Taipan 28 - How well offshore? - Page 2 - 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 > Skills and Seamanship > Seamanship & Navigation
 Not a Member? 

Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


Like Tree11Likes
Reply
 
LinkBack Thread Tools
  #11  
Old 11-05-2003
Jeff_H's Avatar
Moderator
 
Join Date: Feb 2000
Location: Annapolis, Md
Posts: 6,664
Thanks: 5
Thanked 103 Times in 79 Posts
Rep Power: 10
Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about
Taipan 28 - How well offshore?

Mr, Coles assertion that "The effects of large movements of the VCG on the propensity to capsize was surprising small." is extremely dated and represents the understanding of capsizing that lead to the Fastnet disaster. The current understanding of the mechanics of capsize that came out of studies of models, full sized boats, and studies of actual capsizes, suggests that in most cases almost nothing is as critical in preventing a capsize than a low vertical center of gravity. L/D is length to displacement. It is not a linear formula but it is useful as a rough predictor of drag.

Drag is actually much more critical to the cruiser than the racer. In order to over come drag and make way to windward in heavy air, a high drag boat must carry more sail area than a more easily driven hull. This means that to claw off a lee shore, the boat with the large L/D must carry a lot more sail. The problem is that high L/D, long keel cruising hulls generally have a lot less stability relative to their drag than modern race boats which have very high stability compared to drag. A racer has a large crew to deal with sail trim in heavy going but cruisers are usually undermanned making it harder to carry the sail needed to claw off a lee shore. The problem gets worse on boats that are fin keel (by the classic definition where a fin keel is any keel where the bottom of the keel is 50% or less of the length of the boat or the length of the sail plan which ever is greater) with attached rudders, as I believe this boat is. Fin keel boats with attached rudders are notoriously poor at tracking and tend to develop heavy weather helm in high wind conditions making sail trim and sail shortening highly critical.

I can easily assure you that you are completely mistaken in your assumption that you have a AVS (which is currently referred to as the LPS which stands for Limit of positive stability) of 183 degrees. The highest LPS that any boat can have is 180 degrees which means that the boat is unstable when completely inverted (i.e. rotated to a heel angle of 180 degrees from a vertical position of zero degrees) If a boat is in equilibrium right side up it can''t have a LPS of 180 degrees even.

I do not know what "formula for angle of vanishing stability" you found but the only accurate way to calculate the LPS is to calculate a curve of the center of buoyancy at varying heel angles and to calculate the center of gravity. Calculating the center of gravity is relatively easy. You calculate the weight of every part of the boat times the distance from some fixed point. Total them and then divide the answer by the sum of the weight.

Manually calculating the center of buoyancy at varying heel angles is quite difficult as most boats change in trim as they heel. There are often notches in the curve as the cabin and cockpit volumes hits the water which is why most manual righting curves ignore the effects of cabin and cockpits. I know of no surrogate formulas that produce even a reasonably accurate rough estimate of the LPS. Certainly the one you used is not accurates since it produced an LPS that is beyond vertical.

While I do not know what the actual LPS for your boat is, I do know that when the EU was attempting to develop a standard for rating the safety of various types of pleasure craft, the stability committee looked at the issue of LPS on a wide range of boats. RORC era boats of types similar to yours generally had LPS''s around 120 degrees but most RORC boats had lead ballast and aluminum spars which would mean a lower VCG than your boat with its low balalst to displacement ratio, shallow draft, low density ballast and wooden spars, etc.

Look, if you are not performance oriented, boats like yours are a good way to get out on the water. While I tend to own higher preformance boats these days, I still enjoy sailing on older designs. The companionship or solitude of sailing is just as pleasant and the sea is just as beautiful whether you are going a bit slower or not. In moderate 10-20 knot winds, boats like yours are in their element. They will not out perform a more modern design or be easier to handle but they sail at their best. It is really at the extremes, winds under 10 knots and winds over 20 that these older designs become a liability. I am not attempting to tell you that you bought the wrong boat, that is not for me to say, but I am attempting to give you my best understanding of the likely reality of the behavior of boats like yours in higher wind situations.

Respectfully,
Jeff

Reply With Quote Quick reply to this message Share with Facebook
  #12  
Old 11-05-2003
Jeff_H's Avatar
Moderator
 
Join Date: Feb 2000
Location: Annapolis, Md
Posts: 6,664
Thanks: 5
Thanked 103 Times in 79 Posts
Rep Power: 10
Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about
Taipan 28 - How well offshore?

I believe that in a prior thread that asked about the ''ideal'' small circumvagator, I belive that I said "There is no such thing as a 27 to 28 foot boat that is ideal for sailing around the world." By that I meant that to achieve what I consider to be an ideal L/D for a 27-28, the boat would not have an ideal amount of carrying capacity for one or two people attempting a circumnavigation. That does not mean that I think that it can''t be done. With luck and planning and in the big picture perhaps a bit more money than it would cost with a larger boat, it can be done. Do I think that is ideal? No!

