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PostPosted: March 27th, 2010, 6:59 pm 
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Joined: April 28th, 2008, 4:32 pm
Posts: 126
Location: Edmonton
The Ranger is a fine design as it stands (or floats,whatever). Changing the design of the boat is a natural process, it happens when we screw up in building it. Many paddlers believe there is something magical about a high degree of rocker: turns on a dime, more manoeuverable etc. Think about this a minute before changing a good design. Where is the boat in a turn? on the tumble right? so for this type of canoe the rocker is immaterial. Changing the design will alter the dynamics of the boat when paddled flat. So, if you want a canoe that tracks well, more wetted length, not less. You want to make a quick avoidance turn, on the side,stroke past the danger, back to relaxing. Even a department store canoe responds to correct paddling. Just build it as well as you can and enjoy discovering all the attributes the canoe has as built. After building and paddling a bunch of canoes try designing and building your own based on experience and needs.

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PostPosted: March 27th, 2010, 10:00 pm 
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Joined: July 29th, 2009, 9:29 am
Posts: 387
Location: Lower Saranac Lake, NY
Block Coefficient:

A recent post has misconstrued block coefficient: gotten the math backwards.

The "Block' is a rectangle sized to fit the maximum waterline length by maximum waterline width by maximum waterline depth of a given hull. The less of that block the hull fills the better the boat tracks. I.e., the finer the hull form. the better it tracks.

For the less calculus oriented and those unwilling to build a waterproof block to fit each of their hulls, there is a simpler number that helps. Most tandems run ~ 4" deep and most solos ~3" deep when lightly loaded, so we can eliminate that number from the block. We can then easily divide Waterline Length by Waterline Width, [in the same units[, and arrive at a fineness number for comparisons; river boats ~ 6, touring boats ~ 7.

Bow rocker increases speed by moving water downwards, under the hull, more slowly. Stern rocker, again. allows the water to re-fill the "hole" where the hull was more gently, the goal being a "wakeless recovery."

Most paddlers carry their paddle blade past their torsos into what is invariably a sweeping conclusion to their forward stroke. A skegged stern reduces the hulls yawing response to this poor paddling technique. As paddler induced yaw and the drag it causes is a bigger problem than a slight increase in turbulence of returning water, differential rocker is incorporated in modern, professionally designed, recreational hulls. ICF racers don't need the crutch, lord knows about the marathon guys.


Last edited by Charlie Wilson on March 28th, 2010, 11:08 am, edited 2 times in total.

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PostPosted: March 27th, 2010, 10:26 pm 
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Location: Atlanta
Thanks, Charlie, I especially appreciate the explanation of the effect of rocker on speed. Even rowing shells, which when racing, usually do not need rocker for turning, have it anyway, and I assume it is partly for speed and partly to control pitching of the bow.


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PostPosted: March 28th, 2010, 12:06 pm 
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Observations About The Block Coefficient On A Boring Sunday Afternoon

The 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. :wink:

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. :doh:

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.

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