Fairmile D – help needed

[Charles OatesParticipant @charlesoates31738]

Hi David, I've read this thread with interest and kept quiet, as I couldn't offer any better advice than has been offered. If you are perceiving a bit of frustration, it's because it might seem that you are misunderstanding offers of help.

I'll try and clarify, in 2 parts.

1. A model with 4 motors isn't 4 times faster than the same model with 1 motor, my best estimate is that it will be around 30% faster. However, it will draw four times the power ( amps) from the batteries through the speed controller (s)

2 you can't escape from the amount of power needed to make this model go in a satisfactory manner, and that means having the right propellers, which then dictate the power drawn for whatever voltage you use. The advice offered you from other modelers is often given as a result of mistakes we made years ago and intended to save you money, frustration and damaged equipment. With that in mind you will understand if the need to steer you a better way is emphasised.

Lastly, my 2 pennyworth, the agm batteries, sealed lead acid I think? Absolutely the last type of battery I would use or recommend for your boat. They are not suitable for prolonged heavy current, the voltage will fall rapidly. 12 ah rating is only true with a mild load, if you try to take say 25 amps from them, they will fail very quickly. This isn't just an opinion, it's a long established bit of advice that might well have appeared on this forum 50 to 100 times.

Sorry if you don't like what I've said, it's meant to help not to upset you.

Chas

[Malcolm FraryParticipant @malcolmfrary95515]

Having a constant resistive load and a constant supply voltage makes life simple for doing the required calculations. A motor running on pure DC is not quite as simple, because it does have an inductive component that varies with the speed, and further varies with the load, and both these variables are compounded by the design of the motor, which, while moveing, presents lots of varying conditions. Most of us don't bother doing precise calculations – we just derate generously to avoid letting the magic smoke out, because magic smoke is expensive.

Some sage advice about lead batteries was offered earlier. They are great for tugs, only good for demonstrating that the motors work on a fast boat. After that, they are good for providing weight when you need to glue a deck down on a later model. Or for powering a tyre inflator or, via an invertor, supplying standby power for small mains powered items.

[David Hepworth 1Participant @davidhepworth1]

Hi Charles

No. Not upset. I never get upset over what is said on the internet. I save that for real lfe. I was worried that I had upset someone else.

I agree about 4 motors but the previous owner of the hull had already fitted 4 prop tubes…and, as the original Fairmile had 4 props, that's what i'll go with. More importantly will be the props. The current 3 blade brass ones will be rubbish, I know, but they look the part, so i'll live with them for now.

The batteries i'm using are fine for heavy current and deep discharge (I use them in my work).

The effective current is nothing like 25Amps, as i'm running the motors at 12v, and nowhere near full power. I reckon closer to 12Amps effective.

Hi Malcolm

Not ideal for the application, true, but i'll use them to get the boat in the water. I reckon i'll get about 20mins use before a battery change. I have a few different boats and, when I can drag myself to the pond, I like to give them all an airing so 20 mins would be fine. If I find I'm happy with the boat and i'm using it more then I'll change them out for a Lithium battery.

[John W EParticipant @johnwe]

Hi ya there, just out of curiosity is your hull fibreglass – Is it one of Kingston Mouldings' ?

Brass props that are to scale will be efficient, especially if they come somewhere like Propshop or somewhere.

Keep us all uptodate

If you try to make this hull plane across the water, you are going to have to ditch the lead battery, unless the hull and internals are extremely light weight. Many years ago I built an RAF rescue launch – pre-internet days – and it was powered by 2 x 6 volt 4 amp batteries and the hull was the same weight as a 5ft span glider (very light) and it only stayed on the water for a matter of 4 or 5 mins and eventually one of the battery casings split, spitting its internals into the hull as the batteries were that hot you could fry eggs on them.

John

[Paul TParticipant @pault84577]

David

I have been following this thread as I am interested in your adaptation of robotic systems but I am confused about the ability of the Pololu Motor controller to handle the demands of 4 motors.

Whilst the specs do claim the ability to deal with 25A they also state that its control system is for a single motor, which I understand especially when I apply the technical specifications to the changeable needs of a motor in a mobile robotic situation.

You will see why I am interested in this unit as it could potentially save a considerable amount of money but I can't see how a single Pololu controller can supply a constant and reliable current to each of the 4 motors without power spikes on some motors and power dropouts on the others.

Can you point me in the right direction to where I can find the technical specs for your adaptation, I presume that these specs are buried somewhere in the Internet but I can't find them.

Also very keen to see your deep cycle batteries as my limited experience with these thing is restricted to the large leisure batteries used in boats and caravans and I would love to use small versions in my six foot model.

I hope you don't mind my asking these questions but the answers could have deep implications on my own design work.

Paul

 

Edited By Paul T on 17/06/2020 20:01:29

[Dave Cooper 6Participant @davecooper6]

Hi David

Could I suggest a few simple tests before "committing pond" :-

– Run it on the bench in your proposed configuration. If you have the electrical test kit, measure amps (and watts) at different throttle settings. Quickly touch the ESC – insulated part – and feel the temperature.

– Put the props under static load, finger on end of prop boss(es), or, simple scrap balsa wood 'snubbers'. Check ESC temp again.

– Bath test. Feel the pull at the stern with different throttle settings. Check ESC temps.

if you have to quickly withdraw your finger from the ESC in any of the above – it's running too hot ! You need to do something about it…

Battery life /power delivery etc – These days, for a high-speed launch (brushed or brushless motors), I would only really consider LiPo's, although NiMh's (or, equivalent) may also be ok. One way or another, you need to establish your battery life before going on the water. Most modern Tx's have a timer facility – set it to a conservative amount initially.

In short – test, test, test…good luck,

Dave C

[ashley needhamParticipant @ashleyneedham69188]

Dave has it: By now I would have put one motor on an esc with whatever batteries you plan on using and measured the current with the boat in the water on full throttle…bath if necessary.

All the theories are valid, but only the test will show you what’s what. You need to KNOW, not guess.

The problem with all the theorising and advice is that really you are no nearer the answer and the longer the thread goes on, the more confusing it will be.

Ashley

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