The Relathionship between Flexy and Fast

I just tried to post the following as a comment to Rob Mousely's great article "Stiffness and Speed". However, my lengthy response was too much for the comment to be accepted. I felt it worth posting separatly and inviting the forum moderator to move it as seen best.

First I would like to thank Rob and Dale on a very good experiment and for taking the rare opportunity to do some real world and garage based scientific comparison testing. We need much more of this type of thing. I hope that the following opinion will help clarify some of the effects that have been noted. These opinions are based in part on technical analysis and in part on basic naval architectural principles and in part on time spent in different skis including running back to back speed runs at constant and max heart rates on the same stretch of flat water lake with weights added etc. and not least, based on measurement of in-water-rocker with different skis loaded up with lead weights. (This last being difficult as a Mako Millennium tends to become unstable as you can't add all the weight low enough!!)

My opinion on the flexy boat issue:-
Assume for a moment that the boat has no weight. While not true, a 15kg boat weight is minor compared to the weight of the paddler and the assumption makes the following more easily understood. The same principle still applies when the relatively minimal weight of the boat is included.

When a paddler sits in a ski on the water, the weight of the paddler is distributed along the length of the boat in accordance with the distribution of buoyancy (or distribution of below-the-waterline-volume). Unless all of the below waterline volume occurs immediately under the paddler (i.e. very, very fine ends), then some of the paddler's weight is supported by a portion of the boat both forward of and aft of the paddler. For a very flexy boat, this means that the ends of the boat are going to tend to flex upwards as the paddlers weight pushes down. This can also be considered as the middle of the boat flexing downwards as the boat flexes along its length. Thus, for any given rocker when the boat has no paddler on board, a flexy boat will have more rocker when the paddler sits in the boat.

What happens to two otherwise identical hulls, when one has more rocker? For the same weight of paddler, the boat with more rocker will have less of the ends of the boat in the water and more of the middle in the water. Having a greater depth of the middle of the boat in the water will create a slightly wider maximum beam at the waterline. Even more importantly, the average waterline beam along the length of the boat has also increased but at a greater rate than the maximum beam increase.

What happens when we increase waterline beam? Stability increases at approximately the rate of the increase in the average waterline beam cubed! (This last statement is a simplification but the concept holds for the point of our discussion.) Thus a small, say 6mm {1/4"} average waterline beam increase, will give a noticeable increase in stability. For a typical hull, a 20mm {3/4"}increase in rocker can easily increase the average waterline beam by far more than 6mm {1/4"} and lead to a very noticably more stable boat.

Lets take this a step further. What happens to the rocker on that same flexy boat when the boat is in waves? For the most part, the hull tends to be supported by water forces that are unevenly distributed along the length of the boat. In the case of wave crests at the bow and the stern with a wave trough at the middle of the boat, (typical times when a boat feels unstable), the boat is going to tend to flex more than in flat water. Most of the time the wave induced flex is going to be in the direction of increasing the rocker. (If we ignore the weight of the boat, there are almost no occasions where the boat flexes in waves to reduce rocker!) Thus, in waves, the flexy boat feels even more stable than its stiff counterpart.

For the sake of simplicity I have ignored some issues as the basic principle remains the same even with all factors taken into account.

So....... Is a Flexy boat a good thing??? Ahh.... That old question of what type of boat should a paddler paddle. I truly believe that for any given boat, comfort in the boat, stability and just plain good feel, have a far greater effect on speed than hull shape for most of the racing ski's out there today. If, for any given paddler, the comfort, stability or feel of a particular boat allows the paddler to put more productive effort into making the boat go forward instead of trying to keep it upright or trying to steer etc.then that paddler will be faster in that boat than a less "comfortable" boat. So, if a paddler's ability and sense of feel is such that he feels better and is able to put more productive power into a flexy boat, then that paddler will tend to be faster in the flexy boat rather than an identical but less flexy boat.

However....... I also believe that if that same paddler were to find an alternate stiff hull that had a hull form that allowed the same feel and ability to put productive power into going forward, then that same paddler would be faster in most conditions in the stiff boat.

There are three reasons why the stiff boat will be faster for a paddler who is able to apply full power to the water:- (i) The flexy boat tends to absorb power as it flexes. Some of the flex is created by the paddlers efforts and it is this flex that will most rob the boat of potential speed. Upwind flexing would also noticably absorb forward driving power. (ii) The flexy boat with more rocker has a wider maximum waterline beam and greater stability. If all other hull features are identical and if the paddler is capable of applying full power, then a narrower, less stable boat will always be faster. (iii) For otherwise identical hull features and for a normal hull shape, the more rocker the boat has, the greater the wavemaking drag. For a typical surf ski, at speeds in excess of approximately 7 to 8 mph, wave making drag starts to become an important speed limiter. The flexy boat will always have more rocker than its stiff counterpart, even in flat water. At racing speeds, especially for the top racers at speeds of greater than 8mph, a very flexy boat would not be expected to be good at all.

Note (a) - I will concede that for a down-wind-only boat, it may be possible to optimism hull shape by allowing additional rocker to form when the middle of the boat is in a wave trough, however such a boat would loose considerable upwind capability as the constant upwind flexing would rob efficiency.

For a paddler who is able to feel comfortable and able to apply maximum productive power, including steering control, in a stiff version of any given boat, then the stiff boat will always be the faster boat except for those very rare occasions noted in (a).

Note also that a boat with more rocker will turn more easily and with the rocker tending to increase the most at times when stability and steering control is most needed, (middle of the boat in a trough), the feel of a flexy boat may be alluring to some. Consider also that a flexy boat can be built more lightly than a stiff boat and a little of the speed disadvantage of the flexy boat could be overcome by the weight saved.

To close, experiences out on the water in different water conditions will often provide additional feedback. All such feedback should be noted and analysed for potential additional effects. I would be very keen to hear Rob and Dale's comments on my opinion and see if they agree that what I am putting forth may be the feel they are noticing. Also Rob, would it be possible for you to load the two ski's with bags of lead shot while on dead flat water. Place the center of the bags at the point that your crotch sits in the boat as this is close to the longitudinal center of gravity of the average paddler in paddling position. Then measure the rocker. If you wish I will be happy to write a short description as to how to measure the rocker. Just let me know. Anyway, by quantifying the rocker change from "as-built" to "as-paddled", at least in flat water, I would be able to run a quick calculation for the added stability of a stiff vs. a flexy typical surfski hull. (Note, I cannot do this for your particular boat as I do not have access to that particular hull shape.)

While analysis of hull stiffness/flexyness over the years has shown that properly shaped stiff hulls outperform flexy hulls, there is at least one noted incident of a flexy hull improving speed. In the 1950's, Paul Elvstrom of Denmark used to race Finn class sailing dinghies (4 Olympic Gold medals). For a while he raced with a Finn that had a flexy hull in the flatter aft sections. Overall, the boat may not have been the fastest around a race course but on very windy down-wind legs, the hull used to plane significantly faster than the more typical Finn. He put it down to the shape of the hull being modified at high speed in the flexy aft sections.

Apologies again for not having been able to keep this on thread with the original article.
Peter