|Canadian Canoe Routes
|Drag, Resistance of a new solo design
|Page 1 of 1|
|Author:||Awetcanoe [ February 21st, 2008, 12:56 pm ]|
|Post subject:||Drag, Resistance of a new solo design|
I have a few questions that I have not found answers to yet.
I am using Delftship Free version to draw up a new hull that I would like to build later this year.
I have ordered JW's book "The Shape Of The Canoe" but it has not arrived yet. I expect many of my questions will be answered with the information within his writings.
But here are a few questions anyway.
The hull I am designing will have a waterline length in the 16 to 17.5' range. Maximum beam of 2' Prismatic coefficient is about .52 Block coefficient about .385
The intended use of this canoe will be a quick efficient cruise with light loads over open inland waters. By light loads I am looking at about 230 lbs. total.
I have drawn about 10 iterations of this hull now and have found an interesting result in the KAPER program.
The first hull had the lowest drag and as I cleaned up the surface I can not get the drag anywhere as low as the first hull.
I get 1.73# @ 3Kts and 4.38# @ 5Kts on the first,
I get 1.83# @ 3Kts and 5.87# @ 5Kts on a more refined hull.
What I notice between these two results is the hull with the lowest drag shows all zeros under the Rr column.
The higher overall drag has a lower Rf but has results under the Rr column.
Is this an error to have all zeros under the Rr column?
Occasionally as I change the draft .01 at a time the results go haywire but I can get clean numbers at both shallower and deeper depths. This appears mostly in the Rr column also.
An item that I am not sure how to access on paper is what I refer to as "Return to center", by this I refer to how a solo canoe responds to each paddle stroke. I find some hulls remain yawed for a period of time where some return straight immediately at the end of a stroke. These obviously have a much freer glide.
Another, is there a way to access the pitching loads of a hull as it travels, this would be the lift, be it positive or negative as the hull travels. In aircraft terms this may be considered the pitching moment. Does the Vertical Prismatic coefficient refer to this?
In the long run I might just buy a set of plans from JW but I want to see how close I can get to what I am looking for with my own design first.
|Author:||Graybeard [ February 21st, 2008, 7:10 pm ]|
|Post subject:||Re: Drag, Resistance of a new solo design|
......The hull I am designing will have a waterline length in the 16 to 17.5' range. Maximum beam of 2' .....
|Author:||Bryan Hansel [ February 21st, 2008, 7:26 pm ]|
I'd have to see the files to see what's going on with Delft. Sounds like a modeling problem.
|Author:||Awetcanoe [ February 21st, 2008, 7:57 pm ]|
These are some examples of what I am finding.
Here are two files,
And Bob, a seat belt might be a good idea especially after slamming a dirt bank with a Merlin II when I did not get a quick water section right last summer. This boat is not going to turn in anywhere as nice as that Bell.
|Author:||Jwinters [ February 22nd, 2008, 2:46 pm ]|
Not sure what to say about the Freeship results as I don't use it. However. I do know that an effort was made to correct some for the mistakes put into the program by Matt Brose before it was put into Freeship!. Matt was well intentioned but not supported by any test data.
How do your "refined" shape numbers compare to the first shape?
If you like, you can send me the pertinent data and I can run it through my copy of KAPER which runs outside the design program.
I suspect Bryan can sort out your problem because he uses Freeship and is familiar with it.
It is worth pointing out that the results provided by any prediction program should be considered design guides. They do not provide absolute resistance figures despite what some people claim. I had a hard time explaining this to Matt Brose when he modified the original version for use in Sea Kayaker Magazine's boat reviews. It is also important to note that people cannot detect small variations in performance (typically +/- 10% 50% of the time) although some experts claim they can I have never found any that could under controlled test circumstances.
Pitching is most affected by the moment of inertia and waterplane coefficient. Few canoes are ever so fine that they pitch badly so long as the ends are kept light through proper loading. The only factor that I have found in addition to those two aspects is having the center of flotation slightly aft of the center of buoyancy although I suspect this is also fairly insignificant in wide flat boats like canoes.
The Vertical Prismatic probably plays a minor role in pitching and one needs some experience with other shapes before drawinging conclusions.
The low Block coefficient will produce a straight tracking boat and the narrow hull should be pretty quick. You might want a higher Prismatic coefficient though to match it properly.
Yaw is the great pain. I have a description of a test for directional stability in my book that seems to work (once again it is relative data so you need to test several boats for comparison purposes). A lot of the yaw problem stems from paddling technique. Boats sensitive to heel can correct their course during the withdrawal phase of the stroke. What you describe is called Type III directional stability. I have not yet found a canoe had it while remaining upright. I have wathced good paddlers and they are often unconscious of their tweaking the boat to keep it going straight. Comes from practice I suppose.
|Author:||Awetcanoe [ February 22nd, 2008, 7:03 pm ]|
I posted a link above to two files , if you can open them all the info is within. Otherwise I can get as much posted here as I can. I may leave something out and I expect some of the details would be lost. I expect they may be of more value determining if or what the error is in the first design. If it proves to be faulty then there is little need for me to use it as a baseline.
I recognize that one can not get true figures from a program being that there is no accounting for surface roughness or waviness. I do find I can find and log trends as I make changes and to me this is of great value. It does not mean I am making the right changes. That is part of learning though.
Good points in the rest of your reply. I sure wish the Post would get my copy of your book down here.
|Author:||Bryan Hansel [ February 29th, 2008, 9:13 pm ]|
I looked over the file. Nothing seems out of place. I suspect that one of the parameters is out of the range allowed by KAPER, because once draft is increased by .03 feet, the numbers start to look correct. Maybe something to do with the low Cp. Off the top of my head, I can't remember exactly what is allowed by KAPER and I don't have the spreadsheet handy.
|Author:||Awetcanoe [ February 29th, 2008, 10:07 pm ]|
Ahh yes, CP is to be between .48 to .64, that hull falls below that range.
I have been drawing many variations since those earlier drawings and trying to get enough time to read JW's book The Shape of the Canoe. I have not been able to give it a thorough read but have been scanning it to get answers.
I like working with the Delfship program, so far I have drawn over 50 variants in 3 different lengths and am getting a feel for how changes affect the drag. Granted there is a whole lot more to a good hull than just drag numbers but this is an interesting learning curve.
I have a hull I like the look of but have just started fresh again to see what else I come up with.
I sure wish I had programs like this when I was designing and building homebuilt aircraft. I had good drag and stability formulas to work with but they were all longhand. I always felt one had to design at least 10 variants before committing to a final design. That was allot of work on paper. Heck I am a long way from a final design but in the old way of drawing I would have committed to wood by now.
Thank you Bryan and John for getting me started, I will have more questions as my learning develops.
|Page 1 of 1||All times are UTC - 5 hours|
|Powered by phpBB® Forum Software © phpBB Group