serious discussion about electric power in large boats

[john61ct]

OK, I'll let go of the multiplication distinction.

But

Originally Posted by StuM

That's at least the second time you have equated "average" and "up to".

No, I was simplifying in order to clarify the units nomenclature.

The averaging is necessary (totalizing actually) if there is variation in the instantaneous rate unit (watts or amps) in order to get to the static quantity measure (Wh or Ah).

If the rate is actually constant (given a different example use case) then no need for averaging / totalizing.

Now for the first time, we bring the "up to" weasel wording into the discussion about units.

And the nomenclature usage remains the same, again with little to no room for misinterpretation IRL.

> It will not get you 300 Whr/hr if the poster meant 300 W and will require considerably more that 300 W at times if he meant 300Whr/hr

Up to means 300W is the maximum - and as you state, perhaps very rarely reached IRL.

Therefore a higher rate is not possible,

____
"Up to 300W", on the one hand, leads by definition to

"up to 300Wh per hour", on the other.

Same precise meaning.






I guess you are one of those who fall for the "Sale - up to 70% off" signs and then can't understand why everything you look at is only marked down by 10% (Hint: You'll find the one item in the store that is actually marked down by 70% in the most hard to get to location.

[SwellGuy]

Originally Posted by StuM

A unit of rate like watts or amps "held over a period of time" IS multiplication.

Ahhhhhh!!! Reading this was like a breath of fresh air.

[StuM]

Originally Posted by john61ct

Now for the first time, we bring the "up to" weasel wording into the discussion about units.

No, the post several of us were originally critical of used the "up to" weasel wording. I have emphasised it in several previous responses.

The averaging is necessary (totalizing actually) if there is variation in the instantaneous rate unit (watts or amps) in order to get to the static quantity measure (Wh or Ah).

If you use the MAXIMUM value (i.e. "up to") of a variable and multiply it by time, you are not "averaging" or "totalizing", you are deriving an impossible value that must be greater than the maximum achievable.

[danstanford]

I am not the original poster but I would give you more information. Looking at a J-88 Oceanvolt which claims 20nm range which would be good for every bit of sailing I did this year. Virtually every week I leave the dock and sail 5 minutes out into the lake to daysail or race.
My question is about finding a way to go more than that distance without having to worry about getting home if there is no wind. The motor is a 6kw motor which, it appears, would use only a fraction of that power at reasonable speeds. Could I reasonably expect to have a 4000 watt generator drive her at reasonable speeds once the batteries are depleted?

[StuM]

Originally Posted by danstanford

I am not the original poster but I would give you more information. Looking at a J-88 Oceanvolt which claims 20nm range which would be good for every bit of sailing I did this year. Virtually every week I leave the dock and sail 5 minutes out into the lake to daysail or race.
My question is about finding a way to go more than that distance without having to worry about getting home if there is no wind. The motor is a 6kw motor which, it appears, would use only a fraction of that power at reasonable speeds. Could I reasonably expect to have a 4000 watt generator drive her at reasonable speeds once the batteries are depleted?

What's "reasonable" to you?


As a WAG, I'd expect that you could probably drive a Benetau 323 at around 2.5 - 3 knots in calm conditions with a 4KVA generator.

[GrowleyMonster]

Originally Posted by danstanford

I am not the original poster but I would give you more information. Looking at a J-88 Oceanvolt which claims 20nm range which would be good for every bit of sailing I did this year. Virtually every week I leave the dock and sail 5 minutes out into the lake to daysail or race.
My question is about finding a way to go more than that distance without having to worry about getting home if there is no wind. The motor is a 6kw motor which, it appears, would use only a fraction of that power at reasonable speeds. Could I reasonably expect to have a 4000 watt generator drive her at reasonable speeds once the batteries are depleted?

