Announcements
New Discussions
Polar Diagrams for beach catamarans?
by Frank N. 05/04/22 07:38 PM
Previous Thread
Next Thread
Print Thread
Rate Thread
Hop To
Page 2 of 3 1 2 3
Re: Mast theory [Re: Smiths_Cat] #145236
06/10/08 11:55 AM
06/10/08 11:55 AM
Joined: Jun 2001
Posts: 9,582
North-West Europe
Wouter Offline
Carpal Tunnel
Wouter  Offline
Carpal Tunnel

Joined: Jun 2001
Posts: 9,582
North-West Europe


Large squaretop sails like a bit of twist along the leech. Pulling the leech tight on these sails with slow the boat down. The explanation given for this is often the unacceptable increase in drag because of having a large tip vortex.

On the French Sailing DVD Glenn Ashby says in an interview that his top is often around 300 mm twisting off, as taken from what would be a perfectly straight leech.

But you can feel it too. If the leech is to tight up top then the boats hangs up, it feels powerful but not fast. You heel easily but don't accellerate quickly in the gusts You don't get that nice "streaming" feeling that you would get with the right amount of twist.

Try it a few times, you will learn to feel it.

Wouter


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
-- Have You Seen This? --
Re: Mast theory [Re: Smiths_Cat] #145237
06/10/08 11:59 AM
06/10/08 11:59 AM
Joined: May 2003
Posts: 4,451
West coast of Norway
Rolf_Nilsen Offline
Carpal Tunnel
Rolf_Nilsen  Offline
Carpal Tunnel

Joined: May 2003
Posts: 4,451
West coast of Norway
Klaus,

if you are unable to get your top to stall first, it might be that you could benefit from having your sail recut. Usually I oversheet the squaretop by error in light winds and stall the top, not the other way around.
Do you know how much development have gone into that sail? Is it an old sail or a new one? Can the sail have stretched badly?

Re: Mast theory [Re: Rolf_Nilsen] #145238
06/10/08 12:02 PM
06/10/08 12:02 PM
Joined: May 2003
Posts: 4,451
West coast of Norway
Rolf_Nilsen Offline
Carpal Tunnel
Rolf_Nilsen  Offline
Carpal Tunnel

Joined: May 2003
Posts: 4,451
West coast of Norway
Oh, and pictures please.. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Put the boat over on the side, sheet in and put on downhaul, then do a picture upwards from the boom and one from behind the boat.

Re: Mast theory [Re: Rolf_Nilsen] #145239
06/10/08 12:30 PM
06/10/08 12:30 PM
Joined: Sep 2007
Posts: 571
Hamburg
Smiths_Cat Offline
addict
Smiths_Cat  Offline
addict

Joined: Sep 2007
Posts: 571
Hamburg
Quote
Usually I oversheet the squaretop by error in light winds and stall the top, not the other way around.

Thanks Rolf, that's exact the information I needed.
Quote
Do you know how much development have gone into that sail?

I can only guess, that they used an A-cat cut, but the mast is stiffer due to the reinforcement for the second trapez. Sometimes the leech between the top and second batten flutters a bit.
Quote
Is it an old sail or a new one?

2004, pentex mylar from Petrucci, but not sailed much.
Quote
Can the sail have stretched badly?

Could be, but I don't think so (6:1 mains sheet and sailed on lakes in the Alpes).

I will try to take some pictures next week end. (try = have the patience, not to be all the time on the water <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />).

Cheers,

Klaus

Re: Mast theory [Re: Wouter] #145240
06/10/08 12:54 PM
06/10/08 12:54 PM
Joined: Jul 2001
Posts: 1,200
Vancouver, BC
Tornado Offline
veteran
Tornado  Offline
veteran

Joined: Jul 2001
Posts: 1,200
Vancouver, BC
Quote

On the French Sailing DVD Glenn Ashby says in an interview that his top is often around 300 mm twisting off, as taken from what would be a perfectly straight leech.


Wouter,

Do you have source for this DVD? Is it the one on Multihulls-world website? I can't seem to create an order there... <img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" />

Mike.


Mike Dobbs
Tornado CAN 99 "Full Tilt"
Re: Mast theory [Re: Smiths_Cat] #145241
06/10/08 12:59 PM
06/10/08 12:59 PM
Joined: May 2003
Posts: 4,451
West coast of Norway
Rolf_Nilsen Offline
Carpal Tunnel
Rolf_Nilsen  Offline
Carpal Tunnel

Joined: May 2003
Posts: 4,451
West coast of Norway
Hmmm, what kind of battens do you have up there? Tapered and well tested? If the batten leech ends are too soft, you can get some flutter in the leech and more twist than you wanted in the top. Have the top inverted with full downhaul and strong wind?

