Just ran a quick check with my beam calculator. If your engines really do
weigh 1700 lbs each and the length is 4' you should probably go to 3x4 .250"
wall. That will keep the bending stress safely under the 33kip yield
strength and deflection at the end about .290". Both legs and the
connecting piece will deflect so allow one inch total. 3x4" quarter wall
tube is commonly used for support columns in construction so you should be
able to find it easily.
If the engines are under 100 lb you could get by with 3" .250" wall but in
either case gusset plates on either side of the corners would be good
insurance for any welding faults.
--
Glenn Ashmore
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at:
http://www.rutuonline.com
Shameless Commercial Division:
http://www.spade-anchor-us.com
"Paul Wiggins" wrote in message
...
Glenn Ashmore wrote:
Hi Glenn,
Great suggestion!
But ****e, Are you sure the lower leg will hold the weight? I figure the
distance from the rear roof line to the center point of the engine... is
3 - 4'. Seems like the leverage would cause... snap crackle pop.
Heavy walled... what thickness?
thanks!
-paul
If you are going to have to hire a crane anyway here is a suggestion that
eliminates any stress on the floors and makes removal and resetting a
simple single operation that minimizes crane time.
Have a "U" shape frame welded up of 3" square heavy wall steel tubing.
The legs of the U should be the distance from the edge of the cabin top
to the lift point of the engine plus a couple of inches and the bottom of
the U should be the length needed to clear any obstacles plus a few
inches. Reinforce the corners by welding on some gusset plates. Bolt
lifting rings to the ends of the legs. A 20' stick of 3" heavy tube will
set you back about $80 and welding another $50 or so.
When the crane arrives, have it lift the frame using one ring and
maneuver the lower leg into the boat. The upper leg will be over the
cabin top. Hook the engine to the bottom leg and lift. The weight of
the engine will balance the frame horizontally. When it gets high enough
to clear any obstacles have the crane swing back out of the cabin and
lift out all in one operation. Save the frame for use in resetting the
engines.
I set my 550 lb Yanmar under the cockpit single handed using a frame
welded up from 2" tube, a rolling gantry cobbled together from a pair of
old barn door tracks bolted to a 2x8 and a chain fall.