Equinox: Designing the Hull

Steam Happens #26

Click on the photo below to view a new Equinox movie

Will and Equinox at Lees Mills August 2014

Pictures and illustrations from the article in order of appearance

plus a few extras

A spar-mounted torpedo

torpedo-boat attack. As dangerous for the attacker as the target

Various confederate stealth torpedo-boats.  These were usually called "Davids" as opposed to their "Goliath" targets


Herreshoff's Lightning with two spar torpedos

Turbinia at speed

Turbinia on display

Yarrow's Azor

Herreshoff's Stiletto, capable of launching a free-running torpedo

and Herreshoff's Lotus Seeker (below), a private fast commuter capable of launching a great gin and tonic

Oberon, a beautiful modern fast launch from U.K.

Click here to view Oberon running at speed.  You must view these videos:




The Prismatic Coefficient:

In an easily driven hull the prismatic coefficient ranges from just under to just over 0.52.
A boat with a Cp of 0.525 or less is usually considered a very “fine” hull and is optimized for speed in a displacement (non-planing) hull....



V is the immersed volume or displacement of the hull in cubic feet

A is the maximum cross-sectional area in square feet

L is the waterline length in feet.


The Cp thus indicates the longitudinal distribution of the underwater volume of a yacht's hull.

Little Pal

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Will's hull designing illustrations. Evaluating a hull design with simple tools. Please refer to article

Figure 1 Calculation of the cross sectional area at any station by counting squares


Figure 2

Figure 3 The area below the waterline

Figure 4 Locating the center of bouyancy

Figure 5 Graphically calculating the righting moment by tilting the waterline


Figure 5b, a healed version of Figure 5. Graphically calculating the righting moment by tilting the hull and keeping the waterline level

Figure 6 How the height of the center of gravity affects heel stability

Figure 7 Round profile vs hard chine with the same cross section area

Figure 8 Comparing the heel stability of a round bilge vs a single chine hull. 


Stern.  Note hard chine aft of skeg

Hull on trailer. Note balance on single axle and angle of repose to facilitate easy one man launching and loading.



To download the above CAD file of the Equinox hull "hull.EPRT" click here

Download eDrawings Viewer (it's free) to view, rotate, and closely examine the hull drawing file click here.
Install it on your computer and use it to open the drawing hull.EPRT