If your mainsail goes slack in between battens in the leech in light winds, I would be tempted to speculate that there is too much luff round for your mast setup. Easiest and fastest way to test it is probably by putting the boat over on the hard and set mainsheet and downhaul, then adjust diamond tension. You could do the same on the water, but I like to get in the ballpark with the boat on its side and on the hard. Of course, that theory goes against your twist problem.. I would have put the boat over on its side and spent the time to find out what is really going on.

Re: Mast theory [Re: Tornado] #145242
06/10/08 02:19 PM
06/10/08 02:19 PM
Joined: Jun 2001
Posts: 9,582
North-West Europe
Wouter Offline
Carpal Tunnel
Wouter  Offline
Carpal Tunnel

Joined: Jun 2001
Posts: 9,582
North-West Europe

Quote

Do you have source for this DVD? Is it the one on Multihulls-world website? I can't seem to create an order there...


I think that is the same one, but give me a link to the multihulls-world order page and I'll check for you.

I seem to remember that Ricks online store featured these DVD's too !

Here is the link to the Rick White online store offer :

https://store.catsailor.com/ProductCart/pc/viewPrd.asp?idcategory=178&idproduct=13792

Wouter


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: Mast theory [Re: Rolf_Nilsen] #145243
06/10/08 02:30 PM
06/10/08 02:30 PM
Joined: Sep 2007
Posts: 571
Hamburg
Smiths_Cat Offline
addict
Smiths_Cat  Offline
addict

Joined: Sep 2007
Posts: 571
Hamburg
Quote
Tapered and well tested?

Actually the battens are what I found in the sail, when I bought it.

Quote
Have the top inverted with full downhaul and strong wind?

No, never, but I think I never applied full downhaul, because I had already enough twist. Actually in strong winds, I am happy with the sail. However having some thoughts about the battens is never wrong.

Quote
If your mainsail goes slack in between battens in the leech in light winds...

Maybe I used the wrong wording in my previous posting: It does not go slack, it is tight, but the leech flutters. I think that the distance between the battens is just a bit to long. I do not really worry about that.

Quote
I would have put the boat over on its side and spent the time to find out what is really going on

I lack the expirience to judge from the shape of the sail only. I need to go on the water and see the tell tales to understand what happens.

Rolf, thank you very much for the advices. Now, I know, that I should be able to stall the top. And that means that I have still to tweak on the settings. I will have a look at the batten(s) and try if I still can reduce the diamond tension before making them slack.

Cheers,

Klaus

Re: Mast theory [Re: Wouter] #145244
06/10/08 05:35 PM
06/10/08 05:35 PM
Joined: Jul 2001
Posts: 1,200
Vancouver, BC
Tornado Offline
veteran
Tornado  Offline
veteran

Joined: Jul 2001
Posts: 1,200
Vancouver, BC
Bingo!

I've placed my order.

Cheers!


Quote

Quote

Do you have source for this DVD? Is it the one on Multihulls-world website? I can't seem to create an order there...


I think that is the same one, but give me a link to the multihulls-world order page and I'll check for you.

I seem to remember that Ricks online store featured these DVD's too !

Here is the link to the Rick White online store offer :

https://store.catsailor.com/ProductCart/pc/viewPrd.asp?idcategory=178&idproduct=13792

Wouter


Mike Dobbs
Tornado CAN 99 "Full Tilt"
Re: Mast theory [Re: Wouter] #145245
06/11/08 10:18 AM
06/11/08 10:18 AM
Joined: May 2004
Posts: 61
davidtugwell Offline
journeyman
davidtugwell  Offline
journeyman

Joined: May 2004
Posts: 61
A couple of questions to help my education.

1. Rolf: Can you explain too much power/drag. I dont understand.

2. Wouter: I was led to believe the only factor that stalls a wing is angle of attack. I think I understand what you are saying (i.e. tweaking changes the foil shape and therefore changes the stall angle of attack). Or am I misunderstanding you?

David

Re: Mast theory [Re: davidtugwell] #145246
06/11/08 11:46 AM
06/11/08 11:46 AM
Joined: May 2003
Posts: 4,451
West coast of Norway
Rolf_Nilsen Offline
Carpal Tunnel
Rolf_Nilsen  Offline
Carpal Tunnel

Joined: May 2003
Posts: 4,451
West coast of Norway
David,

when you increase the power by making the foil deeper (more draft) the drag will also increase. If we say the windspeed is relatively constant there will be a given draft which have the best lift to drag ratio for that windspeed. If we sail with more draft than optimum, we will be slower than somebody with optimum draft or even less than optimum as they have less power but also far less drag.

Perhaps I should do a graph..

This is all very theorethical and in real life it is most important before you get out on the trapeze. It is especially important in light winds in my opinion!

