To A New Paradigm With FPB

Ultimate Dinghy Revisited: Is a Big RIB The Answer?

FPB 78 RIB Concept 3 1

We have been rethinking the ultimate dinghy concept and are looking at a big custom designed RIB for our new FPB 78. Not having experience with properly designed deep V RIBS, we have a few questions and are hopeful the SetSail community can provide real world answers.

FPB 78 RIB Concept 3 4

This is a very rough concept sketch: to check weight, basic dimensions, and how this design might fit on the aft deck of the FPB 78. We are at 4.85m x 1.95 meters overall or 15.8′ x 6.35’. The hull is a constant 24 degree deadrise V-shape.

FPB 78 RIB Concept 3 2

This dink has three jobs: a shore boat where there are docks at which to disembark, for exploration far from the mother ship, and as a primary escape vehicle instead of a life raft. As such we are interested in comfort, which means ride and dryness, and seakeeping in sub-optimal sea states. The ability to maintain good turn of speed in chop also has value.

FPB 78 RIB Concept 3 3

We are looking at a displacement of around 800/900 pounds (360/410 kgs) rigged, but not including passengers.

Now some questions:

  • How does a RIB like this compare in terms of seakeeping to a more traditional flatter bottom dinghy?
  • Is it drier or wetter in chop?
  • What about beam to length proportions? We are 2.5/1. Narrow by dinghy standards, but a little chubby by the criteria of big rescue RIBs.
  • Any projections on performance and fuel burn with a pair  of 20/30 HP two stroke outboards?
  • We have assumed 4mm/3/16” bottom plate, 1/8”/3mm deck, same for frames at 12”/300mm on center, with 6mm1/4” transom + local stiffeners in our weight budget. We want to be fault tolerant. How do these scantlings sound?
  • Suggestions for tube construction details, materials, what to look for and what to avoid, and vendors are solicited.
  • Tube diameter? We are showing here 450mm/18”. Sufficient or should it be larger?
  • Tube clearance from waterline at rest, with two crew aboard, and when heavy?
  • Deadrise angles forward and aft?
  • Other details on rigging, store, etc are always welcome.

Posted by Steve Dashew  (November 24, 2013)




33 Responses to “Ultimate Dinghy Revisited: Is a Big RIB The Answer?”

  1. Marco Says:
    I would closely look at some one like Naiad http://www.naiad.co.nz/Boat+Range/Commercial/Naiad+Open+RHIBs.html who have a lot of experience in RIB design. All the RIBs I have been in have alway floated at rest with the tubes in the water and above the water when the planing, which allows a sharper dead rise for comfortable travelling, and the tubes then provide stability at rest. I also thought a variable dead rise design was better for planing and requires less power.

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  2. Justin McPherson Says:
    Steve, I would have a look at Sothern Pacific in NZ (http://www.southernpacific.co.nz/cor_aluminium.htm). I recently bought a 3.3m RIB and had it shipped to Fremantle. They were happy to customise the boat. It runs really well and I am very happy. Regards Justin

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  3. Joe Says:
    One of the Miami Police boats is all aluminum, the very front of the tube is made of aluminum and flattened off on the top, to make a step. So for picking up/dropping off they just hold her ahead slow head onto the dock, the side tubes look like hypalon. When considering duty as a lifeboat, although not as fast as an outboard, a small inboard diesel would give much greater range at hull speed, and would give a much lower c of G, having the weight forward really reduces the pounding, like the old mako 236 inboard. Since we’re talking hypothetical, a jet drive would help a lot in skinny water.

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  4. David Says:
    All of my experience with larger RIBs is launching them off of a US Navy ship for drug interdiction operations. My experience is that when the seas were rough I much preferred the motor whale boat for seakeeping ability. The RIB really bounced around a lot and was far more wet on deck. Although coming along side it gave a natural bumper and made it a natural for a boarding platform—if you can keep it along side. Obviously the ability to plane gives RIBs a huge advantage in getting around fast. As for the design features I don’t have much to add.

