reversing with large outboard

I only have experience of small outboards which reverse by rotating the whole unit in it's mount. I presume large outboards kick up if they hit an obstruction so I guess they use something like a shear pin to lock down in reverse which is automatically engaged when selecting reverse. Is this correct?
I realise the answer must be completely obvious to those who use them.

 

yes there is a reverse lock which releases once you engage forward to allow kick up if you hit something.

 

Just remember that an outboard may not just kick up when it hits something as the propeller is driving the gearbox forward when still under water.
A greater force than than that of the propeller thrust is required to make the outboard kick up and that is a substantial force. You can often have the option of locking the drive down ..or not.

 

Motors with hydraulic trim and tilt, which would be virtually anything over 50 hp, don't kick up when striking an obstruction, nor do they kick up when shifted into reverse.

 

The trim and tilt on modern out boards is all hydraulic so the old hold downs are now the rams that hold the outboard in what ever position you want it to be for or shallow you can trim till the prop is just about out of the water and put it in gear and slowly drag the boat !! a touch further down and you get more bite if you trim a little in reverse it has the tendency to actually lift the back up a little and forces the water under the hull !
I LIKE THE Yamaha set up with twin rams for the trim and slow action and tilt is just one ram and comes up quickly !! :idea:

 

Good points . I forgot about power tilt. Tom if you ever hit anything with an outboard at speed you would know that they do kick up. Even the older power tilts kicked up. They had seperate trim rams and the lift rams only had check valves on the bottom side. Thats what i thought anyway but i may be wrong.

 

I believe there is little or no difference between big outboards and stern drives.

One of my summer guests tested the kick up capability of his Yamaha stern drive when he wanted to experience the "planing at night feeling". Barely 80 yards from his point of departure he hit a submerged rock that tore a gash of 10 ft in the hull.

The boat got over the rock, so the stern drive did kick up, but there was considerable damage. The hinge pin sheared off the steering arm, parts were torn off the skeg and prop and the upper housing had several cracks leaking oil, so in fact the drive was total loss.

This was a stern drive comparable with an Alpha-1. The mass of the leg and the limited throughput of the hydraulic hoses and tubes makes it impossible to kick up without substantial damage. For larger drives the chances of survival are even slimmer.

 

the impact protection is in the cylinders of a sterndrive so no oil needs to move through a hose adn it will then go back down to where it was.

Outboards have have several methods over the years but currently all impact in the tilt cylinder and they wont go back to where they were

 

The stern drive I mentioned in post #7 had a valve operated by the gear shift lever on top of the engine, similar to the pre-Alpha1.
I sure like to know how the protection in the cylinder works. With oil on both sides of the piston it would require a very clever valve arrangement.

 

Thanks guys. So it seems like it's a pin, automatically engaged, for outboards which can be tilted by hand, and a valve in the tilt mechanism of hydraulically lifted outboards, which might not protect against a fast impact.

 

Hi CDK
Pre Alpha 1 with trim had the valve to prevent the drive coming up in reverse but it still had impact protection as the oil cant get up the pipes fast enough.
All stern drive trim cylinders work as follows:
There is a piston floating in the cylinder behind the piston in the trim rod.
The trim rod piston has balls and springs in it, heavy on one side and light on the other.
Hit something and the heavy springs let the oil go through the piston and bleed behind it to the floating piston that wont move.
If there is any prop thrust on the drive after the impact the light springs now bleed the oil back still the trim rod piston travels back to the floating piston that is in the original position, simple and it works.
To prevent the drive coming up in reverse the trim pump has check valves.

 

Powerabout, thanks for the explanation.
Found some drawings here: cemrweb.cemr.wvu.edu/~mathews/boat/pdf/.../Chapter%20Eighteen.PDF

Mercuiser warns you not to take the cylinders apart, so I took them for single piston ones, but it seems they really tried to offer some degree of impact protection. I guess they had a sandy beach in mind, not the rock this night rider guy encountered.

 

in that doc you can see the floating piston, item 2 in the exploded view