If they are Gel cells then the voltage may be higher but the ability to deliver the power may take a nose dive. Gel cells are not good for high discharge rates unlike NiMH batteries.

Colin

Advert

2 April 2015 at 18:50 #56986

Bob Abell 2

Participant

@bobabell2

Thank you, Colin

I did think at the back of my mind, that all would not be well, but then, I thought,………..All the tug boat chaps use them, so it may be ok?

What are you suggesting as Plan C?……….Didn`t think I needed fast discharge though?

Bob

2 April 2015 at 18:54 #56987

Colin Bishop

Moderator

@colinbishop34627

Gel cells are OK for low discharge rates and providing ballast in high displacement hulls. There is a bit of difference between a tug hull and a cabin cruiser! Personally I would stick with NiMH batteries.

Colin

2 April 2015 at 18:59 #56988

Paul T

Participant

@pault84577

Bob

Not a spanner…..just a question that Kevin might ask.

The maximum output of the MFA 850 is generated from 12volts (see table below)

Model

Voltage

NO LOAD

AT MAXIMUM EF

FICIENCY

STALL
TORQUE

OPERATING
RANGE

NOMINAL

SPEED

CURRENT

SPEED

CURRENT

TORQUE

OUTPUT

EFF

RPM

oz – in

mN-m

oz – in

mN-m

RE – 850

12.0v

12.0v CONSTANT

9778

1.90

8311

10.82

–

92.13

80.16

61.74

–

614

Paul

2 April 2015 at 19:05 #56989

Bob Abell 2

Participant

@bobabell2

Thanks Colin……….I`m all confused .com now!

I`ve got four 12v NiMH sticks from the big Slo Mo Shun…………Probably ideal for the job?

Pandora`s Box is open again, chaps!

Bob

2 April 2015 at 19:08 #56990

Bob Abell 2

Participant

@bobabell2

Paul

Do we need maximum power?……….We have got four motors to play about with

The 12v sticks may be the way to go then?

Bob

2 April 2015 at 20:17 #56992

Paul T

Participant

@pault84577

Bob

Pandora's box is still closed, all we are discussing is a change of battery voltage to achieve a better performance.

Even though you have four motors to play with the potential maximum power output at 12v is only 320w and Lady Joyce is a big heavy boat, its difficult to estimate without knowing the all up weight of your boat but if pushed I would guess that, to produce average performance, you will require +500w

In theory two motors at 12v should produce 160w but your set up of two motors driving one shaft might limit the power output ( and now we are back to testing with your watt meter)

The 12v sticks might just be the ticket however it will depend upon the amp/hour rating (how fast they will drain when the boat is going full tilt)

Paul

2 April 2015 at 20:23 #56993

Bob Abell 2

Participant

@bobabell2

Just ordered a 10 foot paddling pool………Handy for Amp tests……..Ballast and Waterline tests……..and just messing about in general

Bob

2 April 2015 at 20:35 #56994

Bob Abell 2

Participant

@bobabell2

Hello Paul

What achievable speeds are you talking about, using 320 watts and 500 watts?………All up weight may be 60lb

The model may be heavy, but we don`t need to charge about, except for the odd occasion

We`ll set off slowly and gradually build up the speed in a gentlemanly manner

We could change the pulley ratios too

Bob

2 April 2015 at 20:36 #56995

Gareth Jones

Participant

@garethjones79649

Bob,

Since you asked, Shemarah has a 25 amp Mtroniks speed controller. The maximum current measured with the boat static was 6.5 amps at 10.85 volts, giving an input power to the motor of 71 watts. The speed controller is somewhat over rated for its job in normal conditions but the stalled current of the Speed 900BB torque is 54 amps.

Gareth

2 April 2015 at 20:42 #56996

Bob Abell 2

Participant

@bobabell2

Thank you, Gareth

That`s very interesting………Next question…..What weight is your model……….What batteries are you using?

Are you happy with the achievable top speed?

Bob

2 April 2015 at 21:03 #56997

ashley needham

Participant

@ashleyneedham69188

Bob. There is nothing for it. You are talking big amps here with such a big boat and the wallet has to come out for a big set or double set of Nimh`s. Same with speed controller (or get a racing water cooled type which will handle these amps easily and you would not have to worry about overheating). Fit a clever flappy rudder thing for added manouverability to compensate for no reverse.

There is not much alternative I am afraid.

I would dunk it in the water and see what the motors ACTUALLY draw under load first. Although this is a big boat you may not need so big a prop and so much loading on the motors….

Gareths Shemara is if my memory serves, a short fat hull and much different to yours so I am not sure that his particulars will have much bearing on your issues.

Ashley

Edited By ashley needham on 02/04/2015 21:07:35

2 April 2015 at 21:13 #56998

Bob Abell 2

Participant

@bobabell2

Thank you Ashley

It doesn't take much power to push a boat, so I'm feeling optimistic

We'll soon know

Bob

2 April 2015 at 21:21 #57000

ashley needham

Participant

@ashleyneedham69188

Bob. Dont forget that if, in trials, you find you really dont need so much power, MFA 800`s will substitute the 850s directly, no mods, and they rev to about 5000 rpm which is all you might need. This would also halve your current draw at a stroke.