The point that I have tried to make on the size issue is that cruisers are more accurately sized by displacement first and then by length. All things being equal the longer waterline boat with the same displacement will be easier to operate, be more seaworthy,have greater motion comfort, have a larger carrying capacity, and be faster.

Respectfully,
Jeff
Reply With Quote Quick reply to this message Share with Facebook
  #13  
Old 11-05-2003
Senior Member
 
Join Date: Jan 2003
Location: Severna Park
Posts: 438
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 12
sneuman is on a distinguished road
Taipan 28 - How well offshore?

Jeff,

I''ve used the standard screening formula for AVS (available on the Web), but your point is taken about 180 deg. being maximum - apparently the formula isn''t perfect (or I overestimated draft minus keel by 6"). In any case, I''m sure you''re thinking of the wrong boat: the Taipan is decidedly not a fin keeler, but a modified full keel by any definition. Not sure what you mean by shallow draft either - it draws 4''6" - a full 12" more than the H-28 you speak highly of and 6" more than the comparable Pearson Triton (also 28''LOA and similar displacement).
Reply With Quote Quick reply to this message Share with Facebook
  #14  
Old 11-05-2003
Jeff_H's Avatar
Moderator
 
Join Date: Feb 2000
Location: Annapolis, Md
Posts: 6,664
Thanks: 5
Thanked 103 Times in 79 Posts
Rep Power: 10
Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about
Taipan 28 - How well offshore?

If the Taipan is a close sister to the Newell Cadet, then it is in fact a fin keel (or close to it) by the classic definition of a fin keel. These days people seem to think that any keel with an attached rudder is a "a modified full keel". In the era that these boats were being designed and built, they were considered fin keels with attached rudders. The definition of fin keel during that era was any keel whose bottom was less than 50% of the length of the boat or the length of the sailplan (which ever was the longer). Considering that your waterline is only 70% of your overall length and your rudder post rakes forward and your stem rakes aft, I am reasonably certain that your keel is a fin keel(or close to it) with an attached rudder.

Again, as I have explained, without doing a curve of the center of buoyancy at various angles of heel, and without finding the center of gravity, I know of no surrogate formula that would even closely approximate the LPS (AVS). The one that you were using is clearly in error big time as boats of the general type and design to your own generally had LPS''s around 120 degrees or less.

By modern standards 4''6" draft on a 28 footer is quite shallow. It is true that older designs often were quite shallow. Both the Triton and H-28 used cast lead ballast which helped a little but compared to more modern designs both are quite tender for thier drag. The Triton, like your boat, has a fin keel with attached rudder and is a miserable boat in a blow. In the case of the H-28 she is a nicely modeled hull which greatly helps reduce drag and a low aspect sail plan which helps reduce heeling, all and all a very wholesome design.

Respectfully,
Jeff
Reply With Quote Quick reply to this message Share with Facebook
  #15  
Old 11-05-2003
Senior Member
 
Join Date: Jan 2003
Location: Severna Park
Posts: 438
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 12
sneuman is on a distinguished road
Taipan 28 - How well offshore?

The PT actually has a little less keel ballast than the Taipan, I''m not sure why it matters whether it''s lead, concrete or feathers. Alex Baldwin''s Pearson Triton - very similar in specs to the Taipan - has done two circumnavigations, and the Taipan itself has done at least two Atlantic crossings, both documented here on Sailnet. Guess I''ll just have to take my chances out on the open water. But thanks for the information - I learned a lot through this conversation.
Reply With Quote Quick reply to this message Share with Facebook
  #16  
Old 11-06-2003
Jeff_H's Avatar
Moderator
 
Join Date: Feb 2000
Location: Annapolis, Md
Posts: 6,664
Thanks: 5
Thanked 103 Times in 79 Posts
Rep Power: 10
Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about
Taipan 28 - How well offshore?

Keel material density is important to both stability and motion comfort. In any boat, the lower the ballast the more effective it can be in in developing stability and similarly the lower it is the longer the lever arm for developing a large roll moment of intertia which is important to minimizing roll speed, angle, and roll accelleration.

Cast lead is literally twice as dense as scrap iron and concrete ballast used in boats like yours. In a typical cast lead keel installation on a boat the size of yours, the center of gravity is somewhere between 9 and 12 inches above the bottom of the keel. When you talk about a scrap iron and concrete, the center is often 16 to 20 inches above the bottom of the keel. When you consider that the center of buoyancy on a boat like yours is generally quite deep, 16 to 18 inches below the water line the impact of this lower density is profound. From a stabilty standpoint the decreased density and therefore shorter lever arm means that the concrete/iron ballast will only contribute roughly 70% to the righting moment of an equal weight lead ballast. It has much more profound effect on the roll moment of inertia where the concrete/iron ballast will only contribute roughly 35% to the roll moment of inertia of an equal weight lead ballast. One way to offset this is that boats with low density ballast generally have higher ballast ratios (and higher displacements), greater depth (which your boat is comparatively shallow) and larger keel volumes (which means more drag as well).

