Canoe length, efficiency and speed

There are vessel size and proportion limits in the rules in an earlier post there TT.

 

Intended load is about 375# plus canoe.

 

I don’t know, but if I was pushing a 40# rock across a field and the other guy was pushing a bigger 50# rock, he’d have a heck of a time convincing me that his rock was easier.
Obviously, that’s a little over dramatic. But I think if you’re going to say that a larger, heavier object is easier to move, there ought to be some hard data to prove it. Frictional resistance and hydraulic resistance both increase with speed. I don’t mind being the idiot, I just want to know why.
The “greater displacement in the smaller boat argument” is the only thing that makes sense to me. But even that seems flawed because the smaller boat is ten pounds lighter. It may sit 1/2” deeper in the water, but that’s still less wetted surface that an extra foot or two hanging out front. And frictional resistance is the product of wetted surface area and speed mainly. I don’t know.

 

Your correct that more wet surface is more frictional resistance. But hydraulic resistance is determined by shape (think of the difference in resistance to your paddle moving through the water in normal fashion vs rotated 90° and same wet surface aria). As speed increases, hydraulic drag increases more than frictional drag.

Personally, I would rather roll an eighty pound spherical rock around the track, than drag a fourty pound brick.

Look at your paddle. Narrow is easier to push thru water than wide.

 

And in my example the “shape” is precisely the same. I have decreased the beam by exactly the same amount as the length.

 

Aah, Ok got it. 18'6" is the maximum length. Well that will be easier to carry! Cheers.

 

What TT said in post 30, a 24 ft length and very narrow (skulling boat shape) would be a significant Improvement. It is harder to drag a 500lb stubby metal brick through the dirt, than a 700 lb longish metal rod, even though the metal rod would have a much greater total surface exposure to the dirt, plus weigh more. A small frontal area and narrow profile behind is more efficient, would be my opinion. This can be proven by Towing small models of different the shapes carved from wood behind a motorboat at 5 miles per hour and measuring resistance by using a scale. The difference between the shapes of the two boats you proposed would be very small, if you can't go to 24 feet by the rules.

 

If you want some more concrete answers but don't want to go to the trouble of testing the physical boats then you could consider modelling both hulls in DelftShip (free version). You could then run a Kaper* resistance study (within Delftship) on each hull, at various loadings. You will almost certainly find that the longer hull has greater drag at low speed, but drag on the shorter hull will start to rise steeply at some speed ~5kt. The longer hull drag will rise steeply at slightly higher speed, so there will probably be a speed region where the longer hull has lower drag.

The frontal area argument put forward by hoytedow is a red herring, this element (form drag) is not a major component of the overall drag on boats.

Optimum length will be shorter than e.g a rowing shell because of the minimum L/B ratio rule. This causes surface area to rise faster with length than if beam could be held constant, and does not allow a very narrow entry angle (good for reducing wave drag). OTOH you could play with hull shape to mitigate the increase in beam. For example a longer, beamier hull will be more stable, so a softer bilge could be employed, or a more optimised prismatic coefficient. NB I don't know if this is true of the two canoes mentioned by OP.

*Kaper is just one of the available formulae, but is specifically intended for modelling canoe/kayak hulls.

 

Let us have a look at two examples of fast kayaks to have the possibility to compare them.

First: FW 2000 from Nelo Portugal – 562cm (221,26in) x 44cm (17,32in), volume 301Liter(79,51gal) develops at 4kn 3,75 pounds resistance/drag and at 5kn 6,37 pounds drag which needs an encrease of 70% power.


Second: Looksha II Necky (CDN)- 610cm (20ft; 240,15in) x 51cm (20,08in), volume 392Liter (103,555gal) has at 4kn 3,70 pounds drag and at 5kn 6,39 pounds drag which has to be compensated by an encrease of 72% power.
necky looksha ii - Google 検索 https://www.google.com/search?q=necky+looksha+ii&client=firefox-b&tbm=isch&source=iu&ictx=1&fir=CiqMM-SWrZPOHM%253A%252C2ptJA-TPPNpKjM%252C_&usg=AI4_-kSc62l9CuXo4LPVaFNbvJ4LAzZsag&sa=X&ved=2ahUKEwj7yLTpwKHeAhVBalAKHcbJD4sQ9QEwBXoECAYQBA#imgrc=CiqMM-SWrZPOHM:

Not much difference.

 

My guess is either length built to absolute perfection, with exotic material will be a minimal gain compared to each other in the grand scheme of things. Find a design that has known quantities, and go from there. Both are going to be a wash in the maths significant figures compared to conditioning.

 

Agreed.
It's a bit of a red herring.
The race will be won in preparation and, to some extent, luck.

 

A thread about kayak design parameters which may be relevant: Kayak design parameters. https://www.boatdesign.net/threads/kayak-design-parameters.37739/

John Winters wrote a monograph on canoe and kayak design, The Shape of the Canoe, which is available on a CD. It includes discussion of various hull parameters and their effects on performance. John developed Kaper, a performance prediction method for canoes and kayaks. Kaper is included on the CD and can be used standalone without DelftShip. Licence Agreement and Order Form http://www.greenval.com/order.html

Matt Broze also developed some performance prediction methods for sea kayaks. They can be downloaded for free at Mariner Kayaks Sea Kayaking Homepage http://www.marinerkayaks.com/

 

Here attached is my tentative answer with using my tools (Gene-Hull and VPP) and 2 numerical canoes complying with the rules. The results at high speeds are sensitive to the residuary drag component evaluation, which can be 25% to 35% of the total drag (at 5 mph). To cope with this uncertainty, I adopted a dual approach : computation with the Delft series data on one side (but not really adapted to canoe hulls), at 100 % of their values and at just 70% of their values (to take into account the higher LwL/Bwl ratio of the canoes). This dual approach to show the influence of this assumption on the results and in a tentative to frame the exact values. Other drag components (friction and aero) are more easy and reliable to evaluate.
** The gain with the Canoe 17' is potentially low : drag reduction about 1,7% to 2,5%, but leading (at constant propulsion net power) to just + 0,7 % in speed when around 5 mph (due to the drag slope at this speed which « absorbs » rapidly the drag reduction).
** Canoe 17' is potentially lighter : 28,7 kg instead of 32,3 kg (assuming ~ 5 kg/m2 of hull surface)
** Canoe 17' has a significantly lower GM : 11 cm instead of 23 cm, > 50% reduction.
>>> The low gain on speed + this loss of stability margin show that a priori it does not worth to turn to the canoe 17'
By hoping these order of magnitudes can be helpful,

 

Interesting analysis Dolfiman.
So a 0.7% difference, according to your calculations.
That's a 0.035 knot difference at 5 knots.
In a 16 hour day paddling that would be about half a mile difference.
In a six day race it would be about 3 nm total.

I would question your GM calculation.
That's a huge difference for two very similar canoes.
This was based on a loaded canoe, right?

 

Thanks for indulging me guys. I know I’m splitting hairs, but I can’t help it.