Forums: Whisstock Designs: Rig change? New Topic Post Reply
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Posted: Aug 1st, 2011 at 16:09   |   Subject: Rig change?
We are getting a good start on our 055; frames are done and turned out well. A couple questions: is there a sail/rig plan for 055 with a marconi sloop plan and single mast? Also, we are finding the aluminum for the CB is quite costly here and wonder if a ballasted board could be made from wood? (mahogany ply or glued-up solid?)

Thank you.
Posted: Aug 1st, 2011 at 23:21   |   Subject: Rig change?

Glad it's going well. The frames can be the most tedious part.

There isn't a marconi rig plan, but it wouldn't be hard to do it. The mast would be longer, roughly to the top of the cutter rig gaff and we would need a little different standing rigging. I can provide a plan if you want one.

Yes you can make a ballasted timber centreboard. The gap in the centrecase (25mm) is a bit narrow for a timber board – it would be about 20mm (7/8") to fit easily into the 25mm case. But I think it would be OK. I guess at this stage you could make the case a little wider without too much hassle. If you could get to 30mm (1 1/4") that would be better. But if you'd rather not do that, then we can manage at the existing internal width OK.

If you let me know, I will draw up a plan for you, with the ballast detail (usually a mixture of lead shot and epoxy resin, set in aperture(s) at the lower end of the board.

I guess you could also use galvanized mild steel – but we would then need to reduce the weight! Else you will have problems hauling it up etc. And it will be generally a bit heavy for the boat. To get to much the same weight as aluminium, we would use about 4mm steel – which would be a bit thin. It might be worth considering 6mm. Getting it all galvanized is probably a lot of hassle and expense too .....

So – best to replace it with ballasted timber.

Hope this helps


Posted: Aug 1st, 2011 at 23:53   |   Subject: Photos??
And I would love some photos as you go if you could manage it from time to time!

Posted: Aug 4th, 2011 at 12:30   |   Subject: Rig and CB
Well I do think we want to pursue a single-stick marconi rig. It would get us on the water faster! Also wonder if the CB were made up of 2-9mm plys like the transom, then well WESTed, would that not be a satisfactory fit in the 25mm slot? The "lever" for raising could be metal straps mortised in perhaps? We need to know your additional fee for alterations. Thanks!
Posted: Aug 5th, 2011 at 12:12   |   Subject: Rig and CB
Yes I think ply laminates would be OK. We need to be sure the ply keeps straight and flat. I think just two 9mm would do this OK. Ply does tend not to stay flat. It might be better to use 5 off 4mm laminates, which would in theory produce 20mm. Or 4 off 4mm, plus a centre layer of 3mm = 19mm, which is about right for the 25mm gap in the centrecase.

I will draw up a marconi rig plan and spar plans. No additional fee – I will add these to the plans available. I will also draw the ply centreboard/ballast arrangement – pretty much as you suggest.
Posted: Aug 11th, 2011 at 17:36   |   Subject: transom construction
George, the 055 plans OK a ply transom made of 2 layers of 9mm. But suggested we ask for specific tips on how to do this. We're ready to go! Thanks
Posted: Aug 12th, 2011 at 20:19   |   Subject: Laminate Transom for 055
It is pretty straightforward

Get out the aft layer of 9mm ply to the aft face dimensions.

Similarly get out the fwd layer to the fwd layer dimensions.

Clean up the edges of both layers carefully square.

Lay the fwd face flat on the floor or bench. Lay the aft layer on top of it and position so that the tip of the aft layer is 15.5mm above the tip of the fwd layer. I've posted a sketch and a link to a full size PDF on the blog (can't post HTML on the forum - my own doing!).

Around the sides, the aft layer edge is equidistant each side from the fwd layer edge. So it is central side to side and 15.5mm up from the tip.

Cramp the layers together. Bore off for three screws (one near the tip and one each near the sheer corner each side). Or you could bore 8mm dia holes and use short lengths of 8mm dowel rod. The purpose of either screws or dowel rod is to locate the two layers together in their correct relative positions, and keep them there while they are being bonded together.

This will give you a stepped bevel. Once the layers are bonded together, you will plane off the aft corner of the fwd layer so that the bevel on the fwd layer runs true to the aft corner of the aft layer. This will leave a little triangular gap at the fwd corner of the aft layer which we will fill with epoxy/coloidal silica as we bond the planks on (see the sketch).