Good luck,
Jeff
Reply With Quote Quick reply to this message Share with Facebook
  #17  
Old 11-06-2003
Senior Member
 
Join Date: Jan 2003
Location: Severna Park
Posts: 438
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 12
sneuman is on a distinguished road
Taipan 28 - How well offshore?

that makes sense. very interesting. thx
Reply With Quote Quick reply to this message Share with Facebook
  #18  
Old 11-12-2003
Senior Member
 
Join Date: Dec 2002
Posts: 140
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 13
aflanigan is on a distinguished road
Taipan 28 - How well offshore?

Here''s an interesting website that discusses AVS (angle of vanishing stability) LPS (limit of positive stability) RM or righting moment and GZ (the distance between CG and CB). Note the graph showing GZ as a function of heeling angle.

http://www.sailbuyersguide.com/articles/boatpages/Stability.cfm#sidebar

Here''s another interesting page with an estimation formula, and a table of recommended values for small sailing vessels. Note that for vessels of about 30 feet, an LPS angle of 143 degrees is called for for unrestricted offshore use. Extrapolating to 28 foot would probably bump that to 145 degrees.


http://www.radford-yacht.com/stablty1.html

You can always measure your righting moment directly (GZ times displacement equals righting moment; crudely you can attach a spinnaker pole of known length to the mast base, attach the other end to a halyard, tie a tension gauge to the spinnaker pole end and the dock, and measure the tension at various angles of heel up to about 80-90 degrees. Tension in lbs. times pole length equals righting moment; divide by displacement to get GZ), but estimating the curve past 90 degrees is problematic because of cabin top buoyancy.

BTW, when you use the US Sailing online calulator below

http://www.sailingusa.info/cal__avs.htm

and enter data for the Newell Cadet 27 (Disp. 6900, ballast 2700, beam 7.75)

You get an obviously incorrect answer (negative). When I plugged values into the formula I got a value of about 113 degrees, but this may well be off by 15-20 degrees or more.

Here''s a Powerpoint presentation from Navy ROTC at MIT showing lots of diagrams and info about vessel stability.

navyrotc.mit.edu/www/navsci/NS-102/Mauger%20PPT/ Principles%20of%20Stability.ppt

Reply With Quote Quick reply to this message Share with Facebook
  #19  
Old 11-12-2003
Senior Member
 
Join Date: Jan 2003
Location: Severna Park
Posts: 438
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 12
sneuman is on a distinguished road
Taipan 28 - How well offshore?

Yeah. Those are some of the same formulas I used. It''s the DCB that I''ve had to guess at, and that makes a big difference. By the way, the Taipan has about 1000lbs more ballast and displaces about 1000lbs more than the Cadet. Like the Folkboat, it has cast iron ballast.
Reply With Quote Quick reply to this message Share with Facebook
  #20  
Old 11-13-2003
Senior Member
 
Join Date: Dec 2002
Posts: 140
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 13
aflanigan is on a distinguished road
Taipan 28 - How well offshore?

So the numbers for the Taipan 28 are more like 3600 lbs. ballast and 7900 displacement? Wow.

My gut reaction is Jeff''s concern about low density ballast is well taken. If you''re carrying almost half your displacement as ballast relatively close to the bottom of the hull, it will get you a certain amount of stability, but a much better and stiffer design would put a bulb of lead (weighing, say, a ton, or about 40 percent less than your cast iron ballast) at the end of a 6 foot keel, giving you the same or lower center of gravity, and giving your hull much more freeboard and reserve buoyancy due to the reduced displacement. The Cadet design would be great for getting into the shallows here on the Chesapeake bay while still having decent stability and righting moment, but it seems the lower, lighter ballast might be better suited for serious ocean sailing.

Allen Flanigan
Alexandria, VA
Reply With Quote Quick reply to this message Share with Facebook
Reply

Quick Reply
Message:
Options

By choosing to post the reply above you agree to the rules you agreed to when joining Sailnet.
Click Here to view those rules.

Register Now

In order to be able to post messages on the SailNet Community forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




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
Bluewater defined? dch Learning to Sail 44 07-29-2009 08:20 PM
what determines a coastal cruiser??? troyaux Boat Review and Purchase Forum 18 02-15-2007 12:39 AM
Reflections of an Offshore Passage Bermuda Seamanship & Navigation 4 07-23-2006 09:03 PM
JeffH - Question on cutters el General Discussion (sailing related) 6 09-05-2003 07:50 PM
Choosing an Offshore Boat WHOOSH Boat Review and Purchase Forum 1 07-10-2002 02:07 AM


All times are GMT -4. The time now is 11:15 PM.

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.