Some of you may remember the long thread on hull balance named “Hull Design,” where we discussed the Turner Metacentric Moments method of getting a hull that didn’t dive and yaw when heeled. It appears that the notion of a balanced hull has returned to the full-sized world. From the boatdesign.net forum:
“The latest TP52’s have gone for a fuller and less concave bow entry than before. This has helped spread the volume distribution more evenly through the hull, which means that as the boat heels, there isnt so much of a change in trim and balance, and this allows them to carry deeper (more powerful) sails. The foils can be a tad smaller too as they arent being jacked around so much by the changes in trim, and not so much rudder is needed to keep things on course. The differences are pretty minute tho…”
This from “Andy,” a senior contributor from Edinburgh. The whole (short) thread can be found at:
Does anyone out there pay attention to the location of section centroids ? The centroids move around quite dramaticly at various angles of heel. Typically, but not invariably, low heel angles cause some lee helm because the line connecting the centroids curve outward at each end of the immersed volume, conversely the line curves inward at large heel angles which tends to cause weather helm. Somewhere in between these heel angles there is a sweet spot where the centroid line is straight and helm is neutral. One can fiddle up the shape of the underbody to maximize the range of the straight or near straight centroid lines.
Am I merely tilting at windmills here? I have never seen a discussion of this phenomena on any of the forums that I play in. Please set my head straight about this.
I think there are a lot of student projects in this. I still have not encountered, and no one at the SNAME symposium where I presented my paper proposed, an explanation as to why a hydrostatic measure should be predictive of hydrodynamic behavior. In an unbalanced hull (in control system terms) heel (roll) is somehow coupled to yaw. One good student-level question to investigate is, where does the energy to produce the yaw come from? One possibility is that it comes from diverting the thrust vector. Another possibility is that it is the heeling energy that somehow is converted to yaw.
This could be investigated in a straightforward fashion with moving models of known unbalanced hulls. I was thinking of using park flyer ducted fans to push the boat along without the torque effect of a propellor in the water. A lateral fan on a short mast could be used to induce roll without upsetting the lateral CG the way a sliding weight would. Then you could run tests inducing and holding various angles of heel and see what happens.
Thanks Earl. I have been intrigued by this concept for quite some time. It all started when I was studying Pivers trimarans. His waterlines were essentialy parallel and quite pretty to look at. He used 45 degree deadrise in much of the hull. The waterlines were progressively more narrow as each descending WL was drawn. It was then that I started to think about sheer planes and deflection angles and all that sort of thing. That was a long time ago when Piver was in his heyday.
For those who have ever tinkered with the homely little Plyboats program; the user manual lays some focus on centroid locations at various angles of heel. The author, Ray Clark, was interested in water flow paths and expounded some on the subject. I would expect that skilled designers do so too. Long ago, Herreshoff was adamant about having hollow waterlines forward. I dare not refute him, but it seems to me that hollows will cause unnecesary deflections, and even separations, that surely must require added energy input.
As a certified realist, I have never imagined that I could design a miracle. Terrestial and fluid physics does not suffer fools lightly. So much to learn, so little time.