Chines vs Round bilge – Is there evidence of superiority?

Talking about chines and why they seem to be becoming generic.

There was a recent article on Sailing Anarchy by Britt Ward of Farr Yacht design.  “Taking it on the chine”.

The article included excellent take away points about how chines improve form stability for a hull of the same basic dimensions and how accommodation is able to be moved towards the edges of the boat more.

However, at the same time the article reflects a lot of the traditional talking points about Chines vs Round Bilge.

I’m not sure most of these traditional talking points stand up to real scrutiny.

Times, they are a’changin’

During the 60s and 70s we, in the antipodes, were fed the line that round bilge hulls were kind of the classic look of speed.

And that our chine racing dinghies were poor relatives.  Homebuilt out of plywood.

Flying Dutchmen, 505s and other imported dinghies were the shape of speed.

Bob Miller (AKA Ben Lexcen) wiped out all the other conventional singlehanded dinghies with his twin chine Contender dinghy at the IYRU trails to select a new international class, but was told it needed to be round bilged to be acceptable as a high performance boat.  He adapted the shape.

Contender sailing

Now things seem to have very much changed.

Chines almost totally dominate modern racing dinghy design.

Now the big end of town seems suddenly interested – the “trickle up” effect so noticeable in the last America’s cup.  Big expensive boats are using the tech developed in thousands of backyard built racing dinghies.

However, when thing change in expensive yachts, they suddenly become validated.

So we are seeing a lot of this appearing.  This is Comanche, one of the newest maxis.

Chines or round bilge best -

Comanche Maxi – see link to Sailing Anarchy article above

So are chines as “Draggy” or create the vortices that everyone talks about?

Aircraft vortices are large because of the high differential between high pressure under the wing and low pressure over.  At the ends of the wings the high pressure air wants to scoot to the low pressure side creating a vortex and the end of each wingtip.  Any pressure differential at a boat chine will be very much less.

aircraft wingtip_vortices_flyingindiancom


Be aware I design and build mostly chine boats.  So I do have a horse in this race.  But read on and see if my arguments are convincing.

Are chines bad on this goat island skiff -

Both ends of these Chine vs Round Bilge memes are given reinforcement both by the high level people who say them, and by constant repeating in publications and the interwebs.  But they have very little data to back them up.

There was always the statement that chines “create turbulence” or vortices or eddies.  Even now, there is no way to calculate or measure that.  Unsteady flow is a formidable problem and most attempts at calculation are applied to extremely simple models.

Working out what happens on a real 3D boat and operating so close to a water/air interface makes it incredibly complex.

Some “thought experiments” about chines like the “sea of peas” which attempt to match the lateral and vertical curves of the chineline to hopefully minimise the pressure differential, don’t make sense because of the ever changing flow directions.  The chine is always operating from different depths of immersion, different angle with heel and changing position of the trough/s and peak/s of the hull’s wave system adjacent to the hull as it moves through the water.  And they don’t explain the excellent performance of chine racing dinghies with very shallow hulls and relatively wide hulls like most of the photos here.

I design boats with close to 90 degree chines which should perhaps be the worst case of this “expected turbulence” … however I see very little trace of it in the water.  Sometimes on some boats I see a little swirl of bubbles in some specific situations – on suddenly turning you sometimes see a vortex off the inside chine.  If it is visible sometimes … why don’t we see it all the time?

Or at least visible consistently?

The big test tank

Witness the last generation of non foiling moths – chines had plenty of presence and mixed it with the boats with radiussed chines.

Moth lowrider - are chines draggy -

A-class cats have in the last decade become quite interested in boxy shapes, often with very tight radii and flattened bottoms as they understood that a semicircular hull is not necessarily the optimum cross section.

round bilges or chines catamaran example -

Moths are at the fast end – half a catamaran … so what about the slower end?

Take NS14s.

Australian NS14s are forced towards maximum efficiency by a relatively low sail area.  They are a popular restricted class with a very open rule for hull shape.

Most breezes you are below hull speed upwind at a bit under 5 knots. they are light enough to plane well, but with only 100 square feet of sail and two adults aboard they do spend a lot of time at sub planing speeds. Dead downwind is trim for low drag and compared to spinnakered boats, pretty well head for the mark.

Rather like conventional yachts in some ways.

There was always a predominance of chines, even some quite radical sponsons with much less than 90 degree angles on their tips became popular in some design series.  The intention was that these sponsons would be above the waterline.

There were also some round bilge or almost round bilge boats that were successful.

Here is a bunch of 1990s designs.

chines or round bilge most efficient ns14 -

But they always seem to come back to chines with occasional flirtations with round bilges despite so much time spent subplaning.

The big advantage of chines during plywood boat days was easy building.  “Something easy can’t be fast – right?”.  We really had it in for ourselves.

But the boats above are all composite and there isn’t so much difference in building time now.   But we see chines and bumps which spend time in the water in most upwind and much downwind sailing.

Other classes that allow different hullshapes and have a lower speed range – like the Merlin or the National 12 in the UK have always had a mix of round and chine boats.

National 12 chines vs round bilge -

I save the best for last …

And many Merlin Rockets, also a restricted class allowing large variations in hull shape have been built clinker/lapstrake style … which is the ultimate for big numbers of edges. And they have been very competitive with other hull constructions

Chines or round bilges -

So I tend to go with the tank test on the water, rather than go with what people “say” about round bilge vs chine.


My argument is should not be read as saying chines are always better than round bilge.  And it should not be read as saying bad chine placement is OK – we should try and do sensible things by our normal educated guesswork – it seems to work to produce competitive hulls

But it seems like chines and edges are no big deal for most reasonable speed regimes and likely have advantages because of much better understood reasons for reducing spray release quantity and keeping spray low – anything thrown up will push the boat down.

In the end the lack of evidence of subplaning advantages and disadvantages seems to disprove a lot of what is passed around as if it is understood.

Planing speeds, there are some pretty convincing basic arguments from physics that indicate that chines might be better for at least parts of the boat.

So, choose what you like and go with it.

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3 thoughts on “Chines vs Round bilge – Is there evidence of superiority?

  1. I would of thought that the Hartley TS16 was a perfect example of hard chines, a 16 footer that can out run many larger boats as one can tell by its CBH even though its a none spinnaker class it matches many classes of larger boat with spinnaker.

  2. A perfect example of a successful hard chine boat is the Bunyip 9 moth scow. It has a semi round aft with chines. The chines near the aft acts as an extension of the sharp bottomed transom when at plane with the rounded sections helping to keep the speed of the water constant. The aft part of the chines should be sharp and the rest should have some minor radius. Chines also help reduce the size of the centerboard. Chines are a thing of beauty!

    • Hi Ray,

      Can’t agree more. For those interested in Bunyips and other highly evolved plywood designs there is an excellent group on facebook for Australian derived scow moths. Certainly among the most sophisticated plywood structures ever built allowing hullweights for a 11ft by 4ft+ boat as low as around 35lbs. Normal lightweight ply boatbuilding hits weights of 8 to 10lbs per foot. The scow moths hit under 4lbs per foot. Amazing.

      The group is here and the Bunyip plans and others are in the files section

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