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  • #18916

    David Marchand
    Participant
    Vessel Name: Atlas Pompano 23
    Engines: Yamaha 70 hp 4 cylinder/cycle
    Location: Punta Gorda, Florida
    Country: USA

    A buddy just bought an old Mainship 34 flybridge trawler. The lower helm looks original and steers ok, about 6 turns lock to lock. The upper helm looks like it was replaced and steers at 10+ turns lock to locks which is unacceptable. It is a Seastar 1.7 cu in per rev pump. The lower helm pump has no identification..

    The ram cylinder is about 1.5″ id x 9″ stroke giving it a volume of about 16 cu inches. Using Tony’s hydraulic sizing criteria of dividing cylinder volume by turns and adding 10% for leakage you get a pump volume of about 3 cu in per rev which is what I suspect the lower helm is.

    But I don’t see any helm pumps available from Teleflex or Seastar that big. The biggest is 2.4. Are there any 3 cu in per rev pumps available? Or do I have my math wrong?

    David

Viewing 6 replies - 1 through 6 (of 6 total)
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  • #19021

    David Marchand
    Participant
    Vessel Name: Atlas Pompano 23
    Engines: Yamaha 70 hp 4 cylinder/cycle
    Location: Punta Gorda, Florida
    Country: USA

    Will try to get pics

    Tony:

    I helped the owner move the boat 350 miles from Md to Ny and now someone else is helping him move it up the Erie and Oswego Canals. I will contact him to message me some pics.

    But the upper helm, the one with the excessive locks and some light binding, is definitely a Seastar 1.7 cu ft pump. The ram is a two hose pass throgh rod type with no labeling.

    David

    #19018

    Tony Athens
    Moderator
    Vessel Name: Local Banks
    Engines: QSB 6.7 550 HP
    Location: Oxnard, CA
    Country: USA

    Any chance of posting good pics of both components ??

    The ram and the pump ( front & back)..

    Yes, you could have internal slippage in one or both components–

    When Hard over on the pump / ram–Can you continue to turn with some extra force?

    Tony

    #19007

    David Marchand
    Participant
    Vessel Name: Atlas Pompano 23
    Engines: Yamaha 70 hp 4 cylinder/cycle
    Location: Punta Gorda, Florida
    Country: USA

    Redoing the calcs based on Tony's advice

    So, here are the results of using Tony’s more rigorous advice:

    The distance between the ports is close to 10″, so subtract 2″ to give a stroke of 8″. The ID is 1.5″ based on an OD of 1.75″. This gives a gross cylinder volume of 14″. Then the rod volume is about 2.5 cu in for a 5/8″ rod, leaving a net volume of about 11.5″.

    Then if you take the known upper helm spec of 1.7 cu in per rev, that gives you a lock to lock number of turns of about 7.5.

    The real numbers is closer to 10+. So does that mean that the upper helm pump is just worn and leaking a lot of fluid. Changing it out to a new 2.4 cu in per rev pump would give a lock to lock of about 5.3 which would be great.

    What do you think?

    David

    #18995

    Tony Athens
    Moderator
    Vessel Name: Local Banks
    Engines: QSB 6.7 550 HP
    Location: Oxnard, CA
    Country: USA

    David M. and all,

    Just to help you ID any general cylinder as to its internal measurements when you cannot get a specific make or model number..

    You can safely assume this as to base measurements when you only have EXTERNAL MEASUREMENTS to deal with, AND you are working with a balanced cylinder to where you SEE the EXIT rod on retraction on the cylinder.

    1) If you cannot actually measure the stroke, you can safely subtract OVER 2″ ( averages 2.5-3″) from the C~C input port distance to get the stroke….. I.E. If the HYD PORTS are about 9.5″ C~C, the stroke is about 7″.. This applies to basically all cylinders of this design…

    2) Measuring the internal piston bore , EXTERNALLY.. Safety assume the WALL THICKNESS of the outer ROUND TUBE has a 1/8″ wall thickness and you’ll be close enough.. Measure about 1.75″ would mean a 1.5″ bore.. Most companies use even measurements like 1.5″, 1.75″, 2″ or when metric, use 40 mm, 50mm, or something “even” as to bores. Strokes like 7″, “. 9″ 11″ are common..

    3) Remember this ( seems most for this part–You need to SUBTRACT the volume of the ROD–IE, if you have a 5/8” dia. rod, then that VOLUME to be subtracted from the “bench calculation” to get the actually internal cylinder volume..

    4) In the end, if you have both displacements ( Ram and PUMP).. Let say they are 60CC’s and 300CC’s (5:1) ………….Divide one by the other and then add 10% to the turns “Lock to Lock”, and your be very close if all is 100% internally as to normal hyd leakage ( which is always “10%” on any HD system that is not static and has varying loads & pressures)

    Hope this helps..

    #18955

    David Marchand
    Participant
    Vessel Name: Atlas Pompano 23
    Engines: Yamaha 70 hp 4 cylinder/cycle
    Location: Punta Gorda, Florida
    Country: USA

    Answers below:

    David,
    I think you need to measure the diameter of the ram shaft and the total length of throw of the shaft to do your
    calculation.
    That should give you the amount of fluid required to move the rudder through its total travel. See the picture.

    I did measure and that is how I got a cylinder volume of 16 cu in. Here it is: The OD is 1.75″, so I suspect the piston is 1.5″. The distance between the inlet nozzles is a bit more than 9″, so figure a ram stroke of 9″. 9*(.75)^2*3.14 = 15.9 cu in. 15.9 / 6 turns lock to lock plus 10% = 2.9 cu in per rev.

    A few questions.
    When you turn the upper helm to the lock does it go to a totally hard feeling or is it spongy?
    Can you continue to turn it with some effort?

    Both upper an lower helm go hard at end of lock.

    When you turn either the upper or lower helm, does the other one move? No
    When making hard turns under fairly strong power does the helm feel like it is sort of chattering or slipping?

    The lower helm is smooth although stiffer than the upper. The upper does seem to bind just a little and release as you turn it. I had thought that this was mechanical wear on the pump vanes.

    And of coarse, has the helm been bled of all air? Actually both helms. Haven’t been bled.
    Can you post some picture of both helms and the ram?

    I am not on the boat, and it will take a while to get pics.

    In summary, the upper helm is much looser than the lower and takes almost twice the number of turns to go lock to lock. That is what lead me to conclude that the upper pump when it was replaced was replaces with one that was too small.

    Your thoughts?

    David

    #18925

    Bill Fuller
    Participant
    Vessel Name: Audax
    Engines: Yanmar 4LHA-STE
    Location: San Diego, CA
    Country: USA

    David,
    I think you need to measure the diameter of the ram shaft and the total length of throw of the shaft to do your
    calculation.
    That should give you the amount of fluid required to move the rudder through its total travel. See the picture.
    A few questions.
    When you turn the upper helm to the lock does it go to a totally hard feeling or is it spongy?
    Can you continue to turn it with some effort?
    When you turn either the upper or lower helm, does the other one move?
    When making hard turns under fairly strong power does the helm feel like it is sort of chattering or slipping?
    And of coarse, has the helm been bled of all air? Actually both helms.
    Can you post some picture of both helms and the ram?
    Bill

Viewing 6 replies - 1 through 6 (of 6 total)

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