Trim in/down issue

[series60]

I'm back at the original issue I have with porpoising. My 1984 Mercruiser 305 powered 20' IMP likes to porpoise above 25mph. Shifting weight doesn't help. It improves when I use the tabs but I feel it would improve more if I could get the trim rams to retract completely. If I disconnect the rams the outdrive will drop at least another 3/4 inch. My thinking is if I could get the rams all the way in/down the porpoising would improve. Now I have to state that this ole' girl has been completely rebuilt bow eye to stern including new transom. The outdrive is a combination of newer upper housing (Upper housing 1.32R #OB568855 (Alpha One 0A479507 -> 0D469858 (1986-1990) (not the correct gear ratio but I live with it) and lower is an R model of unknown vintage (assume original 1984). Trim cylinderort #98703 Starboard #98704). I have done the gimble ring mod also.

The first question is how much ram shaft extends when they are all the way retracted. Mine measures approx 3?" of exposed ram shaft when retracted. Looking closely I can see old shadowing on the shaft where it at one time might have retracted to about 1?" of exposed shaft.

Second question? Is there an adjustment for retraction length of the rams?

Hope this is to confusing!

 

[achris]

Re: Trim in/down issue

...The first question is how much ram shaft extends when they are all the way retracted. Mine measures approx 3?" of exposed ram shaft when retracted. Looking closely I can see old shadowing on the shaft where it at one time might have retracted to about 1?" of exposed shaft.

I don't have one in front of me, but 3-1/2" sounds about right. When they were first painted Merc masked the last 1" to 2" so it does sometimes look like the ram would retract further, but they don't.

Second question? Is there an adjustment for retraction length of the rams?

Short of cutting the end off the ram, the length of any ram is a function of the length of the piston rod (the top end of which you see and has the aft anchor point attached).... Once the ram is fully retracted, that's it... there is no more.

Chris.........

 

[Bondo]

Re: Trim in/down issue

I'm back at the original issue I have with porpoising. My 1984 Mercruiser 305 powered 20' IMP likes to porpoise above 25mph. Shifting weight doesn't help. It improves when I use the tabs but I feel it would improve more if I could get the trim rams to retract completely. If I disconnect the rams the outdrive will drop at least another 3/4 inch. My thinking is if I could get the rams all the way in/down the porpoising would improve. Now I have to state that this ole' girl has been completely rebuilt bow eye to stern including new transom. The outdrive is a combination of newer upper housing (Upper housing 1.32R #OB568855 (Alpha One 0A479507 -> 0D469858 (1986-1990) (not the correct gear ratio but I live with it) and lower is an R model of unknown vintage (assume original 1984). Trim cylinderort #98703 Starboard #98704). I have done the gimble ring mod also.

The first question is how much ram shaft extends when they are all the way retracted. Mine measures approx 3?" of exposed ram shaft when retracted. Looking closely I can see old shadowing on the shaft where it at one time might have retracted to about 1?" of exposed shaft.

Second question? Is there an adjustment for retraction length of the rams?

Hope this is to confusing!

Ayuh,.... How Straight is the keel line, under the hull,..?? Especially the last 4' to 6',..??

Sounds like all that rebuildin' mighta put a rocker in the hull...

 

[series60]

Re: Trim in/down issue

Ayuh,.... How Straight is the keel line, under the hull,..?? Especially the last 4' to 6',..??

Sounds like all that rebuildin' mighta put a rocker in the hull...

Keel is as straight as it was based on an old man's eye sight. Really don't have an idea how one would measure it. It's a delta conic hull which isn't quite like a deep V. There was a 'wow' on the port side 4' up from the stern but I was able to take most of that out when setting the new stringers. Guest I will just have to live with it. Generally don't run much faster than 25mph most of the time. Have GPSed it at 36 but not unless I'm on glass.