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

[Poly]

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.

[Cookee]

It's not blow out - the whole back of the boat goes sideways and believe me it gets your attention!

Waste of time going up 4" of pitch if you're not hitting top revs with the 15" so I don't understand how you can get the same RPM? Is the 19 a three blader and smaller diameter by any chance?

The holes in the prop are irrelevant at this speed

Is the boat porpoising at all?

Have a look to see if there is any pitting on the blades on the four blader.

Just my 2P worth!

[Razorbill RIBs]

Originally Posted by Cookee

It's not blow out - the whole back of the boat goes sideways and believe me it gets your attention!

Waste of time going up 4" of pitch if you're not hitting top revs with the 15" so I don't understand how you can get the same RPM? Is the 19 a three blader and smaller diameter by any chance?

The holes in the prop are irrelevant at this speed

Is the boat porpoising at all?

Have a look to see if there is any pitting on the blades on the four blader.

Just my 2P worth!

Thanks all for your responses....props really are proving a black art to me....

Ran some trials today....the effect goes away with the 19 pitch....

The revs definately show the same /similar with both props (as posted on earliuer thread can't get my head around this)....probably inaccuracy in the gauge (?) as has been suggested....therefore, concede it may be a rev limiter, although can't find any mention of a rev limiter doing a brief internet search....perhaps someone who knows could advise.

No pitting of the prop Cookee, and as above re the revs dia of both props the same....bizarre!

Anyway, relieved it doesn't appear to be an engine fault! ALTHOUGH, it threw a brief check engine alarm "SENSOR POWER" as i came off the water Typical Cleared now and hasn't reappeared...don't have my manual to hand so not sure which sensor triggered it....anyone know off the top of their head?

Back out tomorrow....

Thanks again to all

Dan

[Bigmuz7]

Call me a witch .. but (in the absence of being there) I think 15" is too small for that set up and in any case what was it... 15 x what ?

[9D280]

Jumping back to the tacho for a minute - what flavour of merc is it? I'm thinking if it;s one of these new fangled digital things, chances of the tacho getting the wrong signal are slim (it will take the tacho reading form the crank sensor signal rather than the slightly crude but effective counting pulses form the alternator)

I just calculated the ratio of pulses for an analouge tacho set to the wrong number of poles, and the theoretically indicated revs don't add up there either assuming a rev limiter at 5700. That of course doesn't prove the "needle to pulse count" accuracy.

If you can borrow a tacho, might be worth just sanity checking yours.

[Razorbill RIBs]

Originally Posted by 9D280

Jumping back to the tacho for a minute - what flavour of merc is it? I'm thinking if it;s one of these new fangled digital things, chances of the tacho getting the wrong signal are slim (it will take the tacho reading form the crank sensor signal rather than the slightly crude but effective counting pulses form the alternator)

I just calculated the ratio of pulses for an analouge tacho set to the wrong number of poles, and the theoretically indicated revs don't add up there either assuming a rev limiter at 5700. That of course doesn't prove the "needle to pulse count" accuracy.

If you can borrow a tacho, might be worth just sanity checking yours.

Tis an Opti....150

I have a spare smartcraft which if 'd thought I would have put on...good thinking batman!

Just got back from sea....all ran fine. The POWER SENSOR alarm turned out to be a dodgy speed wheel sensor....so disabled that and all fine. Running with 19 and no probs....guess it must have been rev limiter??

Was talking to a dealer earlier who said he's never been able to get a 150 Opti near the top revs of 5700....on tacho at least....

Possibly wrong attitude, but just pleased its all okay.....need to keep the throttle back slightly with the 15 pitch....not a prob when loaded up....

Incidentally with the paddle wheel now going caput, this means that I have had to disable both the pitot and the paddle wheel sensor as well as the trim sensor which has also thrown a wobbler....will get them looked at at some point /when I next go to a dealers...fragile aren't they!

Cheers Dan

[PeterM]

Originally Posted by Dan Worth

Incidentally with the paddle wheel now going caput, this means that I have had to disable both the pitot and the paddle wheel sensor as well as the trim sensor which has also thrown a wobbler....will get them looked at at some point /when I next go to a dealers...fragile aren't they!

Cheers Dan

Can you not tell the guage to read from the pitot at a lower speed ? My paddle wheel (and the depth) becomes worthless at anything over 10knots - so I have set it to switch from the paddle to pitot at 5knots .

I am pretty sure the opti rev gauges are accurate & instant ( not averaged) but with all the electrickery going on who knows !

[Razorbill RIBs]

Originally Posted by PeterM

Can you not tell the guage to read from the pitot at a lower speed ? My paddle wheel (and the depth) becomes worthless at anything over 10knots - so I have set it to switch from the paddle to pitot at 5knots

Sorry, wasn't clear....Pitot got switched off a while ago...kept throwing up check pitot and no obvious issue /blockage....got fed up and switched it off. Then the trim gauge went mental fluctuates betwen trailer and numbers (random number generator stylee ) so that got switched off....now the paddle wheel sensor has been causing alarms and the speedo to do strange things....have switched that off today and problem has gone away!

Will get them looked at when I get chance.....

D.

[Downhilldai]

Originally Posted by Dan Worth

...Was talking to a dealer earlier who said he's never been able to get a 150 Opti near the top revs of 5700....

Mine's the same - can't get it past 5100 RPM. The DDT readout confirms this too.

[Cookee]

Connect your GPS up to the Smartcraft and tell it to use that - paddlewheels and pitots are not that great anyway!

[Bigmuz7]

Originally Posted by Cookee

paddlewheels and pitots are not that great anyway!

Yes thats been my experience too