To A New Paradigm With FPB

The Dinghy Conundrum – What Is The Correct Mix Of Factors?

Circa DInk 16 1

We have recently been asked our ideas for the ideal mix of design factors for a larger FPB dinghy. This got us to thinking about our own experiences, and what we would want in this category of gear, if we were starting from scratch. The comments that follow are based on the assumption that the dinghies will often be used in cruising areas off the beaten path. In this post we will share a few thoughts, and ask for yours.

When we were designing the dinghy for Wind Horse we obsessed at length over weight and being able to drag the boat up a beach. We quickly came to the conclusion that there was no way we’d ever move it more than a half a boat length, even if we cut the weight by a third or more. This lead to the understanding that we’d need a beachable dink to compliment the bigger dinghy. Our Gig Harbor sliding seat wherry served admirably in this context, and at 114 pounds/53kg, was easily dragged up the beach by the rower.

There are certainly lots of very light car-top aluminum dinghies on the market, and you may be wondering, why not use one of these? The answer lies in the type of service we want. The larger dinghy needs to be strongly built, able to take a pounding from running hard in chop, bang into the odd bit of ice, and find the location of rocks. Impact resistance requires more metal, way more compared to the car toppers.

The next issue is length, which directly relates to space on deck. There is a conflict here. You are playing off the space around the bow, and overhang of the swim step, against the benefits of a longer dink. For us that is an easy decision. Waterline brings with it efficiency, sea kindliness, and better steering characteristics. These pay such benefits in the dinghy that we will always opt for the longest dinghy we can fit, but still keeping beam more akin to the shorter design.

Circa DInk 16 3

The boat shown in these renderings is  the standard Circa 13 foot dinghy, stretched by 25%, with beam held the constant. This results in a sleeker underwater profile forward, which translates to a softer ride. The added length brings with it more stability, and better steering. The LOA upgrade offers the option of employing more horsepower, and cruising faster. Sound familiar?

One of the things we want in a dinghy like this is a second engine, so that if the main outboard quits when we are fishing outside the pass of a Tuamotian atoll, or deep in some Greenland fjord, we have a means of getting back to the anchored mother ship.

There are two approaches here: one is a single large engine with a smaller outboard for backup and for use when fishing. Or, you can use  two smaller, but similar engines. There are cogent arguments on both side of the debate, but for us the answer is twin engines. We feel the loss in efficiency that comes with two outboards is made up for by the ability to move the dinghy at a good pace on just a single engine.

Circa DInk 16 2

We are still working through the conceptual details and will fill you in as these come along. We anticipate building in a T-top to provide shade, with roll down windows to give a bit of protection in cold or wet weather. Our preference is to keep the area forward of the steering console clear, so that it is available for carrying gear, and keeps the deck space open should the dinghy be used as a life boat.

We’d want a couple of lockable storage areas for ground tackle and safety gear. And we would certainly need a built-in boarding ladder, to make climbing back aboard easier after a swim or dive off the dinghy. There needs to be provision for a tent or cuddly for environmental protection if this becomes a life raft, and to keep gear or people dry on a long dinghy trip. Granny bars for getting in/out over the bow and side will be needed. There should be somewhere to stow a drogue and its rode and a rack for oars and rods on the sides; under the gunnels would be a good idea.

These three images are a baseline. We’d love to hear your thoughts on the single vs twin outboard question. And, what, if any features you think should be incorporated in the dink design.


