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Old 28 June 2008, 15:45   #1
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First day on the water :)!!

Hi Guys

Replaced the plugs and a few bits and pieces today, she went into the water! Thanks for your advice on a previous email guys!

Shes a 5.25 osprey with twin 60hp 3 cylinder mariners '96.

Advice here would be great -please.
One engine tops out at 5000rpm and the other 4700rpm, as the engines are old would it matter trying to squeeze the last 500-700 rpm out of them? (one is a little bit smokey). Someone on the site said they should be at 5500rpm.
Props are currently 15p. One is damaged, all blades are intact but 2 out of three blades are out of shape not horrific but noticable. This is also the engine with 4700 rpm not sure if thats related to prop damage. But its also the weaker engine (smokier)
Boats currenty doing 28/29 knots

If I go to a bigger prop eg 16p, will that increase top end speed but reduce accelleration but also increase fuel economy, find the prop side confusing

Any advice welcome

Still smiling from the day out

Colin and Elizabeth
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Old 28 June 2008, 16:02   #2
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the 300 rpm could be related to the damaged prob.
how smokey is smokey? and any idea why? are these 2 strokes? autolubed or mixed with the fuel (and do both engines get fuel from the same tank?)
is there any possibility that the difference is in the gauges not the engines?

not sure it makes much difference but are they counter rotating?

If you change the prop pitch up (from 15" to 16") I would expect that the engines find it "harder" to turn the prop and so revs will drop. dropping pitch might actually get you more speed.
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Old 28 June 2008, 16:32   #3
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Before you do anything else, sort the damaged prop out. That could be causing a drop in RPM from both engines. Also do/get compression readings for both engines if you think one is 'weaker' than the other.
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Old 28 June 2008, 16:53   #4
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Quote:
Originally Posted by Polwart View Post
the 300 rpm could be related to the damaged prob.
how smokey is smokey? and any idea why? are these 2 strokes? autolubed or mixed with the fuel (and do both engines get fuel from the same tank?)
is there any possibility that the difference is in the gauges not the engines?

not sure it makes much difference but are they counter rotating?

If you change the prop pitch up (from 15" to 16") I would expect that the engines find it "harder" to turn the prop and so revs will drop. dropping pitch might actually get you more speed.
Hi Polwart
Yes they are two stroke, petrol fed from the same tank which has an oil mixture of 50 to 1 (told to do this by the previous owner), also has oil in reservoirs under each cowl, both working as levels droppping (is this autolube?) one is smokey on tickover and has more oil residue around the exhaust (twice as much residue than the other engine) plus engine looks like it has had more work. Both start and tickover well
Smoke wise, nothing to write home about from either when on the move. Should they smoke on tickover? Can you control the oil flow from the reservoir under the cowl. Sorry for all the questions again, never had a two stroke before
They are not counter rotating
If I drop the pitch to say a 14P would that possibly increase by the revs 500rpm to its optimum range of 5500? Any Idea what it would do to acceleration and to fuel economy?

Thanks again

Colin and Elizabeth
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Old 28 June 2008, 17:02   #5
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Originally Posted by Nos4r2 View Post
Before you do anything else, sort the damaged prop out. That could be causing a drop in RPM from both engines. Also do/get compression readings for both engines if you think one is 'weaker' than the other.
nos4r2 thanks for your advice. The engine with the damaged prop also has a judder as you are going along, could this be the damage on the prop, is it normal to feel a judder/vibration?

Thanks
Eli and Colin
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Old 28 June 2008, 17:06   #6
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Quote:
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nos4r2 thanks for your advice. The engine with the damaged prop also has a judder as you are going along, could this be the damage on the prop, is it normal to feel a judder/vibration?
Yes if it's badly enough damaged. Change it ASAP or it'll trash the gearbox.Don't use it til you get it fixed. Don't play with prop pitch til you've tried it with the damaged one fixed either-if you're dragging an engine due to a damaged prop you won't be reaching peak RPM on either engine.


If you're running the autolube system and it's working I'm not surprised it's smoking with 50:1 premix in there too! Use either one or the other but not both.
The amount of oil the autolube pump delivers is controlled by the engine-ie it varies depending on load.
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Old 28 June 2008, 20:07   #7
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Quote:
Originally Posted by MOON View Post
oil in reservoirs under each cowl, both working as levels droppping (is this autolube?)
yes
Quote:
.,, which has an oil mixture of 50 to 1 (told to do this by the previous owner),
some people like to do this as they are paranoid that the autolube pump will fail and the engine will sieze. Normally though they disconnect the autolube system - as otherwise there will be too much oil, especially at tick over.
Quote:
plus engine looks like it has had more work.
do you know its history? could it have been used as a single before and therefore have more wear?

Quote:
They are not counter rotating
perhaps someone with knowledge/experience of twins can comment if this could account for some or all of the rev difference? it strikes me that one engine will be working in "dirtier" water than the other.
Quote:
If I drop the pitch to say a 14P would that possibly increase by the revs 500rpm to its optimum range of 5500? Any Idea what it would do to acceleration and to fuel economy?
I would suggest you follow Nos's advice and get the 15" fixed and test it again so you are solving one problem at a time.

I imagine you are reluctant to spend 40 quid at steel developments fixing the prop only to replace it straight away if the revs are still too low. Notwithstanding the next comment, you could swap the props from side to side and see if the rev's issue swaps side.

Quote:
The engine with the damaged prop also has a judder as you are going along,

Quote:
could this be the damage on the prop, is it normal to feel a judder/vibration?
yes this is a typical symptom of a bent prop. As Nos' says you really don't want to use a prob that is causing this.
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Old 29 June 2008, 15:19   #8
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Good advice from Nos and Polwart, Geoff could have a new prop with you by Friday, I would wait until the prop is sorted before you try anything else.

Pete
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Old 30 June 2008, 04:56   #9
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Hi Polwart
Yes they are two stroke, petrol fed from the same tank ....... one is smokey on tickover ......
Might be worth chucking them in for a service - if one is smoking (understandable if it's getting a double dose of oil) and the other isn't that might hint that the autolube on one isn't working too well or at all at low rpm. That may be why the previous owner used premix? I would make sure the autolube works on both before removing the oil form the petrol tank....... (Assuming it's a Yam based one, the clymer manual has a couple of pages on how to clean out the oil injection gear, it's not a difficult job, just messy. )

The bent prop is easily worth 300rpm. And it will trash your gearbox if you don't fix it.
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