Naval architect Julian Wolfram uses some able hands, scales and maths to explain a practical way to calculate your boat's stability
Have you ever measured your yacht’s stability?
Adding heavy cruising gear will change your boat’s stability, so it is worth checking, although the names and terms, such as ‘Dellenbaugh angle’ and ‘metacentric height’, might be initially off-putting and leave you flummoxed.
These measures of a yacht’s stability or stiffness – used to compare one boat to another, or modifications that might have done on board – are more reliable than the crude and common ballast/displacement ratio, and understanding them will reveal the impact on your boat of all the additional cruising gear that has been added.
Ballast ratio is a flawed because it takes no account of the shape or depth of the keel, or of how heavily loaded the boat is.
Rather than ballast ratio, a better way to assess the stiffness is by dividing the position of her centre of gravity, as measured from the bottom of her keel (known as KG), by her draught, as this takes into account her both her draught and the centroid of ballast on board.
For any yacht built after 2002 the designer or builder will have calculated and potentially measured the KG for the minimum operating condition and probably for the fully loaded condition too.
This data is needed to do the required Recreational Craft Directive (RCD) calculations.
It may also be available for many yachts from before then if the builder or designer was conscientious.
Interestingly, this information has to be provided, by law, for a commercial vessel in the form of a stability booklet and there is no logical reason why it should be withheld from a yacht owner – but that doesn’t mean you’ll get it.
If you want to compare the stiffness of your yachts with others and can’t get hold of the KG, you will have to do an inclining experiment to calculate it.
An inclining experiment is required for all commercial vessels, including sailing yachts used for commercial purposes, charter and sail training, and is usually carried out, or at least witnessed by, a ship or yacht surveyor.
The inclining experiment yields the metacentric height (GM) which is a primary measure of stability.
Once you have GM then KG can be found using the hydrostatic particulars that are calculated from the table of offsets or the lines plan.
If you can’t get hold of these then you will have to get a 3D laser scan of the boat, when she is out of the water, and a naval architect who has a stability software package to do the calculations for you.
However, doing an inclining experiment is still worthwhile and, on traditional vessels built by eye or for which the lines and hull offsets have long since disappeared, it is the only option for assessing stability.
How to carry out an inclining experiment to check stability
Anyone can carry out an inclining experiment on their own yacht if they wish to check its stability.
It is done afloat, and simply involves moving weight from the centreline towards the deck edge and measuring how much the boat heels as a result.
The weights can be of any sort – jerry-cans full of water, baskets of old chain or the like.
I once did an inclining experiment on a 19m ferro-cement schooner with a weight that consisted of a bunch of students weighed on bathroom scales. It worked well.
Traditionally the angle of heel is measured using a pendulum (plumb line attached to a mast) and recording the offset to the side when the vessel heels.
The pendulum is usually damped using a bucket of water or oil.
Nowadays a device known as a stabilograph is often used which is more convenient.
Alternatively, I have used a long (1,830mm) spirit level successfully when the heel angle is between 3 and 6° and I think this is the cheapest and most practical way for a boat owner to give it a go.
Needless to say calm conditions are necessary to get an accurate measurement and mooring lines should be slack so the boat heels in a completely unrestricted manner.
Ideally the experiment should be done in the loaded condition but with the tanks no more than half full.
The crew are ideal inclining weights and six crew will weigh nearly half a ton (and maybe more in some cases!) and they don’t have to be lifted onto and across the boat.
Mark lines each side of and parallel to the centreline close to the deck edges.
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The crew will stand facing the same direction with feet together, one foot either side of the line and their weight evenly distributed on both feet.
Ideally there should be one long line marked with chalk or tape on each side of the boat.
That may not be possible, however, and two or more lines may be needed; in which case you will have to note who stands where, as the product of each weight and its distance from the centreline is needed in the calculation.
Start by weighing each of the crew in turn on an accurate set of bathroom scales.
Then put the long spirit level across the cockpit with both ends supported so it is level.
Note the distance between the points of support (x mm).
You should be on the centreline when you are checking the level.
Get the crew on board and along the centreline to start with.
Now get them all to take up positions on one side of the boat and carefully chock up the end of the spirit level so it becomes horizontal.
Pieces of plywood and plastic packers down to 1mm in thickness can be used as you need to measure to the nearest millimetre how much you have chocked up the end of the level (y mm).
Ideally the heel angle will be between 3 and 6° and you will have chocked up the end of the spirit level by at least 100mm.
Take the average of the values as the best estimate of GM. It should be accurate to 1 or 2%.
Typical values of GM range from about 0.8m for a 6m coastal cruiser rising to 1.5m or more for an ocean-going 12m yacht.
Read about what makes a boat seaworthy here
Wide hulls with little freeboard should have higher values; any significantly below this range should give cause for concern.
Knowing GM allows the Dellenbaugh angle, to be estimated.
The heeling arm is the distance between the centre of effort of the sail plan and the centre of lateral resistance of the hull and keel.
These can be estimated from a profile drawing, showing the keel and sail plan and worked out using known measurements.
Once calculated, for a 12m long boat a value of 12° would be considered stiff and 18° tender whereas for a 6m boat 18° would be considered stiff and 26 degrees quite tender.
For those who wish to learn more about this I recommend reading Principles of Yacht Design by Larsson, Eliasson and Orych.
How to measure the stability of your yacht
First you have to wait for Mother Nature to give you a calm day: any wind on the rigging could skew the measurements and drive you mad while you’re trying to get the level correct.
Also consider where you’ll do the test; while it is possible to do this on a mooring, the shelter of a marina is best for accurate results.
You’ll also need weight: passers-by, friends or relatives will do as long as they can spare you 10 minutes.
If not, jerry cans of spare fuel and water, sails, dinghy and liferaft will be needed.
The bigger the boat, the more weight you’ll need. You’ll also need a spirit level – the longer the better.
If it isn’t long enough to go across the cockpit seats of coaming, use a flat bit of wood long enough to span the gap.
1. Marking and measuring
Marking the centreline on deck, we used masking tape, but a pencil or chalk line would do the trick. Then tape another line parallel to the centreline on deck, remembering to allow room for feet on the outside of the line.
Measure the distance from the centreline to the deck line. Use this measurement (d) to mark a second line on the opposite side. The last measurement you will need is the distance between the supports of your spirit level (x).
2. Prepare the weights
Next weigh your helpers or equipment to act as weights – a set of bathroom scales is ideal for this, whether for people or heavy objects. Slacken your mooring lines so they don’t affect the way your yacht heels.
Line your weights on the centreline mark and ensure the spirit level is showing your yacht is lying flat in the water. You have to remain on the centreline in the cockpit.
3. The experiment
Now move your crew or weights until they are over the deck line; people should stand with one foot either side of the line, weights should be piled up as best as possible. Now pack up the end of the spirit level to bring it level again, then take a note of the thickness of the packing to the nearest millimetre. Repeat the experiment with the weight on the opposite side. Finally, average the GM figures from both calculations.
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