How can we Design a Better Flopper Stopper?

How can we Design a Better Flopper Stopper?

Stabilizing a large yacht at anchor presents many challenges.  With their almost unlimited budgets and full-time engineers, megayachts usually choose an electronic solution such at TRAC’s STAR (STabilization At Rest), and smaller, light-displacement yachts often opt for gyro solutions such as Seakeeper anti-roll gyros.  Both systems are effective, but they’re costly to install, require full-time generator use and varying levels of maintenance, and create noise.  On the other hand, passive stabilization using flopper-stoppers is time-tested, simple and silent.  It’s also much less expensive.  But current systems are far from perfect.

We’ve owned Starr for 18 years and cruised more than 90,000 nautical miles. The first six years we had no at anchor roll stabilization. After our second trip to the South Pacific, we got tired of “mattress surfing” in rolly anchorages. At times, we literally had water washing over the decks. It was like being on a bad passage, so we installed flopper stoppers. Since then we’ve tried two types: the Magma “Rock ‘N Roll” (similar to Forespar Roll X) and a custom made fixed frame rubber valve stabilizer. Both have shortcomings.


Magma/Forespar type Flopper Stopper, 8.5 square foot area
20 inch x 48 inch, 50 pound flopper stopper with rubber valves, 6.5 square foot area
Rubber valves on open position

In March 2018, off Diamond Head, Oahu, we did some basic testing of the two types of flopper stoppers.

We deployed the flopper stoppers with a Magma 8.5 square foot Rock ‘N Roll on the port side and the 6.5 square foot rubber valve stabilizer on the starboard side. Wind was about 25 knots and seas around 3-4 feet on an +/- 8 second period. We positioned Starr in the trough.

Without the flopper stoppers deployed we were rolling up to 15 degrees as seen in the picture below.

With the flopper stoppers deployed, roll was reduced to about 5 degrees.

Here are underwater videos showing how the flopper stoppers work.

The below video illustrates some of the problems with the Magma-style flopper stopper.

The design of current flopper stoppers is problematic in several areas:

  • Both the Magma-style and rubber valve style are cumbersome (and can be dangerous) to launch and retrieve. This is  due to their large size and heavy weight.
  • Magma-style flopper stoppers tend to dive, collapse, jerk, and tangle. 
  • Rubber valve stabilizers exhibit less erratic behavior in the water. However, since this is not a collapsible design like the Magma, they’re significantly larger and heavier in order to achieve similar stabilization.
  • Problems with both: They don’t submerge as fast as the boat rolls, so the line goes slack and then jerks as the boat rolls the opposite way

We’ve tried several methods for safer launch and retrieval. First, we used a block at the end of the pole to pull the flopper stopper up to the end of the pole. This makes retrieving the pole very difficult, since it accelerates as it approaches the boat and acts like wrecking ball. We also tried launching and retrieving amidships, but doing so easily results in damage to the side of the hull.

The dream flopper stoppers would be:

  • Light weight
  • Quickly retrievable (we’ve been anchored when this kind of weather moved in!  See this video from N62 Infinity anchored in heavy weather
  • Easily stowed
  • More effective in reducing roll (more surface area without being more difficult to launch/retrieve)

Design considerations so far:

  • Use a rigid pole to attach from tip of outrigger to flopper stopper
    • The pole would likely require a downhaul similar to a boom vang
    • How do you keep the pole from skewing in various directions?
    • What kind of valve would open and close most effectively?
  • Does an umbrella-type design make more sense? A series of smaller units rather than one larger? What other shapes might work better? I’d like to see 8-12 square feet of stabilization on each side if it’s possible without compromising the ability to store and handle.

What are your thoughts on how to improve flopper stopper design?

John Henrichs contributed the following. He has a wealth of experience cruising, and like Sharry and I, enjoys anchoring in places that are often rolly!

Hi Don,

Just read your blog on Flopper Stoppers and thought I would let you in on our experience over the past 10 years using all types of Floppers.

On the Nordhavn we used the Prime Fabrication plates that were made of stainless steel and about 4 feet long and 3 feet wide when open. They were ok, but needed more weight in larger swells. When we got the FPB they came with the Magna units and we quickly tore them apart in swells in Fiji. Also the lines pulled out of the holes in the plates. They rusted badly and were pitted after only a few days in the water. Next we tried some copies of the Prime Fabrication plates that were made in New Zealand. Once again in larger swells, we tore the hinges off the plates where they were welded and finally gave them away. Next I contacted Trevor at Prime Fabrication and had him made a set of very large aluminum plates just like Ken Williams had made. They were ok, but needed additional weight on the bottom so they would sink with the roll. With the size they were a pain to deploy and finally my wife found an advertisement for a unit at  These are 3 feet square and have 6 plates that act like louvers. I wish they were a little larger, but once I added about 5 pound of lead to each side of the units, they have performed very well. The main advantage is that with the boat at rest, all the louver plates are laying on the frame in the closed position. Once the boat starts to roll, you have instant resistance and you don’t have to wait for the clam shell of the other types to open. They really stop the roll in small swells and on our boat could be a little better in large swells. I store them disassembled and only takes a few minutes to assemble them. 

