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Author Topic: is my boat to heavy for motors ?  (Read 10884 times)

Pete25

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is my boat to heavy for motors ?
« on: May 12, 2011, 08:38:32 AM »

ok this is a tricky one.. well for me anyway searched on Google so here goes im a newbie to all this so go easy on me if its a silly question  :P

i have my hull water tight (no prop shafts ) and im now wondering if i will have enough power to move the weight of the boat/batteries and so on.

i want to use 3 motors all with 50mm propellers

power will be 12v

the motors i want to use are

Johnson 600 size  3 - pole
12 volts
No Load speed is approx 20,000 rpm.
Dimensions: L65mm, D36mm, Shaft diameter 3.2mm Off load Current: 1.57A

im not looking for speeds of 1000mph  ok2 just a nice cruising speed will do me  :-)

i will weigh the hull later along with the batteries and post results here later today.
the hull is 6ft long

i have 3 led acid batteries and am i right in thinking if i use all 3 but wire them up Parallel  this will give me longer running time ?

hope someone can help  :-)




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pugwash

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Re: is my boat to heavy for motors ?
« Reply #1 on: May 12, 2011, 09:14:51 AM »

Pete Welcome to the forum.  I think you have this post the wrong way round - if you give the dimensions and
weight and type of ship then we can give you a better idea as to the propulsion system needed. i.e. is it a
slim destroyer type hull or a tug etc etc.
Also have you done a float test to see if the hull will support this weight of motors nd batteries


Geoff
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tigertiger

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Re: is my boat to heavy for motors ?
« Reply #2 on: May 12, 2011, 01:37:41 PM »

In theory, even a small motor will move a big boat ...................eventually.

I would have thought the bigger issue will be getting enough headway to steer.
Are you fitting bow thrusters?
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Pete25

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Re: is my boat to heavy for motors ?
« Reply #3 on: May 12, 2011, 02:06:30 PM »

@ pugwash

my boat is a battleship style that is 6 ft long but a foot wide and just under a ft tall the boat is a flat bottomed design too

the hull on its own weighs 3 Kilo's

i have tested it in my pond in the garden and its very stable as it is without no ballast. this will be due to its flat bottomed design.
i put batteries  inside the hull were the batteries are going to be and the water level moved up no more than 5mm on the side of the hull
and 2 acid batteries i want to use Weight together a total of  4.5 Kilo's

so
2 batteries 4.5 Kilo's
Hull  3 Kilo's

as for motors they didn't make any extra Weight.. well nothing worth mentioning  

just for fun i added 3 house bricks and again the boat was very stable and  water level moved up another few mm i didn't want to push my look and add any more but i reckon i could add more if i wanted so hull can handle the more Weight than i want to put in it.

if you need more info let me no and ill find out thank you for your help so far  :)


@ tigertiger
i will be hopefully but i dint think i would really need them  but again what do i no im a newbie to all this and im learning as i go along ha
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wartsilaone

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Re: is my boat to heavy for motors ?
« Reply #4 on: May 12, 2011, 02:08:34 PM »

Hi Pete. I think three of the motors you mentioned combined with the 50mm props will easily be enough for a 6 foot boat although you may need some gearing to put less stress on the motors, give them more torque and allow better control at slower speeds. You Might want to think about using a mixer to control the motors as it can be a real handfull to control tree props at once.

Ali.
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Pete25

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Re: is my boat to heavy for motors ?
« Reply #5 on: May 12, 2011, 02:11:44 PM »

Hi Pete. I think three of the motors you mentioned combined with the 50mm props will easily be enough for a 6 foot boat although you may need some gearing to put less stress on the motors, give them more torque and allow better control at slower speeds. We could do with more info on the boat itself so we can give you more comprehensive advice.

Ali.

more info above. posted the same time you asked :)
i will be water cooling the motors and will probably use gearing but for the minute i want to see what i can get away with  :P
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wartsilaone

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Re: is my boat to heavy for motors ?
« Reply #6 on: May 12, 2011, 02:20:58 PM »

Yeah thats fair enough, I did the same with my boat. Keep it simple while you get your head round it then add stuff later.
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Pete25

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Re: is my boat to heavy for motors ?
« Reply #7 on: May 12, 2011, 02:24:03 PM »

Yeah thats fair enough, I did the same with my boat. Keep it simple while you get your head round it then add stuff later.
i dint think it would be this complicated at first now im starting to get headaches trying to figure things out  {-) hopefully ill get there in the end and will be able to help others like everyone els on here :)
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John W E

