Power loss question

[chuckaduck]

Yesterday we were doing about 20knts at approx 3500 rpm when momentarily the revs increased and the boat slowed

this happened a few times and lasted only seconds

we brought the throttle down to idle for a bit to let her settle and gruadually brought her back on the plane for a few minutes and she seemed fine again

if it was a car I would have said it was a slipping clutch

It is an inboard 3 litre mercruiser with a cobra outdrive

any Ideas ?

 

[orizaba]

sounds like it could possibly be blocked fuel filters as they can cause revs to rise mommentarily then return to normal ,when you throttle back suction drops in the filter some crap falls off the gauze and allows fuel to flow, and all seems ok til it builds up and does it again its worth a look,easier than looking at drives etc i suspect

 

[peterb26]

I think orizaba is completely correct - some sort of fuel starvation.

 

[chuckaduck]

only thing is if the revs rise then the speed should increase in this case the speed dropped

 

[Questor]

Cavitation or similar, maybe?

 

[DAKA]

Most likely cause is from seaweed.

Seaweed wraps around leg/prop

Propulsion is lost

Revs increase as there is no water resistance.

Seaweed slides off or is chopped up by prop and everything is well again.

Sometimes this is accompanied with a sharp increase in temp as cooling water is interrupted.

I think a school of jelly fish also did this to me once.


you can lift the leg to check nothing has fouled but what ever is on usually floats away as you lift it.

 

[brianrunyard]

How would a blocked fuel filter cause an increase in revs but a reduction in speed? Surely a blocked filter would cause a reduction in both revs and speed.
He stated that it's like a clutch slipping in a car.
So he's probably losing drive which takes to load of the engine and causes the revs to rise, weed round prop, prop dive bush slipping, etc.

 

[DAKA]

[ QUOTE ]
prop dive bush slipping, etc.

[/ QUOTE ]

I thought about that and agree could be a cause but I think it would be more evident during acceleration ?

I assume we are talking about typical petrol head and slow gentle acceleration takes place over 20m /forums/images/graemlins/wink.gif

 

[Kawasaki]

Not had it with an outdrive but had a similar symptom with an outboard.
Was the boss in the prop.
Gives the same feeling as the slipping clutch in a car as You mention.
The rubber type material inside the hub of the prop is designed to fail.
Saving damage to prop and gearbox etc if a coming together with summat solid happens.

 

[brianrunyard]

Good point, idle to WOT in one quick movement.
When it happened to me, if I was very gentle I could eventually get up to about 3/4 throttle, otherwise it slipped immediately.

 

[orizaba]

if the filters are partially blocked they can cause all sorts of strange happenings,as to why the engine revs should rise when the filter is blockedi think it maybe that the engine is not running at the right rev with a blocked filter as it would a clean filter,so you have throttle open slightly more to compensate,again crap of filter falls of or moves,revs increase,this is usually just a mommentary increase in revs then it dies back,but it will get to a stage that it dies back completely,then as i said in reply above, when you throttle back crap falls of etc etc,