Using multiple electric trolling motors on a 34' catamaran

Hi guys, I'm building a Woods flica 34' catamaran and I've been toying with the idea of putting 3 electric trolling motors on my 34' catamaran (11,000 lbs when fully loaded). I'm thinking of getting 3 of the Newport Vessels 86 lb thrust 24v trolling motor (https://www.amazon.com/Newport-Vess... motor&qid=1635910425&s=sporting-goods&sr=1-4)

My goal is to have an electric motor(s) to use for trolling and to be able to go short distances (15-20 miles) in a day using solar power alone. I figure with a boat this size, I would need at least 3 to generate enough thrust to push this catamaran along at 2-3 knots. I plan to have 8 solar panels of 450 watts and a 5.6kw lithium battery. I won't have sails or a mast or rigging. With this setup, I figure I can run the 3 trolling motors whenever the sun is out and it should be able to take me about 15-20 miles in a day at about 2-3 knots. I plan to have a 9.9hp yamaha high thrust gas outboard motor as well.

I was also considering getting the Elco 3kwatt 5hp 24v outboard motor as well, but it seems like the company went out of business since they dont' pick up their phones or respond to email inquiries. I want to use a 24v system so that it would be compatible with my watermaker.

Is this idea crazy? What are the reasons why it won't work?

 

I'm curious on the duty cycle of a trolling motor. 2-3 knots and 15 miles is still a lot of hrs for continuous full load use of a consumer grade electric motor.

 

It'll work in calm wind/seas/currents. At least until it doesn't. Can make for great positioning control at least. If you're happy going along at 2-3k go for it.

Trolling motors have a 100% duty cycle, unlike bow thrusters. They do fail, but you'll have backup.

 

Trolling motors will push a boat that large at your ideal speed, however the windage associated with a boat that large will make the boat hard to control. You really need more thrust and the ability to swing a bigger prop.

And you’ll want to steer using a rudder, not the motors. Leave them fixed and add cavitation plates to them.

I would suggest considering a pair of more powerful and efficient brushless motors, especially if you want to run on solar only.

By running larger motors at a lower throttle setting to achieve your ideal speed, you’ll be operating the motors while their controllers are Pulse Width Modulating which significantly reduces consumption. Then if you need it, you’ll have extra power on tap if you find your self in strong current or windy conditions.

For the money, I think a pair of EPropulsion motors would be the best choice. They operate at 48V, so they’ll draw very little amps and you can wire panels in Series with a MPPT solar charge controller to feed them plenty of voltage providing fast recharging and ability to power the motors while charging the batteries when there is sufficient power. They are compatible with external batteries and can handle external solar charge input, so you could have batteries on each motor as well as a house bank and have them all fed by your solar system.

A pair of Spirit motors would work okay, but Evo’s would probably be better.

Motors like the EPropulsion and Torqeedos swing larger diameter more aggressive pitched props compared to standard trolling motors, so they will give you better speed, efficiency and maneuverability.

For reference I use a pair of 24V 120lbs thrust brushless motors to move a 20’ lightweight narrow hull catamaran. I can cruise at 4mph on sun only drawing 25A with a top speed of 7mph pulling 120A.

Figure out how much solar you can mount on your boat, then work backwards to determine max amount of power generation, then pick motors that will give you the cruising speeds you want running at 3/4 throttle.

By running dual motors, each will run slightly more efficiently when looking at manufacturers estimated specs, unless you use them to steer like I do by using independent forward and reverse thrust like a tank.

If you study the specs of the Elco motors, you’ll see that they are pretty inefficient, due in part to their use of a gear reduction, versus direct drive. This also ads noise.

Remember if you want to run on solar only, the more efficient every component is from motor to battery to panel to controller, the better overall performance. Every little bit counts.

 

Wow thank you so much for the detailed response! I will definitely look into those motors thanks!

 

You’re welcome. Sorry I totally missed your solar plan.

So 8x450W=3600w/48V=75A.

That means in perfect conditions with panels at full voltage, a system that size has the potential to provide 75A for a 48V system.

So to be able to run on sun only, you would ideally like to find a pair of motors that draw less than 37.5A each at your ideal cruising speed.

Then factor in that at the same time while cruising in ideal conditions, you would like to be able to put some power back into your battery bank while also covering the loads of the motors, that you won’t always have perfect conditions, and your target motor amp draw at cruise might be more like around 25A each.

This means in the best case scenario, you could cruise on sun only while putting 25ah or so back into your battery bank to cover/offset any house loads or just recoup your losses from a full throttle blast.

Or another way to think about it is that if you’re drawing 50A an hour during cruising, and your system is generating 75A of output, and your batteries are depleted, you are putting about 1/2 hour of cruising time back into your bank while powering the motors by sun only.