To bond the layers together, wet out the bonding surfaces and bond with epoxy/microfibres as usual. Apply pressure with cramps and cramping bars (say 75 x 50 sort of size). Or you can use weights. Or a mix of cramps and weights.

Fit the fashion pieces and sternpost as per the basic instructions.


Posted: May 10th, 2012 at 13:05   |   Subject: THANKS FOR NEW RIG PLAN!
Thank you George for the Bermudian rig plan; I know that is a lot of work, but many builders will appreciate it. As for us, we moved last fall and my new workshop is still under construction. Our 055 is "on hold" for a bit, but hope to launch in '13.
Posted: May 10th, 2012 at 21:39   |   Subject: THANKS FOR NEW RIG PLAN!
Oh - I'm happy to have done it and several builders have already commented that they like the idea. It did produce a surprising amount of change.

Hope the new workshop goes well – I hate being without a workshop – our apartment is in fact what was my workshop (originally the garage and other odd bits) – I had to build a replacement – not unfortunately as big – before I could consider making the conversion!

Posted: Jul 7th, 2012 at 18:54   |   Subject: center of effort on Bermudian
George, I'm certainly no boat designer, but doesn't the center of effort need to be AFT of the centerboard (center of lateral resistance) so the boat will head up when the helm is released? Am I not reading sail plan properly??
Posted: Jul 8th, 2012 at 14:14   |   Subject: Center of effort on Bermudian
No - the CE leads the CLA. This is because the CE is not the centre of pressure on the sails and the CLA is not the centre of pressure on the hull!

They are both the geometric centres of area.

In reality the centre of pressure on the hull is ahead of the CLA. The centre of pressure of a hydrofoil is 25% of the chord length from the leading edge, whereas the CLA is at 50% of the area.

The centre of pressure of the sails is also ahead of the CE but is much more variable in it's relationship to the hull – because you move the sail plan as you square off to reach and run, compared with being close hauled.

Because the hydrofoil of the hull, keel, centreboard and rudder is a pretty complicated shape it is very difficult to work out where the actual centre of pressure is. So we use the CLA and then put the CE a distance ahead of it (called the "lead", as in leading) so that the actual centre of pressure of the hull will fall ahead of the CE so as to give the boat a small amount of weather helm.

The lead has been derived empirically over many years. It varies a bit depending on the rig and is expressed as a percentage of the LWL, as follows:

Sloops: 12% to 14%
Yawls: 8% to 11% (and only counting 50% of the mizzen area)
Ketches: 5% to 7% (again usually only counting 50% of the mizzen area)
Schooners: 5% to 7%

As with a mizzen, we usually only count 50% of the rudder area in the CLA calculation.

As you can see, these really are empirical figures, not scientific, and experience plays quite a large part in the calculation too! With a modern canoe-bodied, fin keel yacht it is possible to derive a more accurate idea of the actual centre of pressure of the hull, because we have a simpler shape to deal with and there has been quite a lot of experimental work done, which is why, mostly, modern yachts are quite well balanced.

Of course, with a small boat, the whole thing depends on where the people are sitting, both fore-and-aft to balance the boat, and athwartships to control the heel angle – so the centre of pressure of the hull is usually all over the place – as most dinghy sailers know instinctively – hence the need to move the crew about depending on the point of sailing.

This is because the centre of pressure of the hull is the centre of pressure of whatever bit of the hull happens to be immersed at any one time, which is only rarely the centre of pressure of the planform.

Posted: Jul 11th, 2012 at 14:00   |   Subject: rig balance
I'm clearly in over my head in my analysis. Thanks for the comprehensive response! The good news for me is that the Marconi sail plan is so good-looking.
Posted: Jul 12th, 2012 at 01:40   |   Subject: rig balance
Well just to confuse the issue even more, the only sailboat that I know of that had the CLA ahead of the CE was the schooner America – she had a negative lead of 1% – and look what she achieved:

Sailed across the Atlantic with the owner chained in his cabin for part of the voyage because it was so scary and the captain wouldn't reduce sail.

Raced around the Isle of Wight and won, carrying away what was to become the America's Cup, which despite numerous challenges Britain has never, ever, over the last 150+ years, managed to win back!

So go figure that!!