View Full Version : Brain teaser.

01-03-2007, 07:40 PM
This actually happened. Four years ago I was called over to survey a machinery casualty on a wooden hulled lobsterboat. It was a very typical boat, Cummins 210, Twin Disc transmission, stainless shaft, 4-blade propeller, no cage.

The owner told me that he had been out lobstering, and had come up to a buoy to pick it up, when the engine bogged down briefly, then there was a crash and a bang, and the engine sputtered to a stop.
Here's what I found.
1) The floor timbers that the engine beds were mounted on were broken.
2) The engine was still bolted to the beds, but they were slewed around about 5.
3) The bell housing was broken, and the transmission had moved aft about 8", until the propeller hit the rudder. The transmission was dangling from the propeller shaft.

So here's the question. What happened?

01-03-2007, 07:47 PM
Picked up a rope around his shaft and it wound in such a way as to cause the damage?

Could be a lot of things, should have talked to your neighbours at Evolution shaft.

01-03-2007, 07:48 PM
For starters, did he throw the engine into reverse just before this happened?

John Turpin
01-03-2007, 08:03 PM
Had he caught a very, very large lobster?

Les Schuldt
01-03-2007, 08:08 PM
"We're gonna need a bigger boat."

Peter Malcolm Jardine
01-03-2007, 08:21 PM

Bill Childs
01-03-2007, 08:42 PM
He wound the lobsta pot line around the shaft until the pot came came up and jammed the prop.

01-04-2007, 08:42 AM
Pot rope stopped the shaft almost instantly, rotational inertia of engine parts and flywheel twisted the trans so hard it broke the bell housing and the engine beds.

If that's not right then the dog ate my homework.

Terry Rhoads
01-04-2007, 08:58 AM
Do the lobstermen set togles in that area? Was the tide low at the time? Maybe his prop picked up the toggle or the line between the toggle and the main bouy, while he was concentrating on the main bouy.

01-04-2007, 11:33 AM
What happened was that as he shifted into reverse to stop alongside his buoy, he picked up a neighbor's potwarp in his prop. By a remarkable quirk he warp started winding around the shaft in the 1.5" between the prop hub and the stern bearing, and wound on turn after turn, just as neat as on a winch drum. When it got to the stern bearing it kept winding on, forcing turns into the space until the prop shaft was forced back, breaking the motormounts, when the engine took a climbing partial turn around its axis, rolling over the port engine bed. That broke the bell housing. The propellor was still turning, and had enough rearward thrust so that it pulled the prop, shaft, transmission, and pieces of bellhousing aft until it fetched up on the rudder. Once free of the transmission, the engine fell back down in place, a bit askew.
The main clue was a spiral of potwarp with 18 turns, just as neat as can be, lying on the ground under the stern gear.
I have never seen this before, and have asked many lobstermen if they've actually seen a boat it happened to. So far, nobody's seen it. So this was a genuine fluke.

01-04-2007, 12:39 PM
Rare, but not unheard of. Years ago I knew a family with a 1929, 42' Elco. They did the exact same thing thing by accident (potwarp around the shaft). Three of the four engine mounts (cast iron) broke rignt off the engine (120hp Lehman Ford diesel)as it was pulled aft, I don't know why the fourth didn't let go as well. I think they ended up with a bent shaft also.

01-04-2007, 01:21 PM
Well, it goes to show there's nothing new under the sun...Or under the stern gear.
If I hadn't actually seen this I wouldn't have believed that it could happen.
If I had to guess a reason for the Elco's fourth engine mount not breaking, it would be that either the mount-to-bed bolts broke, or tore out, or maybe there weren't any bolts.

I did once see an aluminum-hulled crewboat being driven too fast in big waves, plowed into the back of a wave, one V8-92 Detroit broke its mounts and rammed the engine compartment forward watertight bulkhead hard enough to dent it out about four inches. I was told that when they hauled it out, it had bent the Vee-strut all to hell as well.

I think the cause of that casualty was that they'd repowered from V6-71N's to v8-92TI's, which is about doubling the horsepower, and increased top speed from maybe 18 to more like 26. The added engine weight and added speed might have doubled the loads on the engine beds.