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

[CAPTAIN FANTASTIC]

As an ex dinghy sailor I tend to sail my boat as a dinghy ie adjusting the sails continuously, looking for the wind in front of me, ignoring the instruments most of the time, turning a bit too fast, feel for the wind against my face etc. and I can make an old slow boat go relatively fast.

Last weekend, when coming back from Yarmouth, we were doing 8 knots plus; this is fast for a heavy twin keel Macwester; 5 knots is my norm and I even enjoy sailing but going nowhere against the tide.

 

[Twister_Ken]

...and I even enjoy sailing but going nowhere against the tide.

Bizarre - that's when it's time to kedge and get your head down/lunch prepared/do a crossword, etc

 

[Bosun Higgs]

"Would be nice to think yeah thats good, I'll go put the kettle on."

Coimplete contradiction. Good boat speed requires concentration on boat speed and constant tweeks. You cant have good boat speed when down below making tea,

 

[Tomahawk]

Ih I don't know about that...
I regulalry do ten knots whilst sitting on the loo...

 

[SimonFa]

A good place to start is the sail trimming articles here: http://www.rya.org.uk/cruising/handling-sail/Pages/boathandling.aspx?page=1 but there's loads more on the web.

You should also be aware that most boats are designed to sail most efficiently when flat. Whilst a lot of heel look impressive and can be fun for the experienced sailor (whilst scaring the living daylights out of novices) it might not be your fastest STW. This is where the trade off with VMG starts to get difficult to calculate.

My mate who is a far better dinghy sailor than me hates any sort of weather helm. I think he's paranoid but having a lot of weather helm will slow the boat so don't be afraid to reef thinking it will slow you down. I have a large main in comparison to by genoa and find reefing early can increase my STW significantly because I can get the boat flatter and reduce weather helm but I'm still learning how she handles so I may be doing something wrong The down side of reefing early is I can't point as high so lose out on VMG.

 

[Angele]

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.

Sounds like you will have a busy winter ahead of you. At least you won't be wondering what to do with your time when the weather is too carp to go sailing. Get the basics working and you will find the additional information will help you become a better (faster) sailor next season.

Welcome to boat ownership (and boat maintenance).

 

[SimonFa]

As long as we have depth and know where we are it's Ok.

Given all the problems you've go with your instruments, including a new GPS that takes 30mins to get a fix which doesn't sound right, I trust you have checked you depth sounder and know what it reads ie under keel, under sounder or depth of water?

If you haven't I wouldn't go anywhere close to HW or on an ebb until you know exactly how its working and its accuracy.

PS I have about 20cm under my keel when it reads zero, but none of my guests and crew know that, keeps them honest when they are on the helm.

 

[Twister_Ken]

The down side of reefing early is I can't point as high so lose out on VMG.

Possibly not, because you'll be making less leeway.

 

[SimonFa]

Possibly not, because you'll be making less leeway.

Good point, I hadn't really considered that. Something else to find out about under various conditions.

 

[flaming]

Coming late to the party, but a word of warning about using apparent wind as the basis for any targets.

It doesn't work, it's a moving goalpost.

You need to have the log set up, and use the true wind read out for comparative targets.

For example - if I am stationary with sails flapping in a 10 knot wind, I might say, when I sheet in, I should do 5 knots on a reach, because that's what I did last time. So I do, and I accelerate, building apparent wind. So now I'm doing 4 knots, but with 12 knots apparent wind. So my target just changed... You'll end up chasing number all over the dial and getting confused.

 

[bedouin]

In fact I find apparent wind is more useful than true wind as I find the relationship between target speed and apparent wind speed is much less dependent on the point of sailing than true wind (after all it is apparent wind that powers the boat, not true wind).

So I can assure you that it does work fine - you may decide you prefer something else but it is nonsense to say that all other methods are wrong.

 

[flaming]

In fact I find apparent wind is more useful than true wind as I find the relationship between target speed and apparent wind speed is much less dependent on the point of sailing than true wind (after all it is apparent wind that powers the boat, not true wind).

So I can assure you that it does work fine - you may decide you prefer something else but it is nonsense to say that all other methods are wrong.

Not if you want repeatable readings it isn't. You can also trick yourself into thinking you're up to speed when you're doing 4 knots upwind in 10 knots apparent, when you're actually down on speed and could go 5 knots in 11 knots apparent in that true wind speed. There is a reason Polar diagrams are in true wind speed. There's a reason that wind speed limits for specific spinnakers are given in true wind speeds.

And also, no it is not apparent wind that powers the boat... If that was the case you could motor fast in light winds, get a good apparent wind going, then switch off and sail all day in that apparent wind.

The available power of a yacht is the difference in velocity between the water and the air. How it uses that power is where apparent wind comes in, not how much power there is available.

 

[Bobc]

Growling Kougar

That's a bit "Gareth Cheeseman" isn't it?

"I'M A SALES TIGER! grrrrrrrrrrr"

 

[PaulMcC]

Really enjoyed this thread - partly as I had a look at Growling Kougar when I was buying (but I'm too much of a cruiser for a GK29 and too tall), and partly because I've been doing quite a lot of experimentation with my new boat to work out what counts as 'doing well'.

Finding a polar is obviously the way forward but having looked around I haven't found any for a UFO34. (You might have more luck with a GK as I expect there are more racing now.)

Flaming - or anyone else: Is there an easy way to generate your own polar (without writing huge spreadsheets)? I'm thinking of an app or bit of software that does a lot of the work for you?

 

[flaming]

Really enjoyed this thread - partly as I had a look at Growling Kougar when I was buying (but I'm too much of a cruiser for a GK29 and too tall), and partly because I've been doing quite a lot of experimentation with my new boat to work out what counts as 'doing well'.

