Round Profile

[beekeeper]

I asked this questions befor, but it keeps getting lost or overlooked in the rocker discussion. viewtopic.php?f=10&t=7811

It has beem stated on here that boats with a round bottoms are more efficient (faster) than less rounded profiled ones.

Question #1 Two pirogues with identical dimensions except one has sides flared to 15 degrees and the other one's sides are flared to 30 degrees, which one will be faster (more efficient)?

Question #2 If the 15 degree sided pirogue's floor (bottom) was widened so it would float at the same depth as the one with 30 deg. flared sides, would they paddle the same?

I know neither are actually rounded. Any differences may actually be because of the different shaped footprints, but the question is still valid.

beekeeper

 

[Kayak Jack]

Your question is valid; the 15 and 30 degree sides unnecessarily complicate it. A cylindrical hull will be more efficient than any other bottom shape because it was less wetted surface, therefore less friction. Many other variables can - and probably will - be included, intimated, hinted at, and claimed. None of them alters this basic fact.

The closer a hull configuration approximates cylindrical, the less wetted surface it will have.

 

[tx river rat]

The one with the 15 degree sides would be faster because of less wetted area and drag
It would also have less secondary stability.
Any time you widen the bottom you increase initial stability but also loose speed.
The fastest boat I own is the duck it is 20 inches wide at the bottom, has a full 16 ft water line
very little rocker. I havent found a boat that will stay close to her yet.
Ron

 

[JEM]

A rounded shape, in theory, will provide the least friction/turbulence in the water. But a rounded cross-section does not occur in a vacuum. Many other factors are in the speed equation. Mentioned were waterline and the shape of the bottom from a side view. The intended payload, paddling environment, skill and preference of the paddler, hull shape above the waterline (influencing ease of paddling) along with a few other factors (some which I've discovered by dumb luck which I will not share! :mrgreen: ) all influence hull speed through the water.

If you took a hull and just had it mounted in one of those wind test chambers with the stream of smoke they use, then the ideal hull would look like a pencil that is super sharpened on both ends. Good luck keeping your balance in that one.

But alas... we paddling in water that has currents, logs, wind, and big bag of carbon and water riding in it trying to propel it through the water. One of the reasons Ron's Ugly Duck is so dang fast is it fits his paddling skill and desires. Also, his is a flat water, somewhat straight-line racer. Put him in some water with strong currents, choppy water, and tight turns, and he would likely be seeing the backs of everyone's head.

Summary: The hull cross section is but one of many factors that EQUALLY influence speed of the hull through actual water.

 

[JEM]

Question #1 Two pirogues with identical dimensions except one has sides flared to 15 degrees and the other one's sides are flared to 30 degrees, which one will be faster (more efficient)?

Don't take this as picking apart your question. Just need clarification. If the hulls have different flares, then their dimensions differ: either the bottom panel on the 30 degree flare is narrower or the overall hull width is different. Which do you mean?

Question #2 If the 15 degree sided pirogue's floor (bottom) was widened so it would float at the same depth as the one with 30 deg. flared sides, would they paddle the same?
beekeeper

Can you state the over max beam? Is there a max beam?

 

[jdupre']

I got this from The John Winters Page. The man does seems to have a handle on what it takes to get a craft smoothly through the water.

Rf = 0.97 x Cf x Sw x V^2 where:

Rf = Resistance in pounds
Cf = Coefficient of friction
Sw = Wetted surface
V = Velocity in ft/sec
0.97 = Constant for fresh water

If you look at "V" - Velocity, it's impact on the formula varies according to the speed squared ( if I'm reading the symbols right). The other two factors, Cf and Sw impact the formula by their value to the first power. While Cf and Sw are very important, you have to get by that hurdle of the square of the velocity. What I get from that is you have to make tremendous changes in Cf and/or Sw to get any appreciable increase in paddling ease or speed.

