pulling a boat through the water

johnyp · 15 June 2018, 06:44

Hi is it true that you can damage an outboard if left in the water when being towed and not running,

Regards
Johnyp

charliee · 15 June 2018, 06:47

Why tow unless it’s damaged already? I have no idea but would doubt it.

Mikefule · 15 June 2018, 07:42

It would certainly cause additional drag, and the propellor would act like a turbine and that in turn* would put some pressure on the gear box. I would doubt that towing at a sensible speed (say behind a displacement hulled boat at 6 kts) for a while would cause any damage, but my question would be "why?" Keep the drag down, raise the engine, or, if it's a small one, remove it.

*In turn, turbine, ho ho.

Max... · 15 June 2018, 08:21

Make sure it's in neutral if you do!

Striker · 15 June 2018, 08:33

Quote:

Originally Posted by Max...

Make sure it's in neutral if you do!

Won’t having it neutral will increase the drag due to the vortex when it freewheels?

Lots of sailing yachts have prop shaft brakes to lock the prop when sailing.

Why not just tilt the outboard?

Pikey Dave · 15 June 2018, 11:50

Having the casualty engine slightly in the water to create a touch of drag can be a good thing if it's lumpy. It acts like a drogue & keeps the tow straight.

beamishken · 15 June 2018, 13:02

I rebuilt a 90 honda years ago that was full of water after being left down on a twin rig going home on one engine after a breakdown. fault was fairly minor but being ran in the down position filled the engine with water and it hydrauliced when the owner tried to start it

jeffstevens763@g · 15 June 2018, 13:31

you would need to fix it in the straight position or it will vear off and act like a rudder.
dont see any problem down but if getting home in a hurry best up less drag less fuel used by towing boat IMO

jwalker · 15 June 2018, 18:09

Quote:

Originally Posted by beamishken

I rebuilt a 90 honda years ago that was full of water after being left down on a twin rig going home on one engine after a breakdown. fault was fairly minor but being ran in the down position filled the engine with water and it hydrauliced when the owner tried to start it

I'm with beamishken. It's possible to flood the engine up through the exhaust when it's being dragged through the water. Tilt it up.

johnyp · 15 June 2018, 18:29

Thanks guys
It was a pal of mine who has a 25 foot sports cruiser and wanted to tow mine with the engine down I said I had heard it could cause damage and tilted it up.

So thanks for confirming this.

Johnyp

Mikefule · 15 June 2018, 21:04

Well, I may never need to use the knowledge, but I've learned something.

jeffstevens763@g · 15 June 2018, 23:12

How does the engine flood then though the prop?

beamishken · 16 June 2018, 11:28

Quote:

Originally Posted by jeffstevens763@g

How does the engine flood then though the prop?

Not sure how the water gets up into the engine ( suspect via the exhaust) but having seen this honda powerhead destroyed by being in the down position id always tilt it up

Mikefule · 16 June 2018, 14:53

Quote:

Originally Posted by beamishken

Not sure how the water gets up into the engine ( suspect via the exhaust) but having seen this honda powerhead destroyed by being in the down position id always tilt it up

I hadn't thought of it before, but it makes sense now I think of it.

When the engine is running, the exhaust gas is under pressure coming down the exhaust and venting below the waterline. It is not possible for water to go the other way.

When the engine is not running, for a 2 stroke, the exhaust port may be open, and for a 4 stroke at least one of the exhaust valves will be open or partly open. With no exhaust pressure to stop the water coming up the exhaust, there is a danger of water pressure caused by towing causing water to rise up the exhaust and enter the cylinder.

It would only take a small amount of salt water to cause corrosion to steel piston rings, and only a small amount of water of any kind to cause mechanical damage. The engine is designed to compress air and a fine mist of fuel; liquid water is not compressible.

Lee argyle · 16 June 2018, 15:11

Jet boats / skis can’t be towed at speed with the engine off, water can be forced into the engine via the open exhaust valve - you can in emergencies clamp the water intake pipe to stop the above happening

jeffstevens763@g · 16 June 2018, 18:18

I guess the twin rig ken mentions would have been on the plane I can see that but with a low speed tow not so sure but lift it up the way to go. One thing towing we learnt with our 5 m sit rescue boat very lightweight it was towed too fast and flipped over.