BoardCAD flow/mass/volume lines

[createinquiry]

I'm a very novice shaper (4th board) wanting to try my hand at a displacement board. I'm fooling with design ideas using boardCAD and wondering how to best utilize the volume and mass distribution curves. Having used AKU shaper in the past as a trial, I haven't had access to this information.

For no other reason than liking the appearance of the fly race, I'm drawing my ideas from the 12'6"/27.75 board. I'm in the middle of Tennessee, so I'll have nothing but flat water to paddle, unless I drive 8-11 hours.

I realize that my shaping ability will negate a number of these considerations. We have a EPS factory 30 minutes away so I can get 32"x16'X???" foam easily and cheap (~$75 per board). Paying for CNC services and shipping doesn't make sense.

Here are the ideas rolling around in my head:

1) Is the center of mass (red dot on outline) generally in front of the board's center? It makes some sense as the rider's mass behind this point would pull this point back, but I'd hate to be wrong and have a sweet spot for paddling too far forward to control comfortably.

2) Will this volume distribution cause the board to ride too high in the nose? Or by keeping rather sharp V near the nose, does this keep the majority of the volume above the waterline? (Perhaps there is a boardCAD function I haven't seen where you can show the volume distribution at a user placed/hypothetical waterline, rather than the entire board.)

3) I like the "tuck under" line as a reference and it makes sense to me that a displacement style hull will keep these lines towards the middle of the board at the nose. However, at the extreme rear of the board are there advantages to moving these lines back to the middle for release?

4) The flow lines appear to move over the top of the board, so it seems they are good to consider for watershed off the nose and out of the cockpit. Are there also flow lines for the bottom of the board to show how water is likely to be displaced?

Thanks in advance to anyone who has some input to offer about these questions or the design in general.

[SUPpaddler]

The center of gravity is merely that -- where the board balances.  It's where you put the handle, and has nothing to do with performance.  It's fore of center because the thicker nose is heavier.

You need to learn the difference between the center of gravity, center of buoyancy, and center of flotation.  If nothing else, understand that the center of buoyancy (usually called the LCB or longitudinal center of buoyancy) is the center of the submerged volume with the boat's trim perfectly level when you look at your diagrams.  It's where you will stand to produce the planned trim, so it's important to know the location.  If you stand aft of the LCB for instance, the nose will come out of the water.  

To start with, do a little reading att http://www.oneoceankayaks.com/smhydro/hydro.htm#lcb
and http://www.guillemot-kayaks.com/guillemot/information/kayak_design
and http://www.greenval.com/jwinters.html

It's the distribution of the submerged volume that's important, and crucial to performance.  I hope it's not the grey curve in your diagrams.  I'm estimating that would produce a prismatic coefficient of about 0.75 which is completely out of the usable range for any displacement craft.  It would push water, and be like bringing a school bus to a car race.

See the suggested ideal volume distributions in the attached pic.  Use a Sears-Haack, or even parabolic distribution for racing speed optimization (approaching your hull speed of 5.47 mph), and a more cosine-like distribution with finer ends for casual cruising optimization.  Remember, this is the submerged volume, which is all that really counts.  To get your submerged volume, you may have to incrementally determine your waterline level, starting with a guess based on your displacement (total weight).

As an example, here's a board layed out with volume distribution and key parameters and locations marked on the diagram.  

[SUPpaddler]

By the way, since you're not sending the design out for CNC, why not model it in some software that gives you all the hydrostatics and let's you optimize the design.  KayakFoundry is easy to use and you can see that it gives you the waterline, volume distribution, and all key parameters, and also predicts stability and resistance.  It will also print out form patterns that might help in your shaping.

[createinquiry]

Thanks tons, that's some good stuff and I'm definitely going to look into it further. I have a feeling those tabs will be open for a while.

I did understand the center of mass difference, but thanks for further clarification.

The gray line is in fact the volume distribution, but I think that it's of the entire volume of the board, rather than just the area below the waterline. You addressed perfectly what I was trying to verbalize in my second point. I just didn't know the term "LCB," thanks. The ability to estimate volume distribution at the waterline appears to be absent from boardCAD, but kayak foundry looks like a great addition.



How far off is my first interpretation:
Get to your max submerged

[TWsup]

I used shape3D. it has the displacement data, etc. designed my board on it and paid for a 1 year license to get export feature and had templates cnc milled by local guy. he was nice and did it at cost for me.
it was quite a few steps to export the templates...dxf out, into adobe illustrator to lay them out and add features,  export again to dxf.
rocker templates were perfect, but outline template polygons were too long and I'll have to sand them a bit. ( I should have checked them, but was in a hurry).

it seems some of my reading and memory of a yacht design course I took 30 years ago taught me something and I remembered that prismatic coefficient. back then we had to manually calculate by measuring the area of sections. the CAD makes it easy !

the kayak foundry looks interesting..