Think about these two examples : Chunky 14 ft x 29 " pirogue and 16.5 ft x 23" sleek kayak. Pretty much on either end of the practical paddling spectrum. I'm taking a pretty good guess that there is probably about 3/4 to 1 knot of difference in the comfortable paddling speed between these radically different designs. Any small "tweaks" in a design are probably not going to make any measurable or practical difference in the performance.

And, as Matt pointed out, if you throw in choppy water, strong currents, and tight turns, any tweaks might just be nullified or even cause MORE resistance.
Joey

 

[Jimmy W]

A quote from John Winters:

Ask almost anyone what hull section provides the least wetted surface and they will answer "Round". While this is true, its importance has been greatly exaggerated. In fact, for normal shapes the Beam /Draft Ratio has the greatest impact and the waterline should be as narrow as possible within the confines of maintaining acceptable stability.

I can't find the page that I am thinking of, but somewhere on the internet is a page comparing the wetted area of different shapes. With the shallow draft of most pirogues I doubt that the difference between 15 and 30 degrees would be noticed when the boat isn't tilted. It will affect the stability of the boat when it is tilted.

 

[jdupre']

Jimmy, this might be what you're talking about.

http://www.greenval.com/fig1_3.gif

Not a whole lot of difference amoung all of them. Worse to best, you're probably talking a tenth of a knot difference in comfortable cruising speed or mere ounces of paddling resistance at the same speeds. Keeping a fair, clean and smooth hull would probably give you that much or more.

Joey

 

[Jimmy W]

Yup, that is it. Thanks Joey

 

[Jimmy W]

Now can someone explain to me why one shape would be better upstream and another downstream. Seems to me that only hull speed in relation to water and air speed would matter. Going up or down stream would affect speed in relation to the shore, but I don't see why that matters.

 

[beekeeper]

Question #1 Two pirogues with identical dimensions except one has sides flared to 15 degrees and the other one's sides are flared to 30 degrees, which one will be faster (more efficient)?

Don't take this as picking apart your question. Just need clarification. If the hulls have different flares, then their dimensions differ: either the bottom panel on the 30 degree flare is narrower or the overall hull width is different. Which do you mean?

Question #2 If the 15 degree sided pirogue's floor (bottom) was widened so it would float at the same depth as the one with 30 deg. flared sides, would they paddle the same?
beekeeper

Can you state the over max beam? Is there a max beam?

Y'all going pretty fast for me so be patient, please.

Question #1 By identical dimensions I meant rocker,length, bottom, and sides. I thought it was a given the beam would be different, because of the change in the sides angle. I was thinking that he 30 deg. might present a softer chine (edge) and be more efficient. Probably not enough to best the narrow profile of the 15 deg. boat, but see question #2. That advantage might go away if they were built to float at the same depth. Then we would have apples to apples.

Question #2 I don't know what the beam width would be. I was not concerned about the max. beam width for this question. The beam width at the water line would change for the 15 degree flared boat when the bottom width is increased to make the boat float as high as the 30 degree flared one.

beekeeper

 

[beekeeper]

The one with the 15 degree sides would be faster because of less wetted area and drag
It would also have less secondary stability.
Any time you widen the bottom you increase initial stability but also loose speed.

Ron

How is "wetted area" measured? If the 30 deg. boat had water 1" up each side and a 24" bottom, that would be a 26" of boat "wetted". Wouldn't a boat with 15 deg. sides sit deeper in the water with the same width bottom and have more "wetted" area. Could it be, it presents a more efficient ratio of footprint width to length? we are still comparing apples to oranges until we build them to float at the same depth. My 200 hp. bass boat is faster than your kayak until we compare them with equal propulsion (paddling). I keep saying widen the 15 deg. bottom, but making the 30 deg. boat bottom narrower would allow it to sit lower in the water. Oranges to oranges. Which one would be faster? If I'm following Joey, the one with the smoothest hull.

beekeeper

 

[beekeeper]

Jimmy, this might be what you're talking about.

http://www.greenval.com/fig1_3.gif

Not a whole lot of difference amoung all of them. Worse to best, you're probably talking a tenth of a knot difference in comfortable cruising speed or mere ounces of paddling resistance at the same speeds. Keeping a fair, clean and smooth hull would probably give you that much or more.