John Henrichs
FPB 64 Tiger
Formerly Nordhavn 64 Tiger


    Posted at 18:47h, 29 March Reply

    SUBJECT: Re: How can we Design a Better Flopper Stopper?

    The attached is a paper written by a friend. I thought it might be of
    interest to you.

    On Fri, Mar 30, 2018 at 7:21 AM, Starr”s Blog < blogcomments-QNIVY@[...]> wrote:


    Posted at 19:26h, 29 March Reply

    SUBJECT: Re: How can we Design a Better Flopper Stopper?

    Interesting underwater videos. Neither system looks very effective. We have always just used our paravanes. They are reasonably effective at anchor, although we pull them in about 6 feet nearer the boat to prevent them swimming around. As you know from VUP we have a fairly elaborate way of removing them from the water. If you rig a winch and a retrieval line it”s pretty easy. However, if we have to leave a rolly anchorage, we just let them do their normal thing–stay at work in the water. When you get back to the northwest, you might pick up a paravane and try it out…who knows, it might be more effective. If you do be sure to use a swivel on the paravane end of the line.


    From: Starr”s Blog
    Sent: Thursday, March 29, 2018 3:21 PM
    To: umstot@[…]
    Subject: How can we Design a Better Flopper Stopper?

    Posted at 21:09h, 29 March Reply

    SUBJECT: Re: How can we Design a Better Flopper Stopper?

    Ahoy Sailors:

    I recognize that shirt! Wow, wish we were there and yes, don’t want to die. Hi Don and Sharry, hope you are well, great to see the Starr Blog back on the airwaves.

    Cheers, Joe and Elayne.

    Joe Golberg
    The Captain Abides

    > On Mar 29, 2018, at 3:21 PM, Starr”s Blog wrote:

  • Jim crossley
    Posted at 11:12h, 30 March Reply

    Great videos and commentary. 

    Loved the “no rock n roll” sticker.

    I felt mine on N55/11 worked great and yea it was a hassell getting it out i still managed. 

    After watching it several times and to a point my bride was wondering if i was OK.


    i thought of adding more weights to the bottom hinge. 

    Like a line with weights that i could retrive separately. 

    Good luck.

    Posted at 00:04h, 01 April Reply

    SUBJECT: Re: How can we Design a Better Flopper Stopper?

    Hi Don, Hmmm…..I can see the problem, you need weight to keep the
    stoppers hydrodynamically stable yet they have to be large enough to be
    effective. So to be effective and stable they become too large to safely
    handle and could ultimately tear up your gear. How about an array of
    inverted umbrellas made of webbing with a carbon fiber frame. You would
    still need some sort of weight at the bottom of the array to pull them
    down. The advantage would be you could add umbrellas as conditions
    dictate, each umbrella could fold to be easier to store and much lighter.
    Also, not using square metal designs with sharp edges would reduce the
    likelihood of causing damage or injury. you would still need a weight at
    the bottom of the array, but if you utilize enough umbrellas to
    significantly decrease the roll it shouldn”t have to be too heavy. I
    envision the umbrellas as being made with nylon strapping with a foldable
    carbon fiber frame. The strapping would allow the umbrella to sink more
    rapidly while providing upward resistance.
    Loved the videos and thanks for requesting input. If you want to make a
    prototype I think I know someone that could do it. He currently designs
    sea anchors and drogues, his stuff is beautifully done.
    We are hauling out at Canal Boat yard on Monday to replace a thru hull.
    Going on the CCA/PNW Spring Cruise in May, otherwise no definite plans.
    Miss you guys.
    Brian and M.A.

    On Thu, Mar 29, 2018 at 3:21 PM, Starr”s Blog < blogcomments-QNIVY@[...]> wrote:


  • PersonalBagi
    Posted at 08:53h, 06 April Reply

    i am a design graphic from indonesia, btw nice article .

  • Ted
    Posted at 20:54h, 31 August Reply

    Love the blog, This was some of the best information on the internet about roll stabilizers.

    I went through the same process of getting fed up with rolling at anchor and built a set very similar to the flopstopper.

    Have you built the set with rigid poles? I am very interested in how this works, seems like a great idea.

  • Joe shes
    Posted at 01:33h, 18 May Reply

    What about a single hinge set up with a majority of the weight on the hinge side? As if falls it would be angling towards the hinge and always providing tension and snap a bit less? It would squirrel around a bit. You would have to limit how much it opens(45 degrees?)

  • Joe shea
    Posted at 01:34h, 18 May Reply

    What about a single hinge set up with a majority of the weight on the hinge side? As if falls it would be angling towards the hinge and always providing tension and snap a bit less? It would squirrel around a bit. You would have to limit how much it opens(45 degrees?)

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