PIRANHA   Mk I - MFA/Cosmo drills

 

Source:              

Kit:                     

Accessories:    
Notes:            

Most model shops & mail order.
35 
40 (motor, speed control & batteries) + radio.
Superseded by Mk II

 


The MFA Piranha 2  is a compact high performance powerboat.  570mm long, with a 230mm beam, the Piranha 2 is designed to take 2-channel radio control.  two versions of the kit exist; one for electric power, using MFA's own "Rocket 500" powerpack; and one for I/C power, using a 12 size (2.1cc) 2-stroke engine. 

Both versions are tough fibreglass - not the plastic that some other smaller powerboats are made of.   Complete kit - requires only engine or motor, plus 2-channel radio.

STYLING
The MFA PIRANHA is a small fast electric boat.  It's not based on any real  boat and has a sort of sixties style look to it.  The Piranha is a strong,  uncomplicated  and  above all one of the cheapest model boat kits  around.  It's  has been many modellers  first  boat as  well  as mine.  The Piranha  can  be  built  fairly quickly and works perfectly well without  any  real  modifications.    

 

The  boat  is  about 570  x  230mm in fibreglass. Fibreglass   is    usually  known as  GRP which stands for Glass  Reinforced Plastic. The top bit (white) is  made  of  styrene, a thin plastic similar to  the stuff margarine tubs are made from. The Piranha's hull is a Vee at the  front merging to  flat  at the transom.   The boat is designed for the MFA  Hummingbird '15 or 20'  which are  540-550 sized electric  motors, 8 cells  (9.6v) and 2 channel radio.  

 

The  instruction  leaflet  seem  to assume you   already  have  some  boat  modelling  experience but are good enough to get the beginner and the boat  to  the water without too much trouble.  The included plan is full size so  measurements can be transferred directly to the boat, less room for error.  The  kit includes all the ply for the internal woodwork,  a good 4BA steel  and  brass propshaft,  a thin brass rudder,  universal coupling and a  few  other  bits & pieces.  The coupling in my kit was ridiculously small so  I  advise  you to replace it before you even start building.   Buy one of the  interchangeable Huco  or  Ripmax  couplings available in most model shops.

  

 

CUTTING & GLUING    
If  you don't know anything about working GRP,  (and I know even less than  you  do),   endeavour not to get the glass fibre in your skin or   you'll   be  scratching yourself for a week.  The Piranha instructions recommend using Stabilit  Express - very good glue but it's not cheap! Other  possibilities  for gluing include two-part epoxy  glues  (Araldite or Devcon etc ), car body filler paste (Isopon, Plastic Padding  etc)  and David's 'FASTGLAS'  paste used for repairing holes in car bodywork.  The  latter  has  coarse glass fibre strands already mixed in and is  good  for  bridging  gaps and strengthening weak areas.  A similar  mix  'Easibuild',  has  much  finer  fibres in it leaves a much smoother finish and  is  much  easier to work with,  it's available from Prestwich Models.   For the most  part I used 'FASTGLAS'  GRP resin bought from a car accessory shop  because  it's  the cheapest and can be used in inaccessible  places.

 

Note from wife  - GRP  resin  which can  get really messy,   so  don't wear your Sunday best  while working with it.     

 

Before starting on the boat,  test glue some off cuts of GRP so you  get a  feel  of gluing process.  If you use less than the recommended proportions  of  hardener in the resin mix,  the resin takes  longer  to set and  gives  you more working time. If you put more hardener  than  recommended it sets  faster  but  progressively  produces  a more  brittle  joint.  Too  little  hardener  and the resin will not set uniformly,  too much and the mix will get   hot  enough to generate smoke. Be careful but do experiment.    

 

The  shiny surface of the GRP is called a GEL coat.  It is very brittle so  saw  GRP using a coping or fret saw and cut it slowly from the shiny  side  in,  file or sand to final size. If you're lazy like  me, you could  use a  modellers drill with a 1.5 or 2mm HSS drill and use it as  a  sort of high  speed milling cutter.

Be careful, don't get too near  your marked  cutting  line  because it can cut very quickly - in the wrong    direction.  Let  the  drill or saw cut at it's own speed as forcing it will break  the  drill  bit or snap the blade.  Wear safety glasses and a dust  mask as GRP  dust is not good for you! Work in a well ventilated area.  My wife's best  silk scarf made a good dust mask,  (until I got caught) I wore it like  an  outlaw!    

