sudden engine stop when shifting to reverse

[Ken Heyman]

Twice this season, upon shifting from neutral to reverse I have heard a pronounced transmission "thunk" and the engine stops. I'm able to restart and all is well but I have no idea as to the cause. It seems that this happens when I have a bit of momentum (perhaps a couple of knots or more). I believe I am shifting correctly by at the very least pausing in neutral if I was previously in forward gear. Since I bought this boat, I have had a minor vibration or rattle at low RPM that disappears in the normal cruising range. A mechanic thought this rattle might be the shaft slapping the shaft "slapping" the hull at the exit hole. My Cutlass Bearing was new two seasons ago and has plenty of life left. I probably do need to align the shaft but that shouldn't cause this symptom--should it?

What can cause this kind of seizure?

Thanks in advance,

Ken  

[Jeff McKinney]

Obvious things (at least to me):

"Slap" problem-Is the prop balanced? Is the shaft straight? The alignment may be a culprit, although this should not improve when accelerating if it is the cause

Stalling-too few RPMs at time of shift, too many RPMS & not  enough time for the shaft to slow down. Both could overload the engine.

[Stu Jackson]

Ken

Going into reverse while moving forward, even with the pause in the middle, is just like catching a line in your prop or on your shaft.  Most of us have already done that and know the engine stops.

Question is why you need to go into reverse when moving fast enough for that to happen.

In unusual circumstances, like an emergency, it's good to know that that procedure (chuncking into reverse) just doesn't work.

[Ken Heyman]

Thanks all for the response. Stu, while I expect your question was rhetorical--"Question is why you need to go into reverse when moving fast enough for that to happen. " While steaming in the harbor at maybe a couple of knots, we changed our minds as to where folks would "de-boat".
This required a bit of deceleration but nothing dramatic. If I would have advanced the throttle from idle to perhaps 800 - 1000 RPM would this have made a difference? While not frequent , we occasionally do find ourselves using reverse at forward speeds of 2 knots or slightly more. (boat parades etc.) I always throttle back to idle.
The Universal Engine manual points out that in an emergency it is permissible to shift to reverse at high RPM's. I assume of course that this is a poor general practice but it is good to know that this will not ruin your transmission.

Ken

[Randy Stolze]

Ken,

        "Idle speed" in my boat is 1000 rpm. Anything less than that and the engine starts to shake, rattle and roll. I have never had a problem like you describe shifting at that rpm.

         I think what the manual is referring to as high speed emergency maneuvers is cruising speed directly to reverse as in I gotta stop right now.


   Smooth Sailing,

   Randy

[Ron Hill]

Ken : I think that Randy has nailed your problem.  Engine idle speed.

I find that if I'm much above 1500 rpm the shifter won't come out of it's gear unless you reduce throttle.  The other point is that when you shift into an opposite gear while the boat is moving more than 2 kts. , it's one hell of a strain on the engine.  

Next time you try that maneuver, make sure that your engine idle in neutral is about 1000/1200 rpm - if you can't slow the boat speed to under 2 kts.  

[Gary Ambrose]

Here, in Maine, we frequently pick up sea weed ( we call it salad) around the prop. The symptom is a slight increase in engine temp and a slight decrease in boat speed at the same throttle setting.

The remedy is to drop engine rpm, slide the transmission in reverse, and increase the throttle. The seaweed is thrown off.  Sometimes we hear a thunk as the weed is thrown up against the hull. All systems can then return to normal operation. There are occasions when doing this that the engine can stall because of the seaweed binding around the prop. Restart, and a little extra throttle always clears the prop.