Hi Dan,
I was told the same thing about the "Propeller Handbook" by Gerr. But as I discovered back in 2011, and have posted to this group numerous times since, the power requirement predictions are significantly off for for most of the types of conversions being done here.
Here's a copy of the first post that I made that showed the discrepancies in detail. You can see that Gerr sets my power requirements 2.8 times higher than my measured power consumption at 5 kts and 6 times higher than my measured consumption at 3kts. These are not minor differences. So one should always question predictive estimates and attempt to validate them against real world measurements wherever possible.
Aug 17, 2011
Hi Bill,
I went to check out the boatdiesel.com calculators, but it takes a $25 membership just to see them...
I'm familiar with the Gerr formulas, here's the reason that I asked if you had confirmed your numbers once you went electric:
I found a free online "Boat Speed Calculator" based on Gerr's formulas at www.psychosnail.com/boatspeedcalculator.aspx
Plugging in the numbers for my 30' ketch, I get the following results:
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
That seems to match up pretty well with the power curves for the Yanmar SVE12 and 13x10 2 blade prop that used to be in the boat. Full throttle provided 5.5kts at peak power (3200 RPM), just like the formula predicted. I also used your published numbers for Barbara Ann, LWL 46.3', displacment 35,700 lbs and target speed of 7.2kts and the calculator spit out 34.94hp. A little high, but fairly close.
Now here comes the curious part. I've logged the energy coming out of my batteries through a very controlled experimental process. I'm now running a Propulsion Marine 5.5kW drive. It is comprised of a Mars 0913 PMAC motor with a Sevcon Gen4 controller. I've got a 2.55:1 gear reduction and a 13x15.5 4 blade prop. This is the measured performance:
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
And the energy measured is at the battery, the drive system cannot be 100% efficient, so some of this power is lost along the way. Therefore, the actual shaft hp must somewhat less than the observed power figures listed above.
You can see that these power numbers are significantly lower than Gerr's predictions. I agree with you that "shaft horsepower is shaft horsepower is shaft horsepower - needed to MAINTAIN a given speed". All of my measurements, energy and speed, are averages of 4 different passes over a 400 yard course in opposite directions on each of two different days. Even more curious, almost all of the reported performance metrics posted in this group by owners of similar 29'-30' electric sailboats are within 10-15% of my measured numbers. Even if the other reported numbers weren't collected under such controlled conditions, they are consistent enough to support the idea that our hulls take significantly less energy to motor at these speeds than Gerr predicts.
That's why I asked if you had confirmed that your boat actually uses 22,500W to motor at 7.2kts in calm conditions now that it is electric.
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...
Fair winds,
Eric
I went to check out the boatdiesel.com calculators, but it takes a $25 membership just to see them...
I'm familiar with the Gerr formulas, here's the reason that I asked if you had confirmed your numbers once you went electric:
I found a free online "Boat Speed Calculator" based on Gerr's formulas at www.psychosnail.com/boatspeedcalculator.aspx
Plugging in the numbers for my 30' ketch, I get the following results:
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
That seems to match up pretty well with the power curves for the Yanmar SVE12 and 13x10 2 blade prop that used to be in the boat. Full throttle provided 5.5kts at peak power (3200 RPM), just like the formula predicted. I also used your published numbers for Barbara Ann, LWL 46.3', displacment 35,700 lbs and target speed of 7.2kts and the calculator spit out 34.94hp. A little high, but fairly close.
Now here comes the curious part. I've logged the energy coming out of my batteries through a very controlled experimental process. I'm now running a Propulsion Marine 5.5kW drive. It is comprised of a Mars 0913 PMAC motor with a Sevcon Gen4 controller. I've got a 2.55:1 gear reduction and a 13x15.5 4 blade prop. This is the measured performance:
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
And the energy measured is at the battery, the drive system cannot be 100% efficient, so some of this power is lost along the way. Therefore, the actual shaft hp must somewhat less than the observed power figures listed above.
You can see that these power numbers are significantly lower than Gerr's predictions. I agree with you that "shaft horsepower is shaft horsepower is shaft horsepower - needed to MAINTAIN a given speed". All of my measurements, energy and speed, are averages of 4 different passes over a 400 yard course in opposite directions on each of two different days. Even more curious, almost all of the reported performance metrics posted in this group by owners of similar 29'-30' electric sailboats are within 10-15% of my measured numbers. Even if the other reported numbers weren't collected under such controlled conditions, they are consistent enough to support the idea that our hulls take significantly less energy to motor at these speeds than Gerr predicts.
That's why I asked if you had confirmed that your boat actually uses 22,500W to motor at 7.2kts in calm conditions now that it is electric.
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...
Fair winds,
Eric
Marina del Rey, CA
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