Hi All,

Sailing in 12-15 Knots on the weekend, I had a mast rotation malfunction just as I had cranked things on for a start.
As a result, the mast went all the way round. I was out on the wire and didn't want to slow down so I thought I'd
resolve the problem on the next tack. I didn't have any trouble with holding the boat down but seemed to have
developed a LOT more speed relative to the boats I sail against regularly and I was still able to point as high
as any of them.<img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" /> I generally set the spanner to point at the forward edge of the dagger board case which seems to be
the usual on a Mozzie and when I reset it after tacking, the new found speed was gone. This has left me a little
confused on the theory of mast rotation. I would have thought that the shape created would have provided increased
acceleration but would not have been so good for speed.??

I'm intending to have a good play with rotation next time I'm out but I thought it might be a good subject for
general discussion.

Cheers

Hi Simon

Never assume there is a "normal" case. You have to experiment with these kind of adjustments to see what happens - as you unintentionally did. Now you've learnt something to your advantage - thats great.

Every mast, sail and diamond setup is going to behave differently, so unfortunately you need to figure out what will make yours go.

You can start by looking at the sail on dry land as you gradually increase downhaul and mainsheet tension and swing the rotation lever around to see what effect it's having on the sail.

Lever pointing at the centreboards is certainly not a lot of rotation so don't be scared to try more (or less).

Thanks for the comments Tim.

Maybe I have accidently stumbled on the "Turbo" button for my particular sail/rig. I shall do some testing of various positions. It's a good point you make about not assuming.

Now, if I can just find the "press now for good start" and "Auto perfect Tack" buttons, I'll be on my way.<img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Cheers

Simon,
You were lucky, the last few mast spanner failures I have heard of have resulted in a bent mast or total failure.
Mast rotation is a good thing but, be a little careful with the extremes because as you rotate the mast you also increase the load on the diamond wire and spreader arms.
As for the perfect tack button when you find it can you let us know
Peter

Simon,

Any turbo button is a good button. <img src="http://www.catsailor.com/forums/images/graemlins/cool.gif" alt="" />

The extra rotation would have provided an increase in drive force (speed) and moved the drive further forward (less weather helm).

Maybe your sail/mast combination doesn't have too much bend either with varying amounts of downhaul.

Worth playing around with in light to medium wind strength but leave it alone in strong winds. <img src="http://www.catsailor.com/forums/images/graemlins/smirk.gif" alt="" /> As Peter says, you would potentially risk mast failure or mast bend and it would be way too powerful. <img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" />

Regards,

Peter <img src="http://www.catsailor.com/forums/images/graemlins/tongue.gif" alt="" />

Both Peters,

Thanks for the advice re braking a mast... I shall tread carefully.<img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" />

I think I've managed to break at least one of most things this season <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" /> so I'll try not adding the mast to the list. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Cheers

I have a very "flat" (ie. nearly no rake) fixed spreader and the spanner malfunctions result in an inverted mast. Do the higher raked spreaders make this less of a problem?

Nick,

A set of raked spreaders would definitely help avoid an inverted mast. <img src="http://www.catsailor.com/forums/images/graemlins/cool.gif" alt="" />

Raked spreaders induce prebend since the force is applied towards the rear vertical edge of the mast. If your mainsail is a little fuller than desired it would also help flatten the sail and open the leech.

An inverted mast creates a fuller sail but tightens the leech making the yacht very powerful but also choking the airflow and dramatically reducing forward speed. <img src="http://www.catsailor.com/forums/images/graemlins/crazy.gif" alt="" />

Regards,

Peter

This sounds like the solution to my problem...well my sailing problem. Might be time to invest in some new spreaders.

Was out sailing yesterday and before the race i was playing with the mast rotation on the shore and watching how it affects the shape of the sail and i may have a theory as to the increase in your speed. From playing around with the mast rotation, i noticed that the more rotation i induced, the flatter the bottom of the sail got, which makes sense, the fullness of the bottom part of the sail is dependent on the distance between the where the clue is on the boom (outhaul) and the tack at the mast. So if the outhaul remains fixed, and you rotate the mast, your pulling the tack away from clew, lengthening that distance and reducing the depth of the camber in the bottom of the sail, which flattens the bottom of the sail and will give you less drag upwind, meaning a higher potential top speed.

As i was sailing on a flat lake in moderate winds i was trying to get a fairly flat sail, and i found that by rotating the mast a little more than usual i could get more speed out of the boat, slower to accelerate but faster top speed. I tried to apply this to tacking, in that as i came out of a tack and moved forward i would derotate the mast to get a fuller sail to get some acceleration on faster, and as i built up speed i could rotate the mast and allow the boat to reach higher max speeds upwind, not too fast at actually getting into position and pulling all the lines nice and quickly yet, but i think with practice and consequently speed, it could make a big difference.

What ive read about mast rotation from books like Cat racing: for the 90s seems to support this as well, more rotation means a flatter sail, flatter sail means less drag, less drag means greater top speeds. Quoting the aforementioned book: "The rule of thumb upwind is: For the flattest possible sail, rotate to 90 degrees, for a fuller sail, rotate only until the trailing edge of the mast points to the side stay..."

Anyone else with concording or conflicting experiences/opinions?

Michael.

Hey Michel,
Hows the spi going?

Yeah the spin is up! had a blast, used it in a race for the first time yesterday, still have a lot to learn and many questions to ask (ill save those for a more appropriate thread), first race i went out in a rush (arriving late as i live about 1 1/2 hrs away from where the race was held) i had forgotten to attach the pole stays for the spinnaker pole! Didnt realise until id hoisted the spinnaker and pulled the sheet on and the pole was 'Bending it like Beckham', so i clambered out onto the decks with a spare line and jerry rigged up a pole stay in between numerous jibes (the lake is shallow atm due to not much rainfall in the catchment, so its quite narrow), but it was all good fun. The boat seems to be taking the spinnaker very well, i put a high bridle on it even though it probably doesnt need it, and despite guarantees from some sailors at HYC that my boat would never be able to take the spinnaker forces after what theyd witnessed happening to ncik's boat, she seems to be just fine, ive even hoisted it in 20 knott winds with 1m swell, which was .... interesting, the boat seems to be handling the kite very well, just needs a sailor who can handle the kite as well as the boat can now mind you ive taken it out more than 10 times now in nothing under 10 knotts and i havent pitchpoled or capsized yet, mustnt be working it hard enough

Well that's one approach

I always recommend starting with the spinnaker in 10 knots or less and work up gradually. But I guess you can do it in reverse! As you go out in progressively lighter winds you will find the spinnaker makes a bigger difference to your speed around the course.

Good work.

Well ive been trying to do exactly that, but everytime we get a forecast of under 10 knotts, we end up with a 30+ degree day which brings on 15+ knott sea breazes, great for hull flying but not so great when you're trying to learn the ropes of spinnaker handling.