I'll dare to say something about the second question as well. Draft and airspeed are also part of deciding the point of stall to a greater or lesser degree depending largely on the speeds and angles you work with. That is my understanding at least.

Re: Mast theory [Re: Rolf_Nilsen] #145247
06/11/08 06:30 PM
06/11/08 06:30 PM
Joined: Feb 2007
Posts: 266
UK
Cheshirecatman Offline
enthusiast
Cheshirecatman  Offline
enthusiast

Joined: Feb 2007
Posts: 266
UK
Quote
David,

when you increase the power by making the foil deeper (more draft) the drag will also increase. If we say the windspeed is relatively constant there will be a given draft which have the best lift to drag ratio for that windspeed. If we sail with more draft than optimum, we will be slower than somebody with optimum draft or even less than optimum as they have less power but also far less drag.

Perhaps I should do a graph..

This is all very theorethical and in real life it is most important before you get out on the trapeze. It is especially important in light winds in my opinion!

I'll dare to say something about the second question as well. Draft and airspeed are also part of deciding the point of stall to a greater or lesser degree depending largely on the speeds and angles you work with. That is my understanding at least.


There is a good trick you can use this full camber for. In light winds if the leg is long enough and you are having trouble catching/overtaking a nearby boat ahead when sailing to windward, try easing the outhaul and downhaul to increase camber and start to pinch slightly. If done properly you fall back a little but you lift above the other boat. You can then bear off a little and trim accordingly for greater boatspeed. This helps get you past the other boat. If they try luffing they slow down, plus you have more lift from your boards with the increased boatspeed.

Cheshirecatman

Re: Mast theory [Re: Rolf_Nilsen] #145248
06/11/08 09:17 PM
06/11/08 09:17 PM
Joined: Mar 2008
Posts: 37
Western Europe
ClaytonF16 Offline
newbie
ClaytonF16  Offline
newbie

Joined: Mar 2008
Posts: 37
Western Europe
Quote
David,

when you increase the power by making the foil deeper (more draft) the drag will also increase. If we say the windspeed is relatively constant there will be a given draft which have the best lift to drag ratio for that windspeed. If we sail with more draft than optimum, we will be slower than somebody with optimum draft or even less than optimum as they have less power but also far less drag.

Perhaps I should do a graph..

This is all very theorethical and in real life it is most important before you get out on the trapeze. It is especially important in light winds in my opinion!

I'll dare to say something about the second question as well. Draft and airspeed are also part of deciding the point of stall to a greater or lesser degree depending largely on the speeds and angles you work with. That is my understanding at least.


sounds fair....

Re: Mast theory [Re: davidtugwell] #145249
06/12/08 05:20 AM
06/12/08 05:20 AM
Joined: Sep 2007
Posts: 571
Hamburg
Smiths_Cat Offline
addict
Smiths_Cat  Offline
addict

Joined: Sep 2007
Posts: 571
Hamburg
Hi David,

Quote
I was led to believe the only factor that stalls a wing is angle of attack. I think I understand what you are saying (i.e. tweaking changes the foil shape and therefore changes the stall angle of attack).


Actually, there are many factors which let the sail or parts of the sail stall. Some are linked to the local angle of attack, the draft (camber), roughness, mast shape and perturbations and to a small amount to the speed (actually Reynolds number). Others are linked to the plan form of the sail and the its aspect ratio among many others.

The good thing of all these parameters is, that you don't have to care about them as long as you know if and where your sails stalls and how to stop it. Put tell tales in your sail and at the leech. They cause very very little extra drag, so don't be thrifty. I have them at three different heights each with a sequence of four or five tell tales from about 10cm behind the luff to the leech. With this (and a minimum transparancy of the sail of course) you will easly see the effect of sheet, downhaul and outhaul tension and mastrotation and -prebend.

Avoid stalling on the leeward side of the sail at upwind courses, if possible. But you may accept some stalling at the leeward leech if you are on a reach or going downwind. Stalling on the windward side is harmless and an indicator that you can tighten the sheet (if you are not over powered of course).

Hope that answers your question.

Cheers,

Klaus

Re: Mast theory [Re: Rolf_Nilsen] #145250
06/15/08 04:38 PM
06/15/08 04:38 PM
Joined: Sep 2007
Posts: 571
Hamburg
Smiths_Cat Offline
addict
Smiths_Cat  Offline
addict

Joined: Sep 2007
Posts: 571
Hamburg
Hi Rolf and all,

Quote
Oh, and pictures please..


Finally I made some pictures of my sail. I was in a hurry because of an approaching thunder storm, so I didn't have the time to play around and I couldn't test the new diamond setting on the water.
The main sheet was tight, downhaul open, rotation open (at least should be open), outhaul tight and the diamonds close to slack. Analysed with your eyes, should I be able to hook the top leech more than that? Thanks.

