How does a chine work?

[Jim Cate]

We are seeing lots of new designs with fairly hard chines in their aft sections. There are claims of improved hydrodynamics, primarily in increased stiffness. I find this to be non-intuitive, and would appreciate knowledgeable input on the actual means by which this design feature works (or not).

Thanks for any informed input!

Jim

[Weyalan]

Wild-ass speculation here, but given the narrowness(fore-aft) of the blade on a modern bulb keel, I could see how a hard chine, when heeled, could serve to prevent leeway / side slipping...

[scuba0_1]

According to a seminar we went to the newer boats are using them because tnt bottoms are not as round and the chines serve as another rudder. So they say.

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[Delancey]

International 14's in the mid-late eighties when people started planning uphill seems like when the chine (re)appeared purposefully. I always thought it was a planning feature/wetted surface thing.

[Polux]

Jim Cate:
Quote:

Say Pollux, I'm curious: do you think that the chine has any practical effect on the hull hydrodynamics? I find it hard to understand its function, other than as a styling statement. Folks talk about "getting down on the chine and becoming stiffer" and things like that, but I don't see how that works in the real world. If anything, it reduces the volume of hull submerged per degree of heel rather than increasing it compared to a smoother shape of heeled waterlines.

I note that the very wide race boats don't have such features...


Yes, that pretty much resumes the effect of a chine of that type: "getting down on the chine and becoming stiffer". When the boat heels till that chine, it will be necessary a much bigger force to sail over it, meaning to heel more than that, compared with what would happen if it was not there and the hull was rounded.

In practical terms helps to maintain the boat upwind on the most favorable heel angle (according to boat design), the one that is determined by that chine. Downwind makes the sailing much easier diminishing the roll and allowing an easy control of the boat. It makes the job of an autopilot a lot easier.

Regarding racing boats all solo racers use them by the reasons I gave: When a boat is solo sailed it can be sailed faster if it is easier to handle.

Regarding max performance on crewed racing boats most of them don't use them (I mean chines of this type). A rounded well designed hull allows bigger heeling and helps to maximize the effect of the ballast and the weight of the crew on the rail. Downwind the boat is at fast but much more dificult to sail, needing a dynamic delicate balance of sails and crew weight. The exception are boats designed to be crewed in extreme conditions, day and night extensively where the easiness of control can have a speed advantage over a needed all time downwind control. That's the case of the VOR:

http://i804.photobucket.com/albums/y...ps33d5a0db.jpg

Originally Posted by Jim Cate

We are seeing lots of new designs with fairly hard chines in their aft sections. There are claims of improved hydrodynamics, primarily in increased stiffness. I find this to be non-intuitive, and would appreciate knowledgeable input on the actual means by which this design feature works (or not).

Thanks for any informed input!

Jim

I don't see them as non intuitive. I look at them and understand how they work in an intuitive way.

If you look to motorboats you will see that they use chines for the same reason, to increase stability, except that while on a sailingboat the chine is designed to work at the best optimal heel angle for a given design, on a motorboat is designed to prevent any heeling:







As I said on a sailing boat they are there mainly to make sailing easier and that's why they are used on all solo racers and that's why they made sense on cruising boats. An easier boat to sail with a short or solo crew is a faster boat.

Regarding This: "There are claims of improved hydrodynamics, primarily in increased stiffness." it is not a claim, it is a fact: they increase stiffness when the chine hits the water and if we try to pass over it the resistance increases as well as drag. That's why on boats were absolute performance is searched (top racing with top professionals) chines are not used or it is used a very high chine that allows the boat to heel a lot before it enters in function.

Regarding being useful or not in what regards cruising boats, it depends on the design, if it is a good one certainly they are in what regards to make the boat easier to control. If the chine is to high and only enters in function at high angles of heel, unless it is a cruiser racer, it will not make sense because a cruising boat is not normally sailed to those angles of heel.

