What factors determine the maximum speed of a sailing dinghy?

[SailTheMoon]

I haven't delved into the physics behind it, but I understand that for a displacement hull, speed is often limited to the hull speed. How does this apply to dinghies specifically?

 

[barca nova]

"hull speed" isnt a fixed limit so much as the point at which the power needed to go faster starts to rise very fast. But the modern dinghies that I see ( I am not a dinghy sailor these days) look designed to plane in the right conditions and then "hull speed" doesnt apply. The old style day boats that I sailed in the 60s werent designed to plane

 

[seivadnehpets]

"hull speed" isnt a fixed limit so much as the point at which the power needed to go faster starts to rise very fast. But the modern dinghies that I see ( I am not a dinghy sailor these days) look designed to plane in the right conditions and then "hull speed" doesnt apply. The old style day boats that I sailed in the 60s werent designed to plane

You can get any boat to plane with enough power, but a boat that isn’t designed to plane is going to be unstable. My old mirror was a perfect example, the rudder just wasn’t long enough, and if it had been, the tiller wouldn’t have been strong enough, so without excellent sail balance it was always going to go over. Great fun, but not a reliable means of transport. I set it up with a reefable jib, so that I could achieve this sail balance with reefed main, but I moved on to other boats before I thoroughly tested this.
I am now sailing a much heavier Beaufort, that isn’t considered a planing boat, but when it begins vibrating like a nun on heat, you know it’s lifting a little. The extra weight makes it a bit more stable.

 

[Chiara’s slave]

The mirror is very much a planing dinghy. If you’re a larger gent its going to need some breeze. Rudder wise, your blade may not have been fully down. If it was home built, maybe incorrectly shaped. They ate fine little boats. I’d also take issue with the ‘any boat can’ bit. Try an X one design. I’ve had one with the kite up in 30kn of wind, and whilst it was many things, it was not planing. Nor would it in any wind. Nor with any engine that wouldn’t sink it weight wise. They will exceed hull speed by a small margin, they will surf. No planing. Its not that unusual.
It used to be said that hull speed would be 1.4 x wl in feet, answer in kn. Not completely true, but in the ball park. Exceptions apply though. It’s all about wavelengths, and the speed they move. If a hull doesn’t drag a wave, the limit is different. One way of getting around it is a narrow hull. Less than round about 8:1 length/beam, like a catamaran hull. Dart cats don’t plane, but they’ll comfortably top 15kn. These days most hulls are designed to plane. How easily is the question. Before planing happens, think of a hull following the displacement rules.

 

[seivadnehpets]

The mirror is very much a planing dinghy. If you’re a larger gent its going to need some breeze. Rudder wise, your blade may not have been fully down. If it was home built, maybe incorrectly shaped. They ate fine little boats. I’d also take issue with the ‘any boat can’ bit. Try an X one design. I’ve had one with the kite up in 30kn of wind, and whilst it was many things, it was not planing. Nor would it in any wind. Nor with any engine that wouldn’t sink it weight wise. They will exceed hull speed by a small margin, they will surf. No planing. Its not that unusual.
It used to be said that hull speed would be 1.4 x wl in feet, answer in kn. Not completely true, but in the ball park. Exceptions apply though. It’s all about wavelengths, and the speed they move. If a hull doesn’t drag a wave, the limit is different. One way of getting around it is a narrow hull. Less than round about 8:1 length/beam, like a catamaran hull. Dart cats don’t plane, but they’ll comfortably top 15kn. These days most hulls are designed to plane. How easily is the question. Before planing happens, think of a hull following the displacement rules.

I could plane the mirror with balanced sails, but would often sail in stronger winds with reefed main, and then she was impossible to control when she lifted.

 

[westernman]

To go faster:-

• Less weight
• More righting moment (e.g. catamaran and heavy guy on a trapeze well away from the centre line)
• Lots of sail area
• Narrow hull(s) (e.g. catamaran)
• Planing hull shape, or better foils to get the hull completely out of the water
• Plenty of wind
• Flat water (and enough of it)
• Crew with steel balls

Looks like this (dinghy):-

or this (catamaran):-

 

[Chiara’s slave]

I could plane the mirror with balanced sails, but would often sail in stronger winds with reefed main, and then she was impossible to control when she lifted.

I’ve never found a need to reef a Mirror, and I’m not particularly large. As a teenager, I was probably 8-9 stone, with my sister as crew. Sailed them on and off for years, my own children learned on them too.

 

[seivadnehpets]

I’ve never found a need to reef a Mirror, and I’m not particularly large. As a teenager, I was probably 8-9 stone, with my sister as crew. Sailed them on and off for years, my own children learned on them too.

I really only intended to make the point that a planning boat needs more careful design and control as it is less stable than a boat sat in the water.

 

[ylop]

Westernman’s list is pretty comprehensive but he missed an essential point: a crew who actually know how to work the sails and boat to the best advantage. A very good sailor in a mediocre design of boat may well be faster than a ropey sailor in a racier boat. People spend thousands upgrading boats or moving between classes when they actually just needed to invest some time in training!

 

[Baggywrinkle]

Wind speed and the nut on the tiller

 

[dunedin]

I could plane the mirror with balanced sails, but would often sail in stronger winds with reefed main, and then she was impossible to control when she lifted.

