How does a chine work?

[Polux]

Yes,a good write up by a NA even if I disagree on some small points regarding the use of chines on racing boats, not because what I know regarding theory but regarding what I see top NA doing using on contemporary winning racers regarding chines. Farr is being a bit outperformed on the last years regarding that and even if the VOR 65 is designed by him, it is good to remember that his boats lost the two last Volvo Ocean Races. Not many using Farr to design top offshore racing boats these days.

But regarding cruising boats, with the exception of fast performance cruisers that are designed to plane or semi-plan I mostly agree. Beside fashion trends linking them to the images of fast racers and regarding sailing it is about this:

.. there are some advantages in allowing wider total beam aft.. cruising boats, chines can be complementary to the function of twin rudders. .. cruising boats often appreciate the control that twin rudders and chines provide when the boat is overpowered. For these heavier cruising boats there is a drag penalty due to the chine but it allows a big increase in form stability ... This keeps the low speed and low heel drag manageable while taking advantage of the control and righting moment of the chines at higher heel angles. The increased form stability also helps us achieve stability requirements without piling on keel weight that is a significant cost center.
..Ian Gordon,
http://sailinganarchy.com/2014/12/12...-on-the-chine/

This is what I have been saying here: on a cruising boat chines are about increasing control making boats easier to sail and about increasing stability without increasing beam while limiting heel. A more stable boat and a boat that heels less is a more safe boat to go forward to the mast, for instance.
When the chine goes underwater, even if the sailboat resist to heel more, it will be time to take a reef to go faster while diminishing drag.

It could give the idea that after all chines make cruising boats slower and that eventually will be the case if they are raced by a top crew that can control perfectly the boat in all demanding situations but, like it happens on solo racers, on a cruising boat that is almost never the case (being slower) and with a small crew with average sailors the extra control the chines and twin rudders give will allow a much easier exploitation of the boat sailing abilities and that would be translated in an average bigger speed and also on more safety and easier sailing.

[Dockhead]

Originally Posted by Polux

. . . A more stable boat and a boat that heels less is a more safe boat to go forward to the mast, for instance.
When the chine goes underwater, even if the sailboat resist to heel more, it will be time to take a reef to go faster while diminishing drag.

The other reason for reefing at the point is that form stability, unlike ballast stability, stops working suddenly at a certain angle of heel. Once you reach the chine, that's the end of the curve of the bottom, and righting moment starts to fall off rapidly. It's dangerous to sail a boat designed like this at a large angle of heel. The whole purpose of the flat bottom is to let you work the forces on the sails against form stability -- which is greatest when the boat is sailed flat.

[Dockhead]

Originally Posted by clockwork orange

. . . I would think that if you wanted to actually keep the chine clear of the water an inclinometer would be one of your most important instruments.
. . .

I think it's one of the most important instruments on any boat.

Sailing upwind I always have one Triton display set to show heel angle and a graph showing heel angle over the last 30 minutes.

The other really important indicator is rudder angle. I wish the Tritons had a screen showing this and heel angle on one screen.

You could probably use either one as a decent proxy for trim imbalance or overpowering, but the two together are really informative. Not really necessary if you're standing behind the wheel and hand steering, but passage making under sail upwind and always under autopilot it's really essential in my opinion.

Like others, I bet, I had a lot of trouble learning how to sail a boat with a modern hull form correctly. One reason why I changed all my standing rigging last year was I didn't realize until I had been sailing my present boat for two years that I had been sailing her overpowered all the time. Because I was used to older-fashioned hull forms which like the rail in the water. I had a hell of a time figuring out why I had such strong weather helm no matter how the sails were balanced. The light bulb finally went off when I tried sailing upwind once with the mainsail not reefed, but actually put away completely, and I still had excessive weather helm. I realized the helm force was a linear function of heel angle -- totally different from how it works on old-fashioned hulls, where fore and aft sail balance is key to this.

Modern boats need to be sailed flat. Mine 20 degrees or less; ones with chines and wide flat bottoms aft even less.

[UNCIVILIZED]

Originally Posted by Dockhead

Modern boats need to be sailed flat. Mine 20 degrees or less; ones with chines and wide flat bottoms aft even less.

This has pretty much been the standard for several decades now actually. It's a BIG part of why I'm always harping about getting, & reading, the Polars for one's boat. As optimum heel angles are included in any proper set of them.

[Dockhead]

Originally Posted by UNCIVILIZED

This has pretty much been the standard for several decades now actually. It's a BIG part of why I'm always harping about getting, & reading, the Polars for one's boat. As optimum heel angles are included in any proper set of them.

Yes, well, my previous boat would be more than 30 years old by now Full skeg rudder, fin keel but just barely -- very long and with no bulb, very round and deep aft sections, deep forefoot. And a pig to sail, I might add, with all that wetted surface. It was tuned to sail at 30 degrees, and the waterline length increased with heel. So if you wanted to sail at what passed for fast on that boat, you had to pile on the canvas and get her heeled over.

