Evaluating dinghy hulls: Or, why is a laser faster than a moth?

[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?

[robm]

The Laser is longer, which means more waterline length. It is also stabler than a short boat of the same beam. Think waterplane area as providing the stability, not just the beam.

Read Frank Bethwaite's book "High Performance Sailing", apply his equations to the Moth designs and the Laser, and you will see what the deal is.

In theory, flat-out on a plane, the Moth should be as fast or faster, but is a bit sketchier, because it has less stability.

It is in the "forced mode", between displacement and flat-out skipping from wave to wave, that the Laser does better, as it has a longer waterline, more stability, etc.

Bethwaite's book will give you some good ideas about hull design too. How about a Maser cut down from a Taser instead of a Laser? Problem is, cutting that 4 ft off the hull will probably cut off the chine that makes them faster, but it is worth a thought.

[John Meachen]

A Moth will likely be faster in light winds because it has less wetted surface.In light wind displacement mode this is important and as wind speed (and consequently hull speed) increase,wavemaking resistance becomes a dominant factor.A scow Moth will probably plane faster than a Laser but will lose out in the light stuff.All a bit irrelevant these days as a foiling Moth is one of the fastest boats around.

[Paul G.]

Get High Performance Sailing by Frank Bethwaite

[cmjns]

This is perfect. Exactly the answers I was looking for.

[gowansg]

I have 4 classic moths (1 vintage, 2 Cates and a scow moth) and a laser. The laser is probably slightly faster but the moths are more fun and much more interesting unless having a stable boat is your thing. I also have a 505, even more fun when sailed single handed.

[Sailor]

The boat I'd like is a Flying Dutchman, to veer a little off topic. But they are considered the fastest monohulls upwind as far as I know. And have been since they were first introduced as an Olympic class boat some 60 or more years ago.

[robm]

The FD is no longer the fastest dinghy on the water. The modern skiffs are faster. They have more sail area, more efficient sails, more efficient foils, faster, lighter hulls, more beam, and all the crew on trapeze. Catamarans can be faster than skiffs. Foiler Moths are faster yet.

To compare, the Portsmouth numbers are:

Int. Moth 600
Tornado 644
18 ft Skiff 675
FD 879
Fireball 975
Laser 1087

https://en.wikipedia.org/wiki/List_o...smouth_Numbers

[johnw]

The last of the non-foiling Moths were faster than a Laser, or for that matter, a Contender. They were skinny, wall-sided, and reliant on hiking wings, which the classic Moths don't allow, so they no longer have a fleet to sail in.

Anyhow, if I wanted to go objectively fast, I'd buy a ticket on an airliner. Sailing is a sensual sport, and the appropriate metric is how much fin you have.

[cmjns]

I have 4 classic moths (1 vintage, 2 Cates and a scow moth) and a laser. The laser is probably slightly faster but the moths are more fun and much more interesting unless having a stable boat is your thing. I also have a 505, even more fun when sailed single handed.

So what you are saying is that you have way more boats than you need. Let me know if you need any help with that.

[cmjns]

The last of the non-foiling Moths were faster than a Laser, or for that matter, a Contender. They were skinny, wall-sided, and reliant on hiking wings, which the classic Moths don't allow, so they no longer have a fleet to sail in.

Anyhow, if I wanted to go objectively fast, I'd buy a ticket on an airliner. Sailing is a sensual sport, and the appropriate metric is how much fin you have.

Of course, my question was really about evaluating hull shapes, and not about going objectively faster. But your point is taken, and I'm an agreement; I'm more interested than an exciting sail than a fast one. I'm already settled on a moth. I just want to understand my choice better.

[johnw]

I've actually been playing around with designing a Gen. 1 type Moth. I'd want something more stable than a Mistral, but not too much slower.

[robm]

Boat, bike or car, it doesn't matter how fast you go. As long as you are running at the ragged edge of control, it will be exhilarating, whether that is 5 kts, or 25, 80 km/h on bald tires or 180 in a Lamborghini.

[gowansg]

How many boats are too many? I forgot the M-16 score out back. It's not easy to sail 7 boats but all added together they cost less than $1,000 because many were fixer uppers and I built two of the moths from scratch. Old boats are just as much fun as new ones.....but the 505 is still the best yet, a handful.
I've had a Mistral and although fast, are very very tippy. I like my Cates Moth best.

