Sailboat maximum airespeed - SailNet Community

   Search Sailnet:

 forums  store  


Quick Menu
Forums           
Articles          
Galleries        
Boat Reviews  
Classifieds     
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Charts
Clothing
Electrical
Electronics
Engine
Hatches and Portlights
Interior And Galley
Maintenance
Marine Electronics
Navigation
Other Items
Plumbing and Pumps
Rigging
Safety
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here






Go Back   SailNet Community > General Interest > General Discussion (sailing related)
 Not a Member? 


Reply
 
LinkBack Thread Tools
  #1  
Old 01-17-2012
Member
 
Join Date: Nov 2010
Posts: 32
Thanks: 0
Thanked 1 Time in 1 Post
Rep Power: 0
patrickbryant is on a distinguished road
Sailboat maximum airspeed

Colleagues:

After completing a particularly challenging voyage on the S.F. Bay last Sunday, it occurred to me that there must be a reason why my boat progressively slowed down while I was on a close reach into an increasing wind (topping out at about 40 knots). Any comments on my explanation below will be greatly appreciated.

There appears to be a maximum air speed (wind speed) at which the sails of a sailboat on a reaching point of sail can no longer overcome the induced drag produced by the sails themselves plus the parasitic drag of the wind-exposed area of the boat (the combined exposure of the freeboard and all of the other wind-exposed areas). Above that speed, "the curves cross" and the sails can no longer produce enough lift (forward propulsion) to overcome the aerodynamic drag of the boat -- so an equilibrium is reached: the boat will go no faster. If the air speed increases, the boat will slow down.

Lift (propulsion) of an airfoil (wing or sail) increases as the square of the velocity. But the power needed to overcome drag increases as the cube of velocity, so at some velocity, drag overcomes lift (propulsion). The same effect is seen with airplanes: if an airplane that cruises at 100 knots requires 85 horsepower to maintain level flight (lift-to-drag equilibrium), the amount of power needed to make the same airplane cruise level at 200 knots (assuming you could add a bigger engine without increasing the airplane’s weight) would be 85 X (2 cubed) , or 85 X 8 = 680 horsepower. In the case of a sailboat penetrating forward (reaching) in the airmass, when the wind speed doubles, the drag increases by a factor of 8, while the lift (propulsion) produced by the sails only increases by only a factor of 4. Equilibrium is reached at some critical speed, and the boat will go no faster. Increase the wind speed any more, and the boat slows down.

The challenge of reaching into a high wind can be mitigated somewhat by reducing the sail area (reefing), which reduces its induced and parasitic drag more than the reduction in lift, but the parasitic drag produced by the freeboard and other exposed areas (cabin top, rigging, etc.) cannot be reduced. So, regardless of what one does, any given boat has a maximum wind speed into which it can reach before it begins to slow down. The more freeboard and other wind exposed surfaces, the lower is that maximum speed.

Last edited by patrickbryant; 01-17-2012 at 03:36 PM.
Reply With Quote Share with Facebook
  #2  
Old 01-17-2012
casey1999's Avatar
Senior Member
 
Join Date: Oct 2010
Location: HI
Posts: 2,845
Thanks: 4
Thanked 17 Times in 17 Posts
Rep Power: 4
casey1999 is on a distinguished road
I think you are correct. Take it to the extreme. Could you sail to windward in a 150 knot wind? I would say no, the drag from your boats "wing" (sail) and the drag on your boat (plus wave impacts) would slow your progress to the point I think you would go backwards.
Reply With Quote Share with Facebook
  #3  
Old 01-17-2012
Senior Member
 
Join Date: Sep 2011
Location: Massachusetts
Posts: 132
Thanks: 1
Thanked 0 Times in 0 Posts
Rep Power: 4
mm2187 is on a distinguished road
I am not sure that I agree with your way of looking at it. I think it comes down to the boats ability... not equilibrium(granted I do not know any boat that can safely sail in 150 knots of wind). I can not think of any boat that has as much freeboad/mast and standing rigging that equils the amount of sail area possible. therfore there will never be equilibrium from that. Your forward momentium is driven by your boats ability to stay perpendicular to the water as the wind increases your ability to stay up in return decreases your power in your sails which will slow you down.
__________________
Schock - New York 36
Portsmouth, RI
Reply With Quote Share with Facebook
  #4  
Old 01-17-2012
casey1999's Avatar
Senior Member
 
