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Increasing mast rake in stronger winds

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Stefan Lloyd View Drop Down
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Post Options Post Options   Quote Stefan Lloyd Quote  Post ReplyReply Direct Link To This Post Posted: 14 Sep 04 at 7:23pm
Originally posted by Andi

I'm interested to know if any one has any useful web links regarding rig tension as it affects a dinghy in the form of a more rigid jib luff. I presume that the mid point of a slack jib luff will get pulled aft and to leeward by the wind forces in the sail.  My next statements are questions really as I'm thinking aloud:  The aft deflectiion will create fullness in the mid-height part of the jib - bad.

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.

 

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Andi View Drop Down
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Post Options Post Options   Quote Andi Quote  Post ReplyReply Direct Link To This Post Posted: 14 Sep 04 at 7:59pm

Interesting that the Merlins are convinced about raking as a means to depower. ( I wonder if you mean de-power or mean re-balance the helm? )  I'll have to look at their web site to see if they have any theories on WHY this is so.

- Andi

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jeffers View Drop Down
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Post Options Post Options   Quote jeffers Quote  Post ReplyReply Direct Link To This Post Posted: 15 Sep 04 at 8:28am

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



Edited by jeffers
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Post Options Post Options   Quote redback Quote  Post ReplyReply Direct Link To This Post Posted: 15 Sep 04 at 12:36pm

If you rake the mast back but maintain the same shroud tension you should find an increase in forestay tension and hence less jib luff sag.  This is theoretical and I'll check it ASAP.  Is this why the boat goes better in a blow with a raked mast? 

Here's another point, when you measure the rig tension do you measure the shroud or the forestay?  The forestay is awkward but that's where you should measure.  Next time you have the gauge on your boat you'll be able to notice that the shroud tension is higher than the forestay tension, that's because the forestay is at a wider angle to the mast.  This is not theoretical - I have measured it.

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Post Options Post Options   Quote redback Quote  Post ReplyReply Direct Link To This Post Posted: 15 Sep 04 at 12:56pm

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

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Post Options Post Options   Quote Barty Quote  Post ReplyReply Direct Link To This Post Posted: 15 Sep 04 at 2:42pm

Jeffers

Next time you see Mr Bowers ask him if he can explain in laymans terms why mast rake helps in a blow.  I have spent a good few hours (and pints) discussing this with him and as yet have been unable to do it.  4th corners are great but try and explain it without being technical e.g. "the centreboard stops you going sideways" etc

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Post Options Post Options   Quote Andi Quote  Post ReplyReply Direct Link To This Post Posted: 15 Sep 04 at 9:36pm

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

 

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Post Options Post Options   Quote Scooby_simon Quote  Post ReplyReply Direct Link To This Post Posted: 15 Sep 04 at 11:40pm

forstay tension stays the same because (I think, and it's late as I was up at 05:50)

 

1, Base of the mast is fixed

2, Base of the forestay is fixed

1 and 2 mean that the base of the forstay/deck/mast triangle is fixed

3, Mast is bent so (may) move back a litte

4, Mast is bent so (may) move down a little

5, Mast is bent so the 'back' of the triangle (may) have reduced slightly

3,4 and 5 mean that the top of the tringle (may) have moved a little back and down but may still be the same distance from the base of the forestay, thus no change

Shouds are forming a different triangle where 3,4 and 5 mean the distance from the shroud plate on the Hull (or hulls  ) has reduced.

 

QED  (I think)

 

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Post Options Post Options   Quote jeffers Quote  Post ReplyReply Direct Link To This Post Posted: 16 Sep 04 at 12:07pm

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



Edited by jeffers
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Barty View Drop Down
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Post Options Post Options   Quote Barty Quote  Post ReplyReply Direct Link To This Post Posted: 16 Sep 04 at 12:31pm

Jeffers

As I said in my previous message, I have discussed this with Adam.  I have the 4th corner/Jonny and the perfect hill T-shirt etc.  I know the three fundementals (as quoted by Aam) of a sail.  I have a degree in yacht design..........BUT and its a BIG BUT......I can't explain to a beginner why mast rake works without being technical.  Most other things like the example I quoted about the centreboard, are easy to explain without getting in cross-flows, deflection points etc.

Just a thing that amuses and drives me wild at the same time....nothing more!!!!

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