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   As  with  electric  motors the most important consideration  is  not  the  engine  itself but the alignment of it to the prop shaft.  Good alignment  will mean faster,  smoother and quieter running, bad alignment slows down  the  boat,  shakes things loose and may even damage the engine  bearings.  Alignment  requires a certain amount of skill but with a little planning,  time  and  lots of testing & checking,  anyone can get it  right.   While  working  on the boat it's a good idea to block the  carburettor,  exhaust  port and plug hole to prevent dust and debris entering the engine.

    Engine installation is a chapter on it's own. It's hard to say what mount  is  the best as each boat will have it's own particular requirements  but  the diagrams below give a few ideas. If possible fit a flexible or rubber mount as it  kills most engine vibration and is easier on both hull,  bearings,  shaft  and  the  ears.  An  IC engine installation is vitally important  to  get  right,  not  only because of the required shaft alignment but because  it  also  involves setting the boats centre of gravity and shaft down  angle.  The mount needs to be rigid,  removable and robust. You are going to pull  very  vigorously  on  this thing while starting so make  sure  it's  very  strong.     After  the  boats layout has been mapped out,  the engine,  mounting  and  shaft  should  be the first thing fitted and the rest of the  boat  built  around  it.  Don't  forget to leave room for the starting belt under  the  flywheel during setting out.





   After you are happy with the dry alignment,  wait  till the next day before starting to glue it in.  This gives you  a  chance to think through everything again before committing yourself.  Try  applying  tiny  amounts of glue (or whatever fixer you are using) on  the  critical  points of the mount and shaft to just 'tack' them in place  which  enables  you  to  see if corrections need to be made.  Use lots  of  glue  during  the final fix down of the engine mount to make it really  strong, it will need it!!.  When  you've finished,   coat the engine room with gel coat or a  glazing  resin to make the boat easier to clean.

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COUPLINGS  ( Flexy shaft )

    Rubber  couplings  are the most popular coupling used in sports and  race  boats because they are quiet in operation and allow for the movement of the  flexible mount and engine vibration while starting and running. They come with different screw  on  adapters to allow for different engine and shaft sizes.  Suppliers of  multi  boat  equipment are the best source for rubber couplings and  also  flexible engine mounts.   There are a few other type of couplings such as  flexible  disk and coiled spring but I personally don't like these types,  the  former  in very large and prone to wear and the latter  often  spins  eccentrically.

    Flexible  shafts  must  use rigid couplings to  prevent  whiplash.  These  couplings come in three guises,  (1)  collet chuck,  (2) solid block with  grub screws and (3) solid block with square hole and shaft end. Type 3 is  the easiest type but only TM products of Chatteris seem to supply  them  in the UK but there are several in the USA.  The stuffing tube  for  flexi  shaft should be made to follow a graceful gentle curve to minimize  power losses through friction.


(1)  Collet chuck

(2) Solid block with  grub screws

(3) Solid block with square hole and shaft end

See also...


    With  the engine installed and the coupling and shaft carefully lined up,  fit  a  suitably  sized  propeller  for  you  engine  (check  the  engine  instructions  or an engine book for sizes) and leave a small gap  between  the  propeller  nut and the end of the prop tube to  prevent  binding.  A  clearance  gap between the stuffing box and washer,  more than 1mm is too  much  on  a  solid shaft but flexi shafts require as much as 2mm  as  the  shaft  expands  in diameter under centrifugal force which draws the  ends  in.  Try  to get the outboard end of the prop shaft or strudder,  in  the  case  of a surface drive,  to take the thrust load instead of the engine.    A  Teflon,  hard  plastic washer or stainless steel behind the  prop  nut  makes a good enough thrust bearing for sports boats.  Keep the washers as  small  as  possible to reduce drag and don't forget to oil or grease  the  shaft and tighten the propeller lock nut.

    Metal props are better than plastic ones as they don't change their blade  shape under load.   Metal props apart from aluminium  ones require proper  balancing  to  prevent vibration.  I mainly use plastic propellers  as  a  whole range can be bought for the price of one metal prop. When one of my  plastic prop breaks, I just "Tsk", change it and carry on boating at very  little  extra cost.  Plastic props are good enough for sports boaters but  if  you  want  to  go  racing  the metal props  may  well  be  worth  the  investment. Graupner and Robbe make the best plastic props.

Also have a look at....

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

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