How water ballast works

[Peter Taylor]

A library article on the "Physics of Water Ballast" is waiting for approval so (if it allows me) I'm attaching a pdf version of the article.  This arises from David Hudson's thread on a motor for a BRe...
http://www.swallowyachtsassociation.org/smf/index.php/topic,1020.0.html
...where we somehow wandered off to discuss boat stability!  The article is also available from my Seatern blog under 25th February 2015.

In case anyone wonders at the title, long ago my first degree was in physics at Imperial College and I then spent a career in marine physics research.  However the latter was more marine meteorology than anything else and certainly not boat design... so I'm very capable of writing rubbish and am happy to receive suggestions for needed changes or clarification! 

Peter

ps (20 March 2015) Alan Shapcott (from New Zealand) has pointed out that in my  text description of fig3 (how keel boats work), the cog and cob have been swapped over - it should read "For yachts like that the centre of gravity can be below the centre of buoyancy" - a correct version will hopefully reach the Library at some point!
Peter

[Julian Swindell]

Excellent paper Peter, sets things out with great clarity. I shall carry on sailing with full tanks as I need coffee at frequent intervals! I do have a dinghy buoyancy bag inflated in the stern tank of my BC20 as she seems a little stern heavy/stem light and was so from new. The bag just seems to level her out, without too much loss of ballast. I think Matt may have adjusted things slightly on later models (or just painted a different water line on...)

[Matthew P]

Thank you Peter for a very interesting and useful article.
 
You mention the importance of filling the water ballast tanks quickly to avoid interim instability with half filled tanks.  From experience I agree with you.  Also it is tempting to leave the large tank hatch cover open so that the bung can be replaced.  A rising wind and a tossing boat is not a good time to be groping about in the bottom of the cockpit, so the temptation may be to avoid taking on ballast when one should.  This was a contributory factor to me conducting a very public and embarrassing unintended boat self-righting test which has been discussed sufficiently elsewhere in this forum  – and I'm not going there again (see, if you must, "Who Dunnit").

My Bayraider 20 used to take about ten minutes to fill in flat water.  Apart from anything else this adds too much time to getting sailing.  I have since replaced the filler/drain bung with one of twice the cross-sectional area (I'm sorry I do not have dimensions to hand) and she now fills in about 5 minutes.  Maybe Swallow Boats now fit bigger bungs anyway.

Five minutes is still too long to fill the tank.  Has anyone tried turning round one of the rather self balers and remove its one-way flap so that it can be used as a scoop to fill the tank?  My self balers are not particularly effective at emptying the ballast tank anyway, the boat has to be moving quite fast, perversely when I probably want to fill rather than empty, and the balers only seem to empty the first third of the tank volume so most of the emptying has be done by hand pump.

And finally, here is some homework.  As engineering students we were asked to consider the hypothetical case of a small boat containing a heavy cast iron anvil and floating in a small swimming pool.  If the anvil was dropped over the side of the boat into the water, what would happen to the level of the water in the swimming pool and why?  If you think the water level will reduce because the anvil cracks the bottom of the pool causing a water leak you might be right but this is not the sought-after answer. 

Matthew
BR20 Gladys

[Michael Rogers]

I second that. Thanks, Peter. My personal interest in water ballast is retrospective, in that I used it in my first boat (storm Petrel). However I now feel much better informed about boat stability generally, thanks to you: and can go back to the Library for revision if necessary!

It's a marginal issue, but I am unconvinced about any useful 'real life' contribution made by a hollow mast to preventing a boat turning right over. My 12 ft Trouper has a 15 ft CF mast which weighs about 2 1/2 kg, and is quite wide diameter compared with an unstayed mast. At the outset of my Adventure off Durdle Door last summer, the relatively beamy hull lay broadside to a F4+ wind and quite high in the water (because of her buoyancy tank layout) for less than 2 minutes before turning turtle. Apart, possibly, from practice sessions in calm water, wind and wave are almost certain to negate any marginal help from an air-filled mast in a capsize situation. (I plan to invest in one of those airbag thingies which Graham helpfully drew our attention to.)

Michael

[Peter Taylor]

Thanks for your positive comments!

Julian - your buoyancy bag may explain why Matt thought that my adding a bit of equipment very far forward in my BC20 might not be a bad thing! When I was trying to use an electric outboard and had two 110Ah batteries on board I wondered about putting a bag in the ballast tank to offset their weight (which was just forward of the aft tank).  I didn't use one because I was worried it might move around in the tank. I assume yours is jammed in when inflated.

Matthew - my BC20 has a forward facing bailer to fill the ballast  tanks and that worked well for the aft tank (even though it appears to still have it's flap!). However on the BC20 the pipe connecting the two tanks is the limiting factor in how fast they can be filled/emptied.  The electric Whale Gulper pump fitted by SB took well over 30 minutes in emptying and  I now have a Johnson Reversible Ballast pump (supplied by Matt) which I use to both empty and fill the tanks electrically. Notes on the timings, electric power needed, and plumbing installed are on my blog (access them via: http://www.seatern.org.uk/SeaternDiaries/firstyear01.php )
Re. the homework... what was the temperature difference between the anvil and the water in the swimming pool?!

