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Old 08-12-2014, 01:48   #16
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Re: How does a chine work?

The short, KISS answer: Think on Dories, & how they get STIFF when you lean'em over. For more info, details & background, read on.

In terms of speed & drag, full, hard chines typically do add a tiny bit more drag than a rounded hull form of the same length. To some degree, just because of the extra wetted surface area, but...
Look at a round bilged boat from dead aft, & compare it to say, a tire. Now, look at a chined bilged boat from dead aft & compare it to a hexagonal or octagonal "tire". Which one's harder to get to roll, side to side?
And stability means speed, due to the ability to control more sail for a given boat size.
Trust me when I say, that on boats like the VOR's, they run them through software & modeling akin to that of spacecraft, to get those 0.001%'s of speed.

And, no, you don't have to be planning for chines to work. Go out on the water for a day with a good Lightning sailor, or take a Laser, or Thunderbird for a spin. The latter BTW still clean house in a LOT in PHRF fleets/racing. And even some designers were experimenting with chines back in the IOR days, like Brendan Dobroth. The 42' 2-tonner he designed for Ted Irwin for SORC, cleaned house. As she was controllable, finger tip like, at 15kts-20kts+ with a kite up, unlike a lot of boats of that era, thanks to her aft chines.

Plus, yes, chined boats are easier to get to plane. We're talking about (racing) sailboats that can outrun most RIBs, well, unless it's got twin 150's on the stern. The VOR's & other top tier racers hit north of 50kts @ times. So you've gotta' switch your thinking away from conventional lead mine designs. Although Cal 40's have chines, albeit slightly softer ones, & they're out of sight when the boats are in the water.

It even took the designers & builders a LONG time to shift their thinking over more towards making the shift more towards high performance power boats. Since the early/mid 80's, the big, fast boats have perpetually had troubles with core shear. It's not due to poor build quality or bad materials. It's simply the slamming loads being generated by powerboat like speeds, in boats built to older, sailboat, scantling rules.
Yes, they're sailboats (VOR's & such), but... their power to weight ratios, & speed, necessitates different designs. Say, semi-Donzi'ish ;-)


PS: And yeah, Sexy sells (which is the bottom line), so boats which resemble the latest hot racing machines...
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Old 08-12-2014, 03:45   #17
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Re: How does a chine work?

Quote:
Originally Posted by Jim Cate View Post
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
Thanks for asking this question, Jim.

I've wondered the same thing myself.

I think we need a real naval architect to tell us; no one here has answered the question. I don't have the answer, but I have a few speculations.

From what I have been able to find, chines on power boats are needed to direct the spray downward to produce more lift -- in order to help them plane. Some power boats have really exaggerated chines called "lifting strakes" -- name is self-explanatory.

Once the chines get wet, they lose this property:

"The loading situation a high-speed craft encounters in rough seas is commonly idealized as a two-dimensional transverse section of a ship-hull vertically impacting an initially calm water surface. This is illustrated for a v-shaped section in Figure 1. The pressure distribution on the hull is critically dependent of the local relative angle between hull and water surface, and the incident impact velocity. The pressures on the bottom-hull panels increase for increased incident impact velocity and decreased relative angle between hull and water. At small hull-water angles (< 5°) an air-cushioning effect
however results in decreasing pressures. During the impact, the water surface piles up close to the hull and forms a jet in the intersection between hull and water surface. The hydrodynamic pressure acting on the bottom-hull and the water surface pile-up close to the hull is schematically pictured in
Figure 1. The characteristics of the pressure distribution on the bottom-hull may be distinguished by a chines-dry stage and a chines-wet stage. In the chines-dry stage, the pressure distribution is characterised by a quite localized pressure-peak at the intersection between hull and water and a distinctly lower fairly constant pressure over the remaining part of the bottom-hull. In the chines-wet stage the pressure distribution
loses its peaked characteristics and may be described as a fairly uniform pressure distribution, which gradually approaches atmospheric pressure near the chine
.
"

http://www.dynalook.com/european-con...ter-impact.pdf


Some people say that chines increase resistance to heeling. It is true that flatter bottoms have higher initial resistance to heeling, and that will be obvious, I think, to everyone. A chine makes it easier to create a flatter bottom, so maybe people associate chines with flatter bottoms which offer higher initial resistance to heeling -- so the chine doesn't actually do anything itself, but is simply often present in designs with flatter bottoms.

