Numbers & Ratings - Page 3 - 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 Forums > General Discussion (sailing related)
 Not a Member? 


Reply
 
LinkBack Thread Tools
  #21  
Old 11-04-2010
cb32863's Avatar
Lake Sailor
 
Join Date: Oct 2009
Location: Minneapolis, MN
Posts: 713
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 5
cb32863 is on a distinguished road
I will keep the with/without spi in mind, thanks for pointing that out. As I have said before, PHRF is not the only factor in my decision making process. There is comfort level & durability as I plan on going where ever the mood strikes. Which, means a heavier boat. There is the interior as the first mate/admiral will be involved I am sure. Always nice to keep them happy. $$ of course and just what one likes in general. Not looking to race around the planet but I don't want to take forever getting where ever I am going either.
__________________
Umquam Porro

S/V Papillon 1977 O' Day 25


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Reply With Quote Share with Facebook
  #22  
Old 11-04-2010
Jeff_H's Avatar
Moderator
 
Join Date: Feb 2000
Location: Annapolis, Md
Posts: 6,454
Thanks: 3
Thanked 66 Times in 48 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
There are a lot of misunderstandings about the relationship of a boat's PHRF number and its real world speed. As has been mentioned, a boat's PHRF rating is intended a relative gage of its speed to other dissimilar boats. It assumes that the boat is properly prepared (good bottom, good hardware, and good sails). The regional rating represents the relative speed of the boat around an old-style Olympic Triangle course at the average windspeed for that venue. It is at best a very narrow snapshot of a boat's relative speed at particular windspeed. As noted it does not tell much about how the boat's relative speed will compare on any particular point of sail, or in any other windspeed.

While it is true that there can be some politics in a given rating, that is far less common today than it once was since computers allow a more accurate crunching of data and understanding of the norms. That said, new boat designs and custom boats are often over-penalized due to the lack of experiencial data.

PHRF ratings tend to be a little more generous toward cruising boats than full blown race boats, but not for the reasons mentioned. A well equipped boat that has a design that sails well over a wide range of wind speed and direction is easier to keep at speed in almost all conditions. This allows this boat to sail extra distance in order to take strategic advantage of variations in wind speed and direction around the race course. Skilled racers tend to gravitate towards boats that afford and can take advantage of that kind of strategic advantage. There is no fair way to actually rate this tactical advantage, but because these boat have a strategic advantage and are oftyen sailed by more skilled sails, it does show up race result data and so can creep into the rating process making it harder for an unskilled sailor to sail to the resultant rating.

In terms of real life cruising, a 30 second a mile advantage for an equal length boat can be a huge advantage in terms of passage time. While it sounds like nothing, say 12 minutes on a 24 mile leg, in practice, a rating gap that large typically results in much greater gains. In coastal cruising, the faster boat will generally have fewer tacks upwind (each adding several minutes to the passage time) and will hold its speed more consistently through windspeed changes, to sail at reasonable speeds in ligher winds, and therefore be able to sail from gust to gust, and will have the a greater ability to optimize the postion of the boat to take advantage of where the fairer wind is located. While this is not always the case, it means less time spent motoring or the ability to comfortably cover a significanctly greater distance in a day.

One reason for this is that PHRF ratings only look at a boat's relative speed at an average windspeed for that venue, but in most venues boats rarely sail in winds that are that actually at the average speed (San diego being a well known exception). As compared to boats which are rated as being slow for their length, boats which are rated as being fast (low rating) for their length will generally have a much bigger speed advantage in both heavier and lighter wind speeds than they will in moderate conditions.

My sense is that 30 seconds a mile, more realistically takes an hour or more out of a coastal cruiser's typical daily passage or in other words therefore allows an additional 5 to 10 mile as a comfortable daily range. In off-shore passages the advantage can be more dramatic that is predicted by a boat's PHRF rating. A few years ago I crunched the numbers on the results from ARC, comparing boat lengths to PHRF ratings to elapsed times. As broad generality, those boats with a low PHRF number relative to thier lengths had much faster elapsed passages times than their PHRF numbers would have predicted.

