Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 28-12-2014, 12:18   #1
Senior Cruiser
 
unbusted67's Avatar

Cruisers Forum Supporter

Join Date: Dec 2006
Location: Islesboro, ME
Boat: Looking for a new boat
Posts: 2,199
Images: 24
Confused by race boat speeds and boat handling

Ok so you are on an Americas cup boat and you have 10 knots of breeze on the port quarter. You start on a broad reach. As the boat accelerates the apparent wind angle creeps forward to the point where the boat speed is actually exceeding the true wind speed thus creating even more apparent wind? Can someone please explain this to me? Isn't there a law of diminishing returns at play? What happens when one must adjust course and jibe through the true wind angle?


Sent from my iPhone using Cruisers Sailing Forum
__________________

__________________
__________________________________________
Unbusted67 or just Ben
unbusted67 is offline   Reply With Quote
Old 28-12-2014, 12:25   #2
Registered User
 
Suijin's Avatar

Join Date: Feb 2013
Location: Annapolis MD; currently in Oriental NC
Boat: Valiant 40
Posts: 2,938
Re: Confused by race boat speeds and boat handling

Quote:
Originally Posted by unbusted67 View Post
Ok so you are on an Americas cup boat and you have 10 knots of breeze on the port quarter. You start on a broad reach. As the boat accelerates the apparent wind angle creeps forward to the point where the boat speed is actually exceeding the true wind speed thus creating even more apparent wind? Can someone please explain this to me? Isn't there a law of diminishing returns at play? What happens when one must adjust course and jibe through the true wind angle?


Sent from my iPhone using Cruisers Sailing Forum
The AC72s sail so fast downwind that they are actually beating into apparent wind. There is a law of diminishing returns, which is why the boats hit a limit. Aerodynamic drag on the wind foil and hydrodynamic drag limit the boat's ability to accelerate past a certain point. I think they only sailed in 18 knots of wind or less...might have been 15, I don't remember. But they were seeing speeds of up to 45 knots all the same.

And as far as jibing and tacking go, they very often don't, depending on the wind direction, if they carry enough speed through the turn. That was in fact one of the key differentiators in the performance of teams in last years races...those that had it down to where they could foil through a turn excelled.
__________________

__________________
Suijin is offline   Reply With Quote
Old 28-12-2014, 12:28   #3
Senior Cruiser
 
Sailmonkey's Avatar

Cruisers Forum Supporter

Join Date: Sep 2009
Location: Houston
Boat: '76 Allied Seawind II, 32'
Posts: 5,793
Re: Confused by race boat speeds and boat handling

The best way to understand it is to experience it.....on a lesser scale. Try to go sailing on a modern beach cat like an f18 or f16, they often move the apparent wind forward, limited by hull drag more so than airfoil drag.


Sent from my iPhone using Cruisers Sailing Forum
__________________
Sailmonkey is online now   Reply With Quote
Old 28-12-2014, 12:52   #4
Senior Cruiser
 
44'cruisingcat's Avatar

Cruisers Forum Supporter

Join Date: Sep 2006
Posts: 7,459
Images: 69
Re: Confused by race boat speeds and boat handling

Quote:
Originally Posted by Suijin View Post
The AC72s sail so fast downwind that they are actually beating into apparent wind. There is a law of diminishing returns, which is why the boats hit a limit. Aerodynamic drag on the wind foil and hydrodynamic drag limit the boat's ability to accelerate past a certain point. I think they only sailed in 18 knots of wind or less...might have been 15, I don't remember. But they were seeing speeds of up to 45 knots all the same.

And as far as jibing and tacking go, they very often don't, depending on the wind direction, if they carry enough speed through the turn. That was in fact one of the key differentiators in the performance of teams in last years races...those that had it down to where they could foil through a turn excelled.
I read somewhere that the difference in AWA between sailing upwind and downwind, was 4 degrees!


Even more confusing - on the downwind legs it was actually an ADVANTAGE to push against a stronger tide.


Sounds totally wrong, but, because the boats were sailing at around 3 times windspeed, an extra knot of current meant an extra knot of wind, which equalled 3 knots boatspeed - a net gain of two knots...
__________________
44'cruisingcat is offline   Reply With Quote
Old 28-12-2014, 13:49   #5
Registered User

Join Date: Jan 2010
Location: Port Ludlow Wa
Boat: Makela,Ingrid38,Idora
Posts: 1,975
Re: Confused by race boat speeds and boat handling

Arrgh! Its all PFM.
__________________
IdoraKeeper is offline   Reply With Quote
Old 28-12-2014, 13:55   #6
Registered User
 
Suijin's Avatar

Join Date: Feb 2013
Location: Annapolis MD; currently in Oriental NC
Boat: Valiant 40
Posts: 2,938
Re: Confused by race boat speeds and boat handling

Quote:
Originally Posted by 44'cruisingcat View Post

Even more confusing - on the downwind legs it was actually an ADVANTAGE to push against a stronger tide.


