Sizing Twin Electric Pod Drive Motors for a 33ft Cat

[CaptainShack]

Hello!

Hope someone can help break this down for me. I've got a Prout 33CS Quest thats in need of an refit. Mywife and I are very interested in going for a hybrid system and I've been looking over the Epropulsion line of Pod drives as a possible replacement for the 20HP Yanmar and replacement for the Sonic Drive these older boats have that they no longer manufacture.

We run the current single engine around 2600rpm and have motored along at a fine 5+knts. Comfortable in the ICW no big deal.

My question is.. When switching over to twin engines how does the power equate? Are two 9.9hp equivalent 6kw pod drives around the same thrust as a 20hp 2ym20?

Now in a perfect world I'd go for twin 6kW / 9.9HP with the two of the big E175 batteries.
but it turns out the space between my rutter and skeg.. bar, beaching plate? Not sure what you call that under the prouts hulls isn't large enough to fit the Pod Drive 6.0 without hull modification. Don't want the pod drive being lower than those! Looks like it sits about 4 inches lower. Hate to see us run aground and rip the drive off.

So the question... Can we push the boat along with twin 3kW / 6HP equivalent pods or will this cause us issue with power in those tight spots?

[valhalla360]

6kw ~ 8hp, so, that's around 16hp, so you are giving up some power. On the positive side, splitting it across two props is a small improvement but for most purposes, HP=HP.

In most conditions, that's plenty but in some conditions, it will be a bit light. We had a 25hp on our 34ft Gemini and a few times we needed all of it...such as motoring up a narrow channel directly into a 30kt wind doing around 2kt at full throttle. A fair number of folks run with 15hp outboards with Geminis. As long you accept the reduced power and capability, it should be fine.

Be careful of claims of "equivalent HP" most come from the automotive world. The reason a 100hp electric motor can match a 200hp ICE in a car is because, the electric motor has max torque from 0 RPM. An ICE typically has to build up RPM to reach max torque. HP = Torque * RPM, so the low end torque means at low RPM, the electric motor is producing more HP. In reality, your average ICE never puts out it's rated max HP in real world use. Cruising down the freeway a small car might be putting out 50-60hp electric or ICE.

With displacement cruising boats, acceleration isn't a big consideration. Plus since the prop can slip in the water, the ICE can quickly build RPM and get up into peak torque. As a result, HP to maintain cruising speed is the limiting criteria and at steady speeds with motors appropriately sized, electric or ICE are running at or near peak torque and HP=HP. Reality is you almost never see people using full throttle while docking...unless they don't know what they are doing or things are long since out of control.

If you can feed them, electric is a great option. The problem is storing the feed for electric motors is much more difficult if you want any significant range.

[CaptainShack]

Thanks for the info. Seems the only drive that really fits would be twin 6kW drives. Issue with my cat is there slightly to long. They stick out past the hulls skeg.. bash plate... Whatever you call those. Pods Stick past about 4 inches. Would need to come up with a fix.

Thanks for the info on HP

[7H3LaughingMan]

Long time lurker, decided to make an account to see if you have taken a look at Torqeedo. They have a 12kw Pod Drive that they say is equivalent to a 25hp engine, so if your set on using dual pod drives that is an option if you want a bit more power. But from my understanding most pod drives are meant for smaller monohulls, so not sure about setting two up on a catamaran unless your treating them as two separate systems.

Other than that you could look at electric sail drives, sure the motor is going to be on the inside but you might not have to make any major hull adjustments since the part sticking out of the boat should be shorter than a pod drive.

I am provided a link to a bunch of electric sail drives and pod drives that you can peruse to see what else is out there.

https://plugboats.com/guide-to-elect...d-boat-motors/

[PineyWoodsPete]

Originally Posted by valhalla360

6kw ~ 8hp, so, that's around 16hp, so you are giving up some power. On the positive side, splitting it across two props is a small improvement but for most purposes, HP=HP.

In most conditions, that's plenty but in some conditions, it will be a bit light. We had a 25hp on our 34ft Gemini and a few times we needed all of it...such as motoring up a narrow channel directly into a 30kt wind doing around 2kt at full throttle. A fair number of folks run with 15hp outboards with Geminis. As long you accept the reduced power and capability, it should be fine.