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  5. Shannon Says:
    If fuel efficiency isn’t a big issue the running gear (engine & jet) from some of the larger personal water craft or smaller jet boats could work well. Less draft would be good for beaching or shallow water & with the weight mounted low the center of gravity would be lower. It would also be harder for someone to steal a motor. I don’t know how reliable those systems are but they do have some pretty small twin jet boats & they use those same Rotax engines in ultralight aircraft so I would assume they are reasonably reliable & light. Motor & jet probably weighs more than a 2 stroke outboard though(just assuming) If they are light enough you could still run twins. I figure you want 2 in case 1 fails.

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  6. Sarah-Sarah Says:
    Steve-Sue and I used fiberglass RIBs prior to our Circa dink. The big difference is greater stability of the RIB over the Circa dink. We “skipped” over an uncharted rock at speed with our Circa dink and the resulting damage was a number of tears in the bottom which require expensive repair. Had we done the same with a fiberglass bottom, we would probably repaired the damage ourselves with cloth and resin. The RIB we liked best had a floodable fiberglass double bottom which contributed significantly to its stability at rest. The water drained rapidly enough from the double botton to pull a 150lb single skier from a water start with a 30hp OB. This dink was 13 feet by about 5 feet. I don’t know its weight, but we easily hoisted it onto the foredeck or our sailboat with the spinnaker halyard. It had a “saddle” seat behind the center console which allowed a comfortable and secure position for two (one behind the other). The only drawback was the tube being filled with air is subject to leaking. Foam filled tube similar, but larger than those on the present Circa dinks would eliminate this problem.

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  7. Scotto Says:
    with that deep V and fore foot, it will be difficult to beach and enter/exit the boat. The kiwis built some very wicked RHIBs of all sizes, you are in the neighbourhood, use the local talent! Personally I prefer a diesel inboard and jet Steyr/Hamilton would be a good combi possibly too big for your sub-16 footer.

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    Steve Dashew Reply:

    We do not consider any of the bigger dinks beachable. TRying to keep the pre-passenger displacement under 400kg.

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  8. Rick de Castro Says:
    Just a thought – I personally prefer jet drives in that horsepower range: There is really negligible added weight, and the lack of a prop strikes me as a really good thing. Also, to replace a life raft, some sort of cover would be really nice, especially if it had stays or inflatable arches to create more headroom.

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  9. Scott Flanders Says:
    Steve, first of all, you need two dinghies; one for dinghy exploring and a second for just getting to shore in usual situations. Now let’s make your quest super simple rather than design something from scratch. Buy two AB – aluminum bottom dinghies, one 9′ with or without a sub floor and a second should be the largest you can fit comfortably on board. The second dinghy should have a 30-40hp 2 stroke Yamadog with no console or steering. Then have Circa build a very simple tube frame seat and a super lightweight console with a flange that can be 5200′d or Sikaflexed to the sub floor. We built a 14′ Aqua Pro – originally made in NZ but now made in China with initial Big Problems when we were looking, with just the seating and console we mentioned it was super light instead of the usual console and crazy heavy seating & storage. AB makes a commercial version of a 14′ for pro dive use and that is what I would buy. Today we have a 10′ AB – aluminum bottom – with a 15hp Yamadog and we LOVE it as well as a 9′ dink with a 3hp, but today I would buy a 4hp. The small dinghy should be no smaller than 9′ and no larger than 9 1/2′ with a 4hp Yamaha 4 stroke that is available in NZ for peanuts compared to the rest of the world. This is a super light package that can be dragged up a low tide beach easily by two geriatric cruisers such as ourselves. It is particularly good for taking shore ones ashore quickly without the fuss of a larger dink. Over the years most of our dinghies have been upside down for one reason or another and I can get a 2 stroke Yamadog running in no more than 45 minutes using no parts and they suffer no harm. With a submurged 4 stroke, you have a nice anchor. S.