At the pond side it would be instructive to disconnect one each motors from their respective drives, and see just what the performance is like on a pair of 850 (one per ABELL unit), this will give you some sort of yard stick to go by. perhaps taking some smaller props, just to try…say a pair of x50 or x60 if obtainable would be a cheap way of experimenting.

Ashley

2 April 2015 at 21:29 #57001

Bob Abell 2

Participant

@bobabell2

Hello Ashley

I've only got 50mm 3 blade props at the moment and I think this may be a safe size to play with

It should be great fun, testing the various options

The ability to change belts and pulleys should be useful

What is the advantage of using 800 motors?

Bob

2 April 2015 at 21:48 #57002

ashley needham

Participant

@ashleyneedham69188

The advantage is they only rev to half wot an 850 revs to, and take half the current. One 800 or 850 will turn a 50mm prop easily. The are identical, can-wise so you can swap them over dead easy.

Your pairs of 850`s will not even break a sweat turning 50mm props at 9000 rpm. I would fit a pair of X55 props (probably the largest of the cheap x series available)

Ashley

3 April 2015 at 08:20 #57006

Bob Abell 2

Participant

@bobabell2

Thank you, Ashley

Thinking out of the box and radical and controversial, we could have a small prop on one prop shaft and a coarse prop on the other!

And no ESC!………As a novel experiment, a sort of two speed gearbox?

We could stooge round on the small prop and gallop about on the big prop!

I could try that in the pond and observe it's potential

Only thinking out loud really

Bob

3 April 2015 at 09:07 #57007

ashley needham

Participant

@ashleyneedham69188

Nah, boat would steer horribly in one direction.

Making two contra rotating shafts would be the thing, and you could feed power slowly to one prop and then have the second one come in later or simply be on or off..

Or have a working "emergency outboard motor" on the transom!!

Ashley

3 April 2015 at 09:55 #57008

Dave Milbourn

Participant

@davemilbourn48782

Four of these would reduce the maximum power available on 12v to just 148W; realistically maybe 100-120W once all friction losses etc have been overcome.
Bob's model is 5 feet long and made of thick-section marine ply.
I have a 400-sized brushless outrunner which could produce more power than that on its own, using 11.1v. It is fitted into a 23"-long liteply-and-balsa Huntsman 31. **LINK**
Those are the stone-cold facts – there are no opinions involved.
Suit yourself.

3 April 2015 at 10:25 #57009

Bob Abell 2

Participant

@bobabell2

The model is built from 3mm lite ply and weighs 24lb without the batteries

It could almost fly!

Bob

A few quick snaps showing some trinkets that Duckie has found in the £1 shops……He loves a bargain!

Edited By Bob Abell on 03/04/2015 10:50:26

3 April 2015 at 11:24 #57010

Dave Milbourn

Participant

@davemilbourn48782

Bob
Mea Culpa for not reading the thread thoroughly, but there is a lot of difference between Liteply and Interior Ply, not least of which is the price. If you can get five 24" x 48" boards of genuine 3mm Liteply for less than twenty quid (Page 29 Post #6) then I'd be grateful for the address, please. I pay £8.44/sheet for mine from SLEC.
Comparing weights doesn't really mean a lot but, if you insist, my model weighs 32.1 ounces (910gms) without its battery (which adds 208gms, or 7.3ozs). Here's a shot before all of the deck fittings were added.

As for "almost flying" that could be down to a question of interpretation come the day. We will just have to wait and see, won't we?

Dave M

Edited By Dave Milbourn on 03/04/2015 11:25:39

3 April 2015 at 11:37 #57011

Gareth Jones

Participant

@garethjones79649

Bob,

Shemarah weighs about 65 lbs fully ballasted and is powered by a single 12 volt 7.2 amp hour lead acid gel battery. As Ashley says its a short fat hull and rather different to your VGC. I am quite happy with the performance of Shemarah, its not fast but it bustles through the water, probably faster than scale speed. Heres a picture to give you an idea. I think it is at, or close to, full throttle.

Gareth

Edited By Gareth Jones on 03/04/2015 11:37:47

3 April 2015 at 11:58 #57012

Bob Abell 2

Participant

@bobabell2

Hi Dave

My source of lite ply, should really read light ply…….From the local wood recycling shop

You have to take pot luck!……All I know, is that it is much lighter than the heavy stuff!

Having said that, old Bean……I reckon the weight of 24lb….May have raised a few eyebrows?

The moment of truth is rapidly approaching!

Your model looks very nice….btw

Bob

3 April 2015 at 12:20 #57013

Bob Abell 2

Participant

@bobabell2

Thank you, Gareth

I think Lady Joyce will dance the Light Fantastic, weighing only 30lb all up and the four 850's

Bob

3 April 2015 at 16:28 #57014

Paul T

Participant

@pault84577

Hello Bob

When drawing up the original design I had estimated the all up weight including the hull, motors, batteries and ballast to be approx. 30kg (say 70lbs)

Paul