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Lister SR2 Issues

We bought a 45' narrowboat last year fitted with a Lister SR2 air-cooled engine and a 16" prop (pitch unknown). I find it won't go much above 3mph without overheating and over a period of a couple of hours or so the oil pressure gradually drops to 10psi at that speed and looks as if it is continuing to drop slowly. Stopping and manoeuvring are poor. The air intake to the engine compartment is only 290x130mm. The cooling air comes off the manifold, turns a right angle into a rectangular box 130x100mm and 700mm long, then another right angle into two 160mm dia hoses to the outlet which is also 290x130mm. This appears from the manual to be inadequate cooling so would a fan in the duct help and would a new prop be a help? The space between the skeg and underside of the boat is 20".

Asked by: Andrew Strawson  | 5.17pm, Thursday 12 September

WW says:

It is very important for all engines to have a good supply of fresh air, even more so for air cooled engines like the Lister SR series. A restricted air supply can cause overheating and loss of power since the engine will be unable to suck in enough air to burn the fuel properly.
The manual for the Lister SR2 twin cylinder engine states the minimum inlet air duct size at 78 sq inches; this equates to 50,322 sq mm. Your inlet duct comes in at 37,700 sq mm, which is quite a bit smaller and likely to be causing problems.
The air outlet can be smaller since it is only passing the hot air from the engine cooling and not the combustion air, here the manual gives a minimum of 60 sq inches which equates to 38,710 sq mm. The two hoses have an area of 40,212 sq mm which meets the requirement, and the outlet is close; however the box or ducting coming off the manifold will be restricting the flow of air away from the engine. If the duct is over 6 ft long the size should be increased by an additional factor of 1.4.
It looks to me as if you need to put another vent into the engine compartment to supply more fresh air, and look at easing the flow of hot air from the engine.
Overheating can also be caused by the cooling fins being covered with oily dirt. If the engine continues to exhibit signs of overheating after addressing the air flow, these will need cleaning, but it requires removal of the cooling air ducting.
The SR2 engine is rated at 16.3 BHP which should be quite adequate for a boat of this size, once it is working properly.

Rupert Smedley  | 6.54PM, Thursday 12 September

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