Hydrodynamic Drag Of Sail Drive

I was wondering if any of you experts could help me calculate the hydrodynamic drag of a Yanmar SD20 sail drive leg and associated spinning prop while it is dragged through the water under sail.

Could you supply a set of equations typically used for this purpose?

I have a couple more sanity checks to do before i finalize my propulsion system. I want to verify the drag on the boat as a result of a saildrive leg with folding or feathering prop and see how much speed that takes off a boat like mine at 20 knots and at 5 knots...

Also need to compare a straight shaft and strut drag with folding prop.

 

Sorry to bump this, but surely someone here understands hydrodynamic drag?

 

Don't forget to factor in the effect of leeway under sail......this is a migraine inducing guesstimate exercise. What do you have against an outboard with a jacking plate so you can get it just deep enough to work under the prevailing conditions, and the ability to completely get it clear of the water when using sail ?

 

If noone else helps before I get home next wednesday, I can model it for you then. In the mean time, we need to know the dimensions of the leg to create the model and run the simulation. Also need your hull dimensions and displacement to give an idea of the difference in speed it will make to your boat...

 

Spinning prop when sailing is a pain in the *** fro two reasons:
* The drag is ~30...50% less than fixed prop (depend on prop characteristics, numbers from memory), but still dramatically more as that of proper folding prop.
* The constant wear on everything what rotates together with prop, vibration and sound.

If you can afford it, buy a folding prop or Autoprop.

 

Yes, it is a little difficult to figure out, but if a NA can figure out the drag of a rudder or a dagger board, then it shouldn't be very hard to do the same for a saildrive leg.

What I do have against outboards and jacking plates? Nothing at all.

Outboards are the most probable solution to my propulsion issues, but I want to check that the drag I am assuming the saildrive leg and prop has is enough to warrant all the hassles of using gasoline/petrol as a fuel.

Those hassles include: The fuel goes bad quickly and plugs up the carbs, the fuel attracts lots of water from the ethanol, you have to build huge "nacelle" type things under the bridgedeck - I'd rather have it clear, you have to put in some hydraulic or electric actuators to raise and lower the 200+lbs outboards.

Lots of drawbacks, but I'll live with them if the drag from the saildrive is too much.

Attached, please find the dimensions of the saildrive in PDF format.

So does this need to be modeled in a computer program, or are there a simple set of equations I can use that are commonly used to calculate drag on something like this?

A rough number is ok...

 

Thanks, Groper!

The hull dimensions are as follows...

http://multihulldesigns.com/designs_stock/45bdcat.html

Saildrive dimensions are in the above post in the PDF.

 

Regarding the straight shaft- I don't know of any sailboat with a decent shaft angle and normal sailboat fixed prop that can freespin with less drag than locking down. The drop angle of the shaft means that there is no such thing as an unloaded prop. The blade travelling up will be working against the blade travelling down and both will be loaded and the vibration may be as large as running under power.

With a prop on a leg that is at 90 degrees to the stream, this isn't true, but I would never want that amount of wear and tear for a pitiful savings. Remember, if the boat is pitching up and down at all, its the same as a drop angle shaft, and drag goes up. I assume if you've got wind to sail, you will be pitching some.

Some round numbers from fin keeled monohulls (can't recall the source) was that a typical fixed three bladed prop and shaft and strut was 25-30% of the skin drag of the hull if locked. That would imply a speed reduction of about 15% for speeds where wave resistance is not a factor. Or you could get a folding prop and get the same performance with 15-20% less rig. Looking at it from that perspective, folders are cheaper than the additional rig you need to match performance in a boat your size.

Unfortunately, all the above is just pulled from my head from odd bits I've read over the years. Maybe I'll provoke someone into coughing up some source material.

 

Would a "Screw Drive" work to propel from the wind.
Have a 2' wide sail on either side of a mast.
The mast is a big screw blade.
The wind driven into the screw blade turns the Prop down below the boat.

 

Phil. Try it in your boat. Get her up to 6 or 8 knots or so freewheeling, then put it in gear. Instant 2 knot reduction in speed on my 45' monohull. It's a myth that a fixed prop has less drag:

http://catamaransite.com/propeller_drag_test.html

That's an interesting piece of data - 25-30% of skin friction for a standard 3 blade prop. That's huge.

I knew it "felt" large and certainly was surprised to lose 2 knots off my 8 knot boat speed under sail when I put my tranny in gear that day to test the fixed prop thing out, but 25-30% is incredible.

I wonder if it's even more on a catamaran.... with two props...

I still would like to calculate the drag though... just to be sure.

Thud... I don't have a question about something like that. I asked what they standard formulas for calculating hydrodynamic drag are...???

 

The drag of a spinning propeller is simple to calculate if you have open water curves for the prop. You need to estimate the torque required to spin the system and then use this torque to find the J at which the system will freewheel, the corresponding Kt will provide the thrust (drag). If you don't have curves, use Wageningen B-series data to provide a reasonable estimate. This becomes an iterative process in finding the RPM, but you can use the j at which Kq becomes zero as a staring point.

The drag of the gear is simply Cd*A*Q, where you need to make an estimate of Cd based on the shape of the gear. Since the prop probably will create most of the drag, the Cd estimate is not critical.

 

You can use the formulas in ORC VPP. http://orc.org/rules/ORC VPP Documentation 2011.pdf

For SD20 PIPA is about 0.004 and Cd for PIPA is 0.81. Thus the drag 0.5*rho*0.81*0.004*V^2. At 2.5 m/s the drag is 5 N and at 10 m/s 80 N. Thus not much at all.

For a fixed propeller you can't really use the ORC formulation, since that is for a very optimized propeller. Free spinning propeller has about 50% of the drag of the locked one. You need to know the diameter and blade area to even questimate the drag.

For a locked fixed propeller the drag is approximately 0.5*rho*1*Ap*V^2, where Ap is the projected area of the blades and can be roughly calculated from D^2*PI*DAR/4. Thus for a typical 14" propeller drag is ~100 N at 2.5 m/s and ~1500 N at 10 m/s.

For a free spinning one ~50 and ~750 N, which is still an order of magnitude more than just the sail drive or a sail drive with a good folding propeller.

More accurately: http://strathprints.strath.ac.uk/5670/1/strathprints005670.pdf

You really want to have a folding propeller!

 

Wow, thank you for the last two posts. Those are enough to get me calculating the effect of the saildrive and props, as well as comparing them to outboards and complete removal of running gear from the water when sailing.

Again, thank you very much. Exactly the information I was looking for.

 

No. you can't do this. There is absolutely no relationship between a prop generating thrust and one acting as a turbine. There is no info you can carry across that divide.

 

CatB, you didn't read my post. The test you pointed to has zero drop angle. Get them to try it again at 15 degrees down angle, and put a real shaft and cutlass bearing and shaft connector and tranny on it.

I can't try it on my boat because I run a folder