Dinghy buoyancy-bags...are they really any better than large blocks of polystyrene?

[Greenheart]

Before flat-screen TVs, every purchase of a new television left large-ish chunks of polystyrene, half a dozen of which might have provided fairly unbeatable buoyancy chambers, if stuffed into cavities aboard dinghies, and retained there...

...of course, I've seen lovely clinker dinghies with those expensive yellow inflatable chambers under the benches, and the boats' beauty might indeed be slighted by a lot of disintegrating TV-packing, floating in the bilges...

...but for dinghies which have enclosed, unseen chambers (whose flooding is likely to result in foundering), is there any good reason to pay Mr Crewsaver, rather than use stuff like this, below?

View attachment 30140

 

[Rigger Mortice]

So long as that retention system is strong enough to support the boat if it capsizes, why not?

 

[alahol2]

Our dinghy (Bobbin) has solid polystyrene blocks under all the side benches. They are hidden by the front of the benches but I occasionally manage to kick a couple of bits off. They work perfectly well and save having to produce watertight compartments. Only just enough buoyancy to keep the dinghy and occupant afloat though, no real chance of getting back aboard and bailing.

 

[Tam Lin]

I built a ply canoe which had buoyancy chambers at each end. Filled them with empty plastic lemonade bottles. Figured that if the lids were on tight then they would give plenty of flotation. Didn't cost me anything either as the kids were drinking fizzy drinks at the time!

 

[Greenheart]

Only just enough buoyancy to keep the dinghy and occupant afloat though, no real chance of getting back aboard and bailing.

So, maybe I should FILL the bow cavity, seen in the pic? Or at least the bow-corner, leaving a dry space for cigars and dressing-gown.

Where can I find large blocks of polysterene? The ones pictured may have been the biggest that would fit through the little door at the forward end of the cockpit.

 

[Tam Lin]

You can get the foam in cans which expands as you let it out. If you partitioned a bit of the bow off you could fill the shape exactly. Don't know how much it would cost though.

 

[Greenheart]

You can get the foam in cans which expands as you let it out.

Thanks, I remember seeing it. I hope it's closed-cell foam, not just some kind of a space-filler for insulating stud-walling.

 

[Kelpie]

AFAIK polystyrene is usually open-cell and will absorb water with time. Better to use a closed-cell foam like polyurethane (Kingspan etc)- widely available in sheets up to about 6" thick.

There's a great account somewhere of a guy filling his cedar-strip canoe with expanding foam. It didn't go well, but it was hilarious to read

I knew a guy who was, over a number of years, gradually filling his Westerly with empty wind box bladders. Sounds like a great plan.

 

[Stork_III]

My old 470 had polystyrene blocks within the side tank from new built by Parkers, part of the class rules. Blocks must be adequately restrained within in case of large scale holing, not uncommon in competative dinghy racing.

 

[Pleiades]

Polysterene is not great for adding buoyancy - it absorbs water and gets eaten to nothing by a drop of petrol/oil. Closed cell polyurethane is much better but even that deteriorates over time and eventually takes on water and adds weight. Also hinders addition of through deck fittings etc. The advantage of proper buoyancy bags is that you can tell at a glance if there is air in them and there is little weight. You can take them out to do deck repairs etc. Sealed plastic bottles with plastic lids are long lasting, cheap as chips and as long as they are not able to pop out they will give excellent reserve buoyancy. Excellent for your save the planet recycling credentials if that appeals.
Robin
Pleiades of Birdham
MXWQ5

 

[Greenheart]

I knew a guy who was, over a number of years, gradually filling his Westerly with empty wind box bladders. Sounds like a great plan.

I infer that you meant wine-box bladders?...great idea! Eventually the whole boat would be completely bladdered, and the crew too!

I've heard that Mk2 Ospreys float troublesomely high in the water when capsized, and that the rear 'deck' compartments of some examples are deliberately 'slotted' to encourage the rear to dip a little...

...but (don't snigger) here's the rear compartment of mine:

View attachment 30148

...complete with more polystyrene blocks, probably fitted from new...

...now, I accept that the bulkhead at the rear of the cockpit has probably been hacked away by a previous owner, possibly to make enough space to equip the boat for the same bizarre cruising-adventures I've been dreaming about myself...

...but I suspect that the whopping great hole (where I believe most Mk2s have a moulded, fairly permanent bulkhead) will allow disastrously rapid ingress of a quarter-tonne of seawater...so I'm thinking that a few cubic feet of the polyurethane foam belong in there. (Thanks for the warning about polystyrene's limited suitability.) And, possibly she'll want a new semi-permanent ply bulkhead...

