A25 HP and Torque chart for various RPM

[nebulatech]

Hi All!

I'm very close to ordering my new (electric) motor for my A25. I'm trying to decide if I need a reduction gear. As you all know, the reduction gear reduces RPM while increasing torque, e.g. a 2:1 reduction converts RPM in half and doubles torque.

I am trying to find the torque it takes to drive the boat at given speeds, rpm (motor) and rpm (shaft). There is a universal formula to calculate torque:
Torque = HP X 5252 (constant) / RPM

As an example, the MD17C I removed from the boat, which operates at 2800RPM at 35HP, would produce around 65 ft/lb of torque. A 2:1 transmission would double this torque to around 130 ft/lb, take away about 10% for transfer inefficiency, if I understand correctly. I am probably wrong though. One problem I am encountering is figuring the HP at lower RPM's. Sunsetrider started this thread where they state a speed of 7.2 knots at 1800 RPM (motor) with the same MD17C:
http://www.albinowners.net/aog/viewtopi ... 28&t=12449

I don't know the HP produced by the MD17C at 1800 RPM, so I can't calculate the torque. 7.2 knots is above hull speed, and in all likelihood, beyond what I can hope for with an electric conversion. With the age of the MD17C, my efforts at finding such a chart have proven unsuccessful. Does anyone have this kind of chart, HP at various RPM, for their motor, along with what kind of speed they get, and hopefully their reduction ratio and prop specs?

Thank you in advance!

[DesertAlbin736]

I dunno, what about looking up specs on Duffy electric boats for rough ballpark comparison? A 22 ft Duffy electric (their largest size) has a 48V DC motor powered by a bank of sixteen 6V batteries in series-parallel. That gets you roughly 6.5 hrs run time at 5 knots top speed. 200 amp hour 6V FLA deep cycle golf cart batteries weigh about 75 lbs each, so a bank of 16 batteries would weigh 1,200 lbs & give you around 19 kw-hour range capacity. That is even if you could find space to fit that many batteries.

[DesertAlbin736]

Prop vs HP, I guess you could size a prop to the motor, not vice versa. We used to have a 25 foot sailboat that was at least 1,000 lbs heavier than our Albin (1,750 lb lead wing keel). We could motor to 6+ knots with an 8 HP outboard. 8 shaft HP = 10.66 KW. Since electric motors generate high torque at low RPM a 10 or 15 KW motor with large diameter steep pitched prop for low RPM seems like it would do well. Torque=power=amp draw, so the higher voltage motor the better as far as thermal losses & cable size goes. A 10 KW motor at 48V with .95 power factor would draw 220 amps at full thrust. A 96V motor would draw 110 amps. Any way you look at it pure electric means limited range. I believe there are some Albin 25's converted to electric out there somewhere, maybe over in Europe. If you could fine one who has a blog that could give you some good ideas.

[NickScheuer]

Is your MD17C available? I'm looking for a spare engine I can play with.

[dkirsop]

I doubt that torque will be your problem and that you will need a reduction gear to match shaft rpm to the design rpm of the propeller. On my boat the propeller is 16.5" dia. by 12" pitch and is designed to operate at a max. of 1100rpm. Based on performance curves that came with the boat the prop is absorbing about 11HP at this speed. My boat is an early model full displacement hull with a 21 shaft horsepower motor. The motor has a 2:1 reduction gear.

As an aside, I trimmed my prop to 16.0" diameter and had the blades cupped. There was a noticeable increase in efficiency which dropped my cruising rpm by 100rpm. Max. speed remains about the same due to the nature of the displacement hull form.

[nebulatech]

Dkirsop, what is the designed max rpm for your motor?

NickScheuer, I disassembled the motor for removal and have it parted out on ebay. I move about 2-3 pieces/month. If you need any parts, let me know. No charge for albinowners members, just handle the shipping.

