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Old 17-04-2019, 16:02   #16
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Re: Electric motor conversion

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Originally Posted by Adelie View Post
If I need to go upwind that’s what sails are for.

"Upwind" ( i.e with the wind forward of the beam) is not the same as "into wind" (i.e. wind on the bow).


Good luck using sails to go "into wind".
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Old 17-04-2019, 16:58   #17
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Re: Electric motor conversion

I see, too impatient to sail on a sailboat.
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Old 17-04-2019, 18:01   #18
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Re: Electric motor conversion

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Originally Posted by NPCampbell View Post
I'm curious where these numbers come from. As long as you are below hull speed, shaft HP generally increases with the cube of speed. So 50% hull speed should generally equal HP/8 at 100% hull speed. 50% hull speed should require 15.5kW if 124kW is req'd at 100% hull speed. Shaft HP is independent of the method of propulsion. (i.e. HP req'd = disp pounds / (10.665 / SL ratio) ^ 3)

Solar: 50% hull speed or 15.5kW would require around 1000 sq ft of solar operating at 100% efficiency. 1000 sq ft is larger than the footprint of a 45' catamaran.

Battery: 15.5kW would last less than 45 minutes taking 1000 AH of batteries to 100% DOD without running the genset.
The power (kW) come from an Oceanvolt model. While they are theoretical as there's no motor in our particular hull to get empirical evidence, people who have had the theoretical figures and then gone with the company have said that the match with the reality afterwards is very close. I'm using them as a example.

Your HP calulation comes from Gerr (who may have got it elsewhere), who says merely that it's "useful" in predicting power and speed. Importantly, the 10.665 is not something that can be used to apply to all hull shapes and boat types. In short, it's not a universal truth in the slightest!

Regarding solar, and needing more than a 45' cat - we have a 50' catamaran . We certainly plan a reasonable solar array, with about 3.5kW likely.

I haven't looked further at the battery sizing. Your 45mins would not surprise me at all depending on speed, conditions (and battery type of course).
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Old 17-04-2019, 19:17   #19
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Re: Electric motor conversion

Yes , you can get enough energy to run your motor all day but let me know how you plan to cool down the motor when in operation??? Because air-cooled motor don't do well in boat compartments and water coiled ones don't do good at all .
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Old 17-04-2019, 21:52   #20
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Re: Electric motor conversion

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Originally Posted by bluenomads View Post
Your HP calulation comes from Gerr (who may have got it elsewhere), who says merely that it's "useful" in predicting power and speed. Importantly, the 10.665 is not something that can be used to apply to all hull shapes and boat types. In short, it's not a universal truth in the slightest!
I agree. However, even if the 10.665 is hull dependent variable then it still holds that HP varies with the cube of speed.

Quote:
Originally Posted by bluenomads View Post
Regarding solar, and needing more than a 45' cat - we have a 50' catamaran . We certainly plan a reasonable solar array, with about 3.5kW likely.
Wow, that's a ton of solar! You definitely won't be hurting for on board power
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Old 17-04-2019, 22:42   #21
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Re: Electric motor conversion

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Originally Posted by cgm12mgc View Post
We get it, you’re no fan of electric motors. You can stick to diesel... I do in my other boats. People survived for thousands of years without a motor in their boat and as I have stated, mine is to get out of the marinas and under bridges etc. don’t like it? Don’t do it.


Same concept as what you’re saying, I can run my 61’ sportfish wide open throttle for a few hours but i’m up shi*ts creek when I run out of diesel at a burn rate of 120 gallons an hour... atleast with electric I can refill while on anchor 👍🏻
Not at all. For some limited output use cases, it's completely viable. If I need a dingy just to get around the harbor at low speed, it will likely be electric as it's very much viable to operate short distances at low speed and the cost to implement is low and the ongoing maintenance is low.

But lets take your example of a 61' sportfish...you may be able to do wide open for a few hours with diesel...trying that with electric motors, you would be lucky to put out the same power for 5 minutes...but that is a wildly different use case from a displacement hull. You most likely could do hull speed with less than 20% of the available HP in your sportfish (maybe less than 10%). Also a 61' hull will have a much higher hull speed compared to a 35' sailboat. le: speeds in a sportfish to fight a current or storm conditions are far below full throttle.

Let's say you are trying to fight a 5kt current...Your typical 35' sailboat will top out around 7kts, so full throttle will only get you 2kt SOG. If you throttle back to get more duration, you likely come to a stand still. On a 30kt cruise speed (not full throttle) sportfish, you can continue at cruise speed and still are making 25kt SOG...its not a big deal.

