Increasing mast rake in stronger winds

I don't have such web links but I did once spend several hours tuning a keelboat rig and checking forestay tension with a gauge. On a keelboat, pointing ability and windward boatspeed are very related to jib sag, which in turn is determined by forestay tension. I found the relationship between rig tension, mast bend and forestay tension was more complicated than I thought it would be. On that boat at least, you needed a fairly straight mast to maximise forestay tension. If you allowed the mast to bend much, the compression reduced forestay tension. Therefore you could increase rig tension but decrease forestay tension, which I hadn't expected before I tried it.

My point is that there are complicated inter-relations in rig tune, which are often class-dependent. Therefore general tuning guides may make statements which are not true for your class.

Now relating my first paragraph to mast rake, I wonder if more rake allows you to use more rig tension to increase jib luff tension rather than increase mast bend. I'm really not sure. On my keelboat, the whole class used the maximum allowable rake anyway, so it wasn't a tuning variable. However I used to sail Merlin Rockets and there is no question whatsoever that in that class, rake is the most effective way to depower. The whole history of rig development in the class over the last 10-12 years has been to allow easier adjustment of rake. That is why the class has moved from hog-stepped to deck-stepped masts, despite the constructional difficulties this has caused.

I also think your suppositions on why excessive jib sag is bad are correct.

After having a 2 day training session run my none other than Adam Bowers he explained all about depowering (it was a windy weekend).

Basically the effect of raking the rig moves the centre of effort back (which means you need to move back down the boat). Also it helps to open the 4th corner of the sail (along with using cunningham) and allow the wind somewhere to exhaust without creating additional power.

For those who seem sceptical try it! You can easily see the 'flattening' of the leading edge of the sails but stand behind the boat and get someone to pull the cunningham hard on and just watch the 4th corner fall away. I can spot it on mt Laser quiet easily but it twns to be mor noticable on a boat with a full length top batten (such as a Fireball, Scorpion,. Werlin) as the batten just falls away.

On a boat with a jib you also need to make sure you reset the jib cars as this has a huge effect on the jib leech which in turn affects the slot. Basically a reasonable rule of thumb is up and in for light winds (gives you a very open leech) and out and down for heavy winds (closes the leech).

Just my 2p....

Paul

If it weren't confusing enough, there are several guages in use for measuring rig tension.  Probably the most popular is the Loos guage but that comes in 2 varieties - old and new.

If you follow this link on the Laser 4000 site you'll find a conversion chart on page 1.  I think we should say thanks to the guy who took the time to produce and publish it - so thanks.

http://laser4000.lasersailing.com/docs/Tuning_Guide.pdf

Before our wednesday evening race I had some time do measure some rig tensions.  On our RS200 340pounds on the shrouds gives a jib luff wire tension of about 270 pounds with our normal mast rake.  I didn't have time to alter the mast rake to test redbacks rake versus jib luff tension theory. But I did try putting full kicker on and re-measuring rig tension - the shrouds tension dropped by 70 pounds while the jib luff wire tension remained pretty much unchanged.  I can understand the drop in shroud tension - a bent mast is shorter than a straight one so the shroud tension is reduced.  I really am struggling to figure out why this doesn't lead to a corresponding reduction in jib luff wire tension.

By the way full mainsheet tension caused a similar reductioin in shroud tension - mast bend again I suppose.

RS200 officionados suggest that light crews should reduce rig tension in heavy weather so that the mid mast comes up to windward and opens the jib slot, they don't talk much about jib luff sag.

An american 505 website was very imformative and gave me the idea of observing luff sag by using elastic to tension the forestay and then gauge the gap between the forestay and jib luff.  Of course the wind will cause some curvature on the forestay but it seems an idea worth trying.

   - Andi

Barty,

I am unlikely to Mr Bowers until the next Fireball training event next year.

It is hard to explain the effect of cunningham and mast rake without getting a bit technical but here goes...

Basically if you rake your rig (forestay tension stays the same on a Fireball) then what happens is the wind start to travel up the sails as well as across (watch your jib tell tales, the bottom one is normally horizontal, the top one can be as high as 60 degrees from horizontal). The rake accentuates this effect allowing more wind (and with it the excess power) to effectively exhaust harmlessly out the top of the sails.

I am not going to try and explain 4th corners because I was so confused when it was explained to me. Best bet is to rig a boat and then stand behind it (with the sail sheeted in) and watch someone pull cunningham on and let it back off again, it is blindingly obvious, just make sure you are looking up near the top of the sail (top batten on a Fireball will give you a rough position).

If I get a chance this weekend I will try and get some photos becuase I am instructing and that is one area the people want to cover.

Paul