Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 04-03-2013, 07:10   #46
Moderator Emeritus
 
David M's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: San Francisco Bay
Boat: research vessel
Posts: 10,150
Re: Dagger Boards --- I don't quite get it

Quote:
Originally Posted by Limpet View Post
Forgot I started this thread long ago.

Aren't most dagger board shapes symmetrical...in which case there is no lift because the water flowing over the surfaces on both sides is moving at the same speed.
An airplane wing provides lift because the air travels faster over the longer top surface thus creating a low pressure on top surface of the wing relative to the pressure below the wing.
This makes for the most efficient shape but it is the angle of attack that determines the amount of lift. You could take a sheet of plywood, give it an angle of attack and it would produce lift, no flat and rounded side necessary.

A rounded backside helps to preserve the laminar flow, without it there is a greater inclination for there to be a vortice in the same area if it were a flat backside. A laminar flow creates the greatest pressure differential, for the amount of drag, between the low pressure and high pressure sides of the foil.

Quote:
Originally Posted by ElGatoGordo View Post
Not sure what you're asking. I haven't heard of asymmetrical boards. It would only generate lift on one tack...sounds like a pain to me.
An asymetrical board with the high pressure side being on the rounded side would still generate lift, otherwise an airplane would never be able to fly inverted. Doing it this way is just less efficient with there being a lot more drag.
__________________

__________________
David

Life begins where land ends.
David M is offline   Reply With Quote
Old 04-03-2013, 07:19   #47
Registered User
 
ElGatoGordo's Avatar

Join Date: Nov 2012
Location: Kemah, TX
Boat: Jim Michalak designed 25' Caprice and Self Design 32' Shantymaran
Posts: 711
Re: Dagger Boards --- I don't quite get it

Quote:
Originally Posted by David M View Post
This makes for the most efficient shape but it is the angle of attack that determines the amount of lift. You could take a sheet of plywood, give it an angle of attack and it would produce lift, no flat or rounded side necessary.
Hence:

__________________

__________________
------------------
Gordo
ElGatoGordo is offline   Reply With Quote
Old 04-03-2013, 07:50   #48
Registered User
 
dan360's Avatar

Join Date: Jan 2012
Boat: Outremer 5X
Posts: 110
Re: Dagger Boards --- I don't quite get it

It's worth noting, that depending on who you ask you'll get different recommendations on how to use your boards.

Windward board down when sailing upwind for safety first, OR
Leeward board down when sailing upwind for performance first.
Some of both boards down when sailing downwind for stability.

I have a table from VPLP with recommendations at each angle/windspeed combination for what do do with each board (full down, full up or partial), but Outremer's recommendations are in some cases opposite with a preference for safety.
__________________
dan360 is offline   Reply With Quote
Old 04-03-2013, 08:13   #49
Registered User
 
Steve_C's Avatar

Join Date: Apr 2009
Location: San Antonio, Texas
Boat: Soon to be a Cat
Posts: 212
Re: Dagger Boards --- I don't quite get it

Chris White Atlantic 57.
Attached Thumbnails
Click image for larger version

Name:	daggerboard.jpg
Views:	184
Size:	80.5 KB
ID:	56169  
__________________
_________________
_____________
Steve
Steve_C is offline   Reply With Quote
Old 04-03-2013, 08:16   #50
Registered User
 
dan360's Avatar

Join Date: Jan 2012
Boat: Outremer 5X
Posts: 110
Re: Dagger Boards --- I don't quite get it

On that Chris White picture - which hull is that? There's an explicit requirement with that asymmetry which tack to use which board on. Looking at the picture it is Port? (as the lifting lines presumably lead to the cockpit).
__________________
dan360 is offline   Reply With Quote
Old 04-03-2013, 08:30   #51
Registered User
 
Steve_C's Avatar

Join Date: Apr 2009
Location: San Antonio, Texas
Boat: Soon to be a Cat
Posts: 212
Re: Dagger Boards --- I don't quite get it

