New diesel-electric hybrid installation - how to size propellers?

erik818: a propeller without slip won't produce thrust. It is a foil that produces lift. If there was no slip, that would mean it is stuck and will have to then turn the boat around the shaft.

 

Ratings and reality can have various relationships, some of which are useful, other of which can be deceptive.

Engines are generally rated using "brake horsepower" which is amount of mechanical power available at a specificed location. For an inboard internal combustion engine without reduction gear it would be the mechanical power at the end of the crankshaft/flywheel. For a gasoline outboard it is the amount of mechanical power available at the propeller shaft. The rating can be for the maximum power the engine can produce under the test conditions, or it may be a lesser amount which is intended as the power rating to achieve the rated service life under the specified conditions. This is typically done for larger Diesel engines.

Similarly electric motors can be rated based on the amount of mechanical power available at the motor shaft. Again, this can be either the maximum power the motor can produce given the test proceedures, or the amount of power can produce continuously for it's rated service life. Some types of electric motors can produce for a short period considerably more power than the continuous rating. With higher power comes more heat which if the motor keeps producing the higher power and heat will eventually damage the motor. Heat is a major enemy of long life for electric motors.

Then there is the "power" power ratings used for advertising of small hand tools, vacuums and the like which have "universal" motors. These ratings really don't have anything to do with mechanical power. Instead they are based on the peak electrical power flowing through the motor at the instant is is stopped from rotating. And when the motor isn't rotating it's not producing any mechanical power. These ratings do cause considerable confusion though.

 

Marine engines have a rating system to let the user know if the maximum power is rated for continued use or what percentage of.

 

More on electric and IC motor ratings.

The power rating of gasoline outboards with propellers are based on the maximum mechanical power available at the propeller shaft. The thrust verses boat speed characteristics at Wide Open Throttle will depend on the propeller used (as well as how the boat hull affects the inflow into the propeller, etc). A larger diameter, lower pitch propeller with more blades will generally produce higher thrust at lower boat speeds but the thrust will fall off more quickly as the boat speed increases. Gasoline outboards are usually designed with propeller shaft speeds and maximum propeller diameters which work best at the higher boat speeds typical of most outboard powered boats. The "high thrust" outboards usually have lower propeller shaft speeds and space for larger diameter propellers. They will produce higher thrust at slower boat speeds but the thrust will fall off with boat speed more quickly than the thrust from a "normal" gasoline outboard.

Electric trolling motors are typically rated in terms of thrust, with the thrust being the maximum possible, usually at low or zero boat speed. The manufactuer or marketer will then come up with what they claim is the power rating of a gasoline outboard which would achieve similar thrust, presumably at similar speeds. What isn't said is that the the thrust from the electric trolling motor will fall off and then vanish much more quickly as boat speed increases than the thrust from the claimed "equivalent" outboard.

 

SLIP: In St Paul,Mn there used to be and maybe still exists, an excursion company with 2 river cruise boats. Identical hulls and engines, but one had a functioning paddlewheel, while the other was prop driven and a fake wheel.They raced every year during waterfront festival. The prop driven boat always gained a good head start, because of the efficient thrust of its props at low hull speed. Shortly however, the true paddlewheeler would walk right past the prop boat and win the race, because of the paddlewheels great efficiency at speed. Rolling on the water, which is non compressible, with near zero slip. The Navy abandoned paddlewheels exchanging for props because of vulnerabilities to canon fire, and commercial fleets wanted to be "modern" like the navy. No point, just interesting.

 

Paddlewheels have "slip" if they are producing thrust. Correct that water at boat speeds is essentially incompressible. But the paddlewheel is not "rolling on the water". Each blade on the paddlewheel is entering the water and "pushing" on the water. As the blade pushes the water moves around the blade, ie slip.

Are you claiming that the use of props by almost all commercial vessels for over a century is due to fashion? Any other source of information about the claim that paddlewheels are more efficient than propellers?

