USCG RBM, Self Righting versus Handling

This is a question for all of you Power Boat Designers.

Attached is a picture and text describing the USCG Response Boat Medium. There was much fan fare about the self righting capability. I saw a video of the RBM self righting but thought that this may somehow compromise other design considerations when I started to think about how we would self right in terms of CG and CB and all of the possible spillage of equipment and personnel as the boat rolls over.

Does the self righting feature compromise the design in terms of handling, stability, or sea keeping?

Thanks,

Mark

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Vessel Data from ACE Marine:

Principal Features:
Length Overall: 44.5 ft.
Maximum Beam: 14.7 ft.
Draft: 3.0 ft.
Fuel Capacity: 495 gal
Top Speed: 45 kts
Aluminum Plating, 0.25" Hull Bottom
Gilman Ionomer Foam Collar

Self-Righting in all Conditions

Machinery Features:
Two (2) MTU DDEC Series 60 825 HP Engines
Two (2) Twin Disc MG-51 14-SC Marine Gears
Two (2) Rolls Royce KaMeWa FF 3753 Waterjets
Two (2) AuraGen Viper G8500X AC/DC Generators
Vector-Stick Integrated Control System
Two (2) Weapon Mounts, Furuno NavNet
One (1) Towing Reel, Two (2) Rescue Zones

 

I am not a NA - but I have talked with several Coast Guard personnel that have operated the boat and they really like it. Fast, very responsive and stable. I have seen some videos where they did high speed 180 degree turns at high speed, the boat just sort of rotates around and then goes the other way, no big radius turn etc.

 

She is a nice boat. I talked with her designers about one year ago, and they said there was only one case of capsize accident during operation of the fleet. Yes, they do capsize for training.

I believe biggest compromise here is sliding/opening windows in pilothouse.

 

does it have an "S2" condition?

meaning that if somehow it was flipped over (and held there for a moment) would it be stable in that position or would it flip itself back over?

I'm guessing it does have an "S2".

 

To be self righting there needs to be sufficient buoyancy in the boat when upside down, to produce a righting moment. This means having, as you can see, a large watertight deckhouse/wheelhouse.

Being high, to get the buoyancy, means more windage, slows the boat down as you go faster, KG is higher, owing to the higher up 'stuff' required to get that buoyancy, which means the stability is compromised to an extent. But if she rolls over, so what, she'll right herself

The change in KG, when normally "hull born" will result in different motions too. Could be benign, or could be detrimental. But usually means greater roll angles are experienced so either making the boat go from tender motions to eeekkkk….or from stiff to tender etc or any other in-between. All depends upon hull size/shape and of course speed, not to mention the size and period of waves.

 

We designed few self-righting boats. Some notes:

- Self-righting capability can be provided due to cabin volume. In reality, it is difficult to keep cabin watertight; the crew is not likely to lock the doors and windows, etc.
- We used recess in the roof with inflatable bag that inflates on capsize, but this system is another possible weakpoint.

From my experience, most of operators asking for self-righting (or builders using this feature for marketing) will never have chance to use self-righting in reality. In this terms, inflatable bag is better because cabin can be lower (less windage), windows more simple, etc.

To capsize, the boat should be side hit by breaking wave exceeding 0.5B - just a guideline.

 

Thanks to everyone for the feedback.

Alik, yes I would agree about windage.

I was told the base design came out of Europe/Great Britain. Would anybody know if this is true?

Thanks,

Mark

 

More than likely based upon these:
http://www.rnli.org.uk/what_we_do/lifeboats/current_lifeboats/lifeboat_detail?articleid=44200

 

I would have a look at Dutch lifeboats.

 

Isn't this the ACB design they used to make here in Bellingham?

 

Update: 2011-06-30

See attached for details.

It seems that the USCG RB-M was designed by camarc per MMC. If so USCG RB-M is a UK design.

Can any camarc people confirm?

http://www.camarc.com/Patrol-Boats.htm

Mark

 

It is pretty conclusive when the press release says: "..The RB-M is based on a CAMARC design.."