North Sails RESOURCES

International Moth Speed Guide

  • Last Updated: Agosto 26, 2019
  • Print Friendly, PDF & Email

Moth Dinghy expert Rob Greenhalgh introduces this extraordinary singlehanded foiling dinghy and describes the keys to sailing one fast.

Who sails the foiling Moth Dinghy?

The Moth class is for everyone—it’s so exciting, you don’t need to race it. Club sailors can get a huge buzz out of foiling, going fast around the bay, and learning to foil jibe and tack. They might enjoy it so much they never need to join a race. At the other end of the spectrum, there are sailors who have been in Moth Class for a decade or more and love the challenge of racing this exciting, complex craft

Moth Speed Guide | North Sails
Viewed from stern or bow, the foiling moth is a high-speed machine, clearly unlike other sailboats.

What kind of sailors enjoy this challenge?

Moths are development boats, not a strict one-design class, and that’s daunting for some. The technical and evolutionary side of Moth sailing requires a good understanding of how everything works—how the foils interact with the rig setup and how to balance both aspects. Over the last five years, there have been massive speed advancements, which will continue. To be at the top of the game, you have to enjoy the technical side.

But you can buy equipment off the shelf and, if you practice and sail well, soon be up to pace. Though more technically oriented sailors often gain an edge for a while, there are also lots of people who will make changes that don’t help them, so it evens out.

How physical is the sailing?

When you are first getting into it, Moth sailing seems physically hard, but once you’ve done it a while, it’s not too bad. On a windy day with a lot of capsizing, it gets pretty tiring. But it’s a light boat, and the loads are not high. Anyone can do it if you can move quickly at times. There are plenty of Moth sailors in their 60s.

New boats or used, which are better?

My advice for the first-time or second-time buyer is to look for a good second-hand boat that you know will fly. This saves you a lot of effort, because even experienced Moth sailors need a few months to work up a new boat. Sure, it’s always a worry that you might not be getting good enough kit with a used boat, but in my experience, any modern Moth can be made fast. If you want to buy new, a standard boat from either of the two main builders—Exocet by Maguire Boats, or Mach 2 by MacDougall McConaghy—will quick enough for top 5. You can also have a look at the new Bieker Moth by LSF Composites.

The Moth is a development class. What’s happening lately?

At the top end of the class, mast lengths are getting shorter to lower the rig’s center of effort and improve performance. A shorter luff length results in a longer boom length. Rigs have progressively become lower over the last five years, and mast lengths have reduced from 5270mm or 5300mm to the current standard of 5100mm.

Moth Speed Guide | North Sails
The Vi-8 DS 3Di (on top) and Vi-8 LA 3DI feature the same sail area. The LA has more power in lighter winds to foil more quickly; the DS has less drag at high speeds.

What sails do you recommend?

North offers four different sails that meet the Moth sail-area limit of 8.25 square meters. (For more detail, read 4 New 3Di Sails for Moth Sailors.)

The choice of sails relates to your weight, expected wind speed, and another ongoing development in the class—deck-sweeping sails. Because of the class sail-area limitation, in 14 knots of wind we are always trying to shed horsepower and reduce aero-drag. The deck-sweeper sail is better at that, cleaning up areas of drag around the front of the mast, the boom, and the deck area.

Moth Speed Guide | North Sails
North’s Vi-8 DS 3Di sail is a decksweeper-style sail that reduces aerodrag.

If you weigh less than 75kg, you can use the deck-sweeper the whole time. If you’re heavier, like me, in breeze less than 10 knots you’ll need the all-purpose sail’s extra horsepower to get up and foil. The deck-sweeper sail has less horsepower because its center of effort is lower and its shape is not quite as good in those low areas around the mast, boom and deck.

Moth Speed Guide | North Sails
8LA-3Di provides extra power to get on foil.

Which sails do you use?

I use the North Vi 8LA 3Di and Vi-8 DS 3Di, as both fit my mast length of 5100mm. They can be used on both the Mach2 and Exocet.

