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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > my boat dances on a ball
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Thread: my boat dances on a ball Reply to Thread
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Topic Review (Newest First)
03-15-2013 05:19 PM
downeast450
Re: my boat dances on a ball

Adding a stern mounted radar mast was enough to cure our I-28. I was planning to attach a riding sail to the radar mast but don't need one.

Down.
03-15-2013 12:16 PM
PCP
Re: my boat dances on a ball

Quote:
Originally Posted by billyruffn View Post
When I used two pennants running into port and stbd chocks the boat "sailed" a bit on the mooring (in addition to the pennants getting hung up on the anchor). When I ran both pennants over the bow roller it stopped. Why? I have no idea.
The same happens to me. It will dance even less if you use only one chocks on port or starboard side. It has to do with the boat getting to one side and the other of the wind and the force necessary to brig it back I guess.

Regards

Paulo
03-15-2013 11:51 AM
RichH
Re: my boat dances on a ball

Quote:
Originally Posted by Bene505 View Post
I think it's because the attachment point is more forward. By way of an absurd example, if we all had anchor rollers at the end of a 50 foot bowsprit, none of us would dance at anchor. The extra distance lets the tension from the anchor pull the nose into the wind better.

Just a guess. It might be more of an oscillation frequency thing.

Regards,
Brad
Very true, there are some similar dynamically unstable 'ratios' involved with simple trailers towed behind vehicles ... when the towbar is less than about 3 times the length of the trailer tire width such trailers become very 'whippy'; ditto when automotive front suspensions have forward facing axle control arms the shorter the control arms the higher the frequency of oscilation; also, 'tail dragger' landing gear on aircraft are more dynamically unstable than 'tricycle' gear.

And, there are additional oscillation instabilities in the vertical direction that become more aggressive when the trailer towbars arent strictly 'horizontal' between the two ........ and thats probably why using an anchoring bridle which holds the pendent or the rode to 'as low as possible' seems to help dampen out most 'swings'. Probably a good reason to use less stretchy dacron polyester rope rodes than the more common nylon or even 'much heavier' nylon to lessen the 'elasticity' of rodes and attachments if you have a 'swing problem'.

Once you introduce any sort of 'spring' to a mechanical system, whether flexing metal, elastic rope, a chain that forms a catenary, etc. you can surely and usually expect some sort of slow vibratory oscillations to develop. Such almost suggests the usage of wire cable or long 'rods' for mooring and anchoring.
03-15-2013 02:51 AM
Bene505
Re: my boat dances on a ball

Quote:
Originally Posted by billyruffn View Post
When I used two pennants running into port and stbd chocks the boat "sailed" a bit on the mooring (in addition to the pennants getting hung up on the anchor). When I ran both pennants over the bow roller it stopped. Why? I have no idea.
I think it's because the attachment point is more forward. By way of an absurd example, if we all had anchor rollers at the end of a 50 foot bowsprit, none of us would dance at anchor. The extra distance lets the tension from the anchor pull the nose into the wind better.

Just a guess. It might be more of an oscillation frequency thing.

Regards,
Brad
03-14-2013 10:35 PM
hallucination
Re: my boat dances on a ball

I'm happy to report that through several tweaks, we got the evil pole dancer off our boat.

1. Kill the dodger, biggest single improvement.

2. Asymmetrically set a bridle

3. set the boom askew, which helped in two dimensions
a. reduced the rigging humm (mast furl)
b. reduced the outhaul slap

4. kick down the Bimini in the last 10%

Now the rowdy little ****** is well behaved in 20+ kts. I'm sure if I really want to kill the prancy dancer, a riding sail would put the issue in the nursing home.
03-12-2013 07:21 PM
klem
Re: my boat dances on a ball

Smallboatlover, a boat dancing on the ball is a big deal for two reasons. First it is really unpleasant (I have experienced 120 degree swings heeling over 20 degrees on each side). Second, it can greatly contribute to dynamic forces on your mooring or anchoring gear increasing the chances that it will chafe, break, or drag.

billyruffn, I suspect that the answer to your question lies in geometry. Lines (and chain) can only apply a force in line with them. As your bow starts to fall off, some of that force becomes a restoring force to pull the bow back up into the wind and some holds your boat from moving backwards. With a bridle, the proportion of the restoring force is lower at first until the line goes against the bow and then it is equivalent to having the line over the bow roller. The worst is when you have it attached to only one side a few feet aft of the bow. Try this sometime and watch the geometry change as you swing one way and the line goes against the bow and the other and it stands straight out. Ideally, you would have the line attached to a point out in space that is far forward of the bow as that would provide the most restoring force. You should get similar results if you don't have a bowsprit if you pull the bridle up tight as described by RichH as with going over your bow roller. The reason that this is not an issue on cats with their bridles is that the angle is so wide on the bridle that the point where it meets is essentially fixed in space, the geometry never changes. I hope that this makes sense.
03-12-2013 03:36 PM
billyruffn
Re: my boat dances on a ball

When I used two pennants running into port and stbd chocks the boat "sailed" a bit on the mooring (in addition to the pennants getting hung up on the anchor). When I ran both pennants over the bow roller it stopped. Why? I have no idea.
03-12-2013 02:37 PM
smallboatlover
Re: my boat dances on a ball

what the big deal if the boat dances on mooring are you worried about it dancing on the ball when your not on it? i mean it shouldn't be a big deal of it going back and forth. We had a boat in the harbor that the guy left the rudder untied and the boat would go back and forth all day long for the whole summer
03-12-2013 02:26 PM
RichH
Re: my boat dances on a ball

Dancing at anchor or a mooring is a dynamic instability problem that can also be dampened by using a 'stout' bow eye mounted close to the waterline at the bow, or 'bridle' if the boats geometry can apply.
The lower the rode/pennant connection is to the boat waterline, most time will SIGNIFICANTLY dampen or stop the 'dancing'.

My boat is a 'avid dancer'. Since using such a 'bridle system', I no longer need to use an anchor riding sail.

Here's a diagram of what a mooring or anchoring bridle looks like. Note - the bridle has to be pulled 'fairly tight' to under the bow, and may need 'chafe protection'. Sometimes in roaring conditions I'll add a secondary/additional bridle from the midship hawse pipes to the bridle to rode connection under the bow:

03-12-2013 01:14 PM
PCP
Re: my boat dances on a ball

Quote:
Originally Posted by svHyLyte View Post
.
Thanks for having pointed that to me. I have been sailing in light modern boats and they all behave like that, I mean swing a lot around the anchor when the winds over 15/18K. With heavier winds that is even worse. I solved the problem with a series of improvised ridding sails and even with a proper one designed by me. The swing can be reduced for less than half but not completely since the ridding sail has to catch wind to stop the boat from being pulled to one side, and then it starts a movement to the other side, over the wind till it catches wind again.

I can see that the design of that ridding sail is a very smart one. The boat in a frontal position to the wind is catching it on the two sails, on on each side and the boat should be kept stable on the spot without any lateral movement.

Someone have tried it?

Regards

Paulo
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