Buoyancy bag straps

29/06/2010 14:11:54
I've never had to replace the straps on a buoyancy bag before, and now I need to it seems odd to realise I have no idea how to go about it. I see you can buy the webbing by the metre, but how do you join them?

Sew them yourself to the exact right length? Can't be strong enough! Yet that's what I appear to have at present.

Looks like I need a new section in the photo gallery.

29/06/2010 17:46:03

Without a picture of your current set-up it is a bit difficult to be precise but... hand-stitching should be plenty strong enough, provided that you use suitable thread (thin whipping twine might be best).

The most likely place to encounter problems is the attachment points. If they are suspect they need to be sorted out and that may involve epoxying and re-drilling.

One point of which to be wary though - don't make the straps any tighter than they already are, even if you think they need tightening or would look neater, etc. I recently found that the buoyancy bag cover in my Winder had been pulled out of the floor of the boat by leaving a slightly over-inflated buoyancy bag(or so it now appears) to be further expanded by the relentless sun beating down on a boat cover.

29/06/2010 18:02:27
Normally they are no continuous, but are fixed to the hull with toestrap fixing plate screwed in.

p.s. on the Winders, the bag cover pulling the bottom 'batten' off was / is a common problem, there is not much to screw into. I think the 'standard' was to have just 4 or 5 screws, the solution is to apply more screws to take the load. If the fixing can't take the load of an over-inflated bag, then it won't take the load of a capsized and swamped boat either (in my opinion and experience).

29/06/2010 18:57:06
Andrew M
If you tie a knot in the strap to join it underneath the bag with the bag deflated it works perfectly and is hidden under the bag when you blow it up.  You may need a couple of goes to get exactly the right length.

29/06/2010 19:30:33
Use a "tape knot" - devised for joining a climbing sling (i.e. webbing tape).
Overlap the ends (facing in opposite directions), take one and tie a flat thumb knot withthe other end inside the thumb knot.
Now thread the other end through the same knot (backwards) so you have a flat thumb knot with the two tapes alongside each other.
The knot is flat.
Now use electrical tape to secure the loose ends to the standing parts.
This knot will fail at about 40-50% of the breaking strain of the tape. It's certainly worked for rock climbers.

Colin (3387)

29/06/2010 22:36:16
Chris M
i've always used 3x 1.5 metre lengths reef knotted in the middle behind the bag. Haven't lost one ever thats been secured in this manner.

30/06/2010 09:05:45
The only problem with a reef knot is that is quite easy to capsize the knot and then it just pulls apart... (unless you tape the loose ends.
Of course, the stability of any knot depends to some extent on the materials used - some synthetic ropes are not secure with traditional knots.


30/06/2010 09:25:07
Andrew M
but webbing straps seem entirely secure, I haven't lost a bag either in over 10 years, the last one that went was the attachment to the hull in Mordillo, 3202.

30/06/2010 09:45:07
Thanks everyone.

I think the metal plates that attach to the hull are secure enough.

I am surprised hand-stitching is considered strong enough. But the flat knot is a good idea that saves the effort.

Is there a reason why we use webbing instead of rope? I suppose the 'point load' of an underwater bag pulling on the rope might eventually rip it? These type of bags seem pretty tough though!

30/06/2010 11:01:21
Andrew M
Mags, reason for using webbing probably that we always have, it's strong enough and it works.  Cuts into the bag less than rope.

30/06/2010 11:47:31
I think you're right about the ropes and the bags - I've seen (other classes) where the rope has caused a crease and wrinkles in the bag - I suspect that as the bag ages, it gets harder and can crak more easily.

The "Rolls-Royce" fittings were the PVC covers that lashed down to a series of eye (onto a stringer on the hull, then screwed into the inwhale). Not only do they protect the bags, but also have multipoint anchorage.


30/06/2010 12:33:09
I see some newer boats seem to have buoyancy bag covers - why is this? The pockets are handy, but is it just to stop UV degradation of the bag material?

30/06/2010 13:23:59
All of the above like bra straps the more strain the wider they are!

01/07/2010 06:39:24
Buoyancy bag covers stop uv and also stop you catching the bags themselves as you move around the boat - the most common leak is at the seams and the plastic of Holt bags is particularly vulnerable. Get the missus to machine stitch your straps for strength.

01/07/2010 09:13:33
The missus? Actually I'm the only one in our household who can thread a sewing machine! But I confess that when I manually manipulate a needle the results are poor...

Anyway, seems like I ought to consider some bag covers then. Mainly for the useful pockets (but also the UV is a worry when the bags are so costly!). I shall make individually sized pockets for charts, sandwiches, water bottles, spare pants, suncream....

01/07/2010 10:00:26
Robin Szemeti
There you go .. tape knot ... one of the easiest knots to tie, its just a simple overhand knot, and then followed backwards with another piece of tape.

VERY strong and secure ... climbers have used it for slings and nuts for many years.

