PDRacer Sailboat

Articles:
Something Like A Foil
Mounting Skegs & Bilge Boards

Keel / Daggerboard / Leeboard / Centerboard
The underwater fin underneath your boat is what makes it go forward. The wind blows against the sail making "effort", and the fin in the water makes "resistance", and the effect of using the two together produces motion. One unique thing about a box shaped boat is that the side of the hull will produce some resistance, but it really isn't enough to make windward progress very well. You can sail about 60 degrees off the wind without any type of fin at all, but it tends to slip a bit. The most conventional method is to have a single fin underwater as your keel, and a seperate fin that is your rudder. The rudder is for steering, and the keel is for resisting the effort of the sail.

Where To Place Your Fin
Figuring where your keel goes is mostly a matter of finding the Center of Effort (CE) of your sail - this is the centerpoint of your sail area, in the sail rig that you use. The keel is then placed so that the Center Of Lateral Resistance (CLR - center of the keel area) is just a little bit forward of the CE. If you placed them just right, as your boat sails along you won't have any pressure on the rudder, and the efforts will be balanced on each other, cancelling each other out as the boat sails along. If you place the CE too far forward of your keel, then the sail's effort will try to rotate the boat and try to push the bow down wind. If you place the CE too far aft, then the bow will be pushed into the wind as you sail, either effect will result in you having to constantly correct with your rudder. For an excellent explanation of this, see:
Figuring Center Of Effort

Another way to look at the CE / CLR balancing is to think of the sails like they are a see-saw on a kids playground, and the board/keel is the pivot point which the see-saw is sitting on. You want to have equal amounts of sail on both sides of the pivot point, so all the effort will be balanced on it.

Keel to Rudder Distance
Because our boats are so short, you have to also take into consideration the distance from your leeboard to the rudder. The further apart they are, the more of a mechanical advantage your rudder will have. I have found that having the board 4' from the stern seems to work OK, but if you have it around 3' from the stern, your rudder will loose effectiveness on certain points of sail.

Sizing & Shape Your Keel
The most common rule of thumb for sizing the underwater portion of your keel is 4-5% of the sail area size. So if your sail is 60 sqft, then the fin should be around 3 sqft. This is up for debate though, and since any size underwater fins are permitted, you can experiment and see what works best. Fins that are deep and skinny are called "high aspect", and have less drag to them. Fins that are short, but are long fore to aft, are called "low aspect" and create more drag for the same square area.

Side Daggerboard
The easiest board to make and install is a side daggerboard. All you need are a couple of strips of wood bolted to the side of your boat and then slide the board down.

If you change the sail plan you are using, you can move the external trunk forward or aft to match it. You can even have one trunk on each side of the boat, so you use the apropriate one with the sail rig you picked for the day.

Michalak Leeboard
This is a very simple to install, and very functional board - and it seems to be one of the most popular types of boards in the plywood boat building hobby. The board pivots around a bolt, and is held down with a bungee cord, or you can melt some lead into the bottom of the blade to make drop by itself. To keep it from wearing the side of the hull, you should put in a couple of pieces of plastic between the board and the hull. Other boat designs have a small rub strip that the leeboard slides against.

Lead Melting Alternative - Instead of melting, you can put lead in the hole you would have melted into, then pound it into shape with a hammer. Then coat the top with some thickened epoxy to smooth it out. Or you can mix up epoxy with lead shot, and smear that in the void in your board.

The inside of the hull needs to be reinforced to accept the great force that is placed on the pivot bolt.

If you plan on having multiple sail rigs, you can place several leeboard mounts along your hull, so you can move the board forward or aft as needed.

Elephant Ear Leeboard
This is the oldest types of board, basically it is a board that is hung on the gunnel by a piece of rope, and a 2nd piece of rope hoists up the aft edge. You need 2 of them, as you tack, you drop the board that will be on the leeward side (down wind side) and pull up the windward side. The boat presses itself against the board, and friction keeps it in place. You will need to either melt or pound lead into the bottom of the board to make it heavy enough to sink on it's own.

If you don't pull up the windward board, it will flop up and down in the water (called chicken winging) as you sail.

The commercially made Sea Pearl 21 uses leeboards with an interesting pivot mechanism. It looks like 2 pintles that are in a stainless tube, which then has a bolt thru the middle and into the side of the hull. Very elegant solution.

Andy Board
Andrew Linn wanted to use a pair of leeboards. Since the PDRacer is a boat which uses many different sail rigs, Andy came up with a sliding leeboard hanger so he could move his leeboards forward and aft wherever he wanted, and can even adjust their location when sailing. NOTE: if you plan to use this same hanger, make sure that you have a pair. If you use only one, then it will break off when the board is on the windward side because of the leverage force. Single leeboards like the Michalak leeboard above need to have their pivot points in the middle the hull.

Bilge Boards
Very popular in England, the bilge boards are either 2 or 3 short keels that are permanently mounted on the bottom of the hull. They are great for very shallow water, or for having a keel that you don't have to worry about putting up and down.

To strengthen the joint between the board and the hull, you can run a fillet of thickened epoxy on each side of the board, or put on chine logs on the outside, where the fins attach to the hull.

Daggerboard
It is called a daggerboard because the board fits into the trunk in an up and down motion, like you would draw a dagger from it's sheath. The trunk is easy to make, basically a box, but it is very important that you have some type of structure at the top of it to link it back to the hull. This is Ken's boat, notice the seat that goes across and how it is attached to the trunk. A lot of force is put on the board and it acts like a lever with the pivot point being the bottom of the hull.

Centerboard
Like a daggerboard, it has a trunk, but the trunk is very long so that the board can pivot up inside the trunk and fit inside it.

If you are building a shoebox type hull, then I think the offset centerboard is probably one of the best types of boards for a PDRacer. If you mount it a litte bit forward, you can adjust your CLR very easily by adjusting how far down you let your board. The more you let the board down, the further forward the CLR goes. A centerboard kicks up automatically if you hit something underwater, and when you are not using it, you just hoist it up and it stays in the trunk -- you don't have to remember to pack it in the boat when you go sailing, it stays in there all the time. If the trunk is offset to the side, not only does it leave a bunch of space in the middle of the cockpit, but you can deck over from the trunk to the side of the hull creating an airbox for emergency floatation and storage. The obvious problem with the centerboard is that you have to make and install the trunk, it has the most initial work of all the underwater fins.

Keel
A keel is any single fixed fin mounted on the bottom of the hull. Some have ballast, some don't.

Shoal Keel
It looks like a single bilge board as shown above. It can either be ballasted or not. Also optional is either a swing centerboard, or a daggerboard that drops down from it to provide addtional lateral resistance. A Montgomery 17 is a good example of a shoal keel with a swing centerboard.

Wing Keel
This is a keel that has a fins on it that stick out horizontally, or at an angle. They are illegal, all fins of any type have to be vertical on a class legal PDRacer.

Chine Runner
This is a small board or boards that stick out horizontally from the side of the hull. The most famous boat design to use it is the Paradox. Since it is a horizontal fin, it is illegal to use on a class racing boat.

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