Ventilation or slipping hub?

HughN · 21 March 2014, 14:34

I know you shouldn't really do it but today I pushed the throttle forward really quickly. The revs went up, there was a bit of froth and no forward motion - I had to back-off and then open the throttle slowly.

It hasn't done this before and was OK at speed. Is this a situation where a slipping hub might be more apparent or have I been trying to spin the prop in a bubble of exhaust gas or something?

Mariner 30, 2 stroke, 2 cyl.

Revs at full throttle are 5,680.

Festinghouse · 21 March 2014, 14:45

is it on a sib? sounds like ventilation to me which is much more common on a sib due to the movement of the transom

A1an · 21 March 2014, 16:38

Sounds like a hub issue to me.

Draw a reference mark on the prop and bush with a felt pen, do the same full throttle thing again then check your marks, if they don't line up you have your answer.

HughN · 21 March 2014, 18:15

Quote:

Originally Posted by A1an

Sounds like a hub issue to me.

Draw a reference mark on the prop and bush with a felt pen, do the same full throttle thing again then check your marks, if they don't line up you have your answer.

A very fine plan - just need the weather to sort itself out for a re-test!

9D280 · 21 March 2014, 22:15

While you wait for the weather have a close look at the inner and outer diameters. If it looks like the rubber is extruding from the join(s) then chances are it's a gonner.

SPR · 21 March 2014, 23:40

props are not that expensive - and good to carry as spare - so if you can I would suggest buying a spare...means if it is you have spare to stay in water...

Binty_Bexter · 22 March 2014, 01:20

Cavitation, not "ventilation." But it sounds like the hub has stripped. Can be repaired.

HughN · 22 March 2014, 17:59

Mystery solved - and personal prejudices reinforced.

With the boat I inherited a plastic prop (you know where this is going, don't you!). It was used from a shingle beach and replaceable blades seemed a good way to mitigate the inevitable dings.

The plastic boss "fuses" in the hub have broken. I may have hit something at one point - there is a lot of rubbish in the river at the moment.

The really bad point is the fact that the exhaust ports were occluded by the aft portion of the propeller.

A steel propeller has been added to the shopping list.

jyasaki · 23 March 2014, 00:19

Quote:

Originally Posted by Binty_Bexter

Cavitation, not "ventilation." But it sounds like the hub has stripped. Can be repaired.

Actually, ventilation, not cavitation.

Ventilation is gas (either air or exhaust gasses) getting mixed into the water around the prop blades, with a subsequent loss of hydraulic force.

Cavitation is a low pressure area that repeatedly forms and collapses a gas bubble. Forces can be pretty high, enough to erode metal. Cavitation does not cause a loss of thrust, I believe.

jky