When at rest in ideal conditions, you would essentially have a 75A charger for your battery bank.

This is where your battery capacity comes into play. Capacity should be sized based around how you plan to use the boat, typical travel distances, ideal cruising speed, worse case condition speed and power draw, etc..

For example I use my boat mostly to cruise up rivers and around Bays. I like to cruise on sun only, or get to a destination quickly, drop anchor and relax. So my boat is designed to cruise long distance at 4mph sun only, or short distances fast using high discharge small capacity LifePo4 that can charge and recover quickly based on the amount of time I typically spend anchored.

So to put this all together in relation to your boat, a pair of EPropulsion Spirit 1.0 Evo motors for example are 1Kw each and come with 1.2kW batteries. At 3/4 throttle with a 750W draw by each, the motors can run for 1.75 hours or 10 miles at around 5mph. With your proposed solar system and a 48V MPPT charger, you should have no problem running on sun only in ideal conditions as each Spirit motor has a 28A max draw.

I don’t know if you have a sail plan, but another feature of these motors is hydrogeneration. Leaving them in the water under sail power, each motor is capable of generating 100W of recharging at 6kn and a max of 330W at 10kn which could come in handy.

The Navy line of motors, specifically the Navy 3.0 would also be a good fit for you solar plan and would provide even higher cruising speeds, more range, and quite a bit more reserve thrust and power if needed. They have a max amp draw of 62.5A each, so you could run them at more than 1/2 throttle on sun only. Since they are more powerful and swing larger diameter higher pitch props, and you’d be running a pair, a lower throttle setting will still give you the speed you are after since you won’t be loading the motors as much as the smaller Spirit versions. They would also make maneuvering more responsive, especially in strong winds, currents, etc.. The Navy motors being larger, are also capable of providing even more Hydrogenation as well.

Anyway, hopefully that gives you some things to think about. Design for how you see yourself using the boat while under solar electric power and reverse engineer the system from there and you’ll be a happy camper.

 

I would avoid the temptation.

First, don't pretend only the engines are demanding boat power resources and do a full demand analysis using spreadsheets.

I found that refridgerator, lights, switches, anchor windlass, bms, exhaust fan, toilet flushing, cabin water pump, washdown pump, navigation support like mfd and autopilot, watermakers, electric stovetops, vhf radio, all place significant demand on the boat. Solar space for panels, while good is also limited.

People quoting ideals are not doing a spreadsheet analysis.

Then, in order to not draw down the bank; your battery bank needs to have sufficient amp hours. To allow your engines to use half the available resources would be high.

Then, air conditioning? Another major draw, but something you may desire on a warmer nite for an hour.

I can try to find my spreadsheets for you to show you examples of how to design the total system

How are you going to get 8 panels on the rooftop or a structure on a Flica? I realize my boat has limits, but I only had space available for two 310 watt panels on Skoota. I probably could have gotten two 450s up on the roof, but even doubling the roof size; not 8!

So, solar panels are not light. They have a design load of 33 pounds per square foot typ. This means the roof you build mist be capable of supporting them. The panels are about 52 pounds each, figure 55 pounds each with mounting hardware. 8 of them is 440 pounds. And let's say your battery bank size goes from 4 to 8 and each battery is 40 pounds and supporting wiring is an extra 20 pounds, now, you have added 340 pounds there. Suddenly, this system has added nearly 800 pounds to your vessel. Is your vessel weight addition limited?

A 20 gallon fuel tank and outboard weigh how much? 250 pounds would be more than a fair comparison.

So, you added 550 pounds to the boat to support the electric (i didn't add electric motor weights).

So, you have to consider your entire power demands and not the engines alone. You have to consider weight budget, and $$ budget. Three budgets or four. Power generation, power storage, weight, and money.

It isn't simply adding a couple panels..I think your solar controller costs also go up a bunch as well as supporting systems like breakers and wiring. If you are building a cockpit frame for the panels; that adds to the weight and dollar budget.

 

I like the idea of three motors on a cat. Two off the stern and one setup as a bow thruster that can also turn its prop to forward drive. You'll be able to swing a circle in any marina.

Trolling motors typically run at 2 to three knots all day long for fishing. That's what they're made to do. You can usually also get remote control standard for them.

 

If the OP has as many house loads as Fallguy outlined (he only mentioned a water maker), then I would split the solar/battery system.

If he went with Spirit motors which have built in batteries, he would have a closed system for the motors only that could be fed by a small solar system made up of a few panels in series and an MPPT as the Spirit motors/batteries/controller have the ability to take in external solar charge current while the motor is being used.

Then he could have a separate independent system for his house loads that could be at a lower voltage (like the 24V he mentioned).

