Boat speed in relation to apparent wind speed and point of sail.

[tsekul]

Hi there,

Fairly new to this sailing malarkey . We have a westerly Gk29

Trying to get a good idea of what we should be getting in the way of sog on a given point of sail and apparent wind. We are tweaking all the controls trying to get her " in the groove" . But having some kind of idea of what is possible would really help.

Is there some kind of rule of thumb we can work too? Would be nice to think yeah thats good, I'll go put the kettle on.

Lay mans terms would be good, brain turns to mush when I look at fraction, trigonometry etc

Tim

 

[Twister_Ken]

SOG is governed by boat speed through the water (STW) and any tide or current. Not much you can do about the latter. So you should concentrate on STW, from your log.

Best guide - if you can find one for your GK - is a 'polar diagram' which shows the STW the designer would expect at a combination of wind strengths and angles.

By the way, I trust your boat has a traditional GK name, like Gnut Kracker or Granny Knickers or Gnats Koq.

 

[tsekul]

Growling Kougar

Our STW log isn't working at the moment so only got SOG through the GPS.

Only had her a month or so and decided to just sail as much as possible and sort the gremlins out over the winter.

 

[Twister_Ken]

Great Kname!

Given sufficient wind, I'd expect something around 5.5kts upwind and about 7kts offwind - not surfing. Estimate the tide flow from a tidal atlas, and add or subtract it from your SOG to get a rough STW - easier to do up and down tide, rather than across.

 

[mjcoon]

Lay mans terms would be good, brain turns to mush when I look at fraction, trigonometry etc

Tim

Make a special effort over square root! The physics says maximum hull speed is related to sqrt(water-line length); if length is in feet, times circa 1.4 to get speed in knots. Less than 1.4 for a fat cruiser, maybe more than 1.4 for a speed merchant.

Mike.

 

[westernman]

Great Kname!

Given sufficient wind, I'd expect something around 5.5kts upwind and about 7kts offwind - not surfing. Estimate the tide flow from a tidal atlas, and add or subtract it from your SOG to get a rough STW - easier to do up and down tide, rather than across.

To get 7 knots off the wind with that waterline length and beam you will be pushing her very very hard.

I reckon a more realistic "cruising" speed (as opposed to white knuckle stuff with spinaker flying when you should be reefing) I reckon is 5.5 knots off the wind and about 4.5 knots uphill unless conditions are very good and you have a good suit of sails (downwind any old sails will do).

These are average speeds - you will get peaks better than that in gusts.

 

[grumpy_o_g]

Go club racing and watch how well you do against boats of a roughly similar performance. Also read up on sail trim and try and get a respected racing skipper to go out with you. Probably even worth going out on a well-sorted boat and with a good crew and see how they do things.

If you asked me what makes Ben Ainslie a faster person on the water than me though I'd say it's about 25% him be able to make the boat go faster than me in identical conditions and 75% making sure his boat is in the right bit of water going in the right direction at the right time. He'll tack when I don't and vice-versa, sail higher or lower on the wind than me, and perhaps even be in a completely different piece of water to me, as well as just having his boat set up better and helming it more accurately.

 

[Tomahawk]

Are there any other 29's near you?
Or failing that is there a local club that does a bit of racing?

The easiest way to learn is by being close enough to othere boats to see what they are doing and copying them..

But you need to get the log fixed.. The log is the only thing that really tells you when you are in the groove.. GPS reacts toom.slowley to register a .5 kt increase slow down but the log will.

 

[Tomahawk]

Forgot to say...

Welcome to the wondful world of sailing...

 

[tsekul]

Thanks for the replies.
SWMBO and I decided to start sailing and I seem lucky as she enjoys it as much as me. I wanted to do it as a way to relax, in a totaly non competitive way. Problem is I am quite competitive and want to sail as well and effectively as possible. Now we are thinking of a bit of club racing. Hence the question.

Problem is at the moment we still sit there going "are they on port tack or starboard, where's the wind ? etc." so until thats a bit more second nature we try and steer clear of congestion.

We were close hauling at times over 5+ sog yesterday I think with around 18knts apparent I think can't quite remember the wind speed and managed a max on the day(off the wind) of 7.2

Our main disaster during proceedings was on one tack, somehow the clew? of the genoa managed to catch on a small cleet on the base of the mast, not only that that cord that tightens the leech also got wrapped around something ! took a while to unhook everything. Next challenge the spinnaker gulp!

Thunder and lightening appeared out of nowhere so we bid a hasty retreat.

 

[Tomahawk]

So please tell us a bit more...
Like which corner of the country are you in?

I amsure members of the Parish local to you will welcome you..

 

[Sybarite]

Hi there,

Fairly new to this sailing malarkey . We have a westerly Gk29

Trying to get a good idea of what we should be getting in the way of sog on a given point of sail and apparent wind. We are tweaking all the controls trying to get her " in the groove" . But having some kind of idea of what is possible would really help.

