Thread: Dynamic gust response

Hi Brian

The birds have it over us that's for sure. One day while climbing, my dad and I once witnessed some amazing flying by 2 crows above a cliff of the Drakensberg mountains at an altitude of about 10000 ft. These birds were just having fun, and even flying inverted. They were also hooking their claws and soaring in the vortex, one inverted and one right way up. We were so glad that we both saw this because people would have thought we were nuts!

The trouble is that whichever yard we make, the response for our sail can only be for a limited range of wind strengths and gusts. This is where the bird has it over us. It has muscle control as well, which our yards can't have, so we cannot match the bird.

My old bendy yard worked well up to about 8 knots, gusting 12. Above that, it was rubbish. This means that in order to work in all conditions, we have to make at least 3 yards of varying flexibility and have at least 2 sails (ideally 3) to match those spars.. This is a bit over the top, so instead I went with a stiff yard to match my sail and introduce some more control over the sail, ie. vang and sail tension from throat to peak. (Like you did on your scow)

So now my “gust response” has to rely more on me and less on the rig, ie:

1. Watch the water and anticipate the gust
2. Hike like hell just before the gust hits
3. Free the sheet by the amount I think is about right. (The strength of the gust anticipated from the shade of the ripples)
4. Enough downhaul to keep the luff taut, but only a little vang to allow the sail to twist off.

Compromises, compromises. It's all about compromise at the lower end of technology!

thanks for that Bruce.

As you say, we have just one yard, but we can vary the downhaul as wind increases as long as the blocks can still work under the increasing tension. Hence my pleading for people to use ball bearing blocks on the downhaul.

On your point three, about watching and seeing the gust arriving. If you want to keep the same course, then the increase in wind strength will free you and allow the sail to be set further out.

When racing though, for me it's all about sailing higher, with every possible opportunity taken to gain windward ground on the rest of the fleet. So, if I am already hiking as much as possible, then what I do is simply sail higher still, just taking the amount of power I can cope with. By sailing at a constant correct heel angle, luffing in gusts, bearing away in lulls, real gain are made on the fleet.

This is upwind sailing at it's best. Some struggle and heal too much, they have bad weather helm and slow even more. Meanwhile my little boat slips away from the fleet!

Brian

Originally Posted by keyhavenpotter

On your point three, about watching and seeing the gust arriving. If you want to keep the same course, then the increase in wind strength will free you and allow the sail to be set further out.

When racing though, for me it's all about sailing higher, with every possible opportunity taken to gain windward ground on the rest of the fleet. So, if I am already hiking as much as possible, then what I do is simply sail higher still, just taking the amount of power I can cope with. By sailing at a constant correct heel angle, luffing in gusts, bearing away in lulls, real gain are made on the fleet.

This is upwind sailing at its best. Some struggle and heal too much, they have bad weather helm and slow even more. Meanwhile my little boat slips away from the fleet!

Brian

Hi Brian, this is such a great topic!

We call the technique “pinching”. It’s quite fundamental to sailing small dinghies and some big keel boats as well, but is surprisingly little used by a lot of helms. If you ever crew for a helm that doesn’t use the technique in a blow can be very frustrating!

However, it does not work with all boats. In high performance boats with lots of power and leverage like cats, most skiffs, and Moths, pinching is a performance killer. With most dinghies sailed from the gunwale, pinching is a fundamental. However, the good helms also understand the dynamics of a gust and so are able to predict with reasonable certainty which side of the gust they are on and whether they will be headed or lifted. If it’s only a few degrees, it doesn’t matter too much, but on some inland waterways the gusts can arrive like a huge fan and either head or lift through as much as 45degrees. Being able to reasonably predict what a gust will do takes experience and sometimes local knowledge plays a big part as well.

But yes, you’re right, there are times where in certain boats you can use pinching up in gusts to help reduce power and gain height, and bearing away in the lulls without affecting speed too much because you’re keeping the boat flat or at its optimum heel angle. It’s hard work, which is why “working” is another name for beating and close hauled!

I always like Eric Twiname's (probably the best writer ever on how to sail well) example about boat heel.

That you take someone who knows nothing about sailing out on the patrol boat to watch a race and ask them to note the differences between the way the boats at the front sail and the ones at the back.

"they are heeling less" is the usual observation - even though some of the crew weights at the front of the fleet might be lighter. Bruce (and you) have it down pat as experienced sailors.

