Tacking is often a challenge with a multihull. The relative increase in windage of 2 or 3 hulls does not help, plus having at least two ‘long & slim’ hulls in the water that both really want to go straight. But let’s discuss how we might get around this.
First, with a trimaran, it’s important to keep the boat heeled so that you have ‘only 2 hulls’ in the water. When tacking, I think there’s an argument for having the ‘windward ama down’**, as it’s then on the inside of the turn and has less distance to go. That ama will then stay down after the turn as it will now be to leeward but one needs good ‘entry speed’ as the turn is sharper. As different boat designs have different turning abilities, it’s just another thing to experiment with. (**Racing dinghies often use this technique as when the crew come over to the high side and bring the boat up vertical, they briefly create extra pressure on the sails and that accelerates them forward on the new tack).
It’s important to get head-to-wind with still some forward motion, otherwise the rudder becomes useless as you pass through the eye of the wind. This means that the initial rudder motion needs to be gentle, or otherwise, it acts too much like a brake. Once the boat starts to turn, then the tiller can be moved over more aggressively, though preferably not over 40 degrees.
The mainsheet needs to be eased out a little (say 1.5m of line) by the time you are head-to-wind – otherwise, the mainsail will be trying to stop the turn to the new tack. At this point, I’d say, ‘slacken & forget it’ until you are really on the new tack. The sail to really concentrate on is the jib. When sailing the W17 alone, I start the turn and then slacken the main before facing forward and grabbing the TWO jib-sheets, one in each hand. I hold the previously sheeted one and watch the sail until the wind starts to blow on its reverse side. Unless it’s VERY rough, I then let that sheet go completely and immediately pull in the new leeward sheet as fast as possible, to get it tightly sheeted before the wind is back in the sail. (In most light to moderate conditions, the jib does not need to be backed at all). I then check my course and pull in the mainsail & boom to accelerate the boat back up to speed.
When it’s very rough, waves hitting on at least 2 (if not 3) hulls can really slow up the turn and if you don’t approach it with some momentum, it can be hard to get the bows around. It’s all too easy to forget that ANY rudder is useless if the boat is not moving … like trying to steer your car when it’s stopped in a parking lot ;) But if you make it into the wind and then start moving backwards, remember that the rudder can start to work again, but this time in reverse! You will have to very firmly hold on to the tiller though, as the bulk of the rudder area is now AFT of the pivot and will try to force the rudder hard over, but you only want a very moderate angle. And most important if sailing backwards … make CERTAIN that the mainsheet is really slackened off. Once fully turned say 40 degrees to the wind in the direction for the new tack, pull in the mainsail and off you’ll go. Tacking in waves is very difficult without a few sq.ft of foresail, so follow the guidance in the stormsail article for using just a small corner of the jib when sailing and tacking with either double reefs or the storm mainsail. See photo below.
I think it’s fair to mention that it is possible to design trimarans that turn very easily, but there’s a significant price to pay. Such boats have cut-away bows and more banana shaped amas. But unfortunately, these boats are slower, they pitch more and typically show more leeway than does the W17. Designing is always a question of trade-offs, and as the W17 generally tacks just fine, I am totally satisfied with the hull shapes that give extra performance, low pitching and remarkably high resistance to leeway.
Tight mainsheet & leech
The leech area should be firm and flat for at least the most rearward 25% of the sail. I personally believe in having the leech cloth doubled in that area in order to help achieve this and also resist stretch from the high vertical loading down the rear edge of a relatively high aspect ratio sail, especially when attached to a rigid wing mast that does not bend fore & aft, even in a high wind. In such a case, it’s the square-top of the mainsail that eases off in a gust.
Mainsail foot tension
I’ve already mentioned that ‘in my personal opinion’, most boats have too much tension on their boom outhauls. But then, what is required? What is enough and how can we figure this?
Here is one way. If we look at the table (towards the end) in Part 1, the Sail Camber proposed for say 30kts of wind is just 6% (a C/C (chord over camber) ratio of 16.7). But for only 5kts of wind, the proposed camber is 16% (a C/C ratio of 6.2). So now refer to this chart (published earlier with an article on ‘Adjusting sail fullness’ to show how much extra sail material is needed to give a certain chord-to-camber ratio). From this chart, we can see that we will need about 6.6% more foot length to achieve a C/C of 6.2 for 5kts of wind, but only 1.6% more length to achieve a C/C of 16.7 for 30kts wind.
As noted in the small print under the graph, this is for a parabolic curve so we can reduce the % values by 35% (or multiply by 0.65), for a more typical aerofoil shape sail.
