Rotating Mast Trim on Trimaran

[Bosun182]

Hey guys,

I'm a 24 years old Norwegian who has just bought a 1970 custom trimaran which I'm cruising Europe in. I love my boat already, but it is somewhat lacking in sail trim equipment so I'm constantly upgrading whenever I can afford to do so.

The boat has a rotating mast, and up until now I have simply fixed it in the middle with ropes. What I am wondering about is how does one normally trim a rotating mast? I was thinking about setting up a Cunningham, this would let the mast rotate with the boom, is this a standard way of doing things? That would also give me control of boom height, which would be great to have!

A second question, how much should I tighten the stays? Right now the mast is tilted 5-10 degrees to the aft of the boat, so the forestay is tight, while the sidestays are fairly loose. Under sail, the port stay on a starboard tack is very slack and vice versa, could this lead to issues with durability in the longer term?

And finally, the boat is 9.9m, weights 2.2 tonns, and have a mast 14m high from water. I'm considering to mount a 1.7m bowsprit and a 12.2m gennaker, would this be overkill on a boat with such low displacement?

Thanks in advance for answers and tips :-)

[Laidback]

Having sailed and raced a Newick design with a rotating mast - the best results are those that improve the flow of wind over the draft of the main sail. Generally this means turning the mast so that the air is directed onto to the forward 20% of that sail. Additional trimming of the wing mast, the main's shape and the main's sheet will be required. Not to ignore what effect the jib is having on wind flow.

[cal40john]

Well the only multihulls I've sailed with rotating masts were beach cats, but seems the bigger boats should be similar. To elaborate on the last reply, what you're aiming for is to rotate the mast such that the mast looks like a smooth continuation of the sail on the lee side. If you have a rig with no backstay (shrouds back at an angle), then aiming the rotater bar, or back of the mast at the shroud is a good starting point for upwind. Downwind rotate the mast forward of 90 degrees.

The way I've seen mast rotation control done is with a V shaped bar attached to the mast such that the point of the V is aft of the mast.

The picture at the bottom of this article has one near the bottom of the mast:

http://www.corsairmarine.com/UserFil...yIraHeller.pdf




John

[tsmwebb]

Feeling wonky? Try: Aerodynamics of Teardrop Wingmasts

Tom.

[David M]

With a rotating mast, you let it twist more to help flatten out the main. This is because with rotation, the mid part of the mast can bend more forward allowing the main to flatten more.

A mast is obviously not a circular shape, so its ability to bend changes with rotation. You have to imagine why a 2 x 4 bends more when its turned one way versus turned 90 degrees....that's the idea. This is done with racing cats when going to weather in high winds...you let the mast rotate quite a bit to allow the center of the mast to bow forward. A mast that is bowed forward at the middle allows the sail to change shape making it flatter. Creating a correct foil like leading edge is socondary. If you were to look down on a rotating mast that is rotated fully, it would be much the same shape as a jets slats extended all the way, such as when it is landing. In light airs, the shape would be more like a jets forward wing slats retracted all the way.

[cal40john]

Maybe it's a geography difference. You're in S.F. in mostly overpowered conditions, I'm in PNW in light air so I think the added drive of a properly rotated mast is important.

I think they're only doing this on beach cats, but there is a new way to depower. This doesn't necessarily work with older cat designs. The old way of over-rotating the mast requires mainsheet tension, so the leech is closed, the mast side on to the air flow makes for more resistance, and it bends forward towards the jib closing the slot a little.

The new way prebends the mast with raked diamond spreaders and lots of diamond wire tension. Now put a lot of purchase on your downhaul and use the luff of the sail to further bend the mast as you need to de-power. You leave the mast rotated at the point where it looks like it's part of the sail. The result is no increased drag due to over rotating the mast, the mast bends forward and to weather opening the slot, and the leech becomes slightly less tensioned as the tip moves back, allowing the top of the sail to twist off and depower up high where the lever arm is greatest. In extreme cases you can depower further by under-rotating the mast causing turbulence and loss of lift of the sail just behind the mast.

Some high end boats have led the downhaul to the skipper to adjust to keep the boat at the correct power and the mainsheet goes to the crew for major changes like preventing capsizes.


John

Originally Posted by David M

With a rotating mast, you let it twist more to help flatten out the main. This is because with rotation, the mid part of the mast can bend more forward allowing the main to flatten more.

A mast is obviously not a circular shape, so its ability to bend changes with rotation. You have to imagine why a 2 x 4 bends more when its turned one way versus turned 90 degrees....that's the idea. This is done with racing cats when going to weather in high winds...you let the mast rotate quite a bit to allow the center of the mast to bow forward. A mast that is bowed forward at the middle allows the sail to change shape making it flatter. Creating a correct foil like leading edge is socondary. If you were to look down on a rotating mast that is rotated fully, it would be much the same shape as a jets slats extended all the way, such as when it is landing. In light airs, the shape would be more like a jets forward wing slats retracted all the way.

[tsmwebb]

Originally Posted by David M

With a rotating mast, you let it twist more to help flatten out the main. ... Creating a correct foil like leading edge is socondary.

The particular rig and scale play a role here. It might help to know what the OP's mast is like (cord length, thickness, stiffness &c). Generally, I think that the motivation for rotating masts is aerodynamic. The ideal rotation of small cord masts is large and will certainly effect the moment and sails cut for small masts will have more shape to play with. But, the mast's rotation will also enhance the flow on the lee side of the sail. And, there are plenty of very stiff wing masts out there particularly as you get into bigger, faster boats. At 9.9 meters the OP's boat may not tune like a H16... In any case, keeping it centered will not be ideal. Rotating to get a fair curve and/or mostly flowing telltails on the lee side might be a good first cut. Keeping an eye on the speedo while fiddling can provide good feedback too.

Tom.

[DaveOnCudjoe]

Bosun182,
I'd go to the Yahoo groups site for F-boats and ask those guys. Most of the F-31's have rotating rigs and those guys have learned how to get the most out of them. Dave

[eyschulman]

The amount or rotation varies with wind and rig particulars so I wont address here but I will caution you to have some kind of positive control on mast to stop wild gyration in severe conditions since this can cause rig failure. A common method to control rotation is to have a fulcrum arm attached to base of mast with control lines.

[Laidback]

Here is a picture of the controls on a Newick TRIMARAN's rotating mast.
NB - this was not a bendy mast.
One day when the wind piped up to 40+ took all sails down and used the mast alone to power the 15 miles home.

[David M]

On racing cats we called it a mast rotation tiller. Its purpose is to control the amount of rotation. Without it, with any mainsheet tension at all, the mast would rotate all the way which was not necessarily what you wanted, especially in light air. The diamond stays had some slack so the mast could bend some but not so much as to allow it to break.