As I said before, take all manufacturer/installer claims with a grain of salt. However for your usage pattern any prefab kit or turnkey setup will work just fine. But you want to be able to go further. Of COURSE you can use a generator for get-home power. I did it with my Ryobi 1600 watt portable gasoline generator. My manual charging setup was a 2kw Variac and the rectifier from a junked welding machine, and the requisite meters of course. If you want to run a "smart" charger from a small generator to feed a system that is at the same time pushing the boat, you could run into problems. And a "dumb" charger might not be adjustable, and you must be able to adjust the output.



What I did was to use the batteries until SOC reached 80%. Then I noted the current at the then present throttle setting. I set the variac so that the charge current into the batteries equaled that, and noted the system voltage. From there I maintained that voltage by adjusting the Variac occasionally, and did not touch the throttle. Except for the noise of the Ryobi in the cockpit it worked rather well. I just wanted to test the viability of having it onboard as a cheap emergency power source, not to use it on a regular basis. When I let the batteries rest without charging or discharging for 24 hours after returning to my slip, I found that my SOC was about 84%. So I was apparently not in any danger of over or under voltage damage to the batteries.

[john61ct]

Grin Satiator is a great little charger,

both voltage and current fully adjustable.

Very safe and reliable, only 300W each but can be stacked, fully isolated output.

The nominal 72V version delivers a max charging current of 4 A, 36V-103V max

the nominal 48V version delivers 8 amps, max 60V.

About $300

[danstanford]

Originally Posted by StuM

What's "reasonable" to you?


As a WAG, I'd expect that you could probably drive a Benetau 323 at around 2.5 - 3 knots in calm conditions with a 4KVA generator.

Stu, I would prefer to have 4-5 knots but for sure we are only talking about exceptional conditions here. The boat is actually a factory issued J-88 with an Oceanvolt system in it. Oceanvolt SD6 is a 48 volt 6 KW motor as I understand it.

The system does not scare me for the most part but I don't want to buy it and then end up stuck not being able to sell her because others are afraid of the limitations.
Dan

[jzk]

I think that electric power starts to get interesting in boats that can sail 10 plus knots. In those situations, apparently the ocean volt regeneration ability is staggering. There is a Voyage 580 that has the system, and apparently when it is sailing 10 knots or more, it can run all of the electric including the air conditioning.

If only there was a way to store electric energy such that one could motor for days if need be. And, I suspect the electric conversion in that Voyage was north of $150k.

Another problem is that propellers designed for propulsion are not the same as those designed for regeneration.

So far, the concept is awesome. If you want to refuel, just go sailing. Completely self sufficient. But, in practice, it seems it is just not there yet. And those that are paving the way are investing huge amounts of money to do it.

Originally Posted by tom jones

greetings capns
we are in the process of struggling through an engine rebuild/replacement with our 1985 Ford Lehman SP90 on our 43ft Brewer cutter centerboard, approx weight 25K.
as usual big plans with a small budget..
options to date: rebuild the Lehman replace with newer power plant ie Beta 50
In the process we continue to run into discussions re: electric power
trouble is we cant find an adult discussion about the feasibility of electric power in a vessel of our size and weight.
Our criteria: at least 100 miles cruising distance between charges
recharge capacity using 2 Kyracera 130s and Carib WindGenerator
and occasional Honda 2000i portable generator

any insight appreciated as well as experienced/knowledgable forums
its disappointing the lack of actual information and experience onhand.
thanks Tom Jones s/v Blue Trust
tj5035@gmail.com 443 454 4871


s

[seandepagnier]

A bristol 27 takes 150 watts to drive at 1.25 knots in calm conditions.

At 2.5 knots it already takes 500 watts. This is with a 25-40% efficient setup, using only 8 inch propellor. Much larger propellors would help.

If you triple the diameter of the propeller and gear it down 1:9 more, it produces twice the thrust for the same power input at these speeds.

Originally Posted by StuM

What's "reasonable" to you?


As a WAG, I'd expect that you could probably drive a Benetau 323 at around 2.5 - 3 knots in calm conditions with a 4KVA generator.

4kw is too much, it would only take a fraction of that power

[seandepagnier]

Originally Posted by jzk

I think that electric power starts to get interesting in boats that can sail 10 plus knots. In those situations, apparently the ocean volt

You could produce this power at any speed, you just need a larger propeller.

