Rib ran aground

brigsuri · 15 January 2012, 03:50

Aloha,

Ran new 15 hr on 50 hp Yamaha Zodiac 500 bay runner onto soft sand and was able to drag rib back We took about 6 inches of water and the fuel water filter was submerged

Boat restarted but was revving and moving very slowly for 30 mts It started running fine after all the water had drained out and the fuel water ful separator was no longer submerged. on land no engine external damage and no water inside engine Have not checked fuel water separator yet

Question is why did the engine rev and boat not move for all most 30 mts

Appreciate any input

Mahalo!

Suri

Richard Selman · 15 January 2012, 07:13

Prop slipping on its bush after screwing through the sand?

Perhaps when the weight of the water was out, the load was reduced enough to allow it to just grip.

If this is the case, if has slipped badly the bush will have been worn down will go again easier the next time. Each time there is less chance that it will grip. Its probably done its job and protected the engine. Time to get it rebushed or replaced.

willk · 15 January 2012, 10:24

Yes, if you hit the sand while the motor was in gear, then you have most likely done as Richard says. Lucky you - you got away with it. You'll need to get the prop replaced or rebushed ASAP - it will just keep getting worse and let you down some day when you need it most (gunning the engine up big swells, for example).

The fuel system should be fine, but check it out anyway. Those filters are air and gas sealed, so seawater shouldn't get in at surface pressures.

Poly · 15 January 2012, 10:50

Quote:

but was revving and moving very slowly for 30 mts It started running fine after all the water had drained out

I've read this differently from Willk & Richard. I read this as: revving slowly AND moving slowly. They have read it as revving properly BUY moving slowly. Their interpretation is consistent with the prop bush having spun (and saved your gear box). If the engine was revving slowly I would suggest the prop bush is probably fine but you were just asking the engine to try and lift an extra tonne of water onto the plane with you - thats probably something like 3x the load it has to deal with in normal running so its was labouring.

willk · 15 January 2012, 11:23

Quote:

Originally Posted by Polwart

They have read it as revving properly

Actually Miss Cleo, I read it as "revving". When ordinary people say "revving" - they mean "vroom vroom". You would probably call this overrevving.

Consistent with our diagnoses.

BTW, my calculations suggest a maximum of 0.5 metric tonnes of water , but I'd reduce that by an E-Factor of 50%.

jwalker · 15 January 2012, 11:38

If the water in the boat was about 6" deep throughout the boat then you"ll be carrying over half a ton on water when driving off before the boat empties. Too much to ask of a wee 50hp motor and it's propeller. It's likely the prop just broke away from the water because it couldn't drive that load. The prop had too much pitch for the load you were carrying.

Poly · 15 January 2012, 16:24

Quote:

Originally Posted by willk

Actually Miss Cleo, I read it as "revving". When ordinary people say "revving" - they mean "vroom vroom". You would probably call this overrevving.

Consistent with our diagnoses.

BTW, my calculations suggest a maximum of 0.5 metric tonnes of water , but I'd reduce that by an E-Factor of 50%.

Willk bloomin eck you like to dispute the minutia don't you!

The difference depends on a comma which wasn't present "It was revving, and moving slowly" (i.e. move throttle and get lots of noise but no speed) or "it was revving and moving slowly" (i.e. move throttle and it doesn't get to max rpm). Of course internet punctuation may mean your interpretation was absolutely reasonable.

A tonne was a guess, and 6" is probably more like half a tonne (but still a big load on the engine) but 6" of water wouldn't normally cover a fuel separator so it may well have been more.

Nos4r2 · 15 January 2012, 16:37

Quote:

Originally Posted by willk

Miss Cleo

Some days I wonder about why you know some of the stuff you say, though I do wonder if it's as bad as being drawn into watching zit popping vids on Youtube.

Poly · 15 January 2012, 17:11

Quote:

Originally Posted by Nos4r2

Some days I wonder about why you know some of the stuff you say, though I do wonder if it's as bad as being drawn into watching zit popping vids on Youtube.

It doesn't get any better when you meet him in real life either... ...in fact it gets more worrying.