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Re: is my boat to heavy for motors ?
« Reply #8 on: May 12, 2011, 03:19:10 PM »


hi there Pete25
 
In the 60s and 70s the RAF had the English electric jet fighter.  Its party trick was to hurtle along the runway and then go supersonic as it took off -  I think your model may do the same  {-) {-) O0 with them motors and 50 mm diameter props spinning at 20,000 rpm  ,may just be a tad too fast, the motors will be drawing about 20 amps each plus they will get very hot, you should maybe be looking for motors that spin at approximately 11,000 rpm

aye
john
 
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unbuiltnautilus

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Re: is my boat to heavy for motors ?
« Reply #9 on: May 12, 2011, 03:47:15 PM »

Hi, the Johnson motors are fine for fast hulls but I would avoid running 3 in a displacement, scale hull. Two, via reduction gearboxes on 6 volts should be more than enough for your model. I would suggest if you go the way of the reduction gearbox, you consider 2.5:1 or even 3:1 reductions. A better motor choice would be three Graupner 500E or similar, I would still peg back your voltage to 6 volts though.
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tonyH

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Re: is my boat to heavy for motors ?
« Reply #10 on: May 12, 2011, 04:01:15 PM »

Just adding a minor spanner, if the draft is between 3 & 4 inches, the displacement could easily be up to 30kg. How would that effect the choices already mentioned?

Tony
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boatmadman

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Re: is my boat to heavy for motors ?
« Reply #11 on: May 12, 2011, 04:15:54 PM »

And another spanner coming in!

That boat is going to be heavy and awkward to handle between garage - car - water and back.

You might want to think about putting in some free flooding ballast so you dont have to carry as much weight about.

Ian
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wartsilaone

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Re: is my boat to heavy for motors ?
« Reply #12 on: May 12, 2011, 04:42:00 PM »

I'll give you an example. It may help, it may not.
My boat is 4'8'' long by 8'' wide and is about 15Kg. It has twin 50mm props which have quite steep blade angles so they bite the water a lot. I opted for 540size motors with 11:1 gearboxes because I felt I needed torque rather than speed. The props spin quite slowly without the motors stalling. When at scale speed the motors are at around 50% power which means I can wind them up if I need to get out of trouble. Each motor runs off a 20amp speed controller powered by a single 12V battery. Apart from the props and the battery I would just uprate the different components eg I think the speed controllers would need to be 30amps minimum, the gearboxes can be 6:1 if you like. The more suggestions you get, the bigger the picture you will get. The rest is up to you.

Kind regards Ali.   
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tonyH

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Re: is my boat to heavy for motors ?
« Reply #13 on: May 12, 2011, 08:39:54 PM »

Hi Pete, it sounds as if you've got a real beast there :-))

Can I suggest that you need to get a better idea of what the motors have to push first. The reason for this is that in your first post you mention that 3kg dropped the hull by about 5mm. That would mean that if the depth under the waterline is 2 inches then the all-up-weight would be over 30kg but if it's 3 inches then well over 45kg etc. You could find out in the pond by just adding weight to bring it down to the level you want.

As Ian said, you could be getting into quite a heavy ship to move, whether on or off the water.

Any chance that you could post a pic or two so that, maybe, we could help more.

Cheers

Tony
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john s 2

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Re: is my boat to heavy for motors ?
« Reply #14 on: May 12, 2011, 09:18:54 PM »

As regards Motor size. If there are a bit on the big size and overpowered, does it matter? I always thought
that the pupose of an Esc was to vary speed. Some esc's can be range set. Mtronics for instance. This is
assuming your finger doesnt work well enough.Fit what youve got and enjoy. John.
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Pete25

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Re: is my boat to heavy for motors ?
« Reply #15 on: May 12, 2011, 10:16:15 PM »

thank you to everyone that has posted i really do appreciate it  ;)

if i stick with one lead battery 12v  to keep weight down would this run 3 12v motors for a good run time ? that's why i was going to use 3 batteries in parallel

also my pond is 3 ft deep if this alters any of my results :)

i don't have any pictures as they were not good quality However i have a video of the hull here

http://youtu.be/Ee9wJpThPQ4
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john s 2

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Re: is my boat to heavy for motors ?
« Reply #16 on: May 12, 2011, 10:41:52 PM »