Some people think that the chine itself creates resistance to heeling, but I don't see how that could be so, and couldn't find anything which explains why it would.

If a chine is sharp enough, I could imagine that at speed, it would somewhat separate the laminar flow across the hull and create a runner effect -- like a fin. This might tend to stabilize the boat along the longitudinal axis and dampen roll. Or I guess if the chine were not horizontal, but tilted down towards the transom, some hydrodynamic righting moment might be produced (but at the expense of drag). But I could not find any description of any such effect, and I doubt it would be strong at sailboat speeds and with the shallow chines sailboats have, if it exists at all.

A really fast boat which fully planes, like an Open 60, might use chines the same way as power boats -- to redirect the spray and create lift under the hull. This would not occur in normal sailboats which don't plane, and the really mild chines you see on some modern production boats probably wouldn't produce that effect even at 20 knots.


So, as I said, we need a real naval architect to give us a real answer, but I am guessing that chines on normal sailboats don't actually do anything themselves, but they are associated with wide, flat sections aft which DO have a number of effects including higher initial roll resistance.
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Old 08-12-2014, 04:29   #18
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Re: How does a chine work?

A chine changes the bouancy of the lee side of the boat per degree of heel. Just when the chine hits the water there is a slight decrease in this effect but further heel pushes the hard chine down and then buoyancy per degree of heel goes up fast. If the hull were perfectly round then buoyancy would be the same at any angle of heel and the ballast would be the only heeling resistance.

Also, once the chine is in the water it helps with lift so the boat can better resist leeway. The leeward side of the hull starts to look like a more aggressive foil. It isn't as efficient as the smooth turn of the hull but who cares about that when you have lots of wind. Racers want the boat to power up and take advantage of the excess wind.
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Old 08-12-2014, 06:13   #19
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Re: How does a chine work?

Quote:
Originally Posted by Dockhead View Post
... I think we need a real naval architect to tell us ...
From an earlier discussion:
Quote:
Originally Posted by bob perry View Post
... A chine on its own can help stabilty or hurt stability dependant on where it is and how you define "stability". Do you want stability at 10 degrees of heel or 30 degrees of heel?
Here ➥ Hard vs Multi vs Radius vs Round Bilge Chine
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Old 08-12-2014, 06:15   #20
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Re: How does a chine work?

IMHO it is a deco in the cruising context. Any potential benefits lost in obvious disadvantages of the cruising modality of the boat's equipment and crew.

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Old 08-12-2014, 07:59   #21
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Re: How does a chine work?

Jim...Bob Perry summed it up concisely.

In the 80s and 90s I worked with top Dutch naval architects on tank testing the performance curves for Super yachts.


These Architects were leading edge at the time with +40 knt design speeds of motor yachts like Octopussy.
http://www.superyachts.com/motor-yac.../octopussy.htm

In Tank testing our projects which were more normal speeds....What I observed was that using a chine to make the aft end more "barge shape" improved stability at anchor.

They then played with modifying chine shapes at 70% displacement which really had a dramatic affect on resistance and lift at normal hull speed of 15knts.

Tank Testing performance on 1:20 scale models..... for weeks...The goal was to position the lowest wave form that caried as far aft as possible , yet did not create a wave vortex right at the stern which would increase drag.

For powerboats this was all about reducing fuel consumption at given design speeds, but I think this also translates to sailing performance.

Chines do make a difference.
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Old 08-12-2014, 08:17   #22
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Re: How does a chine work?

Too bad John Spencer is not still alive to be able to say i told you so. Its funny that most racing dinghy classes have been hard chine for the last 70 years or so the world over and yet keel boats are just getting there.
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Old 08-12-2014, 08:25   #23
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pirate Re: How does a chine work?

Dunno about all this 'Techy Speak' but having owned a couple or three (wood) hard chine bilge keelers in the past all I can say is Chine is great.. upwind they'd heel till they hit the chine then it was rock solid.. downside slammed a bit.. but quite a few round hulls do that as well..
Downwind I'd regularly cream 9-10 metre round hulls in my Magyar 7.. so much so I'd get accused of running my inboard engine.. could not accept my 5hp OB hanging on the back was my only motor...
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Old 08-12-2014, 08:45   #24
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Re: How does a chine work?

Quote:
Originally Posted by u4ea32 View Post
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.

....
So, it improves roll stability on a racing boat and not on a cruising boat?