Join Date: Oct 2010
Location: HI
Posts: 2,845
Thanks: 4
Thanked 17 Times in 17 Posts
Rep Power: 4
casey1999 is on a distinguished road
Quote:
Originally Posted by mm2187 View Post
I am not sure that I agree with your way of looking at it. I think it comes down to the boats ability... not equilibrium(granted I do not know any boat that can safely sail in 150 knots of wind). I can not think of any boat that has as much freeboad/mast and standing rigging that equils the amount of sail area possible. therfore there will never be equilibrium from that. Your forward momentium is driven by your boats ability to stay perpendicular to the water as the wind increases your ability to stay up in return decreases your power in your sails which will slow you down.
I used 150 knots as a outrages extreme so that one could clearly see no standard boat could make headway in such a storm, and there must be a point where a sailboat cannot make forward motion against a certain wind speed. As wind speed increses, you must reef (reduce sail) so there could be a point where the wind friction of the rig and boat is greater than the forward force gernerated by the sail. Also, the actual force acting to make a sailboat go forward is a small fraction of the total forces acting on the boat- that is why there is a keel, to counter the leeward forces.
Reply With Quote Share with Facebook
  #5  
Old 01-17-2012
Senior Member
 
Join Date: Sep 2006
Location: Texas
Posts: 526
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 9
CapnBilll is on a distinguished road
In any point of sail there reaches a point where the forces exerted on the sail needed to further increase boat speed will only blow out a sail.
__________________
The Sun has Risen on a New Day filled with the Promise of Adventure.
Reply With Quote Share with Facebook
  #6  
Old 01-17-2012
casey1999's Avatar
Senior Member
 
Join Date: Oct 2010
Location: HI
Posts: 2,845
Thanks: 4
Thanked 17 Times in 17 Posts
Rep Power: 4
casey1999 is on a distinguished road
Sailing faster than the wind - Wikipedia, the free encyclopedia
A good link
Reply With Quote Share with Facebook
  #7  
Old 01-17-2012
Member
 
Join Date: Jul 2011
Location: Stockton CA
Posts: 30
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 0
curtcee is on a distinguished road
Quote:
Originally Posted by casey1999 View Post
That's a good article. Thanks for posting it.

Curtis
Reply With Quote Share with Facebook
  #8  
Old 01-17-2012
Faster's Avatar
Just another Moderator
 
Join Date: Sep 2005
Location: New Westminster, BC
Posts: 14,700
Thanks: 69
Thanked 197 Times in 189 Posts
Rep Power: 10
Faster has a spectacular aura about Faster has a spectacular aura about Faster has a spectacular aura about
I think the greater hindrances are hydrodynamic drags... low drag multihulls, esp those on foils are going to go much faster than our lead bombs.

Could be those craft (and ice boats) may get a chance to run into the aerodynamic limits, but since we're working in two distinct elements I think the aerodynamic ones won't come into play for the average boat....

This question is probably a good academic one, though, when dealing with extreme winds in the 100 Knt + range...
__________________
Ron

1984 Fast/Nicholson 345 "FastForward"

".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
Reply With Quote Share with Facebook
  #9  
Old 01-17-2012
GeorgeB's Avatar
Senior Member
 
Join Date: Dec 2004
Location: Alameda, San Francisco Bay
Posts: 1,503
Thanks: 1
Thanked 36 Times in 35 Posts
Rep Power: 10
GeorgeB is on a distinguished road
Patrick, you need to find a copy of the polar diagram for your boat. This will tell you the maximum speed potential for windspeed, point of sail and sail size. Remember, as a displacement boat, you are constrained on the top end by your theoretical hull speed. I donít know what boats you sail on the Bay, but I have had the experience of going into horizontal stalls a couple of times on light displacement boats where a gust pined us over and we didnít have enough displacement/ momentum to carry forward boatspeed. Wound up getting dragged sideways a little bit. On our family cruiser, we do not have the rail crew to hold the boat upright and the constant feathering and square waves/chop knocks some boat speed off.

Most importantly, where were you sailing and what type of boat? I had to take care of some family business and wasnít able to get out on the Bay. Sounds like I missed a good one.
Reply With Quote Share with Facebook
  #10  
Old 01-17-2012
-OvO-'s Avatar
Owl
 
Join Date: Dec 2011
Posts: 217
Thanks: 0
Thanked 2 Times in 2 Posts
Rep Power: 3
-OvO- is on a distinguished road
Time to build a flying vertical wing...
Reply With Quote Share with Facebook
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

 
Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Maximum Time Between Haul-Outs casey1999 Gear & Maintenance 24 12-29-2011 01:35 PM
Maximum Draft in Western Erie MooreVOLS Lake Erie 8 07-25-2011 01:15 PM
Cal 39 maximum RPMs jigs711 Cal 2 07-14-2010 03:39 PM
Yanmar SB 12 wont reach maximum RPM in neutral weststar Diesel 8 02-28-2010 06:22 PM
Maximum mizzen height for canals ClubOrlov Cruising & Liveaboard Forum 2 12-03-2006 08:47 PM


All times are GMT -4. The time now is 05:16 AM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012

The SailNet.com store is owned and operated by a company independent of the SailNet.com forum. You are now leaving the SailNet forum. Click OK to continue or Cancel to return to the SailNet forum.