 

THE ELECTRIC STUFF    

I had read that 'electronics don't like to get wet',    (try using your HiFi at the bottom of the sea) but on the boat's instructions doesn't make it very waterproof. I bought an airtight  Tupperware type  box   (about 90 x 130mm) for  use as a watertight radio box. The  problem   with  these  boxes  is that no  glue  in the  world  seems  to  stick   to  them  apart from Evostick (and that get quite unsightly). If you know of better glue, I would be interested  to  know.   Due  to  the boat's internal layout the rudder post comes right  up into the middle of the radio box!  With a lot of  jiggling  around  I  got  the  rudder post,  radio gear  and  the  mechanical  speed  controller  all inside the radio box!  I couldn't find room for the  radio  batteries so they got stuck on  the lid using 'Velcro' and Evostick, but  this  turned out to be a bad idea as you will see later.  The deck opening  had  to  be  opened out to accommodate the box and top hatch also  had  to  widened  with  Plasticard  so it covers the radio box down to deck  level.

 

As I couldn't get anything to stick to the plastic box I made up  an  internal platform from a sheet  of  3mm  marine   ply  or Perspex. Holes for the servos, speed controller and rudder post are cut into the ply  and the whole lot sits on a couple of supports in the  corners. . The platform is   The  whole lot then  just sits loose on the bottom of the box,  it   can't   move around because your plate  just touches each side of the  box.

 

I used  a  Tamiya mechanical speed controller activated by a normal servo  (see  BOAT ELECTRICS AND A LITTLE BIT MORE. ). This is a though unit  and  has withstood  several 'burn-ups'  due to  weeded-up propellers  etc.  Resistive controllers can get  very hot so leave a  clearance  gap  around the coil so that it doesn't melt  anything  nearby. Lubricate  the  controller  with  silicon grease to  inhibit  arcing. The controller  was fixed to the servo  side with double sided tape. The action of servo this tends to push the controller off after a while so use a plastic cable tie to hold the two  together.  The  cables to the battery connector and motor were run out of the front of the  box  through  a  rubber  grommet and sealed with Evostick and  a  blob  of  silicon sealant.    

 

 

BUILDING SITE    

Because  I was using a proper Huco type coupling The propshaft needed  shortening. Push the bearing out and refit it to  the  now  shorter stuffing tube.  Remember to keep the same overall length from  the  motor to propeller. 

 

  

After cutting everything else out or to size the  first  gluing  job is the most important,  the alignment of  motor,  motor  bulkhead,  coupling and propshaft.  You'll find that I'll keep emphasizing  the  importance of motor alignment as a lot of beginners boats suffer from  bad  alignment  and they wonder why the boat are slow, the  batteries  don't  last and they are very noisy!  POOR ALIGNMENT ROBS BOATS OF SPEED. 

 

Cut  a  small  pilot  hole  for  the  propshaft and open it  out  wide  enough  to  accommodate  the  stuffing tube constantly checking it's actually  in  the  right place in relation to the motor and rudder. Use a straight centre section  for  the  Huco  coupling  while building which makes  alignment   much  easier,  replace  it with the universal joint when the boat is complete. Dry fit the bits together  and  turn  the propshaft over with you fingers  and  the  only  resistance  you should feel is the motor.    Don't oil  the  shaft, if any oil gets on the GRP the resin will  not  stick.  Hold everything in place with sticky tape or clamps when done. 

 

I found a way to check motor alignment is to run a  low  voltage,    say   2  volts,  into the motor.  Measuring  the current drawn by the  motor with a  meter,  best  alignment corresponds  to  the  lowest   amp reading.  The  motor  only  needs  to  turn over slowly so  don't  test your main cell packs on it! The movement will also help with cylindrical  alignment too.

 

By the way, the motor, shaft and propeller  needs to spin anti-clockwise looking from the back.  If your motor runs the wrong way when you push the stick  forward  then  flick the servo reverser on the transmitter or reverse  the  wires  to  the  motor.

 

Once  truly  happy with the alignment you can pour resin over the motor bulkhead and shaft exit  to fix them in place. The advantage of using resin here is  nothing needs to be touched  or moved.  The shaft may need to be sealed to  the outside of the hull with Bluetack or Sellotape to  prevent seepage.      

