mcollinstn

You're assuming that the TOTAL pressure load across the tab will be directed into the hydraulic cylinders. This is a fallacious assumption, as the hinge section will obviously absorb a substantial percentage of the load.

However, I also don't see where you have taken into account anything related to the leverage ratio of the trim rams. Depending on their mounting point, and the angle of their static force vector, the force ratio afforded the rams can change the outcome wildly.

Somebody gimme a side sketch.

mc

Falcon

The pump Shawn started the thread with is a Merc Racing outboard trim pump when used with the hardware (solenoids & mount) shown. It uses the same main pump & settings as the competition pump that trims the MC#6, 3A, & 5 sterndrive and competition trim tabs. The competition pump has the tall cast aluminum mounting bracket with motor support. It has the slow rate pump section, 3600(+/-200)psi-up, 2000(+/-200)psi-dn. Ports are 7/16" thread inverted flare, but may have brass adapters for smaller inverted flare or sae flare.

Standard K-Plane pumps (not competition style) also use the same slow rate pump as the competition & outboard, but with lower output adjustments, different reservoirs (plastic or short metal) and different solenoids. 1900(+/-200)psi-up, 800(+/-200)psi-dn. Ports are 7/16" thread inverted flare, but have adapters for 3/8" inverted flare to fit the standard tab lines.

Standard Mercruiser drive trim pumps use a high rate pump for faster trim rate (taller pump gears, same pump design). 2400(+/-200)psi-up, 500(+/-100)psi-dn. Ports are SAE o-ring style with debris screen adapters for 7/16" thread inverted flare.

I believe all current products use the same drive motor.

shawn
The easiest solution if you have too much pressure for the cylinders is to get some standard K-Plane pump assemblies. You can also try to turn down the pressure setting on the up port as mentioned in an earlier post (3/4 of a turn out). I think you should only have to do this on the UP port, although you could turn down the DN port valve a half a turn or so.

mmwalters
Did you try the leakage test I mentioned (extend tab down, remove and cap low pressure line at the cylinder, run pump again to try to continue extension, leakage past o-ring will come out of the open cylinder port). If you have high cylinder loads that the cylinders are not designed for, the piston o-rings can extrude and tear quickly causing leakage. Also, do you hear the pump suck air sometimes during extension/retraction? If so, you may have air in the system. I know most people just hook things up and cycle back and forth, but it only takes minimal effort on a trim system to fill with solid oil. Let me know if this may be a problem and I will post my method of filling a dry cylinder and pump system.

mmwalters and shawn
As for having enough pressure for the tab, hard to tell. It depends a lot on where the cylinder is mounted. If the cylinder is mounted more toward the hinge, the cylinder handles a majority of the load and it can even get multiplied if a load comes in only on the tail end of the tab. Piston size makes a huge difference in the hydraulic pressures created, but don't forget about the physical loads on the components. Here is a picture of a large tab on a 36 Spectre (Dana). It doesn't have much movement, there is a lot less cylinder stroke than you would think (3").

mmwalters

Falcon No I did not test the cylinders. Before I responded to this post I had resealed the cylinders with new seals from Dana. I had bleed the system before. Now the plan is ... I ordered 2 new cylinders from Trick marine. These have 2'' diameter pistons vs the 1.25 pistons of the Dana units. Will this make a big difference? Now we just need spring to test these changes. As you can see from this picture Not much leverage here.

Falcon

The diameter change will result in over 2.5 times as much capability in the cylinders! Diameter (piston area) is very important. At 1000psi the 1.25" piston will generate approximately 1200 lbs of force per cylinder. At the same pressure the 2" piston will generate over 3100 lbs of force per cylinder! Will the cylinder length change?

mmwalters

Falcon No the length will be the same. If I still encounter problems then I will extend the ram length and rework the mounting points. I hope this will not be needed as it will be necessary to remove the tabs from the boat. It seems this is a large increase in force.
The point I should make is as is the tab works well in flat water it is only in the rough that they work back in. I assume this is due to the tab coming in contact with the water. Does that seem correct?

mmwalters

OK guys this is what I an going to use. I am waiting for the cylinder body to come back from the powder coater. As it turns out I did have to build a longer tim rod. Is this going to work? Of course the one on the left is what I was using {dana marine} The one on the right is go on { trick marine)

Falcon

The cylinder diameter should work and the added length should also help as long as you don't go over center (I don't think so from your pictures). Good luck!

shawn

On my set up, I used the trim tabs as they were and mounted them side by side and cross drilled them and mounted the tunnel tab directly to the two trim tabs. Thus my rams are closer to the transom than Michael's. Short version it that I have too much pressure in both directions right?

mmwalters

Shawn I started with the Dana marina tabs. I did replace everything except the hinges but I kept the geometry the same. I think the added surface area was just to much for the cylinders Maybe 26''x42'' is a little to big. We will see what happens with the new cylinders.
Could be the cylinder you broke was hurt before you bought it? Those Dana cylinders are held together by a dam snap ring, and they never broke.

mmwalters

TTT for Johnny V