MAST BUOYANCY. Seals, and Capsize Recovery

Upper mast - Is it foam filled or just sealed at each end? Failure of a Catapult to right from a total inversion, probably means that water is leaking into one of the mast sections. A small quantity in either could prevent the o righting system from working and turn a simple 90 capsize into a full inversion that requires outside help. This article shows how to renew mast seals and how to recover your Catapult after a full inversion. Our Technical Officer, John Peperell says - “successful sealing of the mast sections is a difficult task, especially the top seal of the lower mast”. Difficult access, because the seal is set well down inside to allow space for the upper mast to slide in. Is your top mast section foam filled? Probaby not, but if your boat will float on its side, that's good. And if, when fully inverted, you can heel the mast and it raises a hull in the air, all is OK... but you need to remember the Capsize Drill. Read on for an easy way to remember the sequence. Note: A fully inverted capsize is always the best leak test, as this puts the mast head under 7mts of water pressure. It will also test the lower mast, and it gives you an opportunity to practice capsize recovery. Any water ingress should be apparent once the mast is lowered. Only do a test capsize if you have rescue cover.

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Following a full inversion of my own boat, which I couldn't right, I gave some serious thought to mast buoyancy. Fortunately my own capsize was on Rutland Water with a Safety boat there in seconds to help me pull it back up. I then sailed another race before going ashore. I then left the boat rigged for some time before taking the mast down in one unit. When lowering it, the weight of water high up took me by surprise, and it came down fast and hard. Next time I'll lean the boat over and remove the mast in sections. After separating the mast, I poked a long screwdriver inside the bottom of the upper mast, which released about a litre of water from behind a very loose plug of builders foam. A lot of water would have drained out, while I was sailing the last race as well. Back at home I found more builders foam further down inside the mast. This was more difficult to remove. But various rods and wires, plus some impacting of the mast end on a soft timber block, eventually removed it. A search on the internet for some reliable information on sealing dinghy masts produced little. So with a lack of DIY information available, I thought I'd illustrate my own method in this article. But if anyone can point out any errors or add further knowledge I would appreciate an email - here. I will then amend the article as appropriate. The information given here involves safety and is given in good faith. But if you make use of it, you do so entirely at your own risk. Some very unscientific tests were done by me on various materials first; E.g. Resistance to water pressure, (by holding various foam types against a high pressure mains water tap). Crushing resistance, (by standing on it), and adhesive qualities, (by reading the sealant guidance notes). If any long term problems crop up, I'll amend this article. No major problems since June 2014. Amendment dates are at the foot of the last page. When inverted, any water in an unfilled upper mast will obviously run down to the lowest end. It then replaces the air from where it's needed, adding extra weight. This reduces the mast “flotation effect” and also the buoyancy leverage length. In a 90o knock down, this will make the mast float lower, and encourage a full inversion. A similar scenario applies to the lower mast section if it floods. This article mainly covers the fitting of seals in the upper mast, but the principal is the same for the lower mast section.

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Capsize Recovery Drill Righting a Catapult depends on mast buoyancy, halyard set up, and correct procedure. First - Release the mast heeling line from its cleats by pressing down hard on the line (Fig.1)

Fig 1: Release the Heeling Line Cleats

Basic Principal

Fig 2: Heeling line actions are from Right, to Left - Push, Lift, Climb.

PLC = Push, Lift, Climb. 1. PUSH – Push the heeling line away from you. Cants the mast towards you. 2. LIFT – Lift the heeling line towards the top hull. Flips the top hull over. 3. CLIMB – Climb onto the trampoline. Drops the upper hull down.

More info and photos here - Capsize Recovery

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Mast Buoyancy -What you should know... At a length of 7.2mts, a Catapult mast, when capsized, reaches pressures almost double that at the surface. So the top mast seal may have to withstand 1.4 bar or 24 lbs/sq in. Capsize recovery depends on watertight mast sections. Testing for leaks. At inland lakes, you can take your mast sections to the shore line and submerge them to look for air bubbles. This will show up any major leaks, but will not test the mast at anything like the fully inverted pressure, but it's a good start. Best is to have a rescue boat at hand, and turn your boat over a full 1800 for several minutes to subject the mast sections to real depth conditions. This is a fool proof method that combines capsize training as well. Back on land, immediately check for water squerting out. Then dismantle the mast and rock each section to either side of horizontal. Listen and feel for any water sloshing about inside. If you can hear water in either mast section, you need to take action... So don't mess about, remove the lower seal as soon as practical to see what's going on inside.