 

Once the resin has set the ply reinforcing wedge can be glued in under the  inboard  end  of the shaft.  I have since found out that this is a  little  unconventional and most boats, both real and model have a shaft support or  skeg   outboard  near  the  propeller  which  helps  damp  down  propeller  vibrations and aid directional control. Holes  for the motor and battery wires are drilled into the radio box  and  both  bulkheads.  All the wiring needs to be completed before the deck  is  fitted because it's impossible afterwards. The  hatch  spring clips  are  also  fitted to the deck while it's still loose.  When   fitting  the  clips,  use washers on the bolts to strengthen the deck  else  they  could  pull right through if you're not to careful. You won't want your new  boat to sink so  cut some  polystyrene to fit in all the unused available gaps and the lid. If the boat now  turns over and fills with water, it won't sink.

 

Next job,  fitting the deck.  Sand down the deck until it's a near perfect  fit  to  the hull.  Seal the joint on the outside with Sellotape or Bluetack  to  prevent  seepage.  Mix up  some  resin  in   small amounts  and  run it in  through   the   three  deck  openings  and  then  move the boat around  so  that  the  resin  flows   around the  joint on the inside and  on  to  the  bulkheads.    Do  this  a couple of times until the joint  is   completely   bonded and you're happy with the seal.  I added a little more resin to the  nose  to strengthen it in case of a head-on crash.   You can  check  the   joint   with   one    of  those  angled  inspection   mirrors  like   your   dentist    uses  before  he starts  to inflict pain!    Stand the boat  upside down  and  leave  the resin to  set  overnight and  sand the joint to perfect finish.    

 

The  radio  box  and radio etc.  can now be fitted.   Install all  the  radio gear into the box and connect up all the link and  wires and oil the shafts.  Test  everything  works correctly and smoothly,  the more time you take  getting  this  right  now will mean more time on the water.    If the  rudder  movement is reversed  then  again  likewise  flick  the servo reverser on the Tx.  If you transmitter  has  no  servo  reverser switch ask someone to fit one (a twenty minute job & about  1.50 for the switch,  see  BOAT ELECTRICS AND A LITTLE  BIT MORE!  or Radio Controlled Model Boats Magazine May/June 1990).    

 

FINISHED?   

Lastly  the   hatch   needs  to be strengthened to take the force  of  you  pulling  on the  dowel.   Cut  out  a scrap of ply and make a 6mm hole  to  take  the  dowel,   line everything up,  superstructure, clips,  dowel and  supports,  mark  it  up  and glue the dowel and supports to the  top  with  Superglue  (fig c).  Now you  can  fit  the rubbing strip using Superglue,  it also hides the dodgy joint you may have made! I  didn't  like  an empty  cockpit  so covered it with a windscreen made from a scrap of   theatrical   lighting  gel  and stuck it on with Superglue.   A good paint job and  any   other   additions  you  can make to  the  appearance  can   only   be   an   improvement to it in my personal opinion.  Total construction time took me  about  40-60  hours,  sticking GRP together with resin is a job  best  not  rushed.    

 

So to the Water     

I filled the bathroom test tank and put in my new pride and joy.... IT DIDN'T SINK! This is  a  good sign.  It floated with the water about halfway up the sides with a  slight nose up attitude.  I checked the radio and motor, everything seemed  to be in order. So after a short run I wiped up all the water splashed everywhere and waited for the weekend.     

 

The lady  wife  and  all  7ft of my good friend Jason came down  the  lake  one   evening in summer '88 for the launching ceremony.   I understand the usual  pre-launch   count-down   procedure  is  to put one hand  on  your   empty   wallet,   salute  the  sunset and say "I name this model ......   May  she  never   sink,   well  not  today  anyway!"  

 

I don't know  what  I  really  expected,  maybe  it would scream across the lake at a phenomenal speed or  would   it  just  move  slowly around in pathetic  circles.   Anyway  I  installed  all the batteries,  tested the radio again and put the boat  on  the water and pushed the throttle stick gingerly  forwards.  IT  WORKS!  The  boat moved off at a fair pace and was off ploughing through the water.  It  didn't scream across the water but I wasn't disappointed it worked.  Speed  was  good,   well I though so but I didn't have  another  boat  to compare  it  against.   I  was  disappointed the batteries only lasted  what seemed a few minutes. Shame! I'll have to do something about that.    

