Repowering? Bigger is better?

[Terra Nova]

Originally Posted by Delancey

...I would like to build my electrical system around a very lavish house bank which I hope to draw on modestly and mostly charge with wind and solar...

The more "lavish" your house bank, the more you will periodically spend on its replacement.

[Stu Jackson]

Originally Posted by Delancey

I am not disputing this, but how do most liveaboard cruisers really use their engines?

My guess is "well I usually run the engine for about an hour in the morning to charge up the batteries and pull down the icebox" has to add up and that in the end it might account for as much or more much engine hours as actually using the thing to get you somewhere.

Most don't, that's the reason for solar. That's the whole point. If you ARE going somewhere under power, then it charges the bank, based on what it needs.

[Delancey]

Originally Posted by Terra Nova

The more "lavish" your house bank, the more you will periodically spend on its replacement.

Ok, maybe this is a chance for me to show more about how much I don't know, but doesn't a bigger bank with lower demands get you longer overall life? Do I have that one wrong?

[Delancey]

Originally Posted by Stu Jackson

Most don't, that's the reason for solar. That's the whole point. If you ARE going somewhere under power, then it charges the bank, based on what it needs.

I didn't mean to come across all hell bent to turn my engine into a generator, more so maybe just looking for reasons to justify an even smaller engine. So for discussion sake, if you take propulsion out of the picture, any thoughts on the best size engine to run a 100 amp alternator? Seems like in that case smaller would be better.

[Stu Jackson]

Originally Posted by Delancey

Ok, maybe this is a chance for me to show more about how much I don't know, but doesn't a bigger bank with lower demands get you longer overall life? Do I have that one wrong?

You are correct. I think that post was somewhat tongue in cheek. Of course, if you buy a larger bank, it will cost more to replace, eventually. However, a lower daily draw down will make the bank last longer. Plus, as Maine Sail is fond of saying, the biggest reason banks fail is poor/chronic undercharging by owners! A good sized bank, good solar and controller, a good battery monitor and you'll be fine.

[Delancey]

Originally Posted by Stu Jackson

You are correct. I think that post was somewhat tongue in cheek. Of course, if you buy a larger bank, it will cost more to replace, eventually. However, a lower daily draw down will make the bank last longer. Plus, as Maine Sail is fond of saying, the biggest reason banks fail is poor/chronic undercharging by owners! A good sized bank, good solar and controller, a good battery monitor and you'll be fine.

Whew, I got the bit up front about the larger the bank the greater the replacement cost but was suddenly worried had I missed something big.

[cwyckham]

Originally Posted by Delancey

I didn't mean to come across all hell bent to turn my engine into a generator, more so maybe just looking for reasons to justify an even smaller engine. So for discussion sake, any thoughts on the best size engine to run a 100 amp alternator?

Well, we've been debating how many hp a 100 amp alternator takes, but it should be about 3-5 hp. So, since you should run a diesel at about 80% load, you're looking at maybe 7 hp.

[Delancey]

So I talked with Farron again at Beta who was very helpful and kind enough to answer all my questions. Here's what he gave me for numbers where my waterline boat speed is is 7.75 knots in flat water-

Model (HP) HP/LT (BS) RPMS
Beta38 (38) 4.7 (7.9) 3,600
Beta35 (32.5) 4 (7.6) 2,800
Beta30 (30) 3.7 (7.5) 3,600

He said the difference between the 35 and the 38 amounted to a difference in the flatness of the peak torque curve, with the one being more flat than the other.

If I have him correctly he was of the opinion that draw of the alternator, being variable, was sort of inconsequential and minimal and that any engine would be fine with a 120. I neglected to ask him specifically a for number for alternator HP draw so, next time for that one.

Some would already know this but the best size engine for using as a generator is one that is likely running at either 1,500 or 1,800 rpms of course. He said they build out a lot of these engines as generators with the 38 producing 10kw and the 30 putting out 8.5kw.

He's sending a ballpark price on the 30 as well as a price for a fancy feathering prop which he figured was worth a solid knot of boat speed when sailing.

[S/V Alchemy]

Originally Posted by dannc

One rule of thumb is 1 HP per 500 pounds of displacement.

18,000/500=36 HP

I have three formulas that Gerr has published that use displacement and they spit out 36-38 HP.

Later,
Dan

That's why I got a Beta 60 for my 15 tonne steel 41 footer. It was still lighter than the Westerbeke 52 it replaced.

It's mated to a four-bladed feathering VariProp, however, as I need torque over speed...what with the inertia and all. Frankly, I will only motor in calms or to get in or out with authority.

[cwyckham]

Originally Posted by Delancey

So I talked with Farron again at Beta who was very helpful and kind enough to answer all my questions. Here's what he gave me for numbers where my waterline boat speed is is 7.75 knots in flat water-

Model (HP) HP/LT (BS) RPMS
Beta38 (38) 4.7 (7.9) 3,600
Beta35 (32.5) 4 (7.6) 2,800
Beta30 (30) 3.7 (7.5) 3,600

He said the difference between the 35 and the 38 amounted to a difference in the flatness of the peak torque curve, with the one being more flat than the other.

