Wave drag caused by propellers

Hi,

I am working on a big sailing yacht, equipped with 2 propellers. They are constant-pitch ones, and not folded as well, so their drag under sails is significant. It's easy to estimate roughly the form drag of the propellers, but what about the wave-making drag?

Waves around the hulls are caused by different pressure in various places around the hull. There are different pressures before and after the stopped propeller - so it will affect the wave pattern around the ship and the wave drag.

How to estimate this additional drag and the interference drag?

I tried to use Michlet, and put the propellers as 2 very short round-sectioned side hulls. The estimated form drag of the propellers is included as the viscous form factor (about 1.6).
The chart shows the drag (in daN) at various speeds (in knots) for 2 positions of propellers - 6m and 7m behind the center of the hull.

As you can see, the position of the propellers appears to be important - but should I believe Michlet in this case? I doubt whether Michlet can estimate correctly the flow around the stopped propellers.

Any better ideas?...

regards

krzys

 

No. It is inappropriate.

 

For a given performance under sail, I think the cost of the extra rig and ballast needed to overcome the drag of the fixed props is much greater than the cost of feathering props and a reduced rig of comparable performance. If the drag of fixed props is 25% of total hydro drag (reasonable for twins on a full keel boat), you should be able to make the boat 10-15% lighter and get the same performance if you switch to feathering props. Those are the numbers I get when looking at the problem. Perhaps some of the pros have better cost analyses.

Why are you using twins? A perfectly ordinary single would do, and stuck behind the keel, the protected prop could hide when not in use. You also seem to have the heft to handle a decent thumper. The additional cost of twins will be considerable. The operational cost will be much higher as well. It all seems exorbitant for a 55'er (guessing from the Michlet LWL). Is this an existing boat, or a design project?

 

Thank you, Leo, for the answer. I was almost sure about that, but had no other ideas.
On the other hand, sometimes Michlet can be forced to work well far beyond its range. In your opinion, this is not such a case? Well, then, the only information I got is, that the problem is important. Have you heard about some papers on it?

It was the wish of the owner. I still hope to convince him, that the single prop behind the keel is better.

regards

krzys

 

If it's a big yacht then the owner can afford to pay for the required research.
Tell him you will hire consultants to do more work