Regarding being better for planning it is not really relevant. Yes they help planning speeds on a short crew boat or with a not top crew because they make the boat more forgiving and easier to sail. Look at the TP52 or other very fast racing boats with a big planing ability and will see that almost all designers don't use chines on them and when they use it is not the type of chine low on the hull that limits roll or supposedly help to plan but one very high that only works at very high heel angles. That would be true, I mean helping planing but then we are not talking about this type of chines but the ones that were used on the 60's and 70's on plywood boats with hard chines at almost the waterline, a bit like motorboats.

Chines on solo boats or cruising boats work like the two small learning wheels on a bicycle: they just prevent major errors, not totally but they increase the safety margin and therefore a solo sailor or inexperienced crew can take more speed from their sailboats.

no chines on the TP52:

no chines on the fastest 40ft crewed racers (Ker 40):

Chines on allrecent solo racing boats designs:

Chines on fast cruising sailboats meant to be solo sailed:

Chines on modern cruising boats, here on an Oceanis 38:

You can see at min 2.20the chine working preventing more heel indicating the boat is near the limit (the wheel is not moving) and on min 2.58 what happens when those indications are not followed with a reduction of sail area and the heel passes over the chine: the boat heels much more very quickly till the Max RM prevents it to heel more. That is what I mean with the comparison with those two learning small wheels on a bicycle: if you rally try and lean over them, they will not resist forever too.
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Old 08-12-2014, 09:19   #25
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Re: How does a chine work?

Quote:
Originally Posted by Jim Cate View Post
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
When the chine immerses deeper it changes your center of buoyancy these damping the roll vs. a round bottom. I'm sure there are other pros an cons.. A smudge pot is another story.
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Old 08-12-2014, 09:23   #26
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Re: How does a chine work?

Quote:
Originally Posted by Polux View Post
So, it improves roll stability on a racing boat and not on a cruising boat?

Regarding being better for planning it is not really relevant. Yes they help planning speeds on a short crew boat or with a not top crew because they make the boat more forgiving and easier to sail. Look at the TP52 or other very fast racing boats with a big planing ability and will see that almost all designers don't use chines on them and when they use it is not the type of chine low on the hull that limits roll or supposedly help to plan but one very high that only works at very high heel angles. That would be true, I mean helping planing but then we are not talking about this type of chines but the ones that were used on the 60's and 70's on plywood boats with hard chines at almost the waterline, a bit like motorboats.

Chines on solo boats or cruising boats work like the two small learning wheels on a bicycle: they just prevent major errors, not totally but they increase the safety margin and therefore a solo sailor or inexperienced crew can take more speed from their sailboats.

no chines on the TP52:

no chines on the fastest 40ft crewed racers (Ker 40):

Chines on allrecent solo racing boats designs:

Chines on fast cruising sailboats meant to be solo sailed:

Chines on modern cruising boats, here on an Oceanis 38:

You can see at min 2.20the chine working preventing more heel indicating the boat is near the limit (the wheel is not moving) and on min 2.58 what happens when those indications are not followed with a reduction of sail area and the heel passes over the chine: the boat heels much more very quickly till the Max RM prevents it to heel more. That is what I mean with the comparison with those two learning small wheels on a bicycle: if you rally try and lean over them, they will not resist forever too.
I don't think the chines themselves resist anything. I have not seen any explanation of any reason why they should.

It is the radius of curvature of the bottom which creates the resistance to heeling or rolling. The flatter, the wider, the greater the resistance and longer lasting (respectively). Until you reach the edge, when there is no more buouancy so no more resistance. Whether the edge of the bottom is a chine or a radiused curve or just a cut-off edge doesn't make any difference as far as I can see.

This accords with what Bob Perry seems to be saying.
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Old 08-12-2014, 09:26   #27
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Re: How does a chine work?

This is a pretty complex subject and there are no firm answers. Ask any naval architect this question and they will answer "It depends on a number of factors. A hard chine does not in and of itself improve general stability".

I think in modern production cruising boats it's basically a styling conceit that also has a practical advantage; it allows the designer to bring the broad conical cross section as far aft as possible while meeting beam limitations for the boat, maximizing interior volume along the way. Since many modern "fat ass" cruisers have downwind weather helm problems due to the resulting asymmetrical waterlines, I suppose a hard chine can add to stability, but I'm going to posit that it's more a function of correcting that waterline profile than anything else but I ain't no naval architect.

