Sizing aluminum cutlass bearing housings

Discussion in 'Inboards' started by q240z, Aug 6, 2013.

  1. q240z
    Joined: Aug 2013
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    Location: Mid-Atlantic

    q240z New Member


    I have been doing a major refit to a 1969 Chris Craft Roamer 46 for the last five years and recently had to fire and replace the guy who was going to do the new shaft and engine install. The last one claimed to be a Naval Academy marine engineer, but I'm uncertain of those claimed credentials now. Anyway, the new welder/fabricator has questions about the robustness of the cutlass bearing housings, which the last guy made out of 6061 tubing stock.

    The original Chris Craft bearing housings were welded directly to the V-struts comprised of 3/4" x 6" 6061 plate, which are welded to a 3/4" base plate that is welded to the 1/4" 5086 hull plating. See pix of the struts in my article about priming the bottom. We had to cut off the original cutlass bearing housings because the 1.5" Aquamet 17 shafts were inadequate for the new Cummins 6CTA 430hp engines that are going in. The gears are Twin Disk with 1.5:1 ratio, so I went with 1.75" Aquamet 22 shafts that yield a 5:1 safety factor. The new shafts are roughly 16' long and are carried by three bearings--one in the stern tube, one intermediate strut and the strut back by the rudders.

    The 6061 extrusion he used for the new cutlass bearing housings has a 1/4" wall thickness. This is identical to what came out of the boat. The original engines were 427 Chris Craft/Fords with 300hp at 4000rpm, but the boat was repowered in 1973 to Super SeaMasters -- 534ci twin turbo and intercooled gas engines producing 400hp at 3200rpm and 657 lbs/ft of torque. The original cutlass bearing housings with 1/4" side wall thickness appears to have held up fine with significant increase in HP from the SeaMaster engines. The Cummins Diamonds, by comparison, are 430hp at 2600rpm and 1029 lbs/ft of torque. These boats also came equipped with 871n Detroits with 320hp at 2300rpm.

    Incidentally, because the SeaMasters had 2:1 gears and the Cummins have 1.5:1, the prop shop estimates that the original 24 x 24 prop specification might work out for the new engines and gears.

    The question is, how thick does the cutlass bearing housing wall have to be to hold the bearings in place, given the engines, gears etc? Is there a standard formula to estimate this sort of thing, as there is with shafts, props etc?

  2. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Out of curiosity I long ago checked the DNV dimensioning rules for I-brackets. It boiled down to a dimensioning bearing pressure of roughly 30 kp/cm2.

    If you apply this loading on your bearing area (length should be > 3 x dia), you get a radial load of 1823 kp. Now, this is a pulsating load and the housing must be dimensioned with fatigue in mind. You have a housing area of about 16 cm2, which gives a tensile stress of 113 kp/cm2. I can't say what the acceptable tensile fatigue strength is for the 6061 in welded condition, but if you contact the forum member Ad Hoc, he will give adequate advise on the material aspects.

    One observation that I find a little worrying is the square corners of the backing plates welded to the hull. They are bad stress concentrators and I see a great risk for stress fractures from these corners. Square corners in welded alu is bad practice. Next question is: what does the inside backup look like?
  3. q240z
    Joined: Aug 2013
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    q240z New Member

    Thanks, Baeckmo!

    I'll wait to see if Ad Hoc weighs in on this thread and ping him if he doesn't show up.

    Regarding the heavy base plates for the struts, all of the big, aluminum Roamers Chris Craft made came with the same approach for attaching struts to the hull...even the 60 footers with Detroit 12-71TI power. I have never heard of a hull failure on any Roamer from causes other than corrosion, and even those are more common to the steel boats than the aluminum ones. There are no backing plates on the inside of the hull; just the 1/4" 5086 hull plating, frames and stringers (on the 46' model from the late '60s). My boat is an exception since it's been on the hard for 25 years, but I would expect that if the square corners were a problem it would have manifested on the hundreds of other Roamers that have been running around for the last 40+ years.
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