On the Oceanis 38 case it is a low one that will enter in function at a relative low angle, the best angle to go upwind, that is a relatively small angle on that boat. It helps to maintain the boat "on a grove" upwind, it increases stability on a beam reach and will help controlling the boat downwind limiting roll.

if you ask me if it will make a big difference if instead the chine the transom had the same shape but rounded without a chine, I would say no, not a big difference but if it works better with a chine, a chine should be used. A better performance on a sailboat is attained by many little things, none of them having much influence per si but that all put together represent a lot.

[neilpride]

Regards regarding regards, a 360 roll scenario in the chines arse is a more violent and slow picture compared to a rounded bottom,

[Jim Cate]

Regarding This: "There are claims of improved hydrodynamics, primarily in increased stiffness." it is not a claim, it is a fact: they increase stiffness when the chine hits the water and if we try to pass over it the resistance increases as well as drag.

Although I wonder about the "fact" of increased stiffness, this is still not addressing my question. WHY does the chine increase stiffness? Why does a sudden change in the deadrise angle increase stiffness? I've heard a lot of performance claims, but no one has yet explained the principle on which the claim is based.

I can intuitively understand how the chine could increase yaw resistance, thus making the boat more directionally stable, but not how there is a magic effect when the chine approaches or encounters the surface of the water and increases the dynamic righting moment. If anything, my intuition says that as the chine is driven under the surface there is less hull volume immersed per degree of roll. This it seems to me would not increase stiffness, and is the basis for my desire for understanding.

Jim

[neilpride]

I guess, just guess, the chine and the flat panel offer a initial resistance to heel, pass a certain limit in the Stability curve and the Bang !!! is guaranted,, just in theory, early IMOCAS if i remember.... so yes they are a stiffer initially...

[Stu Jackson]

Originally Posted by Jim Cate

.......................This it seems to me would not increase stiffness, and is the basis for my desire for understanding.

Jim

Jim, I agree. My usual method of trying to find out if the initial answers don't "hit" my noggin right is to hop on Google!

[u4ea32]

On a non-planing sailboat, a chine is really just a styling gimmick. Its a really nice styling gimmick, in that it makes the topsides look lower, so the boat looks longer and sleeker. There is a reason cars have something like a chine -- feature lines or creases along the side of the car. Just looks better.

On a planing hull, it reduces wetted surface, but more importantly, dramatically improves roll stability.

It is true that a chine provides a tiny bit more stability than a curved topside.

However, a far, far bigger contributor to form stability is having no deadrise along centerline.

[StuM]

I have the same gut feeling as neilpride. Intially,as you heel, the flatter hull is resisting the heel. Once the chine is in the water, you get the opposite effect. So more initial stiffness up to the designed angle, but less beyond it.

[noelex 77]

Intuitively I would expect the chine to increase stability.

If you look at the possible underwater shape of a rounded hull section and a similar section with a hard chine the added buoyancy of the boat with the chine would produce more of a righting moment and increase form stability.

You can see how the chinned hull section (the lower blue line on the diagram) has more buoyancy and a wider effective beam at most angles of heel.
It also increases the volume aft for accomodation which I think might be its primary purpose.

Unfortunatly fuller aft sections make the boat less balanced as it heels and the added outboard bouyancy needs a very deep or twin rudder.

Other considerations are flat panels are less strong than rounded sections and the sharp chine intoduces an area that is more vulnerable to impact damage.

[StuM]

Unless it looks more like this:

[dana-tenacity]

There's nothing new under the sun. This is our new (to us) cruiser by John Spencer. Launched 1998 and I believe his last keelboat design before his death. Kind of a cruising version of Ragtime (Launched 1963? and with a chine)for you Americans.

[noelex 77]

You produce neater diagrams than me Stu

In the shapes you have drawn the chined hull would be stiffer between the two crosses. While it would be slightly less stiff at other angles a designer can still use chine with this sort of profile to stiffen up the boat just when it most needed say between 15-20 degrees.

A couple of points I didn't mention is that the flat sections are also slightly better at resisting leeway, but the immersed area and therefore skin friction is higher with a chined hull. The hull moulds are cheaper and easier to produce. Whether this is better overall means little if it fashionable and sells boats