[Polux]

Originally Posted by Dockhead

The other reason for reefing at the point is that form stability, unlike ballast stability, stops working suddenly at a certain angle of heel. Once you reach the chine, that's the end of the curve of the bottom, and righting moment starts to fall off rapidly. It's dangerous to sail a boat designed like this at a large angle of heel. The whole purpose of the flat bottom is to let you work the forces on the sails against form stability -- which is greatest when the boat is sailed flat.

It depends on the boat. It is true that on the Oceanis 38 for instance there will be s sudden heel after one takes enough efforts to pass that chine, as we can see on the video I posted but it is not dangerous only stupid since the boat will lose a lot of performance.

You imply that the the Max righting moment of the boat (the heel angle where the boat makes a bigger effort to right itself up) is near the angle the chine enter s the water. It is not the case, the max RM on that type of boats is normally somewhere between 50 and 60º while the heel angle where that chine gets immersed is about 20º. Even much more heeled then 60º the boat is making a superior RM than the one that is making at 20º. The average AVS of that that type of boat is not different from the Average AVS of more narrow, different typed main market cruisers and it will be around 120º.

[Polux]

Originally Posted by Dockhead

...
Like others, I bet, I had a lot of trouble learning how to sail a boat with a modern hull form correctly. One reason why I changed all my standing rigging last year was I didn't realize until I had been sailing my present boat for two years that I had been sailing her overpowered all the time. Because I was used to older-fashioned hull forms which like the rail in the water. ...
Modern boats need to be sailed flat. Mine 20 degrees or less; ones with chines and wide flat bottoms aft even less.

On that one I agree 100%. Even if mine has not all beamed pulled aft I am still being surprised as many times when I reef the boat I get a better speed and much less efforts everywhere, including the helm. A big difference to my previous boat that was a 2002 design. On a boat with chines that will be even more noticeable.

[Dockhead]

Originally Posted by Polux

It depends on the boat. It is true that on the Oceanis 38 for instance there will be s sudden heel after one takes enough efforts to pass that chine, as we can see on the video I posted but it is not dangerous only stupid since the boat will lose a lot of performance.

You imply that the the Max righting moment of the boat (the heel angle where the boat makes a bigger effort to right itself up) is near the angle the chine enter s the water. It is not the case, the max RM on that type of boats is normally somewhere between 50 and 60º while the heel angle where that chine gets immersed is about 20º. Even much more heeled then 60º the boat is making a superior RM than the one that is making at 20º. The average AVS of that that type of boat is not different from the Average AVS of more narrow, different typed main market cruisers and it will be around 120º.

Righting moment from form stability disappears suddenly when you run out of bottom curve and will produce negative righting moment after a certain point. The extent to which ballast stability makes up for these effects depends on the CG of the specific boat. A deep keel and a heavy torpedo bulb of course, especially if what is above is all light, can have a huge positive effect on stability.

The point is that the initial stiffness produced by form stability does not translate into ultimate stability.

[clockwork orange]

On many boats regardless of chines or not, performance would be better if folk would just sail them flatter and of course when racing with a full crew they attempt to do so with movable human ballast rather than reduce sail but you cant do this shorthanded or cruising so reduce sail. Ive never had any fancy electronics but the little liquid inclinometers work fine and can be used for heel or trim for low bucks. Every boat has an optimum heel angle (and trim) that presents the lowest wetted surface to the water and in light air sail area to wetted surface is most important.


Steve.

[clockwork orange]

Originally Posted by Dockhead

Righting moment from form stability disappears suddenly when you run out of bottom curve and will produce negative righting moment after a certain point. The extent to which ballast stability makes up for these effects depends on the CG of the specific boat. A deep keel and a heavy torpedo bulb of course, especially if what is above is all light, can have a huge positive effect on stability.

The point is that the initial stiffness produced by form stability does not translate into ultimate stability.

Don't forget that most of these chine raceboats use canting keels which of course would also help keep them at optimum heel angle.

Steve

[Polux]

Originally Posted by Dockhead

Righting moment from form stability disappears suddenly when you run out of bottom curve and will produce negative righting moment after a certain point. The extent to which ballast stability makes up for these effects depends on the CG of the specific boat. A deep keel and a heavy torpedo bulb of course, especially if what is above is all light, can have a huge positive effect on stability.

The point is that the initial stiffness produced by form stability does not translate into ultimate stability.

When you talk about a boat stability you cannot separate righting moment from the hull form stability from the one that come from ballast on the keel.

A stability right moment curve shows you the combined effect of both and I can assure you that those boats don't have a "strange" stability curve with a big loss of stability when the boat is having less hull form stability and more stability from the keel (that provides an increased stability at higher heel angles, with the max between 50 and 60º). Here you can have a look at the Pogo 12.50 stability curve, the boat with chines I posted before.