Be careful of claims of "equivalent HP" most come from the automotive world. The reason a 100hp electric motor can match a 200hp ICE in a car is because, the electric motor has max torque from 0 RPM. An ICE typically has to build up RPM to reach max torque. HP = Torque * RPM, so the low end torque means at low RPM, the electric motor is producing more HP. In reality, your average ICE never puts out it's rated max HP in real world use. Cruising down the freeway a small car might be putting out 50-60hp electric or ICE.

With displacement cruising boats, acceleration isn't a big consideration. Plus since the prop can slip in the water, the ICE can quickly build RPM and get up into peak torque. As a result, HP to maintain cruising speed is the limiting criteria and at steady speeds with motors appropriately sized, electric or ICE are running at or near peak torque and HP=HP. Reality is you almost never see people using full throttle while docking...unless they don't know what they are doing or things are long since out of control.

If you can feed them, electric is a great option. The problem is storing the feed for electric motors is much more difficult if you want any significant range.

For a real world example, our 33' Greenline hybrid motor cruiser has a 7kW elect. motor (5kW gen. with diesel running) in-line with the diesel, separated by a clutch, with a relatively large 48V/240Ah 11.5kWh Lipo battery bank, driving the 165HP diesel's 17x14" 5-blade prop at about 600 shaft RPM through the 2:1 tranny. The boat is about 6 tons in cruising mode w/ hull speed about 7.6kts.

In benign conditions, we can reach only about 5.3kts @ full tilt, and maybe 10nm before recharging , or 15nm throttled back to 3-4kts. So it's mainly for harbor out/in usage, with occasional whale-watching in silence after dieseling out. The biggest advantage cruising is the huge battery capacity and all-electric galley...

Your cat would probably get 1-1.5kts more with 6kW output between the two motors, with a healthy reserve, but battery capacity would be a huge challenge in the confines of the ICW we've traveled so much in another life...

[CaptainShack]

Oh this is really interesting Piney. So let me break this down to look at comparables. Your the closest boat yet to have a system close to what we're thinking. So thank you for jump in on this conversation.

Quest 33 / 33' Greenline
9500lb / 12,000lb
twin 6kw pods / single 7kW elect, 5kW with Gen Running?
48v/17.8kWh / 48v/11.5kWh Battery Banks
/ 7.6 hull speed loaded
/ 15nm range pushing 3-4kts

Is that 16.5hp diesel? Assume not 165hp! The big bank of batteries does sure sound nice, hows your solar setup? Or any? We're a cruising cat so I'm planning on adding a substantial solar upgrade after I finish this deck work.

I'm wondering how much that weight difference will make as well as the extra 6kwH of power. Starting to think this little cat is in a sweet spot for this project. Can you tell me more about the diesel and you mentioned 5kw?

[valhalla360]

Originally Posted by CaptainShack

We're a cruising cat so I'm planning on adding a substantial solar upgrade after I finish this deck work.

Define "substantial" for us.

On our 34ft cat, I figure I might be able to get 1000-1500w of solar (assuming we don't ditch the mast and cover the entire boat. Which you could expect to put maybe 4-6kwh each day.

That would be enough for use to cruise at maybe 6kt for a half hour.

As suggested, the big advantage would be a battery bank sufficient to run air/con and other house loads at anchor for extended periods and then have a modest sized generator to top them up when away from shore power for extended periods.

[CaptainShack]

Substantial - Multiple times more than we currently have.
Two ancient scratched up and rusted panels pulling around 250w I have yet to measure the mounts we already have to see how much we can handle. I'd like to get something similar to what your running 1 to 1.5 coming in. Prior owner seems to have taken half the panels with them.

[valhalla360]

Originally Posted by CaptainShack

Substantial - Multiple times more than we currently have.
Two ancient scratched up and rusted panels pulling around 250w I have yet to measure the mounts we already have to see how much we can handle. I'd like to get something similar to what your running 1 to 1.5 coming in. Prior owner seems to have taken half the panels with them.

I only have 200w but without outboard power, that's plenty.

