Model Boat Mayhem - Forum

Dry Dock / Shipyard: Builds & Questions => Pleasure boats, Sports, Race, Power and Leisure Boats: => Topic started by: andi4x4 on April 03, 2008, 10:59:15 PM

Title: Three motors too much ?
Post by: andi4x4 on April 03, 2008, 10:59:15 PM
Ahoy Shipmates !

 Newbie here with a new build Brave Borderer on a 36" fibreglass hull.

I have just started building from the Model Maker plans that were supplied with the hull.

But, I have a few questions-
 I have set three propshafts into the hull - currently set up with 30mm scale brass props and direct drive from three Speed 600 motors, running in parallel from a single ESC and single 7.2v NiMH buggy/car type battery pack of 3000mah or higher.  Now, in the past I have run this hull with a single 50mm 'X' prop and single Speed 600 ( watercooled ), running through a 2:1 reduction gearbox, single ESC and single 9.6v sub'C' type pack with no problems, a good turn of speed and about 10-15 mins duration from 1500mah pack.  The current three motor,three 30mm prop, direct drive system is quite a bit slower and much shorter duration - 7-8 mins from a 3000mah pack. I expected a drop in speed due to the change of prop type, but this is a considerably bigger drop than I was expecting, and the duration is also disappointing. My questions are, therefore -

1) is a single pack and single ESC not enough for three speed 600's on direct drive ?

2) is it the direct drive that is the issue - should I go back to having a 2:1 reduction on the motors ?

3) should I be thinking about a higher voltage - e.g 9.6v or even 12v ?

4) or should I be thinking about a twin or even single motor set-up running through a custom reduction gearbox to all three shafts ?

Any and all advice would be warmly welcomed !

All the best

Title: Re: Three motors too much ?
Post by: wombat on April 04, 2008, 04:57:29 PM
Hi Andi,

Scratching out a couple of quick calculations - moving from a single 50mm prop to 3 30mm props, you haven't actaully increased the active area of propellor - in fact it may have gone down a bit. You will have to swing the things quite a bit faster to compensate. But calculating the current drain for the battery - you are at the limit for the battery (Checking a 3300mAH sub-c cell it gives the maximum current as 30A - this will drain the battery in about 6 minutes). It could well be the battery current limiting which will prevent the motors swinging as fast as they would like. It may be worth trying a couple of batteries.

Before going for fancy gearboxes, try running with two escs and batteries  - use a couple of diodes to feed the central motor from the other two and see if that improves matters.

The other question is whether you are getting a bit of cavitation on the props - this would reduce the efficiency and reduce the top speed.

Title: Re: Three motors too much ?
Post by: andi4x4 on April 04, 2008, 10:12:28 PM
Hi Wom,

 Thanks for the reply,

 I have today changed the setup from three motors with one pack.

Currently running two motors ( centre motor and prop disabled for the time being ) with a 20a esc on each and a pack to each esc. Pulls about the same power as the three motor/one pack setup, so I consider that to be an improvement !

Was planning to add the third motor back in on a servo operated micro switch to trigger at full throttle only, but, kind like your idea of the diodes too - what rating did you use ?

Think I am getting a little cavitation too - I have not got round to installing the transom flap yet which may or may not help. Also, I had considered the gear reduction because I thought the props were being spun too fast, and I guessed it would reduce the load on the motors/batteries and hopefully increase the duration some. I have also shifted some weight further towards the stern to try to stop the back end lifting so much.

Will try it out at the pond tomorrow evening and see who it is performing with two motors/props and will go from there.

Thanks for the advice, Wom !



Title: Re: Three motors too much ?
Post by: John W E on April 05, 2008, 12:09:10 PM
Hi there Andi

With reference to running your model on 3 motors, I am running an M.T.B. on 3 motors - with three 3 blader 30mm brass propellers.   Also, the power supply for this is 2 x 6 volt 4 amp gel cell batteries the motors are MTroniks 500's.  The batteries are divided as follows; 1 fits midships, 1 fits behind the 3 motors at astern.  You will possibly find, if you put a lot of weight at the stern of your particular model, that she may not come up to the plain properly - she may just bury her stern into the water - due to all of the weight factor being at the stern

This model comes up on the plain very easily, but, the duration of the model is, at the max 10 minutes.   I do have the facility though of shutting the 2 outboard motors off and running solely on the centre prop - and - doing it this way - her duration on the water is increased dramatically.   However, her performance is way down.

The model I am currently building, The Whaleback, is also a 3 motor setup.    I also have the facility on this one of shutting the outboard motors off.  I also hope to be running this on NiCad packs just to see how it will perform.     

The way I control switching on and off the outboard motors - on the M.T.B. I have 2 speed controllers from Electronize and the speed controller which controls the 2 outboard motors - the output transistor and the circuit board has been 'beefed up' to compensate for the extra load drawn by the motors.    The signal wire from my RX goes to a microswitch, switching the signal off and on to the speed controller.   This gives me the facility of switching the outboard motors off and on without having to use heavy duty relays on the power feed side, to the motors.    This aforesaid method would be the ideal method, but, it entails either a very heavy duty micro switch in the region which can handle in the region of 30 amps, or, a relay that can tolerate 30 amps switching. 

I have noticed to date that when using a Futaba RX or a HiTec RX switching the motors in and out I get no 'chatter' from the speed controller relays.   However, when I use the miniature one from Cirrus or the Sanwa one - when I switch the speed controllers off using these RX's - I do get a lot of 'chatter'.

