Hull Speed - Page 2 - SailNet Community

   Search Sailnet:

 forums  store  

Quick Menu
Boat Reviews  
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Hatches and Portlights
Interior And Galley
Marine Electronics
Other Items
Plumbing and Pumps
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here

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

LinkBack Thread Tools
Old 08-02-2002
Jeff_H's Avatar
Join Date: Feb 2000
Location: Annapolis, Md
Posts: 6,579
Thanks: 5
Thanked 95 Times in 71 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
Hull Speed

This is a good discussion but a couple points here.

While some boats with long over hangs increase in speed as the boat heels not all do. In the current thinking, the boats that do increase in speed, do not increase in speed because the waterline is getting longer as previously thought, but because the counter is submerged increasing the buoyance aft and helping prevent the stern from squatting.

Moving crew forward as a boat comes off the top of a wave can help a boat surf sooner. (Similar to walking toward the front of a surfboard.)

I think that it is not just a matter of lower displacements per se but the degree of fineness of the bow and stern. There are boats that are considered semi-planing or semi-displacement hulls. These boats easily achieve speeds well over the theortical 1.34 times the square root of the waterline length. The issue that creates hull speed is about the energy required to push a boat against the resistance of its own combined bow and stern wave. When you talk about semi-displacement hulls great effort is made to reduce the height of the bow and stern wave at speed so that while the boat may not actually climb up on its own combined bow and stern wave (i.e. plane) it takes less energy to overcome the drag of its own combined bow and stern wave. Typically these are boats with comparatively fine bows and waterline beams, comparatively shallow canoe bodies and fairly powerful stern sections. If you look at most IMS typeform boats or the Volvo Ocean race boats that is where they get their speeds well in excess of the theoretical 1.34 number. Almost all catamarrans do not plane but infact get their speed though small wave making (i.e. semi-planning).

Reply With Quote Share with Facebook
Old 08-03-2002
Senior Member
Join Date: May 2002
Posts: 2,538
Thanks: 3
Thanked 16 Times in 14 Posts
Rep Power: 13
Sailormon6 will become famous soon enough
Hull Speed


The authority that Duane cited said "it all depends on displacement." If we assume that is true, just for the sake of argument, then the one quality that distinguishes a heavy displacement boat from a light displacement boat is that the heavy boat moves more water out of its way than the latter. If the hull has to move more water out of its way, more energy is used up to drive the hull through the water. Perhaps a light displacement boat is capable of pushing its own, smaller, bow wave through the water at a faster speed than a heavy displacement boat can push its bigger bow wave through the water, using the same amount of energy. I don''t know whether this reasoning is correct, but it seems to be consistent with what you and Duane''s authority are both saying.

Also, Duane''s authority says, "Everyone is familiar with Anthony Deane''s original formula for heavy displacement hulls, and people are slow to catch that non-planing boats go faster than Deane''s formula predicts, despite our observations that boats sometimes do go faster than they''re supposed to." You said, "Waterline''s affect on hull speed is theoretical and not absolute." Is this an indication that, as boat design becomes more sophisticated, the authorities are coming to the conclusion that Deane''s formula should only be taken as an approximation, and that, with future advances in boat design, the upper limits of speed can be raised, and that perhaps Deane''s formula itself should be modified to reflect current technology and thinking? This would certainly explain the huge amount of anecdotal evidence that boats do in fact exceed the speeds predicted by Deane''s formula, without planing.
Reply With Quote Share with Facebook

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
Windward performance deseely General Discussion (sailing related) 21 04-01-2012 02:42 PM
Wide Transoms...A Discussion Thereon. (was Jeff-H) windship General Discussion (sailing related) 28 05-24-2010 02:22 AM
Hull speed and wide sterns Jeff_H Boat Review and Purchase Forum 24 11-21-2006 11:58 PM
hull construction cgha33 Boat Review and Purchase Forum 4 05-23-2003 07:11 PM
What is the formula for hull speed? I can''t remember. duffer1960 General Discussion (sailing related) 3 05-08-2003 05:17 AM

All times are GMT -4. The time now is 09:03 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) LLC 2000-2012

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