Dragon boat hull speed: "W" hull vs. flat hull

[tom28571]

Originally Posted by Douglas Ingram

It will do no good to argue how the W works, or why one would want it.

The fact is that this "W" shape is the traditional shape of the Dragonboat.

Whether the shape is a byproduct of the construction method or an intentional performance feature no longer matters. Dragonboat racing is all about racing crews, not boats. It is not a developmental class to improve the performance of Dragonboats. The International Dragonboat Racing Federation expressly states this. All the Dragonboats are to conform to international standards for just this reason.

The OP wanted to know if the W shape offered any advantage over a flat bottom for marathon style distance races. I would offer that, no, it will not.

Team coordination and good steering are paramount in Dragonboat racing.

That the W shape bottom has some advantage in sprint racing, which is what most Dragonboat racing consists of, is definitely claimed by serious racers. I don't race Dragonboats so cannot make the claim myself. It takes a lot of work to get achieve this advantage, and just as much work to maintain it.

Any advantage that the W shape my provide, whether real or imagined and by whatever method it imparts its, would not be able to be maintained over and significant distance.

I agree with you Douglas,

Now my only thought is that, if it is ok to build a flat bottom (or mostly flat), then that is what I will do if I am asked to build one. Just watching the performance of various teams in sprint races, I would say that crew training is tops, steering and drummer come next. The boats are all so close that I don't see any advantage between them.

Don't want to argue the air entrainment issue but if it was an advantage would not racing sculls use it? They are faster than the dragon boats (at least the ones I saw) and it would be more of an advantage to them.

[Douglas Ingram]

Originally Posted by tom28571

Don't want to argue the air entrainment issue but if it was an advantage would not racing sculls use it? They are faster than the dragon boats (at least the ones I saw) and it would be more of an advantage to them.

It would be fascinating to try building one!

BTW, if you ever do get involved in building a DB, its not hard to build the hull with a flat bottom and add the W bottom points afterwards. That's how I did mine.

After re-reading my previous post I am ashamed at all of the typos! That's what you get for writing just before going to bed...

[troy2000]

Putting in my two bits strictly as an amateur when it comes to boats, I'd say that if a particular shape is traditional in building something (whether it be a boat or a barn), it's important to figure out why before changing it. There may have been very practical reasons for doing it the way it was done -- whether it be for strength, ease of construction with the original materials, or practical function.

Of course, sometimes there isn't a good reason. It may just be that fashion evolved into habit, for better or worse.....

But getting back to the dragon boats: aside from the ideas already mentioned (about the exaggerated chines helping the boat track; the 'W' shape stiffening the boat and allowing fewer frames; etc), I have one more thought.

Is it possible that the 'W'-shaped hull transfers some of the buoyancy from the center of the hull to the sides, and helps the boat stay level when 20+ paddlers start rocking and rolling? It seems to me that with short paddles and a short stroke, anything that kept the boat from rolling even a couple of inches might make a difference.

Or would the difference between the 'W' and a flat hull be negligible?

[PAR]

I'm not sure what the PPI for each "sponson" would be, but it wouldn't be as much as the flat bottom hull Troy, so I would assume the flat bottom has a higher initial stability.

I guess the real question is how much real measuring is done to see if a new build conforms the the standard. If they come out with a tape measure and check; length, beam, freeboard, LWL, etc. then you could optimize the rocker and volume distribution for best preformance. Naturally, if they drag out some templates, you're screwed.

[Douglas Ingram]

The W bottom on the Dragonboat doesn't add width as a sponson would, its more like dropping the chines along each side down below the bottom panel.

You can see it here on the drawings that I used to build my DB.

For a flat bottom DB, just imagine that the bottom panel is extended out sideways to meet the chine panel.

[tspeer]

You guys are all assuming the W has a hydrodynamic function. What if it simply makes it easier to pull the boat out of the water & up the beach across rollers?

[troy2000]

Originally Posted by tspeer

You guys are all assuming the W has a hydrodynamic function. What if it simply makes it easier to pull the boat out of the water & up the beach across rollers?

