Contra-rotating prop designer

Looking for some suggestions. Working an unusual project that requires a contra-rotating prop set (tractor configuration). We are trying to avoid making too many costly prop sets as trials proceed but are not having much luck with the limited amount of design information we've managed to gather to date. I'm sure that the Volvo IPS prop selection tools are mature and accurate but we're not using a pod drive and so we're not using one ov Volvo's products.

So who is known to be ..or claims to be..a wizard in the art of contra-rotating tractor prop designs?

 

Marine application I assume.

Tractor is not really smart, 10-15% less efficient and the only reason would be for motor cooling. Being a tractor you will really need to pitch up the root, or make the hub very large, both of which reduce efficiency.

I've done several CR sets but I'm not looking for a project right now. You should get the following papers to perpare you for any design discussion.

Contra-Rotating Optimum Propellers Operating in a Radially Non-Uniform Wake, Lerbs, DTMB Report 941, May 1955

The Design of Counterrotating Propellers using Lerb's Theory, W B Morgan (...yes the Dr Morgan at NSWCCD...), in SNAME Transactions Vol 68, 1960.

Thrust Deduction in Contrarotating Propellers, Beveridge, NSRDC report 4332, Nov 1974

 

Thanks for the references!

In the application we're working with, there is no other possible choice than to locate the props on the "nose" of the hulls.

 

I might add the following:

"Contrarotating and Tandem CLT Propellers" by Gonzalo Pérez Gómez et al.; SNAME Propellers/Shafting Symposium, Virginia Beach 1994. I have the impression his institution has been (is?) involved in commercial design projects as well.

My own experience has been connected mainly to the application of tandem units, but the main issue for any kind of impellers working in series seems to be the wake adaptation of the aft impeller, and the power split proportion between the two, be it tandem or CR. The reason for using the tractor configuration would be the better control of inlet conditions when cavitation is at stake, but otherwise I believe the pusher is a better design compromize, when you are free to choose.

If you can arrive at a reasonable wake mapping by calculation, you should be able to pick a pair of standard screws that are not too far away from an optimum. Any decent prop supplier should be able to apply whatever modification needed without too much fuzz. There really is not much of a surprise if you "happen" to measure a set of commercially available CR props

 

Poor dolphins!

 

See my discussion in the spin-off thread.

http://www.boatdesign.net/forums/props/pusher-vs-tractor-puller-propeller-46309.html#post616026

 

Maybe we have to define what we mean here. In my book a "tractor" propeller is a prop that is mounted ahead of its driving/bearing unit. A bow mounted pod unit can thus be "bow tractor" or "bow pusher", depending on the mechanical arrangement.

I take it that BMcF is considering a bow mounted tractor unit with CR props? A logical question is then: why CR?

 

The bows are the "forward pointy end" of two submerged cylinders and that is where the CR props are.

Using single props vice CR has been looked at but the desire to keep both drivelines identical was one of the reasons a CR prop solution was first settled on; using single-disk props would have them both turning in the same direction. The "vehicle" is intended to operate over a very wide range of Fr numbers and "prop walk" would have created significant problems (it was assumed).

So, with all the mechanicals all in place and not subject to change, I'm back to my original quest: to find someone with some tools and experience to help us minimze the number of expensive "cuts" we'll have to make to arrive at the final/best prop design.

 

Can you reveal any preliminary data? If not allowed to publish, I might check a few sources under a non-disclosure agreement. If so, mail me in private.

 

For sure the friction resistance of your hull will be higher than normal, because it will be washed by the high-speed wake of the prop. Don't forget to take it into account.
Cheers

 

For "standard" outflow distribution yes, but with a switch in radial momentum distribution, you may even create a low-pressure zone over the nose of the vessel itself. For instance, if the blade loading follows the forced vortex principle, there will be a radial switch in the outflow. To take it one theoretical step further you could circulate pressurized wake fluid and thus increase the theoretical jet efficiency......

 

I'm reminded of AdHoc's axiom, the client's SOR trumps everything, (or words to that effect).