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Old 03 December 2010, 08:56   #1
Country: South Africa
Town: Secunda
Boat name: BeBe
Make: Bucanneer RIB
Length: 3m +
Engine: 15hp Mercury OB
Join Date: Dec 2010
Posts: 1
Altitude Compensation

Hi everyone, getting my 3,2 metre RIB to plane at 1600 metres above sea level with a Mercury 15hp is a bit of a challenge. Local legislation makes a 15hp a good choice though. I seem to battle to get to 5500 revs, due to a lack of power and the prop is already a 7 inch pitch.

Do I need to change any jets, perhaps the fixed fuel jet, to get rated power and thus the required revs to get into the power band at this altitude ?


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Old 03 December 2010, 09:33   #2
Country: UK - Scotland
Boat name: Wildheart
Make: Humber/Delta Seasafe
Length: 5m +
Engine: Merc 60 Clamshell
MMSI: 235068449
Join Date: Apr 2007
Posts: 4,538
Welcome to ribnet! Are you our highest altitude member?

Yes, you will need to change jets. At that altitude, the air is "thinner" (less oxygen) so you are likely running a bit rich. Have a look on your manufacturer's website or the handbook, it should say somewhere "above X00m, see your dealer for appropriate tweaks" or soomething to that effect.

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Old 07 December 2010, 17:36   #3
starnetman2001's Avatar
Country: UK - England
Town: Sheffield
Boat name: Fluffy
Make: Northcraft, Ribquest
Length: 6m +
Engine: Out 135 Mercuary
Join Date: Mar 2010
Posts: 93
Not sure where you are but can I dive it?

Yes, you will need a smaller jet or a way of adjusting the mix.

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Old 07 December 2010, 18:17   #4
Country: UK - Scotland
Make: HumberOceanOffshore
Length: 8m +
Engine: Volvo KAD300/DPX
Join Date: Oct 2002
Posts: 4,850
Originally Posted by Nitram View Post
...Do I need to change any jets, perhaps the fixed fuel jet, to get rated power and thus the required revs to get into the power band at this altitude ?

A smaller main jet can correct the over rich mixture but you cannot get back to rated power. The power the engine produces is dependent upon the amount of air (and therefore oxygen) it consumes. You've got less air/oxygen at that height therefore you will make less power. The best you can do is to get the mixture correct or a tad rich and prop the engine to reach max revs. If it's excessively rich it will 4-stroke rather than 2-stroke. It's possible that it may 4-stroke on light throttle but 2-stroke under load. A slightly rich mixture on a 2 stroke engine may give a touch more power because the extra cooling this provides to the charge mixture allows more charge air into the crankcase. A week mixture on a hard worked 2 stroke engine will lead to overheating.
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