RE: catalina27-talk: Hull Speed

Tue, 03 Oct 2006 09:10:15 -0700

Title: Message
There's an explanation of Dave Gerrr's hullspeed formula written up at http://www.blumhorst.com/potterpages/hullspeed.html . 
 
And the4re's a comment from Dave himself about it, too.  As Dave emphasizes, the formula describes the top speed a hull may potentiallly achieve WITHOUT planing.  His formula implies that he's made a refinement of the concept of displacement hullspeed.  His formula lends itself to prediciting which hulls are likely to go more easily from displacement to semi-displacment to planing modes.
 
According to Gerr's framework, the C27 is still considered a displacement hull.  But dispalcement hullspeed is now a function of displacement and length (and hullform), not just length.  And as a consequence, it is flexible enough to accomodate the fact that with enough wind power (wind speed and sail area), even a displacement hull such as the C27 will plane/surf down the face of waves at much higher speed.
 
Under Gerr's new formula, an empty C27 weighing 6850 with an LWL of 21.75 has a hullspeed of 6.56 kt, by the "new & improved"  formula.  The multiplier is 1.41, rather than 1.34.  Load up the boat with an additional 1500 pounds of gear, and the non-planing hullspeed drops back down to about 6.2 kts, tthe same number you get according the old formula. 
 
 
 
 

Fair winds, Judy B
1977 Catalina-27 Tall Rig, hull #3459,Bijou.
Sailing San Francisco Bay Area
Visit Bijou’s website at <http://www.blumhorst.com/catalina27/catalinahomepage.htm>
 
Commodore Emeritus, Potter Yachters of Northern California
1985 West Wight Potter 19 #266, Redwing
Danville/SF Bay, CA
Potter Yachters website at <http://potter-yachters.org/>
Potter Forum on the TrailerSailor website. http://bbs.trailersailor.com/forums/potter/index.cgi Judy B's Potter Website at  <http://www.blumhorst.com/potterpages>

 
 -----Original Message-----
From: [email protected] [mailto:[EMAIL PROTECTED]On Behalf Of Phil Agur
Sent: Monday, October 02, 2006 2:12 PM
To: [email protected]
Subject: RE: catalina27-talk: Hull Speed

Lance,

 

Can you take it as part of an Excel spread sheet?  Otherwise I’ll have to do some work to write the equation out long hand in a graphics program.

 

Phil Agur                    s/v Wing Tip

Commodore,             Call Sign WCW3485

IC27/270A                   MMSI 366901790

www.catalina27.org      Vessel Doc# 1039809

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Jones, Lance B
Sent: Monday, October 02, 2006 1:21 PM
To: [email protected]
Subject: RE: catalina27-talk: Hull Speed

 

What is the improved formula? Yes, I too have exceeded the hull speeds on both C-22s and 27s.

 

 

Lance B. Jones _/)
Internet Account Representative
YELLOWPAGES.COM from BellSouth
BellSouth Advertising and Publishing Corporation
2247 Northlake Parkway-8th Floor
Tucker, Georgia 30084-4005
Phone: (678) 406-2531 Toll Free: (866) 213-2617
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Visit our website @ www.yellowpages.com

-----Original Message-----
From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Phil Agur
Sent: Monday, October 02, 2006 4:10 PM
To: [email protected]
Subject: RE: catalina27-talk: Hull Speed

Traditional hull speed assumes your vessel is a traditional heavy plank construction and derives your displacement from there. That’s why a formula like hull speed, which we all know is greatly impacted by displacement (as loaded weight) doesn’t appear to care about your loading at all.

 

If you read Dave Gerr’s The Nature of Boats (that’s Naval Architect Dave Gerr) carefully you can assemble the full formula (I call it advanced, but it really just replaces the traditional assumptions with data) from 3 chapters on boat speed.

 

It is still limited to displacement hulls or should I say hulls operating in the displacement mode. I’ve planned a C22 and it was clear hull speed didn’t apply for a few minutes.

 

The nice thing is the real world and the real formula correlates much better than the real world viewed with traditional wisdom. The traditional for C22 5.89 knots but as you know that’s not a real number, especially for race ready early old style C22.   

 

Phil Agur                    s/v Wing Tip

Commodore,             Call Sign WCW3485

IC27/270A                   MMSI 366901790

www.catalina27.org      Vessel Doc# 1039809

-----Original Message-----
From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Jones, Lance B
Sent: Monday, October 02, 2006 10:49 AM
To: [email protected]
Subject: RE: catalina27-talk: Hull Speed

 

Hummm, what's the difference?

 

 

Lance B. Jones _/)
Internet Account Representative
YELLOWPAGES.COM from BellSouth
BellSouth Advertising and Publishing Corporation
2247 Northlake Parkway-8th Floor
Tucker, Georgia 30084-4005
Phone: (678) 406-2531 Toll Free: (866) 213-2617
Fax: (770) 908-6213
Email: [EMAIL PROTECTED]
Visit our website @ www.yellowpages.com

-----Original Message-----
From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Phil Agur
Sent: Monday, October 02, 2006 1:47 PM
To: [email protected]
Subject: RE: catalina27-talk: Hull Speed

Lance,

 

Are you looking for traditional or advanced hull speed?

 

Phil Agur                    s/v Wing Tip

Commodore,             Call Sign WCW3485

IC27/270A                   MMSI 366901790

www.catalina27.org      Vessel Doc# 1039809

-----Original Message-----
From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Jones, Lance B
Sent: Monday, October 02, 2006 7:54 AM
To: [email protected]
Subject: RE: catalina27-talk: Hull Speed

 

 

Does anyone know what the hull speed of the 27 is?

 

Lance B. Jones _/)
Internet Account Representative
YELLOWPAGES.COM from BellSouth
BellSouth Advertising and Publishing Corporation
2247 Northlake Parkway-8th Floor
Tucker, Georgia 30084-4005
Phone: (678) 406-2531 Toll Free: (866) 213-2617
Fax: (770) 908-6213
Email: [EMAIL PROTECTED]
Visit our website @ www.yellowpages.com

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