Weird engine behaviour

[Matthewb]

OK, this is the scene:
KAD32, runs for 30 mins around the harbour, engine's sweet - no problem.

Out into a head sea, open her up, straight onto the plane, 25 knots for 2 or 3 miles - no problem. (SWMBO was even smiling at this point).

Turn around, following sea, fairly lumpy, planing at 17/18 knots, revs suddenly drop to 2000 ish, stern digs right in, I try to drop the nose with t/tabs, engine suddenly screams back to life without me touching the throttle, revs back up to 3200. Eventually the boat speed increases but now I'm going too fast for the conditions (SWMBO has stopped smiling) so ease off slightly, stern digs in, falls off the plane again, revs drop suddenly...... and on and on it goes. Get back onto the flat in the entrance to the Harbour and she's behaving perfectly.

Someone said its either a) Cavitation or b) Dirty Hull. Either way, no amount of t/tabs or leg adjustment seems to solve it. The disconcerting thing is the way the revs just seem to die and then rocket back up again.

What does the forum think? Is it a sea conditions/boat trim issue or is the engine playing up?

Matt

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[camargue]

Hi Matt,

sorry for your problems

Have you left your boat for a while wit diesel tanks half full, or nearly empty? it could be that algie you get in your dieseltanks is cloggging your fuel filters up. i have a big problem with it on mine. not suprising with 2000 litre tanks. brought it from sunseekers with a problem, and had the tank completley drained two or three times. still didn't completly solve the problem though!
My boats fine, and then all of a sudden, it will looses revs, and then regain them. if it is this diesel algie, i suggest using 'fuel set'. The other weekend the engines were playing up, so when we filled up, we put a good dose in, and it was fine. we weere doing 32 knots with a dirty hull!!

Not bad

Good luck

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[stamfordian]

S******n is on special offer at as*p supplies if diesel the problem,although from yyour discription it isn,t,more likly speed in following sea,but you don,t say if this problem has just started or is an ongoing one.

<hr width=100% size=1><A target="_blank" HREF=http://www.arweb.co.uk/argallery/stamfordian>http://www.arweb.co.uk/argallery/stamfordian</A>

 

[longjohnsilver]

I agree, sounds like fuel problems to me, suggest you take a look at the filters and change them.

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[Matthewb]

But if its a fuel problem, how come I ran for 2 hours at 8 knots around the harbour in the morning and everything was fine, not a single hiccup. Stopped for lunch and a snooze, ran for another 30 mins at 8 knots, then say 20 mins at 25 knots with no probs until I turned round for home?

Fuel is a week old so I can't see it's contamination. But I will get the filter checked out to ensure there's no blockage.

It has happened several times before but always, I think, when there is a bit of a chop. Maybe a coincidence coz it always seems to be a bit lumpy every time I go out...!!! As soon as I got into calmer water the problem seemed to go away.

I guess I need to carry out more scientific testing and scribble some notes as it happens so I can remember the precise sequence of events.

Matt

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[camargue]

the algie might have been partly blocking the filters, and in the harbour at eight knots, the fuel demand may have squeezed through the filter sufficiently.
Doing 25 knots out at sea in rough weather will have shaken the tanks up and broughtup some more algie from the tank, and clogged up the filters more. i found that it got so bad that i was down to 16 knots, with the throttles set the same, the revs rose and fell by about 1000. thats a lot considering the big lumps only do about 3500 at a push.

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[Moose]

Blocked filters will cause that because the engine demands more fuel when it is running faster but the blocked filters prevent the required amount of fuel reaching the engine fast enough. At low speeds the demand is less so the fuel can be delivered correctly.

