Neil, The speed of the boat can be determined ahead of time if you know both the drag-speed curve of the boat and the thrust-speed relationship for the motor. Max speed point is where the falling thrust of the motor matches the increasing drag of the boat.
Torqeedo lists actual measured zero-speed thrusts reluctantly and only because folks misled by the trolling motor spec convention suspect them of withholding comparative information without it. But thrust is not, in any way whatsoever, a useful measure of the ability to push anything that actually moves because it lacks any insight into the speed-thrust relationship. And you are interested in moving. One final criticism of trolling motors: mfgrs publish lb thrust from a theoretical test that usually overstates measurable thrust by about 40%. Torqeedo does not. The Electric Paddle has a measured zero-speed thrust of only around 12 lb but matches a 30 lb thrust trolling motor at 4 mph while using only 1/3 to 1/6th the power. So we have optimized the thrust-speed curve of our product only for easily propelled craft sacrificing its ability to work on a bassboat. The main message is that the thrust-at-speed interaction with a particular hull-type matters. A "thrust rating" in itself is misleading.
I have never seen Torqeedo publish a thrust-speed curve for their motors but it would be helpful to designers. Electric Paddle has a thrust-speed curve- you can find it in a our presentation to yacht clubs in the links section of our website. http://www.electricpaddle.com/links.html
You mention a max rpm for Torqeedo and estimate maximum speeds. I confirm your speeds if the pitch and rpm are correct, and I, like you, suspect that the 720 rpm may be under some reasonable load in which case maximum (zero-thrust) speeds will be somewhat higher. Nevertheless, look up our presentation to see the general shape of a motor thrust-speed curve, enter the two endpoints (zero-speed thrust and maximum speed at zero thrust) and you should have a relatively close estimate of your rig's performance.
We are ignoring the fact that we could also use a power calculation to determine max speed. But that would assume a properly chosen pitch for the motor/ boat combination. Without optional propellers for the Torqeedo, we are forced to use a thrust-speed relationship method.
Try fitting the two points in the thrust-speed domain to a typical curve from our presentation and if you have a drag-speed curve for the boat, you should have a reasonable prediction. Good luck. Joe
--- In electricboats@yahoogroups.com, "F Neil Simms" <f.neil.simms@...> wrote:
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> I got a good deal on a Torqeedo 801 Travel (hooks up to external 24v battery bank), with the intent to use it on a new 'fast' boat I'm building. The boat will be a light 24' x 2.5', and by Freeship resistance calculations should be capable of 8 to 10 mph with a motor that can put out 62 lbs of thrust, which the 800watt 801 can do.
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> What I realized after purchase was that the standard Torqeedo 10" pitch prop likely won't be able to drive the boat at those speeds, what with the motor's 720 rpm max.
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> Anyone have any experience running the 801 on a slippery fast boat that should be capable of speeds in this range? Is the Torqeedo prop/rpm limited at high speeds? Or will it exceed 720rpm in a lightly loaded situation?
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> I would have preferred the new Torqeedo 1003, but they don't have an external cable adapter available for them yet, and the standard model with integral battery (which I don't need) is $2k :(. The 1003 has a max rpm of 1200, which if used with the three blade prop of the 801 I would think stands a good chance of getting the boat to the speeds I'm shooting for.
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> I guess I'll see how the boat does with the 801, and wait for the 1003 Travel to come out and then upgrade. My brother will probably want the 801 as auxiliary for his Rhodes 19 anyway :).
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> Neil S.
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