First facts.
1.
Props on boats are very inefficient. The larger the prop, the more efficient.
R squared relationship.
2.
ICE engines are very inefficient.
About 30%.
3.
Electric engines are very good for motive needs, and all data from tens to hundreds of installations, supports that en electric engine about 1/3 of equivalent ICE is sufficient.
4.
Older sailboats in the 10 m range had 12 hp engines.
With small, inefficient props.
This is/was plenty.
5.
Any amount of data indicates about 1 kW - 1.5 kW of power is needed, in calm conditions, to reasonably power a sailboat, at around 80-90% hull speed.
6.
Counterpoint.
Electric engines are cheap.
They are very efficient (if modern ac synchronous 3-phase,or servo motors, or sometimes some others) independent of power used.
Thus a 10 kW motor wont use (almost) any more energy when powered at 1 kW than a 1 kW motor.
Thus a bigger engine =/= bigger losses.
7.
Speed.
Slowing down by about 1 knot will double your range.
8.
Mass.
Mass is almost irrelevant and has almost zero effect on range.
Adding say 500 kg (ie huge amount) might affect your speed by 0.2 knots with exact same range (10 m boat).
The boat might be say 3 cm deeper and affect wetted suface area = friction by about 2% ? or something similar, even less.
9.
Prop:
This is contentious, somewhat. Almost no-one does this.
It will DOUBLE your range and DOUBLE your investment value.
Pick a prop from a boat double the size.
If a 10 m boat has a 40 cm D prop, aim for about 60-70 cm (for about 2-3 times more area).
The bigger prop will likely have very low rise, relatively.
And thus spin very slow. 1/3 typical speed.
And be very efficient.
And have huge thrust, holding ability, ability to resist current, waves, wind.
By my estimate, the 70 cm or 27" prop might have a 12-15" rise.
And might spin at 2-3-400 rpm.
(9) is how tugs for example are built.
There is an excellent reason for this.
30 cm rise x 300 rpm = 90 m / minute.
= 5.4 km / hr (too slow).
38 cm x 400 rpm = 15.2 km /h (too fast).
- losses at maybe 10-25%.
10.
10 kW usable => 5 hours at 2 kW.
2 kW might be about 4 - 4.5 knots at 27 feet lwl.
Range: 4 x 5 = 20 miles.
At 3-3.5 knots maybe 70% more range, ie 34 miles.
I tried to indicate how I would calculate this, in the That Seems To Be About Right (TLAR method).
10 kW is, as seen, plenty for daysail use, but not for passages.
BUT !!!
IF you add 4 x 350 W solar panels, your range is tripled, or more, in good weather.
For very low cost, about 0.50€ / Watt, plus controllers and bos, say 1€ all-in, approx 1500-1900€.
At 6 panels, you would have enough power for mobility at circa 3 knots, unlimited fair-weather range.
I might plan for a mount hangin over and behind the transom, in 2 levels, that you slide out (or swing down/up) behind or to the sides of the boat once out of harbor.
A suggestion:
Use plastic linear bearings from igus on 40 mm stainless steel rails.
Cheap, lasts forever, cannot rust, resists salt and spray.
Sliding out to sides or behind, depending on aesthetics.
Note moment arms and mass of panels.
A 3D CAD design could be made available (Rhinoceros 3D, export to iges or step).
Example:
https://www.civicsolar.com/product/sunpreme-maxima-gxb350-350w-double-glass-frameless-bifacial-solar-panel
Mass is 27 kg.
Hanging 6 x 27 kg = 180 kg, of which maybe 40 kg per side ouside transom/mount area, needs a sustantial mount.
Use about 40 mm DOM 316L tube (polished for looks).
Conceptually, I would plan for the mount to be strong enough to lift a person, or dinghy.
Note winds will have a big impact, one panel is 1.9 sq m in area.
HTH.
-Hannu
On 10/08/2016 06:09, clamk123@yahoo.com [electricboats] wrote:
It's LOA 30.75 (LWL 27.92) with an 11,000 lb. displacement.
I intend to remove much of the interior weight as possible (obviously engine, tanks, hosing, etc) as well as other features not needed anymore (appliances) to support more stored energy.
I've read many discussions on larger sized props; how would one go about determining the best prop?
What would be an acceptable motor for this size and displacement? Due to where I'm at, there can be substantial currents.
My initial battery bank would be about 10kw, does this sound under sized? I'm not looking to go particularly fast, so I should be able to make it stretch fairly well at low speed, but can go higher if there's a good argument for it. I am looking for distance/duration more than speed.
-- -hanermo (cnc designs)
Posted by: Hannu Venermo <gcode.fi@gmail.com>
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