# Round Bilge vs hard chine

Discussion in 'Multihulls' started by nickvonw, Apr 11, 2011.

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Thanks D..as always you’re too kind with words. I was only pointing out what, well to me, was obvious upon closer inspection.

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### Richard WoodsWoods Designs

just to clarify further

A small pedantic point

The Slenderness Ratio compares length with displacement, not weight. Displacement varies slightly between fresh (1) and salt water (1.025). Not a big deal. However using displacement means you compare a length with a volume, not with a weight, which makes more sense.

A bigger point

Two boats (and we are talking here on the Multihulls page, so not discussing slow monohulls or planing powerboats) may have the same displacement and length, so both have the same SLR

But one may be wide and shallow, and the other narrow and deep. Most multihull designers think the narrower/deeper hull will be faster

Richard Woods of Woods Designs

www.sailingcatamarans.com

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### daiquiriEngineering and Design

Good poins.
The conversion formulae in the post #43 are for fresh water. The conversion factor will be somewhat different for salt water, as you have pointed out.

So,

for fresh water:
• DLR = (30.32 / LDR)^3
• LDR = 30.32 / (DLR^0.3333)

for standard sea water (1025 kg/mc):
• DLR = (30.57 / LDR)^3
• LDR = 30.57 / (DLR^0.3333)

As about the observation about the slenderness ratio - it is correct and that's when L/B ratio gets in. What the graphs do show is that, for fast displacement speeds, a change in L/D ratio has more influence on resistance that a percentually equal change in L/B ratio.

Also, one has to bear in mind that the graphs in Blount's paper are made from systematic hull series. So, one can use the results if the hull shape he/she is designing is of a similar form.

Cheers!

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Slenderness ratio, in the old days was or is the length/volume^(1/3)

Most today use the length displacement ratio- length/displ^(1/3).

The volume is a product of the density and weight(mass). The difference is so small, it is not worth even noting, just as the difference between FW and SW is just 2.5% [or 0.8% at ^(1/3)]. However, so long as consistent datums are used, L/D or SLR is used, so what? Thus whether one uses SLR or LD makes no difference. Converting the LD into SLR yields the same result, except the graph is just moved to the left an little. But once converted back, is exactly the same and vice versa.

So, I’m not sure what your point is?

Says who??

It is not about which is faster, or slower, it is about resistance and this varies with speed. One cannot make a blanket statement since this axiom does not hold true across the Fn.

Take a typical series 64 hull.

At a B/T of 2.0, at a given L/D ratio the residuary resistance is 215 (just reading from my papers, units are not an issue here, just the trend) at a speed –length ratio of 5.0. At a speed length ratio of 2, the resistance is 65.

At a B/T ratio of 3.0, it is 242 and 68 respectively

At a B/T ratio of 4.0, it is 260 and 70 respectively.

So, clearly this says that the wider (B/T=4.0), fatter increases in resistance, compared to the narrow deep hull (B/T=2.0).

At slow speeds, not much difference, but at higher speeds, big difference.

Thus, everything requires to be put into context.

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### oldsailor7Senior Member

Dur.
The late Edmond Bruce nailed it all.
After many long hours of laborious tank testing in the early 1960s he found that L/B ratios below 8 were really draggy due to the induced bow wave. Fat monohulls simply did not have the sail power to enable them to climb out of the hole in the water they were making. It was called their "Hull Speed".
As L/B ratios increased beyond 8 things improved until at 11 they were almost gone ---and by 12 had disappeared altogether so that the only impediment to increased speed was skin friction and form drag. Form drag was largely a function of the L/B ratio anyway, so slimmer deeper hulls were faster.

However------ as L/B ratios began to rise there came a point of diminishing returns. As the hulls got slimmer the displacement was reduced and so boats had to be made lighter or not carry so much payload in order to reduce the form drag and skin friction increase. This point seems to have been reached when L/B ratios exceeded 15. At this point only very light racing multis were effective, as evinced by the Tornado catamaran at 20/1. Richard Woods Gwahir was a perfect example. A very fast 25 footer for racing at L/B 15 , but not very comfortable for pleasure cruising. So the lesson is long and slim is faster--- under L/B 8 is slower but will carry more of Dick Newicks "Modern inconveniences".

