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

Hydrodynamic gurus! I have an inquiry for you regarding dragon boat hull designs. I'm on a dragon boat team and we recently raced a traditional flat hull dragon boat (flat bottom). Below are the boat specs:

Length: 41ft
Weight: 800lbs
Hull spec: Typical spec IDBF dragon boat
Construction: marine plywood/fiberglass

For details on the hull shape see IDBF dragon boat "Standard Model 1222" on page 9 of the document below.

http://idbf.org/documents/watersafety/Boat&PadManufSchemes08.pdf

Our team is considering moving to a more modern dragon boat design with a faster "W" shaped hull. Rather than being fabricated from marine plywood, this boat will be made of fiberglass/core mat and will weigh ~650 to 800lbs. For the sake of this discussion, let's say the boat will weigh 725lbs.

Question: Assuming all things being equal (crew, boat weight), how much faster is a "W" shaped hull on paper?

Factors to consider:

1. We will be paddling the boat in a long distance marathon vs. a sprint race, so minor hull speed enhancements are a plus.

2. The boat will be used for flatwater racing vs. open ocean, thus waves will not be much of a factor.

We have asked the dragon boat manufacturers this question and unfortunately we don't get a straight answer on just how much faster a "W" shape hull is. Hopefully someone has modeled these or similar hull types (flat vs. W) and can lend some helpful information.

Thanks

Joseph

 

I see a Chinese traditional, a Hong Kong standard and an International Racing hull form (incomplete lines), but no "W" shaped hull. The International Racing hull has some lines to suggest in might have an extra chine or two, but without a sectional drawing, I'm just guessing this is the hull you mean.

Without a full set of lines of all hulls involved, it's unlikely you'll get any reasonable replies about preformance potential.

 

Michlet

 

With over 3500 lbs of 22 crewmembers, the weight of the boat is a minor factor in its performance. After watching Dragon Boat races last week, I would say that keeping the water outside the boat is much more important than either boat weight or minor speed potential differences. If a crew member has to be assigned to bail instead of paddling, it becomes even more important. Of course, the freeboard is very limited by the need to make paddling more efficient and that complicates thing too.

The races I witnessed were sprint affairs of about 200 meters with nine barely trained local crews. I saw the bow of several boats dip under waves on the river, which was not your calm backwater affair though. Even in calm water, the surge of 20 paddles making a simultaneous stroke can probably drive the bow down.

One issue I see is the necessity for enough beam to allow 20 side by side paddlers enough room to operate without interference.

I did not see the underside of the fiberglass hulls but the interior had a big longitudinal bulge in the middle which may be the source of the apparent "W" shape hull. I'm guessing that this bulge contained foam for stiffness and flotation and the crew feet would fit on either side of the bulge. Whether the hull bottom was flat or had a small tunnel for a W cross section, I can't say.

 

The "W" shaped hull is generally considered to be the traditional. A flat bottom hull is a convenience for modern building.

The W is harder to build, but I believe that it stiffens the length of the hull. It also catches some air and water within its boundaries. Many believe that this helps the hull go faster with some lift. It won't provide much help until you get the boat going fast enough to get the lift. If the points are deep enough you may get some help in steering control.

I built a Dragonboat a few years ago, so have some experience in the matter. I would not claim to be a current expert. As far as I've known Dragonboats have always been about sprint races, not distance racing. They are not really shaped for long distance efficiency.

I copied an existing design to match the clients fleet, and then drew it up with some very slight modifications. I did build it with marine plywood and fiberglass. It was built as a flat bottom and had the "W" points added during construction. You can see it here: http://redrivercanoe.ca/frames.htm

You probably already know this, but for everyone else: success in a race is highly dependent upon: Team coordination, steering, and a fast start. Steering is critical, once the boat starts to go off course it will keep going through its turn and returning to the boat to its race line is very hard. By then you've probably lost the race.

I hope that this helps!

 

The bottom of the IDBF spec boat in the pdf link is perfectly flat. The newer fiberglass designs have a concave channel on the bottom as shown in the image below.



Precise calculations are not the point. As noted, all things being equal would a "W" hull with a concave channel on the bottom be faster than a flat hull?

Expressed another way, if you had two canoes of equal length and beam and one had a flat bottom while the other had a concave channel bottom, which would be faster (both equally loaded)?

Thanks,

Joe

 

Hello Douglas, your feedback provides some logical rationale on why a W shape would be faster. As you point out with additional speed it could take on a bit of air and act as a lifting body, which would offer less surface area and thus offer less drag.

