Sailing in Waves ....Again

I come back to this topic again having read a very good summary for the theme on Zim's Sailing Blog on Sailing In Waves http://zimsailing.blogspot.no/2013/04/tracking-upwind-in-waves.html about upwind sailing in the bumpier stuff.

I have read up on the topic, entered into much discussion on forums about it and blogged on my experiences and acrued knowledge, yet still there seemed to be some doubts in my mind.

Zim in their blog focus  on tracking up wind, and combine the two schools of thought if you like- active movement through the waves and tracking under power.

The two have often been discussed as even a little mutually exlusive. On the one side you want to have the boat set up for height in order to actively sail up onto the wave crest, and you want the jib draught well forward in order to see/saw your pointy end down and up without stalliing the jib. The other perspective is to progressively power the boat up and sail lower and faster to punch through the wave train and maintain tracking.

When does each method pay, and can you engage both?

Firstly let us set a bit of a scope here, we are talking about normal boats in displacement mode upwind, not higher performance craft. Within then this lies probably over three-quuarters of all sailing hours upwind full stop! However breaking it down into different boats is now important. Also when to engage active sailing versus the more passive, powered up tracking is an important.

We can start where ZIM takes issue with the problem - you want to avoid slamming down into the furrow from the top of the wave and killing the boat speed. In the US this is called hobby horsing apparently, and that is a good description because you are rocking without getting anywhere! So the first most important factor is wave size compared to boat size put that simply>

There comes a point when the wave height relative to the boat length means that sailing on a close hauled course will result in slamming. In an Optimist this will be pretty modest, 50 cm waves perhaps. At the same time a Super Maxi will be just gliding through such a relatively small chop, fully close hauled.   Wave legnth is also of note here, because a long, swell wavelength you would experience offshore will take a different methodology to tackle than short, inshore chop of the same nominal height. Wind against tide will soon build steeper waves in the course of a race even, and necessitate a different sailing style to avoid banging up and down, losing speed as the bow slams into the trough and gets stalled more by the next wave.

There is then a significant wave height, steepness, frequency (speed of wave train) and length which necessitates firstly falling off, powered up tracking and then eventually for all boats, as the sea gets big, active steering takes over.

The next consideration after size,  is type of boat and especially two factors - power to weight ratio and keel design.

A heavier boat will need more power to get over waves in the first place, although it will 'hold its way' to an extent that the momentum will punch through waves better than a light boat which may be stopped in its tracks. The trade off in a heavy boat is then that you can sail powered up, with more twist, on a slightly lower course most likely, will tolerating a good angle to the waves. VMG is optimised, in that the boat sails high enough to hold a constant speed  the waves are not slamming you at this angle, you are cutting them. Hence active sailing is dropped in favour of tracking lower and faster and responding to the wind shifts on the helm rather than the wave-train per se.

Also a boat with a longer keel will respond to being driven low and fast such that the attached flow on the keel is maintained and it functions well to help lift the boat to windward. Rapid, active steering manoerves in a wave train will shake this flow off the keel, especially if the keel becomes exposed to air as you ride the waves. The same is true of a boat where the big old spade rudder helps the boat lift to windward. Steering will unload this.

The converse then is true of a light sports boat or modern racing yacht, with a high power to weight ratio and short chord lengths on boat keel and rudder. The Platu Beneteau 25 responded very well to being see/sawed with an active helm  in waves above about a meter. The keel flies at a lower boat speed, so as you slow down it keeps flying, or if it does stall momentarilly on the face of the wave, then it can reattach flow as soon as the bow is borrne away off the crest. The rudder is pretty neutral and therefore balanced and can be used actively, with some prudence, to steer the boat up and down with each significant wave.

Is  here a synthesis between the two styles? well you just have to ask any serious laser sailor and they will tell you that the two styles are not mutually exclusive and that you may use both modii upwind or change between them at some point. The key factor here being that not all waves are equal - the old seventh wave, and the new rogue-wave theories hold true. A critical wave height for actively steering up and down the wave may come only on those more infrequent waves, otherwise you want to churn on.

Another factor of the wave-train comes into play here. Most often the wave-train does not follow the current DDW wind bearing. It is skew, for reasons a bit beyond the scope of this entry. In other words, if you sail at 45' close hauled to the wind, you will experience going  nose into the waves more on one tack, and the waves more on the beam on the other tack. So you need to be able to track fast on the gentler tack, while probably using active steering on the other tack.

Boat set up then needs more power relative for the wind, which usually means more twist and depth in the sails. For active sailing you will also need more 'knuckle' in the jib - the draught being further forward and allowing for a more varied angle to the wind while still powering.  Given though say the common scenario above , a skew wave train, then you are still free to make adjustments to the running rigging on each tack. Alternatively you may enter a tidal channel and suddenly meet critical wave height for active sailing.

On the softer tack you can then point a little higher and flatten your jib in to give a finer entry and more power. The sails can be less twisted and the slot is then a little narrower and more powered up. The jib entry is easily adjusted by slackening the halyard and tightening the sheet, less twist by moving the jib car forward and of course the main sail will be more easy to reduce twist on for a higher pointing modus, with better VMG. Backstay will then be a function of how tight the forestay and shrouds are, it must firstly make the rig stable enough to sail in what are still very oscillating conditions, and then take up the forestay sag enough for the wind strenght. Generally on most boats these happen long before it bends the topp of the mast and opens the leech of the main up.

In the course of a race then, you can indeed employ a combination of low/fast sail settings and active sailing, and change between the modes as the angle or critical size varies for the day. A lot can be done with the running rigging to shift gears, or rather make for one modus being chosen, even for very short time spans or even just luffing over a single seventh or rogue wave for the day.