Going to windward

[Danny Jo]

What you are reffering to is the difference between true wind and ground wind. Ground wind is almost irrelavent to sailing, it is the true wind (which as you rightly point out is affected by the tide) which is what the boat sees. I have never heard of anyone referencing tacking angles to ground wind, only ever true wind.

This is an effect we see racing, especially with cross currents. As the current builds (typically as you sail out into deep water) you will see a predictable wind shift. This is however not effecting the performance of the boat.

OK, expressed in those terms the lee bowing effect is mediated through an increase in "true" wind speed (as compared with the same wind blowing over slack water) which brings the apparent wind aft and allows you to sail closer to the wind.

 

[dt4134]

I reckon you're mistaken Danny and Flamings first post was correct.

I don't think Danny or Flaming (2nd post) are mistaken, or indeed that you are. It is just that there's a couple of different things being discussed.

I agree with you, with relevence to the OP's post, that using GPS COG in tidal waters to work out tacking angle is flawed. It's one of those rubbishIn->rubbishOut processes.

On the other hand there is a tide wind as Danny Jo says, which is equal and opposite to the tide. It's the difference between 'true' wind and ground wind in Flaming's terminology.

It's certainly not something to neglect when racing although it is usually subordinate to other factors, and is often a negligible factor, but just occasionally it can have a significant effect on a light airs race.

 

[Danny Jo]

I reckon you're mistaken Danny and Flamings first post was correct.. The tacking angle on the compass is the difference between the directions the boat points on both tacks. Now since the tide and wind are constants, so is the resultant wind felt by the boat ie the addition of the tide vector to the wind vector. Adding the boat speed into the equation still leaves the apparent wind identical on both tacks unless the boat is one like mine which naturally for some reason is a bit faster on one tack than on the other

Lee bowing comes into it because of leeway alone, but this isnt seen by the compass.

There are two quite separate issues here: whether lee-bowing can be said to exist or is merely a myth; and whether lee-bowing may usefully be taken into account when beating across, say, the English channel.

As regards the first, consider the special case of a tidal channel up which a "ground" wind is blowing at 12 knots from the south against a 2 knot tide running due south. The "true" wind speed, which as Flaming points out is defined the speed of the wind in relation to the water, is 14 knots. Now suppose that this wind is the product of a meteorologically stable pressure system and is still blowing at the same ground speed 6 hours later, at which time the tide is running north at 2 knots, producing a true wind speed of 10 knots. Leave aside the wisdom of trying to tack down a channel against wind and tide, switch off the GPS, ignore the transits on the shore and just compare the compass headings obtained when close hauled in 10 knots of wind with those achievable when close hauled in 14 knots of wind.

Put another way, trainee helmsmen are commonly taught to "luff with the puffs" and bear away in the lulls. Why is this? Because in puffs the apparent wind comes aft and enables one to sail further upwind.

So when people so confidently assert "she's good on the wind - tacks through a tad under 90 degrees" they are referring to the tacking angle in a good breeze, not what they can achieve in light winds.

As regards the second issue: why bother with lee bowing? If the same effect is experienced whichever tack you use, it is clearly not material to decision making. But consider now another scenario: you are racing against an identical boat over a twelve-hour beat in a steady northerly to a destination due north of you, with six hours of easterly-going tide and six hours of westerly-going tide. Unlike your rival, you choose the starboard tack while the tide is running east, and the port tack while the tide is running west. This gives you the advantage, because that constant northerly will be blowing across the water from a little east of north while you are on the starboard tack and a little west of north (relative to the water) while you are on the post tack, allowing you to sail in a more northerly direction.

 

[flaming]

There are two quite separate issues here: whether lee-bowing can be said to exist or is merely a myth; and whether lee-bowing may usefully be taken into account when beating across, say, the English channel.

As regards the first, consider the special case of a tidal channel up which a "ground" wind is blowing at 12 knots from the south against a 2 knot tide running due south. The "true" wind speed, which as Flaming points out is defined the speed of the wind in relation to the water, is 14 knots. Now suppose that this wind is the product of a meteorologically stable pressure system and is still blowing at the same ground speed 6 hours later, at which time the tide is running north at 2 knots, producing a true wind speed of 10 knots. Leave aside the wisdom of trying to tack down a channel against wind and tide, switch off the GPS, ignore the transits on the shore and just compare the compass headings obtained when close hauled in 10 knots of wind with those achievable when close hauled in 14 knots of wind.

