2 New SIC's

[capobeachboy]

Bill - when it comes to non-SUP paddleboard distance racing on the mainland you wil see about 99% pintail craft unless it's a downwind-specific course (Waterman, Bark, Eaton, Ohana, NCP, Richmond, etc.).  Those types of boards have held the records at the majority of our events for years.  Also, I've never seen anything but a pin tail on a surf ski or OC-1.  All the square tails I've tried in flat water conditions don't release as clean as similar pin tails.
Chris

[noa]

Noa, Pintails do not release better than flat or squash tails (with "release" being defined as minimizing drag), in fact the primary design factor driving the decision for flat tails is release, that and being able to have straighter rails in the same length. One of those things that doesn't seem logical but is still true. Even in air, at low velocity (under a few hundred mph) a pintail generates more drag than a flat (or Kamm) tail. Early race cars all had pintails until they started really going fast, then the aerodynamicists discovered the benefits of whacking off the tail.

Not a simple decision in any case. each design has benefits and faults. As far as I know none of the surfboard designers have access to instrumented test tanks, and the conditions most boards are used in probably would negate any benefit of that kind of testing. But basic hull characteristics are very well known. No matter how smoothly you shape them, Pintails develop uneven flow along their extended sides. the flow variations translate as random sidewards thrust that both increases drag against the fin and causes buffeting. The longtail Porsche 917 at the end of the Mulsanne Straight, randomly jumping ten feet to either side at over 200 MPH is a dramatic example. Porsche engineers literally whacked the tail off with a demolition saw trackside at Le Mans and went 20 MPH faster.

I have seen lots of successful and obviously well developed boards with extended pintails (Bark, Hobie, etc.) and always wonder why. I'm sure that reality and theory collide in the chaotic conditions people actually race in. Not to re-spark an old argument, but the motor is certainly a factor. Put Dave Kalama, Laird Hamilton, or Chuck Patterson on a slightly less than optimal board and they will smoke almost anyone. It's going to take years for the design of downwind/racing boards to approach the refinement of other human-powered racing forms. America's Cup yacht designers use both tank testing and very sophisticated hydrodynamic models to make significant and effective changes that involve less than five square millimeters on their huge hulls. Cut and paddle is a long way from that.

i was scratching my head while reading your post, making hmmm sounds when a friend tapped me on the shoulder and whispered something in my ear. that brought about the following question. if square ends release water better, should i square off the trailing edge of my fins

[PonoBill]

I'm going to work on understanding at least the parameters that affect this design decision. It seems a little illogical. I can see using a pintail to minimize weight and balance buoyancy (and reduce plunging of the bow). a wider square tail will naturally be more buoyant. for boards of the same length, the squarer tail will have straighter rails. the turbulence at the moment the water separates from the squaretail is greater than at the tip of the pintail, and that energy has to come from somewhere, but once the water separates it doesn't seem like it could have much effect on the hull.

the converging rails of the pintail will cause flow separation along their length, increasing drag, and any unevenness of turbulence will cause buffeting and side thrust.

I've signed up on a number of boat design forums to look for answers. I'll let you guys know what i find out. Once i have a question like this in my head i have to follow it through to the end. Maybe as well as being the poster boy for ADD (attention deficit disorder) I might have a minor in OCD (obsessive/compulsive disorder)

[stoneaxe]

We could be in trouble folks. When I was a kid Bill had the top two attic rooms in our house. His bedroom was unremarkable except that everything was remotely controlled from a panel bext to his bed....lazy bugger. Next door...Dr Frankenstien's lab, strictly off limits to little brothers (I think he stashed his porn there). The place where he was building robots from overturned plastic washtubs and 1/2 a radioshack store. Remotely controlled submarines that looked like rocket ships (did you ever get that thing back Bill?). Controlled experiments on rat hybernation! This was all in the early 60's. Of course he's melted a few brain cells since then but this is still a guy that thinks books on quantum physics are fun. Whatever you do please just post the crib notes...

[E.J]

Bill - when it comes to non-SUP paddleboard distance racing on the mainland you wil see about 99% pintail craft unless it's a downwind-specific course (Waterman, Bark, Eaton, Ohana, NCP, Richmond, etc.).  Those types of boards have held the records at the majority of our events for years.  Also, I've never seen anything but a pin tail on a surf ski or OC-1.  All the square tails I've tried in flat water conditions don't release as clean as similar pin tails.
Chris

I'm with Chris on this one.
With my non extensive testing, that I've done. Basically feeling and looking at the release in the tail wake, it makes no sense that anything other than a pintail is fast. In California, the pintail clean release tails prevail!! Check the race results. Prone or Stand -Up.
 Note ( The BOP race conditions were extreme for So Cal and the most stable boards prevailed)And Chuck Patterson had the right board and the strength to kill it, and he deserves the win!!!
I know all the boards from last season, and none of the top 3 in any race, were square or squash.
I'm not sitting behind a keyboard speculating, I'm out racing every race, and in MY opinion square tails even for down wind, are not all that. But, what do I know??/? Me know notta!!!!  My NCP pintail Rules.

Nuff respect   E.J     

P.S     Chris, I heard your having a kegger party on the 20th! it's gonna be a hooter.

[PonoBill]

Bob--Nope, never did get the sub back, it's been at the bottom of Boston Harbor for about 45 years. I've pretty much written it off.

Trust me, I know that a lot of folks are not going to be too interested in all this. Might wind up as an article in Ke Nalu or could be just a summary of what i find posted here. I don't have a practical application for this, I'm not trying to design boards, I just gotta know.

EJ--Could be that pintails are the right choice, or it could just be conventional wisdom. that happens more than we sometimes know. Which way do you peel a banana?  It much harder to peel it from the stem end, but almost all people do, because that's how they see other people doing it.  Airplanes have only had winglets for about fifteen years, though the fundamental data that said they would be valuable existed for nearly a hundred years.

Rowing sculls are pretty refined craft and some of them have squared tails. I'm thinking it might be somewhat a matter of reducing the plunging of the bow on a wave. We'll see, I'll figure it out. It will probably take a while. It would be nice if I could just find some testing data or a good paper somewhere. It's likely to exist, this is the kind of problem that a budding naval architect might undertake for a thesis. The trick is finding it.

I need to talk to Mark Raaphorst. when i look at the Ku Nalu design I see the basics of what I'm talking about. a truncated nose to get the nose volume up and the rails straight as soon as possible, and a blunt tail. Obviously not a racing board, but it will be interesting to hear why he designed it like that.

I have found LOTS of hull modeling software, but it's mostly aimed at building tankers or power boats.

[PonoBill]

In my quest I came across this: http://www.yachtforums.com/forums/future-yachts-concept-boats/10376-preview-new-zealand-yachts-50m-wavepiercer.html

If you've got an hour to blow, download the newest version of Adobe Acrobat Viewer and then download the PDF of the yacht (the URLs are both provided in the posting) You can play with the design, spin it, rotate it, climb inside, change colors, etc. etc.. Nice toys these guys have.

Note the hull design. Non-planing catamaran that cruises at 22kts. For what it's worth the hulls are square in the back, but I don't think much design wisdom translates from the 50 meter power yacht category.