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    Steve Dashew Reply:

    Agree on most points, Scott: We have room for the ten foot AB as a beach boat (had the 11 on Beowulf and loved it). And if we do not do a custom we will probably get a 13 foot ABandmake a standup consol. But would rather find a longer, skinnier shape.

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  10. Scott Flanders Says:
    Geesh, I said 4 stroke as a recommendation in one sentence instead of 2 stroke. I washed my mouth out with acetone so please accept my apology. S.

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  11. Chris Critchett Says:
    Steve, That 24 deg deadrise all the way aft is beneficial on boats run REALLY hard, to the point where the hull is frequently airborne. If that’s not your intent, I’m not sure you wouldn’t benefit from flattening that out a few degrees back aft for better load carrying at slower speeds. Not exactly sure, but I think all of our smaller hulls like this are less than 24 back aft. Maintaining 24 deg and higher forward of amidships is essential, of course. Not sure about ‘best’ ride in seas. I think the shock absorption of the inflatable tubes helps dissipate impacts quite a bit, but to get driest ride you’re going to want hard chines below the tubes to provide a sharp corner for the water to break cleanly off of. Spray and even solid sheets of water will ride up those round tubes and can give a surprisingly wet ride. A good rub rail profile with a molded lower lip will help a lot, too. To maximize space inside the boat, I’d consider a D-shaped tube, as many have these days. You lose that nice round bouncy-seat all around the perimeter, but that’s not all bad. Foam-filled instead of inflatable may be desirable, too, if you plan to abuse it around barnacles, commercial docks, etc. Really, what you were approaching with your aluminum dinghy surrounded by heavy-duty fenders was not a bad solution, IMHO. Or the boats Circa is doing now with the foam collars. My $.02.

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  12. Evan Gatehouse Says:
    To answer some of your questions: How does a RIB like this compare in terms of seakeeping to a more traditional flatter bottom dinghy? RIBs have a more comfortable ride when designed properly. The tubes deform as they hit a wave and absorb shock. Thus a 24 degree deadrise is overkill on a RIB Is it drier or wetter in chop? In the size you are considering, when they are planing about the same. At non planing speeds wetter because splashes only have to make it to mid tube before they can come over What about beam to length proportions? We are 2.5/1. Narrow by dinghy standards, but a little chubby by the criteria of big rescue RIBs. OK – lots of this size rib are 2.2/1 beam ratio. Any projections on performance and fuel burn with a pair of 20/30 HP two stroke outboards? You might not be able to plane with full load and single 20 HP motor. Speed really depends on final deadrise angle! Yamaha 25 Enduro might be your choice of outboard, not the standard 20 or 30 model. If a single outboard can’t plane you then twins are not that useful are they? Better to have a big single like a 40 (less weight & cost overall) and a small get home kicker with a 25″ long shaft like a 6 HP Tohatsu Sailpro. How often have your Yamaha outboards stopped dead? With a 2nd dinghy with it’s own outboard loss of big dinghy outboard is a minor inconvenience until it gets fixed. Suggestions for tube construction details, materials, what to look for and what to avoid, and vendors are solicited. Why try to reinvent the wheel? There are lots of vendors of super robust commercial RIBS. I’d look into Zodiac Hurricane (the Military/commercial arm of Zodiac). They are a separate company with decades of experience building RIBs that are bulletproof. Too big for you but the 733 is legendary and has seen tons of CG use. The other commercial vendor to consider is NAIAD in NZ. Also tons of commercial duty RIBs. Tube diameter? We are showing here 450mm/18”. Sufficient or should it be larger? Just big enough I’d say. Absolutely no smaller. Tube clearance from waterline at rest, with two crew aboard, and when heavy? OK to have tubes immersed at rest – the tubes will be out of the water at speed if properly designed. Deadrise angles forward and aft? 20 at transom, more forward unless you’re the CG and HAVE to go out in horrible conditions where a 24 deg would be more appropriate. For a typical yacht tender you do have the option of backing off the throttle if the seas are huge. Other details on rigging, store, etc are always welcome. Shock absorbing seats? Saddle seats so you sit on your bum but your legs can act as cheap shock absorbers? “Elephant trunk” transom drains for quick draining of swamped boat in surf conditions – do you know what these are?

