triming sails if different wind speeds

[jez33]

i have a 11ft comet sailing dingy which
i can sail quite well my self now but need a tad of advice about trimming the sails. ive read a few books to get the info i want but i find that there are 2 or 3 opinions..

so heres the questions.
on calm days ( 5mh or under) should the sail be quite flat?one of the books i have recommends flat which i think might be because it suggests the wind doesn't have much power in it so a flat sail will excelerate the boat well but not get a fast pace ...

the other book i have says that on very calm days the sail needs to be almost the opposite to flat. mainly to get the draft to be 40-45 % back from the leading edge and have a thick draft depth..

i assume as the wind gets faster then the draft depth could be made less and also brought forward..

any info would be helpful

 

[Peterduck]

The second version is the correct one. In light weather ease the foot outhaul on the boom and allow the sail develop a 'belly'. When the wind hardens, get the sail as flat as you can.
Peter.

 

[Seagreen]

Ditto that. Sails should be fuller in light airs.

 

[MarkGrubb]

Hi Jez33,

I've also heard both and pondered the same questions. Certainly in light airs you need to put more belly in the sails. When I raced, we would sit on the leeward side to heel the boat over and help the sails take shape.

However, I've also heard that in really light airs its necessary to flatten them out to encourage air flow over the sail.

According to my North U Trim book in light airs you want deep sail shapes, however, it goes on to say that if the sail shape is too deep the flow of air across the sail will not hold. So it sounds like deep is great, but as the wind gets even lighter, at some point you will need to start flattening again. I guess if varies from boat to boat.

My advice. Put some telltales on your mainsail and genoa at the luff (it probably already has them), but also put two (or three if you like) equally spaced apart between the luff and leach, then at the leach. Do this at three different heights. You can make the telltales out of wool and stick them on, or buy them.

You will then get a good idea of flow across the height and width of the sail. Then go sailing in light airs and see how different sail shapese affect the flow by seeing if the tell tales stream aft (good) or flick about (bad). Make lots of notes.

And please come back and tell me what you found .

 

[jez33]

hi ,
wow 3 ansers and all have the same opinion,unlike my books lol..

funny you say about telltails AvastMark , i have them and used the boat today ( before reading your anwers) i noticed the leeward side telltails were streaming aft as they should but the windward ones werent doing much at all.. is that maybe because i the wind was very calm ??
i dont think it reached much over 5 mph wind speed

 

[Seagreen]

If you really want to see whats happening to your jib and also your main, you can do this with, as the previous poster suggests, putting short lengths of contrasting polyester wool (so you can see it against the white sails) through the sail, stuck both sides with little dots of sticky number cloth. Any decent sail loft or chandlery should have them.

Now then. You can tell when you get some decent laminar flow over both the outside of the sail, and the inside face, that the sail is drawing right for the wind speed. As the previous post suggests, three rows of three tell tales, spaced evenly over the main body of the sail. The wool on the inside face of the sail should lift only about 70% as much as the wool on the outside, but you must avoid the wool on the inside face hanging limp, or you will have no flow on the inside of the sail and the flow will have stalled. This is undesirable.

Go on, experiment. Actually it costs about a fiver and you'll learn a huge amount about sail trim.

However, most people on the Classic Boat forum are really more interested in single malt whisky and a nice sunset anchorage. Are you sure you shouldn't have posted this the YM forum? They are all rufty tufty racers, they are!

/forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif

 

[dombuckley]

You do not say whether you sail on the sea or inland waters.

At its most simplistic, in light airs full sails give you acceleration, while flat sails give you speed. THe fuller sail generates more power, but needs to be sheeted closer to the centreline, meaning that less of the power is converted into forward motion. On the sea, you need fuller sails, to push through any waves or chop: jib halliard tension slightly lower than normal, mainsail with slack outhaul, no kicker, cunningham on a fraction (to keep draught forward). On inland waters, you can concentrate on boatspeed and flat sails work much better: mainsail outhaul full on, no kicker, no cunningham, but the jib halliard slightly slackened on all points of sail apart from beating.

I can back this up with my own experience of racing on the non-tidal Thames: I would usually be in the top third of the fleet, but whenever there was a really flat day, my 50 year old GP14 would lap most of the fleet. We had a training day, when we were discussing sail shapes in light weather, and it turned out that I was the only one who set their sails flat in the light stuff. Suffice to say, after that I never had quite the same advantage - doh!

 

[jez33]

[ QUOTE ]
If you really want to see whats happening to your jib and also your main, you can do this with, as the previous poster suggests, putting short lengths of contrasting polyester wool (so you can see it against the white sails) through the sail, stuck both sides with little dots of sticky number cloth.

[/ QUOTE ]

they sound like telltails ! if they are then i have telltails on the main already. i dont have a jib on my sailing dingy as it only uses a main on the comet 11ft boat.

yer to answer the reply above, i sail both on lake and on sea shore.only trouble is my boat only has a few sail controls. ie, kicker, outhaul, and downhaul , and sheeting ofcause..

thanks for taking the time every one in replying to my questions. i am reading it and will give all the advice a try. just hope i can remember it all /forums/images/graemlins/wink.gif