Joey

It is getting late but build each boat to float at the same depth (draft) and then compare.

beekeeper

 

[Kayak Jack]

Bee, wetted area is measured in square inches, not linear. If a boat - with a good looking 250 pounder complete with his fishing gear and Grandson - were to be carefully lowered into the water until it came to rest, and then carefully lifted back out, the square inches that got wet is the wetted area.

(The grin on the Grandson's face doesn't count here.)

 

[tx river rat]

Here is a pic of a extremely fast perow

This is a 3 panel and can walk off from a lot of yaks.
The reason for a smooth rounded bottom being faster up stream ,the water cant catch on the rounded shape a sharp chined boat will be faster downstream.
Ron

 

[beekeeper]

Mr. Kayak we are on the same page. I knew it would be width X length. I had stated the boats are the same length. The variable would be the different widths of the wet area on each boat. Seems to me the boat with less flair (same bottom width) would sit lower in the water and have more or at least as much wet area, all else being the same. If it is faster it probably is because the foot print is more streamline (long & lean), not because it has less "wetted area" (friction).
I was trying to relate the general shape of the more flared side to a rounder (softer) chine. If having a round bottom was the main factor in efficiency then it would/could/maybe matter. Round bottom boats are more efficient (faster) because they sit deeper and present a more streamline footprint.
I realise the combination of all the factors contribute to the boat. Pirogues seem to be labeled as "slow". Ron's post dispels this. Build a kayak or canoe to be as stable, turn as easy, draft as little, etc. as the pirogue, or build the pirogue to be fast as a kayak or carry the load of a canoe and I guessing very little differences.

beekeeper

 

[oldsparkey]

Pirogues can be fast if you are into speed. I made one at 17 feet and a friend has it , he paddled with us on several trips. Believe it or not , he would pass the canoes when he wanted to using a kayak paddle and just scoot on down river. Forget trying to stay up with him or even catching him.

Watching him in it made me think it is not the boat but the person paddling it. After all the boat is just an object that responds to the persons ability or capabilities of using it. In the past we have had folks using a pirogue when paddling with someone in a kayak ( plastic one ) .... they got into a race ...... The plastic kayak lost to make a long story short.

Here is the link to it..........
viewtopic.php?f=3&t=6851

Same thing happen a few days ago when the kids were down here to get the kayak and one of my Uncle John ( the stripper) pirogues..... They were having a good time on the water testing them out and Neil would make that pirogue scoot. Even he was amassed at what it could do with the kayak paddle. He is not a single paddle person.

Chuck.

 

[Kayak Jack]

Some paddlers are talking about personal experience. Interesting, but not totally pertinent to answer the question. Taking a fixed length line, bending it into various geometrical cross-sections, is a geometricl reality. Again, interesting, but not totally related to the question.

While total wetted area is part of total friction or resistance, it doesn't tell the whole story. Smoothness or fairness also enters into it. To get usable data about a hull, you have to remove the paddler from the equation. Attaching a spring scale or some similar device to measure resistance is a rational way to approach getting an answer.

Take a look at hull analysis of various boats in Sea Kayaker Magazine, and you will get the idea.

 

[Jimmy W]

An Olympic style K-1 would be faster either up or down stream if turning wasn't required, but most of us couldn't keep one upright.
http://vimeo.com/1234062

 

[beekeeper]

Some paddlers are talking about personal experience. Interesting, but not totally pertinent to answer the question. Taking a fixed length line, bending it into various geometrical cross-sections, is a geometricl reality. Again, interesting, but not totally related to the question.

While total wetted area is part of total friction or resistance, it doesn't tell the whole story. Smoothness or fairness also enters into it. To get usable data about a hull, you have to remove the paddler from the equation. Attaching a spring scale or some similar device to measure resistance is a rational way to approach getting an answer.

Take a look at hull analysis of various boats in Sea Kayaker Magazine, and you will get the idea.

You may have turned pages on me. :?

beekeeper