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  13. David Sutton Says:
    Hi Steve, Using the Solas Fast Rescue Boat is a great starting point. These are typically about 2.75:1 length/beam WITHOUT the collar, but you can blunt the bow a bit. For a boat that size you’ll obviously have to be very careful with weight. I have an aluminum 4.2m Solas FRB and being exceptionally overbuilt I did a lot of lightening to increase load capacity. They weren’t intended to carry four people plus provisions, but are great seakeeping boats and really tough. Twin two stroke 30′s should be plenty of power and give you redundancy. I don’t think you want much over 100kg back there though. Foam collars or hybrid foam with an air tube are superior to air tubes. Tubes are complicated to integrate and you really should ask yourself if you want the flotation or a fender or both. If you are going to have a sealed deck or foam filled hull then you already have reserve buoyancy and really don’t need the flotation. You want the added stability and something soft to keep it off your topsides. Check out Wing for some examples http://www.wing.com/sponsons-tubes-collars.php Mine is foam with a heavy vinyl cover and are trapezoidal, about 10″ thick x 14″ tall. This shape normally keeps them out of the water but would support a couple of people standing on the gunwale. That brings me to self draining decks. It is a great advantage if you can swing it, since it eliminates the need for a bilge pump, but I found it just didn’t work with this size of boat and the extra loads a tender is required to carry. I think 1/8″ plating and stem is sufficient with the transom being 3/16″ and framing 3/32″. These are small boats and as such have a lot of inherent rigidity once everything is locked together. A 1/8″ tread-plate deck would be good. It is worth thinking about use of extrusions for chine and gunwale which incorporate a channel for securing the collar. This gives the structure added rigidity and impact resistance. Looking at your drawings I think the bow is overly deep. I suggest slightly less deadrise which flattens out aft to a small bottom flat. A grounding bar that runs fairly far aft is also a good idea since the plating is relatively thin. A deeper vee than is found on most recreational RIBs is going to be far more comfortable. Love the site and happy to share more info if you like. Cheers, David

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  14. Patrick Lasswell Says:
    My experience with RIBs is Navy based, but I differ with David above because I’ve been caught in the middle of the Atlantic with a dead Motor Whale Boat (MWB) that died because of a steering gear failure. I’ve seen too many MWB’s dead in the water in bad situations to ever advocate for them. RIBs were much more reliable because they were less than fifty years old when we got them. I’ve assisted the Coast Guard in launching and recovering their RIBs for drug interdiction, I’ve performed dozens of boardings from a 16′ Avon RIB, and a dozen more from a Navy 26′ RIB. The handful of MWB boardings I was in on are etched upon my mind, because the stressor of wondering how or if we’d get back always added to the piquancy of missions. Additionally, I’ve spent some quality time on the SeaArk 34′ patrol boat which helped me develop a strong appreciation for the advantages of the SAFEBoat design. My friends in the Coast Guard and Customs and Border Patrol speak with unreserved admiration of the SAFEBoat design, construction, and operation. In my experience, you can load the hell out of a RIB in decent weather, but the first splash of the morning is waiting once you clear the lee. In a survival situation, the higher freeboard of a standard hull vessels worries me as a sail and wave catcher in confused seas. I’d rather risk the absolute stability of the inflated/foam collar over a standard hull in bad stink, but I’d also want to make sure that I had survival suits in locations and quantities where they could not be lost or run short. I’ve been cold and wet in bad situations a lot too many times to risk facing that unprepared.

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  15. Pete Says:
    How about going for “D” shaped inflatable or soild foam tubes, this would give you more internal beam.

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    Steve Dashew Reply:

    Do you have a source for d-shaped tubes?

