Boats and their Flotation

[rainbowrascal]

I was just wondering what keeps your boat above the water line, as there are a lot of boats on the market that cannot handle a lot of water on their decks, especially in the event of a rogue wave or anchoring mishap or leak. There was a recent fatality off Brisbane with a boat that capsized and sank, don't know the exact manufacturer, but I heard it was a cat, this is a major problem that should be addressed by all boat manufacturers. Most large aluminium boats (4-7M) would also sink as the bilge pump would be inadequate to cope with a large volume of water and if you look under their decks there is not much there in the name of flotation. I have been to some of the biggest aluminium boat builders in the state to see how they make their boats as well as working in a boat yard for some time, so have seen a few different brands. I also spoke to a friend of mine who was busy stripping 2 x 115s on the back of a old Sharkcat two days ago. The day before the owner arrived back at the pontoon from a fishing outing and went to get the trailer, by the time he reached his car, the boat had sunk. My own glass boat has a self draining deck and under it are 5 different compartments that are all filled with hundreds of individual bottles. If I took a large amount of water on board, it would rush out over the back transom as well as the scuppers. The other option for flotation is the two pack foam, which I know is used in some glass boats. Not fool proof but a lot better than what I have seen in a lot of boats.The other problem is that when some of these boats overturn at sea or on a dam they only have to float a few hundred millimetres above the waterline which is not sufficient to hang on to. Just wondered if you were in a precarious scenario on your pride and joy, and how it would cope in a bad situation.

[Moonlighter]

Boat floatation is covered by the Australian Builders plate. Came in several years ago.

There are basically two levels of certification.

"Basic" floatation, which means the boat will float one way or another if totally swamped.

"Level" floatation, which means what it says - the boat will float in a level position with its occupants on board. In calm conditions.

The problem in achieving level floatation is that the floatation material needs to be up the sides of the boat and under the gunwales to make the boat at lest theoretically float in a level position.

This leaves no room for side pockets etc. Hence, few boats are sold with level floatation certification these days. There is little demand for it from consumers. It also adds a lot of cost.

Nearly all boats therefore just get "basic" certification, which means the boat will not sink and the passengers can supposedly hang onto it even if it's upside down.

And just because a boat has self-draining floors doesn't mean it can't be swamped and tip over. You get one wave over the bow, and 5 seconds later, the next one. And then the next. No bilge pump can cope with that, and neither can scuppers. The boat gets rolled over by the wave action, and the crew are in the water. With their life jackets on, if the skipper has been acting in accordance with his/her legislated general safety obligation and instructed them to put them on.

Self draining floors are also problematic to achieve in smaller boats, as the floor level has to be lifted, and if you do, you then create another problem with insufficient height inside the boat for safety, so people can fall out easier.

Finally, you need to face the reality that it is actually a quite rare occurrence for a boat to sink or even get swamped. And you have to also factor in the other safety strategies available, such as wearing life jackets when conditions suggest that that is prudent.

Boats sinking or getting badly swamped simply doesn't happen very often and the issues involved in making boats theoretically unsinkable compared to the costs and benefits have to be weighed up. They don't add up, unless you really want to live in a nanny state.

[fishing111]

To my way of thinking all trailer boats should be made as unsinkable as possible. I know some want access to side pockets but if your 50 mile out or 5 mile and the shit hits the fan would you be worried about that bit of lost storage? I don't think it's a nanny state thing, I think all boats should be made as safe as possible below deck and above floatation wise. Car makers improve safety from model to model so I don't think it much to ask from a consumer who buys a boat that the bloody thing floats and remains floating, and not having to wave good bye as you watch it sink in 50metres of water leaving you with the dog fish.

[The Woo]

I'd just step up off my sinking boat onto my life raft.

And I'd still get to use my side pockets.

[Chris Tucker]

This is a particular sore point of mine, too many boats are on the market that pay no more than a lip service to AS1799 and the builder's plate requirements. It makes it harder for the builders that are doing it all properly to compete and puts the consumer at an unnecessary amount of risk.

If you are buying a trailer boat demand foam based basic floatation as a minimum. Use of air voids doesn't do it for me and is advised against by the standard. A good boat boat builder or boiler maker will tell you it is impossible to weld an air tight compartment without a vent hole and said vent means that the void isn't truly air tight. If you have your boat parked in the sun at the ramp and then launch it the air in the compartments will cool and decrease in volume. If compartments are truly air tight this regular variation in pressure will result weld fatigue. Finally in the event of a collision (with another boat, reef or main made structure) your air tight compartments can become ruptured and therefore irrelevant.

This is what the Australian standard says about air based floatation:

"2.5.6 Air compartments
Wherever practicable, integral void air compartments should be avoided as a means of
providing reserve buoyancy. Where such compartments are used, the construction shall be
equivalent to that of the surrounding hull structure. Stress raisers shall be avoided, and all
compartment-to-hull seams shall be positioned to allow inspection. Where air chambers or
compartments are used to provide reserve buoyancy, the requirements of Clauses 2.5.1
and 2.5.4 shall be met excluding the two largest compartments or air chambers.


