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Old 30 October 2018, 03:21   #1
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Tough reverse

Excuse my lack of techy terminology but...is it normal to find it really tough to get a RIB to go astern with any kind of Ďwellyí?
It engages reverse gear no problem under normal circumstances/manoeuvres etc but if I get caught in a fast ebb tide and need to go astern quickly & with any controlled force, itís a real bugger i.e. I have to move the throttle all the way back and then use real force before I can get any kind of effective thrust to counter the fast cross tide.
Does any of that make sense?
Is that normal for a RIB?
Thanks
L
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Old 30 October 2018, 03:35   #2
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I,m guessing it's the way our control box has been set up.
It's not uncommon not to be able to get full throttle in reverse but to be honest you should never really need any more than approx 2000 rpm. even in a strong tide.

I'd have it looked at just in case but I'd tin it's probably ok
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Old 30 October 2018, 03:52   #3
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As well as the throttle range that Last Tango rightly refers to - your boat is about the least hydrodynamic shape possible in reverse, the prop and leg were optimised for forward thrust (eg in reverse you are trying to drive through your exhaust gas). If you are using it in shallow water then it seems to be worse as often you are trimmed up so you are trying to lift the stern with a lot of the power rather than pull the boat backwards.
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Old 30 October 2018, 04:12   #4
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Quote:
Originally Posted by Last Tango View Post
I,m guessing it's the way our control box has been set up.
It's not uncommon not to be able to get full throttle in reverse but to be honest you should never really need any more than approx 2000 rpm. even in a strong tide.

I'd have it looked at just in case but I'd tin it's probably ok
Thanks LT, iíve had it checked & tweaked & no problems found. I agree re the power needed, itís just tricky when I need that bit of extra Ďvroomí to get me out of trouble. I really need to pull down hard on the throttle to get any kind of momentum in those circumstances when I need it to react quickly.

Itíll engage at tick over but thereís no gradual increase in speed in between. Itís either Ďclickí to engage (which isnít enough reverse thrust in a strong tide) or full back on throttle with a really hard pull & push down & nothing in between which then results in a sudden burst of power. Not ideal for smooth controlled manoeuvreability.

I think itís probably just me needing to get used to different handling of a RIB but Iíll keep an eye on the revs next time it happens.
Thanks
L
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Old 30 October 2018, 04:16   #5
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Originally Posted by Poly View Post
As well as the throttle range that Last Tango rightly refers to - your boat is about the least hydrodynamic shape possible in reverse, the prop and leg were optimised for forward thrust (eg in reverse you are trying to drive through your exhaust gas). If you are using it in shallow water then it seems to be worse as often you are trimmed up so you are trying to lift the stern with a lot of the power rather than pull the boat backwards.
Ahh thanks Poly, that makes perfect sense.
Good to know itís not a mechanical problem. I just need to allow myself more time & space.
Thankyou.
L
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Old 30 October 2018, 05:40   #6
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Quote:
Originally Posted by Poly View Post
As well as the throttle range that Last Tango rightly refers to - your boat is about the least hydrodynamic shape possible in reverse, the prop and leg were optimised for forward thrust (eg in reverse you are trying to drive through your exhaust gas). If you are using it in shallow water then it seems to be worse as often you are trimmed up so you are trying to lift the stern with a lot of the power rather than pull the boat backwards.
To put some numbers on this, the fastest your boat is going to go through the water backwards is it's maximum displacement speed....1.4 X the square root of it's waterline length (in feet) = 6.5Kn and it's not likely to do that even.
You can pile as much power on as you want, all it's going to do is boil water....you're pi55ing in the wind.
If the current is stronger than 5kn you're going to have to find another solution.
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Old 30 October 2018, 06:35   #7
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To put some numbers on this, the fastest your boat is going to go through the water backwards is it's maximum displacement speed....1.4 X the square root of it's waterline length (in feet) = 6.5Kn and it's not likely to do that even.
You can pile as much power on as you want, all it's going to do is boil water....you're pi55ing in the wind.
If the current is stronger than 5kn you're going to have to find another solution.
How the hell you know those numbers is beyond me but Iím grateful for the knowledge as I now know for sure thereís no mechanical problem.
No wonder Iíve struggled there then.
Just a case of adjusting my approach to manoeuvres then. Sorted.
Thanks chaps, much appreciated.
L
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Old 30 October 2018, 10:11   #8
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How the hell you know those numbers is beyond me
William Froude did the hard work in the 1800's!
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Old 30 October 2018, 11:37   #9
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How the hell you know those numbers is beyond me but Iím grateful for the knowledge as I now know for sure thereís no mechanical problem.

No wonder Iíve struggled there then.

Just a case of adjusting my approach to manoeuvres then. Sorted.

Thanks chaps, much appreciated.

L



Not wishing to sound glib, but the best way out of your predicament is not to get into it in the first place. As youíve found out, boats are pants at going backwards. Can you forward think your manoeuvre to avoid/reduce the need for reverse. Itís like parking, itís much easier to reverse in & drive out forwards, but how many people do you see driving in forwards & then making a hash of reversing out into traffic & pedestrians?
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Old 30 October 2018, 13:20   #10
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And a wee tip for checking the wind direction when you're berthing.

Most of the yachts will have a "Windex" at their mast head pointing into the wind..... cheeting but hey-hoe.........
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