Spinnaker on a-class & hull strength/stiffness

[Martin Gibson]

I've been wondering about adding a spinnaker to my A-class for some time, but have been nervous about mast and hull strength.

My unscientific feeling is that the mast will be OK; it's strong enough to survive pitchpoling with me on the trapeze. Adding the spinnaker won't change the righting moment that I can apply, so provided I mount the spinnaker halyard near the hounds and so don't add bending loads in strange places, I think the boat will just fall over in lower windspeeds. At least that's the case for steady state loads. Not so sure about shock loading. Any thoughts from mast-builders?

The other area of concern is hull stiffness and strength. A-cats are built with two forestays running directly from the hulls to the hounds. If I do the traditional cat thing and mount the pole at the junction of a single forestay and the bridle wires, this dramatically changes the loading angle on forestay fittings at the hull. Question is, is this loading a major or minor addition to the overall loads the hulls see in standard configuration. Is there the prospect of snapping my hulls at the front beam, or ripping the forestay fitting out?

One solution to this is to use a pole a few inches/cm above the deck which changes the wire angle back to normal at the point it meets the hull. The Dart16's use this system - see the attached picture.


Any musings greatly appreciated.

Martin

[Brent B]

Modern fast boats normally use an asymmetric spinnaker flown from a pole on the centerline. I suggest you work out a centerline pole system and keep your current twin forestays. Actually there is sometimes an advantage with an asym to running the pole a bit to the windward side, and you could easily do this as long as you can swing it to the new side when you jibe.

Brent
Benson Sails

[Martin Gibson]

Interesting idea - hadn't thought of that. Since the twin forestays are further aft than a single forestay normally is, gybing ought to be easier.

There's no jib to clear, so the spinnaker tack could be further aft, and the pole would be shorter.

Still worried about pulling the bows together though. I know the Dart18's had this problem when they retrofitted spinnaker kits.

[CT 249]

Maybe you could go to the Catsailor F16 forum and contact Gary M. who goes by the name of "Altered" there. He converted a Flyer (IIRC) prototype A to a Formula 16 with spinnaker and it therefore probably your best bet.

[Brent B]

Originally Posted by Martin Gibson

Interesting idea - hadn't thought of that. Since the twin forestays are further aft than a single forestay normally is, gybing ought to be easier.

There's no jib to clear, so the spinnaker tack could be further aft, and the pole would be shorter.

Still worried about pulling the bows together though. I know the Dart18's had this problem when they retrofitted spinnaker kits.

_______________________________________________________
Downwind sailing unloads the forestay(s). Max forestay tension occurs while beating, largely due to leech tension in the mainsail.

Dart18 problem may be for some other reason.

Brent

[water addict]

Since there is no jib:
You could attach the inboard end of the pole to the base of the mast, run a topping lift from the end of the pole to the hounds or even lower on the mast since it is so lightly loaded, foreguy to the striker, and 2 lines from the pole end to each outboard end of the forward cross beam. Then the tips of the hull would not be loaded at all from the spin, and you'd cover all degrees of freedom. Since there is no jib you could even articulate the pole some by running the side "afterguys" to clam cleats.

[Brent B]

With no jib, you could tack an asym chute to the bow of the windward hull when you want to sail at deep angles. You would need a second tack line to pull it over to the other hull when jibing. Fun sailing with no pole required.

Brent
Benson Sails

[jam007]

I think that the Swedish A-Cat Association might be able to help.
See A-Cat Sweden Ass.
Håkan Fröjdh sailing with spinnaker.
Video: A-Cat with spi

Anders