[johnw]

This is what I've been chewing on. There's no local classic Moth fleet, so I might build something bigger.

[robm]

You have been reading Bethwaite, I see:
- Narrow on the water line
- Wide on deck
- Hard chines aft, to leave the water behind on a plane.

Are the surfaces developable?

[Chris249]

At the risk of self promotion, I've been interviewing dinghy designers and researching design history for years, and some of the results can be found at

https://sailcraftblog.wordpress.com/

As others have said, the Laser's greater speed lies largely in its length. Length not only gives you the accepted advantage of greater hull speed, but it also allows you to have less rocker and a finer bow than a short boat, all else being roughly equal. Because crew weight is such a big proportion of a dinghy's all-up weight, adding length also dramatically reduces the displacement/length ratio. Even an ultralight 28kg (all up, rigged) Moth has a high DLR because it's floating an adult on a very short LWL.

There's a couple of chapters about the design and development of the Laser in the blog. I got the information from interviews and correspondence with Bruce Kirby (the designer), Hans Fogh (who created the sail) and Ian Bruce (who kicked off the project and did the boatbuilding side). I think it's an under-rated design in many ways.

[johnw]

You have been reading Bethwaite, I see:
- Narrow on the water line
- Wide on deck
- Hard chines aft, to leave the water behind on a plane.

Are the surfaces developable?

Yest, they develop with very little stress. In theory, you could even build in aluminum, which is nothing like as forgiving as plywood when it comes to surface stress. I haven't actually read Bethwaite, but I'd like to.

[Chris249]

You have been reading Bethwaite, I see:
- Narrow on the water line
- Wide on deck
- Hard chines aft, to leave the water behind on a plane.

Are the surfaces developable?

I can't see the image, but the trademark Bethwaite shape relies on being longer than an equivalent 'typical skiff'. That could be problematic in a Moth.

The Vee shaped sections that are characteristic of the Bethwaite boats gives a higher wetted surface and less dynamic (planing) lift than the U-shaped sections seen in (for example) a Bieker design or many Morrison designs. The Bethwaite shape works well with boats like the 49er and 29er because they are about two feet longer than equivalent boats like the Int 14 or Cherub. That means that they are lighter (in proportion to length) and have a higher hull speed, which compensates for the higher wetted surface and lower lift. The Vee sections then make the Bethwaite boats fast upwind, because the ability to cut through waves rather than slam is important at 9er-type speeds.

However, later Moth design went the other way - vertical topsides and a flat bottom with a narrow pintail, combined with an extremely narrow hull of just 1 foot beam. Obviously the wings played a major part, but the smaller size of the Moth meant that it had to be designed around maximising volume and minimising hull drag. Even before foilers, people like Julian Bethwaite were saying that the Moth was pretty much the most interesting and influential dinghies around. The irony is that some journalists have written that foils lifted the Moth from obscurity but in fact just about all the leading dinghy and skiff designers thought that even the pre-foil Moth was anything BUT obscure.

The really interesting thing about the development dinghy classes is how enormously different they tend to be from each other, because length and sail area have such a huge impact on the design; the 12 Foot Skiff is (and has almost always been) a dramatically different design to the 18 Foot Skiff, for example, even when both of them were largely limited only by LOA. It all means that drawing parallels from one class to another is extremely difficult.

I really respected Frank Bethwaite as a man and a thinker, but one has to be somewhat cautious when reading his books because he did have a strong tendency to believe the evidence that proved his views and discount the other evidence. As one example, he thought that NS14 design had not advanced since he stopped designing them, but there is an enormous amount of objective evidence to prove that the newer ones are significantly faster. As another example, many of his stories about the history of the sport are just plain wrong; for example he claimed that there was social pressure against the little catboat Una when she arrived in Cowes. In fact the Royal Yacht Squadron offered a race for Una types, and they were all the rage among Sea Lords, Viscounts, other aristocrats and even the future King of England. The same sort of thing affected his view of dinghy design, such as his preference for Vee shaped sections rather than U shapes. People like Julian Bethwaite and Paul Bieker are, in fact, quite happy to admit that they follow different design philosophies because they are designing for different target audiences, not because the Bethwaite style is superior or inferior.

[paminch]

I've put my two cents worth on this dinghy hull shape question up on my blog:

http://earwigoagin.blogspot.com/2017...ssic-moth.html

RLM