CRUISER RACING - POLAR DIAGRAMS

[matelot]

Polar diagrams are typically comma shaped when you get to around the 30 deg to the wind mark. So if you bear away, then your boat goes a bit faster through the water but in slightly the wrong direction. But there must mathematically be a point of sail where the speed towards the objective is fastest. Trouble is in finding where that point is.

Typically you get to a point where you need to tack to make a mark and you have to judge when. I've convinced myself that waiting for a minute or two after I can fetch a mark leaves me sailing slightly free er and making the mark faster, but truth to tell I dont really know.

How on earth do you determine this point because you simply dont have the ability to try repeated attempts on a mark at lots of different angles.

So what do you do? Do you tack as soon as you can lay a mark or wait a bit?

 

[deaks]

Too early in the morning for me to think about this one ....but i will be keen to learn from others. I suppose a good GPS using VMG comes in somewhere, (assuming you have the exact Lat/Lon of the mark /forums/images/graemlins/wink.gif) or is that only for use mathematically challenged ?

 

[sarabande]

a quick rule of thumb.

for 20 degrees off course, you need to gain 20% more speed, or it is not worth it.

Cosines and all that.

There are very neat little programs which will do it all for you. I'll ferret around and see if I can come up with any.

 

[lw395]

If you have a fully calibrated B+G system, it will calculate your upwind vmg for you, by calculating true wind direction from apparent wind and boat speed. Unfortuntely, there are a lot of sources of error in the calculation.
You can set a waypoint on a GPS, say 10 miles dead upwind, your gps should then be able to tell you your VMG (component) in this direction.
You can calculate your vmg yourself, by multiplying your boatspeed by the cos of the angle off the (true) wind, eg
cos 30=0.866
cos 35=0.819
cos 40=0.766
cos 45=0.707
If you print out a little spread sheet for a range of angles and speeds, it can help you judge. Spend some tune up time with different sail trims and calculate the vmg for what you judge to be a good angle, then up 3degrees and down 3 degrees.
The better way is to do some racing against a competent one design fleet, or at least similar designs, you'll soon improve your judgement.

 

[DJE]

If you have a polar diagram it is easy The point on the polar curve that is nearest the top gives the best angle to sail for maximum VMG. Or to put it another way the point where a horizontal tangent just touches the curve gives the best vmg.

If you don't have a polar (and who does?) a good trick is to record the GPS COG on each tack. When you're approaching the layline watch the BTW to the next mark. When the BTW equals the COG from the previous tack you can lay the mark. Easy.

 

[doug748]

Wait until one your crew says " I think we should tack now " press on.
When another pipes up (always in a mock cheery voice, he thinks you are either deaf or have lost your marbles) "we can lay it now, skip " press on.
When all the crew are moaning and some start to lose interest (looking at the shore, at their boots etc) count slowly to 10 and then tack. You may then lay it (just).
If you have to put in another tack it will be your fault.

 

[flaming]

Broadly speaking you tack when you can lay it. Racing is a lot about minimising the distance that you sail, and since there are very few boats for whom best upwind VMG is not in the groove upwind the quickest course will be where you don't have to pinch or foot off.

Be careful using a waypoint set at a distance upwind to calibrate, this takes no account of wind shifts so can give really misleading data.

 

[DJE]

Here, I found a picture.

 

[dt4134]

Finding the optimum course to windward and deciding when to tack to lay the mark are two of the skills that really make the difference when racing. You just have to learn how to do them if you want to win.

You should have a feel for your own boats best close-hauled course in given wind and sea state. If it doesn't feel right it probably isn't right so you should communicate this to the trimmers. If they know what they're doing they should be able to make improvements, if not you might have to guide them through it.

Boats of significantly different design have significantly different best close hauled courses. It is a good idea to sail your own best course to windward rather than following the boat in front.

Laying the windward mark is a great skill. You have to take into account likely wind shifts and what the tide is doing where you are and at the mark. As a beginner (and often as not a beginner) it is probably best to avoid the lay lines until quite close to the mark. It is much easier to find the correct lay line from ten boat lengths out than from half a mile.

If you overstand you have to sail extra distance and lose out. If you're short you have to throw in a quick double tack. It is better to overstand slightly as tacks will generally cost between two and five boat lengths each, maybe more for a bilge keeler with inexperienced crew.

I'm afraid if you want to develop the skill of judging it by eye you will have to practice a lot.

Crew can help you point higher if things are borderline by hiking properly rather than sitting around on the side deck gossiping. If you have crew who haven't the inclination or fitness to hike full time it is still handy to teach them how to do it for short periods such as the last stages of the approach to a windward mark. Don't ever believe it doesn't make much difference.

 

[awol]

I'll go with this method. 2 extra tacks or heavy pinching cost a lot more than overstanding a wee bit.
If you are leading the fleet you have to make your own decisions but if all the boats behind have tacked that is a clue. If not in the lead you can get an idea from the leading boats - in your case with bilge keels, you will probably stand on a bit longer than the fin keeled plastic-sailed speed machines.

 

[temptress]

[ QUOTE ]
......
How on earth do you determine this point because you simply dont have the ability to try repeated attempts on a mark at lots of different angles....

[/ QUOTE ]

Each boat handles differently there are general rules of thumb but no hard and fast rules you have to figure out what works for you.

To do this you need to go out and practice, then practice some more and then some more practice and then practice some more. Write the numbers down and compare. Then do some more practice. The more you practice the luckier you get…. /forums/images/graemlins/laugh.gif

Or you could just go out and enjoy yourself and learn from EXPERIENCE.

/forums/images/graemlins/smirk.gif

 

[matelot]

[ QUOTE ]
If you have a polar diagram it is easy The point on the polar curve that is nearest the top gives the best angle to sail for maximum VMG. Or to put it another way the point where a horizontal tangent just touches the curve gives the best vmg.

