Economics of Single vs VS pumps

Dirk

Gold Supporter
TFP Guide
Nov 12, 2017
4,758
Central California
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

TFP Expert
LifeTime Supporter
May 3, 2007
12,952
Pleasanton, CA
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.

 

Newdude

Well-known member
Jun 16, 2019
1,961
NY
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.