To get 1.0-1.5kw of solar will require the entire stern be covered with a substantial structure to hold it. The cost of installation would likely cover fuel costs for many years.

[CaptainShack]

I've already got a mount at the back of my boat with no solar on it currently. Looking forward to seeing how much I can fit on what's already on the boats. the 200w's does a great job of keeping the AGM's charged for the fridge/freeezer and lights. But yeah getting a professional to come out I'm sure would cost substantial amount to upgrade. Though with the price of diesel. Not sure how many years it would be!

[PineyWoodsPete]

Originally Posted by CaptainShack

Oh this is really interesting Piney. So let me break this down to look at comparables. Your the closest boat yet to have a system close to what we're thinking. So thank you for jump in on this conversation.

Quest 33 / 33' Greenline
9500lb / 12,000lb
twin 6kw pods / single 7kW elect, 5kW with Gen Running?
48v/17.8kWh / 48v/11.5kWh Battery Banks
/ 7.6 hull speed loaded
/ 15nm range pushing 3-4kts

Is that 16.5hp diesel? Assume not 165hp! The big bank of batteries does sure sound nice, hows your solar setup? Or any? We're a cruising cat so I'm planning on adding a substantial solar upgrade after I finish this deck work.

I'm wondering how much that weight difference will make as well as the extra 6kwH of power. Starting to think this little cat is in a sweet spot for this project. Can you tell me more about the diesel and you mentioned 5kw?

Nope, 165HP turbo-diesel, which will give us about 14kts @ 4000 RPM. The 7.6kts hull speed is a calcuilation from her 32' WL length. We've only 500W solar, but 1800 is a factory option on her long coachroof. We never use the AC in the cool year-round W coast waters - 70 deg. is a heat wave! - but with a couple of teenage grandkids the little solar and big battery give us at least four days at anchor without re-charging with the full electric galley and large fridge/freezer ...

Seems to me that12kW power in your cat should give a nice power margin for adverse conditions with your large battery bank, and a sizable generator will give you peace of mind in ICW conditions!

[aybabtme]

I have a Seawind 1000 XL2, about 35.5ft. I installed two ePropulsion Pod 6.0 (6KW each). In fair condition, I use maybe 3kW to get moving at 3-4kts. I need only about 4-5kW total when the sea is a bit more choppy or I want to move faster. At 12kW I reach about 7kts but that's pretty useless to me.

I would probably be fine with just 2x3kW, but I like the tranquility of having 2x6kW.

Here's a graph that I compiled on one afternoon of sea trial (mild conditions).

[PineyWoodsPete]

Originally Posted by aybabtme

I have a Seawind 1000 XL2, about 35.5ft. I installed two ePropulsion Pod 6.0 (6KW each). In fair condition, I use maybe 3kW to get moving at 3-4kts. I need only about 4-5kW total when the sea is a bit more choppy or I want to move faster. At 12kW I reach about 7kts but that's pretty useless to me.

I would probably be fine with just 2x3kW, but I like the tranquility of having 2x6kW.

Here's a graph that I compiled on one afternoon of sea trial (mild conditions).

Good thing that you installed the 6 kW drives, as by your graph, the 3's would give little reserve for adverse wind or current conditions under power only. How large is your battery and solar setup, and do you have generator backup? Where's your home port in CA?

Our 5kW/48V hybrid motor/gen in-line with the 165hp diesel gives us a max of about 5.4kts in flat harbor conditions, and a max range of maybe 8-10nm with the 11.7kWh battery and our small 500W solar.

[aybabtme]

I don't have a backup generator yet. I want to finish installing the 2200W of solar I started putting up, and then test whether I actually need a genset. If I do I'll use a portable Honda one.

I'm staying in the SF bay so far.

[PineyWoodsPete]

We've had several boats on SF Bay over the years, including a 37' F-P Antigua 37 we took down from Seattle and spent a couple of months at Berkeley Marina, then on S through Baja, and leisurely for a couple of years to the Canal, then far up the E coast. She had twin Yanmar 18 hp's, and would top out about 7.5kts, but we cruised under power on the ICW or windless passages at about 6.

Pete