Why the PDRacer rocker is shaped like it is by David "Shorty" Routh
Designing a boat takes a lot of compromises, and you have to clearly understand what purpose you are designing the boat for.
The PDRacer has a very clear definition, basically it is the simplest boat that can be built for 1 or 2 adults, for use primarily as a racer, but also as a recreational boat.
For more info, see the Origin of the pdracer essay.
This essay describes the factors that went into designing the shape of the rocker (the bottom curve of the hull).
Max rocker at the stern
1 - Water doesn't like to be moved, so the further aft you can put the maximum rocker, the more gradual of a water squishing effort you have.
The water is going to get squished either way, but if you can do it more gently, it will cause less resistance, and be a faster hull.
However you can't have the maximum rocker at the complete stern, because you have to curve back up and let the water go without dragging a transom corner across it.
If you drag the transom corner, it will create vortexes and suck at the stern of the boat.
This sucking force is so great, you can actually feel it --- try rowing your pdracer around with even trim, and a passenger in the front.
Get a feel for how it rows, then have the passenger sit on the stern so it digs in, and row it around some more.
2 - So you can look up at the sail.
When racing, you look at the sail every 30 seconds or so to read the tell tales, to make sure you have laminar flow.
The further aft you can sit, the easier it is to look up.
If you sat in the middle, you would have to crink your neck harder to look up.
Only 6" max rocker, instead of the Bolger Brick 8"
1 - The more rocker you have, the more weight the boat can carry.
The PDRacer is designed to carry 2 adults (or 1 adult and 2 kids), so the weight of the boat + the max crew came to be about 630 lbs.
With a lot of stretching around on AutoCAD, I found that 6" was more than adequate to carry that much weight.
2 - Longer water line. The longer the water line you have, the faster the hull is.
Less rocker means more hull touching the water, so it is a slightly longer water line than a brick.
Flat spot
1 - I built a brick, and let me tell you, with that round of a rocker shape, it really feels like a rocking horse.
As you move about, the brick will very easily adjust itself to match where ever you go.
The flat spot on the pdracer dampens this to make it more stable, and easier to handle, especially for new sailors.
2 - Helps the boat ride on top of the water, and get up on a plane.
Yes, I (and others) have planed a pdracer. If it was a complete curve, it would have more of a tendancy to create more bow wave, and dig in instead of planing.
3 - Flat place to sleep in the hull.
I camped in my brick, and boy did it do a number on my back.
There are Lots of protected water adventures to be had, and for the recreational aspect of this boat, wanted to have it more comfortable to sleep in.
Angled bow
1 - When coming around the windward mark, as you turn to go down wind, the mast acts like a lever arm and trys to push the boat end-over on it's bow.
The angled bow will provide some lift while you adjust your weight to get the stern down again.
If it were a blunt bow, it would provide resistance when digging the bow, slow the boat down and help the bow dig more.
SIDE NOTE: All sailboats have the same tendancy when turning to go down wind, but most others aren't as over canvased as the PDRacer usually is.
So the PDRacer will have a much bigger lever arm effect than what a typical boat will.
2 - Deflect Spray - I built a boat with a blunt bow, and when the waves would hit the bow, the water would get splashed straight up, then land on the deck and roll into the cockpit.
Really annyoing.
With an angled bow, the spray gets deflected forward and makes it more of a dry ride.
Cod's Head Mackerel Tail
Some other sailboats are designed with the maximum depth of rocker closer to the bow, such as the incredibly popular sunfish.
The theory behind the design is to encourage the bow to lift up in the air and plane quicker -- and let me tell you, with the right wind conditions a sunfish will get up on a plane and fly.
You can easily tell you are planning because the water squirts out the side of the hull where that maximum rocker point is.
The negative compromise is that the forward rocker will create more resistance at non-planning speeds, and be slower than the stern rocker equivalent.
Just incase you think it would be more advantageous to have the rocker up front, you can setup your PD to sail backwards, there is nothing in the rules to prohibit that.