Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[Tug Fanatic]

I keep an eye on a very readable blog by Phil Parker who I think posts here. Phil is an experienced modeller editor who has contributed articles to Model Boats Magazine (I really liked his Bantam Tug) so I await developments with interest.


His latest project is to motorise a Playmobil Noah's Ark and he has fitted it with a MFA 385 motor running on 6v with a 60mm 2 blade Propeller (which I am guessing is a Graupner P) and a 1.3A SLA battery which will surely suffer horrible voltage drop & short duration if asked to deliver anything over 1-1.5A.

This would normally be considered (by me at least) a very outlandish combination and it is compounded by the fact that the model is only 12in long plus being dumpy which also forces a very steep propshaft angle due to the enormous prop.

My mind sees overloaded overheating motors, short battery life, prop walking, directional instability and more.

This breaks so many rules it is fascinating. If this works well I will need to rethink my understanding of models!! 


See the blog for more:
http://philsworkbench.blogspot.com/search/label/Noah%27s%20Ark





 

[Plastic - RIP]

It all looks ok to me apart from the stupid prop - he could get a sensible 30mm one for a couple of pounds which would sort out all the associated problems of shaft angle etc.    Strange thought process going on there.  Maybe he wanted to make a hovercraft?

[Tug Fanatic]

It all looks ok to me apart from the stupid prop - he could get a sensible 30mm one for a couple of pounds which would sort out all the associated problems of shaft angle etc.    Strange thought process going on there.

Agree re the prop but fitting the 60mm example has dictated so much more even if it gives a "cartoon" look.

[Plastic - RIP]

Maybe he's just using the bits he found on his workshop floor?   

[Tug Fanatic]

Hopefully Phil will post & tell us!

[RST]

He sgave his reasons for the prop.  I don't see to much issue with his set-up, or the SLA battery.  Only rule I see him breaking is you're not supposed to do anything these days without asking for exactly how it's "supposed" to be done!  Suppose it's sacreledge it's not brushless and Li-Po for some?


I quite like his website.  Read all his Nelson Pilot 40 build before and while I built my kit.  I drew and 3-D printed a small version of a Bantam tug recently based on his pics also.


I disagree about not being able to purchase short propshafts though.  There's 2-3 places that'll make them any length you want, I must have ordered half a dozen 4in and less in the last 18 months.  And I definately don't advocate "poundland" epoxy.  His choice and Kudos for the blog -it's always a good read.


Rich

[canabus]

Breaking all the rules!!!

49" coaster cruiser, 50mm 5 blade prop, 3S Lipo battery, 50mm brushless motor!!!
Planning displacement hull !!!!
Canabus

[malcolmfrary]

Once you know why the rules are what they are, and understand them, you have a chance of appearing to get round them and getting away with it. 
"The prop should have a smaller diameter and fewer blades than the motor has poles." 
I haven't had access to the blog, but from the comments above - the prop is a larger diameter, lose a point.  The prop has a lot less poles than the motor has poles, get the point back.  The motor is running on about half its rated voltage, gain another point.  No mention of the pitch of the prop, floating point could go either way. 
No reason why it should not work.  Due to the low voltage, the motor is not being asked to work hard, the low blade count probably compensates for the lack of gearing.
The problems arise when you apply max voltage to a motor attached to a prop that will try to move too much water, forcing the motor to try to work harder than it was ever intended to.

[Tug Fanatic]

Once you know why the rules are what they are, and understand them, you have a chance of appearing to get round them and getting away with it. 
"The prop should have a smaller diameter and fewer blades than the motor has poles." 
I haven't had access to the blog, but from the comments above - the prop is a larger diameter, lose a point.  The prop has a lot less poles than the motor has poles, get the point back.  The motor is running on about half its rated voltage, gain another point.  No mention of the pitch of the prop, floating point could go either way. 
No reason why it should not work.  Due to the low voltage, the motor is not being asked to work hard, the low blade count probably compensates for the lack of gearing.
The problems arise when you apply max voltage to a motor attached to a prop that will try to move too much water, forcing the motor to try to work harder than it was ever intended to.

The load factor of a 60mm prop compared to a similar design 35mm prop (which seems to be a normal 385 6v recommendation) is around a multiple of 7 which is an enormous difference. I have never heard of such a large prop being used on a 385 motor even on 6v but I am interested to see what happens.

I am all for everyone thinking outside the box & trying something new. If this didn't happen the box would never change. To me all this will be really interesting when I find out how well it worked! Who knows it might move the box limits.

Canabus - that is a Q boat.

[canabus]

Hi Tug
Is the Q for QUICK!!!

Canabus

[Tug Fanatic]

Hi Tug
Is the Q for QUICK!!!

Canabus

Just a term we use. I think that it derived from cargo boats in WW2 that were given guns etc to help them defend convoys & lure U Boats into traps. We use it more broadly & thus have Q cars, Q boats etc which are examples that do something that would not be expected. In you case yes Q = quick!  . Sorry I thought that it was a more common use.

[tonyH]

The "Q" came up in 1915 after the so called "mystery ships" became so numerous that their Lordships decided on that designation because they kept forgetting the names!

