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[Z750Jay]

Hi all,
I have already had a troll through the pages of post to try to find an answer to the question but can not see anything suitable.


What I have is a Robbe fixed kort with a 60mm prop. Now I know it supposed to be something like motor diameter ~ prop size but I have no idea what would fit this as all my experience is with smaller than 40mm props. It's not for high speed, it will be driving a tug like thing I am building as a rescue/mark recovery boat.


Was thinking of the Mobile Marine Models T12 but not sure if it's the correct one.
Any help would be great

[inertia]

This would be a good motor to try. Like the old Speed 720BB Torque but with plain bearings and at a third of the price http://www.componentshop.co.uk/775-dc-motor.html
DM

[Z750Jay]

That one looks good to me thanks! Like the price even better as I have to have two. Just trying to shoe horn everything into the weird hull shape I have. Trying to work out what battery set up to go for. My main restriction is space. As the boat has to fit into the top box of my bike I am restricted to a volume no bigger than 400mm x 300 mm by 160 mm high. That's for the hull, the superstructure will house all the electronics and will be carried separately.

[Time Bandit]

The motor seems to be the Power 755/40 from Robbe. The Graupner has a lot less rpm, which should be better for that kind of boat.
What prop will you use? Brass or the Robbe plastic.
The motor will probably work with the plastic prop and probably overheat with a 60mm brass prop.

Did anyone of you found a source for the Buehler Motors in the UK?
Something like this would be perfect

http://www.hobby-lobby-modellbau.com/onlineshop/product_info.php/language/de/info/p5616_B-hler-Star-Max-II---12-Volt-nur-noch-30-St-ck-.html

[Z750Jay]

Sticking with the plastic ones at the moment. Noticed I had a water pump in my bit box so water cooling might get added(thinking the Robbe unit) if I can find space!

[Time Bandit]

Watercooling in a model with a speed less than 15mph is just a waste of energy.
You need just 3W per kg boat weight for a tug, so if you need watercooling you motor turns too fast.
Try to find a motor with 2500-3000rpm on your voltage.

[Z750Jay]

I agree with water cooling being a bit of a waste of energy. I originally got the pump so I could cool my original springer tug when she took part in the Chichester Canal run. Running almost continuously for 2 hours got the motor a tadge warm to the extent the electronics overheated and shut down.
I might fit the capacity in the build stage ( pump, plumbing to each motor cooling block, but bypass the suction from the sea chest so it runs straight to both discharge overboards)
My current design roughshod are hideously over engineered and weird     - there is not much to do at work with all the Navy being at sea and the mind wanders a bit.
Strangely the top of my sketch pad has the Top Gear Jeremy Clarkson moto POWER!

[Calimero]

Brushless FTW !


I'm running a 800kv brushless outrunner motor under 6V with a 40mm (or 45mm wide area blade for heavy towing) 4-blade prop. I usually cruise around at half throttle. I get 4 hours form an SLA 7.2Ah battery with the 40mm prop. With heavy towing (as in 700lb 13' tow) with the 45mm prop I still get a solid hour and a half off of the battery. Cooling is a non issue.


I'm going to use two 620kv outrunners for the bigger springer I'm drawing with twin 50mm props.

[Z750Jay]

The only thing that puts me off brushless is the cost of them.




But the POWER Mmmmmmmmm.


Already worked out a simple LED charge indicator and switching capacity so that for long endurance runs  I can tow more batteries an a barge.

[Time Bandit]

For me costs are not such a big issue even if they are definetly higher.
But,
the horrible sound and the non-ability to run at low rpm keeps me away of them in non gliding boats where I certainly don't need the power.
Nothing beats a 12pole motor with a high frequency controller. Absolutely noise free and you can count the rpm with bare eye.

[Calimero]

https://www.youtube.com/watch?v=1TmoOZVRXTM


At 4:12 that's the slowest I can sail with my cheapo controller. I don't really need my boat to go any slower than that so I'm satisfied with it.


