Dragon Boat Drag Calculations

Thanks Leo, I was pressing 'r' on it's own, using shift+r does work and produces graphs.

Dragon boat paddlers have to be facing the direction of motion, its a rule set by the organisation.The only arrangements allowed, but rarely used are to kneel or stand whilst paddling. It would be rather awkward anyway if they were facing the other way.

 

I wonder if, related to this question, someone could give me a simple way to produce a rough calculation of drag of a Canadian flat water war canoe (C15), based on changes in weight of the crew?

In other words, I'm looking for a way to calculate how much the drag will decrease for each pound of weight lost from the crew, and the resultant decrease in displaced water that the boat has to travel through.

It doesn't need to be highly accurate - what I'm after is a ballpark figure (comprehensible by a mathematical idiot such as myself) to illustrate how much easier it is to get the boat to travel at a given speed if the overall weight and water displacement of the boat is decreased.

I guess what I'm really talking about then, if I can remember my schoolboy physics from 22 years ago, is a simple way to measure the decrease in energy expenditure to achieve the same velocity, as a result of a reduction in boat mass.

 

Assuming that crew makes up some fraction of the total displacement, and that weight loss will also be some small fraction of the total crew weight - and, yes, this is a simplification - one can use the direct ratio of drag-to-displacement. This is based on the idea that the general "geosim" shape of the boat will not drastically change for small changes in displacement. Of course, this would not be valid for planing hulls, or other geometries that might change significantly.

Having said that, I'm not sure that you will find the outcome satisfactory. Your crew, as a fraction of displacement, creates the power for the drive. Reducing crew weight would be much like trying to go faster by taking out your engine.

Rowing shells would be a different story, of course, as the crew weight is a much greater proportion of the whole. This type of canoe (from what little I recall of them) is substantial, so adding "engine" weight may in fact give you better performance. So, the really big guys may be what you want...

Don MacPherson

 

Thanks Don, your answer is much appreciated. It must be said that, while I can work out displacement easily enough, calculating drag has me confused.

Yes, big guys are the answer, but I was trying to get a sense of how much difference (ie very significant, or not that great) it makes to overall speed if each of those guys is carrying 10 to 20 pounds of 'empty' body fat that could be lost without impacting their performance. In other words, two engines of the same power output, but one weighing - say - 200lbs less than the other.

I guess what I'm looking for is an indication of whether, all other factors being equal, reducing the load in the boat by 200lbs make is 'a lot' easier to propel at a given speed, or just 'a bit', if you guess the total weight of boat plus paddlers as 3000lbs to 3200lbs.

I guess at the very least, you're going to have to displace an extra 200lbs of water per second, which over the course of a 2 minute race would be 24,000lbs of water. Or is that bad maths?! And that's ignoring the changes to the speed at which bow and stern waves start interfering with progress.

 

Academics is all well and good but why don't you simply tow the loaded canoe with an appropriate spring or digital scale next time they are out practicing? Change up the weight and measure again. Have a range of reasonable speeds and gather some of the most accurate data you have available to you.

Just an idea...

-Tom

 

Anyone in Tampa this week? http://www.tampabaydragonboats.com/

 

dissertation finsihed?

Hi Alyx

It has been a while since you posted in this forum and I hope you are still following it.
Just wondering if you did your dissertation and if you can share whatever drag equation you came up with.

Cheers

Jacob

 

I am pretty sure the rowing shells use top speed for their analysis as they come up to speed fairly quickly, at least for the purposes of the final "k" value they are probably there within their first 2 or 3 half strokes. The dragon boats are heavier and might come up to speed slower. Not sure.

I think you need to calculate wave making, form drag, and skin friction all separately. At slower speeds the skin friction dominates. At higher speeds the wave making resistance takes over but the skin friction is always significant. The form drag is a somewhat different animal than the other two, and can sometimes get lumped in with wave making. If you find a good reference that leaves it out, I would leave it out. I would use some sort of empirical data, where skin friction and wave making resistance are calculated separately but you include such things as length, displacement, displacement-length ratio, slenderness ratio, and roughness. A dragon boat is a lot like a ship, so there should be plenty of good data to work from.

The paddle stroke mechanics should be most interesting. The forward and downward motion generates thrust through lift, then the middle part of the stroke is straight backwards generates thrust through drag, and the last part of the stroke is upwards while forwards again and generates thrust through lift again. Also the paddlers try to use good water by some paddling close and even under and some reaching further out so as to avoid each others eddies.