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    Peter Reply:

    Yes, both foam and inflatable.

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    Roger Reply:

    With your drive for low maintenance, I’d go with the foam filled tubes, maybe rotomolded. No pressure = no leaks. Easy to get a wide flat surface to stand on too. Add on some retractable wheels for rolling up the beach.

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  16. Graham Says:
    Hi Steve, it is worth a look at Williams tenders. Many options to choose/customise. Some are under your 400kg requirement. http://www.williamsjettenders.com/

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  17. Bill Kelly Says:
    Steve, Have you looked at Bullfrog boats, out of Washington. Their 15 foot utility boat? http://www.bullfrogboats.com/sport-utility-boat-15.html I had their 10- foot dinghy on my Nauticat 44. Unsinkable and very stable.

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  18. Evan Says:
    Or go with something designed by Fabio Buzzi if you want to go fast :) http://fbdesign.it/a24rib.php (He is a long time offshore champion designer/racer of fast RIBs and conventional motor yachts) I have to reiterate the use of soft collars. Tube deformation is the mechanism that absorbs wave shocks and makes RIBS such soft riding sea boats. Plastic/aluminum/foam collars/D collars don’t deform or deform a lot less.

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  19. marcus p Says:
    http://www.gpengineering.co.nz/rigidinflatableboats.php similar construction tech to Naiad – thats is, bag and bladder for much improved puncture resistance and durability. alloy hull and floor. they use a alloy inwhale/riser off the floor that results in a D profile for the tubes – giving more floor width on the deck. they are experienced with jet propulsion and have supplied boats to MSA standard to NZ Coastguard and Policing.

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  20. Alan F Says:
    another alternative – how about this little cat from Jim Pauling in NZ http://jimpauling-yachtdesign.com/jpyd-news.html http://www.youtube.com/watch?v=DDzNrtFZFzQ

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  21. E Says:
    Don’t reinvent the wheel. A range of 16 alu RIB tenders: http://www.ribeye.co.uk/our-boats/t-series http://www.ribeye.co.uk/our-boats/t-series/ta-430

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    Michael Jones Reply:

    The ribeye models with the flip-down seat are nice. (Though larger, so is the Rib-X custom line http://www.rib-x.co.uk/exige-range.html)

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  22. E Says:
    Or http://www.seaswift.co.uk/category/packages/

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  23. Gene Says:
    I am a Zodiac Hurricane H733 user and commute on her almost daily from the island we have a house on to the mainland school, shops etc. Please look at the the new range of Zodiac Milpro’s SEA RIB Aluminium (SRA) with their Dtube. It can be scaled down to the required length. The previous Dtube FRP Zodiac 220 was very comfortable on short chop and was bulletproof. The concept of wider hull with the Dtube is very much proven and in my view works well for a tender. I also agree that the jockey seats work much better than the benches and they are kinder to your spine. I have Ullman suspension seats, but is may be an overkill for a dink.

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  24. Sarah-Sarah Says:
    Or aakronboats.co.nz

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  25. Rigo Says:
    http://www.oceancraft.com.au

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  26. David Sutton Says:
    have a look at these foam collars… http://www.gilmancorp.com/IntegralBoatFenders_.htm this type of collar is becoming more and more popular. the hollow D shapes are cool because you put an air bladder inside. one of the other advantages is that you clamp them onto the hull rather than using a bolt-rope or lacing. Check out this example too. http://www.boattest.com/resources/view_news.aspx?NewsID=853 Cheers, David

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  27. John M Says:
    Steve, Steve Callihan designed a small dinghie that could double as a life boat. He came up with the design after his experience being adrift for seventy six days. He wanted something that would allow him to actively seek land/rescue apposed to waiting for rescue in a raft. With the rigid hull being 10ft and the overall length being 11.5ft including collar it roughly fits your specification for your smaller dink. http://www.stevencallahan.net/images/spcdesigns/frib.pdf Keep up the good work! John

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Comments or Questions?