# Towing resistance – surface friction

Discussion in 'Boat Design' started by kolloff@get2net, Mar 1, 2011.

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### kolloff@get2netNew Member

In the literature regarding towing resistance typically the total resistance is divided into four different type of resistance:

Surface friction
Energy loss due to wave generating
Energy loss due to eddy’s
Wind resistance

For low speed the surface friction counts for the majority of the resistance forces. 80 - 90 %.

The surface resistance is typically estimated by the ITTC formulation where the friction coefficient, Cf , is estimated based on Reynolds number, Rn, and Ct is multiplied with the velocity squared, wetted surface area water density.

Now, I would expect that just by calculating the surface friction one should get a resistance force which was close to 80 -90 % of the total resistance force? But that is not the case. I have results from model towing tests but the surface friction resistance only contribute with about 10%.
The velocity is quite low – Froude 0.03 to 0.07.
From pictures it is visible that almost no waves are generated.

Is there anyone who can explain what I am missing here?

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### daiquiriEngineering and Design

Depends on what has been tested. A parachute, for example, will have a very low friction drag component even at low speeds.
If you are talking about a streamlined hullform, then I think you should perform the uncertainity analysis of your towing test data.

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### MikeJohnsSenior Member

That's quite a low Froude number.

What was the waterline length of the model, and was the test in a proper towing tank or a very simple force balance setup?

Also it might help to know the type of vessel being modeled.

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### kolloffJunior Member

The Froude number is correct (The vessel is 310 metre long and will be towed with a speed on 4 - 8 kn)

The model test was performed by MARIN, a very well known tank in Holland.

I will try to attach the results:

Regards
kolloff

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### daiquiriEngineering and Design

And what do they (MARIN Staff) say on that regard?

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### kolloffJunior Member

I am not in a position where I can contact them for this project.
(If I could, I would have done that.)

But I believe the results are correct.

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### AlikSenior Member

I would expect wave resistance->0 at that Froude numbers.

Looking at results of tests, it is important to look also at methodology of testing: what turbulization was used, extrapolation formula for CF, etc.

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### daiquiriEngineering and Design

What makes you believe that? I see a data which is out of normal parameters here, so the first thing I would think is - something went wrong here. As Alik said, in that Fn range the wave resistance should go to zero (and surely cannot be the predominant one), unless you have a hull geometry which is not streamlined (see the extreme example of a parachute in my first reply).

You (or someone who is in position to do so) should try to contact the staff at MARIN and discuss that test data before proceeding further with the design based on them, imho.

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### kolloffJunior Member

The hull shape is not very streamlined. But still I would have thought a higher skin friction compared to the overall resistance. Unfortunately I cannot give so much information away but thank you very much for your input. very much appreciated! It have give me something to think about.
I'll see if I can get closer to the problem by pulling some other strings.
Thank you.

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### MikeJohnsSenior Member

kollof the low Fn makes more sense now. Did you get the full resistance curve up to Fn 0.15 or 0.2 ? It pays to see this data in the full light of the resistance curve.
And presumably Marin will have calculated the trip stud resistance and given the RM accordingly?

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### kolloffJunior Member

I have found a picture of a similar vessel.
The dimensions are: L 300, B 61, D 30, draught for towing 8 metre.

Fn 0.2 is equal to about 20 Kn and is not realistic for this kind of vessel. Therefore the model test only goes up to 9 Kn (Fn 0.087)

Mike, you mentioned “trip stud resistance” can you elaborate more about that? As I have not heard about that before.

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### MikeJohnsSenior Member

The turbulence tripping studs or sand strip on the model creates extra drag and that drag needs to be taken into account and subtracted from the total measured drag of the model.

On the full sized ship most of the flow is turbulent and only the first meter and a half or so is linear. On the model the linear flow has to be tripped to tubulent otherwise it would be nearly all linear and the resistance would be too low.

Yes I see why the Froude numbers are low.

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### AlikSenior Member

It could happen that at that speed flow was turbulent even with turbulence stimulators.

I would suggest to applly data from wind tunnel tests of elongated bodies to this task. As wave resistance is very low, it could work.

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### DCockeySenior Member

Laminar rather than linear flow???

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### DCockeySenior Member

That's a very reasonable suggestion.

A guess of a drag coefficient based on frontal area of a scale model of the underwater portion of such a shape mounted on the ground plane in an automotive wind tunnel would be 0.4 to 0.55. That's based on a guess of the underwater shape and experience with automobile aerodynamics.

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