Observations About The Block Coefficient On A Boring Sunday AfternoonThe block coefficient is merely the ratio of the submerged volume over the volume of a block having the same length, width, and depth of the submerged portion. By itself, it tells absolutely nothing about the "fatness" or "fineness" of a given hull (i.e. its length/width ratio).

A jon boat is a good example of a boat with a high block coefficient. With the exception of its upturned bow, it is basically a box sitting in the water. A very good portion of the submerged part of a jon boat will be occupying the block formed by product of the three dimensions used to define it (WL length x WL width x draft). It is just a very "blocky" shape. But a 12' by 4' jon boat will have a block coefficient that is pretty much the same as a 24' by 2' jon boat because the cross-sectional geometry of the underwater shape hasn't really changed. It still a block, the displacement will be the same, but the 24' boat has a L/W ratio of 12 while the 12' boat has a L/W ratio of 3.

BTW, although I'm quite calculus-able, one need not use higher math to determine the block coefficient of an existing hull. Given the effective waterline length and width and the displacement at that waterline, one can determine the size of the block formed by these dimensions. Given the displacement, one can simply divide this number by the density of water (about 62 lbs/cu.ft, depending on water temperature) to get the volume in cubic feet.

For example, take a generic touring canoe with a 16' effective waterline length, a 32" waterline width at the 4" waterline and a displacement at that same waterline of 405 lbs. The block formed by these dimensions has a volume of 14.2 cu.ft. The volume of the submerged portion of the boat when laden with 405 pounds is 405/62=6.53 cu.ft. Therefore, the block coefficient of that boat will be 6.53/14.2=

.46. What that number means in and of itself, I haven't a clue.

As an extreme example, take an inverted cone of such a shape that when it is submerged 4" into the water, it displaces about 405 pounds. The waterline length and width of said cone will both be about 8' 8". The block formed by these dimensions (8.67' x 8.67' x .333') would have a volume of 25 cu.ft. The actual volume of this 104" wide "boat" is 6.55 cu.ft. It's block coefficient - by definition - would be 6.55/25=

.26. Way low. But it will have a wetted surface area of almost 60 sq. ft., and a L/W ratio of only 1. Not an efficient tripper at all, but with only 4.4º of deadrise, it should be fairly stable.

How well will this boat track? Even with a paddler having the 5' long arms necessary to paddle this thing, I suspect he would need to use a

very hard C-stroke.

Let's take this same boat and pinch it in and elongate it like we did with the two jon boats. We can retain the vee-bottom, but since this changes the shape of the waterline from a circle to a long and narrow ellipse, the block coefficient will increase. By the time the boat achieves a manageable 32" waterline, it will be 14 3/4' long and have a respectable block coefficient of about

.45, but it will have extremely blunt ends, so it will not part the water well and it will create enormous transverse waves. As well, it will now have a very tippy 14º of deadrise. But that vee-bottom will make it track like a champ.

So as you can clearly see, the block coefficient has little to do with tracking in and of itself. The L/W ratio, as mentioned in a previous post, must be large as well. It is a geometric fact that vee-bottoms have the lowest block coefficients. They are also known to be the best tracking boats, so there is a definite connection, but the bottom line is that there are many other design features that are as important to tracking as the block coefficient.