Gybe angle vs speed increase required

[Spencebox 5]

Once upon a time on a yacht we used to have VMG relative to zero and 180 degrees displayed most of the time. Very useful for knowing whether to sail in a high mode or slow mode upwind and downwind respectively. Also indespensible for knowing where correct sail crossovers are. It is fairly easy to know if you are on the shortest board to the next mark or not , but these type of crossovers require fast maths that you can leave to a machine and then concentrate on trim e.t.c.

[Pumbaa 9]

For the vmg argument, in days gone past when I've had boats with wind gear, vmg (true vmg not gps) is the one to use getting up or down wind when your target is within your boats dead zone.

 

As for the spinnaker/gennaker thing. We've had trouble hanging on to spinnakers but most of the time we're pulling away. Whether that's due to boat design, simplicity of use, larger sail area, sail cut or what, I'm sticking with it. Also I've only ever seen one other boat like ours and we've been quite heavily modified now so to get a fair comparison is almost impossible, even two boats of the same design can't occupy the same piece of water or be trimmed by the same crew.

 

The fact that the spinnaker is possibly faster in the middle wind range is a surprise to me. I would have thought that it should be gennakers in the lower wind speeds through to a middle cross over then spinnakers for the high wind strengths. We need a scientist with a wind tunnel.

[Guest]

"PULLING AWAY" Stick to sailing. The stuff you just wrote has me confused.

[Pumbaa 9]

"but if the wind is 30deg different to the course heading then you would be working a fast speed to one side og the course and then would have to get back to the mark."

 

Then you don't use vmg

[Guest]

Please explain

[Guest]

Am I enjoying this !!!

[Pumbaa 9]

If the mark lies within the dead zone of your boats tacking angle or too far ddw, if you have, for example a gennaker or you are sailing angles. The very thing you need to know are wind shifts. If the wind shifts so that you can lay the mark, you don't need vmg. If you don't have wind gear gps can help, but it is not as good as vmg because it can't take wind shifts into account. Unless you get it all hooked up, but i don't think we're talking about that. VMG tells you how fast you are getting downwind or upwind, not to the mark. Every body who built these devices assumes that the person who uses it is trying to get to a position that is either dead down wind or within their boats tacking shadow upwind.

[Pumbaa 9]

Am I enjoying this !!!

 

[Guest]

If the mark lies within the dead zone of your boats tacking angle or too far ddw, if you have, for example a gennaker or you are sailing angles. The very thing you need to know are wind shifts. If the wind shifts so that you can lay the mark, you don't need vmg. If you don't have wind gear gps can help, but it is not as good as vmg because it can't take wind shifts into account. Unless you get it all hooked up, but i don't think we're talking about that. VMG tells you how fast you are getting downwind or upwind, not to the mark. Every body who built these devices assumes that the person who uses it is trying to get to a position that is either dead down wind or within their boats tacking shadow upwind.

[Guest]

Sorry I Pressed the wrong button re: above.

 

 

Mr Pumbaa, I am not sure if " every body who built these devices assumes etc.etc" as you say. But my question was as still remains. "what is best DDW. Spin or Gen."

I am happy with the response so far but remain a little cofused with some of the stuff I have read. Thats not to say it is wrong .

 

Interesting though.

[samin 0]

Sorry I Pressed the wrong button re: above.

 

 

Mr Pumbaa, I am not sure if " every body who built these devices assumes etc.etc" as you say. But my question was as still remains. "what is best DDW. Spin or Gen."

I am happy with the response so far but remain a little cofused with some of the stuff I have read. Thats not to say it is wrong .

 

Interesting though.

I think everyone agree's it depends on the boat and wind strength

[samin 0]

Samin. you're argument is flawed.

 

you're Vmg.would be zero if you gybed or tacked thru 180 deg not 90.

 

90 Is 45 Deg either siide of a wind direction. or vari. butants theteof

 

thus you would have a vmg speed.

 

Fastest.Vmg to a wind angle will take you dead downwind in the best time but if the wind is 30deg different to the course heading then you would be working a fast speed to one side og the course and then would have to get back to the mark.

 

cannot agree with you're flawed theory.

Ok so speed to mark won't drop to zero but it will be much lower on one gybe than the other. Using your GPS speed to mark do you ever go to a layline?