.

[/ QUOTE ]

I disagree. Using your own polar from above and trying to judge the speeds and angle on the screen there are three alternatives : 6.0kn at 34 deg (the point you marked), 6.4 at 36 and 6.5 at 40. The upwind speed on these three are 4.97kn 5.17kn and 4.98kn respectively. So its faster to sail free of the angle you suggest.

But thats far from the whole story because of leeway. In my sort of boat, the harder on the wind you are the more leeway you make.

And then of course there is the tide.

So far I've subscribed to the "leave it until 1 minute after the crew start commenting" approach on the basis that I have more speed being a few degrees free and dont lose as others do by cocking it up and tacking too early. I think thats right in my sort of boat, so the question is as much about whats too far overshot as anything.

Sounds as if I need to start taking some notes and trying to learn after the event. I already download the gps track into Mapsource after a race and thats really informative.

Got to say this weekend winter racing is way more interesting than summer pottering. Perhaps because its new to me and I'm all pottered out.
/forums/images/graemlins/grin.gif

 

[DJE]

[/ QUOTE ]
I disagree. Using your own polar from above and trying to judge the speeds and angle on the screen there are three alternatives : 6.0kn at 34 deg (the point you marked), 6.4 at 36 and 6.5 at 40. The upwind speed on these three are 4.97kn 5.17kn and 4.98kn respectively. So its faster to sail free of the angle you suggest.


[/ QUOTE ] Sorry but the point where the horzontal tangent touches the curve is the best VMG. It's basic trigonometry. Also the point I've marked is a fraction more than 35 degrees (judging by the 5 degree markers around the circumfernce).
Agree tide and windshifts complicate matters enormously but these are quite seperate from the interpretation of a polar diagram.

 

[lw395]

The picture is an APPARENT WIND polar. This is not what's needed to optimise pointing, you need to reduce all the data to true wind.
I think apparent wind polars are used to check sail trim, if your boat speed is below the predicted, something is wrong!
A polar diagram is only any help when you believe it matches the boats true performance. Many are generated by prediction programs based on hull shape and rig data. They don't take into account sea state or any deviation from the ideal.
My method is based around 'get all the tell tales behaving, then watch the speedo as you make small changes of pointing angle' or in dinghies 'copy the fast boats sail settings, keep it flat and keep it moving'. feedback from racing alongside another boat is the true indicator, if you can climb without slowing down relative to another boat that's good, as is pulling away without losing significant height. Big boats often don't have enough competition to give that feedback, so the polar is the next best thing, a benchmark.

 

[BlueSkyNick]

[ QUOTE ]

Crew can help you point higher if things are borderline by hiking properly rather than sitting around on the side deck gossiping. If you have crew who haven't the inclination or fitness to hike full time it is still handy to teach them how to do it for short periods such as the last stages of the approach to a windward mark. Don't ever believe it doesn't make much difference.

[/ QUOTE ]

What's hiking? Presumably when you get them sat on the windward rail, and not that you make them get off and walk !! So why's it called hiking?

 

[dt4134]

[ QUOTE ]
[ QUOTE ]

Crew can help you point higher if things are borderline by hiking properly rather than sitting around on the side deck gossiping. If you have crew who haven't the inclination or fitness to hike full time it is still handy to teach them how to do it for short periods such as the last stages of the approach to a windward mark. Don't ever believe it doesn't make much difference.

[/ QUOTE ]

What's hiking? Presumably when you get them sat on the windward rail, and not that you make them get off and walk !! So why's it called hiking?

[/ QUOTE ]

I've no idea where the word comes from but it is used to make the distinction from just sitting on the rail. You have to hitch yourself up onto the middle guardwire and lean right forward and attempt to touch your toes. If it is a really crucial point in the race you can stretch your arms out for that little extra righting moment.

It really does make a difference and if the crew are really trying you can climb up to another boat which you wouldn't otherwise reach.

It is physically quite demanding as your weigh is being taken on your stomach muscles and the back of your thighs. Padded guard wires help (you'll notice these on serious race boats around the marina).

 

[BlueSkyNick]

Thanks JCP. I quite like the idea of a seperate forum for racers, and agree with Matelot's initiative. (I wish he wouldn't use capitals to SHOUT the titles though)

I have started to take an interest in racing mainly through providing the committee boat for our club's autumn series, and have often thought about giving it a go.

Now that you have given such a thorough and interesting explanation of hiking and other such activities, my mind is made up.

I will stick to the bacon butties and coffee, on the committee boat. /forums/images/graemlins/smile.gif

 

[dt4134]

[ QUOTE ]

I will stick to the bacon butties and coffee, on the committee boat. /forums/images/graemlins/smile.gif

[/ QUOTE ]

That's interesting. I always feel a bit guilty about those on the committee boat rolling about at anchor when the rest of us are having fun. Glad somebody does it though.

 

[mortehoe]

Sailing or racing the helmsman has to make the decisions ....

It's the wind on the cheekl/kneck or the fluttering luff on the main or whatever ......

Polar diagrams are for winter logfire snoozing ..........

 

[matelot]

[ QUOTE ]
Sorry but the point where the horzontal tangent touches the curve is the best VMG. It's basic trigonometry. Also the point I've marked is a fraction more than 35 degrees (judging by the 5 degree markers around the circumfernce).
Agree tide and windshifts complicate matters enormously but these are quite seperate from the interpretation of a polar diagram.

[/ QUOTE ]

Would be correct if the scales on the diagram were undistorted but they've done something funny with them as you can see if you compare the speed ticks on the 30 deg and 40 deg line. Anyway its a bit pedantic of me - particularly when I dont have a polar for my boat anyway.