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Twin 2006 Mercury 115 4-stoke on 80ft dinning yacht. Tilt…

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Twin 2006 Mercury 115...

Twin 2006 Mercury 115 4-stoke on 80ft dinning yacht. Tilt cylinders will split when boat is accelerated hard in reverse. Have replaced cylinders, installed thermal valve protection kits, memory pistons. There must be some kind of pressure relief valve or service bulletin addressing this problem. The s,n on the unit that just blew again is 1b522996

Mechanic's Assistant: Sometimes boat issues seem like they'll be really complicated, but end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Is there anything else the Marine Mechanic should be aware of?

I don't believe so

Submitted: 8 months ago.Category: Boat
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Answered in 1 day by:
9/21/2017
Marine Mechanic: Boat Doc, Boat Enthusiast replied 8 months ago
Boat Doc
Boat Doc, Boat Enthusiast
Category: Boat
Satisfied Customers: 206
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H​I, sorry for the delay. Please explain to me what you mean by the cylinders will "split". Do they actually come apart? And if I might ask, are these actually moving the yacht as auxiliary motors?

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Customer reply replied 8 months ago
This is an 80ft double deck pontoon by Destination Yachts. The 2 115 Mercurys are what power the boat. They have single ram tilt/trim hydraulics. The cylinder splits open on the side from hydraulic pressure if the boat is accelerated hard in reverse.
Customer reply replied 8 months ago
that is the early design which is what my units both have, if you look on the tilt cylinder you will see a built in "tube or channel" on the side that runs from top to bottom. That is where the cylinder will split open under extreme hydraulic pressure
Marine Mechanic: Boat Doc, Boat Enthusiast replied 8 months ago

I​ was just looking into the second design which seems to have slightly different valving but the shock tube looks similar, at least in the illustration. I can understand why you are having the issue, but I am shocked that the system can exceed the cylinder pressure limit, especially with just engine thrust. There is a 3200 psi relief valve in there as a secondary backup (first one is only 600 psi, but neither probably see enough "shock" to let the pressure off.) But I am wondering if cutting a coil off the secondary spring would allow it to vent before popping the cylinder (of course the engine would rise, which isn't ideal either. Lowering the pressure relief on the trim pressure would not make a difference because in reverse the two sides of the piston are hydraulically locked.

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Customer reply replied 8 months ago
Agree the engines popping up is not a problem. The only time this happens is if the boat is coming in to dock to fast or issues with wind. Modifying the spring pressure is a possibility. It sucks that I have to remove the hydraulic unit in the water but it's warm here. The fill cap on the hyd reservoir is below the water line. Makes me wonder if the engines should come up a hole on the transom plates. Both engines will occasionally die when throttling back to idle in gear. With electric helm controls that makes about a 10-15 sec delay for restart. The operator will have a tendency to "panic" and accelerate harder when going to reverse
Marine Mechanic: Boat Doc, Boat Enthusiast replied 8 months ago

Oh rental unit, makes more sense now. It would be nice to have them rigged with old style commercial units, locked down, no trim. Raising them would give them a little less bite, might help the situation.

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Customer reply replied 8 months ago
No it's not a rental, it's operated by a resort for dinning cruises, but does have 2 different captains. I started servicing it a year ago. The Merc's only have 1100hrs so hopefully I can correct this problem. Mercury does not acknowledge a problem or want to help figure it out.
Marine Mechanic: Boat Doc, Boat Enthusiast replied 8 months ago

Oh gotcha. I have to admit I've never heard of it, but that is kind of a strange application. I'm an ex go fast guy myself, I'd probably just slap on a few more bigger motors, LOL. I was thinking of those big rental houseboats on the lakes. But they usually just have one motor and they don't let them near any docks.

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