[zAr]

Is there a formula, given only the displacement of a boat (and neither light nor loaded, at that), for calculating how much stowage weight the boat can take on? E.g. how much extra weight leads to how many inches at the waterline, perhaps?

 

[mitiempo]

Look up your boat here and it will tell you how many pounds it takes to immerse the hull 1" lower in the water.
https://web.archive.org/web/20140328112219/http://www.image-ination.com/sailcalc.html

 

[zAr]

Thank you! That's the easy way. It also led me to LBS/IN which in turn led me to the formula:

LBS/IN = Pounds per inch immersion (top)
This is the amount of weight added or subtracted to the boat to change the immersion by one inch. If the LBS/IN is 1000 adding 1000 lbs of gear or cheese (or feathers) will make the boat sit one inch lower in the water. If you know the waterplane area (WPA) you can find the LBS/IN by the formula:
LBS/IN = WPA * 64 / 12

And...

LWL/BWL = Waterline Length/Waterline Beam Ratio (top)
LWL/B = LWL / Beam
This is the waterline length divided by the waterline beam. Waterline beam is not a commonly reported figure. A rough estimate can be made by taking 90% of the overall beam but this ratio varies with hull form.

WPA = Waterplane Area (top)
WPA = LWL x BWL x hull fineness factor
This formula gives an approximation. BWL can be approximated as Beam x .90 and the hull fineness factor (my term) is about 68% for fine ended sailboats and 71% for full ended sailboats (Gerr). Or, if you know the LBS/IN immersion you can work backwards to get the WPA by the formula:
LBS/IN * 12 / 64 = WPA.

 

[mitiempo]

Remember boats are not slab sided. The first inch is different for every succeeding inch - in either direction, heavier or lighter. The naval architect who designed the boat will know the best number for any given design.

 

[mr_f]

If the LBS/IN is 1000 adding 1000 lbs of gear or cheese (or feathers) will make the boat sit one inch lower in the water.

I do not recommend putting 1000 lbs of feathers on your boat. They are not very practical and will take up a lot of space.

1000 lbs of cheese, on the other hand, is reasonable as long as it is good quality cheese.