Air Pressure Testing Once the bottom seal has been removed, get someone to keep some positive air pressure in the open end via an air line wrapped with old rags or a wet sponge etc. as a temp seal. You can then brush on soapy water and look for bubbles. Very little air pressure is needed to Fig 3: Hole position marked for drilling. produce bubbles. If any are seen around the mast top, you'll have to renew the top seal as well.

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Materials and Tools. 75mm thick rigid expanded polystyrene foam. Open, or closed cell, rigid foam should be ok to use for the innermost “dry” bung. This is only fitted to provide a firm base support for the thinner marine plywood seal that follows. 50mm closed cell expanded polystyrene foam or any other type of closed cell foam. This type is required for the outermost “wet” bung so that it can resist water getting down to the marine ply seal which forms the “meat” in the sandwich. An off-cut of 12mm marine ply. This is the main seal/s. I highly recommend marine grade plywood (not WBP or exterior ply from B&Q) just in case any damp gets through to it. Some silicone sealant, the type that's used for sealing sanitary ware. Or better still Aquarium Sealant. One small stainless steel self tapping screw 3 - 4mm diam, and some pop rivets for the mast head casting (if it has to be removed). Some acetone or petrol (not turps) for cleaning the internal mast seal area. A Drill, with a small drill bit, approx 2 or 3mm diam. (may be required). A Coping Saw or Jig Saw with fine blade for small diameters. A foot pump, or 12v inflater, and a short piece of thin flexy tube to suit. A sealant application gun. Some basic hand tools, etc., and a bench grinding wheel would help.

Replacing the Bottom Seal Measure how far up the mast your planned seal will go. Mark this depth on the outside of the mast so that you have a reference point. You will then know when to stop tapping each section of the seal down. Drill a 2-3mm diam hole 10-12mm above the total depth of seal you plan to fit (Fig 3) above. My seal went in about 150mm (6”). Allow an extra 10-12mm space above the hole to clear any surplus sealant that'll be pushed down in front of the bung. (Return to Catapult Home Page) (Return to Repairs and Mods Page)

Fit a small self tapping screw in the hole. You can use this hole to drain water out, or to pressure test the seals later. To pressurise use an air line, foot pump or a 12volt mini tyre inflater and some small bore polythene tube to hold over the hole. Don't go above 0.5 bar (7psi) of air pressure, or you risk making good seals leak. Don't forget to seal the self tapping screw after testing.

Procedure. Copy the mast end profile onto some cardboard by dabbing dark paint onto the end of the mast with your finger and press some cardboard over the painted mast end (Fig 4) Cut out the shape to form a template and check this against the mast end. Do any minor trimming then use the template to mark out your marine ply seal. Cut the marked out marine ply with a coping saw, or a jig saw with a thin blade.

Fig 4: Transfer the mast end profile onto some cardboard...and cut it out.

Fig 5 Don't push the seals down into the mast when test fitting. Or they'll get stuck!

Make fine adjustments to the plywood seals on a bench grinder, or rub the edges on course production paper until they look like they'll fit snugly into the mast with no gaps (Fig 5).

Fig 6: Cut out polystyrene inner bungs.

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Cut out two 75mm thick expanded polystyrene bungs with a coping saw. Use either closed cell or open cell foam for these innermost bungs. They need to be made oversized so that they are difficult to force in (Fig 6). Use a marker pen or similar, around your template, to mark a cutting line onto the polystyrene. Cut 15mm outside your marked line. Cut small slivers off the bungs with a craft knife, gradually cutting back closer to the marked line, but stop about 5mm outside it. Cutting off small slivers leaves a smoother finish and allows you to keep checking that the edges are square and have a smooth contour. Cutting the plugs oversize makes sure that they need some persuading to go into the mast. This polystyrene innermost plug forms an abutment for the following plywood seal. It holds the plywood seal square across the mast, and provides friction to resist water pressure. Make another foam bung, but this time out of closed cell (waterproof) foam. Use material about 50mm thick, or glue thinner layers together to make up. Your choice! Some adhesives could melt the material. So test first.

The Assembled Seal.

