The old seadog at the Yacht Club will tell you the answer is usually about 1 litre more than you bought!
Every other paint manufacturer’s website seems to have some kind of calculator to tell you exactly how much antifoul you need to paint your boat. Plug in the boat’s key dimensions, click on some tins of paint and you’ll be given somekind of number: according to Hempel – I need 7 litres. I presume this indication is for 2 coats – as they recommend only 4 litres of the Tiger Xtra which they market as a “a good performance antifouling requiring only one coat for one season protection” According to Jotun I need 9 or 10 litres…. So how do they work out this number?
The general formula used for these calculations is:
Cn x LWL x (B + D) = Underwater Area (m2)
- Cn = Some constant between 0.50 and 0.85
- LWL = Length at waterline
- B = Beam
- D = Draft
Once you have calculated the underwater area, divide by the coverage area for the paint (a number normally expressed in m2/l) and you will know how many litres of paint you will need.
For Faïal: underwater area = 0.7 x 11.50 x (3.20 + 1.88) = 40.894 m2 = 41 m2. Cruiser Performer has a coverage of 12.5 m2/l, so I should need about 3.27 litres of paint per coat. I used 0.7 for the constant has the Alpa 11.50 has a semi-long keel… So the 7 litres suggested by the Hempel calculator seems very reasonable.
However, we now have two problems – first of all we have used a very rough calculation to give us an estimate of the underwater area of the boat. Second we are relying on the theoretical coverage area of the paint. If at least the dimensions of our boat should not change from year to year… the coverage area of the paint we will achieve will… as this will be affected by temperature and by the enthusiasm and technique of the crew applying the paint. This is why the old seadog is right and you will need one more litre than you bought.
Now the astute purser, will notice in his perusal of the fascinating literature put out by paint manufacturers that the theoretical coverage area varies widely between paints. He might be tempted to consider the price of paint not in terms of £ per litre but in terms of m2 of boat that can be covered per £. Sadly a higher coverage rate is usually a reflection of a lower solid partical content. The solid partical content is usually the stuff that makes the antifoul effective. It is stuff like coprous oxide and other biocides that are held in suspension in the paint… So an antifoul with a higher coverage rate might be less effective… But the amount of biocide in antifoul is the not the only factor determining performance… how the paint releases the biocide is equally if not more important…..