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I would like to post a question that has been on my mind for some time.
Why do cat masts rotate ie they are not fixed fore and aft as most dinghy masts are? My thoughts are that by controlling the rotation, an efficient wing mast type effect is created. Also, the amount of mast bend can be controlled by directing the mainsheet/leech tension to the stiff fore and aft section or more flexible sideways section.
If the above is right, why don't we have some form of control, perhaps like Dart 18s (or whatever they are called). Moreover, would the new DX rig benefit from this.
Perhaps someone could point me at some literature where I could read more about cat rigs. I have just read Bob Fisher and Reg White's book on cats from the 60s where I could find no mention.
Regards
Robin Boardman
1078 & 1758
PS If this doesn't generate a comment from George Stephens then I am one hat down
Well so as not to disappoint Andrew and as I cannot get George's link to open these are my thoughts:
1) Mast rotation. If the mast rotates and the mast forms the leading edge of the aerofoil section you get a better and more efficient shape. The feature caught on first on catamarans but is now being used on quite a few dinghies as well. I think it was first used on catamarans due to the size and power of the rig. This means that the mast has to be much stronger and is therefor a much bigger section. With these big section masts there is more to be gained by letting it rotate.
2) The spanner line is the means by which mast rotation can be controlled and limited. It is standard on most big Cats. On the Sprint 15 we have no control other than the main sheet tension and the downhaul tension, which is not as good (but simple). I suspect that this was a simplification as the original Spark was intended as a fun sailing boat. As it happens it works fairly well and as we are a one design it is the same for everyone. As the sail power (size) increases the force causing the mast to rotate increases (especially in strong winds) and so the benefit in controlling it with a spanner line increases. I have heard it said that it the DX rig probably needs a spanner line to improve it's performance to windward, which is probable limited by too much mast rotation.
I'll be interested to hear what others have to say..........
Cheers
Bob
Thanks for the link George, which by the way was missing a full stop so should have been http://www.sailns14.org/tech.aspx . I now understand a lot more about rotating masts.
Am I right in thinking that the Sprint15 mast over over-rotates, not pointing into the apparent wind but more at the shroud? If it was fitted with a mast spanner, when maximum power was required it would be locked more towards the fore and aft line and then even more towards the fore and aft line to depower. As it is, its over over-rotation will cause turbulence and hence drag. In addition, the mast will be bent more sideways although I am not sure how this will affect the rig.
Either way I shall be looking hard at the mast and its bend characteristics next time I am out. Wonder if I can rig up some kind of rotation control.
Sorry I missed the dot in the link! but your default search engine would probably correct it automatically.
From what I know, catamarans (including small ones) first started using wing masts because their wide beam could cope more readily with the forces and thus make use of the aerofoil effect, unlike a dinghy which has a much narrower beam. As Bob points out, technology has now allowed dinghies to make use of them. Also the 'Open' class of monohull viz. Volvo ocean race/Vendee Globe type boats have a wing mast but additional stays are used to cope with the additional forces.
As for the DX rig which Ray is developing, I note the interesting suppositions which I'll leave for Ray to respond to when he picks up this thread
The mast did rotate on Fireflies, but the precise way it and the boom rotated is different from the mast rotation seen on most cats. It was probably an improvement on no rotation at all, but less efficient than that seen on cats. Regrettably, I can't provide any web address for this. I think I read about it some years ago in an article in Y&Y.
As regards mast rotation on cats, more powerful cats usually have a more sophisticated mechanism which the crew can use to control the angle of the mast. Dart 18s use the spanner line - a rope of specific length which joins the spanner on the mast to the eye beside that used for the mainsheet (I believe the "spare eye" in the Sprint 15 mainsail is there merely because it was cheaper to use the same fitting as for Dart 18s...). The spanner line limits the extent to which the Dart 18 mast can rotate, but I think it provides only an approximation to the best angle of rotation over the range of conditions in which one sails.
Note that any spanner mechanism for a Sprint 15, even in the form of that used on a Dart 18, will require modification of the mast. Whether a Sprint 15 mast would cope with the weakening due to making holes etc and any additional strain from the spanner would seem to require the involvement of a marine engineer...... So fitting a spanner control for the DX sail would seem to involve more than a little expense for the DX sail sponsors...... and expense for every Sprinter who fancied becoming a DXer...... Would modified masts be out of Sprint 15 class?
One further thought - the shape of the Sprint 15 mainsail, in particular the curve in the luff, and the curve in the battens due to their varying flexibility along their length, will have (should have?) been designed to work with both the likely curve on the mast when it is stressed by the mainsheet and the downhaul AND with the significant degree of mast rotation that will occur. While the airflow past the mast across the windward side of the sail is probably more disrupted than desirable due to the large mast rotation, the airflow round the leeward side may be less bad than some seem to think - and isn't it the airflow round the back of the sail that matters more?
When I saw a DX sail at Queen Mary in February, the first thing I noticed was that the luff was pretty much straight. I was surprised that a modification to build in a curve had been left for so long. I gathered that a recut to test the effect of such a curve was being considered - I'd be interested to hear if that change has been made and has made a beneficial difference in peformance of the prototype DX sail.
No sign of a reply from George Stephens yet, so Robin might not have to eat his hat. I didn't know there was another Sprinter with a name so similar to mine!
Thanks for your reply George (and all others to this thread). Many apologies for having made your name plural, after all I should have remembered that there is only one George Stephen!
Your comments about mast rotation and the DX sail are indeed interesting. When I made my first post, I wondered whether mast rotation control was something that could be applied to a Sprint 15 however I now realise it is not so simple.
Just to let everyone know that I have as part of the DX Project been experimenting with a spanner line to improve upwind performance particularly in heavy breezes. I would like to keep my powder dry at this moment in time but am in the process of generating a detachable spanner line suiatable for our cat which requires no drilling of the mast.
Obviously any such device will be out of class but believe there is an advantage to be gained.
Did you ever have the DX luff recut to put a curve in it like that on a class standard sail? I still remember noticing the big difference on the sails we laid on the floor at Queen Mary in February.
For what it's worth, isn't there a curve in the luff of a Dart 18 sail, as well as the spanner line?
Do you intend to produce a demountable spanner that fits round the mast, with no change to the mast itself, so that the spanner could be readily changed to fit both Sprint 15 and DX "Class Rules".