Is there some kind of rule of thumb we can work too? Would be nice to think yeah thats good, I'll go put the kettle on.

Lay mans terms would be good, brain turns to mush when I look at fraction, trigonometry etc

Tim

Just enjoy your sailing and a feel for efficiency will come, just because it feels right.

 

[Joker]

90% of upwind sailing is 'seat of the pants' - you feel when the boat is in the groove. Luff up, and you slow down. Dinghy sailors don't have a log - they hear the sound of the boat through the water and know when they're going faster or slower.

 

[BlueSkyNick]

As has been said, you cannot judge by SOG unless you take into account what the tide is doing to you at the time. It might be having an effect from any one of 360 degs !

What you are trying to work out is VMG - Velocity Made Good - which is the best progress you are making towards a waypoint. You could sail close to the wind, and hence towards the waypoint, at a certain speed or you could sail a little further away from the wind at a higher speed. Understanding which angle and speed is best for your boat is the holy grail. You can get the optimum design speed from the polar diagrams. The actual optimum VMG whilst under way requires either some very quick trigonometry, or speed instruments linked to the GPS.

 

[tsekul]

So please tell us a bit more...
Like which corner of the country are you in?

I amsure members of the Parish local to you will welcome you..

We are based in Falmouth and newish members at Mylor yacht club. We have been crewing a bit for members while we looked at boats. finally took the plunge and bought one. Our initial challenge was getting it back from Cowes. We decided it was a little above our pay grade so got help from a yacht delivery skipper Ken Nutt. We had a great trip down including hugging portland bill in fog. Ken lives in Portland so knew the area very well, we had a procession of yachts following closely behind all the way. Lots of motoring but as we neared eddystone at about 11pm the wind picked up at we flew into the Carrick roads.

 

[tsekul]

As has been said, you cannot judge by SOG unless you take into account what the tide is doing to you at the time. It might be having an effect from any one of 360 degs !

What you are trying to work out is VMG - Velocity Made Good - which is the best progress you are making towards a waypoint. You could sail close to the wind, and hence towards the waypoint, at a certain speed or you could sail a little further away from the wind at a higher speed. Understanding which angle and speed is best for your boat is the holy grail. You can get the optimum design speed from the polar diagrams. The actual optimum VMG whilst under way requires either some very quick trigonometry, or speed instruments linked to the GPS.

there is VMG on the wind instrument but i don't really know what I'm looking at, I am not sure its working as the log doesn't work and I assume you need that, there is a gps repeater on the intrument panel but that is water logged and not working. We do have a newish gps linked to DSC vhf which works fine although it can take 1/2 an hour to get a fix.

We should get all this sorted but at the moment if we have spare time we want to be sailing. As long as we have depth and know where we are it's Ok.

 

[bedouin]

Without STW you are really going to struggle on this - and VMG won't work without STW. There are too many variables for SOG from the GPS to be reasonable alternative.

Have a look for "Polar" curves - preferably for your boat, but any will give you an idea about how it works.

With experience most people build up an idea of what they would expect their boat to do on various points of sail - in terms of STW against apparent wind speed and direction. As a rule of thumb on my boat I reckon that STW of 2/3rds AWS is fairly good, but 1/2 would be poor - up until a STW of about 5 knots, then it takes increasingly more wind to get more boat speed

 

[johnalison]

Yes, I agree. The wind instrument needs to be connected to the water speed meter, as in many instrument packages, in order for a meaningful VMG to be read.

It is very hard to sail to a VMG meter for any length of time but it makes an interesting exercise. To get the best VMG you need to be watching the meter, the wind direction, the boat speed, well as looking at the sails and trying to drink your coffee. We found it quite revealing and that a very delicate and precise touch was needed for the best figures, and that the best course was closer to the wind than our natural assessment. As a result, when sailing to windward my main source of information is now the boat speed and I pay less attention to the apparent wind direction.

 

[pugwash94]

How about ignoring the instruments and looking at the telltales and the "feeling" the boat.
Trim the main so that the lower telltales are streaming and the top one is just curling around the sail every so often.
Trim the genoa so all the telltales - on both sides of the sail - are streaming horizontal.
If you are hard on the wind steer by the telltales on the genoa. And don't pinch! Feeling the boat will tell you the moment you do.
You'll then be sailing at pretty close to best speed.
There are lots of things to adjust to get the best, halyard tension, out-haul, mast-bend, traveler position ...
So:

• Buy a good book on sail trim.
• Try sailing a dinghy - that will give you more help on getting the best from the wind than anything else

Do as much sailing as you can to "get the feel" which ultimately is the best way to sail.
And enjoy it - have fun!

 

[mjcoon]

What you are trying to work out is VMG - Velocity Made Good - which is the best progress you are making towards a waypoint.

That's what I thought the definition was, having first read about it in GPS manuals. Hence didn't, originally, understand comments like the other one in this thread about VMG requiring wind speed and direction. The alternative definition is speed in the upwind direction, which obviously instruments cannot calculate without that extra data.

Mike.