However, presuming a pure flatwater race over a long distance (e.g. marathon) we only kick up to a sprint about 10 minutes out of an hour. Thus, I'm still curious as to which hull has less resistance. The lighter the boat the easier it would be to offer some lift of course.

How much did your wooden dragon boat weigh after completion?

If anyone has any additional notes on W hulls I would be interested in learning more. The same principal would seem to apply to a multi-hull type ski boat.

And Tom, that "bulge" you point out is actually the W hull I am describing. Our marathon races don't splash as much so taking on water, extra freeboard, etc. is not an issue. We have emergency bilges on board in case of a rouge wave, but we have yet to encounter that.

 

I see no advantage to the increased wetted surface and wave making resistance with the W bottom hull form. It's highly unlikely that the top of the W will be out of the water for a very high percentage of the sprint and the speed of the boat wouldn't be high enough for trapped air to offer more then "token" assistance. I'd elect to go with the flat bottom boat and teach the crew as well as permissible. I'd think driving the flat bottom hard, would do more to make her "freer" then the W bottom.

 

Douglas,

That is a great help. Based on what I heard from someone who saw the bottom of a boat, I would not think that the very large bulge I saw in the interior was repeated on the outer hull but I will check further on that. If we get into building a pair of boats locally, I know I will be involved. Actually, I have been putting it off for several years but the festival nature of the event this year shows that many people are interested in it.

I will look closely at the rest of your material and may be back to you for advice if we do decide to build. I agree with Paul that the shape and speed would not appear to warrant any confidence in drag reduction from the W shape. Have been wrong before though. The bow of the boats I saw spent little time high enough to entrap any air, or make good use of it.

 

The W bottom doesn't so much trap air and ride on a cushion, the boat is far too large and heavy to rise that much, but the aerated water is the thing.

Most amateur crews can't take advantage of this, but a good crew can get the boat there. They will tell you in no uncertain terms that there is a moment where the effect kicks in.

I am unfamiliar with Dragonboats being raced in distance events. I would think that the desired boat issues would be different when going the distance.

Regardless of the hull form, for Dragonboating crew coordination and the ability to steer a true course are of paramount importance.

My boat was 42' long, and 48" wide, if I remember correctly. I don't know the weight, I never weighed it. It was probably overbuilt, too, but the client wanted it to be sturdy for the way that it would be used. The crews really liked it, and liked it better than the clients other boats.

 

I agree entirely with PAR's post. W will probably only increase the wetted surface and the friction drag. However, it might actually enhance the course keeping qualities of the boat, as Douglas Ingram has pointed out - so the pros and cons need to be weighted carefully in order to establish which one is more important in the race.

Maybe you could also think about using a mix of the two worlds - flat panels in the forward half to decrease the resistance, and a W shape (ora a stabilizing fin, if allowed by the rules?) in the aft half of the hull for coursekeeping.

I also agree with Tspeer that this hullform appears to be suitable for analysis with Michlet. You could use it to optimize the lengthwise distribution of beam and draft, in order to get that little something that will give you an advantage in the long distance race.

Cheers!

 

I suspect the intention of the tunnel is to force water under the hull to give it lift from a vortex effect and may possibly produce more lift, hence less wetted area. The question is at what speed with the weight in it would this begin to happen, and can the team achieve and maintain it.

The tunnel may well be an easy way to provide seating without having to add an extra long box for that, if the box's weight saved would have an advantage over the gained wetted area. Maybe a flat bottom with super light seating would be better ?


If you consider two teams equally powerfull, what would make the difference to gain the advantage. The longer the distance the more obvious small advantages would become.

 

All this talk of riding a cushion of air is ridiculous. Do you know how much air it would take to even remotely support a ton and a half! Not to mention the air has to be contained to do any good. I suspect the tracking will be quite good on the W hull but I'd rather have a skipper with a steering oar so the correcting drag is only when needed, not continuous. Mashing isolated pockets of air back into the water, does sound especially advantageous.

Tom is right. Michlet should solve the debate handily.

 

Hi Par, I agree with you. It may have been a design intent though. People have funny ideas, which is why I said intention. It's like the guy with the 1600 Golf that goes so much faster when he removed the silencer on the exhaust. Well, it sounds a lot faster.

Imo pockets of air is going to sink the boat rather than enhance anything. Unless maybe if they can do like 100km/hr no air is going to go under that hull.

 

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.