Although a GoPro ain't the best recording equipment motor at slow RPM is extremely silent (aside from the small chirping when starting). At 4:20 the Gopro picks up noise from the rudder servo but nothing from the brushless motor. At 5:37 even at full throttle there's more noise coming from the water itself than from the motor. Under normal cruise there's a faint mid-frequency "whizz" you can barely pickup past 15'.


The only downside for brushless motors is that you need one ESC per motor. I don't really care because I'd go with independent control even with brushed motors. But we're talking about $50 ...


My springer is probably the most silent boat we have at our pond. Aside from offshores/FEs, the worst are geared setups.  The kind of nightmare you don't have to endure with a brushless motor anyway.

[Calimero]

The only thing that puts me off brushless is the cost of them.

$30 per motor, $20 per ESC. That's $100 (of $120 with shipping) for you rig. You could probably pick up some cheaper brushed motors, but well ....

Already worked out a simple LED charge indicator and switching capacity so that for long endurance runs  I can tow more batteries an a barge.

How much run time do you need ? Instead of an SLA battery you could go with a 2S LiPo which has greater power density. But a 40x30x10 box will probably require quite a lot of ballast to get to waterline. You might be able to fit two 6V/7.2Ah batteries giving you plenty of run time.

[Z750Jay]

I am aiming for a run time of about 2 hours with the in situ batteries, When I do the 3 hour endurance run I will have the extra in the barge.


Over here in the UK brushless motors are priced about $70 and up with the esc costing the same. Our brushed motors cost about half that.


Like I said, some of the functionality and ways of doing things may seem strange to most but that's what happens when there are a couple of bored engineers designing stuff (marine electro mechanic for me and my mate designs and builds bespoke electronic boards )
Being fans of Top Gear does not help!

[Calimero]

I am aiming for a run time of about 2 hours with the in situ batteries, When I do the 3 hour endurance run I will have the extra in the barge.

2-3 hours of heavy towing or two three hours of sailing around ?
A 6V/13Ah SLA battery might fit in your hull depending on its actual design. Maybe even two.

Over here in the UK brushless motors are priced about $70 and up with the esc costing the same. Our brushed motors cost about half that.

http://www.hobbyking.com they even have a warehouse in the UK (but not all inventory is available there of course). From their Asian warehouses stuff is at your door in three weeks.
But anyway if you're okay with a brushed setup, just go with it. These aren't demanding applications.

[Z750Jay]

After watching your vid I have come around to the idea of brushless. A bit more trawling the internet (work is sooooooo slow today) I think I can get a 930kv brushless for between £24 and £50 which is a price I am happy with. I already have a couple of 6v 7Ah batteries I can use rigged up in parallel.


I could squeeze another in as well so I would have a grand total of 21 Ah  

[Calimero]

930kv under 6V, that's 5500 rpm with no load. Way too wild on a 60mm prop IMHO.

I'd go with a 600kv motor, 650 tops like these:
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=8484

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=18167

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=8506

[Z750Jay]

I was thinking of putting this bad boy in
http://www.hobbyking.com/hobbyking/store/__29486__NTM_Prop_Drive_Series_42_38_750kv_785w_UK_Warehouse_.html

[nick_75au]

http://www.hobbyking.com/hobbyking/store/__6246__hexTronik_DT700_Brushless_Outrunner_700kv.html


A motor like this will drive a 60 mm prop at 6 volts,

[Z750Jay]

Thanks for the advice. Eventually went with some 650Kv ones.
Lucky for the local boating lake the Clarkson inspired springer thing is on hold as I managed to pick up a MMM tug hull, props, shafts and steerable korts fairly cheap in my local store so the motors I got are going in her.
Mainly because the wife has told me to pick one boat project to complete and put the rest in the loft( spare room currently has the springer thing bits box, the Seaport naff tug to WW2 harbour tug box, the Revell corvette hull, my Dragonforce and several rigs plus the new tug)
I can get away with the DF but the rest are burning through my brownie points !