Outboard Hydraulic Rams

[neil.mccrirrick]

So I was wondering what purpose do these serve? If you start with the engine trimmed all the way down, and then trim up and keep trimming up til you hit the sweet spot, won't that mean the engine is off the rams? I reckon mine is, although in fairness I've not looked.

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[9D280]

I assume you are talking about the two big pokey out things at the bottom of your clamp bracket?

In days of yore, all Power trim looked like that. (after the initial version that consisted of big external twin rams with a cat's cradle of hydraulic hoses). Now smaller engines use a single cyl, but larger machines still need the extra "grunt" given by the three ram systems.
Basic idea goes like this:

When running, the swivel bracket sits against those two rams. They have a big area piston behind them (x2) so the tiddly little hydraulic pump & valves can handle the pressure the engine excerts on them when at full chat. (Pressure = Force/Area - rearrange it to give Force = Pressure x area)

When the engine is trimmed out beyound the point of contact, the middle ram takes over. This has enough grunt to lift the engine, but not enough to handle the force created by the prop thrust. (it is always pressurised, but is incapable of holding the engine against prop thrust, so drops to the rams when the prop is working)

If you trim out slowly on land, you'll also notice the "not touch" point is when the swivel bracket is still (just) in between the clamps. Much further up & you run the risk of snapping it clean off, as it's only held in place by the tilt tube. Hence the design!


Single rams have a bypass port part way up connected to a low pressure relief valve, so that it instanly has a way lower hold pressure and drops back between the clamps when the throttle is opened.


If you are truimmed "all the way up" it may be off the rams when you start, but it won't take much, especially at 140 HP on a 6m+ boat to get it back down again!

Hope that both made sense and was what you were asking about!

[neil.mccrirrick]

Thanks very much for such a good answer all clear and concise and I get it. It makes sense and I think it's worth me noticing the angle of the outboard as it comes off the rams. Clearly moving beyond that and them accelerating is only going to cause a problem eventually as the bracket must be taking the initial strain

[jyasaki]

I thought the twin rams were trim rams, and the center ram was the tilt. Pressure goes to all 3 when coming up from full down, so the response is fairly slow making it wasy to adjust the trim. When the trim rams hit the top, the pumped fluid goes only to the tilt ram, and the motor raises much quicker.

The trim range should be while all 3 rams are operating (i.e. the bracket is still in contact with the trim rams.)

Least that's how I understood it.

jky

[Nos4r2]

If you can get off the trim rams and accelerate without falling flat on your back, you must have a hell of an angle on your transom!

[9D280]

Originally Posted by jyasaki

I thought the twin rams were trim rams, and the center ram was the tilt. Pressure goes to all 3 when coming up from full down, so the response is fairly slow making it wasy to adjust the trim. When the trim rams hit the top, the pumped fluid goes only to the tilt ram, and the motor raises much quicker.

The trim range should be while all 3 rams are operating (i.e. the bracket is still in contact with the trim rams.)

Least that's how I understood it.

jky

Yeah, the perssure is in all three, but the trim tram on it's own can't hack the force from the prop thrust. I think we just said the same thing in a different way!

(you can test how sticky the trim rams are by disconnecting the tilt ram & pumping in & out. When the trim ram hits an extremity, the others move, ideally in perfect synch, but one of them will always lead!)