With the exterior paint work done, I have to get the propeller shafts installed before retiring Tent Model IX. The tent goes all the way to the ground, which keeps out any breeze that might blow away the argon shielding gas from weld areas…and the prop shaft cutlass bearing housings have some very critical welded joints.
Our Roamer originally came with 1.5″ Aquamet 17 shafts that worked fine for the 400hp twin turbo and intercooled Super Seamaster engines that were in the boat when we first got it. But when we decided to upgrade to Cummins 430hp Diamonds, with all of that power coming online at 2600rpm (vs 3200rpm for the Seamasters), the result was a safety factor of three. Since the recommended safety factor is at least five, that meant we had to replace the shafts with 1-3/4″ ones. While the original shaft log would accommodate the larger diameter, I had to cut off the cutlass bearing housings (with brand new, practically unused cutlass bearings installed back in 2009!) and have new ones made up and welded in place.
Struts await new cutlass bearing housings
The good thing about aluminum is that woodworking tools can be used to cut and shape it. A sawsall and circular saw with carbide blades got the old cutlass bearing housings off the boat fairly quickly.
New 1-3/4″ cutlass bearing, with matching housing and collets
Before we could weld the new cutlass bearing housings onto the original 3/4″ 6061 plates that make up the V struts, we had to ensure that the the struts were cut to the proper angle. The collets in the picture above are sized to fit around the new shafts, then slide inside the new housings.
Two-piece aluminum collets can take the heat of welding without catching fire
The cutlass bearings are made of rubber and phenolic, neither one of which responds well to the kind of heat we’ll generate when the housings get welded in. The two-piece collets are a tight fit to the shafts and a snug fit inside the housings, but not too tight…we need to be able to remove them after the welding is done and everything has cooled down.
Pretty slick fit…ready to install
Nice fit to the OE struts
When we raised the shaft/collet/housing assemblies into place, we found the V-strut cuts were pretty much where they needed to be. In the engine room, I verified that the shafts were dead center in the shaft log. I used a jack to raise the housings as high as the V-strut plate cuts allow before blocking them up.
The intermediate strut fit was good
Welding this intermediate strut assembly will prove to be a challenge, since my Miller Spoolmatic 30a has a straight nozzle. With only six inches between the hull and the bearing housing, it doesn’t have much room to allow us to weld inside the V.
Ten passes later, the housings were welded in.
My Miller Trailblazer 280NT and the spool gun performed flawlessly while the fabricator made multiple passes to fill in the “vee’d out” welded joint. On the rear strut, he was able to weld inside the vee as well as outside. For the intermediate strut though, the straight nozzle on my spoolgun just won’t work. We’ll have to take another crack at it with a flexible nozzle the fabricator will bring from his shop. Since we had both housings welded in (as far as we could with my equipment) on the starboard side, we prepared to swap the collets out so we could rig up the housings and shafts on the port side.
Bad news…stuck collet
The two collets on the intermediate housing came right out, but on the rear strut the rear collet took a bit of nudging with a hammer and chisel. Unfortunately, the forward collets just wouldn’t budge…at all…not even with a 10# mallet. So, we have to wait until next weekend to attack again. We’ll use a bit of heat and maybe a hydraulic cutlass bearing press to remove the collets and set them up on the other side. We’ll also have the fabricator’s welding gear that can weld in the tight space of the intermediate struts.
Next up in our 1969 Chris Craft Roamer 46 Refit: Helm Windshield Frames (part I)
1969chriscraftroamer46 
Factor of safety – I was curious as to your calculating the factor of safety for the old vs. new shaft. Did you use the yield point or UTS for the material? I teach strength of materials and just love actual applications rather than textbook stuff.
Love your postings! I should have done this for my work.
Thanks, John!
I used the calculator at Western Branch Metals. It uses yield strength for the calculation.
Where:
D is the shaft diameter
P is the rated horsepower
S.F. is the safety factor chosen
S is the yield strength in torsion
N is the shaft rpm
321,000 is a factor derived from converting horsepower to torque/balancing units
What was the yield strength torsion? Generally only the tensile yield is published.
John
70,000 psi. Aquamet 22 specs are available from the manufacturer: http://aquamet.com/aquamet22.nxg. Aqualoy 22, from a different manufacturer (Western Branch Metals), has the same spec on their website. Is tensile yield only provided for materials other than marine shafting? I can’t imagine how a shaft manufacturer could sell their wares without providing that spec.
Q
Generally the shear/torsion values are derived by a factor of 0.60 times the tension yield. This appears to be about right for this material. This is made by a steel mill Crucible Steel in Syracuse NY about 90 miles from me.
As a metallurgist, I knew several of the metallurgists up there. Thanks for the information, you have researched this very thoroughly! I now have a great research project for my students.
John
Interesting, John!
I’d heard from the (incompetent) mechanic/engineer/machinist I fired a while back that not all shafting is made equally, even in the same series. He seemed to believe that Aqualoy (or maybe other brands, assuming there are others) was made in China, and that Aquamet was made in the USA–the real deal. Do you mean that Western Branch Metal’s Aqualoy and Marine Machining and Manufacturing’s Aquamet are both just rebranded products from Crucible?
6061, eh?
Interesting to be sure.
Cheers!
-Eric
Why do you comment as this being interesting? Probably about the best available. 2000 series prone to SCC, 7000 series not as corrosion resistant.
I always thought my shaft’s were large on my 1981 46′ Bertram, but 2 1/2″ seems like overkill according to what you say is normal. LOL
2.5″ might be large, but it really comes down to a combination of factors–HP, shaft size and material, etc. I like Western Branch Metals’ shaft calculator (http://wbmetals.com/shafting.asp). If you’ve got Cummins 430s with 2:1 gears and a safety factor of 5, with 304 stainless shafts they have to be 2.5″. But if you’ve got Aquamet 17 or 22 in a 2.5″, they can handle up to 900hp with the same gear and safety factor. It’s all in the math!
Cheers,
Q
Very interesting, thanks for that info.
Bill Kelleher