That said, stiffness when you heel past the chine is going to decrease, because you're putting less hull and so less displacement in the water. But a chine, up to a certain modest immersion of the chine will increase stiffness because it has allowed you to broaden and flatten the conical cross section of the hull.
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Old 08-12-2014, 09:29   #28
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Re: How does a chine work?

Quote:
Originally Posted by Dockhead View Post
...
So, as I said, we need a real naval architect to give us a real answer, but I am guessing that chines on normal sailboats don't actually do anything themselves, but they are associated with wide, flat sections aft which DO have a number of effects including higher initial roll resistance.
Well, you need to be blind if you don't see their answer (NAs) regarding the use they make of them in cruising boats and racing boats and I am not talking about outdated Nas but about the ones that are on the cutting edge of modern NA. They are just using what they learned in racing boats on cruising boats, just some time after, not one of them but pretty much all of them.

Regarding scientific explanations I would say that a NA is more interested in practical evidence. They use computer fluid dynamics but are not interested on how it works like that, just that it works like that and that this or that way they can improve a sailboat performance.

Regarding modern chines, the type that are used on the Oceanis 38 it is a recent development, about 10 years and i still remember very well the boat that started it all. A wood/composite 40 class racer deigned by Marin this one:

He said that he had made studies that proved that chines improved stability and had advantages over the created drag. Nobody believed, including me, and all thought that he was making excuses regarding the limitations of the building technique that he had used. The two built boats had a very good performance and it turned out he was right...and the rest is history with all contemporary NA having a new look at the advantages of using that kind of chines on solo racers first and on cruising designs later.
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Old 08-12-2014, 09:34   #29
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Re: How does a chine work?

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I don't think the chines themselves resist anything. I have not seen any explanation of any reason why they should.

It is the radius of curvature of the bottom which creates the resistance to heeling or rolling. The flatter, the wider, the greater the resistance and longer lasting (respectively). Until you reach the edge, when there is no more buouancy so no more resistance. Whether the edge of the bottom is a chine or a radiused curve or just a cut-off edge doesn't make any difference as far as I can see.

This accords with what Bob Perry seems to be saying.
Bob Perry has no experience working with this type of chines and he does not even use CFD to perfect the performance of his boats. I am not talking about the old chines that were used on the 60/70s near the waterline, kind of motorboat chines.

The evidence is simple and its given by all contemporary top Nas: All solo racers have chines the type that are used on the Oceanis 38. The same NA don't use that type of chines on top racing crewed designs. Do you mean they are all wrong and don't know what they are doing? Hardly belivable besides racing results speak by themselves.
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Old 08-12-2014, 09:54   #30
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Re: How does a chine work?

Quote:
Originally Posted by Suijin View Post
...
I think in modern production cruising boats it's basically a styling conceit that also has a practical advantage; it allows the designer to bring the broad conical cross section as far aft as possible while meeting beam limitations for the boat, maximizing interior volume along the way. Since many modern "fat ass" cruisers have downwind weather helm problems due to the resulting asymmetrical waterlines, I suppose a hard chine can add to stability, but I'm going to posit that it's more a function of correcting that waterline profile than anything else but I ain't no naval architect.
...
What you say could be an explanation if racing boats, that have nothing to do with practical advantages but only with performance, did not follow the same hull basic design, pulling all the beam aft.

As it is the case the explanation is obvious and, instead of what you say, the NAs are applying what they have learned regarding a better hull performance on racing boats on cruising boats.

Note that all Nas that make negative comments about the new hulls are the ones that are not linked to top racing and racing hull development for ages while the ones that are applying it to cruising boats are the ones that have real and big experience in the development of new top racing boats, as it is the case with the designers of the Oceanis 38 that Bob Perry called "a blotted shoe" design.

That's true that modern designs have a strongly asymmetric hull in what regards water plane while sailing, but saying that new designs, like the Oceanis 38 have rudder problems it is obviously not true. The boat was tested by dozens of professional testers when it was tested for European boat of the year (the Oceanis 38 won that prize): The conditions were rough and they all said great things bout the boat, including a great response from the rudder and a better ability to sail in bad weather (compared with boats of the same size).

The problem of asymmetrical hull designs was solved by the designer of that boat decades ago, with the introduction of the twin rudder concept. Obviously the Oceanis 38 has a twin rudder.
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