There are some very long and detailed discussions about the relative merit of the capsize screen formula and motion comfort index. Most yacht designers that I have spoken to dismiss these formulas as being a useless relic of a bygone era. I have posted following here many times and I had written for an earlier discussion but it does discuss the basis for considering the capsize screen formula and motion comfort index worse than useless.

It seems that as soon as someone posts a question about the seaworthiness of some particular boat, that a well meaning responder sends them to Carl's Sail Calculator to look at the Capsize Screen Formula and the Motion Comfort Index. And no sooner than a poster questions the seaworthiness of some boat, that someone cites the Capsize Screen Formula and the Motion Comfort Index in that vessel's defense or prosecution. But as I have explained many times in the past, (and I am about to explain yet again) these surrogate formulas tell almost nothing about how the reality of a boat's likelihood of capsize or its motion comfort. In fact they provide so little indication of a boat's behavior that to rely on them in any way borders on the dangerous.


Both of these formulas were developed at a time when boats were a lot more similar to each other than they are today. These formulas have limited utility in comparing boats other than those which are very similar in weight and buoyancy distribution to each other. Neither formula contains almost any of the real factors that control motion comfort, the likelihood of capsize, or seaworthiness. Neither formula contains such factors as the vertical center of gravity or buoyancy, neither contains weight or buoyancy distribution (of the hull both below and above the waterline), the extent to which the beam of the boat is carried fore and aft, and neither contains any data on dampening, all of which really are the major factors that control motion comfort or the likelihood of capsize.


I typically give this example to explain just how useless and dangerously misleading these formulas can be. If we had two boats that were virtually identical except that one had a 500 pound weight at the top of the mast. (Yes, I know that no one would install a 500 lb weight at the top of the mast.) The boat with the weight up its mast would appear to be less prone to capsize under the capsize screen formula, and would appear to be more comfortable under the Motion Comfort ratio. Nothing would be further than the truth.


And while this example would clearly appear to be so extreme as to be worthy of dismissal, in reality, if you had two boats, one with a very heavy interior, shoal draft, its beam carried towards the ends of the boat near the deck line, a heavy deck and cabin, perhaps with traditional teak decks and bulwarks, a very heavy rig, heavy deck hardware, a hard bottomed dingy stored on its cabin top, and the resultant comparatively small ballast ratio made up of low density ballast. And if we compare that to a boat that is lighter overall, but it has a deep draft keel, with a higher ballast ratio, the bulk of the ballast carried in a bulb, its maximum beam carried to a single point in the deck so that there was less deck area near the maximum beam, a lighter weight hull, deck and interior as well as a lighter, but taller rig, it would be easy to see that the second boat would potentially have less of a likelihood of being capsized, and it is likely that the second boat would roll and pitch through a smaller angle, and would probably have better dampening and so roll and pitch at a similar rate to the heavier boat, in other words offer a better motion comfort....And yet, under the Capsize Screen Formula and the Motion Comfort Index it would appear that the first boat would be less prone to capsize and have a better motion when obviously this would not be the case.


There are some better indicators of a vessel’s likelihood of capsize. The EU developed their own stability index called STIX, a series of formulas which considered a wide range of factors and provides a reasonable sense of how a boat might perform in extreme conditions. Unfortunately meaningful results require a lot more information than most folks have access to for any specific design. The Offshore Committee of US Sailing developed the following simplified formula for estimating the Angle of Vanishing Stability (Sometimes referred to as the ‘AVS’, ‘limit of positive stability’, ‘LPS’, or ‘Latent Stability Angle’ ):
Screening Stability Value ( SSV ) = ( Beam 2 ) / ( BR * HD * DV 1/3 )
Where;
BR: Ballast Ratio ( Keel Weight / Total Weight )
HD: ffice:smarttags" />lace w:st="on">Hulllace> Draft
DV: The Displacement Volume in cubic meters. DV is entered as pounds of displacement on the webpage and converted to cubic meters by the formula:
Displacement Volume in Cubic Meters = ( Weight in Pounds / 64 )*0.0283168
The Beam and lace w:st="on">Hulllace> Draft in this formula are in meters. These values are entered in feet on the webpage and are converted to meters before SSV calculation.
Angle of Vanishing Stability approximately equals 110 + ( 400 / (SSV-10) )


