Anti-Roll Bag ver. 2.0

 Although my first experiment with the anti-roll bag (ARB ver. 1.0) was successful, I wanted to try a different bag. To accompany a blog about a brand new bag, you may want to open Papa's Got a Brand New Bag in a separate window for a musical accompaniment.    

I wanted to try some kind of a bag that was an "off-the-shelf" product.  I also wanted to get some actual numbers so that I could compare one version to another, both as to cost and effectiveness.  The cost for my first DIY bag, ARB ver. 1.0, was about $200.  The cost numbers are easy to calculate for version 2.0.  I found a "water diversion tube" on Amazon for $34 (free shipping).  These are intended to be filled with water and placed across on opening to a building in order to stop water intrusion.  In my case, the tube was 12 feet long (probably to protect a garage door opening.)  

At 12 feet long, the new bag is longer than what fits across my flying bridge.  Both ends of the tube have to be pointed up in the air, which makes them empty.  But that is not a bad thing.  First, all the air is bled out of the bag.  When my boat tips, the water rushes to one side and sort of fills up what was the empty space at the end.  Thus, unlike my first version, more water can "slosh" up into the empty tube.  This changes the timing a little and provides a little more bang for the buck than my original flat bag.  In the video of ARB Ver. 1.0, the end of the bag is constricting and doesn't allow the slosh to run up the side.  I thought I would see if that makes a difference in stability.

Here is an article by a naval architect that briefly compares the use of an anti-roll tank to other forms of vessel roll attenuation:

"Passive Anti-Roll Tanks . . .

Space requirements are very difficult for small pleasure vessels (say below 60 feet). Possible undesirable effects on stability, depending on the vessel (large free surface effect). Very unlikely as a retro-fit. Possibly noisy. Relatively complex to design correctly (therefore relatively expensive to design). Relatively inexpensive to build. Relatively simple in use."

Here are the claims broken down numerically:

Con

1.  Space requirements are very difficult for small pleasure vessels (say below 60 feet). 

2.  Possible undesirable effects on stability.

3.  Very unlikely as a retro-fit. 

4.  Possibly noisy. 

5.  Relatively complex to design correctly.

6.  Relatively expensive to design. 

Pro

1.  Relatively inexpensive to build.

2.  Relatively simple in use.

I'll use this NA's Pro and Con assessment of an anti-roll tank in analyzing my ARB.  I've already passed the first hurdle (Con #1, i.e., "space requirements are very difficult for small pleasure vessels say below 60 feet)."  I'm 30 feet LOA and space isn't an issue.  The "space" issue is simply one of personal preference.  Which would you rather have on your flying bridge, a freezer, two kayaks, or a roll attenuation system.*  As we will see, a roll attenuation system can be by far the cheapest to install, maintain, and operate.

Con #3 ("very unlikely as a retro-fit") also hasn't been a problem.  I guess it depends on if one is willing to sacrifice some space for comfort, so this is simply a version of Con #1.  I don't know of a roll attenuation system that does not require any space, so this isn't really a Con for an anti-roll tank, it is a Con for every system.  In fact, I can't think of any roll attenuation systems that require less space than an ARB.  I'm not sure why this is included as a Con only for this particular system.

Con #2 ("possible undesirable effects on stability") is also true for all systems.  Take, for instance, the new concept of hydraulic operated fins on the outside of the hull.  I say "new" because anti-roll tanks of various types have been around for over 150 years.  And, yes, there have been a few incidents of undesirable stability (can't find them on Google, but can imagine it happening).  But when hydraulic operated fins have been around for 150 years, there will be the same or maybe more incidents.  Why?  Hydraulic gizmos on boats (just like electric gizmos) fail given time.  If the fins freeze in the wrong position, or get 180 degrees out of sync, there will be an incident.  So Con #2 also applies to all systems.

Con #4 (the "possibly noisy" claim) is likely based on someone's imaginary perception of what a partially filled tank of water would sound like (in a thin steel tank?)  First, when a powerboat is in operation the engine noise generally covers up the sound of liquid moving in a remote tank, at least in boats under 60 feet.  To the operator, the "noise" would be like that generated by the bow wake or water tanks.  Have you ever heard anyone say that their boat is really noisy because of the splashing water in their water tank?  Based on the experience of millions of boaters, I would change "possibly noisy" to "implausibly noisy."  Certainly not a noisy as running a generator to operate a gyro stabilizer system.  (The NA ignores the noise issue for the gyro system).  And the majority of those dragging paravanes also note that there is a humming from the cable that it transferred throughout the boat.  The "cure" seems to be having a section of chain connected to the paravane, thereby reducing harmonics by increasing drag and fuel usage.  (The NA also ignores the noise and snag issues for the paravane system).  

