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Electric Propulsion
"bowgus" wrote...
Seems to me I once calculated 32 lbs is about 1/4 hp ... if I find the equations, I'll post ... but don't hold your breath :-) I recall once I tried figuring it on my MinnKota 42 once, and got about 2/3 HP. I, also, don't have the calculations handy... |
Electric Propulsion
John R Weiss wrote:
"Jim Woodward" ... Since one HP is 550 ft-lbs per second, that suggests that one hp is 5.5 pounds thrust at 100 feet per second or 6.25 pounds at 60mph (statute miles, here). That feel OK, but my college days are in the distant past -- any comments? Brian Whatcott wrote... Yes, ignoring scaling constants: Horse power = thrust X speed It appears there are a few means of conversion... My MinnKota EX42 is advertised to put out 42 lb thrust at 12V and 36 amps. What is that in electrical power consumption, converted to HP? It also pushes my 21' boat at about 2.5 - 3 knots at full thrust on flat water. What is that in power output or work done? I don't have my calculator and conversion constants handy, but at first glance the electrical power draw appears considerably higher than mechanical power output or work. OTOH, maybe it isn't pulling all 36 amps when pushing the boat at max speed (I assume the prop RPM is electronically governed). that works out (at 100% conversion efficiency) to .57 hp. figure the motor at about 85% efficiency (at best). and you are at just under 1/2 hp. -- Beer, it's not just for breakfast anymore....... |
Electric Propulsion
What no-one has considered here is that powering calcs are usually done from
the other point of view: My boat has X lbs of drag at 5 knots, how many horsepower do I need? Factor in windage (the lee shore situation), foul hulls, appendage drag, prop efficiency (an oxymoron if ever I saw one) and other such truck like Taylor Wake Fraction, and you have a real problem on your hands. Doing it in reverse? Priceless.... Steve |
Electric Propulsion
Stephen:
In this biz, that's as simple as it gets... We have worked for many of the small trolling motor and propulsion equipment companies, so here's some more insight into trolling motors. 1. Trolling motor drives are typically less than 1 hp. One of the most powerful is Minn Kota's e-Drive at 2 hp (see www.minnkotamotors.com and look for the motor spec chart). 2. Thrust figures are indeed for maximum thrust at low speed (as one would expect from their principal market - recreational fishing). These products (including the propellers) are not typically designed to push boats at higher speeds. (As an aside, we designed a custom prop for one of the mid-90's AC syndicates to use in side-by-side testing. They wanted to match drag figures as closely as possible, so they were looking to add some 30-35 lbs thrust to one hull. The existing trolling motor props lost all of their thrust as boat speed increased, so we did a prop with much higher pitch to get a better speed-rpm-pitch match where they needed to be. Who knows, there might be a nice market for replacement props with higher pitch for small boats.) 3. Unlike gas or diesel engines, these electric motors have their highest torque (and power) output at lowest RPM. This further complicates the ability to run at higher speeds, since an increase in revs (associated with getting to higher speed) doesn't buy you more available power. 4. If you need to get over 2 hp into the 5-10 hp range - and you've got some deep pockets - you can consider the Briggs & Stratton gas outboard or a new elec outboard from eCycle (www.ecyclemarine.com). There are other elec outboards on the market, as well. Regards, Don "Stephen Baker" wrote in message ... Don McPh says: snip technicalities Don, The OP asked for a "simple formula"....... ;-) |
Electric Propulsion
Don McPh says:
Stephen: In this biz, that's as simple as it gets... snippage... I know, Don, I know... ;-) Steve Stephen C. Baker - Yacht Designer http://members.aol.com/SailDesign/pr...cbweb/home.htm |
Electric Propulsion
On Tue, 16 Sep 2003 17:56:16 -0700, "John R Weiss"
wrote: "Jim Woodward" ... Since one HP is 550 ft-lbs per second, that suggests that one hp is 5.5 pounds thrust at 100 feet per second or 6.25 pounds at 60mph (statute miles, here). // Brian Whatcott wrote... Yes, ignoring scaling constants: Horse power = thrust X speed It appears there are a few means of conversion... My MinnKota EX42 is advertised to put out 42 lb thrust at 12V and 36 amps. What is that in electrical power consumption, converted to HP? It also pushes my 21' boat at about 2.5 - 3 knots at full thrust on flat water. What is that in power output or work done? I don't have my calculator and conversion constants handy, but at first glance the electrical power draw appears considerably higher than mechanical power output or work. OTOH, maybe it isn't pulling all 36 amps when pushing the boat at max speed (I assume the prop RPM is electronically governed). 12 V X 36 A = 432 W 432 W X 1HP/746W = 0.58 HP IF notice IF 0.58 HP is the electrical rate for 2.5 kt ( = 2.5kt X 1NM/hr / 6080ft/hr X 1hr/60min) = 253.3 ft/min \so the required HP at 100% efficiency is 253.3/550 HP = 0.46HP So the efficiency might be 0.46/0.58 (power out/power in) = 79% which is rather high..... Alternatively, 42 lb thrust from 0.58 HP would give at most 7.6 ft/min which is 0.075 kt. Hence, unsurprizingly, the maker is quoting quasi-stationary testing. Brian Whatcott Altus OK |
Electric Propulsion
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Electric Propulsion
"D MacPherson" wrote...