Sounds totally wrong, but, because the boats were sailing at around 3 times windspeed, an extra knot of current meant an extra knot of wind, which equalled 3 knots boatspeed - a net gain of two knots...
I'm having trouble understanding how that works, at least in terms of a foul current translating into greater apparent wind. I would have thought the reverse was true. Maybe it was that a foul current meant more lift from the foil and as a result better foiling performance?
__________________
Suijin is offline   Reply With Quote
Old 28-12-2014, 14:53   #7
Moderator
 
Paul Elliott's Avatar

Cruisers Forum Supporter

Join Date: Sep 2006
Posts: 3,888
Images: 4
Re: Confused by race boat speeds and boat handling

Quote:
Originally Posted by Suijin View Post
I'm having trouble understanding how that works, at least in terms of a foul current translating into greater apparent wind. I would have thought the reverse was true. Maybe it was that a foul current meant more lift from the foil and as a result better foiling performance?
I think this is correct. And not just foiling performance, but improved (traditional lateral) keel lift.
__________________
Paul Elliott, S/V VALIS - Pacific Seacraft 44 #16 - Friday Harbor, WA
www.sailvalis.com
Paul Elliott is offline   Reply With Quote
Old 28-12-2014, 16:19   #8
Registered User

Join Date: Dec 2009
Posts: 3,016
Images: 4
Re: Confused by race boat speeds and boat handling

Obviously, if the water and wind are moving at the same speed and in the same direction then no sailing is possible. All sailboats take advantage of the difference between the speeds and directions of wind and water. Where there is such a difference there is energy that can be exploited for some use. A sailboat exploits that difference to overcome drag and move in some direction. The race boats have reduced the drag to such a point that they are free to move in unexpected directions. With low enough drag that energy extracted from the difference between the speed and direction of wind and water can do most anything.
__________________
daddle is offline   Reply With Quote
Old 28-12-2014, 16:29   #9
Writing Full-Time Since 2014
 
thinwater's Avatar

Join Date: Nov 2008
Location: Deale, MD
Boat: PDQ Altair, 32/34
Posts: 4,355
Re: Confused by race boat speeds and boat handling

Yup, you tack down wind. My last cat would tack down wind if everything was handled just so, which was quite fun and much faster. Since the boom is not moving far and the wind angle is good, acceleration out of the tacks was brisk.

I always thought of it like tiddly-winks. Sure, there is an upper limit, but it has to do with forces and drag, not windspeed as a limit.
__________________
Gear Testing--Engineering--Sailing

Writing full-time since 2014.
Bookstore:http://sail-delmarva.blogspot.com/20...ook-store.html
thinwater is online now   Reply With Quote
Old 28-12-2014, 16:30   #10
Registered User

Join Date: Sep 2012
Posts: 4,974
Re: Confused by race boat speeds and boat handling

Correct me if I am wrong but I seem to remember that ice boats can get 3-4 times the true wind speed
__________________
robert sailor is online now   Reply With Quote
Old 29-12-2014, 13:52   #11
Senior Cruiser
 
44'cruisingcat's Avatar

Cruisers Forum Supporter

Join Date: Sep 2006
Posts: 7,459
Images: 69
Re: Confused by race boat speeds and boat handling

Quote:
Originally Posted by Suijin View Post
I'm having trouble understanding how that works, at least in terms of a foul current translating into greater apparent wind. I would have thought the reverse was true. Maybe it was that a foul current meant more lift from the foil and as a result better foiling performance?
Yes, it sounds completely counter intuitive. But that's because people like us don't get to sail at multiples of the windspeed.

Think of it this way:

Say you were in an AC72 sailing downwind in 2 knots of wind. You also have a two knot current in your favour.

That would mean you'd be drifting at two knots, with zero wind for your sails.