Richard Selman · 15 January 2012, 17:47

Quote:

Originally Posted by jwalker

If the water in the boat was about 6" deep throughout the boat then you"ll be carrying over half a ton on water when driving off before the boat empties. Too much to ask of a wee 50hp motor and it's propeller. It's likely the prop just broke away from the water because it couldn't drive that load. The prop had too much pitch for the load you were carrying.

I would have thought with half a ton extra load, the prop would have been well submerged and able to grip if it could. Hull was not moving much so it would not have disturbed the flow. Engine would have just lugged if the bush was not slipping, no?

Sounds like the OP did not just strike the engine on the bottom but properly ran-aground (not the only one this week-end!). Prop might have had a good run through the sand.

willk · 15 January 2012, 17:48

Help! Help! The Admin Team are picking on me!

I would like to take this opportunity to correct my usage of the E-Factor. It is a "multiplier" and so should have read E0.50

You'll see a bit more of it over the coming months:

For top speeds in RIBs it's generally E0.85
For sea state experienced by RIBbers it's usually something similar
For fish that got away it can be as low as low as E0.20

A rough rule of thumb for the Exaggeration Factor is the amount that you feel tempted to embellish a story recounted down the pub...

BTW, I've covered fuel filters on my old transom, maybe 8 inches, and kept my feet dry at the controls. The water gathers there.

brigsuri · 15 January 2012, 18:01

Thanks everyone!
To clarify revving at 3500 rpm and was moving at 9 mph for 10-15 mts then slowly gathered speed to 20 mph at same 3500 rpm without me touching the throttle. I was able to rev to 5000 rpm and obtain top speed of 30 mph after that

No external prop damage Hit sand at lowest speed before shutting engine off manually Also 6 inches of water was taken in and I estimated about 20 gallons maybe?

Where I live it may be cheaper to buy a new prop

willk · 15 January 2012, 18:07

Hi,

That just sounds like the weight of the water* onboard holding you back off the plane.

* E0.15

Erin · 15 January 2012, 21:22

Quote:

Originally Posted by Polwart

The difference depends on a comma which wasn't present...

...which can be quite fundamental sometimes. I recall a book on the subject called 'The Panda eats, shoots and leaves'

Quote:

Originally Posted by Wikipedia

A panda walks into a café. He orders a sandwich, eats it, then draws a gun and proceeds to fire it at the other patrons.

'Why?' asks the confused, surviving waiter amidst the carnage, as the panda makes towards the exit. The panda produces a badly punctuated wildlife manual and tosses it over his shoulder.

'Well, I'm a panda,' he says, at the door. 'Look it up.'
The waiter turns to the relevant entry in the manual and, sure enough, finds an explanation. 'Panda. Large black-and-white bear-like mammal, native to China. Eats, shoots and leaves.'

jwalker · 15 January 2012, 23:50

Quote:

Originally Posted by Richard Selman

I would have thought with half a ton extra load, the prop would have been well submerged and able to grip if it could. Hull was not moving much so it would not have disturbed the flow. Engine would have just lugged if the bush was not slipping, no?

But it doesnt work that way. The prop is under maximum load when the boat is stationary and the throttle is opened. The load reduces as the boat accelerates. The faster you go, the lower the load on the prop blades.

dnv · 16 January 2012, 06:42

Quote:

Originally Posted by jwalker

But it doesnt work that way. The prop is under maximum load when the boat is stationary and the throttle is opened. The load reduces as the boat accelerates. The faster you go, the lower the load on the prop blades.

That's bollard-pull condition, the design point for tugs.

Sent from my iPhone using Rib.net

Richard Selman · 16 January 2012, 06:57

The maximum load on the prop (or torque on the bush) would occur when the engine is delivering the most power - which would normally be at high revs for an outboard. Exception might be for very dynamic load changes such as changing from turning in water to hitting sand or rock. If the power absorption of the prop at the given water speed and shaft speed is greater than the power output of the engine at that shaft speed then the boat will not accellerate and the engine will not climb up its power curve.

We normally don't have much excess power at low speeds and therefore are not much use as tugs despite our relatively large engines.

I would have thought a proper tug would be set-up to deliver its maximum power at a much lower water speed. You presumably don't use all your available power at your maximum speed when not pulling anything.