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EL MOOSEO

 

[plombier]

Anyway of checking if the fuel pick-up pipe in the tanks is sound. Sounds like fuel surge if it only happens in a chop but OK when in smooth water. IMHO

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[jfm]

i think it is all fine

Matthew:
I spect there's nothing wrong. Your fuel filters must be fine because it will blam along for hours flat out. I think you are just driving at the spot where your boat drops off the plane. If you slow it to this speed and have a tiny bit too little throttle, it will slow down till it just falls off the plane. Then the stern digs in etc, all the symptoms you say. Playing with the tabs just gets it back on the plane. Once it is on the plane again it will accelerate very significantly without you touching the throttles. This is all totally normal. You have to work the throttle a bit more so the boat has enough speed to keep planing, and if you feel it coming off and the stern sinking, step on the gas to keep it planing. else throttle right back and go slow. The boat wont be happy at the point where it's nearly but not quite planing. you need to go faster or slower than this speed. all fast boats are like this, to varying extents.

Of course I'm guessing from your description. If you take an experienced fast boat driver out with you they could confirm whether what I say above is your "problem" instantly.

(Anyway, at least my diagnosis is cheap to fix)

 

[Matthewb]

Re: i think it is all fine

Ummmm - looks like I have to do two things then:

1) Check out the fuel filter before I next go out.

1) Eliminate the speed/trim possibility - as you say it might be the quickest/cheapest. I'll try to drag a pal out who has more experience and see if he thinks I am just being a bit too timid in choppy seas at that critical speed between planing and not quite.

Thanks for the advice. I'll let you know how I get on.

Matt

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[jimh]

Re: i think it is all fine

this is my diagnosis too. Can be overcome with careful throttle control - lots of frequent adjusment to keep her on the plane. You can normally hear and feel when the boat slows and the revs drop, and similarly when she races away and the revs rise, these need to be counteracted with more / less throttle.

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[Doffy]

Re: i think it is all fine

Try going slighty across the waves to give you a longer run at the flatter water.


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[[2068]]

Re: i think it is all fine

Hi,

I've found that in following seas I need to "work" the throttle fairly assertively to prevent the effect you are describing. What I do is to use almost full throttle as I climb up the back of the wave, then ease off substantially as I surf down the face of the wave: that way you can keep an (almost) constant speed.

If you leave the throttle at a constant setting, what happens is that as you climb the wave, you slow down, the revs drop, and below about 2500rpms the Turbo isn't much use, so the revs drop more, down to 2000rpms or so. Finally the compressor (supercharger) thinks it would be a good idea to kick in, big shove of torque, lots of acceleration, too fast !, you back off the throttle, the revs drop... rinse and repeat.

dave.

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[Matthewb]

Re: i think it is all fine

Dave,
Maybe if you're down on yours in the next couple of weeks I can coax you into coming out with me for 30 mins so I can show you what happens and try out your remedy. I'll make sure I've got some of your favourite tipple onboard !!!

Matt

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[h4nym]

Re: i think it is all fine

Blimey - u sea folks! Into ur hard work aren't u!

When I've been out on the big water, if it gets lumpy I slow it down. Much easier to go thru the water than to try and beat it!

H

<hr width=100% size=1>Life balance?

 

[BrendanS]

Re: i think it is all fine

Even slowing down doesn't mean you don't have to work the throttle. Some boats respond better, some less.

After being out in conditions which meant some seriously steep high waves, some book learnt techniques came into their own, alongside learning as I went.

When waves are taller than your boat, you zig zag. Throw helm over left as you go down, as you reach bottom, throw helm over right, power on as you go up. Power off as you go over the top and.....all this at displacement speed. I probably never went over 5 knts. Front on was horrrible.

It all made sense after a while,and was quite easy going. This approach assumes you have plenty of fuel, though slow displacement speeds are usually quite fuel efficient and you really had to be able to power up the wave, or it would be quite dangerous

The upside is that you don't get any slamming or damage. Saily types move at this sort of speed most of the time, and you get there eventually. Much easier to go with the flow, and just take what is thrown at you , rather than try to force the boat to go faster than feels comfortable

What I hate is slamming conditions, where the waves are so short and steep that you're going to hit hard. In some ways, big swell is easier to cope with

<hr width=100% size=1>Err, let me know if Depsol enters the forum, I'll go and hide