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Author Topic: MFA Piranha  (Read 10399 times)

cuppa

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MFA Piranha
« on: December 10, 2011, 12:02:32 PM »

Have just been told my wife has bought me an MFA Piranha kit for Christmas. So it will have to go the top of the 'build' list!

I was wondering if anyone could recommend as reasonable power setup for it?

My young grandson will be using it so it needs a reasonable turn of speed but does not need to be 'ballistic'. I would rather have longer runs at a speed that is entertaining rather than shorter 'fast as possible' runs.

Budget for the motor/esc is about 60 and I don't mind either brushed or brushless although i know very little about power systems as applied to model boats but have some experience of powered model gliders.

Thanks for any suggestions.

Cheers
          Jon
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gregk9

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Re: MFA Piranha
« Reply #1 on: December 10, 2011, 01:56:36 PM »

How about a 555 or 600 motor, 35mm prop and a 20amp esc. that should stand the test of time, as at least if he outgrows the motor, the esc wil take a further upgrade of both motor and prop.
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Nige52

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Re: MFA Piranha
« Reply #2 on: December 10, 2011, 02:03:09 PM »

We have half a dozen in our club, all with different set-ups. We have basically agreed that a Graupner speed 600 Race with water cooling is the best all rounder. The hull isn't really design for brushless as it can't handle the much faster speed (I will now get shot down in flames  {-)) We have tried brushless and although they're quicker, they can't handle the twists and turns so well as the speed 600. But of course, it's your decision in the end  :-))











Hope this helps?
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Subculture

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Re: MFA Piranha
« Reply #3 on: December 10, 2011, 02:59:12 PM »

Is the poorer handling on brushless power owing to a speed control issues?

Nige52

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Re: MFA Piranha
« Reply #4 on: December 10, 2011, 03:08:58 PM »

I don't think so, but we don't really know, I suppose with trim tabs and turn fins it might improve them, but built as per instructions to a standard layout for racing purposes, brushless has no advantage, not for us anyway...
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Subculture

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Re: MFA Piranha
« Reply #5 on: December 10, 2011, 03:15:34 PM »

For a sports/fun run boat they should give more running time assuming the brushless motor is equivalent in power to the brushed version, with virtually zero maintenance.

Nige52

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Re: MFA Piranha
« Reply #6 on: December 10, 2011, 03:22:41 PM »

We need instant reverse though...most brushless esc's don't have this, and if they do it's a double de clutch to get it. Like I said before, we can only offer advice from actual experience of these boats and their performance. And as the OP doesn't want all out speed, the speed 600 race is as good as any, have you got a brushless in your Piranha, what does it go like for you?
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cuppa

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Re: MFA Piranha
« Reply #7 on: December 10, 2011, 03:24:15 PM »

Thanks for the advice

I will go for a speed 600 - I have a Jeti 30 amp esc that should do nicely if I add water-cooling.

It should be fun for the lad.
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Nige52

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Re: MFA Piranha
« Reply #8 on: December 10, 2011, 03:39:26 PM »

Yes, if you pay a little extra for the Race version, they are a bit quicker than the standard 600. We use 7.4v Lipos in them with a low voltage detector off ebay for a couple of quid. The 5000mah 2 cell lipo's last quite a time, but you can use standard 6 or 7 cell packs. A 35mm sub surface prop seems to work fine although some of the lads use a 40mm. Be careful of the X props though, they can burn out a smallish esc pretty quickly, most of us use 25a Mtroniks or better.
Hope this helps
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Nige52

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Re: MFA Piranha
« Reply #9 on: December 10, 2011, 03:44:24 PM »

And make sure you fix the lid on well, also a watertight lid will stop it sinking when you overturn it.... ok2
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cuppa

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Re: MFA Piranha
« Reply #10 on: December 10, 2011, 04:32:35 PM »

Thanks for all the advice.

It is much appreciated.

Cheers
          Jon
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Tug-Kenny

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Re: MFA Piranha
« Reply #11 on: December 10, 2011, 06:16:45 PM »


He will love that Jon.  You'll be a star in his eyes.  :}


ken
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cuppa

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Re: MFA Piranha
« Reply #12 on: December 10, 2011, 09:41:58 PM »

He will love that Jon.  You'll be a star in his eyes.  :}


ken


Thanks Ken

He sure will love it.

Cheers
          Jon
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cuppa

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Re: MFA Piranha
« Reply #13 on: December 11, 2011, 10:08:42 AM »

I have now ordered a 7.4 volt 5000mah lipo pack, a low voltage alarm, speed 600 race motor, cooling coil and a motor mount.

Gosh - Lipo's have sure come down in price since my days with model gliders! I shudder to think what a 5000mah 2 cell pack would have cost a few years ago........

Must admit I am looking forward to building this and watching my grandson operating it!

It is wonderful that he is interested in making and operating models rather than spending all his spare time with his games machines. Probably I am old fashioned but I reckon it is far better that he has a hobby that requires imagination, teaches him practical skills and shows him the rewards of working at something........
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mrbiz

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Re: MFA Piranha
« Reply #14 on: December 15, 2011, 06:06:35 AM »

Be carefull the brushless set up does not suffer a dunking in the water!! You may get away with the standard ,nicad speed 600 and switch set up. mr biz
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cuppa

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Re: MFA Piranha
« Reply #15 on: December 15, 2011, 08:22:13 AM »

I am doing the build with a mind to the kind advice I have been given here:

The piranha will have a taped-down watertight hatch at deck level so if he flips it over all should be well. So far all i have done is clean the hull to remove any release agent left from the moulding process, done a hull leak test (all was well) and opened up the slot for the propshaft.

