[capta]

It has been suggested on other sites that if a 40 odd foot boat is pitching in an anchorage, this might be reduced by hanging a bag of rocks off the stern and possibly the bow also.
Just off the top of my head, I would guess that this bag would need to weigh a thousand pounds or more to have much effect, if any.
Can one of you really, really smart engineering types on here chime in with a bit of fact based information on this, please. Thank you.

 

[SVAuspicious]

It has been suggested on other sites that if a 40 odd foot boat is pitching in an anchorage, this might be reduced by hanging a bag of rocks off the stern and possibly the bow also.

Oh my.

I don't think weight would help. After all, we try to reduce weight in the ends to reduce pitching moment. You could talk me into a bucket that provides resistance to motion without significant weight.

 

[mr_f]

Auspicious,

Please do not try to discredit this hypothesis. I would very much enjoy being at anchor watching people tote a thousand pounds of rocks in their dinghies in the sort of conditions likely to create unpleasant pitching.

 

[krisscross]

In order to reduce the pitching movement of the boat you need to re-arrange your load distribution, placing proportionately more weight aft. Think of it as a hobbyhorse, where a fat guy sits in the back.

 

[SHNOOL]

Think of it as a hobbyhorse, where a fat guy sits in the back.

Well I certainly solved this problem then... I am the fat guy always sitting in the back, although hardly a 40 footer.

 

[RichH]

Adding a thousand pounds of rocks to one of the ends will increase both the period and the amplitude of the pitching.

Whatcha need to help stop the pitching is either a 'flopper stopper' or a 'paravane' ... and with no rocks required.

 

[Jaramaz]

The idea is not as bad as some of you want it to be. Those active in sailing some 3-4 decades ago may remember that about the same theory was quoted when pitching was to heavy during sailing - the cure was said to put something heavy (as a crew member) at the crosstrees in the mast.
And it worked - sometimes.

When pitching heavily there is a resonance, caused by wind, waves and the boat, mainly its moment of inertia. The moment of inertia can be changed be re-distribute weights on the boat, which might move the boat out of resonance with the waves & wind.
To this one should also note that the moment of inertia is proportional to the mass (weight) * r*r where r is the distance between the centre and the centre of the weight (google Steiners theorem if you want to know more). Thus ... distance is more important than weight, the further out the more moment of inertia is changed.

No 1000 pound is needed to make this change on a 40 ft boat. It is often perfectly enough with some few 100 pounds ... conclusion is given, eeh?

Now, all this is nice. In reality sea waves are not of one frequency, but have a wide distribution (a wide normal distribution if anything). In order to really avoid resonance effects, moment of inertia should be changed rather dramatically. So, these few 100 pounds, could they be kind enough to move up to the top of the mast?

OK, if all this works out well, what happens then when wind changes? Oh, new possibilities for resonances!

There are other methods, actually. But weights work, in theory and practice.

/J

 

[sony2000]

Reducing the distance the bow rises up, will lessen its fall.
So in cabin weight should go forward even if it lightens the stern. The stern never has a pitching problem.
You may want to try hanging an empty, wide lipped bucket ,from the bow chain. Place it up to 3' from the bow to avoid contact, and low enough that the bucket never rises above the sea during the pitch up. It should dampen the initial pitch.

 

[krisscross]

Whatcha need to help stop the pitching is either a 'flopper stopper' or a 'paravane' ... and with no rocks required.

These are used to dampen the roll while anchored, but I guess you could rig a bowsprit (if you don't have one already) and hang it from the bow.

 

[harmonic]

Usually I would change anchorages,Just saying.

 

[newhaul]

I set a stern anchor to keep us bow quarters to prevailing waves seems to lessen the effect

 

[Minnewaska]

If a 40 footer is pitching so much that you find it uncomfortable, you're going to need to be more concerned about dragging your anchor or chafing something. I wouldn't want anything hanging overboard that was going to slow down dealing with either.