We raced Mirrors with full sail plus spinnaker in all weathers with two very light weight kids. Only capsized 3 times in 2 very busy seasons - and one was in huge waves when other bigger boats had abandoned racing.
But a lot was about preparation and technique. Bin elastic rudder downhaul and pull rope downhaul bar tight.
Release all controls before pinning gunter (in those days) bar tight on mast, only then pull on downhaul, then outhaul then kicker.
Well mannered boat.
Later happily sailed West Eleven with family of 5 round Gulf de Morbihan.

 

[Ed Sned]

The mirror is very much a planing dinghy. If you’re a larger gent its going to need some breeze. Rudder wise, your blade may not have been fully down. If it was home built, maybe incorrectly shaped. They ate fine little boats. I’d also take issue with the ‘any boat can’ bit. Try an X one design. I’ve had one with the kite up in 30kn of wind, and whilst it was many things, it was not planing. Nor would it in any wind. Nor with any engine that wouldn’t sink it weight wise. They will exceed hull speed by a small margin, they will surf. No planing. Its not that unusual.
It used to be said that hull speed would be 1.4 x wl in feet, answer in kn. Not completely true, but in the ball park. Exceptions apply though. It’s all about wavelengths, and the speed they move. If a hull doesn’t drag a wave, the limit is different. One way of getting around it is a narrow hull. Less than round about 8:1 length/beam, like a catamaran hull. Dart cats don’t plane, but they’ll comfortably top 15kn. These days most hulls are designed to plane. How easily is the question. Before planing happens, think of a hull following the displacement rules.

Sorry but being a pedant, it's 1.4 x square root of waterline length for displacement hull speed
It is of course an approximation, and the 1.4 bit if often flexed for different types of craft - entry and exit angles and shapes make a big difference - but its does stand up to a reasonable amount of empirical testing (in a former life as a naval architect, I spent a fair bit of time around towing tanks).

 

[dunedin]

Of course nowadays the fastest dinghies are on hydrofoils. And then the speed is not constrained by size as much as weight and windage - as little as possible of both.
Hence the tiny 11 foot International Moth is massively faster than much bigger planing dinghies, even the exotics like the Olympian 49ers.

 

[Chiara’s slave]

Sorry but being a pedant, it's 1.4 x square root of waterline length for displacement hull speed
It is of course an approximation, and the 1.4 bit if often flexed for different types of craft - entry and exit angles and shapes make a big difference - but its does stand up to a reasonable amount of empirical testing (in a former life as a naval architect, I spent a fair bit of time around towing tanks).

Thanks. I wrote the sq root bit in my head, but not on the screen. Thanks for the correction, not pedantic at all.

 

[Chiara’s slave]

Of course nowadays the fastest dinghies are on hydrofoils. And then the speed is not constrained by size as much as weight and windage - as little as possible of both.
Hence the tiny 11 foot International Moth is massively faster than much bigger planing dinghies, even the exotics like the Olympian 49ers.

And massively more difficult to sail. 49ers are for mortals, Moths are for demi gods, and extremely optimistic chaps.

 

[dunedin]

And massively more difficult to sail. 49ers are for mortals, Moths are for demi gods, and extremely optimistic chaps.

Times change and different skills.
People thought a Contender would be impossible to sail, as single handed trapeze. Then the likes of the Musto Skiff came along and added a huge spinnaker. And plenty of people - all massively more talented than me - mastered the Skiff in all conditions.
Not convinced (from the comfort of an armchair) that a Moth is necessarily more difficult to sail than a Skiff, or 49er, just different.

 

[ylop]

And massively more difficult to sail. 49ers are for mortals, Moths are for demi gods, and extremely optimistic chaps.

There’s a small nest of Waszps in Oban Bay, which seem to have a lot of fun at quite impressive speeds. I struggle enough working sails never mind foils you can’t see but I have often wondered if these are as hard to sail as Moths or have they managed to make foiling accessible for mere mortals?

 

[westernman]

Of course nowadays the fastest dinghies are on hydrofoils. And then the speed is not constrained by size as much as weight and windage - as little as possible of both.
Hence the tiny 11 foot International Moth is massively faster than much bigger planing dinghies, even the exotics like the Olympian 49ers.

But you can't beat a big and light catamaran on hydrofoils with a dinghy on hydrofoils.

 

[oldbloke]

Sailing a hydrofoil Moth is fairly easy. Turning round and coming back is another matter

 

[dunedin]

But you can't beat a big and light catamaran on hydrofoils with a dinghy on hydrofoils.

Well actually, you probably can - a cat is potentially just extra windage once foiling, albeit more stable and therefore easier to sail during the transition stage.
The SailGP use cats, but very specialist ones - but again perhaps more for stability pre foiling, and width of foil base. Americas Cup moved to monohull, but with very complex moveable ballast.
A monohull Moth with moving ballast of the sailor is probably faster than a cat would be.

If could stay airborne, fastest of all would be no hull, dropping the floats once airborne (and capsizing / sinking at end of race).

And of course kite foiling sets all the speed records - again showing big and multihull is not necessarily added speed.