I should speak a little bit about "sloshing" because an anti-roll bag differs from a tank. The "slosh effect" is a real thing and can be a problem, though not as big of a problem as is often stated.  Here is some info if you want to nerd out about the slosh effect.  It should be noted that "slosh" is both a noun and a verb.  We can have the verb (i.e., the slosh effect) without the noun (i.e., the slosh sound).  How?  The slosh sound is generated by the interaction of the liquid with the atmosphere.  If there is no interaction, there is no sound.  Think about trying to hold on to a big water balloon as it squirms around in your hand.  The water moves back and forth inside, but there is no slosh sound because the water balloon contains no air.   That is one advantage of an antiroll bag over an antiroll tank.  Of course, if one can soundproof an engine compartment one can soundproof an antiroll tank or bag.

On the Pro side in the above article, the NA notes that an antiroll tank is "relatively inexpensive to build and relatively simple to use."  As already noted, my ARB ver. 2.0 cost $34 and it is definitely "simple to use" (it required filling up with a hose.)  Filling it probably took 5 minutes and cost nothing at my marina.  There is no hydraulic system to activate, gyro to engage, generator to fire up, or paravanes to drop overboard.  Not even a button to push.  And the article calls that relatively simple?  What could be simpler?  Certainly no other roll attenuation system.

Which brings me to another plus that isn't included in the NA's critique.  Maintenance.  My $34 bag had a little leak even when new (grrrr).  It needed to be topped off every couple of weeks.  I have tried to fix the leak, but I expect that the bag will wear out over time from motion in rough seas.  I'll guesstimate that it will need to be replaced every other year.  So the annual maintenance cost for this ARB stabilization system is equal to one-half the installation cost.  $17 per year and hopefully the next one won't leak.  Check out the maintenance schedule, cost, and down time for a gyro system.  Hint: it's more than 5 minutes and $17/year.

And the NA forgot Pro #4, the ARB works at anchor (without running a generator).  Is there another roll attenuation system that works as effectively at anchor as underway?  The NA forgot to include that one, likely because none exists.

But according to the NA we still have two remaining minuses for the use of antiroll tanks.  The article claims:

5.  Relatively complex to design correctly.

6.  Relatively expensive to design.

These two Cons are really just one claim.  In fact, all of the negative claims against the use of antiroll tanks for roll attenuation are just Con #5 restated.  Yes, an antiroll tank system can be "relatively complex to design correctly."  That's the only downside and term "correct design" subsumes all of the other Cons. 

"Correct design" requires the intelligent use of some space (Con #1).  If not designed correctly, it could have "undesirable effects on stability" (Con #2). Skilled designing could be required for a retrofit (Con #3).  An improperly designed system "could be noisy" (Con #4).  Designs created by an expensive naval architect could be expensive (Con #6).  Even more expensive if designed incorrectly and needing redesign (duh).  Of course, all of these difficulties also pertain to every other type of roll attenuation system available.  And all of the other systems are going to cost more than $34.  And all other types do not allow for inexpensive incremental experimentation in tuning the system as allowed with an ARB.  It turns out that "correct design" of an ARB can be a DIY project for the small recreational vessel.  A gigantic Pro only available for the ARB.

For instance, the installation process for some forms of recreational vessel roll attenuation systems begin with 1) "haul out your boat" and 2) "cut a hole in your hull."  Sounds costly and makes me more nervous than a bag of water on my flying bridge deck.  If you don't want to haul out and drill holes, then "install a $50,000 gyroscope and the required $15,000 generator."  That's more than I paid for my boat.  Or "fabricate 20 foot masts operated by electronic winches" and then "drag stuff through the water" to make it work.  Really?  Is there is someplace that doesn't have low bridges, crab pots, abandoned gill nets, submerged rocks and floating snags?  What fun is that?

I recently saw all of the popular roll attenuation systems at the Seattle Boat Show (excepting the ARB, of course).  The booths reminded me of "Psst, hey buddy, I have a bridge to sell you."  Commercial systems have to be really, really expensive.  How could a business make money selling a $34 vessel stabilization system?  It is actually the $34 cost that is the biggest Con of an anti-roll bag to a naval architect, a commercial business, and a boat yard.  Where's the profit?  An ARB business couldn't even afford a booth at a boat show.

Next, I need to experiment with what a $34 roll attenuation system can do.  Specifically, what must it need to accomplish to be worth the time and expense of "installation?"  We could look at it mathematically.  If the installation of a $52,000 hydraulic fin roll attenuation system reduces an 11 degree roll to a one decree roll (a delta of 10 degrees), what must a $34 ARB system do for an 11 degree roll?  If I've done the math correctly, a $34 stabilization system should reduce an 11 degree roll to a 10.9925 degree roll based on a simple cost/benefit analysis. Any additional roll attenuation is a win for the ARB system.  I should have some test results in Anti-Roll Bag ver. 2.1.  Anti-Roll Bag ver. 2.1.

  *  Roll attenuation systems are often called stabilization systems.  I'll try to stay away from that misnomer.  The stabilization of a vessel generally remains the same and a roll attenuation system merely makes things more comfortable by damping the vessel's roll.  Still, "stabilization system" is so commonly used that I'll likely use the term inadvertently.