The other thing you might look at is the effect of the acceleration and deceleration on the average drag, and whether or not that is improved by the ooching motion of the paddlers between strokes, or whether their ooching motion is all just part and parcel of getting maximum power out of their next stroke. I think part of the rhythm has to do with biomechanics, how the body stores and recovers energy elastically by bouncing forwards and backwards again, with their knees and hips and back, but it may also serve to keep the boat going at a more constant speed for less average drag than a more varying speed.

You could also look into strategies where different groups paddle out of synch, if they could leave a gap in the middle of the boat in order to do so, and why this may or may not work in theory or in practice.

The drummer. Should he be ooching, or just focus on his drumming?
What part of the stroke should correspond to his drum beat, and why?

 

Thanks for responding Jamie. After my posting I checked the forum members list and noticed that Alexz is not a member any more so wasn't expecting anything.
The reason for my interest is that I coach dragon boat racing and try to understand what impact changes to certain factors will have on boat speed. For fast crews, average speed is just above hull speed (abt 4m/s). But it is little more complicated than that as during the stroke cycle speed varies from 3 to 5.3 m/s (see https://backsixpush.wordpress.com/2012/11/15/boat-speed-and-individual-strokes-surprising-results/) and thus a significant portion of time the boat travels significantly above hull speed. One of the teams I coach is a breast cancer survivor team and their average race speed is about 3.3 m/s so still travel a reasonable proportion of time at greater than hull speed.

As you pointed out, dragon boats take longer to get up to speed than rowing boats. Dragon boats typically take about 15 seconds to get to full speed. Stroke rate varies a lot between teams and between start and finish. Some Asian teams achieve stroke rates of 130 spm while other top teams do about 90spm but may be quicker towards the end of the start sequence.

Because I am mainly interested in the impact of changes on velocity rather than velocity per se, I found that a spreadsheet like Hullcalc is quite useful as I can play around with variable and observe the changes in velocity. Leo Lazauskas Michlet software will probably provide more specific results, but I haven't been able to spend enough time with it to understand it fully.

As you mention paddle stroke mechanics are the interesting part and what coaches spend most of their time on. But there's not much scope to paddle “under'" or further “out” to avoid eddies. So effort is mainly on stroke length and force. However, you mention paddling asynchronously which is something I have been thinking about and why I was interested in what formulae Alexz came up with. The idea is to take a “model paddler” and compare theoretical performance results for paddling in phase and out of phase. It's a great embarrassment for a team (and coach) to be paddling out of phase so it'll be hard to convince them to even try it. Furthermore as most of the teams I coach win medals, they need to be sure that their results are not jeopardised. So I figure I need to show an improvement on paper of 3-5% or more. Your suggestion of taking out 1 row will probably not get us there as it reduces available power by 10%, partially compensated for by a 10% weight reduction. However, a “caterpillar” action is quite feasible to do and many beginner teams do this naturally – until the coach tells them to paddle properly. So need to crunch the numbers, but mathematics don't excite me and I have yet to find a convenient way of doing the calcs (i.e. first converting acceleration data into speed, then calculating power at different points in time, then segmenting the power & applying an offset before adding the data and recalculating boat speed).

Regarding your final comments:
Drummer – in a race the drummer executes the race plan and provides motivation for the team. However, during training the coach is often on the front of the boat as well (we leave the drum & seat off) while the drummer just keeps time with their voice.

What part of the stroke should correspond to the drum – Interesting question. I actually train the drummers to do the opposite i.e. the strokes set the pace and the drummer hits the drum on the down part of the stroke, but communicates with the strokes regarding increase/decrease of the rate. The beat coincides with the visual cue of the top hand going down so that probably is the most effective way of keeping paddlers in time. I guess it also has a slight psychological effect in stimulating the paddlers to apply maximum force immediately after the beat.

If we just had an outboard motor on the boat the calculations would be a lot easier …….

 

Thanks for all your knowledge and insight. Very interesting. I have only observed dragon boat racing and training from the rowing/kayak club I helped out at a few years ago, fixing boats. The caterpillar action sounds interesting. Good luck with the F=ma stuff. Out of curiosity, how is it best to trim the boat? Heavy people middle I presume?

 

You are good at asking interesting questions James.
Left/right balance is the most important. I generally try to get a level trim. The boats are quite long and symmetrical so not so quite forgiving as far as trim is concerned. Tall people go in the middle. The seats are slightly higher there. Front & rear choises are also driven by paddler characteristics as the hull slopes down 7 deg at the back and up at the front. This affects paddling technique for some people. Then of course we need good strokes in the front and little people at the back (as the big ones don't fit there). Then distribute the remainder to get level trim.

 

Fascinating. I wish you and your crew all the best of success for all your efforts this summer. Cheers.