Reversing Gears at High Rate of Forward Speed?

I did the seatrials (7 days worth) on the 2nd boat in the US with MAN common rails back in 2005. We had MAN of North America's top engineers as well as twin discs, the transmissions supposedly were designed to where you could go from full speed foward to full speed reverse and the computer would compensate and it could go full speed foward to full speed reverse without an issue. We did not test this. However they did play with the computer mapping for both the transmission and engine.......

Hi,

I am quite amazed that this damage is attributed to this action.

Was there any damage to the gearbox internals or the underwater gear?

BTW: It would have only been the gearbox that changed direction not the Engine itself.

You aren't the only one.

Those mounts are equally strong ahead as well as astern and the propeller generates more thrust ahead than astern so that claim is bogus as far as I am concerned. If going astern could pull the engine off the mounts just imagine what would happen if someone went full ahead ... I don't care what some judge said, a crash stop didn't cause the damage in either of those cases.

Crash stops are not a structural issue with boats, there may be issues with wear on clutches due to slipping while the shaft and wheel are reversed but that has no impact on the rest of the hull. And don't forget the mass of a small boat propeller and shaft is not very great. The time delay in electronic controlled gears is to protect the clutches. The power consumed is far less that was being transmitted at full ahead and the prop works in a fluid, not like a tire on the road.

I am sure the noise and vibration caused by cavitation is scary to some but it isn't dangerous. Even 60,000 hp ships can do it with little effort and no damage.

If the boat is swinging a large diameter propellor that didn't cavitate much, I could see that happening when going from full speed foward to reverse, or from wide open in neutral into gear.

Back when I used to drag race, we used to break solid motor mounts here and there. That was from letting the clutch out with the engine at high rpm's with slicks on (launching at the drag strip). Street racing with street tires on, it never happened because they would spin and not stress everything as much.

Propeller size has nothing to do with it, and if you have found somebody who makes a non-cavitatiing propeller I think there are a lot of military guys who would like a piece of that company. If the engine has enough power to turn the prop full ahead it has enough power to make it cavitate going astern.

A fixed pitch prop has less thrust astern than ahead and unlike your hot rods, a propeller isn't stuck to the water like a tire on pavement.

If you ran the engine up to full rpm and dropped the transmission into gear, ahead or astern, you would probably damage the clutches long before you generated enough thrust to break the engine off its mounts.

Gents,

This discussion has gotten off-course, so several posts are being split into a new thread titled: "Reversing Gears at High Rate of Forward Speed?"

Hi,

Capt Bill 11 The fact that there was damage to the underwater parts must surely make it more likely the boat hit something than was simply put astern whilst underway ahead. A bent shaft that jams can sure give the gearbox internals a hard time. Maybe the gearbox seized and wrenched the Ford off it's mounts then the forward way on the boat pulled the whole lot back a bit.

In all my years working on boats/ships and in ship/yacht yards I have never seen anything like that purely from the gear lever being shoved into reverse by accident. In fact I can't say I have ever actually seen damage like that on a boat before.

Me too, I attribute it to ill maintained hardware. The mount bolts broke, they looked like they were there for 20 years. The direction of thrust would have also changed. The precipitating event was undoubtedly going into hard reverse, that does not mean that result should have happened. The underlying cause was hardware outside of its service life limit.

Most likely the propeller and rudder took a bit of damage.

... that anyone on this planet would think that a fixed pitch propeller can generate enough thrust in reverse to break engine mounts designed to withstand more than ahead thrust.

Propellor size does have something to do with it. You are spinning a lot more rotating mass/weight on the end of the shaft. Not even considering cavitation or any of those factors.

I understand that all propellors slip and 20% slippage is considered good for a propellor on a normal boat. But going from full throttle neutral to in gear puts a hell of a strain on motormounts, because you still will have some bite with the propellor.

I've seen numerous people over the years go from high throttle neutral on boats to in gear and in most cases the clutches remained intact, motor mounts also.

I have also seen someone go from wide open foward to wide open reverse on 1 engine, and it did shift, and the boat decelrated spun around in a circle (while it was at 25 knots) everyone fell on the floor, and the transmission was fine, the motor was not and it bent valves and piston rods but still ran this was on a 32' SF with 318 chryslers. I dont know if the motor mounts survived.

You ran a 318 Chrysler with bent connecting (what you termed piston) rods? WOW!!! That's impressive...damaged valves even and it kept on running!!!

I did not run it personally, I was very young at the time maybe 8-10yrs old, but would never forget what happened. It happened on my father's 32' Permacraft during the ft. laud boat parade......in the 80's.....after the parade and some drinking they were running up the ICW, his buddy accidentally grabbed the port shifter and yanked it into reverse. Outriggers were in the down position, boat spun around......the outrigger knocked this old guys globe lamp off of his dock piling and it was hanging in the water, lit, by the wiring......and the old guy was screaming at them. While they surveyed the situation. Port engine restarted and knocked, but ran at idle and we limped home, I can't remember if my dad ran the port engine or just shut it off and we came home on 1 engine. Needless to say the transmission was fine and the running gear......the engine had bent valves and a bent connecting rod or two but did run at idle with a knock in it, that I remember. I do know it ran to the yard at idle on both engines with the knock because my dad was replacing them with 350 Mercruisers anyways. Needless to say that was the last boat parade we had the boat in after a decade of them.........

BTW Henning, yes it did still run with a few bent valves and piston rods. I did not say that it ran well.

Cavitation reduces the load on the prop, it is a bit like a slipping clutch in your car.

What does the weight of the prop have to do with it, if the setup is designed to run with that prop no matter what you do with the gearbox it should not be able to separate the engine mounts and do all that damage.

Where is the Propeller thrust absorbed in one of these little boats? I certainly do not think those engine mounts were ever designed to be combined thrust bearings and rubber mounts.

I am also in agreement with Hennings comments and observations regarding the general state of the rest of the engine bits and pieces we can see in the photos.

In a standard small boat setup the engine mounts take the thrust and transmit it to the hull. The thrust bearing in the gearbox transmits the thrust from the prop to the gearbox housing and since that is usually bolted to the engine it goes from there to the mounts and then to the stringers.

Unless you have one of those separate thrust bearing thingies or drive the gearbox through a Cardan shaft ala some V drives.

I don't buy Henning's position at all. Like I have repeately said, there cannot be more thrust astern than ahead no matter what or how quickly it is applied and those mounts would not all fail like that from astern thrust. Something external to the boat caused the loads that pulled that shaft back with enough force to cause that damage. If those bolts were so weakened they would have failed in the ahead direction when the throttle was advanced quickly or a seaway induced a higher load.

As far the old Chrysler, well duh, how about passing through neutral at full throttle let the engine overspeed enough to destroy the parts that typically get destroyed by overspeed. Putting it astern probably reduced the damage that would have occurred milliseconds later if it remained in neutral.

After spending some time looking at the photo of the broken bolt, it's a shame the ex-commander never got a metallurgist to look at those bolts. Judging by the looks of the adjusting nuts it is possible that over the years attempts to correct alignment issues resulted in overtorquing the bolts to the point of failure.

I agree with this assesment.It looks like someone lost a court case based on BS also the( thingy is a Kingsbury thrust bearing)

The thingie I was writing about is that thing marketed to the same folks who buy shaftsavers and retrofit dripless shaft seals.

Unless the boat has a direct reversing engine and was built 60 years ago the likelihood of it having a Kingsbury bearing is pretty slim. Even then, most smaller boats used a tapered roller bearing for that job.