Moth Speed Guide | North Sails
The decksweeper (Vi-8 DS 3Di) creates an endplate and lowers aero-drag. It is slightly faster once foiling.

How does foil choice affect sail choice?

Moths have two foils, a larger foil for lighter winds and a smaller foil for stronger, and that’s a choice you have to make on the beach. In my case, if I expect to be on my foils all day, I’ll choose the smaller foil and the deck-sweeper sail. If the winds are lighter or less reliable, I will often choose the big foil and the LA sail. And if I expect light air to start but a building breeze all day, I’ll likely choose the big foil with the deck-sweeper sail.

Tuning the Moth Dinghy

What are the keys to tuning a Moth?

Generally speaking, you should set up your boat to carry as much rake as possible. How much is that? You’ll know it’s too much if you can’t get under the boom in tacks and jibes. This is not something you do underway, and if the boom is making boat handling more difficult, make the rig more upright until that smooths out.

What other key adjustments do you make before launching a Moth?

Batten tension is one. For Batten 1 (the top batten) down to Batten 5, just take the creases out of the batten pockets—don’t over-tighten them. For Batten 6, remove the crease and then add two more turns by twisting the batten key two full revolutions in a clockwise direction. For Batten 7, take the creases out and add three turns.

The outhaul is also non-adjustable underway. Rig the boat ashore with the outhaul loose. Pull on max vang and max cunningham, and then pull the outhaul tight. This will give you a good all round setting.

What other settings can you recommend?

Here the some of the number from my boat, but don’t worry if your measurements are different. Comparing measurements boat to boat may not matter at all. The important thing is to start recording them and experimenting.

  • Spreader Dimensions – show a drawing or a photo
  • Front Prodder 340 (adjustable +/-50mm)
  • Side Arm 410mm
  • Deflection 220mm
  • Shroud Base 1500mm
  • Mast Length 5100mm (excluding plastic plugs top and bottom)
  • Back of mast to clew ring 2340mm

Moth Speed Guide | North Sails

How do you set up your foils, actuator wand, and gearing?

This could be the subject for an entire article, but here is an introduction. A Moth has main foils on the daggerboard and a wand in the bow that automatically adjusts the angle of the foils as you fly higher and lower. The rate at which your foils respond depends on how you set the gearing in the bow between your wand and the foils, and this is something you’ll adjust depending on your expectations for wind speed and wave conditions—the rougher it is, the more sensitive you want your gearing to be.

Moth Speed Guide | North Sails
Sailing in rougher water requires faster gearing between your actuator and main foils.

Foil gearing is a critical control for waves; make sure it can be adjusted for rough waters. To get through waves you need faster gearing. You’ll also change foil size depending on the wind forecast. I have two sets of main foils and use the smaller one on windier days. However, if the wind is light for the first race and predicted to build, I’ll go with the larger foil because the speed penalty of not being able to fly with other boats can trumps all other considerations.

Upwind Moth Sailing

What is most important when sailing upwind?

On any point of sail, the vang and cunningham are powerful controls, and need constant adjustment for different conditions. The goal is to generate power for low-riding and to get up on the foils, and then flatten the sail as the wind builds.

One of the challenges of the Moth is it has a maximum 8.25 square meters of sail area, and upwind in most conditions, that’s more sail than you need. Downwind, you have the opposite problem; your speed reduces the apparent wind so much, you’re always starved for power.

What is low-riding mode and how do you sail fast in light winds?

In general you don’t race in winds when you can’t foil, but it may happen that the wind lightens up substantially during a race. Sailing in low-riding mode is an art in itself, something like balancing in a canoe because the hull is so narrow. I always recommend practicing sailing that way.

What does it take to fly a Moth?
To get on the foil, you’ll need a little windward heel to start. If you have marginal flying conditions, shift your weight just slightly farther aft to get more angle on the main foil. Your goal is to get to critical speed to get lift on the foil. Your rig can create vertical lift, so try to tip it over on top of you, but not too far. It’s a bit of a fine line.

