Is this 'Blow Out'?

[Razorbill RIBs]

We've been running the 15 pitch 4 blader today with a full boat (non dive party). The extra hole shot with the swell and rough bar proved really useful!!

However, on the run back into the estuary we had (the only) opportunity to go WOT. When it does run WOT it does a strange 'surging' of power (on and off type frequency) and changes engine tone (grumbles) as it does it....I would suggest it changes rpm by about 100rpm up and down between 5100rpm and 5200rpm (roughly) seems to go away if you drop off WOT.....No engine alarms or obvious mech probs....is this what I've heard refrerred to as 'blow out' of the prop?

Intend to stick the 19 on tomorow (tide wouldn't allow us to stay afloat today) and see if it goes away.....

Any one able to shed any light on this?

Thanks v. much

Dan

[BogMonster]

What is the max rpm the engine is supposed to do - it isn't an engine rev limiter cutting in and out is it?

[Tim M]

Sounds very much like the rev limiter kicking in

[Nos4r2]

Originally Posted by BogMonster

What is the max rpm the engine is supposed to do - it isn't an engine rev limiter cutting in and out is it?

Unlikely at 5100 rpm.

[chewy]

Does it happen when your climbing a wave or any particular sea state?

[Razorbill RIBs]

No, not rev limiter, but that sort of effect....tech literature gives max RPM as circa 5700rpm IIRC. It doesn't happen at any particular point on wave etc. and only at WOT.....power back a little and goes away....

I was sure i'd read about this on a boat forum (hence the 'blow out' query), but buggered if i can find it.

We opened up to WOT on the final run in to the estuary as it had flattened out and allowed...tide was on a strong ebb (not that I think that has any bearing).

Strange one...going to run out with the 19 tomorrow and see if it still happens....I'm leaning towards it being the prop casuing it as no engine faults and it ran faultlessly otherwise.

If cookee or another pefromance ribber is looking in, is the 'blow out' something the racing guys struggle with or did i imagine that too?

D

[Bigmuz7]

Originally Posted by Tim M

Sounds very much like the rev limiter kicking in

Sounds like that to me too and your gauge just doesnt read properly for some reason.. does the prop have gas vents ?

[Razorbill RIBs]

Originally Posted by Bigmuz7

Sounds like that to me too and your gauge just doesnt read properly for some reason.. does the prop have gas vents ?

Assume that means the holes on the hub? If so, the SS Laser II 19 pitch does, but the (new) ali 4 blader doesn't....

Would have thought the tacho was okay? It's a new smartcraft one....shows similar revs with the 19 and no rev limiter type issue there??

Also, found the thread where i'd read about blow out...hope it's okay to link to Boat Mad here? http://www.boatmad.com/forum/showthread.php?p=177430

This is on an Opti 200....dead similar /same symptoms....

Will swap over to the 19 tomorow and see what happens

[Polwart]

Originally Posted by Dan Worth

Would have thought the tacho was okay? It's a new smartcraft one....

Is it damped though? So you are actually getting the "average" revs over maybe the last 20 seconds which isn't showing the peak (because the limiter has cut in?)

[jwalker]

It could be a number of things, rev limiter is the obvious but could also be fuel starvation or other engine problem. It's not blow out. Blowout occurs when the water can no longer follow the surface of the gearcase and prop hub and can be sudden. Effective drive from the prop ceases and it is a dangerous and undesirable situation. It normally only happens at very high speed.
Fuel starvation is my guess.