Economics of Single vs VS pumps

[Dirk]

Mark, I'll have to defer to your expertise on the pumps. But I can say unequivocally that doubling RPM will not double a boat's speed, not even close, with the exception of certain types of hulls, like planing hulls, which, within a certain range of velocity, will lose a good deal of drag. For most other types of hulls: "A hull that is designed as strictly a displacement hull can be propelled close to its hull speed, but as the hull speed is approached the power requirement rises exponentially." This fact is what I (mistakenly or not) was applying to pool plumbing.

None of this off-topic conversation changes the following facts. Back when all a pool pump was called on to do was push water through a filter for x-hours a day, a single speed pump was all that was needed. And if that's all the OP is doing (and he can get around CA's codes), then a single speed pump might make good sense. But the point I got sidetracked making is: if the OP might want to do other things with his pump, like run a spa, a suction or pressure automated cleaner, a heater, an SWG, a waterfall, or any number of other water features, then a single speed pump would not be the best choice, not only in terms of operating cost, but also for optimizing flow rates, individually, for any of those other uses...

 

[mas985]

I agree that the closer to hull speed the boat gets, the lower the efficiency due to wave-creating effects and an increasing hull-water contact area (i.e. bow and stern waves). But at a certain point above hull speed, the hull-water contact area starts to decrease and efficiency will increase with increasing speed, assuming a large enough propulsion system.

But even when the boat speed is well below the hull speed, where hull-water contact is nearly a constant, RPM and speed can be somewhat proportional. You can see this in the following measured data where the boat has a hull speed of ~8 knots and below 2000 RPM the RPM vs knots is somewhat linear. 2000 RPM is ~6 knots and 1000 RPM is ~3 knots. I can't vouch for the data but it is the type of behavior I would expect for calm water conditions.

 

[RMcGirr83]

Then there's stainless steel vs aluminum props. Just trying to muddy the waters a bit more.

 

[Newdude]

Then there's stainless steel vs aluminum props. Just trying to muddy the waters a bit more.

And just regular old gravity and wind resistance. The average boat outweighs and is larger than average car. Most by a lot.

But at a certain point above hull speed, the hull-water contact area starts to decrease and efficiency will increase with increasing speed,

I had a 21 ft Sonic speedboat that did 75mph. The back bottom of the hull had a flat 2ft wide triangle. That’s the only contact it had at speed. On a choppy day it only had that contact skipping across the tops of the waves. But the hull has to be specifically designed for that. A 34 ft Grady/Whaler is designed to plow through 6 ft ocean waves, not dance across them.