Moth Speed Guide | North Sails
Heel the Moth to windward to keep the boat more stable and reduce leeway.

Why do you tip the Moth to windward when flying?

We sail with the rig always tipped to windward because it keeps the boat more stable and reduces leeway on the main foil. Use very subtle steering and trimming of the main.

How do you trim the Moth sailing upwind?

Correct trim is the absolute key to good boatspeed. It’s very easy to over-sheet and stall the front of the mainsail. Always have the inside tell tales lifting. A common mistake is sailing under-powered and over-trimmed. As the wind increases, flatten the main by tightening vang and cunningham to accommodate the higher apparent wind speeds.

How do you adjust sail trim for gusty winds?

You need your rig set-up to be user friendly. That means having enough vang and cunningham to straighten the sail. If twisted, the center of effort can move up and down the sail. It takes a certain level of confidence to get to the point where you can let go of the mainsheet to tighten the kicker and cunningham.

Downwind Moth Sailing

What is the key to good Moth speed downwind?

Downwind we need a massive gear change as apparent wind speed reduces when you round the weather mark. Upwind, at speeds of 15 to 20 knots, you’ll be sailing in very high apparent wind speeds and fully depowering. Downwind, apparent wind speed is much lower. After bearing away, let the cunningham off completely, over-ease the vang and then ease the cunningham again. The over-eased vang really allows the cunningham to ease up the mast. I then bring some vang back on too avoid too much twist. People sometimes sail too twisted downwind, which makes the sail flat and less stable. I also pull on a tiny bit of cunningham to clean any creases, and as the wind builds, I bring on both controls slightly.

Moth Speed Guide | North Sails
Sailing downwind, be sure to maintain a big entry angle in the lower part of the sail and avoid over-trimming.

What is your focus when trimming downwind?

I focus closely on my lower tell tales, keeping them flying 100 percent and making sure I ease enough to keep the sail ‘hooked up’ with airflow attached. I look for a nice big entry angle on the bottom part of the sail and clean up any horizontal creases by subtly tightening the cunningham. From there, it’s a matter of very subtle trim and steering. If in doubt, ease the sheet and be sure you have good flow going around the front of the sail. If you run into a light patch, just head up to keep the flow going on the main. If needed, power up the main even more.

Get the gearing really slow on your foils to minimize drag. The boat will go faster and apparent wind will increase, moving the wind forward and letting you trim harder. Especially when it’s a bit lighter downwind, be sure to pre-empt a drop in apparent wind by easing main and bringing the bow up!

Moth Tacking, Jibing And Gear Changing

How do you foil jibe a Moth?

In all maneuvers, focus on where your body has to end up and getting your hand and foot positions correct. When jibing, think of the foils like they are an airplane; banking its wings is the only way it can turn. You need to tip the boat to leeward as you go into your jibe. As soon as the foils are banked, carve into the jibe with the rudder; the more heel you have, the harder you have to turn. Then move across the boat.

Going into the jibe, I put my hand as far along the tiller extension as possible when I pass it across to the new side of the boat, and my body naturally follows. Lead your body with your arm and you’ll come out with the tiller behind your back where you want it and can do the hand transfer. Practice steering behind your back and quickly changing hands out of the jibe. Also, focus on trimming the main nicely out of the jibe so the sail ‘hooks up’ and you get power quickly. Jibing in light air can be hard but once you’ve got it, you can jibe just as easily in 10 knots or 20 knots.

Sailing the boat downwind in all wind speeds is quite different. When it is windier, you tip the boat to windward more, move aft slightly, and don’t power up the sail as much. If you can’t foil jibe, it’s probably not windy enough to race.

Keep in mind that it’s easier to jibe than tack. Sometimes when sailing upwind, we’ll jibe rather than tack to ensure we can stay on the foil.

Moth Speed Guide | North Sails
When foil tacking or jibing, you need to flatten the boat. For jibing, bank into the turn to leeward. You’ll turn the boat more aggressively when your body is ready to move across to the new wing.

