Thursday, March 23, 2017

Why is a Laser Faster than the Classic Moth - Or Is It?

Over at the WoodenBoat Forums there is this thread that recently popped up that was started with this question by "cmjns":
"I've spent a lot of time ogling classic moth designs, from the Gen II mistrals to the Gen I Mint, and I'm a bit smitten by the Farr 3.7 and the Contender (very different boats, I know). So I'm curious: among all the shapes swirling around in my head, some of them reportedly are much faster than others. And the moths in particular *look* fast but aren't nearly as fast as a laser, which nevertheless "sails like an aircraft carrier" in comparison.

"So my question: how do I evaluate a hull shape? What characteristics are associated with downwind vs. upwind performance, and what equates to overall speed? And why is a laser faster than a moth? Is it the sail? Is it the length? Is it the shape? What happens to a laser when you cut it down and put a moth sail on it (maser)? Does it slooooooow down?

The answer in a nutshell. A Laser is faster than the Classic Moth because it is longer (as several of the commenters to this thread noted - see "Chris249" comments for a more detailed hydrodynamic explanation). The Laser also has more sail area - the Classic Moth rig is more-or-less equivalent to the area of the Laser Radial. And No... making a Laser into a Maser doesn't make it faster than a Laser. (It does make it more fun in my opinion.)

I think the crux of "cmjns" question deals with hull shape and, specifically, why the flat Laser shape is so different from our fastest Classic Moth hull shape, the Mistral or Duflos designs which have low wetted area, narrow waterline beam, rocker forward, with a large amount of flare to the gunwhale. Does that mean the flat Laser shape is inherently "faster". Are the fastest Classic Moth hull shapes an anomaly?

First up is the Laser shape as put into computer format by PAR Design:
PAR Yacht Design - Boatdesign.net

For a comparison is the following four views of the Mistral hull shape as modeled by Andrew Slavinskas. Not a flat area to be found. The transom is a very circular shape.
Andrew Slavinskas


Andrew Slavinskas


Andrew Slavinskas


Andrew Slavinskas

What gives? It turns out the characteristics of a Classic Moth Gen II fast hull (low wetted surface, narrow waterline beam, high flare to support hiking power) translates very well into a longer, very fast conventional hiking singlehander - in fact the fastest conventional hiking singlehander. It is the RS300 and it was designed by Mothist Clive Everest back in 1998. It is not length that gives the RS300 an advantage. (It is only slightly longer than the Laser - +.07 meter, giving the RS300 the same length as a Melges 14.) It is a hull with very little flats anywhere, where wetted surface is pared back ruthlessly, where the waterline beam is as narrow you can get away. The dirty little truth! For a design close to twenty years old the RS300 is faster than the latest, much ballyhooed RS Aero and the D Zero. Some RYA Portsmouth numbers:
  • RS300 - 973
  • RS Aero - 1024
  • D Zero - 1029
  • Farr 3.7 - 1039 (Yes, I know the Farr is a trapeze singlehander.)
  • Laser - 1097
  • Europe Dinghy - 1145 (I would expect our Classic Moth Mistrals to be slightly faster than this number.)
The downside to this hull shape. As with our Mistral design, the RS300 is very tippy and has a tendency to trip up her skipper with regularity. The RS300 is worse than our Gen II Classic Moths in that respect because the RS300's modern rig is much more powerful than our vintage low-aspect rig. As such the RS300 has carved out a small niche among those singlehanders who want an athletic, challenging, conventional (no-foils) full-on racing dinghy. The market for the RS300 is small enough that RS has given the RS300 production over to another low-volume builder.

So our fast shapes in the Gen II Classic Moth fleet are not an anomaly. There are exactly the shapes you would design if you want the fastest hiking, non-trapeze, non-sliding seat, non-foiling singlehander. However, if  you are trying to sell a lot of dinghies, you just might not find a large enough crowd that would enjoy all that wind-swimming.

From the Earwigoagin archives, you can find other RS300 posts.

A very well done RS300 promotional video:





10 comments:

Tillerman said...

Fascinating. I had never realized before that the RS300 is the fastest hiking, non-trapeze, non-sliding seat, non-foiling singlehander. Maybe I should sell my RS Aero and buy one?

George A said...

So, on a Portsmouth number basis you're saying that despite the Laser's water line length advantage that a well sailed Mistral should be able to go toe to toe with the Laser. I think that's correct, especially in light air. It would be interesting to put Jeff Linton in his Mousetrap against Dick Tillman in a good Laser.

