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Old 07 June 2011, 10:50   #1
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Fuel Consumption vs Tube Pressure

Hi,
Another daft question;

Whilst still looking for a new outboard, I installed a new fuel gauge and noticed I am burning far too much fuel compared to say January. The rib is moored on the water.

I have never topped up the air pressure and the tubes are not as firm as they used to be.

So have you guys experienced increase in fuel consumption whilst the tube pressure is lower?

when I mean increased fuel consumption is like burning 50% more...
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Old 07 June 2011, 11:24   #2
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Do your tubes actually contact the water when you're underway?
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Old 07 June 2011, 11:32   #3
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The hull isn't covered in weed and other such rot is it? I can't imagine squishy tubes being half as fuel efficiency harming as a layer of wildlife being dragged along
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Old 07 June 2011, 11:52   #4
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Quote:
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Do your tubes actually contact the water when you're underway?
Yes, the rib being good old Humber it always touches the water. So maybe when the pressure is a bit lower, it is touchinng more?!!!


Also, do you guys know what's the normal psi level of the tubes? I am buying a pressure gauge to check this.
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Old 07 June 2011, 14:26   #5
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Is your hull antifouled and if so when was it last recoated?
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Old 07 June 2011, 16:34   #6
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Yes, sounds to me as if you've got some growth on the bottom. If you've antifouled the hull, did you do the tubes too? If not, you need to. I've done mine for the last four years and it seems ok. Internationals 'Navy' is exactly the same colour as my Humber's tubes and you can't see the join.
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Old 07 June 2011, 17:04   #7
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I have never topped up the air pressure and the tubes are not as firm as they used to be
erm...pump em up and try it ?
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Old 07 June 2011, 17:24   #8
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Quote:
Originally Posted by Vandad View Post
Hi,
Another daft question;

Whilst still looking for a new outboard, I installed a new fuel gauge and noticed I am burning far too much fuel compared to say January. The rib is moored on the water.

Is the increased fuel consumption as read on the new fuel guage, or actual use?
If you're judging this on the fuel guage reading - are you sure the new fuel gauge is accurate? It may not be coincidence your "increased consumption" has happened after installing a new gauge

I'd full her up, go for a run or two, fill her adain and see how much fuel she uses per NM/ hour based on the usage between top ups.

Many fuel guages are veet inaccurate. could it be a problem with the sender in the tank?
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Old 08 June 2011, 03:41   #9
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A friend of mine ruined an end cone on a zodaic rib by not inflating the tubes to the correct pressure (At least this is why the end cone failed according to the repair guy (inland inflatable services in sligo)) He has a good rep as far as I can tell. So keep those pressures right or it could cost you more than just additional fuel

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Old 08 June 2011, 04:04   #10
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A friend of mine ruined an end cone on a zodaic rib by not inflating the tubes to the correct pressure
Yes, low pressure is very sore on tubes. It causes the tubes to ripple and twist violently underwater. This can lift tapes and remove rubbing strakes if the conditions are bad enough and the speed high enough.
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Old 08 June 2011, 09:38   #11
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A friend of mine ruined an end cone on a zodaic rib by not inflating the tubes to the correct pressure
Also kills top end. A buddy picked up a good 5 to 10 mph when he actually put air in the tubes. 15.5' Zodiac RIB.

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Old 08 June 2011, 10:04   #12
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Also kills top end. A buddy picked up a good 5 to 10 mph when he actually put air in the tubes. 15.5' Zodiac RIB
I must be missing a trick in this thread...Humbers run with their tubes in or on the water and Zodiacs run 5 to 10 mph quicker with inflated tubes.

Try as I might I have no recollection of RIB's planing with tubes in or touching the water. Do Humbers run in displacement mode with submerged tubes?

WTF am I missing???
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Old 08 June 2011, 10:06   #13
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willk is right on, 3,5 psi is correct pressure, lower pressures will cause fabric fatigue and failure. If you check most new tubes have an additinal wear patch on bottom of tubes in rear where cone starts to try and strenghten this area of common fatigue. Keep your tubes to correct pressure..
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Old 08 June 2011, 10:26   #14
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That pressure sounds right for PVC tubes but I would have thought that the hypo tubes likes 2.5 psi
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Old 08 June 2011, 10:30   #15
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WTF am I missing???
Just that some RIBs plane with the bottom of their aft tubes touching the water all/most of the time. It's got nothing to do with displacement mode - they simply touch the water all the time.
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Old 08 June 2011, 10:33   #16
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Just that some RIBs plane with the bottom of their aft tubes touching the water all/most of the time. It's got nothing to do with displacement mode - they simply touch the water all the time.
Yep, I can see that some RIB's run with a small amount of tube touching at the rear...but if I increase the tube pressure from say 'a bit flat' to 'proper' am I going to gain up to 10mph and halve my fuel consumption? Nice thought...but I don't think so...

Couldn't help myself...the Humber displacement mode comment was tongue in cheek
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Old 08 June 2011, 10:44   #17
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Leapy, are you related to my wife? I ask as you have made 1+1=3

I said low pressure damages tubes
John said his bud got higher top end on properly inflated tubes
No-one said low pressure resulted in a 50% increased fuel usage

The OP's problem is that he hasn't established his actual situation before asking a question. i.e. pump up the tubes and measure fuel consumption properly. He doesn't appear to know if his hull is clean or not? Until these things are checked, it won't matter what we suggest as it could be all or any one of the problems.
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Old 08 June 2011, 11:31   #18
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never used a pressure guage on a rib in m'life. When it feels right it is right. Do your pressure check on the tubes, (big volume, low pressure vessels) and not long after it will be different depending on sun, wind and water/air temp. If its early morning allow for expansion when the sun gets on it. In the evening its gonna go a tad floppy as it cools down.

find it strange that you have never put air in the tubes - it was sooo cold in january!
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Old 08 June 2011, 15:35   #19
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Leapy, are you related to my wife?
Possibly...please send photo and vital statistics for verification
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Old 08 June 2011, 15:40   #20
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please send photo and vital statistic
If you don't behave, I'll just send the wife herself. That'll soften yer cough
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