Using forward and reverse thrust of dual motors on a catamaran if mounted apart should allow the boat maneuver in a circle. I don’t think a 3rd motor is necessary and the motors the OP originally asked about are not adequate for a boat of his size IMO.

From experience I can tell you if you tried to turn in a circle in a boat as large as his and was hit with a strong crosswind, he probably wouldn’t be able to make it across the wind with small trolling motors.

Now factor in wind chop, currents, wakes from other vessels, etc.. and you have a dangerous situation.

Those who have tried using trolling motors on large multihulls have encountered a lot prop slippage at higher rpm. You really need a larger diameter higher pitch prop. Adding a cavitation plate has helped to keep the props from sucking air from the surface.

Now if the original PO only needs a system for fair weather and intends to use the ICE outboard the rest of the time, then that’s a different story. In that case, one of MinKotas engine mount motor systems might be a good option. It could be mounted to the cavitation plate of the outboard, and they come with a wired remote and a separate controller which might be easier to manage than 2X trolling motors when quiet slow speed cruising is desired. However being a brushed design, I can tell you they aren’t nearly as efficient, but they are easier to service and parts like brushes, brush holders, and armatures are purchasable through Johnson. I have a few of these and have rebuilt them. Unfortunately the controller is a potted brick, so if that goes bad you have to spend a chunk of change. But it’s another option that would provide 160lbs of thrust without the need to mount more things to the boat hull while allowing for steering using the ICE outboard.

 

using panel rated output as expectation of power at disposal is doomed to disappoint.
you'll get just couple of these hours on a cloudless day and still assuming panels aligned to sun.
on a practical installation on a boat you will likely never see rated output - even in ideal conditions. Let alone as a consistent expectation for practical use.

Here is what I used on my solar calc for flat installation. Basically in summer 1kW rated pabels give about 4.5h of peak equivalent ie 4.5kWh / day. I used 90% of this as the basis for my own boat calcs.

These numbers in the chart are for Finland which is way north but the summer days are looongg so the kWh is surprisingly similar further south. Spring and Autumn will be better in south.

There are brushless reasonably powerful trolling motors at low cost. Haswing seems to have decent rep but I have no personal experience. 2x ~ 1kW ones are not too pricey.

back to the kWh numbers. If your array is 3.6kW it would mean about 14kWh or so daily energy. 20 miles (nm) at 3 knots is 6.7 hours so ballpark would mean about 2kW at disposal. I would reserve some for losses but ballpark is right.

3 knots doesn't seem unrealistic with 1.5-2kW - in calm water. 5kWh battery is reasonable buffer but wouldn't cover a full drive in a cloudy day.

I would use LFP (LiFePo4) batteries. So much lighter.

 

https://haswingoutdoor.com/trolling-motors/protruar-g-39.html

the "3hp" model seems to draw about 1200W so 2x that, the ballpark is not bad, assuming the reserve capacity for when wind picks up comes from the 9.9 gas motor.

This is geared for lower rpm which in theory is good as long as it's well made and reliability is good.

They have high rpm direct versions but they seem to be out of stock.

 

That consumption draw is sales hype and not true. 3HP is equivalent to about 2.2kW at the prop. The actual draw at the battery bank will be considerably higher due to losses on the electric motor, controller, wiring and connections.

 

Those motor brick below the water type outboard trolling motors have significant appendage drag, inefficient weed shedding props, and interference to the water flow reaching the prop- from the brick in front, (X3!)... A single trailing shaft motor would be considerably more efficient, because it minimizes these factors, in my humble opinion. I bought and used a trailing shaft kit many years ago in the USA, that allows someone to mount any type of motor up to 15 horsepower (whether IC or Electric), but they are no longer in business. There was a range of props available depending on the power and RPM of the motor that was to be used. A search may turn up other suppliers, for those that might be interested in a fairly simple DIY assembly.

JMHO, but 3 knots is not enough for a significant solar contribution most of the time, unless someone is operating in a sheltered area.

 

This prop may be helpful and is low enough cost for a trial. Scroll down for a review that includes some measurements. There are also other Marine props in their collection that might be helpful if this particular one doesn't work. Caveats, as I do not currently use Troll Motors myself..

https://www.apcprop.com/product/10x5m-lh/

 

Hey guys, after some more thought, I'm now debating whether to centrally mount my Yamaha 50hp gas outboard and put two Newport vessel 86lb thrust trolling motors (one on each hull) vs putting the Yamaha 50hp gas outboard on one hull and both of the Newport vessel 86lb thrust trolling motors on the other hull. I would need to balance the weight by putting the battery and other things in the lighter hull. The trolling motors would really only be for maneuverability in marinas. What do you guys think? Thanks in advance for all the advice I really appreciate it!!