Round Bilge vs hard chine

Can you explain what you think the 'terrible limitations' are in relation to multihulls?

 

Careful Dean.
You are walking into a quagmire

 

The answer is it depends........

It will vary with the Hullform, the Froude number and the design parameters, and the number of chines.

It's harder to design a good chined hull with say one maybe softened chine but it's certainly possible . Chines lend themselves very well to some hullforms. Properly placed chines have even been shown to reduce the wake field (and resistance) in displacement hulls.

The problem is usually designers simply converting a round design to flat sheet construction with poorly thought out chine placement and no thought to the general flow field around a hull.

 

And with the type of vessel too. Think of stability and roll damping requirements, for example. A chined hull will dampen the roll faster than a round-bilge one.

Next (why I had asked whether it is a sailboat or a powerboat), a sailboat will mostly travel at some angle of yaw due to the side drift (necessary for the keel to produce the lift). A water moving over a chine which is set at an angle to the flow will create a vortex, which in turn will create some additional drag. How much of it and how important it's contribution will be to the overall vessel drag is debatable and is dependent on how important it is for the design team to squeeze out that last bit of a second over the upwind beat. It adds to the additional friction drag a chined hull will have over a round-bilge one, due to a bigger wetted surface (as has been mentioned in the previous posts).

Even powerboats can feel the effect of the chine-induced vortex drag, if their hullform is such that their chines are set at an angle to the flow. See for example this test report on a so-called S-catamaran hull: http://www.iket.fzk.de/cube/lib/files/dd2ae0e496ff46360bb6691ddc1d0a0f.pdf
Take note of this part of the comments about the measured resistance (page 5):

"Disappointingly, the measured total resistance of the S-catamaran in the original configuration turned out to be much higher than that of the reference catamarans at supercritical speeds. (...) The dynamic lift (here as side force) generated by yaw and curvature is inevitably associated with considerable vortex shedding and induced drag, not properly accounted for in the foregoing theory. Moreover, curvature and yaw may promote boundary-layer separation and hence enhance viscous pressure resistance, also not considered in the present potential-flow model."​

Cheers!

 

I think that Daiquiri nails it.
In short a chined hull is not so bad providing that the chined section closely follows that of a rounded hull and the chines are aligned with the waterflow as much as possible.
A perfect example is the positioning of the chines on Lock Crowthers wooden Buccaneer series of sailing trimarans. Their performance speaks for its self.

 

Slavi

With displacement boats there’s very little extra roll damping between chined and round forms, the chines are just not acute enough to generate separation and the chine edges are always rounded sufficiently. The cross chine flow velocity is quite low for roll events and there is little extra entrained water due to chines, the appendages are orders of magnitude more effective there.

As for wetted surface difference it can be immaterial, the arguments usually presuppose the round hull is a half circle, in reality the sections in a ‘round bilge’ design often have large extents with only slight curvature, taking the sections and discretising the curves can be illuminating you may find you have no increase in surface area at all overall for the same displacement.

Separation due to diagonal flow etc is more of an issue since the flow velocity is higher than for the roll event, but even then providing the flow doesn’t experience an angle less than around 165 degrees significant separation doesn’t tend to occur. In reality we will be in a turbulent flow layer anyway for the more acute chines aft.

Wetted surface is also affected by the height waves are encouraged to climb on the hull side above the mean water level, some round hull forms encourage a large dynamic WSA while a chine may shed the same at the first chine above the waterline.

I think with displacement sailboats by the time it’s moving fast enough for these issues to become apparent they also become immaterial since other factors predominate in the drive drag equation. Providing the design is at least sensible.

It’s not really sensible to take any one design criteria and make it significant on its own, there are always qualifying factors that may or may not make it significant.

 

Many sharks have chines.... I just saw a several of them up close yesterday... amd what about the boxfish? That fish is one of the most aerodynamic animals on the planet.

 

Yep Mike, I said it is debatable and there we are now...

Just few observations:

• How acute are the chines you're talking about and how much rounded? If they are rounded, are they still to be considered chines? A flow over a wedge will always results in separation (which might be with or without subsequent reattachment) and/or vorticity, both of which are - drag.
• As about roll damping, I remember reading a japanese technical report about seaworthiness of fishing trawlers which claimed that the measured roll damping coefficients for round-bilge hulls was half the value of hard-chined ones. Talking about monohulls, of course, as multis resolve their rolling problems by other means.
• I agree that keels and appendages are more effective in roll damping, but we should be considering just bare hulls, if we want to make any meaningful comparison.
• What is that 165° angle you have mentioned, sound very huge? Also, you say "significant" separation - but then what is the meaning of the word "significant" in cases when design details could decide between a victory or a loss in a race (everything else being equal)?
• As about wetted surface increase, I'm enclosing here a case of a Laser-like dinghy in both round-bilge and hard-chine version. I have kept the same LWL, BWL, Displacement, Cp, Cb, in order to get comparable hulls. The result is a 6% increase in wetted surface of a hard-chined hull respect to round-bilge one. It is consistent with some attempts of hull transformations I have done in past, all of which would result in an increase wetted surface. Maybe I'm not good enough in this task, perhaps a solution exists where both hull types have an equal wetted surface, but the fact is that up until now I was consistently getting around 5-6% increase in WS.