I decided to go with the reduction gear, mainly because it uses a belt between two pulleys, which provides some safety to the motor against a jammed prop. My main reason not to use it was it requires the motor to be installed higher than the shaft and I hoped to have a flush deck, eliminating the fiberglass engine cover. I believe I've come to terms with it.

For those interested, the MD17C, operating at 1800rpms, is generating approximately 22hp. This is an approximation derived between me and Joe DeMers at Sound Marine. He is going to be helping me out interfacing the 30mm prop shaft into my reduction gear. I've said it before, but Joe is a gentleman.

Finally, addressing DA's comments about range, obviously, range is going to be very limited without a genset, but there's no concrete formulas to just plug in your stats and get an accurate answer. I wish there was, but I guess I'll be the Guinea Pig.

Interesting to hear about other electric A25s. I'll have to try again to find one.

[dkirsop]

The maximum RPM for my engine is 2200.

[tribologist]

The main reason to go with a reduction gear is that you get away with a much smaller motor if you run at higher rpm. In rough terms, power is linear to weight and rpm. I have a 250 kW PM motor in the lab that I can carry if I like. The caveat is that it get that power at 36,000 rpm. At 1800 rpm I could maybe get 15kW from it. (there are some losses at higher frequencies). At these power levels you can easily use a timing gear drive.

[NickScheuer]

Hey, nebulatech! I'm indeed looking for a part for my MD17C. In hooking up the throttle cable subsequent to having installed a rebuilt transmission I can't seem to find what I believe is a ball on a post to which a spring-loaded fitting on the end of the cable fits. The disconnection was don last Fall and I do not remember the details. I just the throttle lever has a hole in it and the spring-loaded sleeve on the end of the throttle cable is not compatible with it. I think I need a ball on the end of a short threaded post. Does your MD17 have such a fitting?

[nebulatech]

Hi Nick,

I can check what I've got when it stops raining (1-2 days?). All the parts are in a large storage crate, which has a bunch of stuff in front of it that I will have to move out to the driveway. I have the throttle setup. I believe I would have taped any small parts to the main assembly and cables. Below is a photo I took prior to removal.

Do any of the boxes (red, blue, orange) look like what you need? The orange one wasn't attached like it was supposed to be, so I may be missing parts from that.

cables.jpg

[NickScheuer]

I need the ball that the spring-loaded sleeve shown in the orange box snaps onto. The connections appearing in the other two boxes are complete and look just like yours. I may have been unable to understand how simple the throttle connection is (a simple sliding sleeve) and felt it necessary to dismount the ball post from the pivoting lever on the fuel system. In hindsight I should have just left the ball-stud alone; it wasn't really obstructing anything.

[NickScheuer]

And many thanks, nebulatech. No real rush; I need to mount a new prop and rehang the rudder before we go anywhere.

[nebulatech]

Man that's a bummer. That cable wasn't connected when I got the boat so I don't know where to look. Can you show me where it goes? Is it on or around the fuel pump? Is it on this chart:
https://www.marinepartseurope.com/en/vo ... 15120.aspx

Or one of the charts from this page? If you can tell me the item number on what page, that should enable me to locate it.
https://www.marinepartseurope.com/en/e- ... MD17C.aspx

[NickScheuer]

I don't recognize what Im looking for on the exploded view in regard to the cable. In the photo you posted I believe your motor is missing the pivoting lever that the end of the throttle cable should connect to. Just outside your orange square to the left you see what looks like a vertical shaft with a slot on top (colored gold or yellow). I believe the lever fastens to that shaft and the cable snaps onto a ball fitting at the end of the lever. The position of the cable end in your photo is almost the same location it would be if the ball were there. I'll photograph my motor tomorrow.

[NickScheuer]

Hi nebulatech:

The reason the ball joint does not appear in the exploded view you posted is that it shows only the fuel system. you need to go to the view showing "controls". The ball joint appears in a subassembly near the top and is listed as "ball joint" #28. You can also see the other parts for the pivoting throttle lever, none of which appear on the fuel system drawing. The lever and stops for throttle travel all fasten on or near the stud I mentioned on your motor.