Now, on the new build (i.e. the Whaleback) I have incorporated a mixer along with 3 speed controllers from Electronize and on this one I am using a twin micro switch setup to switch the 2 outboard speed controllers off via the signal input - this does not affect the control of the mixer or the centre motor.

I am currently trying to negotiate for a mixer that is capable of reducing the speed of the centre motor; when operating all 3 motors together you will find (the same as I do) you have an affect when turning to either port or starboard - the model will have a preference to turn tighter in one direction than the other - this is due to the fact that your centre prop will be the same hand as one of the outboard ones.  Therefore giving one and a half times more thrust to that side of the model, in a turning scenario.   In real life to compensate this, when doing manoevering they reduce the revolutions of the centre prop.

You will also find you will always get a certain amount of cavitation; with smaller diameter propellers.

Hope this is of some help.

john e
Title: Re: Three motors too much ?
Post by: BarryB on April 07, 2008, 09:44:51 AM
Hi John

My Perkasa (51 inch) has 3 props - the outside 2 turn in same direction - the centre prop is "other hand" - so I don't get the biased turn problem you are getting.

Centre prop control: Is it me you are "in negotiation" with?

If so, you probably already know about my #826 mixer which can be built from scratch or will fit (with slight modifications) the ACTion mixer board.  You must be happy with a soldering iron!

Title: Re: Three motors too much ?
Post by: John W E on April 07, 2008, 10:41:08 AM
Hi there Barry,

Just wonder can you explain what is wrong with the ACTion mixer chip.   I myself have used several of these over the years, along with Hunters mixer system, in a good few models ranging up to near enough 6 foot in length.   I cannot fault an ACTion mixer chip.

The only modification that I would want to do for the ACTion mixer is, for this one particular model I am building - and on this model all the propellers turn the same hand.   Oh by the way, your two outboard propellers should turn opposite hand on normal vessels.  :)  If it is supposed to be a 'scale vessel'.

The modification I am referring to above, is, when the model is at speed and it comes to turn either port or starboard, instead of the centre motor driving full wack, the centre motor drops down to 50% centre power, so, it has no or very little influence on the two outer motors, which are aiding with the steering movement.    The guy I am negotiating with is also building for me a sound unit; similar to JJC's unit; only with the sound of three Merlin Engines cos we cant get Napier Sealions sound chips  :) .

John E
Title: Re: Three motors too much ?
Post by: FullLeatherJacket on April 07, 2008, 04:33:39 PM
If so, you probably already know about my #826 mixer which can be built from scratch or will fit (with slight modifications) the ACTion mixer board.  You must be happy with a soldering iron!

Sorry to go a little off-topic but I feel I ought to reply to the comments made in the last two posts.

As Captain Barry says on his website, he last tested an ACTion Mixer in 2002. There have been a number of software upgrades since then. We have sold 97 of these in the 11 months since Liz and I took over ACTion, which must say something positive about it. Harbor Models have just ordered another ten.

Notwithstanding, I shall be happy to supply anyone who wishes to build one of Barry's mixers with a kit of hardware, including the unique epoxy-glass printed circuit board and the special ABS case, but minus the MMX100 PIC chip. Price on application. I presume that Barry will supply not only the replacement PIC (@ 12) but also all of the necessary building and operating instructions, assembly drawings and wiring/setting-up diagrams which cover his modifications but which don't seem to be on his website at the moment.

BTW we have obtained and tested examples of both of his  mixer chips in the last six months.

Suit yourselves - it's a broad church. I guess you pays your money and you takes your PIC?  ::)

Back to the matter in hand!

Andi 4x4 - If you send me your E-Mail address via a PM I can send you a diagram in PDF format which shows how to do what you are proposing without using microswitches. Clue - it uses one of Wombat's own designs!

Title: Re: Three motors too much ?
Post by: andi4x4 on April 07, 2008, 05:22:38 PM

FLJ -- PM sent ! Thanks for the offer !

Title: Re: Three motors too much ?
Post by: FullLeatherJacket on April 07, 2008, 05:57:40 PM
Reply sent. We aim to please  8)
Title: Re: Three motors too much ?
Post by: BarryB on April 08, 2008, 09:20:43 PM

I don't think we are far off topic as we are trying to offer advice to the group (1st poster "andi4x4" and others) between us.
So, thanks for offering choices to the group

Craig always said that he was quite happy that I offered my chip as a plug-in option, so you are carrying on your good tradition of helpfulness.

The #826 chip I offer, which acts as described on my web site, is a mixer with functions which were first asked for by a buyer in the USA - I have only sold about 3 of those in 5 years - so I guess not much demand !!
I like programming, so I do a few specials for people if I can. But as you probably know, programming and testing takes a long time and regretfully I have to turn away most requests unless local & I can see the results working.

So, "andi4x4", if you are local to Reading or Oxford, maybe I could offer some solutions?

I can't compete with ACTion's quantities sold as I only do this a serious hobby, not a business - I gave up the stress of running a business (unconnected to models) many years ago!

Dave, Will you be at Beale show in May 3 & 4 - I will say hello again to you if you are.

Title: Re: Three motors too much ?
Post by: wombat on April 08, 2008, 10:02:44 PM

One of my designs eh? That would be the P91 then!!!

More expensive than two diodes but far more efficient - less power loss and you can set where the motor switches in.

So guys, what is the ideal mixer then?

Title: Re: Three motors too much ?
Post by: FullLeatherJacket on April 08, 2008, 11:47:47 PM
Dave, Will you be at Beale show in May 3 & 4 - I will say hello again to you if you are.

'Fraid not, matey. Next show is Sleepy Hollow this weekend, followed by Doncaster in June.

Yep - that's the one. I'll be interested to see what comes of your second question.