I realize that this debate is a mine ground, and it probably pays more to just stay aside and watch the event. Each person will have his opinion on the subject, and each will have some personal reasons and arguments to believe in one or another story - also because the numerical data to back various claims are scarse and often contradicting. It's all very debatable indeed.

Cheers!

 

Richard Woods has an interesting article on his website on the subject.

 

Slavi

Well the laser dinghy is not quite what I had in mind

Simple static roll period tests show that a chined sailboat seldom has a different roll period to it's round sistership. There's a quite detailed paper that compared roll at anchor for sailboat of chine vs round that came to the same conclusion. Neither the damping nor extra entrained mass are present practically.

Factors which define the pressure field such as angle of entry, prismatic coefficient and rate of change of volume are not effected by the presence or lack thereof of chines .
And what we are looking to reduce is just vorticity. Experiments have indicated for vessels operating under around 9 knots that flow over 165 degree angle change in hull plate doesn't induce any extra resistance. That doesn't mean the chine angle can't be more acute, its the angle the flow vector experiences at the chine encounter. A deeper forefoot single chine form can have only one chine to cross astern and then only for a very short distance from the chine longitudinal run to the curve up at the stern (if it's a transomed vessel) and then only a small volume of water flowing into the sink at the surface.

Rounding of the chine I referred to is just local rounding ie not a sharp transition, but a small radius.

I'd be interested in your fishing vessel report, from experience if there is a very square form with a high block coefficient it does have effect and also lends itself to extra roll damping fins at the chine to good effect. But again if it has a V shaped hull the tale is different.

 

What about the multihull case where boats go about twice that speed?

Any data on that?

 

CB

By the time you have that much driving force ( 18 knots) most hull drag related to WSA and minor turbulence becomes considerably less significant in the total drag equation anyway. It's in the slower, lighter air conditions where small changes will make the most obvious difference.

Chines can be placed to produce virtually nil added turbulence and if they look right to the designer with an eye to flow lines, then you won't go too far wrong. IMHO

 

Thanks, Mike. Great thread in general.

 

This is what Woods has on his website on the subject... Although I haven't been able to duplicate this in freeship yet, it sounds plausible.


Although my Strike 16 and 18 trimarans both have hard chine hulls, when I started developing a new racier small trimaran design I initially drew a round bilge main hull. However I thought it would be worth comparing it to a hard chine hull shape, as obviously the latter is much easier to build and trail.

Fortunately a computer makes it very easy to modify existing hulls. A couple of mouse clicks changed the spline tensions and converted a round bilge hull into a hard chine one with no other input from me, (although I did need to adjust the draft slightly to keep the total displacement the same)

I was very surprised by the results.

As I had kept the length and displacement the same the Slenderness Ratio stayed constant, as did the Prismatic Coefficent. These are two of the main factors affecting wave drag. The other major factor needed to reduce wave drag (and thus to increase top speed) is a high LWL/BWL ratio.

So my first surprise was that the WL beam of the hard chine hull was less than for the round bilge hull, hence the LWL/BWL ratio is higher/faster. Much more surprising was the fact that the wetted surface area was essentially identical, and WSA is of course the main factor affecting low speed drag.

So the implication is that a chined hull will have similar speed to a round bilge hull in light winds, and be faster in a blow.

As I say, that was for a small hull drawn essentially by the computer, not by me (so obviously the final design still needs my input). However logic says that these comparisons will also apply to larger boats. And it did show that a hardchine hull is better than people think, and that a double chine hull (as used on Flica, Romany etc) is better still. Certainly it has convinced me to use it for my new racing trimaran.

 

Mike, I've been on board round-bilge boats, and on board hard-chined boats (planning hulls, single chine). The difference in roll behaviour is big.
Perhaps we are not talking about the same hull types. Perhaps you're talking about multi-chined hulls which closely follow the shape of a similar round-bilge smooth hull, or perhaps your considerations regard boxy but essentially round-bilged ships.

When I say chine I mean a relatively sharp (r < 10 mm) transition between surfaces set at different angles, usually higher than 200° (up to 250 °). The pic shows the example of a hard-chine version of laser seen above, which has a pretty moderate chine angle.

Cheers!