Originally Posted by
Ghost
That’s interesting. And may well explain what’s typically been seen with dp drives. As you note, there is undeniably more drag. The question becomes whether the counter-rotation creates efficiency to overcome that.
Aside from old Volvos, it does, except at the highest speeds. At low and midrange speed, prop slip is greatly reduced, and performance is much better at the same rpm. At the highest speeds, single 3-blade props usually get down to slip of 11 percent or less. They have good bite there, thus good efficiency, and the lesser drag of the single prop wins the day for top-end speed.
On the old Volvos, there seems to be a hydrodynamic obstacle around 60. N2O saw this big-time in his high powered 16. I suppose water flow may get bad at that speed and interfere with the prop’s grab. The DP might have more bite and might overcome this, if that’s what is happening. And thus might yield a bit more WOT speed, as you’ve experienced.
(Curious Bill, when the Dee used to hit the wall in the low 60s, did the revs seem to climb up without adding speed, suggesting a loss of bite? Or did the drag just seem huge and the motor just wouldn’t make more RPM?)