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Will adding an electric outboard to my 39' catamaran provide the desired benefit of added propulsion when under sail?


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I am considering adding an engine mount to the transom of one of my 39' catamaran's hulls so I can move the electric outboard that is on my tender and (hopefully) get added propulsion when trying to sail to windward.  Like most cruising cats, its performance is poor when trying to make my way to a windward destination under sail, particularly out in ocean waves. I generally run one of my diesel engines at around 2200 rpm, which, I believe based on the engine's manual, provides roughly 10 hp of propulsion.  That added propulsion increases the boat's speed 1-1.5 knots which makes the boat ride much better through the waves and obviously gets me to my destination faster. I could continue to do that, but have upgraded the boat with a considerable number of solar panels (4 kW) and a large lithium battery bank (45 kWh) (to enable my wife, me and our guests to have air conditioning without having to run a generator) and it occurred to me that my 6 kW electric outboard might be used to provide the added propulsion that my diesel was providing. This would be preferable to me because I'd like to eliminate the noise and exhaust of the diesel when I'm going from one place to another and I'd like to minimize visits to marinas in foreign ports where the fuel quality is questionable.

Before I drill holes in the transom of my catamaran to add the engine mount, it occurred to me that I should get input from those more knowledgeable than I am about physics and electric engines and possibly even those who have tried something similar.  I also know from experience sailing a fast trimaran that if the boat's gas outboard is pushing the boat and then the sails take over with the engine still running, the engine revs much higher due to the reduced load and doesn't do anything to increase the speed of the boat after the boat speed reaches around 7 knots. My catamaran will sail close hauled at about 5 knots in 12-15 knots of wind. I been assuming that applying roughly the same amount of horsepower from my electric outboard that I've been applying with one of my diesels running will produce about the same 1-1.5 knots of added speed,  but I suspect I may be missing something and find that my electric outboard's 10 hp is doing next to nothing in terms of added propulsion, and instead is just wasting my precious battery reserves.  Thoughts?

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1) What size/model is your engine? You have to consider prop curve power absorption. The engine may produce 10 HP @ 2200 RPM but the prop probably can't absorb it. Here is a random 28 HP Volvo. At 2200 RPM the engine can produce ~22 HP but the "average" prop (dashed line) can only absorb ~8 HP.

Basically single pitch props are designed to absorb full power at max engine RPM. At lower RPMs they are not as efficient. .... So the prop on your boat might only be absorbing some smaller %.

Also the 6 kW outboard motor probably has a prop optimized for pushing a tender at higher speeds than 5 knots when running closer to full power. So it might not be a good match for a whole cruising cat.

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2) How long do you want to run the electric outboard?  4 kW of solar probably gives you only 3 kW on a typical day.  6 kW outboard, 3 kW from solar panel + 3 kW from battery bank; say 12 hours is 36 kW.h from the battery which doesn't leave much left for your A/C.

(we are ignoring losses here)

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I have Yanmar 3YM30AEs (29 hp at 3200 rpms).  The Yanmar manual has that same type of information you provided and it shows around 10 hp (2 in your attachment) at 2200 rpms.  Your comment about the prop of the 6 kW outboard being optimized for a light tender--but not to push a heavy cat--makes sense and has me concerned.  I know that people use similar sized electric outboards to push heavy displacement boats (very slowly), so it might be possible to swap out the propellor.  Although not a very desirable thing to do prior to every trip, it's fairly simple and quick on the electric outboard compared to a gas outboard.  I will contact the manufacturer to see what they think about the prop issue.  Thanks.

As to the power to run the outboard on a "passage" to the next destination (i.e., average time around 3-4 hours underway), I think I have enough.  The 4 kW of potential solar production generates 15-18 kWh of electricity on the average sunny day.  Four hours of burning 6kW/hr would leave me with a bit over 20 kWh in my batteries, enough to run AC in our cabin and our guests' cabin at night and all the misc other things that eat up electricity on the boat.  It would take about 2 sunny days of recharging from the panels at our next destination to replace what I spent during the trip.  It would be unusual to not spend at least a few days wherever we end up, but, in cases where I need to do longer runs or move on before the batteries are recharged, I would run one of the diesels.  So, the electric outboard added propulsion plan will work 95% of the time, according to my calculations.  But that's only if the 10 hp electric outboard actually serves as a substitute for the 10 hp I'm getting from my Yanmar at 2200 rpm, which may not be the case.  Apparently Salona is a firm no that the plan won't work.  Not sure why he/she thinks that, but, like most people, I tend to love it when answers are black or white.

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23 minutes ago, basketcase said:

how much time will the outboard on your transom spend in the water as you pitch in the waves?