Tweezerman said...

Tillerman,

Keep the RS Aero. Tippiness is inversely related to the popularity of the dinghy class. You need to design a certain level of user-friendliness into your hull in order to attract sailors. Very tippy dinghies have a small, fanatical user base. More stable hulls attract larger crowds that can be just as fanatical. The fastest dinghy is not the end-all, be-all parameter - fun racing with a good group of mates is ultimately what most of us are looking for. Remember my post on the Retro Singlehanders?

Tweezerman said...

George,

The Laser is faster than a Classic Moth. As you say, the Mistral Classic Moth might be able to hang in very light air but length wins in this battle. The Europe Dinghy PN of 1145 might be a good basis for our Classic Moth Gen I division (though the Olympic sailors and intensely coached juniors of the Europe Dinghy have set a very high standard that is probably reflected in that PN - versus our older, club sailing Classic Moth group). We might be able to calculate a PN for our Gen II division given that they seem to consistently finish with a 2-3 minute lead over Gen 1 in a race of approximately 20 minutes in length.

Tillerman said...

Thanks for the advice Tweezerman, but I was only joking about buying an RS 300. I had gathered that the RS300 is very tippy and that it almost seems to have "cult" status in the UK. Even wrote a blog post about it in 2015. Is the RS300 a Cult?.

cmjns said...

I'm "cmjns". Thank you for understanding what my question over on the forum really was about. I was aware that I was doing a clumsy job of asking it, and you dodged all the red herrings to get right down to the crux of the issue I was grappling with. And thank for your writing this post, which answers the question beautifully.

Tweezerman said...

Thanks cmjns.

You also have to realize that when you add a trapeze or sliding seat, the extra power flips the equation and the flatter shapes (a'la the Contender, or a Bethwaite shape) becomes the faster hull shape. Flatter shapes can attain the higher speeds you get with increased power.

Tillerman,

Thanks for reminding me of your RS300 post. I had forgotten about it. You quoted what I consider the best sailing dinghy review that I have ever read.

johnz said...

cmjns, if you're still out there, I couldn't find my WoodenBoat password.

I sail both a Laser and a Mistral Classic Moth and racing the Classic Moth is definitely more fun, mainly because Moths are much lighter so they accelerate more. When you're sailing you feel acceleration more than you feel velocity (F=ma). Classic Moths leap frog down the reach legs so we sail triangles a lot.

Moths also have more controllable rigs. In 20+ knots I can flatten out the Moth sail, put the traveler down and keep blasting along. In a Laser, Radial included, I'm struggling - at 140 lbs.

I still enjoy the Laser because we sail all summer at my club on Tuesday nights with 15+ boats on the line every week. We only race Moths five or six times a year and it takes some driving (NJ, NC, FL, MD).

If you like to putter on boats and don't mind driving you should definitely consider the "more fun" boat ... maybe in addition to your Laser. - John Z

cmjns said...

Oh, no! Here we go around again! :D

Ok, so I don't *quite* understand how trapezes flip the equation, Tweezerman. If I took an RS300 and knocked off the sides, I might replace them with a trapeze or a seat to get my body out to the same distance from waterline. So setting aside the foils, an international moth is just an RS300 with hiking wings in place of steep sides. But that wouldn't alter the physics at the waterline, right? So why does adding a trapeze or a sliding seat suddenly make a flatter boat (laser, contender) faster than a V'd low volume hull? Why wouldn't you take a Mistral, lengthen it to 15 ft, put a really powerful rig and a trapeze on it, and suddenly be going faster than a contender?

Tweezerman said...

Ah! Now we are getting into the theoretical vs. the practical. Yes, your longer Mistral with trapeze is theoretically faster than a flatter hull but, for the practical you need some righting moment out of the hull to allow a trapezing helmsman to get inboard safely when the wind lets up or wind shifts radically. At some point, difficulty to sail greatly outweighs the theoretical benefits. I've seen a photo on the Internet of someone putting a rig and trapeze on a rowing shell. A publicity stunt no doubt. Theoretically this craft should be a screamer; practically this craft was good only for flat water and a steady breeze where the helmsman could remain stable out on the trapeze. In the late 1980's, I and a friend played around with an International Canoe hull shape that had less righting moment than the one-design Nethercott; faster, but considerably more difficult. All design is a compromise.