Finding a polar is obviously the way forward but having looked around I haven't found any for a UFO34. (You might have more luck with a GK as I expect there are more racing now.)

Flaming - or anyone else: Is there an easy way to generate your own polar (without writing huge spreadsheets)? I'm thinking of an app or bit of software that does a lot of the work for you?

There are spreadsheets to fill in available online, but then you're still looking at Polars resulting from measured results. These are useful if you're looking to have a set of polars to aid tactical decision making, rather than know if you're up to speed.

My best bit of advice would be to buy a really good trim book (I like the North Sails one) then experiment. Settle onto a course in a steady wind and move cars, sheet tensions, kicker etc, and see what you can get. Remember / record where you got the best result, then try a different point of sail. If you're looking for best windward performance the VMG function on your wind instrument (if linked to the log) is the best bet to measure performance by. I know some people like to use the VMG to a GPS waypoint that is far away upwind. This however doesn't really work, as windshifts that you may not even detect will have a big effect on the VMG.

Then just repeat the exercise in different wind speeds.

Or just pull the sheets until the sails stop flapping and put the kettle on. Whichever gives you more pleasure....

 

[bedouin]

Not if you want repeatable readings it isn't. You can also trick yourself into thinking you're up to speed when you're doing 4 knots upwind in 10 knots apparent, when you're actually down on speed and could go 5 knots in 11 knots apparent in that true wind speed. There is a reason Polar diagrams are in true wind speed. There's a reason that wind speed limits for specific spinnakers are given in true wind speeds.

And also, no it is not apparent wind that powers the boat... If that was the case you could motor fast in light winds, get a good apparent wind going, then switch off and sail all day in that apparent wind.

The available power of a yacht is the difference in velocity between the water and the air. How it uses that power is where apparent wind comes in, not how much power there is available.

Sorry - you seem to have a basic misunderstanding of the physics, the power from the rig is generated from the influence of the apparent wind on the sails. If you are travelling down wind at the same speed as the wind there is zero apparent wind and so your rig is generating no power. And yes, you can get lift from the sails by motoring in light airs. However that is not particularly useful as the AWA will soon come too far forward for the sails to generate any power.

This is not a trick - 4 knots in 10 apparent (STW 40% of AWS) is not the same as 5 knots in 11 apparent (45%) so unless you think 40=45 you aren't going to be "fooled" into thing you are up to speed.



However if you are

 

[tsekul]

Given all the problems you've go with your instruments, including a new GPS that takes 30mins to get a fix which doesn't sound right, I trust you have checked you depth sounder and know what it reads ie under keel, under sounder or depth of water?

If you haven't I wouldn't go anywhere close to HW or on an ebb until you know exactly how its working and its accuracy.

PS I have about 20cm under my keel when it reads zero, but none of my guests and crew know that, keeps them honest when they are on the helm.

Oh yes, one of the first jobs was lead line to check how accurate the depth log was, and also whether it was under keel or water line.

 

[tsekul]

Really enjoyed this thread - partly as I had a look at Growling Kougar when I was buying (but I'm too much of a cruiser for a GK29 and too tall), and partly because I've been doing quite a lot of experimentation with my new boat to work out what counts as 'doing well'.

Finding a polar is obviously the way forward but having looked around I haven't found any for a UFO34. (You might have more luck with a GK as I expect there are more racing now.)

Flaming - or anyone else: Is there an easy way to generate your own polar (without writing huge spreadsheets)? I'm thinking of an app or bit of software that does a lot of the work for you?

lol at too tall, I am 6'5" it was one of the only sub 30' boats with any head room that we could afford.

Really pleased with GK. Feel like we are learning properly, everything in adjustable ! no furler, racing groove so we're setting and raising the head sail everytime and changing sails whilst out. Really enjoying playing the traveller and the head sail cars to either gain or back off the power and adjust heel angle. All a bit trial and error at the moment.

We took it really easy the first few times we went out making sure we were conservative with our sail selection, but last time thought we'd push it a bit lots of heel lots of fun. She does prefer to sail quite flat as the previous owner mentioned and sails faster that way.................I think .

One thing left us scratching ours heads at first, after learning in 35' newish jeaneaus etc. No topping lift no lazy jacks and no proper main sail cover

 

[tsekul]

A good place to start is the sail trimming articles here: http://www.rya.org.uk/cruising/handling-sail/Pages/boathandling.aspx?page=1 but there's loads more on the web.

You should also be aware that most boats are designed to sail most efficiently when flat. Whilst a lot of heel look impressive and can be fun for the experienced sailor (whilst scaring the living daylights out of novices) it might not be your fastest STW. This is where the trade off with VMG starts to get difficult to calculate.

My mate who is a far better dinghy sailor than me hates any sort of weather helm. I think he's paranoid but having a lot of weather helm will slow the boat so don't be afraid to reef thinking it will slow you down. I have a large main in comparison to by genoa and find reefing early can increase my STW significantly because I can get the boat flatter and reduce weather helm but I'm still learning how she handles so I may be doing something wrong The down side of reefing early is I can't point as high so lose out on VMG.

We've been looking at you tube videos on sail trim, some good ones from north sails from the 80's , lots of bushy moustaches and girls with big perms .

 

[flaming]

Sorry - you seem to have a basic misunderstanding of the physics,

Pretty sure I don't...

Yes, Apparent wind is what is actually moves the boat. But the measure of the available power in the system is the difference in velocity between the wind and the water. So I'm going to continue to use that to judge how fast I should be going.

As you accelerate the apparent wind will change. So at what point do you know you're up to speed? It just can't work as an arbitrator of speed to come back to.