Drag coefficient PVC over water

In summary, a boat with a Cd of about .08 would require a motor with about 240 lb of thrust to travel at 5-10 mph.
  • #1
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Hello. I'm looking for a drag coefficient of a high molecular PVC material over sea water (fresh or ocean water)

I'm designing an electric outboard motor for an inflatable boat I got and was wondering how much ft-lb thrust I would need to go certain speed (~5-10 mph), considering weight (~530 lbs with all equipment) and area ( 129" x 55" =[tex]7095 in^2[/tex] )
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  • #2
I think you are confusing coefficient of drag with friction coefficients.

The Cd for a boat hull is dependent on Reynolds number and Froude number. Surface conditions do have an effect as long as the flow is fully turbulent. However, the effect of surface roughness on drag greatly depends on the flow situation i.e. of a blunt body vs. a streamlined body. In a streamlined case, the surface roughness tends to increase drag (airplane wings need to be very smooth). In blunt bodies, the surface roughness tends to decrease the drag (golf ball). I would think a boat hull is a streamlined body, so that means that you'll have to have a reference for your geometry at varying roughnesses to properly account for that aspect.

For boat hulls the Froude number [tex]Fr = \frac{U}{\sqrt{lg}}[/tex] comes into play, but I'm not a marine designer, so I can't say for sure how much dependence there is on it. The amount of drag produced due to a wave formed at a two fluid boundary is a second drag factor you need to account for. I would estimate that it is as important as the reynolds number though if the speed was high enough. One book I have has a chart for a particular boat hull (with and without a bow bulb) and it's Cdw vs. Fr. I would think it would be your best bet to see if you could get something like that from the manufacturer of your inflatable.

I would say that unless you want to do some model testing, which is normally what is done, you'll have to take a swag and estimate the geometry using known geometries in tables of known Cd's and then apply a fudge factor for the wave effects.

I don't think there's any easy way to get somewhat reliable numbers here. Perhaps someone who has access can model it for you and run some CFD code on it.

P.S. You just need to know the pounds of thrust required, not ft-lbs.
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  • #3
Right, thank you Fred.

Would a motor with 70 lb of thrust push this type of design to about 5-10 mph?

  • #4
I honestly have no good feel for what a boat needs to push it. I'm a land lover. Did the inflatable come with any recommendations for motor size?
  • #5
yes max 3.5 HP, however that is a 2-stroke gasoline engine rating. For electric motors that would mean about 240 lb of thrust
  • #6
this formula is really handy and it might help this formula is called Crouch's formula
V=the boat speed in knotes ( 1 knot is about 1.15 )
C= the constant
disp= displacement in pounds
and hp= hourse power

i hope it help's you should beable to change it around
  • #7
What is "the constant?"
  • #8
constant of about 180 for cruisers I am not sure for others though
but you just need to make C the subject ..
  • #9
Well, that gives you one equation with two unknowns...C and HP.
  • #10
With HP its just the output from the engine, the formula is just to give you a idea of the speed the boat can travel at so it is saying if you can get the output of the engine and the displacement of the boat so as in how much the boat has to go through the water and then the constant...

its the smae thing as for this other formula i have it can give you a fairly good idea of how fast a car can travel... that is somthing eles but if you would like it just ask.
i think i have cleared up your question

Related to Drag coefficient PVC over water

What is drag coefficient?

The drag coefficient is a dimensionless quantity that represents the resistance of an object moving through a fluid, such as air or water. It is a measure of the drag force that is exerted on the object divided by the product of its frontal area and the dynamic pressure of the fluid.

How is drag coefficient calculated?

The drag coefficient can be calculated using the formula: Cd = Fd / (0.5 * ρ * V^2 * A), where Cd is the drag coefficient, Fd is the drag force, ρ is the density of the fluid, V is the velocity of the object, and A is the frontal area of the object. This formula is derived from the drag equation.

What is the drag coefficient of PVC over water?

The drag coefficient of PVC over water can vary depending on the shape, size, and orientation of the PVC object. Generally, it falls within the range of 0.2 to 0.7. However, for a more accurate value, it is recommended to conduct experiments or simulations specific to the PVC object in question.

How does the drag coefficient of PVC over water affect its movement?

The drag coefficient of PVC over water plays a crucial role in determining the drag force exerted on the object and, therefore, its movement. A higher drag coefficient means a higher resistance to movement, resulting in slower speeds and shorter distances traveled. Understanding the drag coefficient can help in designing more efficient and streamlined PVC objects.

Can the drag coefficient of PVC over water be reduced?

Yes, the drag coefficient of PVC over water can be reduced by changing the shape, size, or orientation of the object. For example, a more streamlined and aerodynamic shape will result in a lower drag coefficient. Additionally, adding a hydrophobic coating or using additives in the PVC material can also help reduce drag and improve its movement over water.

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