Positive Flotation

(Posted 10-28-2016)

(See bottom of page for post-voyage updates)

Statistically, the chances of a well found offshore sailboat sinking are fairly remote.  Which is a good thing for offshore sailboats under 25' long, as they seldom have enough storage space or weight capacity to accommodate a life raft if the boat does sink.  Even if they did, taking to a life raft has been shown to be a questionable strategy in most cases.  More often than not, the abandoned boat is found floating sometime later.  Better is to make every effort to prevent the boat from sinking.  More on that on our Emergency Preparedness page.  Our original thinking was to give the boat positive flotation.  However, that required using so much interior living and storage space as to be impractical.  Instead, we made 8 flotation bags with enough buoyancy to float the boat. 

Most of this page describes the process of making the flotation bags.  The bags were not difficult or expensive to make and they can be stowed in a relatively small space.  Despite those advantages, we're not sure we'll take them along.  The primary reason is that in the highly unlikely event of being holed by hitting a floating object like a log or container, we'd want to use every available second to stop the leak.  If we were instead to inflate the bags, they would take up most of the cabin space, making accessing and stopping a leak impossible.  The primary reason we can think of for using them would be in the even less likely event of being holed by a whale.  That would likely open a hole too large to cover.     

 In practice, the inflated bags filled up virtually all the cabin space, even before provisions were stowed.  Even then, they didn't all fit and some would have needed to be secured in the cockpit.
Entering the cabin to fix a leak would have been impossible.

For those who may be intrigued with the concept though, we've decided to post this page.   

The first step is to calculate the volume of positive flotation needed.  The best reference we've found is an excellent article on the Glen-L website.   It guides you through the process of calculating necessary flotation, including a safety margin.  Our 25' sailboat, including equipment, stores for 2 people on a major ocean crossing, plus a safety factor, would require about 60 cubic feet of flotation.  60 cubic feet is equivalent to a cube about 4' on a side.  Packing that volume of foam into the boat wouldn't leave much space for stores.  Inflation bags are more practical.   

To be effective, the bags would need to be inflated rapidly.  An average sized scuba tank with 80 cubic feet of air would be ample to fill all the bags for a boat up to about 4000 pounds displacement.  Inflating the bags inside the boat would probably make more sense than having exterior bags somehow strapped to the boat, as they would be subject to chafe and damage should they be needed during heavy weather.  Each bag would need to be well secured and distributed so as to float the boat more or less level.   

After some experimentation, we came up with a relatively quick and inexpensive technique for constructing the bags.  With practice, each bag took us about an hour to make.  The bags are constructed of vinyl coated polyester fabric.  This is a tough, waterproof fabric, weighing 18 oz. per square yard.  We were able to find it locally at a foam and fabric store for $8-9 per running yard.  Online it tends to run about $16 per yard.  We experimented with a variety of glues, including PVC pipe cement, all purpose cement for PVC, CPVD and ABS, and X-15 shower pan liner adhesive.  None worked as well as HH-66, a product made specifically for gluing vinyl coated fabric.  We purchased a quart of HH-66 for $17 from Sailrite.  A quart was ample for gluing 8 bags.  The fumes are strong, so gluing should be done in a well-ventilated area.  Although one person might be able to construct a bag, two people make the process far easier.    

Flotation bag providing 550 lb. of positive flotation. 
Bag dimensions before inflation = 30" wide x 70" long.
Bag dimensions after inflation = 19" diameter x 53" long.

Tools and materials, including acetone, gloves, HH-66 vinyl cement, scissors,
pen, bulkhead fitting, framing square, straight edge, painters tape, wall paper roller,
wax paper and rag

Use framing square to square off both ends of fabric.  Most vinyl coated
polyester fabric comes in about 60" width.  If using full fabric width, finished bag
will be about 17" shorter than cut length.

After marking fabric, cut to line.

From one corner, mark for bulkhead fitting.  Shown is 8" in from each edge,
but locate to suit.

Use nut from bulkhead fitting to mark hole

Cut into middle of hole mark.  Note the fabric pattern in this photo. 
We put this on the inside of the bag.

Then cut around perimeter of hole mark

Using straight edge, draw mark 1" in from both ends

Put painters tape down each mark

Do same on both long edges as shown here

Mark mid-point of each end.  Our fabric was 61" wide, so mark is at 30-1/2". 
Also mark 1/2" either side of mid-point mark (these marks not shown)

Also mark mid-point of one long side.  In this case, our bag was 70" long,
so mid-point was at 35".  Fold over opposite long side and mark even with
first mark.

