Tough reverse

[Lisilou]

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 ‘welly’?
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, it’s 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

[Last Tango]

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

[Poly]

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.

[Lisilou]

Originally Posted by Last Tango

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, i’ve had it checked & tweaked & no problems found. I agree re the power needed, it’s just tricky when I need that bit of extra ‘vroom’ 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.

It’ll engage at tick over but there’s no gradual increase in speed in between. It’s either ‘click’ to engage (which isn’t 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 it’s probably just me needing to get used to different handling of a RIB but I’ll keep an eye on the revs next time it happens.
Thanks
L

[Lisilou]

Originally Posted by Poly

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 it’s not a mechanical problem. I just need to allow myself more time & space.
Thankyou.
L

[Last Tango]

Originally Posted by Poly

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.

[Lisilou]

Originally Posted by Last Tango

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 I’m grateful for the knowledge as I now know for sure there’s no mechanical problem.
No wonder I’ve struggled there then.
Just a case of adjusting my approach to manoeuvres then. Sorted.
Thanks chaps, much appreciated.
L

[Poly]

Originally Posted by Lisilou

How the hell you know those numbers is beyond me

William Froude did the hard work in the 1800's!

[Pikey Dave]

Originally Posted by Lisilou

How the hell you know those numbers is beyond me but I’m grateful for the knowledge as I now know for sure there’s no mechanical problem.

No wonder I’ve 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 you’ve found out, boats are pants at going backwards. Can you forward think your manoeuvre to avoid/reduce the need for reverse. It’s like parking, it’s 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?

[Last Tango]

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.........

[Lisilou]

Originally Posted by Poly

William Froude did the hard work in the 1800's!

I’m actually reading that link. Oh dear God...I’m a nerd!😱

[Lisilou]

Originally Posted by Pikey Dave

Not wishing to sound glib, but the best way out of your predicament is not to get into it in the first place. As you’ve found out, boats are pants at going backwards. Can you forward think your manoeuvre to avoid/reduce the need for reverse. It’s like parking, it’s 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?

Not glib at all Dave...you’re right. I’m very familiar with the spot it happens so I know just how it can catch you there. I was daft to think the RIB would respond the same way as our mobo would have.

Loads of room to manoeuvre there so I can easily avoid the situation again.
L

[Lisilou]

Originally Posted by Last Tango

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.........

All for cheating me. The 1st place my eyes go when coming into berth is to flags or mast tops. Think I might even get myself a bit of ribbon to tie to the A frame.
L

[Pikey Dave]

Originally Posted by Lisilou

All for cheating me. The 1st place my eyes go when coming into berth is to flags or mast tops. Think I might even get myself a bit of ribbon to tie to the A frame.

L

Look for the tide direction too, running around bouys or pontoon legs. Use the wind & tide to your advantage if you can, no point fighting it.

[jonp]

I often have to back up at the entrance to our bar crossing to wait for gaps in the swell to head offshore, doing so in such a narrow crossing backing against 6-7 knots of current and chop are a challenge. By trimming up slightly the engine pullls the stern not only backwards but also raises it, stopping me from taking water over the back. This is the same as what we do backing up on fast game fish such as marlin, a little raised trim before backing but not to raised trim much.

This is our crossing on a calm day with no outgoing tide.

https://m.youtube.com/watch?v=zD8N5025BOM

This is what it's like most of the time, we have many fatalities here.

https://m.youtube.com/watch?v=nV_mNecx0-k

[GuyC]

The Hamble River has got quite a lot of pontoons in the middle of the river which see quite a lot of tide. At this time of year with many of the boats ashore there are ample places to practice going bow first / stern first etc. I did my PB2 on the Hamble and whilst reversing against the tide was tricky to begin with, you do get the hang of it quite quickly.

If you start off trying to hold station just downstream of the end of a pontoon, you'll get the hang of the throttle and steering inputs required and if it all goes wrong you aren't going to immediately impale yourself on the pontoon as you will go down tide.

I found looking forward and using the steering to hold the bow in place gave me the best control and I could ferry glide the boat left and right. As soon as your angle to the tidal stream opens up to more than a few degrees, the tide whips the end of the boat around and you briefly become a passenger.

The Froude number relates to the length of the wave that you create as you go through the water. Displacement speed is where you have the bow in line with a crest (which is generally always the case), the trough of the same wave is around midships and the next crest is as the stern. Any faster than that and you start to transition to planing. It requires a significant amount of power to make the boat climb up that wave slope and start to plane. This is where getting over the hump comes from.

[Lisilou]

Thankyou all for your replies. Hugely helpful. Practice is key so I’ll be making the most of the less busy winter days out in the Solent.

JonP...that’s some swell. Not something I’d fancy at all ��
L