Then say you're in the same spot 6 hours later, with the same wind, but now the tide has reversed. Effectively you'll have 4 knots of wind to sail with. And those boats can sail at 2-3 times windspeed.....
__________________
44'cruisingcat is offline   Reply With Quote
Old 29-12-2014, 23:04   #12
Registered User

Join Date: Sep 2010
Location: Boat in Turkey, Beach cat in Israel
Boat: Lagoon 400 & Nacra 6.0 beach cat
Posts: 630
Re: Confused by race boat speeds and boat handling

Quote:
Originally Posted by thinwater View Post
Yup, you tack down wind. My last cat would tack down wind if everything was handled just so,
How do you tack downwind? Did you mean that the apparent wind was in front of the beam while you sailed down relative to the true wind?
Can you estimate the angle difference between the true and apparent at full speed?
I would guess that on a cruising cat the difference, while noticeable, cannot be large enough to move the wind from the quarter to the front.

I sail a fast beach cat and estimate the difference to be at the scale of 20-30deg. But we sail routinely at 15-20 knots of speed. On a boat sailing 6-8 knots the difference is supposedly much smaller.
__________________
Mark, S/Y Bat-Yam
meirriba is offline   Reply With Quote
Old 29-12-2014, 23:26   #13
Registered User

Join Date: Dec 2009
Posts: 3,016
Images: 4
Re: Confused by race boat speeds and boat handling

Quote:
Originally Posted by meirriba View Post
How do you tack downwind?
A tack is turning the bow of the boat towards and past the apparent wind. Not the true wind. The performance cats are able to maintain speed such that the apparent wind remains forward. If they fail to maintain sufficient speed then the tack suddenly becomes a gybe.

One needs to use vector arithmetic to understand what is going on. The three vectors are TrueWind, BoatWind and ApparentWind. BoatWind is the reciprocal of the BoatVelocity. One can easily make little drawings of arrows to graphically understand and prove the winds controlling these new maneuvers.
__________________
daddle is offline   Reply With Quote
Old 30-12-2014, 00:17   #14
Registered User

Join Date: Sep 2010
Location: Boat in Turkey, Beach cat in Israel
Boat: Lagoon 400 & Nacra 6.0 beach cat
Posts: 630
Re: Confused by race boat speeds and boat handling

Quote:
Originally Posted by marinduque View Post
A tack is turning the bow of the boat towards and past the apparent wind. Not the true wind. The performance cats are able to maintain speed such that the apparent wind remains forward. If they fail to maintain sufficient speed then the tack suddenly becomes a gybe.

One needs to use vector arithmetic to understand what is going on. The three vectors are TrueWind, BoatWind and ApparentWind. BoatWind is the reciprocal of the BoatVelocity. One can easily make little drawings of arrows to graphically understand and prove the winds controlling these new maneuvers.
Thanks for the explanation...
The original poster claimed that his cat was routinely tacked downwind and I wondered. If he has an AC boat than fine.
However, I did the analysis for a 12kn 135deg wind (on the limit of broad reach) and 8kn boat speed and got almost exact beam wind. This is not enough to tack in this situation (when you slow down during the manoeuvre).
So I come back to my original question. How do you tack downwind a cruising cat in what conditions,
__________________
Mark, S/Y Bat-Yam
meirriba is offline   Reply With Quote
Old 30-12-2014, 00:23   #15
Senior Cruiser
 
Jim Cate's Avatar

Cruisers Forum Supporter

Join Date: May 2008
Location: cruising SW Pacific
Boat: Jon Sayer 1-off 46 ft fract rig sloop strip plank in W Red Cedar
Posts: 11,471
Re: Confused by race boat speeds and boat handling

Actually, a lot of folks call what you are describing as "tackig downwind" when they are actually gybing all the time. The name isn't important, but the act of sailing higher angles rather than DDW does work for many boats. And for those that can reach higher speeds, it really is tacking!

Jim
__________________

__________________
Jim and Ann s/v Insatiable II , lying Port Cygnet, Tasmania once again
Jim Cate is offline   Reply With Quote
Reply

Tags
race

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
What's Safe Sailing Wind Speeds for 26ft Full Keel Boat canadianllama Seamanship & Boat Handling 46 29-06-2017 16:06
Cruise speeds and terminal hull speeds? djtopper Multihull Sailboats 13 07-09-2013 15:10
S&S34 Cruising speeds & equipment Steve Pereira Monohull Sailboats 6 12-12-2008 18:49
Epoxy - grinding surface speeds delmarrey Construction, Maintenance & Refit 11 06-08-2008 22:01
Real cruising multihull speeds Woods Designs Multihull Sailboats 47 20-07-2008 04:47



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 12:34.


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.