# Drag coefficient PVC over water

1. May 18, 2005

### cronxeh

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" =$$7095 in^2$$ )

Last edited: Oct 8, 2005
2. May 19, 2005

### FredGarvin

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 $$Fr = \frac{U}{\sqrt{lg}}$$ 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.

Last edited: May 19, 2005
3. May 19, 2005

### cronxeh

Right, thank you Fred.

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

Thanks

4. May 19, 2005

### FredGarvin

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. May 19, 2005

### cronxeh

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. May 21, 2005

### Sowhatalot

this formula is really handy and it might help this formula is called Crouch's formula
V=C/((disp/HP)**.5)
V=the boat speed in knotes ( 1 knot is about 1.15 )
C= the constant
disp= displacment in pounds
and hp= hourse power

i hope it help's you should beable to change it around

7. May 21, 2005

### FredGarvin

What is "the constant?"

8. May 27, 2005

### Sowhatalot

constant of about 180 for cruisers im not sure for others though
but you just need to make C the subject ..

9. May 27, 2005

### FredGarvin

Well, that gives you one equation with two unknowns...C and HP.

10. Jun 3, 2005

### Sowhatalot

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 displacment 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