There is a system which generates many kilowatts from 2 knot currents by flying an underwater glider with control surfaces. This glider travels at 4-5 times the speed of the current and has a small generator turbine on it which it transmits power up the tether line.

You could fly one behind the boat and generate at the lowest speeds, then switch to a normal tow generator at 3 or 4 knots using a 30 inch propellor.

At 7 or 8 knots switch to a tiny 16 inch turbine to continue to generate. Better to use separate propellor for generation than propulsion for best efficiency.

regeneration ability is staggering. There is a Voyage 580 that has the system, and apparently when it is sailing 10 knots or more, it can run all of the electric including the air conditioning.

If you have wind to sail 10 knots, what is the point of air conditioning?

Another problem is that propellers designed for propulsion are not the same as those designed for regeneration.

You can design a propellor which is a compromise of both, but to generate efficiently at different speeds better to have different sizes.

[StuM]

Originally Posted by danstanford

Stu, I would prefer to have 4-5 knots but for sure we are only talking about exceptional conditions here. The boat is actually a factory issued J-88 with an Oceanvolt system in it. Oceanvolt SD6 is a 48 volt 6 KW motor as I understand it.

OK, I missed that the boat was a J-88, I was basing my guesstimate on the Beneteau 323 in your profile/avatar. The J boat is only half the displacement of the Bene so your 4-5 knots is feasible.

[CarlF]

GrowleyMonster

I generally agree with your comments. To get reasonable efficiency out of a marine diesel you have to be running at the right spot in the torque curve (50%-60% max RPM typically) and be comfortably below hull speed. But the same has to be said of diesel-electric. The genset has to be matched to the load and speeds have to be kept down.

I don’t agree that the current grid delivers low carbon electricity in most cruising areas. In many islands (Nantucket, Bahamas) it’s diesel.generated - although offshore wind has changed this on Block Island and in ten years on Marthas Vineyard. Also, utilities have to generate well above demand for voltage and frequency stability - and this is 24 hours a day. Of course the demand is spread across many users but when you charge from the grid it’s only fair to include some loss factor for this 24 hour standby availability of shorepower. With an engine (or diesel electric) it only emits when running the boat - rarely more than 2-3 hours a day for a cruising sailboat with a Captain who actually likes to sail.

A problem I’ve noticed with some diesel electric boat designs is the genset is oversized in the same way most boat propulsion diesels are oversized. This is inefficient at normal cruising speeds and even more inefficient when using the genset for hotel loads. Of course, you could install a big and small genset but that is an added expense and complexity.

But I agree with you that the diesel-electric has advantages and could make sense with the right design. My main problem with the the OP’s question is the frequent assumption by people looking for electric propulsion that a cruising sailboat could power 100 miles a day from solar panels or shore power charged batteries. I expect we agree on this

[danstanford]

Originally Posted by boat_alexandra

A bristol 27 takes 150 watts to drive at 1.25 knots in calm conditions.

At 2.5 knots it already takes 500 watts. This is with a 25-40% efficient setup, using only 8 inch propellor. Much larger propellors would help.

If you triple the diameter of the propeller and gear it down 1:9 more, it produces twice the thrust for the same power input at these speeds.


4kw is too much, it would only take a fraction of that power

So, assuming a cruising speed of 5 knots, how much power do you think would be required for continuous operation?

For my purposes I assume the solar could be counted on to augment this power source and also I don't think the regen discussion matters in this context because I would only need this when there was no wind (or dead the wrong direction) to get somewhere more than 20 nm away.
I sent these questions to Oceanvolt directly and will reply with their thoughts assuming they reply.
Dan

[StuM]

Originally Posted by danstanford

I sent these questions to Oceanvolt directly and will reply with their thoughts assuming they reply.
Dan

You will almost certainly get a reply from a sales-droid who will give you a very rosy picture. We've seen exaggerated claims plenty of times from OV on similar threads here. (Just search CF for "Magic Horsepower"