Although the occasions when I'm island hopping and using the outboard for the added propulsion will rarely involve my transoms submerging, I have no doubt that the engine will get very wet frequently.  Fortunately it's IP67 waterproof which means it can be completely submerged in up to 1 meter of water for 30 minutes without water making its way in.  One of the big reasons I like these engines.

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It will be a little precarious having the dinghy alongside with the boat sailing at 5 knots and the outboard wide open.  I'm sure I can get one of my kids to do it. ;-)

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Why do you run only one engine? Ive never heard of that before. Is there a hole in mah learnin'?

I have only one two week bareboat charter of a cruising catamaran as experience in that category of vessel. But I worked for the dealer and the, very experienced, brokers for Lagoon we had never suggested to me to only run a single engine when we went over my fuel consumption/speed calcs while I was planning the trip. We went from Phuket to Langkawi and back in two weeks. Lots of motoring. I used about what I had calculated for fuel, because we attained the expected speed at the expected cruising RPM (with both engines). The fleet captain told me I only used about half of the fuel used by the average charter party for that time period, and we covered more ground. We figured that was because I had consulted the manual and knew what the proper cruising rpms were supposed to be. Most charter parties just push the throttle till shes smoking, and need a lot of soot cleaned off the transoms and the dinghy on the davits. We did not have that problem at all. 

I did operate a larger power cat for a few seasons, never considered to run that on a single engine either. 

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41 minutes ago, Baldur said:

Why do you run only one engine? Ive never heard of that before. Is there a hole in mah learnin'?

I have only one two week bareboat charter of a cruising catamaran as experience in that category of vessel. But I worked for the dealer and the, very experienced, brokers for Lagoon we had never suggested to me to only run a single engine when we went over my fuel consumption/speed calcs while I was planning the trip. We went from Phuket to Langkawi and back in two weeks. Lots of motoring. I used about what I had calculated for fuel, because we attained the expected speed at the expected cruising RPM (with both engines). The fleet captain told me I only used about half of the fuel used by the average charter party for that time period, and we covered more ground. We figured that was because I had consulted the manual and knew what the proper cruising rpms were supposed to be. Most charter parties just push the throttle till shes smoking, and need a lot of soot cleaned off the transoms and the dinghy on the davits. We did not have that problem at all. 

I did operate a larger power cat for a few seasons, never considered to run that on a single engine either. 

I only run one engine when I'm trying to get to a windward destination under sail in moderate conditions.  I'll run both engines if I just want to get somewhere faster when there is no wind or when I'm stuck out in nasty conditions trying to make my way upwind.  Running one engine at 2200 for the extra 1-1.5 knots that makes my cat ride better and make better progress was just something I heard gives me better fuel economy instead of running both engines at, say, 1500 to get that same amount of propulsion (just a guess). I sometimes run the one engine at its optimal rpm (2700-2800) and, now that you mention it, probably should all the time when trying to sail to a windward destination.  Based on the graph that Zonker attached, it definitely makes sense to run at the optimal rpm to get the biggest bang for my buck.  Thanks for that input.  If I ran both engines at their optimal rpm, I might as well just take the sails down because they won't be doing much at that point.  I've definitely had the thought that I'm getting too old to beat to windward and might as well just motor when I need to get someplace upwind.  But, I'm not there yet. :-) My goal is to find a way to shut off my diesel altogether when I need that 1-1.5 extra knots when sailing and use my electric outboard that's on the back of my dinghy (but with a lower pitch prop) to achieve that goal.  

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1 hour ago, Baldur said:

Why do you run only one engine? Ive never heard of that before. Is there a hole in mah learnin'?

Pretty common thing to do when motor-sailng on a cat... No point running both if you only need a little bit of extra power, the one engine running at higher load will be more efficient (and you put less hours on them overall if you alternate which one you use).

I would think the electric motor should be able to deliver closer to it's max power over a broader range of RPM than a gas outboard because of the high torque at low RPM capabilities.

7 hours ago, Zonker said:

4 kW of solar probably gives you only 3 kW on a typical day. 

Depending where you are you should be able to get considerably more than that. Even a horizontal panel (assuming mounted flat on deck) should achieve several sun-hours equivalent (3-5?) on a reasonably sunny day so you should be able to get over 10kWh per day if it's sunny. 

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2 minutes ago, Airwick said:

Depending where you are you should be able to get considerably more than that. Even a horizontal panel (assuming mounted flat on deck) should achieve several sun-hours equivalent (3-5?) on a reasonably sunny day so you should be able to get over 10kWh per day if it's sunny. 

The 4 kWh of panels will produce 15-18 kWh on an average sunny day.  I get 10-12 kWh on an average sunny day just from my five 400 Wh panels on my arch that hangs off the back of the boat that, although flat, don't get any shading.  Cats have many limitations, but space for lots of solar panels isn't one of them.  