So if the mainsail foot is initially pulled tight, we need to ease off about 1% [0.65 x1.6%] only to give a relatively small 6% camber for high wind. But in only 5kts of wind, we will need to ease off 4.3% [0.65 x 6.6%]. This indicates that ‘cranking up’ the foot ‘super tight’ is not justified. (But in a high wind, keep a check on the actual camber in case the sail stretches, requiring tighter adjustment). So to summarize this example; to adjust a mainsail foot from one suitable for a strong 30k wind to the camber required to more efficiently match a 5kt wind, one needs to slacken the outhaul 3.3% (4.3-1) of the foot length. For a W17, this equates to 3.3% of 2400mm, or moving the clew forward 80mm .… rather more than most sailors do I suspect. It might be useful to mark these two positions on your boom, for 5 and 30kt winds (almost full stretch) .. so that you have some reference points.
This will not significantly affect the leech tension as this is more directly dictated by the vertical mainsheet tension. The photo at the end of this article, shows a W17 sail sheeted down firmly but with the foot still only lightly tensioned, so I hope this encourages you to experiment with your foot tension and not give more than you really need to maintain a foot camber that matches the wind speed in that lower area.
This easing of the foot works best with sails that are loose footed. If your sail goes into a boom groove, try pulling it out for light weather and only attach the sail at the tack and clew. (One could argue that a loose-footed sail with a small piece of cloth going horizontally to the boom track, might be even more efficient by preventing loss of air under the sail - but that would be measured as illegal sail area for most class boats. Anyway, smoke tests have shown that air typically flows more up the sail than down, so I’d personally not consider this a major issue - unless future tests prove otherwise).
As a tight mainsail actually resists bearing off, it’s a good idea to ease out the mainsail significantly, before, or as, you bear off. This will require slackening first the mainsheet and then the traveler, being as the traveler will probably be reluctant to slide out unless the high mainsheet tension is first relieved. One can quickly sheet down the boom again, once the traveler is out. Also, bring your weight aft a little, as it both assures the rudder stays deep and also raises the bow slightly ... both helping the downwind turn.
Wing Masts, their control and adjustment
First, let me be totally upfront with readers and admit, that although I have over 30 years experience with rotating masts, I presently have only 6 seasons of regular sailing with a true wingmast. So if someone with the experience of Randy Smyth or Chris White ever contradicts what I say here, please listen to them ;) With that ‘out of the way’, let me share what my experience has been.
First of all, I am sold to-date on the advantages of a wing-mast, and am convinced that my boats sail more efficiently upwind because of their use. The most forward 12” of any sail is very critical when sailing upwind and that part can be better controlled with a rigid wing-mast rotated by a mast tiller that can precisely position the wing mast relative to the sail, for the most efficient result. But let’s first look at what sectional shape may serve the best.
Mast Section Shape
If we first look at the raw resistance of a wing mast compared to other basic shapes, we can certainly see the advantage of a wing mast as long as it's pointed into the airflow.
We are looking for a sectional shape that will primarily allow the soft sail to be in perfect continuity on the leeward side, so that air speed is maintained at the maximum. This will slightly lower the pressure (as pressure & velocity vary inversely) and also support a good quasi-jet flow off the leech. It’s interesting to note how the fore & aft chord of the mast, changes the required angle of rotation to achieve the same overall aerodynamic form with the soft sail. This diagram (once shared by aerodynamicist Tom Speer) gives values for the variations. Note that a regular rotating mast might be about 5% of the total sail, compared to say 10% for one of my wing-masts ... reducing the required rotation from 53 deg. to 35 deg. The latter is likely to be much closer to the on-coming direction of the apparent wind …. typically around 30 degrees for a well performing multihull.
Although theoretically the front edge could be semi-elliptical, my own wingmasts have a more radiused nose, as one can argue that due to constant variations of apparent wind direction, a wider, more radiused nose will ‘overall’ be less likely to stall than a finer, more elliptical nose that’s no longer pointing directly into the on-coming air. The side wings of the mast will have a very slight curvature and lead to a track or bolt-rope tube that’s as close as ‘physically practical’ to the wing wall for the smoothest flow. For this reason, the bolt-rope tube is more efficient than a track, but it’s understood that many deep-sea cruisers feel more at ease with slides that ‘may’ allow the sail to come down faster. I say ‘may’ as some slides are even worse than a good rope-tube fit in my experience. Depends as always on the design and quality – and resulting cost. Fully battened sails with luffs over 10m are often fitted with batt-cars to get the sail down more easily.
Certainly for small tris (under 10m), I am quite at ease with a rope tube, as long as the bolt-rope itself is fairly stiff. As I also recommend and use a rotating boom, this gets rid of the projecting slides that can foul on various lines around the mast.
The last dimension of significance is the depth or ‘chord’ of the mast itself. As you will see in another article on wingmasts (Wing Masts on Multihulls), I do not personally recommend a chord in excess of 3 times the mast width as then the side area can become hazardous in very high winds. (But see the referenced article for ways to control this). IF it were practical to only have the mast supported at the top so that it could have full rotation to weathercock, then the 3:1 limit could be increased to even 5:1 with further potential performance gain. (Randy Smyth has been playing with this on his light, experimental boat “Scissors” but at last check, it was not certain if this can be effectively ‘scaled up’ for larger craft).
