Hard chined displacement hull

[JordieS]

Hi everyone,

I was just wondering if a hard chined displacement hull would produce more drag than a rounded displacement hull.

Would the hard chines produce lift, and therefore more drag on a displacement hull?

Thanks

[Ad Hoc]

By displacement hull, i assume you mean a low Froude number hull?

If so, then a hard chine shall be a bit more draggy, owing to the chines being submerged. The chines cause separation of flow which creates eddies which in turn adds drag. Therefore a round bilge hull is better at lower Fn.

The chines wont produce any lift. Firstly the area is far too small secondly not going fast enough. To even consider "lift" is for higher Fn hulls, like above 0.8-1.0 into the fully planing region. The amount of lift generated by chines is so small one doesn't consider it. The hull bottom has significantly more area than the piddly chines.

[MastMonkey]

Given similar factors a hard chine will usually have greater wetted surface. But, not all things are equal. A boat with chines is likely cheaper to build so for the same cost you can get more length and thus more speed. That is my understanding at least.

[JordieS]

Doesn't displacement hull always mean low froude number hull? Aren't they always meant to produce least resistance?

Also how much drag would the chines create, would other underwater appendages produce more drag?

Thanks

[Ad Hoc]

Originally Posted by JordieS

Doesn't displacement hull always mean low froude number hull?

Not necessarily.
A container ship owner would consider his hull a high Fn compared to a tanker! Both are displacement hulls. Thus, everything needs context.

Originally Posted by JordieS

Aren't they always meant to produce least resistance?

No. The clue is in the type of hull "displacement". Which means when under way the hull is supported by hydrostatic means not hydrodynamic.

But, least resistance for what??..it needs context.

Originally Posted by JordieS

Also how much drag would the chines create, would other underwater appendages produce more drag?

How long is a piece of string???
All depends upon the size/shape of boat and the chines of that boat. But in the region of ~5% at lower Fn.
Appendages always add drag..again, depends upon the size and their location. But generally in the 5-10%.

[Alik]

There were tests performed for hull models of sailing yachts in 1970-s, for chine hull, double chine and round bilge with same parameters. Results are interesting, on higher Fn chined hulls are likely to have advantage and at low Fn difference between double chine and round is negligible. Published in Russian, will try to find that paper.

[bertho]

interesting point Alik,
most important is probably the quality of the hull design.. some guy do beautiful water lines with chine, not surprised is very little difference in term of resistance in the water, after, is just aesthetic question .? mode ?, .and construction cost..
rgds
bertho

[Ad Hoc]

Originally Posted by Alik

There were tests performed for hull models of sailing yachts in 1970-s, for chine hull, double chine and round bilge with same parameters. ..... Published in Russian, will try to find that paper.

Or just look at "Performance of Prediction - Fast Craft" by D. Bailey In RINA publication of Nov.1974. Conf on Performance Predication of Small Craft.

His graphs clearly show the difference between the two. Going from minimal difference at low L/D ratios to a much bigger difference at higher L/D ratios.

But this is all now "well known" and endless papers on the same subject, in English too

[Alik]

Originally Posted by Ad Hoc

But this is all now "well known" and endless papers on the same subject, in English too

OK, You studied from English papers, I did from Russian. That's where the first catch references come from

Besides do You know that the same guys who tested those round/sharp chine models were the first ones to publish systematic series of yacht hulls (22 models), one year before Delft? It is called NKI yacht series. This series is not widely used now but quite interesting.

[Willallison]

And of course there are (as always) other considerations too. Cost of construction has already been mentioned. Add to that things like initial stability, accomodation spaces, etc etc. No single element can be considered in isolation.... that's why it's called a design spiral. It all depends on the requirements set out for the vessel....

[Alik]

For displacement hull, benefit in resistance of 1% gives increase of speed of 0.3%. Say, if we get extra 5% of resistance with sharp chine hull, what is speed loss?

So, one does not save too much due to rounded shapes. But yes, they are attractive and have higher resale value.

[philSweet]

J, does this help you-

http://www.scribd.com/doc/50375823/R...Hull-Forms-1-1

some charts at the end.