Tuning your Impala
In sailing, as in life, there are seldom absolute answers, but more often several different compromise solutions to any given problem. This is particularly true of tuning the lmpala‘s rather basic swept back spreader rig. I frequently long for infinitely more controllable in-line spreaders with runners and checkstays, only to remember that they also give infinite possibilities to get the shape wrong and drop the rig over the side!
So lets examine these compromises with the help of Peter Kay of Parker and Kay Sails. However, first here’s what NOT to do.
DON’T over sheet the main or genoa
DON’T overtighten any halyards.
DON’T assume that what is right now will be correct In 2 minutes time. lf you are going slowly, ease one thing at a time until your speed improves.
Over to you, Peter.
The sailmaker’s job is to shape panels of flat fabric in such a way that, made up Into a sail and set flying in a breeze, the finished sail will flex and distort Into a fair and efficient aerofoil. The sail will usually take up its designed shape in the middle of its intended windspeed range.
Trimming is largely about manipulating the rig to compensate for wind speed related changes (cloth stretch, forestay sag, mast bend etc) so that the sail sets as closely as possible to the original designed shape. A well designed sail can then be trimmed so that its flying shape can deviate either slde of its designed shape to give good performance throughout its intended wind speed range, the main has to go from 0-30 knots and therefore needs to be capable of greater shape change than genoas with their more limited range of designed wind speeds.
Apparent wind 0-8 knots.
The aim is to create attached flow and keep it, so camber needs to be minimised. Genoa — using the
halyard, make sure the draft location at the top speed stripe is forward of 50%. It will be cut for some luff sag so a straight forestay will push the draft too far aft. At the bottom end of the range, sheet aft to flatten the foot and twist the sail. Slacken leech line if leech is hooked. Main – bend the mast to flatten and twist the sail. Outhaul flattens lower 3rd of sail + straightens exit.
Thanks Peter. To achieve this I want slack lowers so the caps induce prebend. lf the wind increases I‘ll be in trouble! I try to “unload” the forestay by taking the spare genoa halyard forward to the stem and winding hard (lt probably just stretches!). The genoa is sheeted aft in the lower range and the traveller ls all the way to windward, boom just below, and top batten parallel to the centre line. Backstay flattens and twists the top of the sail (hooray!) BUT tightens the forestay (groan!).
Apparent Wind 11-18 knots.
This wind range generates the least difference between the ”designed” and “flying” shapes. Depending on crew weight and wave conditions, maximum power can be induced; this means low twist and deep cambers. At the upper end of the range, efforts to keep the forestay straight will keep depth and draft location under control (Max depth 45% from luff at top stripe}. Tightening the halyard moves the draft forward. Increased wind speed shifts it aft. Move the Genoa lead forward to add camber, ease the main outhaul to deepen bottom 3rd of sail, sheet down the leech of main until speed drops off.
l want a straight mast and tight forestay for power sailing ie wind on lowers and caps. Traveller 1/3 to halfway up the track for pointing, drop it progressively for footing. As much mainsheet tension as the speedo will allow to maintain forestay tension. Just enough backstay to make top 3rd of main look right.
Apparent wind 18-24 knots
This is the designed wind range for the No 2 therefore few compromises are required for the headsail.
Keep draft at 43%, sheeting aft (and possibly wide) to flatten and twist at higher wind speeds. The rnast needs to be bent to “blade” the rnain, outhaul back on.
Here come the compromises! Hard lowers keep the mast and thus the forestay straight, resulting in a genoa near its designed shape but too full a main. Looser lowers flatten the main but the mast comes out of column, the forestay sags and the genoa looks horrid! Try LOTS of vang (induces low mast bend) a straightish mast and play sheet and backstay in the gusts. Remember to release the vang before you bear away!
Apparent wind 24-30 knots
This is the designed wind range for the No 3. Keep draft at 40% and the forestay straight. By now sail
trimming has become a much more dynamic process to cope with gusts. In an ideal world the sailing
forces and rig tensions should automatically cause the middle of the mast to come marginally to windward and the top fall off to lee, helping to further depower the bladed main.
Theoretically, very hard lowers should allow the mast to behave in the way that Peter describes. In
practice they put a reverse bend into the unloaded mast which seems awfully dangerous. J24′s sail with the lee shroud flapping loose which contradicts the advice from Kemps. In practice Menace goes very fast in a blow with a caps/lowers ratio of 39/37 and little or no caps flap. If you want to do well in these conditions you need a full crew of 6, all sitting out as far as legally possible, and preferably severely circumferentially challenged! At the top end of this range it seems sensible to reef as you should be able to maintain higher mainsheet tensions for a given wind strength, thus reducing excessive forestay sag.
Apparent wind 30+ knot!
ln these wind speeds I am convinced the boat goes fastest with a straight mast, a No 4 and several reefs.
Usually it is not until the race is almost over that l begin to understand a given set of conditions. lt might then be several weeks, months or even years before l experience those same conditions again.
By this time l have forgotten those hard earned lessons! The moral is two fold.
Measure everything, and write down your conclusions. After two years you will have a vast fund of knowledge to refer to and will be able to write a far more informed guide than this.
I look forward to it!
Phil Meakins & Peter Kay.