The correct engine!

[Peter Taylor]

Hi Andy,

You are confusing mass and weight, and buoyancy comes into this too!  Momentum, and the force needed to accelerate or decelerate a body depends on it's mass, not it's weight.  Mass is the amount of stuff, in our case the boat, gear, water in tanks, etc.  The mass remains constant whether the boat is in the water or on it's trailer (unless you've drained the tank!).  The scientific definition of weight is a measure of the gravitational force on the boat.  On the moon your boat would weigh much less than on the earth because the moon's gravity is less, although it would still have the same mass.  Non-scientifically, here on earth we tend to define weight as, say, the reading of a spring balance.  Using that definition, if you put your boat in water it doesn't weigh anything whether the ballast tanks are full or empty.  But adding water to the ballast tanks has increased the total mass of the boat.

Filling the ballast tanks makes the boat sink lower in the water because before you filled them they were full of air and adding to the boats buoyancy.  Filling the tanks will tend to lower the centre of mass of the boat compared to the centre of buoyancy.  Thus the boat becomes more stable when tilted, whether or not the water in the tanks is lifted above the level of the surrounding water.  When you accelerate the boat (using either sails or motor) you have to apply more force when the tanks are full because you have to accelerate the mass of water in the tanks as well as the rest of the boat.  Similarly when you plough into a wave and it tries to stop you, the same force of wave will have less effect if the tanks are full (because of the extra mass of water) than when they are empty.

Hope that makes sense?
Peter

[Nick Peters]

Hi Peter,

Great explanation. Simple and clear, need to have that to hand at the next boat show!

Cheers,
Nick.

[David Hudson]

"Archimedes' principle", innit?

[Peter Taylor]

Thanks Nick...

however thinking about it, my introduction of the concept of the net weight (as measured by a spring balance) may cause some confusion with regard to how stability works, to try to correct that...

Even when it's in the water the boat has weight, but that weight is exactly cancelled by the buoyancy force.  So the spring balance registers zero. That buoyancy force is created by the boat displacing it's own weight of sea or lake water which indeed is "Archimedes Principle" as David says. The buoyancy force is always equal to the weight of the boat because if it is less, the boat sinks lower until enough water has been displaced for the buoyancy to counteract the weight.

The weight acts through the centre of mass (also known as the centre of gravity) and the buoyancy force acts through the centre of buoyancy which is located at the central point of the part of the hull's volume that is under water. People often don't realise that, apart from sailing yachts with heavily ballasted keels,  the centre of mass is normally higher than the centre of buoyancy. That is particularly true for large ships where there is a lot more above the water line than below it (as we saw with that car transporter which ended up on the Brambles bank).  Ships only remain upright as long as you don't tilt them too much; beyond a certain point there is no return!

What matters for stability is how the centre of buoyancy moves in relation to the centre of mass.  I really need to draw some diagrams to illustrate this. When the boat is upright the two centres are directly in a vertical line.  As the boat heels the centre of buoyancy moves outwards towards the downward side of the boat. Provided the ballast (including the crew) does not move across the boat, the centre of mass stays in the same place in relation to the rest of the boat.  However if the centre of mass is higher than the centre of buoyancy,  the heeling of the boat will also move the centre of mass further across towards the downward side of the hull.  Imagine a boat where the centre of mass is some way up the mast - in that case it's obvious that it will moves outwards as the boat heels.

For stability it is important that during the heel the centre of buoyancy moves out faster than the centre of mass.  This is achieved by keeping the centre of mass as low as possible (and the boat as beamy as possible). As long as the centre of buoyancy is further out, the two forces, of gravity and of buoyancy, act to rotate the boat back into an upright position.

The actual force of this righting tendency depends (as you might expect) on how great the distance is between gravity pulling down and buoyancy pushing up. The way this changes with the angle of heel is what is shown in a "gz" curve where the g is gravity and z is the distance.  But for a given distance (that is a for given value of gz), it also depends on how big those two forces are, and that depends on how heavy the boat is.    According to Matt's Water Craft magazine article ("What's the point of Water Ballast?", Water Craft 109, 30-33, Jan/Feb 2015) the water ballast in a Bay Raider increases the gz by about 30% (by lowering the centre of mass).  But the tendency for the boat to come upright is more than doubled mainly due to the extra weight of the ballasted hull (pulling downwards) and hence the larger buoyancy force (pushing upwards).