[canabus]

Hi Tug
I did know of Q boats in WW2, but Q cars in Australia we call sleepers !!!
Outside it's like a crappy old car, but under the hood is a beast!!!
Canabus

[Plastic - RIP]

My old Corvette with a 7.2V Speed 400 & 50mm prop....   Useful for avoiding ducks.

[canabus]

Hi Plastic
It all shows the lack of info on motor rpm verses prop maximum rpm.
It's about time some one gave us a ball park idea on this!!!
Cooking up motors and speed controllers is a bit costly!!!

Canabus 

[Plastic - RIP]

No -mostly it chugs along at scale speed on about 1/4 throttle - the battery last ages in that mode.

[Tug Fanatic]

No -mostly it chugs along at scale speed on about 1/4 throttle - the battery last ages in that mode.

Usage often isn't taken into account. Thanks for pointing out that at 1/4 throttle that you are effectively running at 1/4 volts & 1/4 amps which , of course, means that you can drive your big prop & your battery lasts ages. At full throttle I suspect that your motor will heat up horribly - does it?. Being a tug guy I tend to look at what happens after 15mins of full throttle pushing against a pond wall.


I agree with canabus about the lack of data. Endless beginner questions about motors /props/ batteries and the often conflicting advice given highlight the issues.

When I posted the original topic I hadn't thought that running at 1/4 throttle might be the intention & that full throttle load might be not relevant. I would still be interested to know what full throttle amps & volts are. 

[Plastic - RIP]

I've no doubt the motor would get hot if I was thrashing around the pond - but I don't - but it's always nice to have excess capability and not use it rather than not have anything in reserve when it's needed.   


I've never understood this need for everyone to be flat-out all the time.   The stick doesn't need to be at 100% to be having fun.

[malcolmfrary]

The load factor of a 60mm prop compared to a similar design 35mm prop (which seems to be a normal 385 6v recommendation) is around a multiple of 7 which is an enormous difference. I have never heard of such a large prop being used on a 385 motor even on 6v but I am interested to see what happens.

I am all for everyone thinking outside the box & trying something new. If this didn't happen the box would never change. To me all this will be really interesting when I find out how well it worked! Who knows it might move the box limits.

Canabus - that is a Q boat.

It will be interesting.
I think that the idea is that where a 385 would normally be hitched to a 35mm prop with 4 blades, it now has a 60mm with 2 blades and is only being offered half power. For moving a short, dumpy, hull, it might tick all the boxes, but the danger is that someone with limited attention will miss out the important bits that allow the motor to survive, like low voltage and few blades, and be surprised when a 5 blade prop on 12 volts cooks the same motor.

[canabus]

Hi All
Posted my ideas on matching props to brushless and brush motors on the model boats forum site.

Canabus

[Tug Fanatic]

It will be interesting.
I think that the idea is that where a 385 would normally be hitched to a 35mm prop with 4 blades, it now has a 60mm with 2 blades and is only being offered half power. For moving a short, dumpy, hull, it might tick all the boxes, but the danger is that someone with limited attention will miss out the important bits that allow the motor to survive, like low voltage and few blades, and be surprised when a 5 blade prop on 12 volts cooks the same motor.

Until this thread it had not clicked in my head that some modellers intentionally use prop / motor / battery / controller combinations that will not support continued full throttle use even if they didn't intend to use it often.

You learn something every day.

[Shipmate60]

Thinking even more "out of the box" I would fit 2 x small motors and use tank steering.
Even 2 x servo motors using elextronisc fron servo as speed control.


Bob

[phil_parker]

Maybe he's just using the bits he found on his workshop floor?

Sorry, only just spotted this thread (Thanks Pete M for pointing it out). Thanks for all the nice comments.

Anyway, the prop is partly down to using up old stuff. I wanted something red and plastic under the red plastic hull. The cartoony look is also part of the appeal to me. Yes, it's silly, but then this is a silly project.

I've been on holiday for 10 days and then stupidly busy with work but there will be an ark update next week. While the drive system works well, the floating bit didn't, so there has been remedial action carried out, with Poundland epoxy (sorry).

[RST]

Just to say Phil,

I was looking at your website again -it's great for inspiration!  I've parked my little 13" Bantam which I used your pics for and 3-D printed, I think it's just a bit small to keep water out.  But I dug my Speedline pilot boat out again to finish-off as I was looking through your build for inspiration again (I kind of solved your issue with the deckhouse fitting the deck though!).

The ark looks a good little fun project.  It seems to work well enough in your pool also, as long as it floats LoL.


Rich

[phil_parker]

Just to say Phil,

I was looking at your website again -it's great for inspiration!  I've parked my little 13" Bantam which I used your pics for and 3-D printed, I think it's just a bit small to keep water out.  But I dug my Speedline pilot boat out again to finish-off as I was looking through your build for inspiration again (I kind of solved your issue with the deckhouse fitting the deck though!).

The ark looks a good little fun project.  It seems to work well enough in your pool also, as long as it floats LoL.


Rich

Rich - That Bantam looks great. Why do you think it's too small to keep water out? It's about the same size as mine which works fine.