Rigid Inflatable Boats

[kach22i]

I'm seeking to better understand the advantages and disadvantages of RIB designs.

How do they work?

Here is a typical claim:

http://www.e-inflatableboats.com/Rig...ble_Boats.html

Rigid inflatable boats are the best performers, and hence, the most expensive. Their rigid fiberglass hulls attached to inflated tubes combine the lightweight stability and buoyancy of inflatable boats with the speed, maneuverability, directional stability, and fuel efficiency of rigid boats............................These features of the sponson provide enhanced sea keeping ability, better absorption of shock from waves on impact, easier boarding of other vessels, high visibility, increased swamp buoyancy, and increased stability while stationary and under way.

Wave impact absorption

Stable

Buoyancy

The list goes on, however I can't seem to find a lot of actual testing or concept engineering on-line.

What I'd like to read about is how the design specifics of RIB design have been developed over the years.

What are the typical standards?

That is to say, what air pressure is best and why?

What's up with the closed cell foam filled boats, what do they gain, what have they given up?

How hard is too hard (air or foam)?

How soft is too soft (air or foam)?

Aside Note: I own a hovercraft, and may seek to make a connection regarding wave impact. In the hovercraft, the least resistance may often be best (long story), in a RIB I have no idea how the air spring rate is established, what typical pressures are, and why.

EDIT: Please provide links to good articles, that would be a good starting point for me.

[kach22i]

More on the foam stuff:

http://www.special-operations-techno....cfm?DocID=607

The design uses a combination of either a tough polyurethane outer coating, or neoprene Hypalon, over a hollow block of flexible closed-cell foam and an inner inflatable bladder. Program descriptions note that the “H-920 can be equipped with Zodiac’s patented DuraRIB II foam tube collar system that allows the boat to remain operational even with bullet punctures.”...........................Designed to replace the Marine Corps’ aging fleet of rigid raiding craft, the SURC is capable of moving an infantry squad of 13 Marines while providing a limited weapons platform in the riverine environment. The craft measures 38 feet in length (with transom platform) with a beam of 10 feet 2 inches (foam flotation collars can be removed for C-130 transport). An inboard engine with water jet propulsion system and a static draft of only 2 feet are designed to facilitate tactical flexibility in a riverine setting.

As with the SURC, SAFE Boats’ new Defender class RBS 25 features a fabricated 5086 Mrine-grade aluminum hull, with a collar flotation system constructed of solid closed-ell polyethylene foam, coated in scuff-resistant UV stable polyurethane. With a molded length of 25 feet and an overall length of 27 feet 6 inches, the Defender RBS 25 has a beam of 8 feet 6 inches and a draft of 2 feet 3 inches (engines raised) or 3 feet 3 inches (engines lowered).

“We have utility patents on foam stabilized watercraft,” explained Scott Peterson, vice president, SAFE Boats International. “It’s a conceptual patent, and how it’s worded is ‘foam stabilized watercraft.’ If there’s foam on the side of the boat that aids in stability and buoyancy, legally we’re the only people that can build it.”

[cyclops]

Can you imagine how much space is taken up by the internal and external foam collar? Rough water ride is none existant because they " sneak in ". Do not expect this boat to compete with standard boat designs. Do you NEED a bullet proof boat on your cruises? A very special boat design for a very special need.

[kach22i]

Originally Posted by cyclops

Rough water ride is none existant because they " sneak in ".

Yep, and "roostertails" can show up on surface radar, which is why waterjets are popular on the larger ones (plus stopping/turning ability).

Ride is unimportant in the military craft - a given.

So maybe there is a new question here? Does one automaticly give up "stability" for ride comfort?

[kach22i]

Originally Posted by cyclops

Can you imagine how much space is taken up by the internal and external foam collar?

On the smaller craft, they appear to be laying down on the sponsons, equal number of guys on each side.

