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Weight calculations

I am going through more calculations to see how the weight distribution is looking now that we have most of the equipment finalized for the new boat. I thought it might be useful for anyone planning a new boat or outfitting an existing boat for a cruise, to see how I have tried to predict the impact of all our stuff on the waterline. You can apply this method to your boat (or dreamboat) too!

In all the far off anchorages in the world we see cruising boats that have way too much stuff on, and waterlines painted higher to deal with the extra weight. But most designers would not be happy to see this - for reduced performance and possibly impaired safety. I want to minimize this so have been trying to get the best of both worlds - add the gear I want but trim the weight so she will still perform well.

I just love all this stuff!! But if the calculations and all are not your thing, you can skip to the punchline - or jump to the pictures of the hull with the keel being put on!

Calculating the effect of more weight
First step in this calculation is to find out how much the weight will effect the boat. Here is a very simplified method that can be applied to any boat. The first number we need is to find out the surface area of the boat in the water. By this I mean the area of the hull at the waterline. Assuming you can't just ask the designer - here is a simple method to calculate it yourself (also assuming you have a plan view drawing of the boat, Photoshop and a computer...)
  1. bring the plan drawing of your boat into photoshop
  2. you need the scale so you can measure things on the plan. My method is to resize the drawing so the "info" pallette shows the correct dimensions. Since the Southerly is 12.8 meters long I set the scale so the hull on the plan measures 12.8 cm by 4.03cm. Now anything I measure on the drawing will be at a scale of 1cm to 1 meter.
  3. for the weight calculation we need to determine the area at the waterline. If you have the waterline on the drawing this will be easy. If not (as I didn't) then you have to guess.
  4. use photoshop to draw a few rectangles on the drawing so that they approximate the area at the waterline. As you draw each rectangle, note the length and width on the "info" pallette in photoshop. My first rectangle is 3.4 by 5 meters - the main shape of the hull. 5 x 3.4=17 square meters. Now write this down.
  5. Continue making rectangles until you have a rough shape that approximates the waterline of the boat.
  6. Add together the rectangles. 17+1.6+2.8+3+2.2+1.75+1 = 29.35

So we have a rough idea of the waterline area of the boat. 29.35 square meters rounded out to 30.

Now for the fun bit. We know the surface area of the boat so we can easily calculate how much weight will push the boat down by 1 centimeter. Its just 1cm multiplied by 30 square meters = 0.3 cubic meter. A 1 cubic meter of water weighs 1000 kilos (saltwater is slightly more but we will ignore this for simplicity). So 300 kilos of extra load would lower the boat down in the water one centimeter. Obviously this calculation is very rough but it does give us what we need to approximate the effects of weight on the design. For every 300 kilos we add to the boat, we will push her 1 cm lower in the water. (For US measurements this means 1680 pounds will push her down by one inch).

300kg of additional weight will push her down by 1cm

Additional Equipment - What does it all weigh?
Here is my list of equipment with weights. These are in addition to the prototype of the Southerly 42 that I have photographed. If you are looking at your own boat to which you plan to add a bunch of equipment - you can use the existing waterline to start your calculations - then add the weights of the gear you plan to buy.

  • Additional batteries for main house bank - 84kg
  • Yamaha outboard 8hp - 27kg
  • Mastervolt 3kw genset - 100kg
  • Watermaker - 34kg
  • Scuba compressor - 56kg
  • 2 scuba tanks - 28kg
  • Avon dinghy - 43kg
  • Washing machine - 65kg
  • Additional Chain - 94kg
  • Radar Arch - 40kg

Total weight = 571kg / 300 = 1.9cm

So all this equipment will push the boat down by less than 2cm. Not so bad as I had thought really!

The next calculation is to see if the weight will be unbalancing the boat. This is much easier since the main problem normally comes if you add all the weight to the bow or stern. Then you can push the boat down in the stern or bow. You also will likely get worse motion if a lot of weight is added to the bow especially.

Here is a quick calculation where I have drawn blocks scaled to the weight of the various added equipment. I have added them on the side view showing where the equipment is. Note the heavy 100kg genset, plus the main battery bank are both right in the centre of the boat. This helps a lot. At the bow is the additional anchor chain, and the stern has much of the rest. Again a rough calculation but it shows that we might expect to be a bit more down in the stern, but not much.

I must point out that these are very rough calculations but its something anyone can do to at least approximate the effect of extra equipment on the waterline. And just doing the list of equipment weights can be quite informative.
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