There is no negative stability unless passed the AVS point, with the boat inverted and the stability grows progressively till the MAX RM at a much bigger heel angle to the one the chine goes immersed (about 20º).

The designer is the same of the Oceanis 38 and even if this is a more sportive boat with a bigger B/D ratio, the curves would not be much different in shape, only on this one with probably a slightly better AVS and a better final stability. The first part of the curve, till Max RPM will be similar in shape.

[Polux]

Originally Posted by clockwork orange

Don't forget that most of these chine raceboats use canting keels which of course would also help keep them at optimum heel angle.

Steve

Regarding solo racers, were most of this type the chines are used on racing boats, only the IMOCA (open 60's) and the proto class on the mini racers use canting keels, all the others, that are in much bigger number, have them forbidden by the rules (Class 40, series mini racers and Class 950).

[Dockhead]

Originally Posted by clockwork orange

On many boats regardless of chines or not, performance would be better if folk would just sail them flatter and of course when racing with a full crew they attempt to do so with movable human ballast rather than reduce sail but you cant do this shorthanded or cruising so reduce sail. Ive never had any fancy electronics but the little liquid inclinometers work fine and can be used for heel or trim for low bucks. Every boat has an optimum heel angle (and trim) that presents the lowest wetted surface to the water and in light air sail area to wetted surface is most important.


Steve.

The problem with "just reducing sail" is that once you even barely start reefing a roller furling headsail, the shape goes to hell and you lose tons of performance. So you have a choice of losing performance because you are heeled too far over (and your rudder is acting like a brake) or losing performance because your headsail is reefed. Not an attractive choice.

The only way out is to change down the headsail to another size, but why have roller furling gear in the first place if you're going to have a sail inventory? How many cruisers can manage sail changes under way?

And this is exactly why large SA/D ratios, great for racers, or maybe racer/cruisers, are wrong for cruising boats, except perhaps in latitudes where you rarely experience good breezes.

Up here with our good wind, the typical racer cruiser with SA/D of 20 or even more, which will fly with a full crew and sail inventory, with a short-handed family crew, on the other hand, struggles all the time.

My boat is made for these conditions, with very light displacement for her size (under 190 D/L), and modest sail plan (16.5), so is hard to beat in our typical 20 - 25 knot breezes. But even so, the sail area is still too much for many days, which is why I decided to bite the bullet and have a high aspect blade jib made for my boat, out of carbon laminate, which should push up the useful wind range 4 or 5 knots, I hope.

The big question is whether to put it on a separate Solent stay or hoist it in the main furler, a PITA with sails this size and not realistic short-handed at sea.

[Dockhead]

Originally Posted by Polux

When you talk about a boat stability you cannot separate righting moment from the hull form stability from the one that come from ballast on the keel.

A stability right moment curve shows you the combined effect of both and I can assure you that those boats don't have a "strange" stability curve with a big loss of stability when the boat is having less hull form stability and more stability from the keel (that provides an increased stability at higher heel angles, with the max between 50 and 60º). Here you can have a look at the Pogo 12.50 stability curve, the boat with chines I posted before.

There is no negative stability unless passed the AVS point, with the boat inverted and the stability grows progressively till the MAX RM at a much bigger heel angle to the one the chine goes immersed (about 20º).

You keep misreading what I write. I said -- the effect of form stability becomes negative, not that the sum of form and ballast stability becomes negative. That -- as I said -- depends on the boat. As I wrote, a deep keel with a heavy bulb will compensate any such effect, which is exactly what is done on all of these boats.

To see how form stability works without any compensating ballast stability, all you have to do is look at dinghies without ballasted keels and their AVS. nearly all of their stability is form stability.

[Polux]

Originally Posted by Dockhead

The problem with "just reducing sail" is that once you even barely start reefing a roller furling headsail, the shape goes to hell and you lose tons of performance. So you have a choice of losing performance because you are heeled too far over (and your rudder is acting like a brake) or losing performance because your headsail is reefed. Not an attractive choice.

The only way out is to change down the headsail to another size, but why have roller furling gear in the first place if you're going to have a sail inventory? How many cruisers can manage sail changes under way?

And this is exactly why large SA/D ratios, great for racers, or maybe racer/cruisers, are wrong for cruising boats, except perhaps in latitudes where you rarely experience good breezes.
....

A sail can be furled to a point without a being loss on performance. I agree that over that point the sail will have a bad performance. The ones that have fast boats will change sails (on the furler) according with the place or season where they are sailing. For instance on Italy on the summer I sail with a 140% genoa on the furler, but already after being in Greece and before going to the Cyclades or Dodecanese islands, where normally it blows from F4 to f8, I change the the big genoa on the furler by a jib.

I can sail with full jib till F7 (upwind). With more I furl the jib.

Many boats have also two stays with two furlers with different sized frontal sails. I have a furler and a removable stay but I am in the process of installing two furlers.