Questions for RIB Owners from an Engineering Student Team

[Poly]

Originally Posted by Last Tango

I think some kind of pressure indicator built into the valve caps might be of interest, even just over/under pressure warning (at the right price) but beyond that......
"sledge hammer and nuts".

That's not a bad suggestion although given most people can tell be touch (or sight with the accuracy a valve cap might manage?) it might still be a dead end.

It might seem like a daft project for Team Capstone - but actually this is great training for a team of designers - explaining to the "client" that their idea is a waste of time is going to be a big part of their career! Reshaping the project along the lines Last Tango suggests should show your supervisors and "client" that you've listened to the market feedback and responded [of course it might be you find a military/commercial interest but I'd be surprised - rarely do those people want systems that might need extra maintenance]. If you need another suggestion then probably a research experiment to determine what the "optimal" pressure actually is, and some apparatus to measure it. e.g. does over pressure result in harsher ride? does under pressure result in accelerated fabric wear? is it worse on a trailer? etc. Without knowing the answer to that I can't see how you'd spec the pressure for the "system" anyway.

[office888]

Originally Posted by Poly

If you need another suggestion then probably a research experiment to determine what the "optimal" pressure actually is, and some apparatus to measure it. e.g. does over pressure result in harsher ride? does under pressure result in accelerated fabric wear? is it worse on a trailer? etc. Without knowing the answer to that I can't see how you'd spec the pressure for the "system" anyway.

Over pressure stresses the construction of the the material, but it will not affect the performance of the boat.

Underinflation will lead to premature wear of the rearmost seam, decreased ride quality, and the possibility of tearing the tubeset off the GRP hull in rough swells.

The inflation pressure is typically determined by the stretch / strength of the material and the strength of the bonding mechanism for the seams.

[jeffstevens763@g]

better if you filled the tubes with a gas less affected by heat keeping pressures more constant if it bothers you that much.
pressure relief valves are great to prevent failure, topping up to pressure with a pump now & then not a problem do it on the car regular. cost of such a system would out way the benefits OMO

[Poly]

Originally Posted by office888

Over pressure stresses the construction of the the material, but it will not affect the performance of the boat.

anecdotally people tell me otherwise, that too hard = less cushioning, which since the recent explosion in shock mitigation is interesting. indeed if you listen to the criticism of "rigid tubes" one of them is that this feature is missing. It would be nice to see some data that proves or disproves this.

Underinflation will lead to premature wear of the rearmost seam, decreased ride quality, and the possibility of tearing the tubeset off the GRP hull in rough swells.

I understand the "risk" but how much under 10%, 20%, 50% to see problems? Again I suspect that most has been worked out by gut feel, a wee bit of trial and error and doing what everyone else does. It is slightly surprising that weight of fabric, length of boat, diameter of tube, boat loading etc. don't result in different recommendations like car tyre pressures do.

[jeffstevens763@g]

i sent this to Paul tilley recently and he was kind enough to reply
reason i dont have pressure relief valves so wanted to know over pressure test % then i can work out how much expansion at a given temp.

would you be so kind to let me know what the % test pressure is above the working pressure of a rib or sib tube please.

thanks in advance regards jeff
Jeff pressure tests are a bit of a mine field,
MOD always have working pressure 1.5psi
max pressure 2psi
test pressure 2.25psi
test requires a loss of less than 20% pressure over 24hrs
This varies from manufacture to manufacture but is the ISO standard of 50% over pressure .
Regards Paul

[HUMBER P4VWL]

Originally Posted by willk

3 chambers as constructed or a couple of baffles calved? I've never heard of a grown up RIB with less than 5 chambers!

Went out yesterday (3rd time this season) and you're right, 5! ;-)

[office888]

Originally Posted by Poly

anecdotally people tell me otherwise, that too hard = less cushioning, which since the recent explosion in shock mitigation is interesting. indeed if you listen to the criticism of "rigid tubes" one of them is that this feature is missing. It would be nice to see some data that proves or disproves this.

I understand the "risk" but how much under 10%, 20%, 50% to see problems? Again I suspect that most has been worked out by gut feel, a wee bit of trial and error and doing what everyone else does. It is slightly surprising that weight of fabric, length of boat, diameter of tube, boat loading etc. don't result in different recommendations like car tyre pressures do.

iPhones have accelerometers, it would be easy to test the g-forces involved with under vs overinflation vs hard boat.

