Michael Storer Boat Design


Where to place the Centrecase relative to the Sail Area


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One of the more outrageous risks we took with the OZ PD Racer was as a response to the amount of sail we wanted to put on this little 8ft (2400mm) boat.

(By more outrageous - it was the innovation I was most worried  about - there was a very big sigh of relief when the boats behaved and performed so well)

We figured that around 80 square feet would be a sensible step up from the 45 to 70 square feet of most of the existing boats.

In the end our mainsails were a true measured area of 86 square feet (8 square metres) but mounted on simple, but bendy, wooden masts the boats proved both fast and easy to handle.

The combination of the short hull giving a very low wetted surface and the generous sailplan resulted in startling light weather performance - the boats move really well with even a tiny breeze as can be seen on the videos.

A problem with the large sail area are that with the mast as forward as possible there are only two choices - the sail has to go extend up or extend back.

Extending up means bigger heeling loads making the boat harder to hold up in stronger winds not a good choice for good all-round performance

Going back means the centreboard has to move back too.  The drawing right shows the extra sail area compared to many of the other boats and the amount of movement of sail centre and centreboard required to keep the boat in balance.

Normally centreboard cases are somewhere between one third and one half way back from the bow.  In our boats the centreboard case is in the back part of the boat.  It actually looks all wrong - but it worked well in practice.

Robert suggested that the centreboard would be too far back - whereas I was fairly sure it was in the right place to balance the sail area.  He was grappling with the same problem I had when I moved the board back that far!

Hi Robert,

You are a good observer!!!

The board is a bit behind the normal visual position of the centreboard in most boats - but it is the right place to balance against the sail centre of effort.

That's all that defines whether a boat is "hard mouthed" or the opposite - at least in small boats.  BTW, providing that the centreboard and sail are in about the correct position (ie within about 4 inches on the puddleduck) the biggest determinant of "hard mouthedness" is whether the boat is being sailed heeled or not.  Sail it flat and it will balance if everything else is approximately correct.

The biggest difference between good small boat sailors and poor ones is how much they heel the boat.  You can see this in any racing fleet.     The guys at the front are flat, the ones at the back are heeling.

In light winds SOME heel - 15 degrees is nice to reduce wetted surface and to allow gravity to calm the sails down - but by the time the wind is filling the sail reliably the boat should be being sailed dead flat.

Note that I was quite wrong about the heel in light winds.  The PD Racers just dont like being heeled much at all.  Heeled more than a few degrees the corner of both the bow and stern transoms disturbs the water badly - you can let them heel a bit but you have to watch that the corners don't dig in at all.

But back to the design side of things ...

In the last few years in out and out racing yachts there has been a tendancy (healthy for once) toward big mainsails with big roaches and small jibs with little overlap (genoas that overlap the mast by much have finally been recognised as the inefficient sail area they really are).

The end result is that keels in these boats have been moving further and further aft to keep in line with the sail centres.

Biting Midge (my partner in crime who bankrolled the OZ PD Racer project) is about to tell you all that these ideas were first developed by Aussie big multi designer Lock Crowther in the late '70s and early '80s.  Monohull designers are only just catching on to what has been now a long established trend in multis.

In the PDR it allows us to carry a big mainsail with significant transom overhang and still keep the centres in line - if we had got it wrong the boats just wouldn't move as well as they do in the pic right.

Two sails is just too much complication to our minds - and single sails are generally more efficient - they can be trimmed moment by moment by the available single crewperson for a start.

I have kept the rudder a bit bigger than I thought necessary (length is about what I thought right but it is quite a bit wider (which also enabled us to make the accurate template shaped centreboard and rudder in one piece to make the shaping easier and more accurate and cut them apart after shaping.  This is to provide more steering force as the rudder, being much closer to the centreboard (the pivot point when turning) has less leverage.  Its like a lever - if it is short (less distance from fulcrum) then the input force has to be greater.

It's all about interrelationship and making decisions that have a positive effect for the boat and it's building in total.

But you are right - we are running a little risk in bringing it further aft - but it is a calculated one - I think there will be little problem, but when the boats go in the water we will know.

For these two prototypes we have made fairly elaborate facilities for altering the rake of the mast - so we can get the balance perfect.

Once we have worked it out the partner and step will be drawn up to be much simpler than the current one (even though our boats are lighter than almost everyone elses the more elaborate partner and step is just toooo much extra work).

But, if there are problems we can move the cases - about half a days work for the two boats.

Best Regards
Michael Storer

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