Michael - having spent most of my sailing life in dinghies I've had quite a few capsizes!  When masts were wooden turning turtle happened less often than when they became alloy.  I think it is generally accepted (David Hudson may have views on this) that having a lot of buoyancy in the side tanks makes a dinghy much more likely to turtle. If the capsized dinghy floats at an angle such that the top of the mast and sail dip under water then, if the hull is blown downwind, the water pressure on the sail forces the boat to turtle even if the mast is buoyant.  John Claridge Boats have redesigned the side tanks on a new version of the Seafly dinghy specifically to minimise that happening.  The idea is to allow the hull to partially sink so the hull (when on it's side) is vertical and the mast and sail lie parallel to the water.  This also makes it easier to get onto the centreboard from the water.  The alternative solution is, as you suggest, is some form of mast top buoyancy.  Given that Matt favours fat headed mainsails, maybe new sails could have a buoyant top panel incorporated.  Even a little buoyancy that far up would help.

One further comment on turtling - if you have continuous jib sheets (do some SBs?) it might be worth fitting righting lines.  Normally tucked away under the gunwale, they could make getting the boat back upright much easier.

Peter

[Michael Rogers]

Peter, I'm sure you're right about mast and on-their-sides hull angles. Being on my own and more than a bit startled by my circumstances during my Adventure, I failed to appraise the situation with a sufficiently calm, objective eye: but I do clearly remember the hull being, briefly, so high in the water that the mast was below horizontal. As a remedy, and as I said in the discussion at the time, combining airbag and life-jacket technology in a masthead thingy is really very neat.

[Graham W]

If you look at this video of (an admittedly lightweight) epoxy ply BR20 being deliberately capsized, you can see how far out of the water she sits on her side, and how the top of the yard points below the water (from about 1'30" and also 3'00" onwards).  https://www.youtube.com/watch?v=yYrFXsHzvcg 

[Julian Swindell]

One feature of most Swallowboats is that they are designed to keep the outboard out of the water when they are on their sides. This necessarily means they will float fairly high. But the weight of an outboard might make them float a bit lower in the water than shown on the video. Similarly, full tanks will also keep it lower and easier to right before it goes right over.

[David Hudson]

Turning turtle isn't fun, especially when the centre board disappears into its casing. If your crew is on their ;(avoiding gender reference); toes, a continuous jib sheet should be to hand. As Peter says, this a problem with many modern dinghies.

RS have incorporated a flotation panel in the top panel of the Venture, their training dinghy introduced in 2013. Looks a lot better than the suspect looking sausage shaped thingies often seen on Toppers and the like.

There is also the psychological aspect to the event. There is a big difference between a quick righting, spinnaker filled and water disappearing through where transoms used to be and struggling to assess the scope of the problem while floundering next to an upturned boat. Perhaps Matt might put mega transom flaps on the boats?

One post turtle consideration is depth of water. I have seen the mast foot on a '5o5' come through the hull of an old boat.

Keep on drifting, a thread eventually becomes a yarn.

[Matthew P]

"Homework:  As engineering students we were asked to consider the hypothetical case of a small boat containing a heavy cast iron anvil and floating in a small swimming pool.  If the anvil was dropped over the side of the boat into the water, what would happen to the level of the water in the swimming pool and why?  If you think the water level will reduce because the anvil cracks the bottom of the pool causing a water leak you might be right but this is not the sought-after answer." 

Hi Peter, the temp difference between anvil and water is a creative idea but the big factor is to do with relative density and buoyancy.

Matthew
BR20 Gladys

[Michael Rogers]

Matthew, was the anvil dropped right to the bottom, and the line slacked off?

[Matthew P]

Matthew, was the anvil dropped right to the bottom, and the line slacked off?

Good question Michael - yes, the anvil was dropped to the bottom of the pool with no line attachment.  The boat remained floating, upright (no sarcastic comments please anyone), with no other item removed or added to it, including water.   

Matthew

[Graham W]

I'll show my ignorance of the laws of physics by speculating that the water level will go up, because the amount of water being displaced by the anvil when it's in the boat has a much greater volume than the anvil itself once that has been chucked into the pool.  If you then dive in to try to retrieve it, that's a different matter.

Despite Peter's learned article, I still think that cruise liners look like they ought to fall over because they appear so top heavy.  Especially after learning that most of them only have a draught of 8m or less.

PS I just googled the answer to the anvil puzzle.  It's probably better for all concerned if I stick to history.

[Matthew P]

I'll show my ignorance of the laws of physics by speculating that the water level will go up, because the amount of water being displaced by the anvil when it's in the boat has a much greater volume than the anvil itself once that has been chucked into the pool.  If you then dive in to try to retrieve it, that's a different matter.

Thank you for a brave effort Graham. Right thinking - wrong answer!

When the anvil is in the boat it displaces a volume of water equivalent to its weight (weight I think, not mass, Peter?). When the anvil is in the water, unsupported by the floating boat, it displaces a volume of water equivalent to the anvil's volume.  Because the anvil is denser than water the displaced water reduces so the water level in the tank goes down - admittedly a very small amount.  In future I'll stick to dropping rocks into ponds just for the fun of making a splash.

Matthew

[Michael Rogers]

I'm doing the homework 'blind', i.e. I haven't looked to see whether anyone else has posted an answer (honest).

The level will remain the same. The total amount of water displaced by the boat and the anvil remains the same whether the anvil is in the boat or at the bottom of the pool.