Each air compartment used to provide reserve buoyancy shall be—
(a) provided with a drain plug complying with Clause 3.2.3 at its lowest point; and
(b) permanently marked with the following words or their equivalent:
‘CAUTION: This air compartment is essential to the flotation of the boat. Do not
puncture or attach fittings.’


These requirements apply to all of the air compartments used to provide reserve buoyancy,
including the two largest air compartments. "

[GBC]

If you are really concerned about this stuff and think you might put yourself in a situation where you might need it, under deck is not where you want max floatation in a trailer boat when taking on a 'large' amount of water as first mentioned. A self drainer will roll quicker than a bilged trailer boat when filled with water, and most commonly the 'scuppers' end up under water and can't bail the boat quick enough to save it before it rolls. There are some pro style trailer boats that have big enough scuppers and no cabin soles lower than the rear deck, but if your mass produced 'self draining' boat has a couple of 2" holes in the stern with 'slop stoppers' over them, and a cabin up front the size of a small swimming pool, be afraid. If you lose you motor and cap one over the stern then you are going to be in a world of hurt in 2 seconds flat. I have been in this situation.
Rigid inflatable, inflatable or boat collar is what and where you need floatation if you are going to have a decent wave cap into your pride and joy and half fill it. Anything else is pretty much a joke in comparison. Underfloor sealing/foam is going to give you something to hang onto while you're in the water with what used to be your boat, but don't assume much else.

[Chris Tucker]

You make my point exactly with regards to builders ignoring the standard. A small drain in each corner won't do it.

Here is what it says about cockpit drains:

3.4.2 Drain area
Where drains are provided, the minimum effective cross sectional area of each drain shall
not be less than 800 mm2. The total effective area of all drains in a well or cockpit shall not
be less than 700 mm2 per square metre of cockpit or well deck area including the area of
any lockers or compartments that drain into the cockpit or well.

3.4.3 Drain construction
Drains of the scupper type should be designed as follows:

(a) Drains should be easy to clear, without restriction, and with as few bends and fittings
as practicable.
(b) Flexible sections may be used to avoid fatigue failure. Care should be taken to ensure
that drains situated below the weather deck that incorporate a flexible section are
designed in such a way that failure of the flexible section will not result in flooding of
the hull.
(c) Drains may discharge above or below the waterline, but if below the waterline they
shall be provided with a suitable fitting to prevent back-flooding.
(d) Where required, deflectors should be used to prevent water backing up when the boat
is under way. Drains shall be constructed as follows:
(i) Where non-return flap fittings are provided, they shall be clearly visible and
accessible.
(ii) Materials shall be protected against salt water corrosion and shall comply with
Clause 3.13.
(iii) Where strainers are fitted to drains, the clear area of the strainer shall be at least
50% greater than the required minimum effective area of the drain discharge.

[sowden1942]

Hi. I own a 4.5mtr centre console tinny and i have just had a boat collar put on it for this exact reason. If i get swamped the boat will stay upright and float.

[Moonlighter]

All I say, is keep some reality and perspective on this. And for heavens sake, don't advocate for more and more regulation to fix a problem that, in reality, is both almost impossible to fix completely anyway, or is a relatively rare occurrence, or, has other equally suitable solutions such as the occupants wearing their life jackets when they should.

The Seppos already shake their heads about our compulsory life jackets (in certain conditions) and compulsory EPIRBS...... Nanny state stuff.

If you're really worried about this, buy the best quality life jacket you can find and wear it every second that you are on board. That way if you get swamped on the bar etc you will not drown.

Meanwhile, leave me to decide what I will do, thanks very much. Big and ugly enough to make my own decisions.

Even in the U.S. And Europe, where boat building standards are far higher than here, boats can still get swamped and crew get thrown out.

And even in your rigid inflatable etc, if you get swamped in croc infested waters and manage to stay level but full of water, the croc will swim right in there with you. Good luck with that!

BTW a 50mm bung in each corner of a boat totals about 4000mm2. Enough for a 5m2 cockpit. Most boats under 5.5m will be fine. 2 x 35mm scuppers will total nearly 8000mm2 enough for a 10m2 cockpit. But it's almost irrelevant anyway, because if you get a wave break into your hull, it will be rolled over anyway. You could have 100mm bungs or scuppers and they won't help that.

[Mitch73]

I had a 5.8m shark cat. Which was build to be massively buoyant. The twin hulls both have 3 seperate airtight compartments. Which would give it substantial lift as well.

What at is shark cats #1 problem ?
rotten floors.

Soon as floor rots and most times even after it is replaced. The air tight compartments are no longer air tight.

Or or someone puts a bilge pump in.

Nil bouyancy.

I would assume sure it's the same with a lot of old boats.

Mitch

[Andy56]

Foam filling has other advantages. It makes the hull quieter and increases the Impact strength of the hull If applied in a closed rigid compartment. If your going to put it in your tinny, you are better of with pieces cut to size. On others with air chambers, two part expansion is the go. Just pick the right foam.
I disagree with bare minimum and everyone to their ability. If it can be better, why not I say. Especially if a know problem such as soft floors appears