To me this is obvious. I am quite certain but might be wrong. (No, not really).
1. The ICE system is quite inefficient.
At top speed, about 50% of generated power gets to the prop.
Lest is lost in transmission and parasitic losses (engine auxiliary loads).
Parasitic losses are water pumps, laternators, oil pumps, belt losses, heat etc.
2. At low speeds, the efficiency is much lower, as small ICE systems are optimised for top speed running.
This is excarberated because the ICE poarasicitic losses are more visible at los speeds, and ICE tech. is less efficient at low RPM.
Also, there has never been a strong compelling commercial reason to optimise small boat ICE systems for low speed running.
As running costs are very low at low rpm, why bother.
3. The efficiency numbers are also consistent - practically all are within 10%-15% as Eric said.
This further points to the well known wisdom that they are all simlar. "Same- same, but different."
Furthermore, as we have seen on multiple conversion, electric Hp appear to be 2-3 times "better" than ICE Hp.
IF, on a given boat, shaft, propeller, the only difference is the ICE system vs the electric system, thus the inefficiency is within the ICE system itself. Has to be.
4. Given that the trends are all very close, these also poitn out to the well-known fact that most hulls are very close to each other in efficiency.
Ie hulls dont matter that much, given a basic shape.
Ie a double-ender, canoe shaped, long slim hull, is efficient (sailboat), and a short, fat, squat, heavy is less efficient (Nordhavn).
All the rest matter very little.
(In a sailboat, hull balance matters a lot, and comfort varies widely depending on hull shape, but we ar elooking at efficiency only, here).
This is further strengthened by the fact that there are tens of thousands of hulls with similar performance, and the numbers much up very closely.
The only things that have a big, meaningful effect are length waterline (basic speed or hull speed), and basic shape (multiplier), and mass (multiplier).
Both of the three effects are mostly negated by the speed-squared exponential power required after a given, very low power needed.
5. The power of the early 200-400 ton trading ships, in the early 1900s, was about 80-150 hp.
When the age of sail transitioned to steam.
6. As a conclusion LWL is the best way to increase effiency and reduce overall costs and speed made good (if marina cost is not taken into account).
7. Bill Southworth -
As you have excellent data, measurement systems and experience, could we see that is the actual consumption of power at 0.8 and 0.9 hull speed (hull speed) with the 100 kW conversion ?
If the trends agree, it shoud be approx.
20-40 kW, or 30-50 Hp, vs the 72 Hp from the ICE, going from 5.5 - 7.2 knots, or 80% to 100% of hull speed.
Plus any additional electric load (that may be significant) Many bigger boats have 5-15 kW loads on full-time, especially if they have stabilisers and air-con.
Best,
hanermo
3kts = 2.97kW or 3.98hp
4kts = 4.37kW or 5.86hp
5kts = 6.81kW or 9.13hp
5.5kts = 8.74kW or 11.72hp
3kts = 9.54A at 52.4V = 500W or 0.67hp
3.9kts = 19.2A at 52V = 999W or 1.33hp
4.4kts = 28.9A at 51.5V = 1488W or 1.98hp
5kts = 48.5A at 50.7V = 2459W or 3.28hp
5.7kts = 78.8A at 49.6V = 3908W or 5.21hp
What do you think? Feel free to share this info with the rest of the brain trust behind your boat. It does make one wonder...
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