Mast Angle Adjustment
Lighter winds require a sail with greater camber, meaning that the wing-mast will need more angle with the boats centerline, with less camber and mast angle needed in stronger winds. We can also see (from the above diagram) that the chord measure of the wing-mast significantly affects the required mast angle relative to the boats centerline with small masts needing greater rotation.
So with an auto-tacking mast-tiller that’s cleated to the boom (as used on the W17), the tiller will be tightly cleated in high winds and far less so in light ones.
When sailing directly downwind, the relative mast angle can be eased off to advantage, as then the mast can act as a vertical fence to slow the loss of air off the forward edge of the sail, though this may happen seldom, as most multihulls are more efficiently sailed downwind on broad reaches, by tacking and gybing.
While a wingmast can add control to the camber of a mainsail, only jibs with some form of boom offer control of the camber for the lower part of say a blade jib of less than 100% the foretriangle. Being someone who thrives on sailing efficiency, I searched for a way to solve this. This came from an experience back in the 1980’s when I was racing sailboards. Here is the background story if you're interested.
Back in the 1980’s, it was hard to not notice the large flat area of the standard ‘windsurfer’ sail below the wishbone, which resulted from the outhaul tension required to achieve a straight, flat leech area. In high winds, this was ‘a no-issue’ as the wind deformed the sail enough to give some camber. But in low winds, the foot of the sail was really ‘straight’, giving a flat sail low down with virtually no camber at all. So as an experiment, I rigged an adjustable line from the boom-end to the mast foot and by tensioning this, I could induce whatever camber I felt was appropriate for the lower part of the sail, without affecting the required tension down the leech. My sailboarding mates simply called it ‘mike's line’.
One summer soon afterwards, I entered a series of 6 sailboard races over a period of 2 days and despite the very light winds for the whole weekend, the regatta went ahead as planned. So I rigged ‘my line’ and proceeded to race. After winning the first race by a significant margin, I started to realize just how efficient my sail had now become, with this increased draft low down. But after winning the first 5 races, competitors started talking. (I knew this had little to do with any personal skill). They debated, ‘was my board faster than theirs or was it my sail?’ (They had not noticed the small line at the foot that I had cast loose to come ashore).
So for the last race, I decided to have a little fun. I had taken the series anyway so had nothing to lose. To the two sailors finishing just behind me, I said “look, one of you take my board and the other take my sail and I’ll make up a rig from whatever you don’t use”. So they were happy, and eager to beat me for at least the last race. So I cobbled together a so-so board and sail which were really not too great, but I again rigged a ‘mikes line’ to take tension off the foot of the sail. But it was same result, as I again finished ahead of both. This totally convinced me that this line was indeed a great addition for light weather. (However, on a sailboard, a tape might be better as the small rope did cut into my shin!)
Sorry for the long story, but figured you deserved to understand the history ;)
But now, on my W17, I started to notice the same issue with the lower part of the jib so I rigged a similar line from the tack to the clew of the jib in light weather and measured the result. From what I could tell using my GPS, it gave me (on average) about 0.3k to 0.4k more speed in very light airs.
Now, that may not seem like much, but it’s actually huge, representing at least a 10% increase.
So after sailing 2 miles upwind, this would place me about 1000 ft ahead of a boat without this line, whose name I had now shortened to a ‘mikelin’ (and most easily pronounced ‘mik-a-lin’).
These images show two different ways to rig a mikelin to achieve the same result, and its effect on the foot camber. On the left is a loose foot-line with a central tension line to a cleat on the weather side, while on the right, is a sketch of the two options, with a direct adjustable line, between the tack thimble and the jib clew shown at the top.
Once the jib sheets are released, the mikelin goes slack and just rolls up on the furler without problem. No, I do not rig it or need it in winds over about 8kts but it sure helps in the lower ones. Readers should try it and it should not be long before serious light-wind racers catch on to its advantage. In future, I plan to make new blade jibs for my W17 & W22 with a tab extending 25mm downwards at the clew, so that an attached mikelin does not foul with the foot when tacking.
Now the jib clew can be sheeted quite firmly at the right sheeting angle and also apply vertical load down the leech to keep that as flat as needed …, yet the lower 1/3 of the sail now has the ideal camber for these very light conditions. This mikelin is useful up to about 8kts of wind but above that, there’s typically enough wind force to create the 12% camber needed. The photo above shows it working to give a far better camber to the lower part of the jib than we would normally see, and the difference in performance in low wind conditions, is quite measurable on a GPS, as noted above. Just a note though. The mikelin is less effective with a genoa as the longer foot automatically gives more camber than a shorter blade jib.
I set mine up with some loops so it can easily be tensioned and tied off, so that when the jib clew is pulled tight towards the rear, the foot has the right camber for the wind condition.
This final photo, shows how efficiently the mainsail can set behind a rigid wingmast with the right rotation. This mast was the prototype for the DIY carbon fiber wingmast design that I now have available here: CF WingMast Manual
(Your comments, personal tips and questions are always welcome ..
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