 

It's  a  very  stable runner  and  can  turn very tight  corners   without   complaint.    At  full  speed,   full rudder can be applied and I couldn't  turn  it over but it's not recommended for mental stress reasons.  At slow  speed it  can  turn  in about two boat lengths but I did give the rudder a  lot  of movement.   Trying to run astern pulls the stern under and  floods  the deck.      

 

PROBLEMS    

The  first  thing to go wrong was the coupling supplied with the  kit.  It  exploded when the boat came to close to shore and the prop run aground.  A  Huco / Ripmax coupling was then installed ( see above ). The replacement coupling  has withstood some serious misuse without any sign  of distress.  The brass rudder blade did come loose a couple of times when   the boat was in shallow water but this was easily re-soldered.    

 

A  second problem soon showed up.  After a short run the main cells  would  loose  power  for  no  apparent reason,  even  while on the bank  and  the  radio  switched  off.    This turned  out   to  be a combination of a  few   things,  

 

1. The  mechanical speed  controller  is  not a  piece   of    precision  engineering   and   the  'backlash'    is  quite  noticeable on the  wiper  arm. 

 

2.  The wiper arm by design has  to  rub  on the coil resister with  quite  some force to ensure  good  contact.  

 

3. Servos don't like  metal   to  metal touching  as  this  causes radio  interference thus  making  the   servos  twitch.  

 

The upshot of all of this  is  that  the  servo  doesn't  move   the  wiper   arm to  dead  centre  of  the  resister   every   time  (which  cuts  the  circuit) and thus part of the resister remains  in  the   circuit and causes  the  cells  to  discharge.   I fitted a power switch between  the cells and controller - not knowing any better at the time.    On  my  model,  I    combined the power  and radio cells  together in one  2  pole- 2 way switch.    If you  don't know about switches ask  someone in  your  local boat  club to help you.    

 

Batteries

The Piranha is   quite fast on 8 cells but runs much better on 10 and gets upon  the  plain  in  a  couple  of  boat lengths.  In order to save money on the  main  batteries  I  opted  to  buy  eight  individual  rechargeable  cells  ( D cell torch type) and connect them a via spring type battery  box, but  this was given up as a bad job  after  the a particularly unsuccessful voyage.  I hit a turn buoy and  the  cells  were  not knocked loose from the holder which left me with  a  dead  boat  with  not  the  slightest breeze on the water.  I   tried   throwing  stones  just  pass the boat in the hope  that the ripples  would  persuade  the boat back in.  This is not  a  good idea.   Yes, the  boat  took  a  direct  hit from a very hard flying bomb.  My wife asked me 'why was  I  trying  to sink my new boat?'  After several hours  it drifted in on  it's  own  and I was pleased to see the only  damage done was to crack  the  gel  coat surface.  I decided to make up cell packs  and discard the battery box idea.    

 

I  got  hold of some cheap second-hand NiCad cells  from  a   model   shop, most of which were good.  These  cells I  made up  into sets of five by soldering them together with brass strip, two pick-up  wires and covering them with heat shrink plastic.  Two packs are used at a  time  which  takes  the total voltage up to 12v.  I  then   fitted   Hi-fi    speaker    wire  connectors  available  from electronic component  outlets   like   Tandy,  Maplin etc. These are used to quickly connect up the  home  made   battery  packs  which  I  made  less  the  standard  model  battery  connectors. This left me with fully charged batteries with bare wires!!!! - This is not a good idea!!! Spend the extra money and get good batteries and use proper connectors.

Sticking the radio batteries ( also individual batteries in a spring type holder ) on top of the radio box was also a bad  idea  as it kept getting in the way and the holder split twice and had  to  be  replaced.  Soon the batteries became corroded and also had to be replaced.  It's better to get them in the radio box if at all possible and use a proper soldered receiver pack.