[jfblouin]

RIB have somes advantages:
- low weight (need less HP for same speed or go faster with same HP) easier to move with trailer
- low free board (helpfull for diving operation) and give a good near to water feeling.
- boat to boat impact proof (you can put your boat side by side with another boat without dammage)
-good seaworthy with big reserve of stability and flotability (a safer boat lenght for lenght)
- sport ride (like motorbike)

RIB have some disadvantages:
- high cost (you get small boat space and no convinience at high cost)
- need high maintenance on rubber collar (many glue job each year)
- give wet ride
- small usable boat space for same size boat

You have to choice a boat for your requirements

[kach22i]

Originally Posted by jfblouin

RIB have somes advantages:

RIB have some disadvantages:

That's as far as I ever get on the topic. I'm looking for more information such as:

1. Air Pressure

A: How much is too much?

B. How much is too little?

C. Why is there such a thing as too little?

D: Test data - give me some.





2. Foam filled version

a: Advantage over pure air

B: Disadvantage over pure air

C: Test data - give me some.

[kach22i]

Odd Find:
Foam boat, kiddie style...............................


...............and for my next great idea, the Nerf car, although I think "Saturday Night Live" did that one already. They did the Chia pet car and Adobe car, right?

Showing my age.

[cyclops]

You will have to contact each boat company to get answers to your questions.

[jfblouin]

The average pressure is 3.5 psi.

Too much pressure cause high stresse on joints

Too less pressure raise the fabric wearing and risk of pinching the fabric when you hit a solid object

Foam collar are puncture safe but weighty.

[kach22i]

Originally Posted by jfblouin

The average pressure is 3.5 psi.

That information helps a lot, thanks.

Here is what a guy (Barry Palmer of Sevtec) said, who once build a hovercraft using inflatable sponsons.

http://groups.google.com/group/alt.r...aefaf70042297e

..................... and pressure was maintained at around 1 to 1-1/2 psi.

I think he used half the pressure, because the hovecraft is not in constant contact with the water, and the weight of the craft is not bearing on it in standard (hover) operation.

[cyclops]

The only similar thing about the 2 different craft is the air pressure.

[kach22i]

Originally Posted by cyclops

The only similar thing about the 2 different craft is the air pressure.

I've owned both, a hovercraft off cushion is like a bad raft. Also I've had waves break the bow of the hovercraft, followed by a wall of water flying over my head and smack my spinning fan blades with a frightening sound.

I really don't like waves washing away my skirt pressure/cushion and making part of the craft a boat in displacement mode - no offense to the many boater's here - I prefer to fly, even if it's only six inches above the surface.

I think in a very heavy sea state they might react more a like than not. Although common sense prevents me from testing this theory in field at the moment.

[cyclops]

That is a problem when the skirt is all 1 chamber. The military has several chambers to elimanate those problems. Money is no problem.

[kach22i]

Originally Posted by cyclops

That is a problem when the skirt is all 1 chamber. The military has several chambers to elimanate those problems. Money is no problem.

True, but in addition to multi-chambers (Sevtec has them on small craft) the large size helps and more importantly the length helps.

Small craft:
Lets say a 1-meter high wave forms and crests in front of a 2-meter long craft (hover height 15mm/6 inches) - it will take out most of the cushion as it passes below.


Large Craft:
Now imagine a much larger military LCAC encountering 2.5-meter high waves which form and recede in the craft's path. The LCAC is around 30 meters long (100 feet) and has a cushion height of 7 feet/2-meters. Proportionally the waves are smaller than the small craft example, but perhaps even more important is the wave will crest and recede within the length of the craft, making bisecting cushion dividers useful.

Back to RIB's:
I guess what I'm trying to get my mind around is that a hovercraft "off-cushion" in heavy seas is dangerous, as the craft may flip over. The hovercraft has lots of buoyancy just like a RIB, and a flat-ish bottom like most of the RIB's. So how can the RIB's have any claim to stablity in heavy seas?

It it just the fact that you keep moving despite being tossed about and swamped with water coming in?

EDIT:
Here is another way of dividing up hover chambers (from Spain):
http://www.neumar.com/Research.htm