Discussion in 'Boat Design' started by APP, Apr 30, 2012.

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### APPJunior Member

Hi,
Regarding the planing capability of a boat it is written that:
"Ideally any planing hull should have bottom surface loadings of no more than about 50 lbs per square foot if it is to plane easily".
{50 Pounds per Sq Foot is = 244.12 Kilograms per Sq Metre}.

The questions to discuss are:
(a) if this is widely acceptable? and
(b) if it can apply to single hulls of a power Catamaran?

Further, how can be defined exactly the "bottom surface"? I assume it shall be the area between two ususally underwater longitudinal chines forming the V shape (plus the longitudinal strakes, if any), or not?

Thanks for any reply and suggestion you might have,
Regards
APP

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2. Joined: Oct 2008
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Firstly, says who?

Secondly, when you know whom it was that said this, what did THEY define as 'bottom surface'.

And then finally:

..never assume, obtain hard facts!

Good luck

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### daiquiriEngineering and Design

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### AlikSenior Member

Design load on bottom is defined by impact loads on seaway, not quasi-static loads at planing! Proper references would be Spencer and Heller-Jasper, used in most of HSC rules.

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Well..its late and i'm tired, but if i have done my conversion from horrible imperial units into metric, 50lbs/ft^2 = 2.4kN/m^2. Which is nothing at all!

Ive just finished doing an 11m cat that has 25kN/m^2 loading and some time ago, at other end of the spectrum, a 40knot monohull interceptor with 95kN/m^2 and a large high speed cat at 88kN/m^2....me thinks 2.4kN/m^2 is a tad underestimating bottom loadings!

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### APPJunior Member

1) Yes, I recall the article was in http://www.bwseacat.com/index_files/Page427.htm

2) You wrote ... a large high speed cat at 88kN/m^2....What kind of? can you please give us tomorrow LWL, BWL and displacement of this CAT? What did you measure exactly for hull surface in m^2?

3) It is clear that the Bwseacat does not say which is the surface they mean but is most probably the V surface, and for loading they may intend the displacement with a boat fully loaded, otherwise it cannot make sense.

4) I would prefer calculations with something more practical to refer to as the Waterplane area. For example the Waterplane Loading is the displacement divided by the waterplane area. According to David Gerr, waterplane loading is a good indicator of comfort in a seaway [Higher WPL more comfort]. Nothing was said about planing capabilities related to boat weight. On the other hand the deadrise angle is missing in the estimates, so the whole issue, linking weight to planing, seems rather complicated, or not?

Regards
APP

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Here's an old chart from Don Blount showing relative hull size (bottom area) vs full load displacement. Don defined bottom area as "Ap" which appears in the top right of this chart. Ap was the "projected area bounded by the chine and transom".

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### AlikSenior Member

...

Indicator of comfort on seaway is vertical acceleration level. Search for Savitsky/Koelbel paper, they methods to evaluate bot comfort and structural loads on planing hulls. This is one of fundamental papers boat designers use.

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### AlikSenior Member

As to bottom load factors - figures provided by AdHoc; we have 80-120kN/m2 for different designs around 50' designed to LR SSC and ISO12215-5, design speed 45-50kts. This has nothing to do with 'bottom loading' used for performance estimates!

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Alik gave you the perfect reply.

The WPA ratio is simply a ratio of waterplane area divided by displacement. It is a dimensionless ratio that ostensibly categories vessels. Swaths for example are in the 0.8-1.5 range. Pretty much everything else from the QE2 to a wave piercing high-speed cat fall into the 5-6 range. You get the odd vessel that is in the 2 or 3 range, but these are often novel hull forms.

Bottom line WPA will lead you to establish motions relative to a sea state and this must also be linked with other characteristics of the vessel to calculate the natural frequency of pitch/heave/roll etc, including speed, as this affects the frequency of encounter with said sea way and hence the motions.

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### sottorfmember

I agree with Alik, APP in his original question is referring to the works of Donald Blount (most likely referenced by Dave Gerr): there is an optimum range for bottom loading for a planing monohull, where the bottom area is projected area bounded by the chine and the transom. For a planing hull if the bottom area is too light the seakeeping will be bad and if too high, the boat might not get on the plane and have poor performance.

On the Donald Blount website you can find a nice little calculate which incorporates the Blount/Koebel design guidelines. As Alik points out these are fundamental to planing hull design.

Now to answer the original question, are these applicable to planing catamarans? The limits will be slightly different to monohulls. The maximum bottom loading will be less than monohulls as to heavy bottom loading will result in the boat slamming on the tunnel in waves. Definitely to be avoided if possible. The minimum bottom loading for stability will be less becasue the low aspect ratio catamaran planing surafaces will not produce as much lift.

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### AlikSenior Member

For monohulls, this will comprise CDelta=0.2...0.3 and to 0.5 for some heavier boats.

For catamarans CDelta=(Volume displacement)/(2*Beam of chine of demihull)^3 and the numbers can reach 0.7...0.9 for cats. So my experience is different to what sottorf said

Again, CDelta is used for performance estimates; CDelta is included in original Savitsky-Brown formula for vertical accelerations.

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### DCockeySenior Member

The ratio is waterplane area divided by displacement^2/3, and to be independent of units the volumetric displacement needs to be used.

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### APPJunior Member

Hi,
I noted you are a HYSUCAT designer. Is it possible to find out at what speed (knots) the same cat (a typical cat of yours) with HYSUCAT system starts to plan and the speed of the same cat without such a HYSUCATsystem? Is there any definition of the meaning "starts to plan"?
Thanks
APP

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### sottorfmember

Hi App, as Alik pointed out, planing is defined as starting at around Volumetric Froude №, FnV = 2.5-3.0 (depending on whose definition you use). FnV is a function of the displacement of the hull. If you are supporting part of the boat weight on foils, the effective displacement of the hull is reduced. In other words the same cat hull will be operating at a higher Froude number than without foils. So if the foils are carrying say 50% of the boat weight, then the speed at which the boat will start planing will be 12% lower than without foils.

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