 

Once  I was happy with the mechanics and running of the boat I started on the  quest for more speed.  Changes of propeller increases speed but the larger  the prop the shorter the running time,  35X to 40X is the best range.  The  original  motor  was a bog standard 550,  I then tried a racing  type  540  motor,  a  SPRINT 12 but  the motor didn't seem to have enough torque  and  performance.   After  a  few  runs  the SPRINT was removed  and  the   550  refitted.  A  more  powerful  motor and/or batteries would take  the  hull  nearer  it's  full potential.  I  never  got  the  boat  anywhere  near  instability so I imagine it could have taken quite a bit more power than I  ever used. But as this was my first boat I didn't know how to get it to go  faster  with putting more cells in or a bigger motor, (more of this problem in another page).

 

RESCUE ONE    

This   boat has proved very useful in later days as tug for my dead IC boats.   The idea  came  to  me when my next boat the SHADOW,  stalled on a very cold winters  day  in  the  middle of the lake and it wouldn't drift in.   I  fitted   a   bollard   (a  plastic  screw)  in the deck at the transom, attached a line   of about  3    or  4  meters  and a small float at the end.  The line is trailed out  behind  the boat and the boat made  to   circle  a  dead boat on the water.   Once  the     line   snags  on  the  propeller or rudder,   I circle the boat  a  couple  of times to make sure  it's  secure and then  tow the dead boat in  backwards.   It works a   treat.   It's  pulled in dozens of dead boats on  the  Osier Lake  in  Peterborough,  some of them quite large.  Only  thing  though  is be careful  not to snag your own  boats  propeller on the  line  by going backwards or you'll have  two  dead  boats on the water.   That's  when it usually starts to rain... 

 

 

CONCLUSION    

The PIRANHA is a  good  boat  to learn about model boat building,  maybe a  little too much, but not an impossible kit for the first time modeller. If  you  want  to learn all about GRP electric boats then this is the kit  for  you  but starting in ABS is easier and quicker.   As it's style gives away  it's  age but you  could always build  your  own.  One last thing  If  building  the  Piranha,  build  it as light as possible, I've seen one built better than mine and it  was and it ran very well.    

EPILOGUE    

The  Piranha has earned an honorary retirement now and was replaced by  a semi-scale  tug the ORTON built from a ONTARIO kit (to be covered in another page).   I will always have fond affection for this boat as I learnt  so   much  about the many different aspects of boat modelling.  I've  had hours  of  running time with this boat and   I  did succeed in building a working  model boat and it's a great  feeling  to see something you built,  work.   I'm sorry there are no on the water pictures but I didn't know I was going  to do this web site before retiring her - to the bin. Sad end but that's life!   

 

LESSONS LEARNED   

1. Make   sure the first boat you build is an easy build or you could  quickly  become   despondent, disheartened, bored etc.

  

2. Resistive coil speed controllers need a eye kept on them all   the  time as they prone to burning up and draining your cells.  On a  next  boat I'll try a different type of controller.

 

3. Size does matter.. a small one give longer performance, larger ones goes faster but not as long.... I'm talking about the Propeller!!

 

4. Check   the   radio   gear   frequently,   and strip down the gear from time to time to check for water   getting  in.   Make  sure  the  prop  shaft  is  well  lubricated, install  an oiling tube.  

 

5. Build  'getability'. 


FINAL SPECIFICATION
 

550 motor  35X - 40 propeller 

Tamiya resistive speed controller  10 cells (12v) 

Accoms 2 channel radio 

External power + radio switch


MODIFICATION POSSIBLE
 

Higher powered motor 

Gear box 

Surface drive 

Extra cells 

Metal propellers 

IC conversion! 

Appearance!


PERSONAL RATINGS
(out of 10) 

Value ...........................

Kit Quality ...................

Kit Design ..................

Ease of Building .............. 

Finished appearance ......

Handling .................

 

8 (Very reasonable) 
6 (simple, nothing special) 
5 (simple, nothing special) 
6 (Easy apart from working with GRP)
4 (Bland, no decals or anything) 
8 (Can handle high power motors) 


NEXT PROJECT

The  PIRANHA never turned over or sank which is more than I can say for my  next  boat,  a  SHG SHADOW.  I now wanted to build a faster boat  and  one  powered with a  IC engine. The  most  complete IC kit on the market that I  could  find  was the SHG SHADOW.  I bought  one and smuggled it  into  the  house late one after the Mrs had gone to bed.     

 

Well  all  of this is just my opinion,  but what do I know! 
 



Other Piranhas I found on Ebay.co.uk