How to select motors for catamaran?

Discussion in 'Propulsion' started by Afilimon, Nov 4, 2018.

1. Joined: Nov 2018
Posts: 6
Likes: 0, Points: 1
Location: Lithuania

AfilimonJunior Member

Hello everyone,
I'm new on this forum and I'm very happy to found this forum.
(sorry for my English)
Last year we with my team working on the Catamaran project.
This is electric catamaran with two modified 86lbs trolling motors powered by 24V 200AH deep cycle lead acid battery system. For control we developed ours control system. Weight of fully loaded boat is about 500kg, battery works about 2h with about 3-4 km/h speed. Maximum speed is 7km/h with current draw of 80A. In this moment I'm looking for how to prolong battery working time.
And here occurs few questions about that what is what when electric motor is running with propeller underwater. And how to choose required total lbs for my boat.
Most difficult thing in this moment for me is to understand how two motors is working together on the same boat. For example: If lets say boat with usual shape is driven with one 86lbs motor with average speed of 4km/h.Current for example wit 24V system is 25A. What happens if I turn on another motor with same parameters? Did total current per two motors must be 25A or 50A to keep the same speed? Did motors must share load or not?. Water dynamics is confusing me at this point. Because measured current on my boat with max speed is about 80A. 40A on each motor. How motors are interacting together to keep same speed? On some literature is written, that speed of the boat depends mostly on motor speed and propeller pitch. So maybe to keep same speed in me case is naught to have two 55lbs motors with same propellers and RPM like my 86lbs motors. Hope you guys can help me understand this questions.

THANK YOU

2. Joined: Oct 2010
Posts: 10,386
Likes: 1,048, Points: 113, Legacy Rep: 702
Location: Australia

Mr EfficiencySenior Member

You will likely extend your range running one engine, if it gives you enough speed under the prevailing conditions, you certainly won't double speed with both in use.

fallguy likes this.
3. Joined: Sep 2018
Posts: 5
Likes: 0, Points: 1
Location: Brisbane

Mark MorwoodJunior Member

I'm no marine engineer, but I can probably give you some basic info to help you get started on understanding the problem and your options.

1. To make a boat go faster, it requires more power, but the curve is not linear. See Wave-making resistance - Wikipedia https://en.wikipedia.org/wiki/Wave-making_resistance for some examples.
2. Longer hulls will enable higher speeds before the curve goes very non-linear (the speed were this happens is often referred to as hull speed)
3. Assuming you adjust two motors or one motor to draw the same total power, you will probably get close to the same speed for one motor or two. This assumes your electric motors and drive system have similar efficiencies at the different power settings. This would not be true for internal combustion engines, but I suspect is close to true for your electric motors
4. Weight will affect your wetted surface area which will affect your drag. Lighter will be faster, but such a large proportion of your weight may be in the passengers that small changes in boat weight won't make a big difference
5. The propellor matters, so if you change engines and propellors you are changing two factors.

To answer your explicit question, if you add a second motor, you should be able to maintain the same speed with 1/2 the power coming from each motor. So same total drain on your battery assuming similar efficiencies at different power settings.

Going forward, I would suggest keeping two motors for control and redundancy reasons, and doing experiments with different power settings with your current motors. If you find you do not need the full power of your current motors, you could then replace them with lighter, less powerful models, assuming they have a similar efficiency. If you do some experiments and find that to get the speed you want, you are on the steep part of the power curve, longer hulls may help. However my guess is that at the speeds you are talking about, you are well below "hull speed" with those hulls.

4. Joined: Nov 2018
Posts: 6
Likes: 0, Points: 1
Location: Lithuania

AfilimonJunior Member

Mr Efficiency, I can't use single motor, because by controlling the speed of each motor separately I'm controlling total speed and direction of the boat. Without any mechanical steering wheel.
Mark Morwood, By measuring total current within PWM range 0-100% I'm also found out, that required power is not linear. But most interesting thing for me is hull speed. I never heard about this before. Now I have read few articles about this and wondering how to calculate hull speed for catamaran? Because for usual boats formula is: hull speed in knots = 1.341 * square rot of water line length if feet. In my case: 1.341*3.62= 4.85 knots or 8.98km/h. Did this is valid for catamarans?. Also you are righ about the weight, one way to reduce weight is use lithium batteries instead of lead acid, but this is really expensive for 24V 200AH.
But I'm still not clear about two motors on same boat. With two motors on same shaft it's quite clear- motors its just sharing the same load. You said, that in my case to maintain same speed motors also must share same load. When we decided to use two motors I was thinking the same. But it seems not true, or there is something else related with the water dynamics. Because at maximum speed I measured 80A total current. Or should not to be 40A? Rated current for single motor is 40A. Unfortunately because of the construction of the catamaran is not possible to measure speed with the single motor. I'm not sure what to do next. It seems try and error method is the only way. On the second boat we already mounted two 55lbs motors, but we are not sure whether we will bring the catamaran to the water this year.
So my question:
1. Did my mentioned formula about hull speed is valid for catamarans?