 

The propeller driven boat with a fake paddle dragging has the brakes on.

 

We're up to 50 tons with 2 x 33hp engines? Now you are just being silly...

 

Folks say there are 2 topics no one can discuss but only argue about, religion and politics, because people don't "THINK" about those only "FEEL" about them. This gave rise to the philosophy in Industrial Relations of T-truth vs F-truth. The True-truth and Feels like the truth never will agree. Maybe boat design and propulsion should be another verbotten topic. Sternwheelers were and are, enormously more efficient than props and a little research will bear it out. They DO roll on the water at speed, the boats speed thru the water so nearly equivalent to the rotational speed of the circunference of the wheel. Research it yourself, but before any new idea, old idea, or information can take root in your mind, first you must relax somewhat your fierce attachment to your cherished notions.

 

The difference between "nearly equivalent" and "exactly" is the slip of the paddlewheel. Small paddles = less slip, large paddles = more slip (Assuming the paddles are submerged equivalent amounts.) Same as for propellers.

And the paddles have to submerge to provide thrust, not "roll on the water".

Good advice. For some individuals those cherished notions are the accepted wisdom - and - and for some individuals those cherished notions are counter to the accepted wisdom, perhaps cherised as tokens of their ability to think differently.

 

Anyone know of good experimental data or analysis of paddlewheel efficiency?

I expect that in some situations some paddlewheels are more efficient than some propellers. Speed, thrust/power and relative size of the paddlewheels and propellers would presumably be factors.

 

My original post was "Shortly however, the true paddlewheeler would walk right past the prop boat and win the race, because of the paddlewheels great efficiency at speed. Rolling on the water, which is non compressible, with NEAR ZERO slip." and then my post "They DO roll on the water at speed, the boats speed thru the water so NEARLY equivalent to the rotational speed of the circunference of the wheel".
Never have I claimed paddlewheels had zero slip, but I do claim they have less slip than props, which also suffer from cavitation at speed resulting in more slip.
Paddlewheels and props are at opposite ends of the spectrum. Paddlewheels are innefficient at starting a hull moving, much splashing and wasted energy, but gain efficiency as they accelerate the hull. Props begin very efficiently, but lose efficiency as they gain speed.
Efficiency is the inverse of slip.
Conventional wisdom? Whose?
I have 43 years experience on the water. Ships, tugs, even once captain of a paddlewheeler. I have more than 10,000 days underway. I'm licensed Master Steam and Motor Vessels Oceans, Master of Towing Oceans, Inland, and Western Rivers, 1st Class pilot Great Lakes between Gary,Ind, Duluth, Mn, and Cape Vincent ,NY. Thats all 5 lakes and connecting rivers.
I earned a right to my opinions, but I learn more all the time because I am willing to hear convincing points of view.
Sorry pal, but "No it's Not!" ain't a convincing argument

 

Slip of paddlewheels varies depending on relatively loading, speed and geometry; same for propellers.

Some paddlewheels will have less slip than some propellers in some conditions. And some propellers will have less slip than some paddlewheels in some conditions.

It is not correct that all paddlewheels have less slip and are are more efficient than all propellers.

 

You really need to be more specific. We are not looking for the average cruiser to be driven as his first boat by a rich lawyer who needs to get to the yacht club against the tide in time to change into his blazer to make the first martini! We just need propulsion to make over 100 nm/day ocean passagemaking. This is an easily-driven sailing trawler. In light airs, we already know for sure she can make 7 knots with this system. Good old fashioned seamanship will keep us off lee shores, if that's your concern.

 

Perhaps. Certainly a 2 paddle paddlewheel would be A poor example of efficiency, though why anyone would invest in constructing any innefficient design would raise curiousity as to their agenda.
Propellers have been highly developed over a century of monopoly use. Paddlewheels were effectively abandoned and enjoyed little developement since then because research is mostly market driven.
If you are looking for common ground, we have one. We can agree to disagree.