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Rake back the mast ....... a mystery ! #192598
10/04/09 10:05 AM
10/04/09 10:05 AM
Joined: Oct 2008
Posts: 31
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cooper engineer Offline OP
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cooper engineer  Offline OP
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Please help me !
I have tried in much forum and pages web but are't successful to find a convincing answer to my question:
Rake back the mast the mainsail aerodynamically improves the performance? ..... and why ?

Thanks

Last edited by cooper engineer; 10/04/09 10:14 AM.
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Re: Rake back the mast ....... a mystery ! [Re: cooper engineer] #192600
10/04/09 10:52 AM
10/04/09 10:52 AM
Joined: Apr 2006
Posts: 699
SE Pa. or Chesapeak Bay
HMurphey Offline
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HMurphey  Offline
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SE Pa. or Chesapeak Bay
Depends on the boat .... Yes, Daggerboards??? No Daggerboards ....

The H16 likes to have alot of mast rake ... No daggerboards

The TheMightyHobie18 likes only a little mast rake ... Yes, daggerboards


To mathmatically prove it, takes some very involved calculations to prove why involving both hydrodynamics and aeodynamics. But it involves where the "center of effort" is on the sail and where the "center of effort is on the hull and underwater foils ... and how they combine.

Harry

Re: Rake back the mast ....... a mystery ! [Re: HMurphey] #192601
10/04/09 11:25 AM
10/04/09 11:25 AM
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cooper engineer Offline OP
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cooper engineer  Offline OP
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Sorry , yes Daggerboards ...TheMightyHobie18 ..... but how mach "little" mast rake ? where is the limit ?

Re: Rake back the mast ....... a mystery ! [Re: cooper engineer] #192604
10/04/09 11:56 AM
10/04/09 11:56 AM
Joined: Jun 2001
Posts: 3,293
Long Beach, California
John Williams Offline
Carpal Tunnel
John Williams  Offline
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The limit is your ability to get a firm leech on the sail. Block-to-block on the main system with a firm leech is as far as you can go.


John Williams

- The harder you practice, the luckier you get -
Gary Player, pro golfer

After watching Lionel Messi play, I realize I need to sail harder.
Re: Rake back the mast ....... a mystery ! [Re: John Williams] #192606
10/04/09 03:03 PM
10/04/09 03:03 PM

S
Scarecrow
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Scarecrow
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Cooper, mast rake hanges a few things and as a result a full explaination is complicated. But the main reason it helps is to do with lift distribution.

with the mast upright the centre boards are providing most of the hydrodynamic lift that stops your boat going sideways. In order to develop this lift your boat must go sideways a little (leeway) so that the angle of attack of the symetrical foils is sufficient to generate lift. If you rake your mast back then the cntre of effort moves aft so that now the rudders are also being asked to supply lift.

by splitting the load between the boards and rudders you have reduced the amount of lift required from the centre boards and therefor the amount of leeway you'll need to generate said lift. As a result you'll point higher.

As JW says the limit is defined by when you go vlock to block on your main, but before that happens you may find you've developed to much weather helm, making your boat hard to sail.

Last edited by Scarecrow; 10/04/09 03:04 PM.
Re: Rake back the mast ....... a mystery ! [Re: ] #192607
10/04/09 03:15 PM
10/04/09 03:15 PM
Joined: May 2003
Posts: 4,451
West coast of Norway
Rolf_Nilsen Offline
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West coast of Norway
Originally Posted by Scarecrow


As JW says the limit is defined by when you go vlock to block on your main, but before that happens you may find you've developed to much weather helm, making your boat hard to sail.


Hard to sail = slower, just to point that out smile

There are also some zany mathematics on how pressure distribution changes as the leading edge of a foil is raked aft, how it becomes more stall tolerant and loads of other stuff that is relatively "far out".
My thumb of rule on mast rake: Rake the mast aft until you can let go of the helm when going upwind in your dominant conditions and with crew/sails trimmed for best VMG.

We experimented with rake a lot while sailing the Tornado. For us the class standard ment that the rudder(s) at times was overloaded and lost grip. Then we had to sheet out to regain control. When we corrected this we became faster. We sailed at 200kgs crew weight which was not the norm. Find the setting that suits you and that is your setting cool

Re: Rake back the mast ....... a mystery ! [Re: cooper engineer] #192608
10/04/09 04:28 PM
10/04/09 04:28 PM
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Budapest, Hungary, Europe
CatSailingHu Offline
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Budapest, Hungary, Europe
Originally Posted by cooper engineer
Sorry , yes Daggerboards ...TheMightyHobie18 ..... but how mach "little" mast rake ? where is the limit ?


My really simple rule (no mathematics.. ) :

rake the mast forward! until you can manage the boat in downwind without pichpole or without to much risk at the given conditions (wind and waves).






Sanyi
Nacra Infusion
www.catsailing.hu
Re: Rake back the mast ....... a mystery ! [Re: cooper engineer] #192612
10/04/09 05:37 PM
10/04/09 05:37 PM
Joined: Feb 2007
Posts: 266
UK
Cheshirecatman Offline
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Aside from the H16 trim application where you can vary the CLR, a heavily raked mast can help create a rollover vortex to delay separation on the low pressure side.

Cheshirecatman

Re: Rake back the mast ....... a mystery ! [Re: Cheshirecatman] #192622
10/05/09 12:36 AM
10/05/09 12:36 AM
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cooper engineer Offline OP
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cooper engineer  Offline OP
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Originally Posted by Cheshirecatman
Aside from the H16 trim application where you can vary the CLR, a heavily raked mast can help create a rollover vortex to delay separation on the low pressure side.

Cheshirecatman


Finally a technical explanation!

Can give a detailed explanation of this vortex?