My experience is that excessive flex will start to show up at -20%... +/-10% usually still keeps the tube at "ideal" inflation.

The weight of fabric, length, and diameter don't affect calculations too much. The construction of the fabric doesn't alter it too much, since the weak spot will always be the glue.

Most cars these days specify a range of 30 to 40 psi.

Most inflatable boat manufacturers choose "3.48 psi / 240 mbar" as their ideal pressure, but some also choose 3 psi / 200 mbar or 3.6 psi / 250 mbar. The highest I have ever seen for main buoyancy tubes is 4.0 psi / 275 mbar, although I do not remember the brand... it was an East Asia construction PVC boat.

[Pikey Dave]

Originally Posted by office888

iPhones have accelerometers, it would be easy to test the g-forces involved with under vs overinflation vs hard boat.



My experience is that excessive flex will start to show up at -20%... +/-10% usually still keeps the tube at "ideal" inflation.



The weight of fabric, length, and diameter don't affect calculations too much. The construction of the fabric doesn't alter it too much, since the weak spot will always be the glue.



Most cars these days specify a range of 30 to 40 psi.



Most inflatable boat manufacturers choose "3.48 psi / 240 mbar" as their ideal pressure, but some also choose 3 psi / 200 mbar or 3.6 psi / 250 mbar. The highest I have ever seen for main buoyancy tubes is 4.0 psi / 275 mbar, although I do not remember the brand... it was an East Asia construction PVC boat.

My RC is 2.5psi


Lions led by donkeys

[selkiediver]

1) What size of RIB do you own/operate?

A: 4.7M Avon Searider

2) How many compartments constitute that vessel's collar?

A: 3

3) What is the general category of use for the vessel? Recreational? Commercial/Industrial? Enforcement/SAR?

A: Recreational (mainly scuba diving) but occasionally pleasure trips such as dolphin watching with friends.

4) What equipment and methods do you currently employ to inflate/maintain the compartment pressures? For example: How do you assess collar pressures? By 'feel' or sound? A portable or permanently installed gauge? When required, how do you top up the compartments? A foot pump? Portable compressor (DC or AC supplied)? Compressed gas cylinders? On-board compressor and distribution system?

A: Assess pressure by feel, ideal pressure for me is drum tight. Pressure is topped up using a foot pump directly into the tubes via the Leafield C7 valves.

5) Approximately how much did your current system cost to purchase?

A: Nothing as the foot pump came with the boat.

6) How frequently do you check and/or top up the pressure in the collar?

A: Every time I launch the RIB, before and during RIB operations. As the water chills the air inside the tubes, it contracts and the pressure drops so in summer will usually have to top up pressure at least once.
I keep my RIB at home on a trailer so when not using it, will partially deflate the tubes so that during warm sunny weather, they do not become over pressurised.

7) With your current system, approximately how long does this process take on average? Please include time to assess pressures, set up your inflation system, inflate the collar compartments, and take down.

A: Between 5 & 10 minutes, depending how much air is needed.

8) On a scale of 1-10, how satisfied are you with your current system? We'd appreciate any justification you are willing to provide. What do you specifically like or dislike about it?

A: 100% satisfied (so a 10, assuming 10 is completely satisfied, as you did not specify which end of the scale constitutes satisfied). The foot pump is a very basic piece of equipment so very few things to go wrong. A mechanical /electronic system has so many variables of things to stop working that it would not be worth my while/expense installing such a system. I would probably carry the foot pump as a back-up in any case.

9) How interested would you be in replacing your current system with one that would not require bringing any equipment to the vessel, would automatically alert the operator to low collar pressures, and could quickly inflate the collar to specified pressure with a single press of a button? This could be done dockside or on the water.

A: I think I would need a lot of convincing. If one of the tubes suddenly becomes soft, there is usually a reason behind it, such as a puncture or a leaky valve. A fancy system would keep topping up the air pressure but would be unlikely solve the underlying problem.

10) Could you suggest any specific functionality or features that would, in your opinion, add significant value to such as system?

A: If I was installing such a system, a main feature would be a read out of the pressure in each tube, either digital or manual.

11) Could you estimate the total cost (system + installation) at which such a system would become significantly less appealing than your original assessment? (I realize cheaper is always better, but at what point would cost prevent you from seriously considering purchasing such a system?)

A: £100-£200.