Yep, sorry I meant to state that. port side with the bow to the left of my feet. very clearly these boards are meant to be used one at a time.
__________________
_________________
_____________
Steve
Steve_C is offline   Reply With Quote
Old 04-03-2013, 08:38   #52
Registered User
 
Tar34's Avatar

Join Date: Dec 2011
Posts: 291
Re: Dagger Boards --- I don't quite get it

Rethink the obvious....You want lift? You got lift. Less drag? That too! No it's not Broken

__________________
Tar34 is offline   Reply With Quote
Old 04-03-2013, 08:55   #53
Registered User
 
ElGatoGordo's Avatar

Join Date: Nov 2012
Location: Kemah, TX
Boat: Jim Michalak designed 25' Caprice and Self Design 32' Shantymaran
Posts: 711
There you go. I'm way to lazy for that, but out would clearly be a race/performance advantage
__________________
------------------
Gordo
ElGatoGordo is offline   Reply With Quote
Old 04-03-2013, 16:29   #54
Senior Cruiser
 
44'cruisingcat's Avatar

Cruisers Forum Supporter

Join Date: Sep 2006
Posts: 7,453
Images: 69
Re: Dagger Boards --- I don't quite get it

Quote:
Originally Posted by Limpet View Post
I can certainly understand that the Dagger Board approach yields greater speeds sailing down wind when the boards are up, but I don't quite understand the claim that the boards allow closer pointing to the wind.

With the Main sheeted all the way in, as you put the bow closer and closer into the wind, there is a smaller and smaller component of force pushing the Cat forward. The Main does not know if the Hull has a Keel Or Dagger boards and the forward forces are the same.

So how is it dagger boards can yield closer pointing? Is it simply the reduced resistance thus the component of forward force is more effective? If this is the case, then I don't see how Monohulls can point closer, as they have a heavy displacement will lots of resistance to motion.
Getting back to the original question, dagger boards won't neccessarily let you POINT higher. What they do is let you SAIL higher by reducing leeway much more effectively than mini keels.
__________________
44'cruisingcat is offline   Reply With Quote
Old 04-03-2013, 17:07   #55
Senior Cruiser

Cruisers Forum Supporter

Join Date: Dec 2006
Location: Seattle
Boat: Cal 40
Posts: 2,401
Images: 7
Re: Dagger Boards --- I don't quite get it

Quote:
Originally Posted by 44'cruisingcat View Post
Getting back to the original question, dagger boards won't neccessarily let you POINT higher. What they do is let you SAIL higher by reducing leeway much more effectively than mini keels.
And high lift to drag devices (daggerboards) allow you to have much less surface area, smaller area board is as effective or more effective than much bigger surface area keel, which means less drag, higher speed and so better progress to weather overall.
__________________
cal40john is offline   Reply With Quote
Old 04-03-2013, 19:12   #56
Registered User
 
Tar34's Avatar

Join Date: Dec 2011
Posts: 291
Re: Dagger Boards --- I don't quite get it

Quote:
Originally Posted by cal40john View Post
And high lift to drag devices (daggerboards) allow you to have much less surface area, smaller area board is as effective or more effective than much bigger surface area keel, which means less drag, higher speed and so better progress to weather overall.
Sparkman Stevens applied the high aspect foil in it's Tartan 34C design
__________________

__________________
Tar34 is offline   Reply With Quote
Reply

Tags
dagger boards

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Fender boards exranger Construction, Maintenance & Refit 28 08-06-2009 20:23
Dagger board dilemma West Coast Cat Multihull Sailboats 8 07-03-2008 14:51
new to these boards nancynana Meets & Greets 14 13-11-2007 17:52
Squeaky Floor Boards Holding Pattern Construction, Maintenance & Refit 16 06-05-2007 04:06
Can someone delete the SPAM on the boards? AnchorageGuy Forum Tech Support & Site Help 8 19-11-2004 14:02



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 09:55.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.