Round Bilge vs hard chine

What displacement, loa and beam are we using as a constant?

That is probably one huge set of variables that need to be at the center of any discussion of this type.

If we are talking about a normal boat, such as say, a 40' x 20' with inboard engines and a displacement of 8 tons, does this hard chine theory still hold up?
Or... Is it just valid for an AC45?

 

Somebody tell me cause I don't know. Is an AC45 a hard chined boat?

 

I was talking about the America's Cup 45 as an example of a boat that is nothing like a standard boat. Pure racing machine with little displacement.

 

Round or Square? Seems to me an example was stated to show that extreme racers worked well as a square hull, I just want to get the examples straight. I assume that at this level of design there have been expensive tank tests to help design the boat. We seem to be talking about relatively small performance effects based on the anecdotal stories being trotted out.

So lets get a list of square hulls vs round and see if anything can be recognized. While I believe everyone should be able to say "good enough for me" that does not objectively show the benefits of one over the other. There are also a huge number of variations that can be brought out, note the Buccaneer 24, "round hull" made from flats with chines. Probably hard to distinguish from pure round, but I have no facts.

 

Doug's point is well-taken. I built Slider's hulls as dories, because by adding a chine to approximate a semi-circular section, the displacement went way down, if I tried to keep the hulls 10-1. I needed that displacement to carry two people and a big camping outfit, without having fat hulls.

I can't remember where it was but in a thread a couple of years ago, someone posted pics of the last AC winner, and it had dory-like hulls, though with very soft chines. This was in response to someone claiming that semi-circular hulls were always faster, and that was why they were used by racers to the exclusion of all other hull types.

 

Enclose some graphs that might help for better understanding... This is design study for one of our designs, namely 30m powercat.

Horizontal axis is volumetric Froude numbers,vertical axis is resistance in kN. Red lines are for round-bilge catamaran, blue - for sharp chine catamaran. Dashed line correspond to light displacement; solid lines - loaded displacement. Resistance calculation methods used: Muler-Graf for sharp chine hull, Molland for round bilge. On this graph, l=L/V^0.333 is 'relative length', where L - length at waterline, m; V - volume displacement in m3.

One can see that for relatively light catamaran with slender hulls round bilge shapes provide reduced resistance, though sharp chine are more advantageous for heavier hulls.

 

Alik,

Great post, thank you.

In contrast, Rick W's record setting human powered cats use hard chines under light loading and low power but certainly not low speeds for displacment hulls.

-Tom

 

The choice of hull shape would mainly depend on FnV and L/V^0.333, but also from technological considerations...

 

Ray,

Looking at Alik's graph what "I" does Slider sail at - loaded and unloaded? Perhaps that means daysailing and camping? Perhaps we have one prediction that can be examined relative to a real boat. To understand the graph vs Slider we would need to know the Froude # at 5, 10, and 15kts.

Of course Slider and you had specific desires and requirements that drove the design, this chart will not mean the design is "wrong", just you had different goals than the best possible theoretical efficiency.

I'm glad Sub Tom brought up Rick W's HPV boat(s). It's a pity he's on the wrong side of the earth (just a joke) to compare directly to a round bottom. I would build one to match just to do a tow test to see how they compare. After all, theory, equations, hydrodynamics simulations are all beat by a simple test on exactly comparable hulls.

 

You need to remember that alik s graph is only representing 2 different designs, its by no means conclusive evidence that these results would repeat under more varied design constraints. its not gospel ...

 

I'm too tired to think about it right now-- we're trying to get Slider ready for the Everglades Challenge in a couple of weeks, but in general terms, Alik might be right, because Slider is a heavy boat-- a 16 foot cat that displaces 1100 pounds max-- a lot. And she isn't very fast, because her rig is quite modest. This was a decision taken because I wanted to trailer the boat in one piece, and not worry about capsize in most conditions. She was drawn to compete with other beachcruisers, most of which are relatively slow heavy-displacement monos.

 

This has been done on numerous occasions. Plenty of technical and not so technical papers out there on just this subject alone.

And, as already noted, it depends what your length displacement ratio is and also, more importantly, your target speed. Since the resistance varies in the Fn range.

If you have one single SOR it is easy. If you have many to juggle with, is where it gets tricky and fun at the same time. It is called design

 

The late Edmund Bruce had it all nailed in his papers in the AYRS journals of 1966. He had the run of the naval test tank facilities in the UK and performed hundreds of tests from which he obtained the results which are accepted as the norm today.

 

Ad Hoc,

I am not a part of the Nautical Technical fraternity. Can you supply a couple of references to such papers? Thanks.

 

Remember this is an old thread. If you go pack through the pages you'll see much of this has been discussed already

Alik's graphs show that the most important factor is to keep the weight down. Resistance nearly doubles in the 1- 1.5 Fn range between loaded and light. Yet the difference between round bilge and chined hull is only about 10%

So you cannot really make sweeping comments. You have to be more specific about your SOR, as Ad Hoc, Alik, Catbuilder and Groper have already said

Even then, ultimate speed isn't everything. Flat bottomed boats tend to be used inshore while for offshore work a Veed forefoot is more seakindly. Also of course power and sail hulls have different characteristics

Not sure how helpful a technical paper will be to a non NA. There has been lots of work done on low DLR hulls as these are typically used in fast naval vessels. And most pre 1950's motor yachts also had long thin hulls. The deep V planing hull in a post 1960 invention

Richard Woods of Woods Designs

www.sailingcatamarans.com