# Calculating frictional drag

Discussion in 'Boat Design' started by Cpalm, Oct 5, 2011.

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### CpalmJunior Member

What are the common drag coefficients used for calculating drag from wetted surface?

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### philSweetSenior Member

What are you trying to do? Post a picture of the surface and explain how it moves and describe what it is moving through. You might want to shift to the aero and hydro subforum here and scan the threads. There are quite a few pointers to relevant material there.

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### CpalmJunior Member

Basically I am trying to estimate the frictional drag from the wetted surface on my boat, moving forward through water.

Using this drag formula:

D = 1/2*fluid density*velocity^2*Area*drag coefficient.

I assume there is a drag coefficient for water against common hull surfaces (bottom paint, gelcoat, etc.)

Is this the correct approach?

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### DCockeySenior Member

Clean bottom paint vs clean gelcoat will have a negligable difference in drag. Fouled bottoms are a different story.

Area for ship and boat frictional/viscous drag is usually defined as the wetted area.

Most common drag coefficient used for boats and ships is ITTC 1957 which is based on Reynolds number:
Cf = 0.075/((log(Re)-2)^2)
It's around 12% higher than the Cf for an equivalent flat, rectangular plate. The difference is sometimes claimed to roughly account for additional viscous drag due to 3D effects. There are different thoughts on what length to base the Reynolds number on. Simplest is static LWL. 70% of static LWL is frequently used for sail boats.

Frictional/viscous drag generally dominates up to a Froude number of 0.3 or so. Wave making drag becomes dominant at higher Froude numbers.

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### philSweetSenior Member

We're making progress. Is it a motor boat or a sail boat. What does it weigh, how long is it, and what speed is it going. Is it on plane, semi planing, or non planing. Photos would really help because it isn't nearly a simple as you're hoping it to be. If you are trying to figure out how much power you need just say so. That is a lot easier.

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