Sail trim & VMG

[Silkie]

Usually beating singlehanded in anything of a breeze on a luxuryless pocket cruiser is far too involving to allow any kind of scientific sailing but I had the opportunity on a recent cruise to spend a couple of hours watching VMG while playing with sail trim and angle of attack and the results were quite surprising - to me anyway.

With sails trimmed to the best of my ability and all tell-tales flying my speed through the water was good but my VMG was not. Max VMG was attained with the wind quite a bit further forward so that the windward genoa tell-tales were well lifted. There was enough wind (not dead on the nose and therefore both a making tack and one with either a small positive or negative VMG) that I was just comfortable with full sail, the water was flat (Sound of Mull) and a fairly big tide was contrary.

I put this down to my ancient baggy sails but is there anything else I'm missing?

[jim.r]

bit of leebowing

[ash]

AArrrggghhhhh - you would think that I would learn - I spent ages composing a reply, then opened PhotoBucket in another tab without first doing a Ctl A, Ctl C on my text - CPU started running at 100% so closed PB tab - IE8 closed down the other tabs and my words were gone!

Don't think that I could recreate my masterpiece again!

the wind quite a bit further forward so that the leeward genoa tell-tales were well lifted.

Is this not contradictory?

Reading your post as a whole, I assumed that you found that sailing closer to the wind than the telltales would indicate, but presumably at a lower boat speed gave you a better VMG.

I found this on the www - http://www.oceansail.co.uk/Articles/VMGTable.pdf - the angles are from the rhumb line so we are likely to be looking at the 40 / 45 / 50 / 55 rows.

This page explains the table a bit better http://www.oceansail.co.uk/Articles/VMG ... ocId155930

If you create a waypoint in your GPS which is 100 miles or so from your position, and is positioned by bearing so that it is directly upwind then you can monitor VMG to windward on both tacks by using GO TO.

Ash

[aquaplane]

Are you saying that you get a better VMG by pinching into the wind or by bearing off a bit? The leeward telltales stalling seem to indicate that you are bearing off but not easing the sheets to suit.

On the Centaur pinching is counter productive, we just go sideways more, but that's bilge keels. We go better, and the compass and (GPS) track are closer, if we ease the sheets a little and sail a bit faster but tack through ~100°. I have not taken too much notice but I think we make nearer 5° leeway than 10°, it's hard to tell on Windermere as it never blows from the same direction for more than a minute.

This is our 4th season with the boat and the sails don't seem as stiff as they were when we got her but they seem to set nicely.

Going where the wind and tide takes me is a much better option.

[marisca]

Here is the polar for a Contessa 32 (allegedly)

Curves are for wind speed in knots. Angles and wind speeds are true.

If I understand it correctly, and I ain't at all certain, it shows e.g. at 8kts of wind my best VMG is at 50 degrees and the angle decreases as wind increases. All very good but my hi-tech instrument suite only gives true angle and speed.

[ash]

Here is the polar for a Contessa 32 (allegedly)

If I understand it correctly, and I ain't at all certain, it shows e.g. at 8kts of wind my best VMG is at 50 degrees

Did you get this information directly from the polar in one step, or did you use the boat speeds from the polar and a calculation or a table like the one in my link to determine that the best VMG is at 50 deg true.

Ash

Edit Worked it out for myself - the vertical scale is VMG (not boat speed as I had thought)

This is a link to a polar for the Vega but it doesn't show a range of wind speeds - http://albinvega.oddis.com/albin_vega_p ... agram.html

Ash

Edit 2 - I hadn't worked it out fully! So the curve is the boat speed (use the semicircle as the scale) at different headings to the true wind. Taking a horizontal line across to the vertical scale gives the VMG to windward at that heading. So for the Contessa, at 8 knts wind at 90 true, the boat speed is about 6.3 knts but the VMG is zero.

Ash

[marisca]

Straight from the diagram. As I understand it the highest point of the curve is best windward VMG and the lowest point, the best downwind VMG. So with 8kts true the best upwind angle is 50deg* and the best downwind 135deg.
One problem is I can't remember who I nicked the data from to generate this but it is CO32 specific. As to its overall accuracy, it seems to be about right in very smooth water though that's somewhat difficult to find with 20kts wind.

*The algebra gives one solution as an apparent wind 12kts at 30degrees with a boatspeed of 5kts and that feels about right.

[ash]

Usually beating singlehanded in anything of a breeze on a luxuryless pocket cruiser is far too involving to allow any kind of scientific sailing but I had the opportunity on a recent cruise to spend a couple of hours watching VMG while playing with sail trim and angle of attack and the results were quite surprising - to me anyway.

With sails trimmed to the best of my ability and all tell-tales flying my speed through the water was good but my VMG was not. Max VMG was attained with the wind quite a bit further forward so that the windward genoa tell-tales were well lifted. There was enough wind (not dead on the nose and therefore both a making tack and one with either a small positive or negative VMG) that I was just comfortable with full sail, the water was flat (Sound of Mull) and a fairly big tide was contrary.

I put this down to my ancient baggy sails but is there anything else I'm missing?

OK - that's better - so you 'pinched' up to windward a bit and found that your VMG ( to a destination?) improved. What happened to your speed? Both through the water and over the ground. Did it drop? By much?

Does VMG towards a destination which is not directly upwind provide any useful info? Surely only if you will make the destination without tacking otherwise you need to somehow average out the VMG on both tacks.

