Weight distributuion


greatketch@...
 

Danny,

I haven't done the experiment of moving heavy things from the middle of the boat out to the ends as a sailing experiment, and not sure that will every bubble up to the top of my priority list. I will be happy to speculate on the results...

I was taught that weight in the ends of the boat was always a bad thing.  That does not mean that this statement is true, but here is my thinking.  Assuming the trim of the boat stays the same, moving the weight out to the ends, as you said, increases the pitch moment of inertia.  That has a number of effects, and to my mind the most important one is it reduces the boat's natural pitch frequency.  She'll "hobbyhorse" slower--not less--just at a different frequency.

For most boats that would be a bad thing, because a "good" boat will have a pitch frequency high enough that it rarely gets triggered by waves, unless they are very short and steep.  Reducing the pitch frequency brings it into the range where it is closer to "normal" waves.  Also, with more mass, the oscillations will take more energy to stop, i.e. they will last longer. It is really amazing how quickly a boat can come to nearly a full stop when she is pitching at her natural frequency, all the energy that should be moving the boat forward, get used to just pitch her up and down.   

If we are changing the fore-and-aft trim of the boat at the same time it could get a lot more complex.

But, ultimately the dynamics we are talking about here are so complex and have so many moving parts the answer isn't easy to guess.

The only experiment I ever did that was even remotely similar was re-rigging a boat from stainless wire to dyneema--fairly dramatically reducing the roll moment of inertia.  What a difference! With reduced heel she carried full sail in 5 knots more wind, leeway was less, it just made her a better sailing boat.  And this wasn't a race boat, but a 40 foot cruising ketch.  I didn't notice a change in the roll frequency, but I didn't try to measure it either.

Bill Kinney
SM160, Harmonie
Fort Lauderdale, FL.


---In amelyachtowners@..., <lokiyawl2@...> wrote :

Danny,

   This is an interesting area of discussion that can affect the way that we use our boats.  I understand the simplistic explanation that you provided but I think that there is quite a bit more going on and suspect that the actual data from testing might seem a little confusing when those tests are done in waves of  varying period and amplitude.  

   I know that on an older traditional design such as Olin’s Dorade that spreading out the weight can work out well.  Dorade was in fact so comfortable (and fast, the restored Dorade is in fact still doing very well racing)  that trips across the Atlantic were chosen intentionally to have the wind forward of the beam because of this fact..and because the boat rolled downwind terribly. (grin)   I think that the reason spreading the weight worked for that type of boat (under most conditions) is because the bow was very fine with little buoyancy as compared to more modern wider boats and with the pitch heavily dampened by both a heavy mast and heavy ends that the bow did not lift enough to initiate hobby horsing.  This makes for a very wet boat of course and I suspect that if the wave period happened to be close to the natural pitching moment of the boat that the weight spreading was probably not a good thing, but this is just a guess.    The Amel hull is so different that perhaps none of this will translate over but it would be nice to know for planning purposes.  Perhaps when I get my boat back to Florida waters I can attempt some testing but perhaps some other Amel owners have already done some experimentation?  

  I completely agree with you about trying to keep the heavy stuff as low as possible.  My heaviest items will also reside in the bilge.  

Best,

James
SV Sueno,  Maramu #220


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