01/07/2010 11:38:52
Garry R
Is this a great deal of fuss about nothing.  Tying the tapes together with a reef knot - tie an overhand knot in each end of the tape and then tie a reef knot.  Simple - it won't undo.

01/07/2010 12:50:15
Using the right sort of knot isn't fuss over nothing! Though certain situations don't really matter as much as others, I'll grant you.

The tape knot is good as it doesnt use up too much length, and copes better with slippery material.

01/07/2010 13:57:08
Ben 2529
If you go with bags - same offer is there. I made one recently for a bow bag in my Proc 12. I used sewn on loops for attachment of tie lines. If you brought the boat along we could really personalize them - oh yes, and an old Main sail.

02/07/2010 14:15:17

The only problem I've had with straps for stern buoyance bags is the occasional problem of getting the tiller extension caught under one.
It doesn't happen for long - just long enough to realise that the next few seconds will involve something big & wet RIGHT NOW!

Still - it less embarassing that getting the extension caught in the life jacket (of your crew). That tends to destroy the extension, your crew's trust, and the rest of the fleet - laughing!


16/03/2011 11:22:17
When looking at the quoted dimensions of buoyancy bags, I assume the 2 measurements they give are of the deflated bag? An inflated bag would need 3 dimensions (WxHxD), surely?

So any idea how to guess the inflated size from the deflated size?! I presume the 2 measurements would decrease by 20% or something? And the new measurement would be about half of the smallest original dimension?

16/03/2011 11:40:19
Hi Mags

Why do you need the inflated size, as the maximum inflated circumference of the bag is twice its width?

16/03/2011 11:42:05
Assuming you are looking for length of webbing to secure the bag ;-)

16/03/2011 12:51:36
Doh! Twice the width, of course...minus a bit for the seams

Trying to calculate what size bag will fit in a certain hole (not a Merlin) thats why.

17/03/2011 13:42:16
Our old boat has straps with a folded over sewn loop in the end where a piece of dowel fits tightly. Strap pushes through a bridge fitting (like toestraps) and dowel goes in. It makes removal easy.
On the more recent boat the straps are fixed but have a two part easy release clip just like the boat cover with one adjustable end to tighten it all.
Tacking or gybing and getting the end of the spinny pole down the helm's buoyancy aid is even more spectacularly embarrassing as he is carried over the side out of the boat!

17/03/2011 16:10:06
Garry R
To save all the needlework I cut the straps to the required length approximately, heat seald the ends to prevent fraying and tied a simple overhand knot in each end.  The strap is then folded over and pushed through the bridge fitting and a dowel pushed through the strap loop. The strap is fitted through the loops on the bag and the other end is treated the same way pushed through the bridge and dowelled and finally the bag is blown up.  If it is too loose then deflate the bag and shorten the strap or loosen if too tight.  As long as there is tension between the dowel and bridge it won't budge.

21/03/2011 11:22:47
Oops, we were both wrong above. The quoted sizes for buoyancy bags are the inflated length and diameter (assuming a 'pillow' bag is a rough cylinder).

Has anyone used plastic thingies like you get on the understraps of your boat cover, to secure b. bags? That would be the least effort and maximum flexibility.

21/03/2011 14:04:29
Garry R
I thought about that but someone said that they can pull out.  Can't see it myself as the cover from Rain and Sun has clips that take a fair load of pulling tight without coming adrift.  There are three of them on each bag too.

21/03/2011 15:33:57
Andrew M
Mags there is a LOT of force on the strap when the boat capsizes.  As I have never had an issue with a reef knot coming undone and it is adjustable I have never used anything else.  The knot is behind the bag tied before it is blown up, needs a bit of trial and error to get in the right place.  NB though most bags only have loops for 2 straps the stern bags in a Merlin require 3 to satisfy the rules.

22/03/2011 15:13:00
What some people do is start at one end, go straight across to the other side, then diagonally across to the middle (other side), straight across again, then diagonally across again to the far end, then straight across, then come back to the starting point with diagonals to and from the middle.. As said before, you need to be able to allow for adjustments to tape tension to find the correct amount, but it spreads the load when stern bags take so much weight. When you have done it, put a photo in the gallery.
Hope this helps.

22/03/2011 15:30:30
Yes, if you've ever tried to push even a small bag of air under the water you will be surprised at the amount of upwards force. But as Garry says, those cover buckles seem pretty strong too....?

My issue is that these bags will need to be easily removed and replace, when this vintage boat (not a Merlin) is exhibited. I could sew loops in the ends of the straps, and then shackle them to the anchoring points (if I get the lengths just right).

ARG! Too many options!

23/03/2011 19:09:48
Mags, bang one of these babies in each end and then tie or shackle or carbine hook through to the mounting points.

I can sort that out FOC when you eventually pick up the fishman's anchor that is waiting for you.

24/03/2011 08:40:09
Dan Alsop
I've always tied mine as already recommended.   This video shows you how to tie a flat knot:


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