Flopper stoppers, as they are affectionately called, are poles that can be deployed on either side of a vessel. From their outside ends vanes are hung into the water. The vanes are designed in such a way that they resist upward motion by displacing water while easily falling down into the water with little resistance. In this way, the roll (side-to-side motion) of a vessel at anchor can be slowed and/or reduced.

The poles on Gray Matter rattle against the connection point on the hull because of vibration induced by wind on the aluminum poles due to their long length vs. a relatively thin wall thickness.

This is how I got my rattle to stop.

Background

I bought Gray Matter as a 10 year old boat. She is my first mono-hull. My previous yacht was a catamaran. As such, roll at anchor was rarely an issue. We are full-time live-a-boards and our cruising style keeps us away from docks most of the time so I was happy to see that Nordhavn had provided some means to reduce roll at anchor - flopper stoppers.

On Gray Matter, when we bought it, all eight lines needed to deploy the stoppers were double-braid block and tackle setups. I learned over time that from the factory, at least some of these lines would have been stainless steel cables. Therefore, I felt it safe to assume that these lines I had were not factory and the original stainless lines had been lost at some point in the boat’s past. Because of this, during our refit directly after purchasing, I decided to replace the entire setup with Amsteel (Dyneema) lines.

New Lines

I would create eight lines, all some fixed length, with snap shackles on each end and color coded whipping to tell them apart. It seemed to me that the fore, aft , topping and hanging lines should never need to change length - and these adjustable block setups seemed like unneeded complexity.

As most Nordhavn owner’s likely know, the line suspending the vane, would have benefited greatly from an adjustable block and tackle setup to ease deploying and retrieving the vanes. I certainly plan on going to that type of setup in the near future. However, the fixed length property of the lines I made, was not why my setup rattled.

Since the existing lines I had onboard all utilized variable length block setups, it left me with the question: What should the fixed length of each of these new lines be?

Fore & Aft: This seemed simple enough, they should be the identical length to center to pole fore and aft.

Topping: This also seemed straight forward, it should be exactly the length needed to attach the pole to the top support while it’s still in its cradle on the side of vessel. This way I could attach the topping line before ever unbolting the pole from it’s securing mount - no worries of dropping it.

Hanging: Long enough to hold the vanes low in the water, but not below the keel of the vessel. Deep enough to allow for large swings, but not so deep that they would bottom out skinny water.

Feeling great about my choices, I had all of these lines made and put them away until we left the dock for the 2019/2020 Caribbean cruising season.

Rattling

The problem with the rattle began immediately upon first use. Both poles had the same rattle off and on at times. One or the other pole would rattle for a 2-5 seconds than stop for 10 seconds to and hour. Then the process would repeat. As we racked up days at anchorages the rattle would go between almost never happening to happening constantly. I could often see the poles vibrate up and down when observed.

I have a very low tolerance for sporadic noises around me. Like, they must be eliminated at all costs, kind of tolerance. What to do.

Tube Thickness

These poles are made from thin wall aluminum tubing. Increasing the wall thickness to prevent the vibrations from the beginning was my first thought. And I have no doubt that would correct the problem. However, it’s no small undertaking. I would need to source aluminum tubes of the appropriate thickness and diameter, have the ends machined to accept the cast aluminum end caps, and powder coat the pieces once complete.

So, after posting to the NOG to hear from other owners and giving it some thought I figured I would try some other stuff first.

Pole Angle

Because of the length of the topping line I chose, my poles extended outward from my hull nearly parallel to the water or, to put it another way, nearly perpendicular to the pin that holds them to the hull. This is what one of the connections of a pole to the hull looks like on my vessel.

There is quite a bit of clearance in the joint between the connection bar and the pin closest to the hull to allow for insertion and removal of the pin, even when corroded. This is what machinist’s call a loose running fit. If we zoom in and exaggerate the clearances, you get a better picture of how the vibrating up and down movement of the pole is transferred to the connection bar and, in turn, to the hull..

As the pole vibrates up and down it moves the connection bar up and down, not by much, but enough to produce a rattling sound - amplified by the hull.

Connection Bar Preload

In the horizontal position, the connection bar is preloaded in the down direction only with the weight of half of the pole. The vibrations the pole experiences have enough force to overcome this preload force, getting the connection bar to move up and then back down - producing the rattle.

Increasing the angle of the pole will increase the downward preload on the connection rod. Since the amount of force the pole generates when it vibrates doesn’t change in relation to its angle, the only question is:

What is the minimum amount needed to raise the angle of the pole to create a preload on the connection bar that it’s own vibration force cannot overcome?

After I spent a day trying different heights, it turns out not much is needed: about two feet.

Conclusion

I have a suspicion that I haven’t blazed a new trail here. I now believe these stoppers were always designed to be deployed at an angle like shown. I should have investigated their use in more detail prior to creating my own fixed length lines and shoving off the dock!

This did solve my problem and I’m enjoying my time at anchorage more and more - rattle free.