[samin 0]

Going to a layline is not a priority. the priority is to get to the bottom mark in the fastest speed and least distance without chasing the wind.

 

In a situation where there is a breezeline to "go for" or a obstruction in your desired course then sailing Wind VMG is fastest.

 

EG: in and offshore race where an island is in the way of a direct course to the next mark. in this case you could sail the fastest VMG to the wind but!... i would saill the fasest VMG to the next way point (projected or set) to get around the island. (to shorten the distance travelled)

 

to answer your question though...

 

if you rounded the top mark and the wind shifted 90 deg then yes your VMG to the mark "Heading" would be on the layline.

 

If that was say a port gybe and the wind backed, any amound of degres up to 45Degrees total, then that port gybe is stii the fastest up until 45 deg of shift then after that the other gybe is favoured.

 

With you're theory if the wind backed 90deg at thhe top mark you would sail you're "winnd VMG" and sail off on angles or gybes at between 0 and 125 degress to the true course to the bottom mark. Soooo you would sail of on port gybe at say 30deg to the bottom mark and be on the right hand side of the course and snookered in thhe right hand corner hoping for a shift to gget you back to the centre of the course or bottom mark.

 

you can do that but it is high risk in a "persistant shift" scenario and a OK risk in an oscillating shift if the breeze returns to original direction. If it keeps moving in thhe same direction you could be knackered compared to a guy who took a higher course.

 

The key in an oscilating shift is to get down the course as fast as possible and keep to the centre to keep you're options open (not be caught on the wrong side of a new shift)

 

 

Persistant shifst are different but if the breeze, in the example above , shifted beyond its initial 90deg shift then you would take as much hieght on the port reach to set yourself up for a better position, to windward, as the breeze came around to a "on the wind" situation.

 

 

Spin's or A sails the theory is the same but you havve to determine which sail fit the best scenario for Angle and speed.

 

The only way to determine that is to test which saileworks best at different sailing angles / wind speed /boat speed (VMG to wind. Once you have worked these out and have them documented then the selections beome very easy except for the crossover points. Inn most cases you then select the best sail for" what you have" and not what "might happen" and change if things change..

 

EG if the wind did veer 90 at the top mark and the breeze was 20 knots then a A sail would be the go versus a spinnaker (for a Mono). but if the wind was true to course with small "variable" 10 degree shifts) and very light, then a Spinnaker (for a mono) woud be faster due to the easy of gybing in the shifts and also covering less distance in high risk reaches out to the sides of the course (where you could sail into a hole or away from the bottom mark/fleet.)

 

Having said that if thhe breeze on one side of the course is favourable (you are 90% sure it will be a winner) then going for a flyer (with an A sail, or whatever) out to the goood breeze would be beneficial and VMG to the mark is a no go until in the new line of breeze.

 

For Multis i would say the same applies but the decisions are based around the type of A sails you have and the angles/ speeds they produce. A flatter sail that requires a higher angle will need a different set of condtions and course angle than a lighter weight full sail that may sail deeper angles. so planning and decision makeing is different but VMG to the mark is paramount tacticall as well as time wise.

 

 

all pretty obvious stuff.

 

So basically a GPS wont do VMG so not sure why your so excited about it?

 

A normal compass will give you the shifts better than a GPS will (its quicker to respond)

 

Only time I use a mark Waypoint on my GPS is when I cant see the mark... i.e too far away or foggy/dark, otherwise I just sail the shifts/ Breeze lines and my best possible VMG, GPS doesnt help at all

[TimB 7]

Samin said "So basically a GPS wont do VMG..." huh??

VMG = velocity made good. For a GPS which has been directed towards a MARK then VMG represents your best possible speed TOWARDS that mark (or waypoint or whatever else you want to call it). The GPS doesn't care where the wind is or anything else. If you were motoring then your best VMG would be directly towards the mark. Sailing you have to play angles to find the best VMG. However as others have said once you hit the layline your GPS VMG will drop to zero and then go negative cause you are sailing AWAY FROM the mark. It's all basic geomatry.

 

Don't confuse GPS VMG which is towards a mark or waypoint with wind instrument VMG which is directly to windward or down wind in this case. They are not the same thing unless the mark is directly downwind.

 

My 10cents worth.