B.M.B. Mariner

[bernie]

I have just acquired a genuine 'barn find' aluminium 12' boat which I estimate dates from the immediate post-war era. It still has a 1974 river licence disc which I suspect was the last time it got wet!

It's fitted with what I believe is a B.M.B. Mariner inboard with a direct drive coupled to a shaft and a feathering prop arrangement for selecting forward/reverse. The tiller is a curious, side-mounted arm that folds down inside the boat when not in use and up to the horizontal to provide a 'push for port, pull for stbd' steering arrangement.

After reading some very informative threads on this forum I've found that I need to get the 'Twin Spark' coil rewound but hopefully that is all that needs doing. The engine turns freely with good compression. I'm tempted to strip the water pump and gearbox but leave the rest of the engine intact as I suspect that I may have trouble sourcing any spares required for a full strip and rebuild.

I must admit I'm intrigued by the design of the 'top end' with no separate cylinder head and the side-mounted spark plugs. Was this a common design or unique to this family of engines I wonder?

A couple of pics attached and I look forward to any replies from others who have come across these engines.

Regards,
Bernie

[Ferrier120]

Hi Bernie
The Mariner was introduced in 1933 with a 3:1 reduction gearbox which changed to 2:1 in 1946 and was in production up to at least 1950. The majority of Middy and Mariner motors I've come across have brass instruction plates soldered to the top of the tank. The motor looks to be in good condition, but it may be advisable to check the water circulation which may require removing the lower core plugs to ensure there is no build-up of sediment, I've seen so many cylinders with blown water jackets due to this.
I am unaware that new Twin Spark coils are available so you'll have to find someone to rewind your existing coil - last time I inquired it was in the region of £100.
The major components for the power head are common to both outboard and inboards. One other thing that may be of use is that most of the fixings are British Standard Cycle Threads.
Hope this is of use.

[Oyster 49]

That's a really interesting find, not many of those BMB inboards about. I have an outboard version similar to that, which I bought for spares, but is actually complete, still in its box.
I'd agree its worth cleaning out the water jacket, the cylinders are a weak spot for corrosion.

[bernie]

Many thanks for your thoughts. I'm going to clean up the outside and then I'll check the waterways as suggested.

I would usually do a full strip and rebuild on projects like this but I think the downside is possible damage to long-seized parts and the likelihhod of having to make all the gaskets from scratch. Lack of spares availability would be a concern.

I'd assumed that the big end bearings would be rollers but thought I read somewhere these were white metal.....
That sounds wrong for a 2-stroke but maybe someone can clarify?

Thanks again,
Bernie

[Ferrier120]

The con rods are solid bronze - no white metal or bearings.
Gaskets are paper except for the ones between the silencer box and engine which are copper and heat resistant material (asbestos type).






















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[bernie]

Thanks - I decided to do a full strip and rebuild after all.

Really pleasantly surprised how easily it has come apart so far and a really pretty engine.
A couple of points of interest though.

Opposite the water inlet connection to the water pump there is an adjustable needle valve (pic attached).
The only purpose of this would be to restrict the flow of cooling water - why would that be required? Might the engines run too cold and need the flow reducing to help them run at optimum temperature?

The pistons have no pins to prevent the rings rotating althought the ends of the rings are stepped. Just wondering why?

Thanks,
Bernie

[Ferrier120]

A couple of great questions to which I don't have an answer, but I agree with your thoughts on reducing the water flow to ensure the engine doesn't run too hot or too cold especially when BMB were selling their engines all over the world. It would be nice if someone had an owners handbook that might confirm this.

[bernie]

Hopefully an easier question what would be the recommended mixture for these engines please?

Thanks,
Bernie

[Ferrier120]

For a 1940s Britannia Outboard it stated 16:1
For a 1950s Britannia Outboard it stated 20:1
For a 1950s Swordfish Outboard it stated 25:1
Unfortunately this is all I can find at present, when I get a bit more time I'll see if I can find some more info

[bernie]

Brilliant thanks.

I'm not sure of the age of my engine as I haven't been able to find a reference to the serial numbers but think I'll start conservatively with a richer mix after the rebuild.

I'll post a link when its running again.

Bernie

[bernie]

Looking quite pretty now. Just waiting for the coil to come back from a rewind and we'll go for the first start since about 1974!

[bernie]

Coil back from rewind - we now have sparks!

Does anyone have any timing info on these engines please? There are some likely looking numbers stamped on the flywheel visible when the plate is removed but no apparent reference point for alignment if they are intended for timing.

I'll try a first start with about .012" points gap and opening a few degrees BTDC.

Thanks,
Bernie

[Ferrier120]

With reference to fuel mix, I have found a sales leaflet from about 1953 which states one third of a pint of oil to the gallon - but personally I would stick to 16:1 as a little extra oil in not going to hurt.

According to a handbook for a 1950s Britannia outboard with Villiers ignition, the points gap should be between 0.012" and 0.018" and the magneto plate (stator plate) 'should be set so that the contact breaker points are just about to open when the pistons are 5/32" to 3/16" before top dead centre'

Hope this helps

[bernie]

Perfect - just the info that I needed, many thanks.
Had a go at starting to day but no joy. I'll reset using these figures over the weekend and try again.

Regards,
Bernie

[bernie]

I was hoping to be able to post a video clip of the engine running by now but no joy - I may need some help please.
With a healthy spark and the timing as suggested I'm struggling to work out why it won't start.
I managed to get it to fire for a couple of seconds by squirting easystart into the plug holes but that's it.

The engine may have been apart before so I may need to check on any errors that could have been made putting it back together again.

The only variable I can think of is the orientation of the bronze plate that sits over the rotor on the lower part of the crankshaft.
I've assembled it with the fan-shaped cutout facing the inlet port in the casting as I asume that it's job is to direct the fuel mixture to the rotor that then pulls it around and into the crankcase as the pistons rise. Can anyone confirm that please?

Anu other suggestions are of course welcome!!

Thanks,
Bernie