Page 12: of Maritime Reporter Magazine (August 2023)

Back to the Drawing Board

T e Internet is Not As Useful as We May T ink

By Rik van Hemmen was discussing torsional stiffness in ship’s hulls with one of our intern engineers and pointed out a torsional stiffness problem with a certain hull design section since

I it could not inscribe a decent sized circle. (See ? gure above). I expected it to be a comment that would be confusing to a young engineer and proceeded to explain that torsional stiffness is related to gyradius which is powerfully related to radius and radius is related to circles. Inherently the stiffest shape in torsion is a circle and the closer to a circle the better.

A square tube of a certain circumference is much stiffer in tor- sion than a rectangular tube of the same circumference and that can be con? rmed by noting that the inscribed circle in the square tube is bigger than the inscribed circle in the rectangular tube.

This was explained to me when I was a young engineer and it has stood me in very good stead in quickly assessing

Image courtesy Martin & Ottaway torsional issues. While I was explaining this to the intern, he started googling inscribed circles and torsional stiffness and young designer I was working for Johan Valentijn and was came up with … nothing. We did ? nd lots of equations for laying out a new sailboat design and asked him if he had any torsional stiffness of thin-walled tubes, and the online version good references on mast placement. Johan said: “Don’t both- of “Roark’s Formulas for Stress and Strain” even made a mar- er. Put it on station four.” ginal reference to inscribed circles in a few of its torsional At Valentijn Inc., all boat stations were 10% of the bow to equations, but I could ? nd absolutely nothing that made men- rudder stock, so it ends up at 40% waterline. As a young pin- tion of my very useful engineering rule of thumb. head engineer that confused me because he had barely seen

This made me wonder how many other neat engineering the design. “Uh, how do you know Johan?” tricks will disappear in our world of CAD, FEA and AI. “Because all boats that I design have good balance with the

I have mentioned “Beam is Cheap” in a prior column, but mast at station four.” He then explained that when he worked even this well-known axiom is not readily found on the in- at Sparkman & Stephens, they always struggled with helm ternet. As a matter of fact, the only way I could ? nd a refer- balance on boats (Yes, even S&S designs were not always per- ence to “Beam is Cheap” in the context of ship design was to fect out of the box) and he simply took all S&S designs and google “Rik van Hemmen Beam is Cheap”. plotted mast station against weatherhelm and only boats with

Another rule that I ? nd extremely useful is that 44 ft. is the the mast at station 4 had perfect helm. maximum length that is needed for a seaworthy oceangoing Johan said it made no difference if the boat was a cutter sailboat. Smaller is possible, but once you hit 44 feet you are or fractional rig boat, and even claimed that it worked for adding luxury instead of seaworthiness. I googled: “Smallest Bermuda rigged yawls. I have never designed a yawl (quite reasonable size for an ocean crossing sailboat” and there were frankly, who would anymore?) so cannot answer for that, but a lot of articles that discussed sailboat size. They often came even today when I look at very modern sailboats, I keep see- up with smaller sizes, but interestingly nobody advocated any- ing station 4. When I google “mast location for a sloop” I do thing longer than 45 feet, a subtle con? rmation of that truth. get a reference to station 4 for masthead sloops but not for

In the spirit of service to the engineering profession I will fractional rigs where they advocate station 3, which, to me, provide two more engineering truths. looks very far forward and may work for a dinghy (where

The ? rst one relates to mast placement on sloops. As a body placement can control the boat), but I would have no 12 Maritime Reporter & Engineering News • August 2023

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