Easier than pulling a flat bottom across rollers? I don't think so....

[daiquiri]

Found it! The Wikipedia page about dragonboats explains the reason for that odd hullform. Basically, it derives from building method of traditional dragonboats, which were made from three logs - the central one placed slightly above the lateral ones. The lateral logs were acting like sponsons, because the particular paddling technique (http://dragonboats.tripod.com/pages/..._technique.htm) required an increased stability. So the modern hullform is merely the imitation of the ancient one, adapted to modern building techniques.

Citation (from http://en.wikipedia.org/wiki/Dragon_boat):

"Canoes are derived from hollowed out tree trunks (either single log, or single log supported by one or a pair of outrigged float pontoons or else catamaran style double logs.); or from birch and other deciduous tree bark shells stretched over wooden frames. Traditional wooden dragon boats, however, derive from rafts of three lashed-together logs which have been hollowed out and are like the long and slender bamboo rafts consisting of lashed bundles of hollow bamboo stalks which can still be seen in China today. The center log acts as the keel with another log attached on each side of it acting like a pair of sponsons for stability, preventing the center log from otherwise rolling. It is the three lashed, rafted logs of old that give the Hong Kong style of dragon boats its characteristic hull form cross section underwater seen today, which is like the shape of the letter "W". Whereas a canoe or outrigger paddler sits INSIDE of the hull formed from a hollowed log that is wide enough to sit IN, dragon boat paddlers sit ON TOP OF the hull formed, traditionally, of the three log raft. The keel (the longitudinal centerline plank) is constructed above the level of the two, downward sloping garboard planks attached on each side of the keel, so there is a kind of tunnel hull shape running down the centerline. This unique design feature is a vestigal throwback to earlier primitive lashed-log raft forms on which modern hulls are based. Traditional wooden boats are slender and heavy, typically weighing in at approximately 1,750 pounds for a 22-person hull. As the sport of dragon boating has increased in popularity and spread to countries outside of Asia, many countries have switched to using dragon boats constructed of fibreglass and plastic resin, which are significantly lighter."

Cheers!

[Village_Idiot]

I'm on the side that flat bottom should be faster and more efficient. If offers the most load-carrying capacity with the shallowest draft and least amount of wetted area (friction). As mentioned, keels will help to keep the hull on course.

If there truly is a positive effect by trapping water/air under the hull, perhaps a better design would be a flat bottom with ice runners (narrow vertical ridges) on the chines. The runners would serve to hold water under the boat without drastically increasing wetted area, and would serve to keep the hull on course.

[daiquiri]

Come on folks, I think it is so overwhelmingly clear and logical that air entrapment has nothing to do with this hullform that it shouldn't be used as an argument. Unless you want to waste some time on empty talks...
There can be no air below the hull with this hullform and at paddling speeds, hence there can be no friction reduction or else. We can talk about stability if you want, about course keeping, about tradition, about construction methods or about anything else. But let's stop talking about this air entrapment stuff, it is a total nonsense.
Cheers!

[JosephT]

Thanks for all the responses. As an aerospace geek focused more on aerodynamics vs. hydrodynamics the same principle applies with drag. The more surface area the more drag. It's good to debunk the notion that a W hull somehow provides lift. With a boat + crew (4,000+ lbs) I doubt much if any lift would be generated unless you're in the eye of a hurricane.

One of my dragon boat captains acknowledged that steering a W hull is easier, so that reinforces their design intent.

During our marathon we finished in record time with a flat bottom hull and 3 highly skilled steers, so keeping on track wasn't ever an issue. The steering oar alone did the job just fine.

Thanks again for all the great feedback.

[keith66]

On the 25th September is the Great River race from Greenwhich London to Richmond, though primarily a race for traditional fixed seat rowing boats a good number of dragon boats take part. At 22 miles its a distance race that can be very rough but those dragon boat crews really go for it. At the end of the day the strongest fittest crew that does not sink wins!