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Old 07 July 2013, 15:35   #1
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3 kts down, where's it gone?

Puzzling really, nothing had changed in my SIB setup, same 3.4m SIB and 9.8 Twat, same loading etc but we used to touch 17.5 kts and consistently now in flat water we max out at 14.5 kts.

AFAIK engine is running sweetly, prop perfect, fuel fine, carb was cleaned out about 20 hrs use ago.

Any ideas?

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Old 07 July 2013, 15:53   #2
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Tide going other way ?
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Old 07 July 2013, 16:03   #3
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No, just to confirm this is consistent across various trips and sea times and on the river too.
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Old 07 July 2013, 16:05   #4
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Too many mince pies over the winter?

No, srsly, trim position, load balance, pressure gauge faulty?
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Old 07 July 2013, 16:14   #5
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Nah, all the same there too Willk.

Same trim, same load, pressures all double checked...
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Old 07 July 2013, 23:24   #6
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What are you using to measure your speed, GPS, pitot?
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Old 08 July 2013, 01:34   #7
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Hi Dave, Garmin GPS.
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Old 08 July 2013, 03:01   #8
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Throttle cable adjustment? Found mine had loosened itself and wasn't getting full throttle.
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Old 08 July 2013, 05:23   #9
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I'll check that, otherwise could it be gummy jets?
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Old 08 July 2013, 15:39   #10
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Or prop light spun could be beginning to happen and worsen with more use. Borrow same prop pitch and test if possible. Engines doesn't work same on every day basis, it's a environmet outside temp, water temp, atmospheric pressure issues, but not that picky so to loose 3 knts though.

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Old 09 July 2013, 07:16   #11
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when was the last time the plugs were changed? I had a step change on my old Yam when I did that....

Also my portable genny went from "no go" to "first pull" with a new plug.....
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Old 09 July 2013, 07:19   #12
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Or prop light spun could be beginning to happen and worsen with more use.

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Loco - what does this mean please? The prop is mint.

9D280 - relatively recently - 15 hours max. I clean them every now and again too and they are as new AFAIK...
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Old 09 July 2013, 12:39   #13
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Is prop in mint new condition, or it's a old prop looking in mint condition. Should have said spun hub. The middle rubber core that fixes/secures prop body into threaded inner core begins to slip, loses it's vulcanazing condition and prop delivers less rpm, very noticeable when at wot.

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Old 09 July 2013, 13:42   #14
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I guess you need to check the engine RPM at WOT, ours will lose 3mph if you reduce the RPM by 500.
The problem with that is whether you know what it was before. Do you have a tachometer fitted?
If the engine is achieving maximum RPM then it is likely to be the prop drive as suggested above.
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Old 09 July 2013, 14:08   #15
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Or use metres/second and it won't seem as bad. ;-)
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Old 10 July 2013, 07:24   #16
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Quote:
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Is prop in mint new condition, or it's a old prop looking in mint condition. Should have said spun hub. The middle rubber core that fixes/secures prop body into threaded inner core begins to slip, loses it's vulcanazing condition and prop delivers less rpm, very noticeable when at wot.

Happy Boating

Had that years ago on a prehistoric Johnson - We left at full chat - 3 hrs later it could hardly keep us on the plane - and engine was singing nicely!

Way to check visually - have a look at the prop hub. If you can see what looks like oozing stuff coming out from either of the rubber - metal joints it' a gonner. Compare with the other joint (i.e if its oozingfrom the inner annulus of the rubber, compare with outer.

It's basically there to absorb vibration & reduce the impact load on the geartrain if it hits something.
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Old 10 July 2013, 07:39   #17
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Will check that - plugs out and are in lovely nick, throttle working correctly to stops. I need to get a tacho I think...
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Old 10 July 2013, 08:00   #18
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OK, prop off, square drive with short breaker bar on the splines and a big effort moves it maybe a few degrees so guessing all is well there.

Prop really is mint, it's an 8.5 so factory standard I think?
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Old 10 July 2013, 09:04   #19
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Just done the carb, clean as a whistle too - no gum or problems there.

Ho-hum...out Saturday so will see how it goes then.
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Old 10 July 2013, 15:12   #20
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I've had spun hubs with mint prop looking condition, to be sure go for the wot hub slip test. Remove prop, at back : with a white marker, draw a straight line or mark 2 small dots facing each other, put back on engine, with you alone go for a short wot spin, remove prop and check if marked straight line or dots have moved from where they were initially marked.

If so, buy a new same diam pitch prop if you are confy with the peformance of your boat engine combo. Hub generally starts sliping a little and worsens with more use and load at wot. The hub besides absorbing vibration & reducing the impact load on the geartrain if it hits something is what drives the splinned lower core rotation directly to the prop body to spin. To work top, should not slip even a milimeter.

This is the best time if going for best engine/prop optimization, would need to have tested previous prop with a tach at wot to check max achieved rpm, then going for a +,-pitch prop if needed for optimizing near max rpm stated for that engine.

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