...though it won't be easy, colour-matching that green! Hence my recent enquiries about alternative decks/repainting.

 

[prv]

Never mind the buoyancy bags - this thread appears to be evidence that you finally, at last, finalement, HAVE A BOAT!

Congratulations

It's only been, what, three years?

Best of luck with her, and looking forward to more practical questions in future.

If you ever decide to rig her as a staysail schooner just for a laugh, let me know and I'll come and help sail

Pete

 

[Lakesailor]

I did the maths. The volume of plastic bottles or polystyrene you would need to create positive bouyancy on a keel boat makes it pointless.
On a dinghy you'd need a lot less, but as suggested already placing it in the right spot is tricky. If the dinghy floats too high after capsize you'll have a hell of a job righting it, or even reaching it.

 

[William_H]

I did the maths. The volume of plastic bottles or polystyrene you would need to create positive bouyancy on a keel boat makes it pointless.
On a dinghy you'd need a lot less, but as suggested already placing it in the right spot is tricky. If the dinghy floats too high after capsize you'll have a hell of a job righting it, or even reaching it.

I would advocate building side tanks into any dinghy. These can in addition to providing buoyancy allow the dinghy to tip a lot further before allowing water into the cockpit and provide useful stability when the hull is full of water. They also provide a seat to sit on at the gunwhale.
Yes it will mean the hull floats high when on it's side but I can't see this being a problem rather making it easier to right with pressure on the centre board. When fully inverted yes it will be more difficult to bring to lying on it's side but still should be doable.
Side tanks of course could be fitted with some foam or inflated bags. good luck olewill

 

[Greenheart]

...this thread appears to be evidence that you finally, at last, finalement, HAVE A BOAT!

Congratulations

Thanks a lot, Pete. I've put the conversion to schooner-rig on hold, for a while...still mulling over electric propulsion, of course...

...the only immediate plan is to assemble all the parts on a calm, warm spring day...so, progress may still be some months away!

Regarding flotation, I've seen those inflatable launching-rollers, which struck me as handy for hauling out without a trolley, far from place of launching...

...and they're obviously very tough, so they'd survive the rough and tumble of secondary use as strapped-down flotation-chambers too.

And, they might prove amusing, tied a few yards behind the transom on hot breezy days, for bathing beauties to cling onto as the Osprey soars by the beach...

 

[oldsaltoz]

As others have noted you need a closed cell foam for areas that be a problem to get after the build is completed.

I have been using pool sticks for years when replacing the deck on power boats, They are closed cell and allow full drainage via the bung in the transom.

Very cheap and last for years and will dissolve in fuel or other solvents,

Good luck and fair winds.

 

[Daydream believer]

Only a minor point - but
If using inflatable buoyancy be careful to ensure it is fixed properly
If you put straps round the tanks & they deflate a bit the straps may allow the tanks to rotate around the thwart so they go on top rather than underneath when immersed in water
This greatly reduces the flotation height of the dinghy in the water
The straps need to be screwed to the top of the thwart stop them allowing this to happen

 

[prv]

Regarding flotation, I've seen those inflatable launching-rollers, which struck me as handy for hauling out without a trolley, far from place of launching...

...and they're obviously very tough, so they'd survive the rough and tumble of secondary use as strapped-down flotation-chambers too.

You're in illustrious company with that idea - it's precisely what Frank Dye recommended.

I think he said something about having an odd number of them, with more on one side than the other, to make the boat less stable when inverted. I can check in his book if you like?

And, they might prove amusing, tied a few yards behind the transom on hot breezy days, for bathing beauties to cling onto as the Osprey soars by the beach...

We used to do that with fenders behind charter yachts in the Med. Until my dad said something about shark bait, anyway. Never seemed quite so much fun after that

Pete

 

[Kelpie]

I would advocate building side tanks into any dinghy.

Perceived wisdom in the Wayfarer class is that the introduction of side-deck buoyancy in the MkII boat made it more stable when inverted.
Some say that the old woodies, with just a bow and stern tank, were the easiest to right, in part because they floated lower in the water making it easier to get on to the centreboard.

 

[Lakesailor]

That was what I thought. My Solo floated high and reaching the centreboard was tricky. I had a righting-line tied to the centre thwart which you could toss over the upper gunwhale to make it easier.