All the mast seals are made similar

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Note – If you insert the plywood seal more than 50mm into the mast, you'll have trouble getting the sealant gun nozzle in to reach it, unless you fit some tube on the end. None of these measurements are cast in stone, but bear in mind that the centre plywood seal should be tight.

Doing the Work Remove any old mast seal remnants for about 150mm into the mast. Abrade lightly with fine abrasive paper. Working outdoors, check that there are no naked flames about -Then use acetone, cellulose thinners or petrol for cleaning. Wrap a cloth around a screwdriver or similar to get deep inside. Check your intended seal depth against the external pencil marks. Mark now if not done earlier. Fit the 75mm innermost polystyrene bung. Partly insert it into the mast to check that it will fit tightly. Pull it back out and give it a good smear of silicone sealant all round. Wear some disposable plastic gloves to stop your hands getting covered in silicone. Start to ease the bung in bit by bit. Once it's level with the mast end you will have to use a hammer shaft or a piece of wood to tap it down further. Measure down to the bung's top surface and compare with pencil marks on the outside to know when to stop. Next fit the central plywood seal after thoroughly coating it's inside face and edges with sealant. Tap it down with a hammer shaft etc, stopping when it comes solid against the 75mm bung. This will square it up and support it. Coat the visible face with silicone sealant. Now force a fillet of sealant around the edges with your applicator gun (Fig 7). Wipe off any surplus sealant and leave overnight to cure. Next morning fit the outer (50mm closed cell) polystyrene seal. Smear it with Fig 7: Silicone sealant around edge of seal silicone sealant and squash it firmly down. Hold a thin screwdriver blade down the inside of the mast while inserting the bung. To release trapped air, or it won't go in. Cut off the projecting 25mm level with the base of the mast using your Stanley knife. *Update- My previous article suggested a silicone sealant layer over the top, but this shrunk away from the sides of my mast after a few months. The seal can be left exposed (Fig 7) if you prefer to keep an eye on it. (Return to Catapult Home Page) (Return to Repairs and Mods Page)

TESTING – TESTING – 1,2,3. Leave to fully set for a week, then pressure test if you wish, by brushing or spraying a weak mix of washing up liquid and water all over the mast... Remove the drain screw. Press a small bore polythene pipe, connected to an air pump, over the 'drain/test' hole. Keep the tube pressed on tightly while pumping. This alone may be enough to produce masses of bubbles at any leakage points. If you want to inspect the mast while it's being held under pressure, remove the polythene pipe once you have some pressure in, then, quick as a flash, insert a cocktail stick (if a 2mm hole) or slap a lump of blue tack or similar over the hole if larger. Tape it in place, or just borrow a finger off someone, to keep the air pressure in while you do a quick “bubble check” inspection. You won't need much pressure to locate any leaks. Be careful not to over pressurise and create leaks that didn't exist before! Use very little pressure as the seals are designed to resist pressure from outside, not inside.

Replacing the upper mast top seal. This pretty much follows the same lines, but it's a little more involved because of having to remove the mast head aluminium casting first. The pop rivet heads need drilling off with a 3-4mm drill bit. Then the rivet shanks need punching through into the mast. If the cap was sealed previously, you may need to heat around its edges to release the sealant. Wire brush off all debris and clean with acetate etc. Check no naked flames near. Then proceed as for the bottom seal. See Figs Cut back the top closed cell foam bung a little to allow the aluminium top casting to set back in. I would then suggest filling the underside of the top casting with sealant or foam as well, because the mast crane (hook) projects into the casting and will lets water in. If water gets trapped over winter, it could freeze and crack the casting or the mast. I hope this has given you an insight into what this job involves. Pay special attention to obtaining a good interference fit with the marine ply seal. There's no guarantee of success though. Even professionally installed seals, done in perfect conditions with the best materials can still fail. So do your best and give it a good test with a full inversion after all the silicone has had time to cure.

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Fig 8: Upper Mast top plywood seal, before adding the fillet of sealant around the edge, and the final foam bung.

Fig 9: Ready for the mast head cap to be riveted on. Note: The photo shows unsuitable builders expanding foam in place. This was replaced with a closed cell foam bung.

Feedback is welcome so that I can improve the articles and pass information on to others. Good sailing, Syd Gage. Catapult 506. Contact - [email protected]

First published June 2014 Updated Dec 2016

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