Thank You

5. Joined: Dec 2016
Posts: 7,795
Likes: 1,733, Points: 123, Legacy Rep: 10
Location: usa

fallguySenior Member

This steering method is drastically stealing power. As I use a trolling motor, I can tell you that direction changed via engines steals power two ways. First, you are making too many course corrections. Second, motor head movements are also using power. Steering with a rudder or wheel will probably give you a big return.

Plus, conditions permitting, you can run on one engine, saving more.

6. Joined: Dec 2016
Posts: 7,795
Likes: 1,733, Points: 123, Legacy Rep: 10
Location: usa

fallguySenior Member

The efficiency is very simple to understand.

Let's use an arbitrary efficiency of 80%. Discounting all else, you would be running one motor at 80% efficiency, then another. So let's say speed at one engine is 4 and you can run 2 hours or 120 minutes. The efficiency is basically lost time. At 100% efficiency; you would run longer like 150 minutes. Running two motors at the same time means the efficiencies are lost simultaneously. Of course, this is oversimplified, but a general truth.

In general terms, the slower you go the more efficient you become. No polemics please!

The only time this is not true is certain hull designs have distinct speeds that are inefficient.

7. Joined: Nov 2018
Posts: 6
Likes: 0, Points: 1
Location: Lithuania

AfilimonJunior Member

Hi Mr. fallguy
It's very strange to hear that this method uses so much power.
Did you understand me correctly? Please see to the picture below:

Motors are fixed with no other moving parts. I'm not understand what you mean "motor head movements". And about course corrections- on the river I'm correcting course with short pushing of the joystick every 10-20 seconds. Technically it means reducing power for single motor by about 20-30%. In my opinion this is relay stable...
I'm confused at this moment.

8. Joined: Nov 2018
Posts: 6
Likes: 0, Points: 1
Location: Lithuania

AfilimonJunior Member

"In general terms, the slower you go the more efficient you become. No politics please!" Yes because at slower discharge rate lead acid batteries gives more total power because of Peukert's law.

9. Joined: Dec 2016
Posts: 7,795
Likes: 1,733, Points: 123, Legacy Rep: 10
Location: usa

fallguySenior Member

Okay, so simple. I assumed your motors were on powered heads; it doesn't change much that they are not.

Again, assume your system is 80% efficient.

You have to use two motors to steer and to remain going straight, right?

If you use only a single motor the efficiencies are not lost at the same time. You would run basically half as fast twice as long using only a single motor. And you cite Peukert's law....!

Consider it simply.

7kmh both motors running at assumed 80% efficiency for 120 minutes; 14km covered

3.75kmh one motor running at assumed 80% efficiency, each motor runs separately and your run time is 240 minutes; 15km covered

A guess, but probably close.

The efficiency of the motors is higher when each motor can operate alone, barring some hull dynamics, winds, currents, etc.

Of course, the only true way to run the example is to test it. But, in general, you gain efficiency going slower with a single motor if you have a steering system that is not based on power. A simple thin low drag rudder would be the easiest way to get closer to ideal efficiencies.

You may not like this vs joystick steering, and the only way to verify the return is to know each of your speeds and the rudder can't create a problem like drag or operator moving a lot, etc.

The easiest gains are steering and running a single engine; most likely.

10. Joined: Dec 2016
Posts: 7,795
Likes: 1,733, Points: 123, Legacy Rep: 10
Location: usa

fallguySenior Member

The other thing that happens when you increase run times is you can add solar. Now, I reaize this wasn't mentioned, but I have a 13w charger on my trolling motor and it helps me gain a bit back; especially when not using the electric motor for an entire day.

In your situation, if you had room for solar, (more than 13W), a longer running time also allows for greater solar recovery. So, if you can run for four hours versus two; a solar system also provides some gains.