 

[Jeff_H]

Reducing the distance the bow rises up, will lessen its fall.
So in cabin weight should go forward even if it lightens the stern. The stern never has a pitching problem.

(I apologize that this was written for an earlier discussion on this topic in which a similar point about where weight should be placed to minimize pitch.)

I am afraid that is not how it works, at least not in a wave train. Putting weight towards the ends of a boat does nothing good for a boat. To explain, boats pitch around an instantaneous pitch axis (which is called 'instantaneous' because the actual longitudinal and vertical position changes as the boat's pitch angle changes).

And for any given pitch angle there is a moment of inertia, which is the derived from all of the weights on a boat, and their distances from the instantaneous pitch axis. Whether you move a weight 10 feet forward of the pitch axis or 10 feet up from the pitch axis, or 10 feet aft of the pitch axis, or ten feet down from the pitch axis for that matter, it has the same effect on the pitching moment of inertia. No matter where in the boat that weight is located, the further it is located from the pitch axis, the greater the pitch moment of inertia. The more that the weight in question weighs, the greater the pitch moment of inertia.

When a boat with a larger pitch moment of inertia encounters a single short duration wave, (all other things being equal), it will move more slowly, but through a larger pitch angle than a boat with a smaller pitch moment of inertia, because the boat with the larger pitch moment of inertia stores more kinetic energy (momentum) and that kinetic energy carries it past the point that the wave is still lifting it or dropping it.

In a wave train, a boat with a smaller pitch moment of inertia (all weight concentrated near the pitch axis) will tend to more closely follow the rise and fall, and face angle of each wave. A boat with a larger pitch moment of inertia will tend to get further out of phase with the waves, and so have harder impacts with the waves and so encounter more force, which in turn causes it to want to pitch much more than the frequency and height of the wave would suggest.

So a boat with a larger pitch moment of inertia will tend to have a more comfortable motion in a single wave, but a much less comfortable motion in a wave train.

Of course countering the rotational forces, and motions, are dampening forces which derive from the shape of the hull.

Jeff

 

[OPossumTX]

I once saw an about 20ft home built trailer sailor with a horizontal wing on the bottom of the rudder, the wing was maybe two square feet in area. I asked the fellow loading that boat what that wing thing on the rudder was for. He told me that it was to stop the boat from pitching so badly when he went out past the break water. I asked him if it worked and he told me that it helped some as he went back to lashing down his mast.

So, one thing you might try is putting a horizontal wing on the bottom of your rudder. Will it work? Shucks! I don't know, maybe so!

Have FUN!
O'

 

[FSMike]

I think the 1000 pounds of rock method has since been replaced by the new water ballast anchor theory.http://www.sailnet.com/forums/images/smilies/wink.gif

 

[outbound]

How about tying two snubbers to the chain. One attached to the bow cleat the regular way and the other attached to the midships cleat. Let out enough so boat takes seas at an angle. Boat will still pitch but speed of the motion should decrease just like when you bear off sailing.
Agree with Jeff (as always) anything that degrades gyradius is a really bad idea. Agree if you can move that would be a good idea.
My boat doesn't pitch much but she sails some at anchor or on a mooring. Have found using two snubbers to rig a bridle decreases it a bunch.

 

[sony2000]

Jeff_H, The shifted weight forward and raising a bucket of water were thought to improve the cutting action of the hull going into the wave, as well as, generating less exposure of the bow to the winds uplifting it even more.
Yes, even this small effort to limit the pitching, may leave the sailor uncomfortable.

 

[outbound]

Problem is putting weight in the ends makes the boat go slower. Also generally safer if boat stays on surface at or near designed waterline. When boat doesn't rise to meet the coming sea it slows down and there is an increase in boarding seas.
Slamming into head seas is probably less fun than pitching up and down a bit.

 

[SloopJonB]

The answer to the central question of this thread is the term "Polar Moment of Inertia"

Look it up, apply the lessons to your boat and pitching will be minimized.

 

[Hudsonian]

Good idea; wrong location. The 1,000# of stones needs to be at the head of the mast.