Put another way, trainee helmsmen are commonly taught to "luff with the puffs" and bear away in the lulls. Why is this? Because in puffs the apparent wind comes aft and enables one to sail further upwind.

So when people so confidently assert "she's good on the wind - tacks through a tad under 90 degrees" they are referring to the tacking angle in a good breeze, not what they can achieve in light winds.

As regards the second issue: why bother with lee bowing? If the same effect is experienced whichever tack you use, it is clearly not material to decision making. But consider now another scenario: you are racing against an identical boat over a twelve-hour beat in a steady northerly to a destination due north of you, with six hours of easterly-going tide and six hours of westerly-going tide. Unlike your rival, you choose the starboard tack while the tide is running east, and the port tack while the tide is running west. This gives you the advantage, because that constant northerly will be blowing across the water from a little east of north while you are on the starboard tack and a little west of north (relative to the water) while you are on the post tack, allowing you to sail in a more northerly direction.

You're describing a good tidal stratergy, and the effect of the tide to shift the true wind. But, this is a shift so you would be knocked on the other tack. The tacking angle, defined as the difference in the compass heading on opposite tacks, will be the same at slack water and at full flow. In other words, if your tacking angle is 90, and the ground wind is from the north then you could do 045 degrees on port, and 315 on starboard at slack water. Now if the tide is going east at a velocity that gives a 10 degree shift to the east you'll only do 055 on port, but 325 on Starboard. That's still the same 90 degree tacking angle.

As to the difference in angles in different wind speeds, well of course. Our Polars tell us we should be doing about 45 degrees (to the true) in the light, dropping to about 37 degrees optimum, then rising as the waves take effect. But comparissons are not effected by the tide, as the wind you measure is the true wind, not the ground wind. So if my 37 degree angle is achieved in 15 knots it doesn't matter whether that's a 15 knot ground wind at slack, or a 17 knot breeze in line with a 2 knot tide, my compass angles will not have changed.
So given that you measure tacking angles as you tack, which in comparison with the tide is instant, you can completely ignore tide when comparing tacking angles between one sail in 15 kts, and the next one in the same breeze.

 

[Danny Jo]

Ah, the great myth of the Lee Bow surfaces again...

So given that you measure tacking angles as you tack, which in comparison with the tide is instant, you can completely ignore tide when comparing tacking angles between one sail in 15 kts, and the next one in the same breeze.

I concede the latter point, but not the former.

What we seem to have established is that the existence or otherwise of the lee bow effect is one of semantics, or perhaps of the conceptual framework one applies to sailing. I find it easier to conceptualise the lee-bowing effect simply as one which brings the apparent wind further abeam (in my favour) than to do a mental vector analysis on the effect of tides on true wind.

 

[Wakatere]

All,

Thanks for your suggestions.

I've looked at the rig this weekend (a pity I didn't get to use it!) and it's clearly all over the place. Nothing is really tight and the baby stay is noticably slack.

I guess I know what to do next!

Thanks

Charles

 

[Bosun Higgs]

What we seem to have established is that the existence or otherwise of the lee bow effect is one of semantics, or perhaps of the conceptual framework one applies to sailing. I find it easier to conceptualise the lee-bowing effect simply as one which brings the apparent wind further abeam (in my favour) than to do a mental vector analysis on the effect of tides on true wind.

I agree. Tacking angle has to be measured against the true wind seen by the boat and in tidal waters that isnt the same as the wind that would be seen on land. Its the addition of wind vector and tide vector if you want to look at it that way. It's then easy to see that sailing as close to this "sea wind" as you can will make one tack better than the other if your objective as directly up wind in "land wind" terms

I dont understand though why tacking angle should vary with wind speed assuming no change in sail configuration. The polar diagrams I've looked at show differences in the angle at which best vmg to windward is achieved, but thats not the same as tacking angle. Or is it?

Here is an interesting site that allows you to generate a polar for your own boat if like me you dont have one already.