Of course, this also assumes the slower speeds are acceptable, etc.

11. Joined: Nov 2018
Posts: 6
Likes: 0, Points: 1
Location: Lithuania

AfilimonJunior Member

Solar power is another question what we solving in this moment. But this is plans for the future. We have plans to install 5 square meters of solar cells on the boat. I make lot of calculations and believe it's possible to get about 4kWh of energy per day in my region. Technically its about 2 or 3 hours of travel. But as i said this is the plans for the future.
In this moment we working at selection of the motors. So next steps what shall we due is making some modeling with computer. One nice lector promises as to help with this. Also in next week we will try to load the boat into the water, and test 55lbs motors. To make things more clear...

12. Joined: Sep 2018
Posts: 5
Likes: 0, Points: 1
Location: Brisbane

Mark MorwoodJunior Member

It's not very accurate for relatively thin hulls like you have. Your actual "hull speed" will be higher than the formula suggests. In addition there can be complex wave interactions between the two hulls, but I think at the speeds you are talking about that should not be a significant issue. If I understand correctly you wanted a speed of 7km/h and the calculations showed a hull speed of 9km/h so you are fine.

Your setup with twin engines, using differential power to steer seems very reasonable for your planned use. It is commonly used approach to manoeuvre full size catamarans at low speed around docks etc.

I do not agree with the above comments that you will see big differences in efficiency between two engines running at x/2 watts to get a particular speed, or 1 engine running at x watts to get the same speed. There is obviously some extra drag from the extra leg in the water, but my expectation is that would not make a big difference in your use case and the speeds you are talking about.

13. Joined: Dec 2016
Posts: 7,795
Likes: 1,733, Points: 123, Legacy Rep: 10
Location: usa

fallguySenior Member

You don't see an upside to running twice as long?

14. Joined: Aug 2002
Posts: 16,911
Likes: 1,766, Points: 123, Legacy Rep: 2031
Location: Milwaukee, WI

gonzoSenior Member

The power supplied by the batteries will be the same regardless of how many motors you have, as long as the electrical power input is the same. However, the power output will change. One propeller can be made more efficient than two, if it is possible to install it in an ideal configuration. For example, the drag of two shafts will be larger than that of a single one.
It is correct that according to Peukert's law a battery will output more power at a lower rate of discharge. However, that is not related to whether the power is input to one or multiple motors.

15. Joined: Nov 2018
Posts: 6
Likes: 0, Points: 1
Location: Lithuania

AfilimonJunior Member

1. The speed created with trolling motor depends mostly on the pitch of the propeller and RPM of the propeller.

2. Trolling motor creates the pushing force which is measured in lbs. This is static unit to describe pushing force. Force depends on motor power, RPM and propeller construction.

3. I saw lot of videos on the YouTube where peoples are testing trolling motors inserted in to the container with the water. The current at the maximum speed set point is more or less equal to the rated current (not maximum). It says to me, that for the motor it is enough to be under water to use rated power. It does not care the motor is moving somewhere or not. From here I draw conclusions, that exist thing like efficiency. I explain later why.

4. The boat is the object on the water. When he is moving forward the water is creating resistance created of several components. Boat Resistance http://smalltridesign.com/Trimaran-Articles/Boat-Resistance.html. To keep the fixed speed the pushing force must be equal to the resisting force. For me this sounds logically so I believe this is true.

This means that there are three players- speed, lbs and efficiency.

Relation between them i understand like this:

Example 1: boat is moving 7 km/h and the water is creating 172 lbs resisting force. So to keep the speed fixed, motors must create pushing force Which is 172 lbs. At this point I do not see big difference how many motors creates this lbs- one two or ten.
This case is perfect situation, because the motors uses as much power as boat requires.

Example 2: boat is moving and for target speed water resistance is 172 lbs. The motors is creating only 120 lbs force and pushing the water per propellers at same 7 km/h speed (same rpm). But the pushing force created with the motors is too small, and boat does not achieve 7 km/h. At equilibrium between of resisting and pushing forces speed becomes fixed, but less than target speed.

Example 3: boat is moving 7 km/h and the water is creating 172 lbs resisting force. Motors is creating 250 lbs pushing force, but pitch and RPM are the same. Motors are pushing the water per propeller and creating the force equal 250 lbs. Because they are already achieved maximum RPM, the all power to produce exceeded lbs is pure waste power. I'm not so sure at this point, but i have no ideas how more understand behavior of the motors.