Its difficult to be precise about run time as there are so many variables. Certainly with three 12v 7amp hr
batteries youd have a long time say 2 to 3 hrs? Id fit one to start allowing for more if run time not sufficient.
Please remember that Gel Cells do not like to be run flat. It will shorten their life. John.   
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wartsilaone

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Re: is my boat to heavy for motors ?
« Reply #17 on: May 12, 2011, 11:28:26 PM »

That hull looks familiar. Maybe someone on the forum has used one of these.
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Pete25

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Re: is my boat to heavy for motors ?
« Reply #18 on: May 13, 2011, 12:23:29 PM »

i have posted he video of my hull on this forum in other sections. i got the design from looking at real full size battleships  :-)

will this battery be ok to use with my motors ? and could i use 2 of them in parallel if i wanted.

12V 7.5 Ah sealed lead-acid batteries
7.5Ah (7,500mAh) measured at standard 20 hour rate.

do i add each amp of a motor
Johnson 600 size  3 - pole
12 volts
No Load speed is approx 20,000 rpm.
Off load Current: 1.57A

as i want to use 3 motors all three is a total of  4.47 amps
so do i find a 12v battery round 4 to 5 amps ? ummm

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wartsilaone

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Re: is my boat to heavy for motors ?
« Reply #19 on: May 13, 2011, 12:41:43 PM »

My boat has run for 5 hours on a single 12V 7A Lead Acid battery using the MFA919D motors I Mentioned earlier. I also have a spare but never needed it. From my experience this type of battery should be fine as I wouldn't think your motors will be running at full tilt all the time. Make sure you get the right speed controllers that can handle the motor current.

Ali.     
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Pete25

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Re: is my boat to heavy for motors ?
« Reply #20 on: May 13, 2011, 01:55:49 PM »

thank you for that :) i have a car battery charger here. (never used) and was wondering if it would be ok to use to charge one 12V 4.5Ah Lead-Acid Battery
the charger says  12v  2.7A  DC 4 AMP  Would it be ok to charge this battery with this ?
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wartsilaone

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Re: is my boat to heavy for motors ?
« Reply #21 on: May 13, 2011, 02:37:57 PM »

I think you can use it but beware. It depends on the output. If it is 2.7Ah a 4.5Ah battery would need less than 2 hour to charge. if it is 4Ah it would need about an hour. I think that how it works. My mains charger dose 700mah so it takes 10 hours to charge a flat 7amp battery.
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wartsilaone

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Re: is my boat to heavy for motors ?
« Reply #22 on: May 13, 2011, 02:45:09 PM »

I prefer to slow charge batteries rather than ramming juice down their necks.
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Pete25

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Re: is my boat to heavy for motors ?
« Reply #23 on: May 13, 2011, 03:09:29 PM »

ow.. well the charger has a fully charged led indicator on it so ill just try it and keep an eye on it..  :-)
again just want to thank everyone for your help and comments :) i will keep you posted on this website as the build goes on.
i should receive most parts in a weeks time then i can get a start on building drive shafts and installing motors and so on. thanks again
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John W E

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Re: is my boat to heavy for motors ?
« Reply #24 on: May 13, 2011, 03:20:35 PM »

Hi ya Pete25

A couple of questions first:

1.  On your battery charger, is there a switch which states GelCell charge/auto charge for car batteries?    If not, and it is just purely a car battery charger, it may do some damage to your gelcells charging it with this

2.  When you are quoting your amperage on motors, as 1.5 free running - this is the actual amperage that the motor will draw when running with no propeller or anything else connected to the motor - it is free running.  Once we connect a propeller shaft and propeller to the motor, this creates resistance to the motor and then causes the amperage to climb up - so you will note if you connect an amp meter between the battery and the motor whilst you have the propeller/propeller shaft connected you will see the amperage go up towards 1.75 amperage or thereabouts.    Now you stick the propeller into the water causing more resistance to the motor and you will the amperage climb up even further, possibly towards the 10 and 15 amps per hour, depending upon the size of propeller fitted.    With the motor now drawing between 10 and 15 amps, the life span of your battery or running time will decrease drastically.

As a quick rule of thumb, divide the running amps of motor under load by the amp per hour of the battery.     

That will give you an approximate run time on the water.

Last but not least, looking at the video of this hull - are you going to put bulkheads/stiffening inside to support the skin structure.   If not, you will find this will become very flimsy.

aye
john
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