Thoughts on differntial Cp foreward/aft

[ancient kayaker]

Originally Posted by LP

. . . What comes to mind immediately is the placement of your section cuts, but that doesn't really fit your description.

Ah! The mystery is solved! because the hull is summetrical I only had intersections defined for half the hull. Of course, adding the other half completed the curve. I feel slightly foolish, but it will wear off - it always does. Phil: great link for portaging. I was thinking of attaching a bike helmet to the floor . . .

[Tim Hall]

Originally Posted by philSweet

If it's a solo canoe, lower bows and part decking are effective.

This is a solo canoe, and I do plan to partially deck it. Bow stem is 17" (above max draft) and stern is 14".

Originally Posted by philSweet

One other consideration- if you intend to portage the canoe in the usual fashion, you need a thwart at or just forward of the CG - which means you can't kneel there when paddling.

Good point. I haven't figured this out yet, but I want kneeling to be an option...but I also intend to portage.

Regarding Cp range :

Originally Posted by LP

Phil, could you embellish on this thought?

Yes, I'm curious too. I was perusing the Bear Mountain catalog of designs and noticed many of the canoes and kayaks are in the 0.56 range. This seemed a little high to me, as it indicates more wetted surface for a hull that's probably not going to pushing its 'hull speed' all that often.

My overall Cp unwittingly came to 0.57, and I feel like I need to drop that down a little.

[philSweet]

I can't hope to help you choose between .57 and .56 Cp with regards to a single design point on an original linesplan. Check it out at a few different displacements a see how it trends. That would be more constructive IMHO. Don't be shy about the displacement interval, you want the big picture. Change the disp by 20% or so. I have very little experience in kayaks, rather more in canoes, but I haven't designed and tested a whole series of them, so I would refer to as many other designs as I could and see how I sat with regards to established norms. Sounds like you are doing that already. Someone pointed out that its ok to err on the high side a bit (must of been in another thread). It sucks to find out your boat won't plane down a wave face when you're hanging in the air suspended by the ends of your boat in the middle of a big train of waves.

[DCockey]

Originally Posted by Tim Hall

This is a solo canoe, and I do plan to partially deck it. Bow stem is 17" (above max draft) and stern is 14".



Good point. I haven't figured this out yet, but I want kneeling to be an option...but I also intend to portage.

Regarding Cp range :

Yes, I'm curious too. I was perusing the Bear Mountain catalog of designs and noticed many of the canoes and kayaks are in the 0.56 range. This seemed a little high to me, as it indicates more wetted surface for a hull that's probably not going to pushing its 'hull speed' all that often.

My overall Cp unwittingly came to 0.57, and I feel like I need to drop that down a little.

Be careful equating Cp and wetted surface area. For a given displacement and length a shape with a higher Cp may well have lower wetted surface area. It depends on what the differences are that are increasing the CP.

[DCockey]

Originally Posted by philSweet

....
One other consideration- if you intend to portage the canoe in the usual fashion, you need a thwart at or just forward of the CG - which means you can't kneel there when paddling.

Off topic- the proper way to portage- http://www.porchlight.ca/~aferg/home_htpac.html

Use a removable thwart or yoke for portaging. Adirondack guideboats are portaged using a carved shoulder yoke. The ends of the yoke fit into brackets just below (with the boat upright) the gunwales and is removable. Or if you prefer use a straight thwart or tube which fits into brackets below the gunwale.

[LP]

Originally Posted by philSweet

... I guess I ought to at least try to answer the original question regarding the merits of a different fore and aft Cp. If you envision two hulls, one consisting of the fore section mirrored front to back and the other consisting of the aft section mirrored front to back, you can look up the ideal Cp of each based on length for each and the design speed.

Phil, I hope you don't mind me asking. Where do you pull this data from. I like the concept, but would be interested in it's source. Can you elaborate?

[Tim Hall]

Originally Posted by LP

Where do you pull this data from. I like the concept, but would be interested in it's source. Can you elaborate?

Phil, if you're still following this, I was wondering if you could point me toward any references regarding speed/Cp relationships also. I've seen a couple different curves relating Cp to SLR, but of course this if from sailing design sources. I imagine it's relevant to paddling also, since a canoe travels in displacement mode(?)

I'm putting the finishing tweaks on my curves, and I'd like to examine my hull with the procedure you've outlined above...seems like a sound approach.

[philSweet]

The stuff below is pasted from Eric Sponberg's document that can be seen in its entirety in post #264 of "Center of flotation calculation and implications?" thread.


It turns out, after some 150 years or so of analysis, that performance is closely related
to Cp. That is, there is an optimum range of Cp for various speeds of the boat traveling
through the water. You can see a table of speed/length ratios versus optimum Cp in
Skene’s Elements of Yacht Design (by Francis Kinney, 5th ed.) pg.284, which I repeat
below:
Speed/Length ratio Cp
1.0------0.52
1.1------0.54
1.2------0.58
1.3------0.62
1.4------0.64
1.5------0.66
1.6------0.68
1.7------0.69
1.8------0.69
1.9------0.70
2.0------0.70
Larsson/Eliasson shows a similar range in their book on page 83, Fig. 5.22, in which
they plot optimum Cp against Froude Number. Froude Number is very similar to
Speed/Length ratio, and if you convert Froude Number to Speed/Length ratio, you will
find that Larsson/Eliasson’s curve is a bit lower than Skene’s curve tabulated above. As
is true with many things, therefore, there is some wishy-washiness in the guidelines.
Nothing is hard and fast.

[Tim Hall]

Phil, thanks. Just want to let you know in analyzing Cp this way, forward and aft Cp's were almost dead on to what you described. Very minor tweaking to angles at the sheer and keel made everything match up perfectly, and I feel I've arrived at a very sensible hull form...with the help of forum users here. Thanks to everyone for your input.