Click for Delta Click for Alexseal Click for Elling Click for Westport Click for JetForums

Reversing Gears at High Rate of Forward Speed?

Discussion in 'Technical Discussion' started by Capt Bill11, Jan 17, 2010.

  1. Capt Bill11

    Capt Bill11 Senior Member

    Joined:
    Feb 27, 2006
    Messages:
    1,252
    ADMIN NOTE:
    This thread was split from another discussion.


    You would more than likely rip the engines off their mounts or at least break some of the mounts and drive line gear. Unless they had some sort of "Drivesavers". And in that case they would sheer and you'd just stop dead in the water.

    Or, as has been mentioned by someone else, the electronic controls would not let the engines go from full forward speed into full reverse instantaneously.

    But I don't see how the transom would come apart in any case.
  2. Capt J

    Capt J Senior Member

    Joined:
    Jul 11, 2005
    Messages:
    6,149
    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.......
  3. Capt Bill11

    Capt Bill11 Senior Member

    Joined:
    Feb 27, 2006
    Messages:
    1,252
    Well, here's the result when a Ford Lehman goes from fwd to reverse in the blink of an eye.

    [​IMG]

    [​IMG]

    [​IMG]

    Of course the charterer claimed this all happened while the engine was in fwd.

    The judge felt otherwise. :)
  4. K1W1

    K1W1 Senior Member

    Joined:
    Sep 30, 2005
    Messages:
    6,394
    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.
    Last edited: Jan 18, 2010
  5. Marmot

    Marmot Senior Member

    Joined:
    May 20, 2007
    Messages:
    3,001
    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.
  6. Capt J

    Capt J Senior Member

    Joined:
    Jul 11, 2005
    Messages:
    6,149

    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.
  7. Marmot

    Marmot Senior Member

    Joined:
    May 20, 2007
    Messages:
    3,001
    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.
  8. Capt Bill11

    Capt Bill11 Senior Member

    Joined:
    Feb 27, 2006
    Messages:
    1,252

    Well, there were no marks what so ever to the running gear, rudders or the bottom. (But once I pulled the boat I found the shaft bent at a point right between the back of the prop and the strut. As well as the strut being slightly bent.) But again, no marks indicating a strike of any kind on them. And I found the transmission in reverse right after the fact.

    The charterer claimed he was well within the channel at the time, (they always are) did not hit anything, heard a loud noise and immediately pulled the throttles back and shifted to neutral. ("NEVER into reverse".) As he was doing so he claims to have heard another loud noise. In any subsequent questioning he always claimed all this damage happened while the boat was in forward gear.

    When asked if that was all true how did the engine end up falling backwards after breaking off it's mounts, his answer was "well it must have fallen backwards due to the rearward slope of the engine on it's mounts". He also claimed that all this must have happened because the engine mounts failed due to "lack of maintenance". He never did have an answer as to how an engine with forward thrust on it could move/fall backward against that thrust.

    He finally admitted to me he was not running the boat at the time this all happened, his father in law was. And that he reached over when he heard the first "noise, bang" and chopped the throttles.
    Interestingly, the way the helm is laid out, 42' Grand Banks, if you are seated to the left of the person at the upper helm, you would have to push the person behind the helm out of the way and reach across them the get to the throttles on their far right side. But the gear levers are on the helmsman's left with in easy reach of the person on the left.

    When asked about this, the charter at first claimed he did not reach across or move his father in law out of the way. When I reminded him of the helm lay out, his story changed.
    Of course by the time we got to court much more of his story had changed. Including the part about his father in law running the boat. Suddenly only he was at the helm, never his father in law.

    He also had an "expert" surveyor as a witness with him in court, or should I say on the phone with him, who agreed with the way everything the charterer claimed happen had happened. And with the fact that failed engine mounts were the cause of all this. Even though the "witness" had never seen the boat before or after the indecent, or even seen any of the photos of the damage I had sent the charterer at his request. In fact in court the charterer claimed never to have gotten the photos or receipts for repairs I sent him at his request.

    There was internal damage to the transmission. Borg-Warner.

    Sad thing was, this man, (the charterer) was a retired Naval commander.
  9. YachtForums

    YachtForums Publisher/Admin

    Joined:
    Dec 22, 2002
    Messages:
    17,816
    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?"
  10. K1W1

    K1W1 Senior Member

    Joined:
    Sep 30, 2005
    Messages:
    6,394
    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.
  11. Henning

    Henning Senior Member

    Joined:
    Sep 22, 2009
    Messages:
    942

    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.
  12. Marmot

    Marmot Senior Member

    Joined:
    May 20, 2007
    Messages:
    3,001
    ... 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.
  13. Capt J

    Capt J Senior Member

    Joined:
    Jul 11, 2005
    Messages:
    6,149
    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.
  14. Henning

    Henning Senior Member

    Joined:
    Sep 22, 2009
    Messages:
    942
    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!!!
  15. Capt J

    Capt J Senior Member

    Joined:
    Jul 11, 2005
    Messages:
    6,149
    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.
  16. K1W1

    K1W1 Senior Member

    Joined:
    Sep 30, 2005
    Messages:
    6,394
    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.
  17. Marmot

    Marmot Senior Member

    Joined:
    May 20, 2007
    Messages:
    3,001
    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.
  18. Marmot

    Marmot Senior Member

    Joined:
    May 20, 2007
    Messages:
    3,001
    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.
  19. 61c40

    61c40 Member

    Joined:
    Feb 15, 2009
    Messages:
    73
    I agree with this assesment.It looks like someone lost a court case based on BS also the( thingy is a Kingsbury thrust bearing)
  20. Marmot

    Marmot Senior Member

    Joined:
    May 20, 2007
    Messages:
    3,001
    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.

Share This Page