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### Richard WoodsWoods Designs

I think we are saying the same thing but in different words

Maybe I oversimplify, but that's for brevity and to suit a less academic readership

Richard Woods of Woods Designs

www.sailingcatamarans.com

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### MikeJohnsSenior Member

:idea::idea::idea:

That's the crux with Naval Architecture.
For example ( to the thread subject): You add a chine and in a seaway you'll get cross chine flow and some turbulence but then you may also get a reduction in heave pitch and yaw and maybe a better bow wave separation from the hull, and overall you may have a substantial reduction in total resistance for some sea states and some speeds, and maybe an increase in others. Then the skill is in applying the results practically for the vessels SOR.

Taking each hydrodynamic attribute and optimising them individually without considering the total system is often unhelpful for the complex and closely coupled events that make up ship and boat hydrodynamics.

When you start comparing the results of a detailed RAO from a wave basin after your smooth water tests looked good and you start to understand how important model testing is and how many of our predictive methods are no more than loose indicators until you have a similar hull to tie your prediction parameters to.

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### nickvonwJunior Member

thanks all for the very precise and thoughtful contributions

so basically hard chine and round bilge difference to performance is pretty minimal, in boat under 50 foot

and a lot is asthetics

unless you start going very high performance hull design where these slight differences can be the difference in performance designers are trying to obtain

cheers

nick

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### daiquiriEngineering and Design

If you have read the replies, you'll find that it depends on other issues, not the hull length alone.

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### nickvonwJunior Member

yeah I have read all replies and for the purpose i am thinking of (a 40 foot cruising bluewater catarmaran) it seems with all things considered if i were to build a hard chined boat it could still be relatively quick, strong and faster to build than a round bilge design

thanks again for all the info

nick

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### Corleyepoxy coated

I think the posters in this thread and boatbuilders themselves would agree that the savings (in time and materials) from having a chined hull in lieu of round bilge on the overall time it will take you to construct your boat will be minimal. On a daysailor I can see real savings in time because the hull is a larger portion of the total job.

I know it seems counterintuitive because you look at a boat and say well its got a mast and hulls and the rest are just little details but those little details will be where you are spending the bulk of your time when constructing a bluewater catamaran, fairing, painting, installing hardware, engines, wiring, plumbing the list goes on. By all means build a chined hull but dont delude yourself will be saving bag loads of time and effort by doing it.

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Exactly.

One must be holistic when comparing designs. To take one "part" and to scrutinise that one part and then extrapolate it to become the "whole" part, is wide of the mark and missing the point of "design".

Design is a multidisciplinary process, all balancing and all compromising to achieve the one desired goal, or SOR.

Round bilge, hard chine…each has their merits and each has their disadvantages. But, looking at them from just say a hydrodynamics sense ignores the fabrication, it ignores the structure, it ignores the space, it ignores the cost, it ignores the motions, it ignores purchasing…and so on.

The choices you make along this journey to achieve your goal has so many inputs each with different outputs. At the end of the day, if the SOR, your dream, your goal, your objective etc has been satisfied, then it is a good design. No two boats are the same, because the SOR/Dream/Goal whatever you wish to label it, are different.

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### oldsailor7Senior Member

Sorry Corley.
I can't agree.
Between 1962 and 1979 I built 29 boats and compared to a well designed multi chined hull, double (or triple) diagonal planking was a pain in the anus.
I much preferred moulded f/glass foam sandwhich hulls to either.

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### Corleyepoxy coated

Thats ok Oldsailor I'm fine with disagreement to my thoughts and I suppose it does depend on the build method maybe what I should of said is depending on materials round bilge or hard chine can consume a similar amount of time in the construction process.

The variation in overall build time also depends on how much equipment the boat is going to carry, if you were happy with an absolutely bare bone cruising boat it would make a large difference, appointments that are considered virtually essential now were unthought of luxuries in the 60's and 70's.

So overall I dont agree with your assesment but horses for courses in a modern composite (bluewater) cruising catamaran design the hull construction method is a relatively small portion of the total work and time investment. Lots of projects are abandoned at the lockup stage because that is when it starts to sink in how much work, time and money is still required to complete the project for a lot of builders.

15. ### CatBuilderPrevious Member

I don't know if I agree with this.

I am currently building a round bilge boat and I feel it's every bit as fast as a hard chine would have been.

Look up the Farrier method of heating and bending foam in a female mold.

It goes along quite quickly and the end result is a nice, round bilge:

http://www.f-boat.com/pages/construction/index.html

Take a look at the Farrier page and scroll down. Read the whole thing.

It's quite fast, actually. Now that I have the kinks of the process worked out, I am producing half a hull, already glassed inside in about 3 weeks. This is for a 45' catamaran.

I started building on Feb 20th and plan to join my first set of hull halves together in about 2 weeks.

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