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How much speed could you gain if you let the solar power a cattle prod stuck up your ass? Maybe admitting white privilege  will give you another half knot. 

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3 hours ago, Airwick said:

Depending where you are you should be able to get considerably more than that. Even a horizontal panel (assuming mounted flat on deck) should achieve several sun-hours equivalent (3-5?) on a reasonably sunny day so you should be able to get over 10kWh per day if it's sunny. 

You might be confusing power (kW) or the instantaneous output of the panels with energy storage (kW.h)  

At a given time you might average 3 kW of power output with a 4 kW rated array.  If you store if for 1 hour you've stored 3 kW.h

Yes you could easily generate 10 kW.h or more in a day.

I was suggesting the power for the 6 kW outboard could come from a combination of an average output of the panels (3 kW) + 3 kW from the battery to give 6 kW. If you take 3 kW out of the battery for 10 hours, then you have used 30 kW.h of the battery bank's 45 kW.h

 

Yes, but the type of prop that might not be optimized for low speed/high thrust if it was designed for a dinghy tender. You'd want a 3 blade prop of about 12" for 6 kW output for pushing a heavy boat.

4 hours ago, Airwick said:

I would think the electric motor should be able to deliver closer to it's max power over a broader range of RPM than a gas outboard because of the high torque at low RPM capabilities.

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5 hours ago, CommonBlue said:

 Running one engine at 2200 for the extra 1-1.5 knots that makes my cat ride better and make better progress was just something I heard gives me better fuel economy instead of running both engines at, say, 1500 to get that same amount of propulsion (just a guess)

Yes, that's correct.

6 hours ago, Baldur said:

Why do you run only one engine? Ive never heard of that before. Is there a hole in mah learnin'?

Very common on cats. Keeps the motor hours down and because the drag curve for boats is exponential with 1 engine you might go 6 knots but with twice the power you might only do 7.5 knots and burn twice the fuel.

Our 40' cat only had a single diesel in one hull and a small outboard on the back beam on the other side for manuevering in marinas and fuel docks. 99% of the time the outboard didn't get used. We did a circumnavigation that way and the only drawback was the lack of use of the outboard (because we so seldom went to marinas) was the outboard wasn't very reliable due to lack of use!

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With that much solar and storage I'd seriously consider replacing one of the diesels with an electric.  Lots more $$ and certainly not economically rational, but it would allow you to greatly reduce the amount of diesel engine run time.  

4kW of solar fits on a 39' cat?  Would love to see pics of the array if you've got them handy.  

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6 hours ago, GH41 said:

How much speed could you gain if you let the solar power a cattle prod stuck up your ass? Maybe admitting white privilege  will give you another half knot. 

Haha, yeah, probably. Cats are very sensitive to weight.  

There is more-often-than-not a braggy/snotty aspect to mentioning cool new toys.  If you've somehow risen to the enlightened state of not wanting to show off your new bike, that's great.  I'm truly happy for you.  I have to confess I'm not there yet and do get a certain amount of pleasure showing off new toys.  I also have found that there are at least a few others who find them as fascinating as I do, which makes it a worthwhile share.  If, on the other hand, you're the kid who's jealous that you don't have the same new bike and just want to trash talk the kid who does, then go fuck off and accept that your shit stinks too.  

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3 hours ago, socalrider said:

With that much solar and storage I'd seriously consider replacing one of the diesels with an electric.  Lots more $$ and certainly not economically rational, but it would allow you to greatly reduce the amount of diesel engine run time.  

4kW of solar fits on a 39' cat?  Would love to see pics of the array if you've got them handy.  

I looked into the hybrid options, but I just don't think the technology is there yet to give me what I want.  And, as you mention, it would be very expensive to swap everything out and add a massive generator.  My diesels are extremely efficient and come in very handy in many situations.  The "hybrid" approach I'm considering (moving the electric outboard on my dinghy to a mount on one of the transoms) costs far, far less money and gives me some redundancy in case one of my diesels acts up.  I'm even considering adding another electric outboard to the other transom so I can maneuver the boat better without starting up the diesels and have a spare engine for my tender.  At only 67 pounds, I have less concern about making my already relatively slow cat even slower by adding the weight.  This seems like a good alternative to the other hybrid options out there.  There is also the potential for hydrogeneration, but I don't realistically see myself dragging an engine to generate electricity very often, if at all.  And... I would anticipate getting a lot of value from the conversation-starting aspect of having two electric outboards hanging off the back of my boat. :-)

I only found a few pictures of the array when it was being installed.  I've attached one for you to look at.  The Key West welder did a fantastic job at a very reasonable price.  The panels have the ability to also pick up and use light from below.  Although billed as capable of producing 400 w/hr. the panels have generated close to 500 w/hr each.  It's nice when a manufacturer actually overdelivers on its promises.

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