I will repeat: IF YOU ARE WILLING TO LIVE WITH THE REDUCED PERFORMANCE, IT'S VIABLE. But understand it is significantly reduced performance.
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Old 18-04-2019, 04:21   #22
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Re: Electric motor conversion

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Originally Posted by bluenomads View Post
Ease off to 90% of full speed and (nearly) double your time.
Ease off to 80% of full speed and quadruple your time.

People with a 45'/14t cat can motor 4.5-5kn with solar alone - that's people out there doing it, not charging their armchairs.

I'm still charging an armchair too right now, but figures we've received are that, since our nominal hullspeed would be near 9kn, so
nominal hullspeed = 124kW (let's pretend that is 100%)
90% hullspeed = 83kW (59% of max amperage)
80% hullspeed = 30kW (21% of max amperage)
70% hullspeed = 22kW (16%)
60% hullspeed = 11kW (8%)
50% hullspeed = 6kW (4%)
Now your numbers will be different, but the idea is to look at the progression as your go slower.

If you travel at maximum, you won't last long on batteries alone (the comment on only getting 2 hours with the genset running is purely a case of poor choice of matching speed-motor-genset-batteries and not inherent to electric motors!).
BUT, if you're will to come down a little in speed, then there's a lot of possibilities for matching ok speed with distance under batteries alone, or under batteries with genset (maybe solar input too).
So at nominal hullspeed, if the OP had 2 hours of battery draw available at my figures above (an HUGE amount at that speed, but the importance isn't the 100% value it's how much it increases with speed), then the use on batteries alone is:
Nominal hullspeed = 2hrs
90% hullspeed = 3.4hrs
80% hullspeed = 9.3hrs
70% hullspeed = 12.7hrs
60% hullspeed = 25hrs
50% hullspeed = 47hrs
WITHOUT genset!

There's more and more doing it and it works.

In our reality, we see a sweet spot at 60% of hullspeed or about 5.5kn using 11kW total. An 18kW DC genset would work fine giving continuous motoring as long as the diesel lasts (and much quieter and a bit more efficient than diesel only propulsion). We could decide to go 1kn slower, and use about half the power meaning running on batteries-only for short time/distances between endpoints is feasible. Want longer battery-only without genset, then just buy more of them...

Ooof! This is very "uncatamaran-like."

The weight of this system is ridiculous and will definitely ruin the performance of the boat. Also, if you want more range just keep adding batteries?? At 70lbs each even for lithium ion?

Should just get a monohull for this setup.

A pair of outboards is thousand of lbs lighter and you're still burning fossil fuels to run a generator so no environmental advantage.

3.5kw Solar 480lbs
13kw generator 870lbs
Wiring 100 lbs
26 KWH lithium ion Battery Bank 1000lbs
10kw electric motors x 2 is 48lbs (x2)
100 gallons diesel Fuel for generator 700lbs

That's 3,250lbs to get the indicated performance.

Pair of 30hp outboards 187lbs (x2)
100 gallons fuel 700lbs

So 3,250lbs of weight for so-so propulsion where you have to reduce output constantly or 1,000lbs of weight for full, continuous output.

No brainier.

This system is horrible for a catamaran. It's suited to monohulls or condomarans only.
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Old 18-04-2019, 14:50   #23
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Re: Electric motor conversion

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Originally Posted by Chotu View Post
Ooof! This is very "uncatamaran-like."

The weight of this system is ridiculous and will definitely ruin the performance of the boat. Also, if you want more range just keep adding batteries?? At 70lbs each even for lithium ion?

Should just get a monohull for this setup.

A pair of outboards is thousand of lbs lighter and you're still burning fossil fuels to run a generator so no environmental advantage.

3.5kw Solar 480lbs
13kw generator 870lbs
Wiring 100 lbs
26 KWH lithium ion Battery Bank 1000lbs
10kw electric motors x 2 is 48lbs (x2)
100 gallons diesel Fuel for generator 700lbs

That's 3,250lbs to get the indicated performance.

Pair of 30hp outboards 187lbs (x2)
100 gallons fuel 700lbs

So 3,250lbs of weight for so-so propulsion where you have to reduce output constantly or 1,000lbs of weight for full, continuous output.

No brainier.

This system is horrible for a catamaran. It's suited to monohulls or condomarans only.
not sure where you're getting catamaran from but mine is a 45' monohull
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Old 18-04-2019, 17:36   #24
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Re: Electric motor conversion

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Originally Posted by cgm12mgc View Post
not sure where you're getting catamaran from but mine is a 45' monohull

From Bluenomads stated figures for his powercat.



Obviously, you won't be putting 3.5kW of panels on a 45ft monohull sailboat
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Old 19-04-2019, 00:04   #25
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Re: Electric motor conversion

Yeah. StuM is correct. I saw 45' Catamaran, talk about 5-6 knots motoring a catamaran, Blue Nomads, etc and presumed a catamaran discussion.

The weight, of course, has less impact on a monohull.
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