A good sports boat is a quiet boat - so fit an effective silencer. The aero type silencer is not good enough! A second silencer needs to be fitted or the aero silencer swapped for a manifold and a long straight silencer. I've found 'P&R' or 'Peace Plus Power' and SoMoSo silencers to be very effective (are they still around?). Steel, copper or brass manifold are much stronger than aluminium ones and you can make your own without too much difficulty. Silencers and manifolds should be joined together with large bore silicon tubing and the manifold to engine joint sealed with a small dab of clear silicon sealant or caulk. (Silicon sealant is sold in DIY shops as bath edge sealant). Clean all surfaces thoroughly with wet-&-dry sandpaper then a solvent before using the silicon. Be careful not to over tighten the manifold screws as it is very difficult to remove a snapped bolt or repair a striped thread. Silencers need to be drained of "gunk" from time to time by standing the boat on its transom overnight with a small container under the exhaust pipe outlet.
A tuned pipe can be used instead of a silencer, and it can significantly improve an engines performance. Tuned pipes are funny things but have been covered in a couple of model engine books but the best information I've read on them dates back to the January 79 edition of MODEL BOATS written by John Goodyear, (below are a few links to other web sites on tuned pipes)
There are a few different
types of tuned pipes silenced, un-silenced, water cooled, mini
pipes, etc. Some pipes are quieter than others and the
MPBA have a
strict noise limit for any boat of 80db, so make sure you're boat
within the law. Tuned pipes will give a power boost over a range of
revs depending on the size and shape of the pipe, each pipe fits a
certain size engine. A pipe with a sharp centre section give a large
boost over a small range of revs compared to a pipe which bulges
providing a slightly lower boost but over a wider range of revs.
Pipe 1 - "Peaky"
Pipe 2 - Wider power band
The important factor to setting up a tuned pipe is the length from the widest centre section of the pipe to the centre of the engine. There is a long winded scientific calculation that can be made to work out the correct length but most boaters use a second method. Start with the pipe and manifold at it's longest length and cut bits off until a noticeable increase in speed can be seen and the pipe comes "on song". 'On song' means "it sounds right!" It's a little hard to explain but an experienced boater will tell you what to listen for and what the changing pitch reveals. The sound of a tuned pipe 'on song' reminds me of the exhaust "pinging" of those Lambretta scooters that used to be around in the sixties.... if that's any help!! A pipe "on song" does sound good though.....
If possible make the pipe to manifold joint telescopic as in the drawing above. The pipe can be adjusted a couple of millimetres at a time, in and out and this will prevent burnt out silicon tube and cutting the tuned pipe too short.
A good test to check your tuned pipe is to run the boat flat out in a straight line then turn it sharply at 90o to the right and see if the boat looses speed. If "on song" it should lose very little speed and pick up very quickly. Adjust the needle setting, pipe and prop combination until sharp turns can be taken with minimal loss of speed.
I personally don't like to see tuned pipes sticking
out of sports boats as they are not very "scale". If you boat is quite
large then a pipe may be hidden inside the hull, some boaters accept
it sticking out the transom just a little. The Americans have a
number of specialist pipes for outboard engines that are made into a 'U'
shape which can wrap around the engine so making it easier to hide below
decks. The Europeans have a number of short tuned pipes for 3.5cc cars
which should fit in all but the smallest boats. Also worth
investigating are "Magic Mufflers" from Australia and the British Irvine
mini power pipes. The Irvine
power pipes need no setting up at all as they come in one piece,
complete with manifold. Great care needs to be taken when mounting
these pipes as resonant vibrations tend to fracture the manifold
section if the pipe if it is not secured down in at least two
places. Some boat modellers pre-empt this by cutting the manifold in
two and joining the two with silicon tubing, this seems to work quite
happily. The Irvine pipe works extremely well and I highly recommend
them for use on any side exhaust glow engine but do use a secondary
silencer as well as the exhaust note is quite piercing.
Full length tuned pipe as fitted to my heavily modified Robbe Unlimited
fitting ( old fashioned ) of a full length tuned pipe ( with internal
silencer) on my first 'multi boat', a SHG Laser.
Mini tuned pip + after silencer fitted in a "over modified" ABC CESA 1882
For more info on Tuned pipe, have a look at...
Glow engine fuel is a mixture of methanol and oil. As the engine has no separate lubrication, the oil is mixed in with the fuel and lubricates the moving parts as it circulates around the engine with the fuel. Unfortunately most of the oil comes out of the exhaust pipe as a smoky gunge which seems to get everywhere. The oil in the fuel is either castor or synthetic based. Some people swear by caster fuels and others swear at it and use synthetics oil based fuel, e.g., Dynaglow. Castor is said to protect the engine better than synthetics but is more oily. Synthetic fuel although more expensive puts less drag on the engine so it should allow it produce higher speeds. Synthetics seem to be gaining in popularity as it's an easy way of getting more speed out of your boat without doing any work to it. Racers favour synthetics but I'm not into racing so I usually stick to castor oil based fuels e.g., GN.
Most boaters use fuel with nitro methane added. Nitro methane when burnt produces oxygen thus causing a better burn of the fuel in the engine. The better the fuel burns the more power it develops and the faster your engine will run. Over here in Britain we tend to use small amounts of 'nitro' in our fuel say 5 or 10% but our American friends regularly use 40, 50 and 60% when racing. I like 5% as it gives just that extra kick but still as safe as straight. For a new engine always use a "straight" fuel i.e. a fuel that contains no nitro methane, it will put less stress on the engine. Always filter the fuel before putting it in your boat.
info on carbs & setting up engines...
A steady flow of fuel to the engine is a basic
requirement to all IC boats. 'Klunk' type fuel tanks are ideally
suited to sports boats. The klunk weight on the end of the
take-up tube follows the fuel as it sloshes from side to side. All model
shops sell klunk type tanks, try the
SLEC square tanks as they don't
require the manipulation of small brass pipes into strange shapes.
The fuel flow in some boats needs a little help so the fuel supply can be slightly pressurized by using a pick-up from the exhaust system. With a pressurized set-up the flow to the carb can remain relatively constant regardless of the attitude of the fuel tank or fuel level. The needle valve will need screwing in a little more for pressurized systems to prevent the engine flooding. Use thick silicon tubing for the fuel pipe as it is less likely to kink and give problems. Constantly check your fuel pipe for wear, damage & hole - it can save hours of time when trying to fault-find a problematic engine!!!
Well all of this is just my opinion, but what do I know!