(As an aside, we designed a custom prop for one of the mid-90's AC syndicates to use in side-by-side testing. They wanted to match drag figures as closely as possible, so they were looking to add some 30-35 lbs thrust to one hull. The existing trolling motor props lost all of their thrust as boat speed increased, so we did a prop with much higher pitch to get a better speed-rpm-pitch match where they needed to be. Who knows, there might be a nice market for replacement props with higher pitch for small boats.) YES! There is definitely a small but viable market! There are a BUNCH of us with 21' SeaPearl sailboats who use various electric motors as auxiliaries. I am almost positive I could get significantly more speed with a higher-pitched prop, but have found nobody who makes one. Any chance of getting one made for a MinnKota EX42 for a trial? |
Electric Propulsion
John:
Let me think about it for a bit. Can you define "BUNCH" for me? How to approach this will depend to a certain extent on the potential numbers involved (i.e., one-off vs production). Regards, Don "John R Weiss" wrote in message ... "D MacPherson" wrote... (As an aside, we designed a custom prop for one of the mid-90's AC syndicates to use in side-by-side testing. They wanted to match drag figures as closely as possible, so they were looking to add some 30-35 lbs thrust to one hull. The existing trolling motor props lost all of their thrust as boat speed increased, so we did a prop with much higher pitch to get a better speed-rpm-pitch match where they needed to be. Who knows, there might be a nice market for replacement props with higher pitch for small boats.) YES! There is definitely a small but viable market! There are a BUNCH of us with 21' SeaPearl sailboats who use various electric motors as auxiliaries. I am almost positive I could get significantly more speed with a higher-pitched prop, but have found nobody who makes one. Any chance of getting one made for a MinnKota EX42 for a trial? |
Electric Propulsion
I would guess initially a dozen who might take an exploratory plunge, with a few
dozen possibilities after that. I've copied this to our e-mail group, so I might get a better idea of immediate interest. Pearlers: How many might be interested in a source for a high-pitch prop for the electric motors on their SeaPearls? ------------------ John Weiss Seattle, WA remove NOSPAM from reply address "D MacPherson" wrote in message ... John: Let me think about it for a bit. Can you define "BUNCH" for me? How to approach this will depend to a certain extent on the potential numbers involved (i.e., one-off vs production). Regards, Don "John R Weiss" wrote... "D MacPherson" wrote... (As an aside, we designed a custom prop for one of the mid-90's AC syndicates to use in side-by-side testing. They wanted to match drag figures as closely as possible, so they were looking to add some 30-35 lbs thrust to one hull. The existing trolling motor props lost all of their thrust as boat speed increased, so we did a prop with much higher pitch to get a better speed-rpm-pitch match where they needed to be. Who knows, there might be a nice market for replacement props with higher pitch for small boats.) YES! There is definitely a small but viable market! There are a BUNCH of us with 21' SeaPearl sailboats who use various electric motors as auxiliaries. I am almost positive I could get significantly more speed with a higher-pitched prop, but have found nobody who makes one. Any chance of getting one made for a MinnKota EX42 for a trial? |
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