Fold one long side over so it is centered over mid-point marks on each end.
Then, tape securely to table or floor so as to stretch this edge slightly. 
It's important that this edge be flat, without any wrinkles or puckers.  

Wearing gloves, clean edge with acetone.

Apply HH-66 cement to long edge

Then apply cement to other long edge

After cement is no longer tacky (2-5 minutes), place wax paper over cemented
edge as shown

Fold other long edge over so cemented edges are directly over other and
marks are lined up.

Carefully pull wax paper lengthwise out from between cemented edges.
Cemented edges will adhere instantly upon contact, so do this slowly and
carefully, checking frequently to be sure no puckering occurs and edges
are aligned with each other.

Use wallpaper roller with firm downward pressure to thoroughly seal

Pull tape from seam.  Other piece of tape along inside seam can also be
removed, but we just leave it inside bag permanently.

Pull wax paper out from under seam

On each end seam, place tape over short section without tape

Clean end seam with acetone, then apply cement to entire seam.
Two sets of hand are mandatory here.  Make sure no part of edge touches
another part until glue is ready for contact.  Then, carefully align edge and
insure that long seam is centered over mid-point mark on end.

When entire seam is cemented, press seam thoroughly with roller, pressing
down especially hard at seam edges and where the long seam overlaps
the end seam.

Lay another piece of tape 2" in from each end of the bag.  I'm using
a 2" side ruler in the photo above.

Clean with acetone and apply glue to the 2" wide section between tape and
the end of the bag that's opposite the hole for the bulkhead fitting.

Carefully fold glued area over and press down, then roll thoroughly
as on previous seams.

As noted previously, it is important to roll on both sides of where the long
seam overlaps this end seam.  This juncture is where finished bag is
most likely to leak.
See below for an easy solution to leaks. 

Before gluing end of bag near hole, install bulkhead fitting into hole.

Rubber washer on bulkhead fitting goes on inside of bag.  Plastic nut goes on
outside as shown.  Tighten nut securely by hand.

Flotation bag complete except for valve


Valve parts include (left to right) 1/2" threaded PVC ball valve,
1/2" PVC close nipple, 1" x 1/2" PVC reducing bushing.  Teflon tape in background.

Bushing goes into bulkhead fitting, pipe nipple goes into bushing and ball valve
goes on other end of nipple.  Teflon tape works well for all the threads
except between bulkhead fitting which has NPS (straight) threads and bushing
which has NPT (tapered) threads. 
For connecting those parts, use a PVC compatible pipe thread compound.

To inflate, screw 1/2" PVC adapter (1/2" male pipe thread x 1/2" barb) into ball valve, then
slide piece of 1/2" non-metallic plastic conduit over barbed end.  Connect to
hand pump, or inflate a large plastic garbage bag, then wrap neck of bag around
conduit and squeeze air into flotation bag. 

For quick emergency inflation, connect
conduit to a scuba tank.  

To inflate with garbage bag, hold bag open and swing arms
to fill bag.


Then gather bag at neck, insert conduit into bag and squeeze bag.

If higher pressure is desired, connect conduit to hand pump. 
Our idea was to use a scuba tank for rapid filling.

Use soap bubbles to check for leaks.  The 2 most likely places are where
long seam joins end seam and around the valve.  We've had good success with
squirting about 12 fl. oz. of Slime into bag through valve.  Inflate bag, then
position it so Slime flows over leak area.  Once leak stops, remove
valve and pour Slime back out of bag. 
Valve area leaks are fixed by tightening the fittings or applying more
teflon tape or pipe compound.

8 flotation bags deflated for storage.  When inflated, they provide approximately
5000 lbs. of flotation.

Post-voyage comments (updated 10-14-2017):

This page is a testament to the futility of providing positive flotation to a small, relatively heavy displacement boat, especially one with a small cabin.  Installing watertight bulkheads and carrying a life raft would have been a more practical choice.  

Our thinking on this issue has changed in the course of the voyage.  In short, While ideally we'd still prefer that the vessel be it's own liferaft so that rescue wouldn't be necessary, the situation has changed considerably over the past few decades.  Rescue at sea used to involve massive and expensive searches that put rescuers at risk. 

That doesn't necessarily have to be the case these days.  If one has a satellite messenger that continually shows one's position, a ship in the general vicinity can often be routed to pick up survivors without need for a major search and rescue operation.  It should be noted of course that the difficulty is compounded in heavy weather or if medical attention is needed.  

If we went to sea again in a boat without positive flotation,  we would take a life raft.  For more thoughts on this, see our "Update to Sailing Offshore" page.  

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