Re: Rake back the mast ....... a mystery ! [Re: cooper engineer] #192623
10/05/09 05:31 AM
10/05/09 05:31 AM
Joined: Feb 2007
Posts: 266
UK
Cheshirecatman Offline
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UK
With the limited rake of a sailboat mast it is more likely a series of inconsistent vortices which will help delay separation. The result is greater tolerance to sail trim and less tendancy to stall.
I believe there is benefit to be had from deliberately generating more controlled vortices to do this. This has been the subject of previous threads.

Cheshirecatman

Re: Rake back the mast ....... a mystery ! [Re: ] #192627
10/05/09 06:44 AM
10/05/09 06:44 AM
Joined: Nov 2005
Posts: 1,203
uk
TEAMVMG Offline
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Originally Posted by Scarecrow
Cooper, mast rake hanges a few things and as a result a full explaination is complicated. But the main reason it helps is to do with lift distribution.

with the mast upright the centre boards are providing most of the hydrodynamic lift that stops your boat going sideways. In order to develop this lift your boat must go sideways a little (leeway) so that the angle of attack of the symetrical foils is sufficient to generate lift. If you rake your mast back then the cntre of effort moves aft so that now the rudders are also being asked to supply lift.

by splitting the load between the boards and rudders you have reduced the amount of lift required from the centre boards and therefor the amount of leeway you'll need to generate said lift. As a result you'll point higher.

As JW says the limit is defined by when you go vlock to block on your main, but before that happens you may find you've developed to much weather helm, making your boat hard to sail.


That all makes pretty good sense, but would also mean that moving the whole rig backwards would have the same effect. [yes that would mean moving the front beam]

there does seem to be some benefit from having the mast angled back from the vertical. Could it be that some lift is generated? A bit like the long spinny pole on a H16 rakes the spinnaker luff and generates lift?


Paul

teamvmg.weebly.com
Re: Rake back the mast ....... a mystery ! [Re: TEAMVMG] #192679
10/05/09 04:07 PM
10/05/09 04:07 PM

S
Scarecrow
Unregistered
Scarecrow
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S



Some light reading on the aerodymanic component of mast rake.

Marchaj


Re: Rake back the mast ....... a mystery ! [Re: ] #192684
10/05/09 04:30 PM
10/05/09 04:30 PM

A
andrewscott
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andrewscott
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A



Available from these sellers. - 1 used from $999.99

$999.99 ???????????? unless the book comes with a new main sail..... WhatTheHECK ???

Re: Rake back the mast ....... a mystery ! [Re: ] #192694
10/05/09 05:34 PM
10/05/09 05:34 PM
Joined: Feb 2007
Posts: 266
UK
Cheshirecatman Offline
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Cheshirecatman  Offline
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Re: Rake back the mast ....... a mystery ! [Re: Cheshirecatman] #192704
10/05/09 07:27 PM
10/05/09 07:27 PM
Joined: Oct 2005
Posts: 951
Brisbane, Queensland, Australi...
ncik Offline
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ncik  Offline
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A-class cats with their highly efficient and very vertical masts would disagree with the vortex theory.

Scarecrow's explanation is as technical as needed for improving upwind performance. The trick is finding the right balance for fastest times around the course. Balance of lift to drag as well as upwind and downwind performance.

Re: Rake back the mast ....... a mystery ! [Re: ncik] #192706
10/05/09 08:33 PM
10/05/09 08:33 PM
Joined: Dec 2007
Posts: 40
Herbie53 Offline
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Herbie53  Offline
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+1 - 2 Day Acat regatta a couple weekends ago, first day I was going upwind as fast or faster than anyone, only to get my clock completely cleaned downwind (3 or 4 boats per leg). Moved the mast forward 1 pin for day two and was able to mostly hold my position downwind (always somewhat of a challenge since I'm 215#) in spite of usually deadly (to me) light air. I think optimum rake is a very situation and boat specific thing... more is not always better.

Originally Posted by ncik
A-class cats with their highly efficient and very vertical masts would disagree with the vortex theory.

Scarecrow's explanation is as technical as needed for improving upwind performance. The trick is finding the right balance for fastest times around the course. Balance of lift to drag as well as upwind and downwind performance.

Re: Rake back the mast ....... a mystery ! [Re: Herbie53] #192709
10/06/09 12:59 AM
10/06/09 12:59 AM
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Posts: 31
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cooper engineer Offline OP
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..... as imagined, there isn't a definitive solution.

Re: Rake back the mast ....... a mystery ! [Re: cooper engineer] #192731
10/06/09 08:37 AM
10/06/09 08:37 AM

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andrewscott
Unregistered
andrewscott
Unregistered
A



Originally Posted by cooper engineer
..... as imagined, there isn't a definitive solution.

There is no 1 asnwer to a question with so many variables....

Even with all the variables... the great sailors almost always finish in the top spots... so there is non-definitive solution !

Re: Rake back the mast ....... a mystery ! [Re: ] #192803
10/06/09 05:11 PM
10/06/09 05:11 PM
Joined: Jun 2001
Posts: 733
Home is where the harness is.....
Will_R Offline
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Will_R  Offline
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Home is where the harness is.....
Also, don't forget a/b Uni's and the location of your daggers. The further aft your daggers are, the more rake you will HAVE to run to balance all the CE's.

Re: Rake back the mast ....... a mystery ! [Re: Will_R] #192859
10/07/09 10:49 AM
10/07/09 10:49 AM
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Posts: 5,590
Naples, FL
waterbug_wpb Offline
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is that like the 'shared lift' concept of the ARC/supercat designer?

so if you moved the boards well forward, and kept the rake more vertical, does sailplan efficiency increase? Kind of like swept wings vs. straight wings in aircraft?


Jay

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