How do you foil tack a Moth?

When tacking you have to move quickly, bringing the boat relatively flat once head to wind. Start luffing slowly, taking the edge off your speed; make sure the boat is flat when head to wind. As my body moves across the middle to the new wing, I’ll increase the rate of turn, steering quite aggressively through the eye of wind, but my body has to be ready to cross the boat and receive the power on the new tack. You need to come out tipped to windward.

Here’s how I break it down:

  • Ten seconds before tacking, consider slowing the boat slightly and sailing slightly higher.
  • Plan for a slow luff head to wind, followed by a faster turn rate once confident that crew weight can be positioned correctly.
  • Heel to windward before the tack but flatten the boat once head to wind. Move your weight inboard. It is important here to sheet the mainsheet on centerline so that it is not over the leeward wing when you pass underneath.
  • As you move across the middle of the boat, pass the tiller extension around the back and into a position halfway along the leeward wing.
  • When you are confident you can move to the new side, increase the rate of turn, landing on a wider than close-hauled course.
  • There are a lot of G-forces during this period, so expected to get ejected a few times!
  • Don’t try to piece the whole tack together immediately. Practice the first phase of luffing head to wind, getting your weight to center, and switching the tiller extension across.
  • Remember to ease mainsheet out of the tack and have your hands positioned so that the sheet can be eased quickly. The sheet is more important than the tiller on exit.
  • I set my mast rake to allow 70 to 75cm between the boom and the deck where I cross the boat. Tacking is important, so make sure you have room under the boom! For tacking practice, I recommend having the rig more upright.
  • Don’t adjust any control lines right before or after tacks, unless it is very light and foiling is marginal.

Name 5 key gear changes in a Moth

  1. To foil early in light air, power up by having the vang and cunningham as loose as possible. However, it is easy to over-deepen the front of the sail, requiring further bear-away to get good flow on the sail. Don’t overdo it.
  2. Downwind always requires a deeper sail shape than upwind, regardless of wind speed.
  3. Once foiling, immediately pull on vang to stabilize the sail and cunningham to clean the creases. Even though the wind is light, we are quickly doing 14/15kts. The AWS is high so we need to change sail shape to achieve good speeds.
  4. As the breeze builds, constantly increase vang and cunningham loads to achieve the desired sail shapes. Max vang should be achieved in 15 knots and max cunningham in 18 knots.
  5. The Moth gets overpowered quickly, and from 18 knots upwards a softer tip or different sail is required. North’s LA and DS designs have a slight variation in the luff rounds to cover the range, with the DS being aimed at 14 knots true wind speed. For lighter sailors, a softer-tip allows the mast to bend earlier and depower the sail.

How do you start a race in a Moth?

Starts are a bit chaotic. Even for long-time racers, a Moth start will be a new experience. People get up on their foils with about 25 seconds to go and do a big turn-up at about 3 seconds, crossing the lining at 20 knots. A key thing when you’re on your foils is to have the ability to slow down while maintaining control. You need to practice that. Getting to the pin too soon is dodgy, especially with boats starting on port tack.

Why do so many Moths start on port tack?

Moths travel at 15 to 18 knots upwind, so tacking is a very costly part of an upwind leg. Reducing the number of tacks during a beat is very efficient, although this depends somewhat on how well you tack and how confident you are in making your tacks.

How many ways can you capsize a Moth?

You will capsize a Moth every possible way when learning. But I can sail for days now without capsizing, unless it is windy and choppy. The key to recovering quickly is to get on the daggerboard and do a dry capsize, climbing back over the wing. Sometimes that’s hard to do, especially when you pitchpole and find yourself 20 meters out in front of that boat.

What is the most common boat-handling mistake in a Moth?

Capsizing! Foil tacking, in particular, is very difficult. A common mistake is trying to foil tack when it’s breezy. Drag is so high that as you go through head to wind, everything slows down and it’s easy to capsize. You may be better off planning to do a touch-and-go tack.

Read More – Moth Newbie: Dan Neri learns to foil