If I have him correctly he was of the opinion that draw of the alternator, being variable, was sort of inconsequential and minimal and that any engine would be fine with a 120. I neglected to ask him specifically a for number for alternator HP draw so, next time for that one.

Some would already know this but the best size engine for using as a generator is one that is likely running at either 1,500 or 1,800 rpms of course. He said they build out a lot of these engines as generators with the 38 producing 10kw and the 30 putting out 8.5kw.

He's sending a ballpark price on the 30 as well as a price for a fancy feathering prop which he figured was worth a solid knot of boat speed when sailing.

What is BS in the chart above?

So what did he recommend for you, the 35 or 38? Why?

Feathering props are awesome.

[Delancey]

Originally Posted by cwyckham

What is BS in the chart above?

So what did he recommend for you, the 35 or 38? Why?

Feathering props are awesome.

Sorry, BS is boat speed.

Without offering an explicit explanation he recommended the 38 over the 35. As the flatness in the peak torque curve in the 38 means that it's sweet spot has a wider range, I am even at more of a loss to understand why they even offer the 35.

Yes, feathering props look like they have a lot going for them. I have a fair amount of experience with folding props but these look like a step up. More $$$$ than I might have expected but seems like worth investigation.

One thing I notice when looking at the performance graphs is at that 1500-1800 rpm range, which is also well below peak torque, any of these engines is running at maybe 1/3 max hp when used as generators and they typically do so for a very long life at those fixed rpms.

[Andrew Troup]

I guessed BS was Boatspeed. What did you guess ?

Delancey asked how I planned to integrate my hydraulic drive, keel, anchor windlass etc with my electrical system ...

Well, mate, I'm a mechanical engineer. If I could buy or build hydraulic nav lights and cabin illumination, I'd do it like a shot.

As it is I'll carry kerosene and candles as backup... and use electrics for (LED) lighting and "thinking" only, with a couple of exceptions:

I may retain the electric starting capability on the diesels (which are small, and equipped with decompressors which I plan to be able to trip remotely, hence easy on batteries), but hand and hydraulic will be the bottom line for starting.

If I can run my particular hydraulic pumps as motors (which I'm pretty sure will be OK, but I need to hook up and verify) the hydraulic starting is just a matter of valving. My hydraulic valves will all be manually operated, not electrically. Backup to directional-control valving will be by quick-connects.

My hydraulic drive will fit through a cylindrical well whose lip is above the waterline, so it can be withdrawn for access to the prop, even when underway.

I plan to build a prop with separate blades which plug into the boss via tapered splines. This will mean the prop can be manually repitched on long trips to a compromise between what is optimal for motoring, vs for using the hydraulic drive to regenerate and recharge the hydraulic accumulators.

I don't plan on using electricity for bilge pumps. I won't run electrical wiring anywhere near the bilges, except for speed and depth where it's unavoidable.

My diesel stove has an electric fan to assist with startup, but it will start OK without.

My LPG (for the cooking hob) will have a manual shutoff valve, not electric.

For autopilot, I plan to have a couple of small-boat tillerpilots, one installed in a watertight compartment, operating the servo oar of the windvane steering.
The other will be in reserve, in the spares locker.

They seem pretty reliable as long as they are kept dry.

They need to be hooked up with a linkage permitting them to overstroke at each end, because the servo oar does not need anything like the stroke of a tiller, and if the discrepancy is taken up by fitting a long tiller arm to the servo oar, the response is much too slow. (And the whole deal takes up unnecessary space)

This minimises power required for the autopilot because the boatspeed (through the servo oar) provides all the muscle; the electricity just does the thinking.

I'm not sure about radar, it's certainly not on my "must-have" list, and because my electricity supply will be limited. Especially now there is AIS.

[Delancey]

Fascinating. I am intrigued and can't wait to see as your project progresses. I myself am a big fan of John S. Letcher and maybe share some of your outlooks but am obviously dealing with a different stack of cards.

You mentioned 30hp all up for a 7,500kg 10m waterline boat which isn't too far off my displacement, or did you mean weight? I have no idea how much I actually weigh at present and with all our crap onboard.

Also, do I read you correctly there will be no alternators or dynamo in your system to produce electricity from your diesel?

[a64pilot]

I feel certain they offer a 35 because a major costumer who buys many engines wants a 35. I'm sort of surprised there is any difference between the two often the only difference is the RPM at which they are rated

[cwyckham]

Originally Posted by Delancey

Fascinating. I am intrigued and can't wait to see as your project progresses. I myself am a big fan of John S. Letcher and maybe share some of your outlooks but am obviously dealing with a different stack of cards.

You mentioned 30hp all up for a 7,500kg 10m waterline boat which isn't too far off my displacement, or did you mean weight? I have no idea how much I actually weigh at present and with all our crap onboard.

Also, do I read you correctly there will be no alternators or dynamo in your system to produce electricity from your diesel?

Displacement and Weight are the same thing.