Years ago in the early days of sailboarding I watch an international slalom event in surf off Merimbula beach. Most guys were from windsurfing/waveriding backgrounds. The exception was Bruce Kendall, a NZ sailor of high repute who had come into sailboards from the racing classes. You could instantly see the difference - during each gybe he never lost power - he'd obviously spent a lot of time practicing mark roundings.

The others, even though good sailors on the straights lost out badly in the corners - imprecise curves, excessive carving in the turns, stalling, slow sail handling, hitting the exit at the wrong angle to reaccelerate.

Kendall - well you could have set music to what he was doing. He ended up winning every one of his heats and winning that section by a considerable margin.

The point is ... anyone could see the difference instantly even if they knew nothing.

Good observation is highly useful even from the shore.

Best wishes
MIK

Originally Posted by Boatmik

I always like Eric Twiname's (probably the best writer ever on how to sail well) example about boat heel.

That you take someone who knows nothing about sailing out on the patrol boat to watch a race and ask them to note the differences between the way the boats at the front sail and the ones at the back.

"they are heeling less" is the usual observation - even though some of the crew weights at the front of the fleet might be lighter. Bruce (and you) have it down pat as experienced sailors.


MIK

How do the mini transat type boats work in this regard ? Are they not designed to be sailed heeled when on the wind ?

Cheers
JDH

Hi JDH

I imagine it's a trade-off. Basically they are just big skiffs and they will be fastest when sailed flat, so they will be reefed and have the canting keel trimmed accordingly. They are designed to be downwind flyers, but they still seem to do a damn good job upwind.

Howdy,

In a sense the mini transat and the open60s are being built with massive amounts of beam compared to boats previously. Some of the open60s have been about 22ft wide.

If they were sailed flat the massive amount of wetted surface would make them very slow. But it gives them a lot of stability and they are sailed heeled with a large part of the hull out of the water. Makes water ballast work really well!

Also weather routing can usually keep these boats in strong winds most of the time where there is enough power from the sails to overcome any extra wetted surface. They are placed in the middle of strong winds and sail faster than the weather systems to keep finding optimum conditions as they jump from one weather system to another. Almost like surfing waves in a dinghy where you use the speed on one wave to position yourself for a fast ride on the next - this case the "waves" are the weather systems.

The other aspect is that length is restricted in both these classes. So how to get more power?

Go wide. Power increases with something like the square of the beam. Make a boat 40% wider and it will be twice as stable and be able to carry twice the sail area.

With the canting keel maxis the opposite approach is evident. They use a 250hp diesel to cant their massive keels from side to side. So a lot of their stability can be invested in the keel bulb. As a result they tend to have quite a skinny hull to reduce wetted surface. Also they don't have any length restriction and length is good for speed if you have enough power to drive it.

They have to go this way because they sail reletively short courses where there is not much geographic choice of path. The 600 miles of the Sydney to Hobart, they can't go further WEST because of the land and can't head out East a long distance in chase of a weather system because the systems are not going the right way (they go East, rather than South to Hobart) - hard to catch waves when they are broadside to the boat! So they have to accept at least in part the strong and weak wind conditions that come their way.

So the Mini Transats and Open60s are really scows with a pointy bow with the assumption they will be sailed very heeled upwind. Note how some are starting to show a chine in the topside to get water release and stability when heeled and travelling fast and to provide a less distorted bilge shape when heeled so the boat is easier (relatively) to steer.

The big joke of course is on the people using either of these approaches for boats for the rest of us such as some of the big beam and canting keel cruiser racers. The above shows it is a flawed approach - reliant on a lot of equipment and very big sails and creating a boat that bounces around a lot more than a more modest hull.

The Maxi hull approach is better, but who can afford it, or who wants a 250hp diesel banging away in the cabin whenever you are sailing?

Best wishes
MIK

Hey MIK

Did you happen to bump into local sailboarding identity (now deceased) Algie when in Merimbula.
Great place by the way - I'm there now.

Picko

Hi Picko,

I was only a day visitor - mostly I've sailed boats with rudders - though there was some great racing Wednesday Afternoons on the Harbour at Iron Cove.

I was in Mooloolaba because my brother was photographing some of the event from in the water - IN the water - he did a bit of that back in those days.

Best wishes
Michael