There is a convenient calculator at http://www.sailingusa.info/cal__avs.htm


It should be noted that the AVS is only one indicator in evaluating the likelihood of capsize, meaning it only predicts the point at which the vessel wants to turn turtle. It does not predict the amount of force that would be required to heel the vessel to that limit, nor the force to re-right the vessel, nor does it predict how the shape of the boat might encourage wave action to roll the boat closer to an angle at which it no longer wants to return.
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay and part-time purveyor of marine supplies
Reply With Quote Share with Facebook
  #23  
Old 11-04-2010
Jeff_H's Avatar
Moderator
 
Join Date: Feb 2000
Location: Annapolis, Md
Posts: 6,454
Thanks: 3
Thanked 66 Times in 48 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
Quote:
Originally Posted by cb32863 View Post
There is comfort level & durability as I plan on going where ever the mood strikes. Which, means a heavier boat..
Just to be clear, in and of itself weight does nothing good for a boat. It does not mean that the boat is stronger, more durable, more seaworthy, or more comfortable. Weight is often the result of poor workmanship, overly heavy interior appointments, and poor engineering. Making a boat heavier adds stress to the boat's structure, but without proper engineering, good build quality, and material selections, weight in and of itself, does not necessarily add the commensorate amount of strength. Seaworthiness and motion comfort, most directly come from weight distribution and buoyancy distribution, rather than from simply making the boat heavier.

Jeff
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay and part-time purveyor of marine supplies
Reply With Quote Share with Facebook
  #24  
Old 11-04-2010
cb32863's Avatar
Lake Sailor
 
Join Date: Oct 2009
Location: Minneapolis, MN
Posts: 713
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 5
cb32863 is on a distinguished road
Jeff, as always, very well put. I had read your post some time ago about Capsize Factor being not all that worthwhile. Mainly I guess I am looking for a way to pare down the potential list to something manageable and not to eventually waste my time looking in the wrong place or wasting money. I know, due to responses on this thread, that the PHRF is not real-world and that was something else I was looking to gain from this thread. What "numbers" are valuable, what ones are useful but not really carved in stone, and those that are not all that meaningful. Cutting through the BS so to speak and this thread has helped with that.

I am going to continue to research and read and learn as much as I can before making the "That's the one" decision. I have a few years, maybe more, to do that. I appreciate everyone's input and I hope other people are finding this discussion educational as well.

CB
__________________
Umquam Porro

S/V Papillon 1977 O' Day 25


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Reply With Quote Share with Facebook
  #25  
Old 11-04-2010
cb32863's Avatar
Lake Sailor
 
Join Date: Oct 2009
Location: Minneapolis, MN
Posts: 713
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 5
cb32863 is on a distinguished road
Quote:
Originally Posted by Jeff_H View Post
Just to be clear, in and of itself weight does nothing good for a boat. It does not mean that the boat is stronger, more durable, more seaworthy, or more comfortable. Weight is often the result of poor workmanship, overly heavy interior appointments, and poor engineering.
....
Jeff
I should have been more clear on that. I have read many a post about heavy boats that are not really stronger or sail well. My thinking was that a boat that is built for more rigorous sailing is going to be a bit heavier than one engineered to be in more protected areas. Again, my inexperience with design and build characteristics is showing. Something I plan on changing.
__________________
Umquam Porro

S/V Papillon 1977 O' Day 25


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
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
Challenging ASA ratings? jephotog Learning to Sail 29 10-02-2012 11:37 AM
AGC fuse ratings brak Gear & Maintenance 7 04-16-2009 08:44 AM
Battery Ratings Waugh Gear & Maintenance 16 04-01-2008 12:47 PM
CE Ocean Ratings sab30 Boat Review and Purchase Forum 10 01-20-2008 02:18 PM
Boat ratings dm567 General Discussion (sailing related) 6 07-27-2006 08:22 PM


All times are GMT -4. The time now is 01:04 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