Have assembled the aluminium and fibreglass motor mount and am currently working on a water cooling system for the 30 amp esc (In the end I am using a Turnigy 30 amp aircraft esc but it will be water cooled to keep the heat levels down. I don't want reverse on the boat at least at first because at the moment the lad has a habit of running rc boats flat out in reverse and swamping them!

I am also looking at the rudder post because I don't much like the plastic one in the kit (needs a large hole in the hull and I reckon we would be better off with a simple brass tube rudder post with a reinforcing plate in the hull.

When the propshaft and rudder are fitted (hopefully by the weekend) I want to make a fibreglass servo mounting plate. It takes me a long time to build things because i cannot use power tools for medical reasons and so everything has to be done by hand. Not a handicap though because I get there in the end - just takes a while.     
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mrbiz

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Re: MFA Piranha
« Reply #16 on: December 15, 2011, 01:02:16 PM »

I had a airboat with a brushless set up and it turned upside down. I lost the lipo, the receiver,and just managed to get the motor working. A word of warning do not short the lipo as it will explode!! and the modern receiver goes on the blink with all those chips in it. mrbiz
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cuppa

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Re: MFA Piranha
« Reply #17 on: December 26, 2011, 09:34:08 AM »

Well into the build now. The motor mount, propshaft and rudder are fitted. Have made a mounting plate for the battery and a mounting for the servo.

Will post some pics later.
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cuppa

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Re: MFA Piranha
« Reply #18 on: December 26, 2011, 09:43:33 AM »

Here is a picture of the internal layout:

The motor mount is a really nice bit of kit. Very well made and made the motor installation really easy. Battery mounting tray is 1/8 ply coated with epoxy. Servo mount is also 1/8 ply epoxy coated.
Next jobs are to open out the hatch in the deck, add the motor wires, fit motor supression and then attach the deck to the hull. So far this has been a very enjoyable and easy build.
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pompebled

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Re: MFA Piranha
« Reply #19 on: December 28, 2011, 08:51:52 PM »

Hi Jon,

Nice work so far!

I can't see in the picture if you've used just resin to attach everything in the hull, or if some glasscloth is added to reinforce the joints.
If it's just resin, I strongly advice to add glasscloth, rough up the surfaces and degrease well before gluing.

I've been in the same boat a number of times and I've seen motormounts break loose due to the stress of running the boat in rough circumstances, followed by a sinking boat, as the motor coming loose tore the stuffing tube free from the hull...

Regards, Jan.
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cuppa

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Re: MFA Piranha
« Reply #20 on: December 28, 2011, 10:01:02 PM »

Hi Jan

Thanks for the advice.

All the parts are fitted to the hull with glass cloth and resin. The hull was cleaned and the mounting areas were roughed up prior to glassing in the components.

Cheers
          Jon
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cuppa

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Re: MFA Piranha
« Reply #21 on: December 31, 2011, 10:33:45 AM »

Well - the deck is now fitted to the hull and I am trying to figure out how to do a neat job of fitting the plastic rubbing/buffer strip around the hull to deck joint. It seems to need to be pulled really tight in order not to 'crease' up on the corners. problem is that the instructions suggest gluing it on with silicon but the bond would not be very strong and would allow the strip to creep over time if fitted under tension. The alternative seem to be  to fit the strip under low tension and slit it on the corners so that it will lay flat or to use a stronger glue such as epoxy. Ah well - I will ponder this one a while............ Any suggestions/advice will be most gratefully received...

In the mean time the driver figure is being painted and a transparent hatch is being made for the deck.

Cheers
          Jon
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Nige52

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Re: MFA Piranha
« Reply #22 on: December 31, 2011, 10:55:08 AM »

Jon,
Boil a kettle of water, put the rubber into a bowl and pour the boiling water over it. Let it soften up for a few minutes then remove it from the bowl. Dry it off with a towel and you'll find that it will stretch like a gud 'un. Where it raises up on the bends, apply some super glue and a clamp to hold it down, DON'T cut the rubber to size till it's ready to be joined with super glue  :-))

HERE IS MY ADVICE FROM MY WEBSITE:

Quote
And finally, a tip!

The rubber fender is a hard plastic rubber that doesn't bend well. So boil a kettle, fill a bowl with scalding water then put the fender in for a few minutes.
Get a cloth and a tube of super glue and a few small modelling clamps together while it softens, oh, and a craft knife.

Remove the rubber from the water and using a tea towel or similar pull the rubber through it, drying inside and out, stretching it at the same time. The rubber goes floppy enough to bend it round the 3 corners, without having to cut it. Apply super glue and a clamp to keep it flat and it wont raise up.

Starting at the bow, bend the rubber in half, this gives you the spot where you must glue it, so you have equal lengths going down both sides and joining in the middle of the stern. I found that by doing this, I had about 2" of rubber that needed cutting off, 1" either end. If you try to put the fender on without softening and stretching it, you'll find it's barely enough to go round the boat......I super-glued it as I pulled and stretched it around, making sure it WOULD go all the way round as I did it.
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cuppa

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Re: MFA Piranha
« Reply #23 on: December 31, 2011, 11:57:58 AM »

Many thanks Nige.

That would not have occurred to me in a million years!

Your advice is greatly appreciated.

Cheers
          Jon
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Nige52

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Re: MFA Piranha
« Reply #24 on: December 31, 2011, 12:53:58 PM »

Before you apply any glue, do a test to make sure you stretch the rubber long enough to go round the whole boat. A member in our club realised too late as he had glued it without checking and ended up with a gap at the back of the boat.. :-)
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