International Mirror Class Association of Tasmania.


Toll Tasmania: Supporting Tasmanian Mirror Sailors

 

 

 

 

Subject: RE Mast efficiency

Hi Norm,

 The Wolfson Unit for Marine Technology and Industrial Aerodynamics  (http://www.wumtia.soton.ac.uk/ ) is part of the Research Institute for Industry in the School of Engineering Sciences at the University of Southampton here in the UK. The Unit operates a consultancy service in, ship design, yacht design, small craft design, naval architecture, marine technology and industrial aerodynamics. It has a number of towing tanks, wind tunnels, a water tunnel and other research facilities.

Ian Campbell is one of their very experienced researchers and has worked on, amoungst other things, research programs for some of the syndicates in several recent America's Cup campaigns. He is ideally placed to comment as he also used to race Mirror dinghies with his children over a number of years at Spinnaker SC. He built Mirror 69667 "Bold Forester" from a kit in 1995, winning the UK Eastern Area championship that year and qualifying for the UK team sailing at the 1995 World Championships in Pembroke. I was Ian's local measurer during this time so I know him quite well. In 2001 I purchased "Bold Forester" from him and campaigned it at the 2001 and 2005 Worlds.

It's interesting that you Aussies don't try and get your gaffs to rotate ! In one message Martin Grose says "... holds the gaff hard against the mast .... and eliminates gaff rotation" as if that was a good thing. My view is that gaff rotation is a key factor in an efficient gunter set up. I can't speak for everybody in the UK, but I go to considerable lengths to get the gaff to rotate. I'm not alone, in his book "Mirror Sailing" Guy Wilkins says (page 32) "...A certain amount of movement does not seem detrimental as it allows the gaff to rotate towards the wind, but don't allow it to drop back...". I think the UK "gaff rotater" par excellence was Malcombe Goodwin whose son Clive won the Worlds in 1995. Malcombe put a lot of thought into the shape of the back of the mast around the sheave which is a crucial factor in getting the gaff to rotate.

I'm attaching a couple of photographs of "Bold Forester" sailing at the 2004 UK Nationals. In  photo 6358 you can see the amount of gaff rotation achieved on a broad reach/run. If you look carefully, just above the jaws you can see just see the mast (black) appearing to the left of the gaff. That demonstrates how far the gaff has rotated. I estimate that it is at a similar angle to the the boom (i.e. in excess of 60º to the centreline). At this angle the gaff is probably fouling the shrouds at the top of the mast. Left free to rotate, and in the absence of any leech tension, the angle of the gaff is determined by the summing the forces imparted by the sail local to luff groove on the gaff over the length of the luff groove. As leech tension increases the angle of the gaff reduces and tends towards the angle of the boom. In photo 6282 you can see the same boat upwind, the gaff is close to the back of the mast. I hesitate to blow my own trumpet, but I believe this gaff, one of  four designed and built by me, represents the state of the art in Mirror gaff design. This is the benchmark with which I am comparing the Bermuda rig. What you appear to be comparing it with in Australia sounds less than optimal. I think this is one main reason why we differ in view on the relative performances of the gunter rig and the Bermuda rig.

A few other points:

 

I disagree with you when you say  "...direction of wind flow – approximately 40 to 45 degrees upwind...". It's the apparent wind angle, not the true wind angle, which should be used when considering the air flow. When sailing, the apparent wind angle is shown by the burgee or windex. Bethwaite (page 199) gives a figure of 25º to 30º apparent wind angle for a yacht tacking through 80º to 90º, so I would suggest a figure of around 30º for a Mirror sailing upwind. Below the top of the gunter mast, the flow direction will be modified by the presence of the jib. I think it will be approximately parallel to the jib (i.e. less than 30º). Regarding the lacing gap, as I've said before, if the air in the seperation bubbles is stagnant, then I would expect it to be at atmospheric pressure, so I can't see where a cross flow large enough to have a significant effect on the efficiency is going to come from. Given that changing rig is a unlikely to be preformance netural, I feel duty bound to ensure that a further, unnecessary advantage is not handed over to the Bermudan rig. Here in the UK every book and Mirror tuning suggests having the mast step as far forward as possible, the centreboard as far aft as possible and the rig raked as much as possible without excess weather helm. As I've laid out in the comments on the proposal, with a Bermudan rig mast cross section, the mainsail will be about 25mm nearer the front edge of the spar. The same effect as would be achieved by moving the gunter mast step forward by 25mm. This would allow you to sail with more rake for the same helm balance. You say the performance change will be small compared that produced by the rig change. You maybe right, but by moving the Bermudan mast position back by 25mm we ensure the sail plan does not move, and thus should eleminate the possibility of a performance difference due to rake. It's a simple change and demonstrates that we are taking what steps we can to eliminate a potential performance difference.

 

Regards

Martin Egan

IMCA Convener of Rules & Chairman of Rules and Technical Committee


Picture 6282

Picture 6358