# Towing resistance – surface friction

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

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### AlikSenior Member

Thare are also test results available for 'mirrored' models of hull bottoms.

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### DCockeySenior Member

Another approach: there may be non-negligable wavemaking drag due to separated flow at the stern, similar to an immersed transom. So another approach to develop a drag estimate would be the "skin friction" plus an estimate of the drag increment due to an equivalent immersed transom. Then the question becomes how big would should the equivelant immersed transom be. A guess for a starting point might be 3/4 of the immersed depth of the vessel or 6 m. At 8 knots the Froude Number based on 6 m depth would be 0.54.

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### AlikSenior Member

I suggest this would be viscous drag, not wavemaking.

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### DCockeySenior Member

Pehaps I should have been clearer. The separated flow at the stern will influence wavemaking and potentially cause larger waves than if the flow was attached. Energy is used in creating the waves, hence drag due to the waves. The relation to visocity is through the location of separation affecting the wave formation.

There is also drag due to the the pressure behind the bluff boddy being lower than if the flow was attached. For a bluff body in a wind tunnel the recirculating flow and pressure in the separation bubble is controlled in part by the boundary layer characteristics at the separation and resulting shear layer which are directly related to viscosity. The result is drag due to pressures on the afterbody not balancing the pressures on the forebody. This would be the dominant factor affecting drag for bluff body of the shape shown if the surface of the water was rigid.

A vertical immersed transom is a different situation since the base pressure is atmospheric above a clean separation at the periphary of the transom. The situation for a shape such as the vessel under consideration is likely to be mix of the two. There is almost certain to be a separation bubble with recirculation behind the vessel, but the separation bubble may not extend to the water plane, and the area above it would be at atmospheric pressure. The result is also drag due to the pressure.

Last edited: Mar 2, 2011
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### AlikSenior Member

Understood, but below Fn<0.1 this effect is negligible.

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### Leo LazauskasSenior Member

Are the tests being performed at such low Froude numbers to use in a Prohaska plot in order to estimate the form factor?

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### DCockeySenior Member

My understanding the relevant Froude number for the waves associated with an immersed transom is the Froude number based on transom immersed depth, sometimes refered to as Ft, not the Froude number based on overall length. For an effective immersion depth of 6 m at 8 knots Ft= 0.54. I don't know how significant wavemaking will be at this Ft. Based on a look the photos in "Qualitative Investigatin of Transom Stern Flow Ventilation" by Maki, Troesch and Beck at that Ft the wavemaking is not significant and the water level at the stern would be close to the static level. In that case considering the vessel as a bluff body would be a good approximation, though atmospheric pressure at the free surface means the flow and pressure in the separated region will differ from that behind a mirrored hull.

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### kolloffJunior Member

Thank you for the explication!

I am actually familiar with the various type of “device” for creating the boundary layer in ship model testing. I just didn’t know what it was called in English. (English is obvious not my first language

Cheers,

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### Leo LazauskasSenior Member

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### AlikSenior Member

Leo, I believe that propulsion unit will have more effect on transom flow than flow separation itself, at such low Fn.

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### Leo LazauskasSenior Member

Thanks, Alik.
I thought there might be more to the tests though.

Another way around the difficulty with the flow behind the transom is to do bow-down tests, i.e. at an attitude where the transom is just fully-emerged. I'm suspicious of such tests, but others have found them useful.
Of course, that could also mess with the flow into the prop.

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### kolloffJunior Member

That is interesting comment. The stern shape will for sure course some separation effect. However, the transom do not have such a large area as the models shown in the paper :"Qualitative Investigatin of Transom Stern Flow Ventilation". The breadth of vessel's under-body do narrow-in but have a bit "hard" shoulders though.

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### kolloffJunior Member

There is no propulsion system on this vessel. It is towed out to the operation location.

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### AlikSenior Member

I believe those transom terms at Fn in question could be neglected for reliable performance predictions.

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### AlikSenior Member

If so, there is effect of tugboat wake/propeller stream

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