LWL and Hull Speed of Displacement Hulls

Great documents and other reference material
Post Reply
User avatar
boffo
CompetentCrew
Posts: 80
Joined: Sep 18th, '07, 18:11
Location: This user is now known as 'Porridgepots'

Online calculators

Post by boffo » Feb 2nd, '08, 22:39

there are a few online calculators of theoretical hull speed based on Length at WaterLine (LWL)

eg
http://www.sailingusa.info/cal__hull_speed.htm

and
http://arnoldpollikoff.com/DisplaceHullSpeedCalc.htm

Note that it is not the boat length, but length at the waterline, hence many racers like to heel their boat over as this will increase the length of the boat in the water.

I found a spreadsheet somewhere too, that not only calculates your hull speed, but uses this as part of motor and propellor sizing info

for "those-who-shall-not-be-named" 'sailing' boats which have planing hulls rather than displacement, the online calculators are no good to you, but the spreadsheet has adjustable constants for different hulls, so do the calcs for the new 500hp motor you're dying to try!

:lol:
Attachments
propcalc.xls
Spreadsheet for calculating prop size- also Hullspeed
(26.5 KiB) Downloaded 165 times

User avatar
boffo
CompetentCrew
Posts: 80
Joined: Sep 18th, '07, 18:11
Location: This user is now known as 'Porridgepots'

Theoretical Hull Speed is not max

Post by boffo » Feb 7th, '08, 15:44

Here is a well informed article on hull speed calculation from the boat design forum

http://www.boatdesign.net/forums/showthread.php?t=1220

It makes the point that you can go faster than the speed calculated using the LWL formulae, but it just chews up a lot more energy than is efficient.

His main point is that the formula actually calculates the speeds at which the bow wave is the length of the boat, and does not calculate the maximum speed attainable by the hull
This is not a law on the maximum attainable speed but only the formula giving the hull speed at which the created wave length equals the hull length on the waterline. If it is a sailing vessel then the boat speed is quite obvious but if the boat is powered then a propeller travels through the water at the speed of advance. The speed of advance is the hull speed times (1 – Taylor’s wake) so it is nearly always less than the hull speed.

Lots of interesting info about correctly sizing a motor as well

Post Reply

Return to “Reference Material”