Centralized crew weight to reduce pitching?

Oh, Jake! You have challenged one of the mantras of sailing! You're not supposed to ask WHY it is so; you're just supposed to accept that it IS.

But I am glad that you asked the question, because I have always kind of wondered the same thing.

Great question Jake;The way it was explained to me, keeping the weight out of the ends allows to boat to return to proper trim faster. This makes sense because with weight in the ends the pitching motion will have more momentum. On the other hand you are right in that having weight in the ends will make the boat less inclined to start pitching in the first place.

If you have Bethwaite's book the subject is addressed on page 307. He describes spreading the crew weight to prevent the natural frequency of the boat matching the frequency of the waves.

Dan,

Great spot Dan!...to be honest, I hadn't quite made it to page 307 yet ...upon review, Bethwaite does touch on this a little. Too bad it's only a page worth out of the 400 page book!

With some dirty paraphrasing, Bethwaite explains how in a race (on a monohull) where the wind shifted and all but died, they were racing in light air with a moderate chop left over from the previous conditions. The boats were pitching back and forth such that the sails would move backwards in excess of the wind speed (1 to 1.5knots) leaving the boat completely without power. Being the super-sailor he is, he recognized this problem in that the frequency of the waves perfectly matched the hull length, shape, and inertia. To change this equality, he asked his crew to go as far forward as possible as he went aft in order to change the inertia of the platform. Their boat suddenly quit pitching and they quickly built a 50 yard lead by the time everyone else caught on.

OK, so this doesn't exactly support the 'mantra' of keeping your weight centered but he also briefly touches on how in more typical situations, keeping your weight centered on the boat aids the hull in attacking the waves. My interpretation of this effect leaves me thinking that centralizing your weight has more to do with keeping the hull resistance low through the water than keeping the rig stabilized....this makes sense to me a little. In general, speed on a sailboat is about creating power while keeping drag at a minimum. If you are keeping the boat from pitching fore and aft by spreading out your weight fore and aft, the sails do see steadier air but you will be driving your bows into the waves because the boat's resistance to pitching (angular or rotational inertia) is higher leading to high hull drag. However, if you center your weight, the hulls actually pitch a little more but this allows them to traverse the wave action with a more efficient angle to the water surface and therby achieve a less drag through the water. As Bethwaite pointed out, however, there may be times when centering your weight is a disadvantage. But then again, this is not the explanation being used in all the training and reference material I have seen; they say that centralizing the crew weight helps to avoid excessive pitching. Comments?

Hi Jake,

What you are discussing is sometimes called the "dumbell effect".
In more scientific terms it involves inertia and momentum.

Remember that our boats are constantly pitching fore and aft and up and down because of the wind and waves.
When the bow is pitching downward is has momentum(mass* velocity).
The more momentum it has in the downward direction, the deeper it drives into the water and the more force
it takes to bring it back up.
All of this is wasted wind energy and drag.

Bethwaite is quoting the exception to the rule, saying that when the waves/boat length etc are such, it may be more important to slow the boats pitching cycle, out of phase with the waves and lessen pitching by not allowing it to build up, than to simply lessen the ability of the structure to store energy by bringing the "flywheel weight" (crew)closer to the "shaft" (Pitching axis)
Lower energy storage means that the bows/stern will not be driven as deep each pitch, (the pendulum will not swing as far), so the top of the mast will not travel as fast for a given cycle time. However, if this quicker cycle approaches the waves', the boat starts a harmonic with the waves and gets worse and worse. In this case, move crew apart and away from center of boats mass (light boat) On a heavy and big boat, maybe the maximum resistance to pitching would be to all move to an extreme position, far from the boats center of mass. (Whatever makes the longer dumbell and the buoyancy least dispersed too..)

Just like a pendulum or metronome, if the boat starts to hobby horse move your weight to the pivot of the action, around the centre beam, which will increase the resonant frequency of the structure to a value higher then the current resonant frequency which is feeding the hobby horse. If you have a metronome to play with its clearly illustrated.
Darryn
Mosquito 1704
South Australia

Darryn,

Now that does make sense! If you look at a mechanical metronome (if you can find one today) and you move the counterweight closer to the center of rotation, the metronome speeds up....this would be the same as centralizing the crew weight on a catamaran. As you move the weight further away from the center the metronome slows down. The aspect of this relationship that I had missed is that although the metronome slowed down as the weight was moved further away, the motion of the swing arm was more pronounced...i.e. deeper. So while spreading your weight out further on the boat will slow down the oscillation of the platform, the motion becomes greater and more pronounced. I guess when I heard "reduce pitching" I kept thinking about the frequency of pitching and not of the height of the oscillations. Thanks to all!

So, do you think this one is going to show up as a Catsailor article?

prolly.

And all this time I thought the "dumbell effect" was a non-spin boat rounding "A" and tacking to port in front of a large contingent of spin boats.