# Calculating resistance for lake and river boats

Discussion in 'Hydrodynamics and Aerodynamics' started by B K, Oct 29, 2015.

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### B KJunior Member

Hello!
I am new to this forum so I apologise if there is a similar topic somewhere.

Does anyone know any reliable methods for resistance calculation of flat bottom lake and river boats under 20m of lenght.

I tried to calculate it with several methods but i don't know if any of them are applicable for this problem...
I think Compton's and Fung's methods gave me good results.
The results were similar and in range which seemed reasonable...

But I'd like to know for "sure".
I'm quite new to this field of river boat building and fresh out of college...
Any help will do.

Thanks!

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### AlikSenior Member

Look at a) parameters of series and b) type of hull shape when choosing the series. I would use at least 3 methods and would be pessimistic with untested hull.

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### Ad HocNaval Architect

Establish the length-displacement ratio....and then find residuary resistance of similar and go from there

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### fcfcSenior Member

Is there not a depth effect or channel effect to consider ?

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### Ad HocNaval Architect

Indeed. But considering the above request, i think it's beyond him to consider and work out these effects.

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### B KJunior Member

First of all, thanks for your answers.

There is no serie and no specific data to consider, that's why I ask.
My boss drew the hull lines from his experience because they do the job.

@fcfc and Ad Hoc:
I don't have any real usefull info on the resistance of similar boats.
Also, the depth effect is not really an issue here.

I tried to find any kind of numerical method for lake and river boats with flat bottom but all methods seem to concentrate on typical seagoing hull forms.
Also i tryed to do some work with some barge resistance methods but it don't fit also.

The thing is, that those kind of hull forms are basicly planning hulls, but with really big half entrance angle at the bow.
And keeping that in mind, it's logical that no method for planning hull will do.
Because they seem to be founded on traditional planning hulls...

So I actually think I'll try and make my own method.
It' either that or CFD.

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### Ad HocNaval Architect

AS noted, you only need to find:-

So,, that requires the waterline length and displacement, that's all. Are you saying you cannot find any boats anywhere that have similar LD ratios as your boat?

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### baeckmoHydrodynamics

BK: The strongest influence on resistance in the pre-planing speed region stems from the slenderness ratio, as Ad Hoc has mentioned, and Aliks answer goes in that direction as well. That means that you may use statistical methods based on regression or factor analysis without digging too deep into specific hull details, as long as the lines are decently "boat-like".

One such method (I have checked it for validation against some existing vessels, and the results are ok) is mentioned in the following report:

"Resistance Prediction for Hard Chine Hulls in the Pre-Planing Regime" by D. Radojcic et al; published in the Polish Maritime Research no 2, 2014, p 9-26.

It is available on the internet.

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### B KJunior Member

Thanks baeckmo, i'll do some calculation and compare it with what I have for now...

https://upload.wikimedia.org/wikipedia/commons/f/fa/Plitvice_Lakes_electric_boat.jpg

This is what the boat looks like, more or less.

@Ad Hoc:
I can find some data on the propulsion and max speed but not on the resistance of similar boats.
It seems that this area is not that well researched, maybe because it is not as profitable as seagoing vessels.

I'll try and find some conection through L/D, S/L and some other ratios.
But same LD ratio for a seagoing chine hull and a "box shaped" lake or river boat may lead to quite different conclusion and result.
That's why i'm a bit sceptical but maybe I overcomplicated the whole deal.

One thing's for sure.
After this many methods, there will be some kind of a conclusion.

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### Ad HocNaval Architect

BK

You're over thinking this, it is much easier than you realise.

The shape of the boat for your type of boat will not have any major influence on the resistance owing to its speed so long as you obtain data from similar types. I have no idea what speed the vessel you linked to does, but it does not look fast at all, which is another bonus if true.

Your model hull, I assume you can calculate the frictional resistance of the hull...yes?

If so, all you now need is the residuary resistance. So what is the L/D ratio for your boat....that is simply:

length (m)/displacement ^1/3 (tonne)

So for example if the waterline length is say 7.5m and the displacement is say 15 tonne the L/D ratio is 7.5/15^1/3 = 3.0

An L/D ratio value of less than 4 is a very short fat heavy boat, a value between 4-5 is a heavy boat, a value between 5-6 is a not so heavy boat over 6 is getting lighter..and so on.

Using data, from a variety of sources, as noted above, you calculate the L/D ratio of similar vessels. Then plot the residuary resistance of kN/tonne for each L/D ratio hull of the database hulls v Fn.

Then you simply interpolate between curves of L/D ratio to match yours. Then hey presto you have the residuary resistance. Then add this to the frictional, and that'll give you a very good approximation of the resistance.

Thus, where to get the source data of hulls?....by any of the methods noted above. Namely any known Series of hulls via technical papers. So long as the hulls in question are similar in terms of L/D ratio and at your operating target speed/Fn in a similar fashion..that's all you need

For example. If you found a series of planing hulls....which have L/D ratios similar, and the data covers your target Fn..then the fact they are planning hulls matters little. The L/D ratio is important. As you are comparing like for like, at the same target Fn.

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