If you add ballast to a boat in the form of a closed heavy box of a certain weight, it doesn't matter whether what's in the box is water or lead (provided neither the water or lead can move around in the box).  The advantage of lead is you can use a smaller box and you can get it lower down in the hull.  The advantage of water is you can add it or remove it more easily.

...wow! I only meant to write a sentence or two!  If it's thought worthwhile I could take up Andy's challenge and create a library article with some diagrams.  Matt covered much of this in his Water Craft article, although, I imagine to avoid confusion given limited space, he didn't introduce the concept of "centre of buoyancy".

[Michael Rogers]

I for one would welcome a Library article, with diagrams, Peter, but already this bear of very little mathematical brain feels better informed. Thank you.

A small correction if I may - the Water Craft article was by Nick Newland, Matt's father and a naval architect by training. I think he had to do with warship design before small boats took him over. He held my hand, metaphorically speaking, through my first Swallow Boats kit build.

Michael

[Andy Dingle]

Wow! Thanks for such a detailed reply Peter - and a big thank you for that, and for your impending library article with some diagrams? You'll have to do it now! It will be worth the half pint of shandy I buy you next time we meet up in way of apology for making that suggestion.
I clearly have little understanding of what is going on all around me as I happily sail along - all those forces coming into play all around me as I sit ignorantly picking my nose and wondering when the next cup of tea will be.
That'll teach me to idly day dream away all those physics lessons back in the 60's ....

Thanks

Andy

Baycruiser23 No.25 'Equinox'

[Nick Peters]

What I like and will find useful is the relationship between COG and COB, because when you think of stability in these terms, then it is very clear why internal ballast works, and then the benefits of that being water as opposed to lead (for the dayboat, small yacht) follow neatly.
It is difficult to get this over concisely when chatting to potential converts....in a way that will be meaningful for them and help rather than confuse the process of becoming an owner! At times you see the water ballast feature switch people off, because it is unfamiliar and maybe an unnecessary complication for some.

Cheers,
Nick.

[David Hudson]

Back in the early 80's, the "Mini Transat" boats pumped water ballast athwartships to aid righting moments.

[Peter Taylor]

Back in the early 80's, the "Mini Transat" boats pumped water ballast athwartships to aid righting moments.

Yes and so do enclosed car transporters and other ships - pumped systems can be embarrassing if they go wrong (photo attached)!!!

A small correction if I may - the Water Craft article was by Nick Newland
Michael

Thanks, Michael, I'd missed reading the byline under the photo and assumed it was by Matt (though I wondered how he found the time to write it!), I'll correct the attribution in the Library version.
Peter

[michaeln]

In case it is of interest - the new Volvo Ocean 65s have three venturi filled water ballast tanks. Two  aft 800L tanks under the cockpit sides and one 1000L tank forward of the mast. There is very little information on how the tanks are utilised - as these are one of the variables in these new 1 design boats. However, I have seen two references to the forward tank (new to the 65s) as having been added to make the boats less twitchy and to improve upwind performance and reduce the bow falling off to leeward in big seas.

[michaeln]

I should have added that the water ballast augments a canting fin keel (overall draft 4.7m) with a 3465kg bulb!

[Peter Taylor]

In case it is of interest - the new Volvo Ocean 65s have three venturi filled water ballast tanks. Two  aft 800L tanks under the cockpit sides and one 1000L tank forward of the mast....... references to the forward tank (new to the 65s) as having been added to make the boats less twitchy and to improve upwind performance and reduce the bow falling off to leeward in big seas.

Of course I don't actually know why those particular tanks have been added, but what they will do when full is increase the moment of inertia of the hull. Moment of inertia is the resistance to a rotational force.  In other words the boat will indeed be less "twitchy".  Note though that it's not just the forward tank that will contribute to that effect.

That has reminded me of two separate conversations with Matt.  In one I mentioned that a heavier boat might be desirable when beating through head seas and Matt responded that a lighter boat should always be faster.  However when I was discussing permanently installing some gear right up in the pointy bit of the "V-berth" area (for want of a technical term), Matt's response was that a bit of mass there might be a benefit.  Matt's two responses are not necessarily contradictory if what helps you punch through the waves is moment of inertia rather than momentum. Yet more to think about!

Peter
p.s. if Seatern had that fin keel I would be constrained to sailing in water less than 4.7m deep!

[David Hudson]

I have just about caught up with the intellectual content of recent post on this thread. I usually coast along in glib mode, often offending people on the way. I have now moved up a gear or two and hope my contribution is relevant.

