# Resistance prediction of planing hull in pre-planing speeds

Discussion in 'Boat Design' started by Trixini, Aug 2, 2016.

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### TrixiniJunior Member

Hello everybody,

I've got a relatively small and "fat" hull (8m length, 2.8m width). The hull represents a classical hard-chine planing hull and resistance predictions for speeds higher than 20 knots according Savitski are working quite well.

However, I've got a load case in which I put a lot of people in the boat (resulting displacement 7500kg). I'd now like to do an "endurance" prediction for low speeds (e.g. 5-7 knots).

All formulae I found is either not valid for this type of hull shape (e.g. classical holtrop/mennen or other predictions), not valid for the desired speed (e.g. Savitski) or way to complicated for an early estimation in the pre-design stage.

I found the paper by Mercier "RESISTANCE OF TRANSOM-STERN CRAFT IN THE PRE-PLANING REGIME" as well as the paper by Radojcic "Resistance Prediction for Hard Chine Hulls in the Pre-Planing Regime".
But both papers are quite scientifical plus there seems to be a mess between metric and imperial units.

Looking at other threads in this forum, I found this rough estimation (Link: Here) which seems to not be fitting my main dimensions.

Is there some sort of simple preliminary estimation I'm missing? I'm not looking to get down to the last Newton, but an overall impression would be nice.

Thanks in advance for any help.

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Hi Trixini

Welcome to the forum.

The graph in the link you provided will be more than adequate as a good first approximation. Your LD ratio in the "heavy" condition is 4.1...thus just lift off from the graph.

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### TrixiniJunior Member

wow, quick response, thank you already!

Just to get it right: the units required are meters and tons? (why this is not stated in the diagram is beyond me...)

And the volumetric Froude number would be somewhere around 0.7? This would result in a Resistance/Weight ratio of approx. 0.04 . Again, this would be an unit-issue as it is not stated in which unit either resistance or weight need to be...

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Unfortunately it is bit of a mix of units. LD ratio - length to displacement ratio in metric, is shown that being length /displacement ^1/3....length = m, and displacement = tonne.

The SLR or slenderness ratio, is usually used in imperial old units...which the Americans still use today.

Fv = Froude volumetric...that is dimensionless.

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### TrixiniJunior Member

Okay, I figured that the output likely is something like R/W = [lb]/[lbs], so I'm correcting the weight as well as the resistance-force...

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I just checked the original paper - on my shelf.
Shows W is displacement ..but no units and R is obviously resistance, again no units. But since this is an American paper with imperial units everywhere, it is a safe bet, lb/lb

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### TrixiniJunior Member

Thanks...

I hate this unit-mess, maximum confusion assured.

Case closed for now, thanks again for your quick help.

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### Manfred.pechSenior Member

Helpful: "Leistungsvorhersage schneller Rund- und Knickspantboote" of B. Müller-Graf, VWS (Versuchsanstalt für Wasserbau und Schiffbau, Berlin), in "Jahrbuch der Schiffbautechnischen Gesellschaft", Bd. 74, 1980, SS 381-408.
Berücksichtigt werden: "Wellenwiderstand, Wellenbildungswiderstand, Wellenauftrriebs- oder induziertzer Widerstand, Spritzerwiderstand, Zähigkeitswiderstand, Tangential- und Reibungswiderstand, Ablösungs- oder zähigkeitsbedingter Druckwiderstand, Anhangswiderstand, Luft- und Windwiderstand, parasitärer Widerstand, Widerstand infolge Restseegang, Widerstand infolge Kurskorrekturen, Rumpfwiderstand ohne Anhänge." Fundierte wissenschaftliche Erörterung auf der Basis jahrzehntelanger Erfahrung mit allen notwendigen Formeln und Berechnungen.

No unit-mess!

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### TrixiniJunior Member

do you happen to have that publication?

That would save me a trip to HSVA for looking up in the yearbook myself ;-)

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### Manfred.pechSenior Member

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