Boat equivalent of wheelspin.

[So]

We took out today a boat with a Suzuki 40 outboard motor.

It went well until it ran out of fuel. Once refuelled it behaved oddly. At just under 5k Revs it would suddenly increase in revs but the boat immediately slowed.

Something appeared to be slipping.

Was this the propellor “slipping” in the water?

It happened at the same time as the sea was becoming calmer if it’s of any relevance. Tidal effect of some sort?

Newbie question I am sure. But I am quite interested.

[FiF]

Probably cavitation, or could be ventilation if you'd not lowered the outboard properly after refuelling.

https://uk.boats.com/how-to/propeller-cavitation-p...

[tr7v8]

Or the prop hub slipping under load. Designed to protect the driveline if the prop hts something solid. In the old days it was a shear pin but these days it is a bit more sophisticated.

[So]

Thanks guys.

It had been okay early in the day, but I did raise the engine up when we refuelled because it was near some rocks and I didn’t want to whack the propellor. But it was the hire chap who put it back down.

I did wonder if the engine had some sort of clutch. It was was a bit like clutch slip. I could think of no reason why running out of fuel would make a clutch slip though.

[Equus]

FiF said:

Probably cavitation, or could be ventilation if you'd not lowered the outboard properly after refuelling.

https://uk.boats.com/how-to/propeller-cavitation-p...

;yes:

Many years ago I was involved with Imp-engined racing hydroplanes, attempting to break the speed record for the class (which at the time was just over 100mph).

The Imp engine had such a peaky power curve, and such a lack of torque, that it wouldn't pull through the 'hump' and plane with the propeller pitch we needed to use. My solution was to cobble on the guts out of a Sodastream drinks machine, to blow CO2 into the the propeller, on demand - basically induced cavitation.

[gazzarose]

Outboards dont use any form of clucch. Some outdrive legs and inboard gearboxes do, but no outboards that I know of. The Suzuki 40 doesn't have a sheer pin, instead it uses, like most engines above about 2.5hp, uses a rubber bushed propeller hub, but if that had slipped then it would be happening at all rpm and you would struggle getting any sort of speed. What boat were you on? As others have said it is likely cavitation, either caused but too much trim up, the engine mounted too high, incorrect loading of the boat ie too much wait at the bow or a damaged propeller. None of which would be affected by running out or refilling of fuel.

[So]

gazzarose said:

Outboards dont use any form of clucch. Some outdrive legs and inboard gearboxes do, but no outboards that I know of. The Suzuki 40 doesn't have a sheer pin, instead it uses, like most engines above about 2.5hp, uses a rubber bushed propeller hub, but if that had slipped then it would be happening at all rpm and you would struggle getting any sort of speed. What boat were you on? As others have said it is likely cavitation, either caused but too much trim up, the engine mounted too high, incorrect loading of the boat ie too much wait at the bow or a damaged propeller. None of which would be affected by running out or refilling of fuel.

Could the boat have been a Suzuki too? It was a centre console design I believe with a flat canopy to keep the sun off. I think the clocks or maybe a badge had Suzuki on them / it.

I had the passengers on the foredeck so it could have been a weighting issue I guess.

[rolster]

I would suggest, if it was a hire boat and you ran the tank dry, then you probably pulled allot of dirt from the botttom of the fuel tank into the fuel filter blocking it up a bit. This would then induce a very rough running and cycling of revs as the fuel flow varied depending upon demand.

[So]

The “tank” was a plastic affair the size of a Jerry can, slung in the forward hull space.

The hire company neglected to mention it contained not enough fuel for 4 hours of motoring. Which was the duration of the hire. Perhaps I should have asked.