Those who were involved in the 1980, 5o5 World Championships off Hayling Island may remember Steve Benjamin. Steve, from the States, was crowned World Champion that year.

He had less luck while preparing his new Lindsay dinghy for the Australian Worlds in '76. He spent time trying to eliminate the fore and aft pitching moments in his boat, assessing the effect of water taken onboard by the spinnaker chute. This involved placing more and more 6 packs of Miller beer on the dinghy's bow. This led to his crew installing turtles for the spinnaker. Further detail escapes me but I expect the Miller went the way of all beer.  Carbon masts were a long way off at that time.

Unfortunately the dinghy did not make Worlds. My helmsman had to tell him that his boat had been destroyed by a loose container while at sea.

There, almost relevant.

[jonno]

In the past I've advocated the little Suzuki 2.5 for the Bayraider. It's always been powerful enough and it's light.

I'm not so sure anymore. Recently I've been at sea (around Mull) worrying I might run out of fuel. The integral tank is small. Swell and spray have kept me from topping up. Running out of fuel would be awkward or worse.

So I now think for coastal cruising, a bigger motor with its bigger external tank is preferable.

Or is there a way to get around this? Is there a safe way to refill the tank when it's bouncy and wet? Or has anyone successfully linked up an external tank to a little outboard like mine?

John

[Jonathan Stuart]

John,

As we discussed at Sail Caledonia, I am also in the minority that likes the Suzuki 2.5hp motor - it's light, reliable, economical and has sufficient power for a BR/BRe in all but the strongest headwinds and foul tides. But I share your concern and didn't find a complete solution for this. However, here are my thoughts.

First, while it's always reassuring to know you can't run out of fuel, I've found the motor very economical. For example, we sailed from Weymouth to Poole last summer and the wind failed after just a few miles. Looking back at my log, I see that we travelled 30nm of which at least 25nm must have been pure motoring or wind-assisted motoring. We used 3 litres of fuel, i.e. approx. 2 top-ups, but we didn't have to contend with headwinds, etc. That still required refuelling at sea but I was happy that I could time the refuelling somewhere convenient, i.e. a more sheltered area. I run the motor at no more than half throttle because beyond that gives little extra speed and I suspect burns disproportionately more fuel.

I also use 2 litre "Fuel Friend" fuel tanks (designed for overland motorbike expeditions) which are easier to use at sea and to stow (I put them in Gorilla buckets in the aft buoyancy chamber) than the larger fuel tanks. Here are links to examples of the ones I use:

http://www.ebay.co.uk/itm/Motorcycle-Plastic-Fuel-Can-2L-Black-Fuel-Friend-/291626612770?hash=item43e64cf822:g:vMoAAOSwf-VWWWsM

http://www.ebay.co.uk/itm/Motorcycle-Screw-On-Fuel-Can-Spout-with-Cap-for-Fuel-Friend-1Ltr-1-5-Ltr-2Ltr-/291661835767?hash=item43e8666df7:g:F1IAAOSwGotWm1pG

But you could add an external tank for a better solution. Here are links to YouTube videos that show how to do that:

https://www.youtube.com/watch?v=ZS0sn46-ITQ

https://www.youtube.com/watch?v=nYxJQewMGI0

I started down that road but never completed the work because after my Weymouth to Poole journey and buying the Fuel Friends this became less of a concern for me. But I did buy a fuel cap and attach a fuel tank connector to it - see attached photo. You can have this if you want (let me know and I'll post it to you) although I did find that Suzuki changed the threads on their fuel caps so it may or may not fit your motor (mine's a 2012 model).

From memory, I think those YouTube videos show a fully unattended solution that added a gravity-fed external tank. I was erring to an even simpler solution which was to have an attached tank sited on the cockpit floor that I used to top-up the outboard's tank with a few squeezes of the primer bulb. I have a few spare fuel lines & bulbs in the garage and measured which one pumped the most liquid per squeeze.

I can't remember the amounts being shifted per squeeze, but was happy that it was sufficient. I guess that to complete that solution I just needed to connect a tank to my modified fuel cap and provide a way for air to escape from the outboard's tank when fuel is added. Ideally I would have added a little screw-in air valve to the cap for that but, being lazy, had considered just loosening the cap slightly to allow for this.

This all needs a little more thought but I think there is a solution here. I hope that helps.