Ash

Edit - Thinking further - VMG to a destination must provide useful info - even for one leg. If I bear further off the wind (and off the rhumb line to the destination ) then the boat speed will increase but the proportion of the distance travelled which is actually getting me to my destination will reduce and the difference in VMG will depend on whether the increase in speed compensates for the reduction of useful distance travelled.

Ash

[mm5aho]

I've always found this difficult to understand, mainly because of the change in apparent wind direction caused by the diffeence in speed (wind and boat).
That's demonstrated by being able to point higher into the wind in a gust. That increased wind speed changes the apparent direction of the wind, enabling a diffent angle of attack into that wind.

[ash]

I've always found this difficult to understand, mainly because of the change in apparent wind direction caused by the diffeence in speed (wind and boat).
That's demonstrated by being able to point higher into the wind in a gust. That increased wind speed changes the apparent direction of the wind, enabling a diffent angle of attack into that wind.

It's easier if you scale draw it on paper.

Increasing the ratio of true wind speed to boat speed will bring the apparent wind aft towards the beam so you can turn into it.

Does anyone have an opinion on the idea that the wind veers during a gust so you'll get more 'lift' on Stb than on Port?

By the same token - does the wind back just before the gust?

Does the true wind speed drop just before the gust?

I've often noticed during those days when it is sunny but gusty that just before the gust hits that my current course is too high and that I need to bear away to keep the sails filled - this means that I need to turn further to get the correct wind angle during the gust. Is this all down to changes in apparent angle caused by changes in true wind speed in which case it would happen equally on both tacks or is there backing / veering involved?

What's the best way to deal with the lull / gusts? If I hold my course then I'll lose boat speed, the sails will rattle, and the gust will hit with a bit of a bang. If I bear away, then I keep up the boat speed but it will have more heel until I've steered on to the higher course. I tend to prefer the bear away, then steer up but the crew gets upset when I put the toe rail in the water! I tend to reduce the sail plan to suit the gusts which leaves me under canvassed in between.

Ash

[Silkie]

Apologies for the leeward windward error above but to drift for a moment..

Does anyone have an opinion on the idea that the wind veers during a gust so you'll get more 'lift' on Stb than on Port?

By the same token - does the wind back just before the gust?

Does the true wind speed drop just before the gust?

I've often noticed during those days when it is sunny but gusty that just before the gust hits that my current course is too high and that I need to bear away to keep the sails filled - this means that I need to turn further to get the correct wind angle during the gust. Is this all down to changes in apparent angle caused by changes in true wind speed in which case it would happen equally on both tacks or is there backing / veering involved?

What's the best way to deal with the lull / gusts? If I hold my course then I'll lose boat speed, the sails will rattle, and the gust will hit with a bit of a bang. If I bear away, then I keep up the boat speed but it will have more heel until I've steered on to the higher course. I tend to prefer the bear away, then steer up but the crew gets upset when I put the toe rail in the water! I tend to reduce the sail plan to suit the gusts which leaves me under canvassed in between.

Ash

I had exactly this scenario beating up the Firth of Lorne on Sunday into a very gusty F5, some sea and almost no tide but what there was was theoretically with me and against the wind. Wind was eastish and I was on starboard with 2 reefs and 4 rolls. The wind appeared to back significantly before the gusts luffing the genoa which then filled with a mast-shaking bang as I bore away and the gust hit. We weren't hard on the wind but about as close as a Hurley 22 can get in the conditions. Boat size is significant of course and what was a hard sail for me would have passed without comment on a 40 footer.

When I've already got 2 reefs in I ease the main in the gusts.

Amazingly we were going very fast (for 17' on the waterline) but it wasn't very relaxing with sails luffing and banging and leaches motoring.

[Silkie]

- so you 'pinched' up to windward a bit and found that your VMG ( to a destination?) improved. What happened to your speed? Both through the water and over the ground. Did it drop? By much?

Ash

Sadly, although I had leisure to observe, my cockpit data recorder was inoperative so I only have impressions rather than graphs.

It's difficult to be precise anyway because of the lag. As soon as you put the helm down the VMG increases dramatically and then gradually declines as boat speed falls off and vice versa. Nevertheless the effect is significant and this was confirmed by the tacking angles on the GPS track.

I was mostly wondering if I was being too simplistic in attributing this solely to the state of the sails. I was "windbowing" the tide the whole way frinstance.

[ubergeekian]

Does anyone have an opinion on the idea that the wind veers during a gust so you'll get more 'lift' on Stb than on Port?

It's theoretically sound. Ish.

The idea is that since the world is moving anticlockwise, viewed from above the north pole, surface winds in the northern hemisphere are dragged round with it and therefore tend to be a bit anticlockwise compared to upper winds. The thickness of the transition layer depends on the wind speed. In a gust the upper air effects get closer to the ground and you therefore get more of the upper wind direction, which is a bit veered.

This is well demonstrated at a lab level. However, I hae ma doots about it on a larger scale. The earth's rotation is only 15 degrees per hour, and that's not very much "twist" to give the air. I'm not convinced that ironing it out will make much difference.

That's a just a simple fluid mechanics model. There may be a meteorological one I'm missing.

Incidentally, did I post that I recently found a really good way of balancing performance on both tacks? It's called "sight up the mast and then adjust the shrouds so the top half of the mast is not curved off to one side. Now Jumblie handles and points the same on port and starboard. D'oh.

It did make me wonder, though ... the racing boys spend a lot of time fiddling with fore-and-aft mast bend, so why don't they adjust sideways bend too?