks interesting... and I am not averse to having a bit sticking out. However... If I do go with a centerboard it won't exactly be centered... I want to keep the keel intact, so it will be off center by a bit. My concern there with having some of the boar exposed when up is something jamming between the keel and the board. I still may revert back to my tried and true daggerboard, with a bit of weight on the tip to keep it down and protect it. Has anyone had any luck with a reverse angled board... not much angle but just a little? Seems to me with enough weight to keep it down if it contacts something it will as least raise a little if not pop up a lot. If the leading edge at the bottom of the board were angled also to help with the push upwards?

 

For board shape, style of construction of the board & trunk etc, it'd pay to look at how they're done on Searunner Trimarans.
The boards are simply large slabs of laminated plywood, planed to shape, & glassed over or just epoxied. And the pivot for them is rather simple. On the inside of either side of the trunk, one beds in a thru hull fitting, so that the mushroom heads of the fittings face each other. Thus, the threaded parts with caps are accessable from inside the vessel. And one just uses a piece of stainless or bronze rod through a hole in the board for a pivot pin. KISS
Once the board's in place, including the uphaul & downhaul lines, a cap is screwed & bedded onto the top of the trunk. So that the boat stays dry inside.

The downhaul line includes a "fuse", to let you know when you're getting into shoals, or happen to strike something. It's simply a piece of thinner cord woven into, or seized onto the downhaul, which is strong enough to carry the load without a problem, but sized to break when the board strikes something with significant force.

When installing the fuse, it's rigged so that the fuse is shorter than the section of the primary downhaul line to which it's seized. So that when under tension, there's some slack in the primary downhaul which is being carried by the fuse.

When you're sailing, most of the force holding the centerboard in place comes from the lift developed by the board, holding it in place against the trunk. The line I used for a fuse on my 31' Searunner had about a 200lb breaking strength, & it never gave me any trouble.

Also, you might look at Derek Shuttleworth's arrangement for daggerboards. I believe that he has a kickup system for them as well. And likely there are similar setups in other multihull designs which have daggerboards.