Reply
 
Thread Tools Search this Thread
 
Old 04 March 2013, 15:15   #1
Member
 
neil.mccrirrick's Avatar
 
Country: UK - England
Town: Oldham
Boat name: Nemanda
Make: Aluminium
Length: 6m +
Engine: 140hp Tohatsu
MMSI: 235065639
Join Date: Jan 2008
Posts: 869
Outboard Hydraulic Rams

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.

Sent from my iPad using Rib.net
__________________

__________________
If you don't like my opinion, I have others.
neil.mccrirrick is offline   Reply With Quote
Old 05 March 2013, 11:26   #2
Member
 
Country: UK - Scotland
Boat name: Wildheart
Make: Humber/Delta Seasafe
Length: 5m +
Engine: Merc 60 Clamshell
MMSI: 235068449
Join Date: Apr 2007
Posts: 4,610
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!
__________________

__________________
9D280 is offline   Reply With Quote
Old 05 March 2013, 13:49   #3
Member
 
neil.mccrirrick's Avatar
 
Country: UK - England
Town: Oldham
Boat name: Nemanda
Make: Aluminium
Length: 6m +
Engine: 140hp Tohatsu
MMSI: 235065639
Join Date: Jan 2008
Posts: 869
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
__________________
If you don't like my opinion, I have others.
neil.mccrirrick is offline   Reply With Quote
Old 05 March 2013, 16:37   #4
Member
 
Country: USA
Town: Oakland CA
Length: 3m +
Join Date: Feb 2005
Posts: 6,653
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
__________________
jyasaki is offline   Reply With Quote
Old 05 March 2013, 17:38   #5
RIBnet admin team
 
Nos4r2's Avatar
 
Country: UK - England
Town: The wilds of Wiltshire
Boat name: WhiteNoise/Dominator
Make: Ballistic 7.8/SR5.4
Length: 7m +
Engine: Opti 225/Yam 85
MMSI: 235090687/235055163
Join Date: Jul 2005
Posts: 12,645
RIBase
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!
__________________
Need spares,consoles,consumables,hire,training or even a new boat?

Please click HERE and HERE and support our Trade Members.

Join up as a Trade member or Supporter HERE
Nos4r2 is offline   Reply With Quote
Old 06 March 2013, 04:50   #6
Member
 
Country: UK - Scotland
Boat name: Wildheart
Make: Humber/Delta Seasafe
Length: 5m +
Engine: Merc 60 Clamshell
MMSI: 235068449
Join Date: Apr 2007
Posts: 4,610
Quote:
Originally Posted by jyasaki View Post
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!)
__________________

__________________
9D280 is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -5. The time now is 23:40.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.