Keel placement on a PD racer

Hi!

I had this idea of puting two small delta shaped keels on the pdracer, one on each side, so that their areas add up to a little over 2% (2.3% to be exact) of the sails' area (4.45m^2 or 48 square feet). I've attached a screenshot. The x on the sail is the point of effort.

At the same time, I wanted to make a balanced rudder, so in a way it would be part of the sideways force resistance. This would compensate the keellets being relatively far forward and add surface.

All three surfaces would be able to pivot in the event of hitting somthing in the water.

(The issue is that the lakes I want to sail here in Sweden tend to have shallow areas and also random boulders directly under the surface. You are out in the open water minding your business and suddenly there's a giant effing rock 10 cm under the surface.)

What do you think? Is this going to work? Any pitfalls to consider?

Mario

 

The PD Racer has options for pivoting leeboards and centreboard, all you need to do is make the rudder blade pivot, wich is actually also common on them.
Any reason why you would want to reinvent the wheel?

Types of Daggerboard leeboard centerboard etc https://www.pdracer.com/keel/

 

Counter question: what in that page qualifies as a wheel? The ideas shown for the shallow water case are very vague and known to be rather bad.

This boat building thing takes quite some time, I'm hoping have something that isn't a complete disappointment at the end.

 

The centerboard and leeboard are both time honored solutions. We also know exactly what to do to make them work well (aspect ratio, profile, slot, etc.).

As for the wheel, how should I put it, in what way do a pair of pivoting "keellets" differ from a pair of ballasted centerboards?

Dual centerboards and offset singles have been done numerous times, usually for the simple reason of keeping the middle of the boat free for camping (Ixylon for example). There's no magic in it, boards and rudder must balance the chosen rig, so your "keellets" will end up longitudinally exactly where a single board will. Total area of a dual board setup is usually slightly more then a single one because they are usually also shorter.

If you want the best performance you go with high aspect ratio profiled boards and tack them. Basically you take the best daggerboard design, make two of them and mount them on pivots then raise one of them up according to the current tack. If you want to play with rig and rudder area the best bet is to install the boards outboard (making them leeboards), so you can play with the pivot point without having to constantly rebuild the case. There's a slight penalty for not having an endplate wich can be somewhat compensated by angling the board one or two degrees forward.

 

I am trying to find a compromise for shallow water. The performance I am after is clearing shallow water. I am under the impression the performance of this: https://www.pdracer.com/keel/bilge.jpg is not stellar at all. I was hoping the small deltas could be better. Anyway I don't know, so that's why I am asking...

 

Make an offset centerboard or a leeboard, weight the tip with lead. If you hit something the board will swing up riding over the obstacle, then pivot down by itself. Problem solved. All that will happen while sailing is a loud bang, you will soon learn to ignore it and keep on sailing.

 

Hm... Okay.

Does this position relatifve to the sail look reasonable to you? (and thanks!)

 

That centerboard is too far forward. Look at typical boat designs for locations that work well as a starting point.

 

Lee boards are your best and most sensible option. Make then a bit more than 2% of the area of the sail.
Gonzo is right, the boards that you show are too far forward.
Follow the plans for the PDR. The PDR has been around for a long time and the improvements, if any, have been thoroughly sorted out by the many builders.

Rumar's "Reinventing the wheel" is merely a metaphor and not intended as a slight.

 

Re wheel, I understood it that way, no offense taken.

The thing is that AFAICT there are no real plans, nor a good way to access the knowledge of those builders. I've found very little information tbh. Which is a bit of a bummer. I have the plan for the simple 18 and it has some mistakes. And the sail is not a balanced lug but something requiring a huge mast, so I changed that. There are some rough plans for lug sails on the pdracer site, but they are gigantic sails. And then a bunch of pictures.

I've put the leeboard directly under the center of effort of the sail like in the simple 18 plan. Is that better?

 

You will probably find better helm balance by moving the lee boards slightly aft of the CE of the sail. 3 or 4 percentage points are a fair starting point. The advantage of the lee boards is that you can move them without doing much if any structural changes.

 

Okay! Seems like I'll attach the leeboard in some way that makes it trivial to move arround.

And thanks everyone!

 

That is not the center of effort of the sail, only the geometric one in profile. The sail moves to windward which increases weather helm. The center of lateral resistance has to be calculated based on the location of the real center of effort, that will be outboard of the hull and to leward.