Cheers,

Klaus

Attached Files
148320-PICT0001.JPG (145 downloads)
Re: Mast theory [Re: Smiths_Cat] #145251
06/15/08 04:39 PM
06/15/08 04:39 PM
Joined: Sep 2007
Posts: 571
Hamburg
Smiths_Cat Offline
addict
Smiths_Cat  Offline
addict

Joined: Sep 2007
Posts: 571
Hamburg
and #2

Attached Files
148321-PICT0002.JPG (156 downloads)
Re: Mast theory [Re: Smiths_Cat] #145252
06/15/08 06:25 PM
06/15/08 06:25 PM
Joined: Jun 2004
Posts: 502
Port Noarlunga, SA, Australia
D
Darryn Offline
addict
Darryn  Offline
addict
D

Joined: Jun 2004
Posts: 502
Port Noarlunga, SA, Australia
Sail doesn't have a good shape.

Using a tape measure with the sail laying flat on the ground I mark the sail at my max draft position. I use sticky back sailcloth, 3 on each side, contrasting colour to sail colour. I would relocate your tell tales to Max draft, one 15cm closer to the mast and a couple on the leech side of Max draft, same spacing. Should be easy to tune your sail with that setup and a couple of leech ribbons.

I read a quote from Dennis Conner once to the effect of "only an idiot uses the battens that come with a sail". Typically blunt but it is a good point. Battens need work too.

Darryn
Mozzie
1782

Re: Mast theory [Re: Darryn] #145253
06/16/08 01:24 AM
06/16/08 01:24 AM
Joined: Sep 2007
Posts: 571
Hamburg
Smiths_Cat Offline
addict
Smiths_Cat  Offline
addict

Joined: Sep 2007
Posts: 571
Hamburg
Hi Darryn,

thanks for the hints. Please could you specify what is wrong with sail shape and battens (e.g. too much camber, too flat, too hard, too soft, etc.)? Maybe you have a picture of a well shaped sail, which you want to post.

There are tell tales in the sail, but the poor quality of the digital camera makes them hard to see. However I know where and when the sail stalls (just above the boom). But I am not (yet) able to stall the sail at the top as well (e.g. cannot close the leech enough?).
Thanks.

Cheers,

Klaus

Re: Mast theory [Re: Smiths_Cat] #145254
06/16/08 05:41 AM
06/16/08 05:41 AM
Joined: Jun 2004
Posts: 502
Port Noarlunga, SA, Australia
D
Darryn Offline
addict
Darryn  Offline
addict
D

Joined: Jun 2004
Posts: 502
Port Noarlunga, SA, Australia
Looks to me like max draft is too far back at the bottom of the sail and too far forward towards the top.

At the second from top batten the leach seems to hook inwards. I would try a stiffer batten.

Try sanding the bottom batten to encourage some bend at max draft. Not much sanding. The batten also doesn't exit the batten holder on the luff squarely. Loose?

Looks like it needs more downhaul. Some more pics with increasing amounts of downhaul and mainsheet tension would be interesting. Force your rotation to your normal upwind setting. Might be all your sail needs as there is not much mast bend. Position your boat on a beat when you take the pics, need some wind too, hold the boat down. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Marking your sail and moving your telltales takes the guesswork out of setting your sail up.

Strangely I have some pictures of mainsails from the same angle <img src="http://www.catsailor.com/forums/images/graemlins/smirk.gif" alt="" /> I will find a real ugly one and we can pick it to bits.

Darryn
Mozzie
1782

Re: Mast theory [Re: Smiths_Cat] #145255
06/16/08 06:07 AM
06/16/08 06:07 AM
Joined: Jun 2004
Posts: 502
Port Noarlunga, SA, Australia
D
Darryn Offline
addict
Darryn  Offline
addict
D

Joined: Jun 2004
Posts: 502
Port Noarlunga, SA, Australia
Good sail, but not setup right.

Whats wrong?

Darryn
Mozzie
1782

Attached Files
148349-ugly.jpg (112 downloads)
Page 2 of 3 1 2 3

Moderated by  Damon Linkous 

Search

Who's Online Now
0 registered members (), 17 guests, and 89 spiders.
Key: Admin, Global Mod, Mod
Newest Members
toomanyhulls, madina9900, johngava, badcatsailor, wehnet
8119 Registered Users
Top Posters(30 Days)
Forum Statistics
Forums26
Topics22,395
Posts267,036
Members8,119
Most Online1,650
Sep 10th, 2019
--Advertisement--
Powered by UBB.threads™ PHP Forum Software 7.7.1