Posted by Steve Dashew  (April 21, 2013)




33 Responses to “The Dinghy Conundrum – What Is The Correct Mix Of Factors?”

  1. marcus p Says:
    i think single engine is the way to go now – much like your rationale for the fpb (prior to the wing engine installations). new outboards – esp. the japanese manufactured ones – are so good, that auxiliaries are a dieing breed. you can now buy motors with 4stroke and efi right down to 20hp with little to no weight penalty over a 2stroke. failures now are linked to sub-standard maintenance and are often not circumvented by twin motors – think water in fuel (will affect twin engine installations unless fully separate fuel systems), battery problems (unlikely to have twin batts in a dinghy) or seized steering (again twin motors on a dinghy are likely to share the same cable). stick with one engine and be vigilant with maintenance. you could even put a diesel outboard on the dink for the same weight as twins – if youre happy with the price, noise and vibration…

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  2. marcus p Says:
    for areas where security is problematic, a lockable compartment maybe of use to secure items on board. and for the same reason, an engine kill-switch or steering lock to make it a bit harder to drive away with.

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  3. Gerhard J. Says:
    Hi Steve, if I had … I would shop at http://phantomjetboat.com/ It’s not a dink, but a very good shallow water boat.

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  4. Steve Bellamy Says:
    “Sea legs” is a NZ company who build a nice, 6m aluminium D-tube dinghy with retractable wheels, so you can drive it straight up the beach. No wet feet or back strain. Expensive, though perhaps not in the overall context of the cruising life.

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  5. Bob N Says:
    All sounds good. Some suggestions: Flotation tanks and, given the amount of motors that are stolen, means of locking the motors to the boat in a manner that makes them extremely, as in extremely, hard to steal. Given the size of the dinghy, some built in fuel tankage if enough could be had to make it worthwhile while not detracting unduly from load carrying.

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  6. Ben Woodford Says:
    How do you feel about diesel outboards for a dink rather than gasoline? Also, In your experience would a drop ramp for beach loading/unloading be worth the trouble?

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    Steve Dashew Reply:

    Love the diesel concept, Ben, but we don’t know of any in current manufacture. FPB 64-4 has a rebuilt diesel, but it has not been without problems. Drop bows work great in protected waters. We need more of a point for running in chop.

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    Phill Reply:

    Steve, I draw your attention to Baltic Machinery, Klaipeda,Lithuania http://www.balticmachinery.com Operating since 2002,an MSA certified company, rebuilding Yanmar diesel outboards.Personaly I am seriously considering a Yanmar diesel outboard,could you advise me of the problems encountered.

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    Steve Dashew Reply:

    DOes anyone have real world experience with these rebuilt Yanmar diesels?

  7. Matt Marsh Says:
    Another thought on twin engines: Using identical outboards makes for a much simpler spares inventory than using one big outboard and one tiny one. And since at least half of the outboard engine failures I’ve seen have been due to contaminated fuel or bad fuel hoses, I would rig them with independent fuel systems and separate, twin fuel tanks. Another option, one I am leaning towards for our new boat, is a single gas outboard for main propulsion and a pair of electric trolling motors as auxiliary propulsion, perhaps with a small (~4hp) 12V generator to give emergency get-home power on the electrics. That would give access to a lot of shallow, quiet places where you can’t or don’t want to use a big gas engine.

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  8. ron Says:
    my suggestion would be identical twins small enough to rope start (30- 40 hp) and a small solar panel to trickle charge cranking battery, transom extensions as in use on the diesel powered dink ,these to add to fuel capacity of twin internal fuel tanks, and use as reboarding ladder, long oars and matching oar locks so as to row standing up facing forward (removable and stowable oar locks) then blow foam flotation into every open space or void…this starts to sound heavy but with such a nice design to start with I think would be bullet proof

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  9. Kent Says:
    I think the answer depends on how far help is. If someone is available to rescue you within a reasonable length of time then the single engine argument makes sense. But if you are alone I would want redundant propulsion. As to a drop ramps Munson Boats makes them. But these are larger boats. http://www.munsonboats.com/index.html If cost and weight were not a consideration a small high volume jet drive powered by a small diesel has a certain appeal.

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  10. Peter Says:
    Here you go Steve, I don’t know what they are like. http://www.klaxondieseloutboards.com/ Regards Peter

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    Steve Dashew Reply:

    Peter – do you know of any real world experience with these diesel outboards?

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    Peter Reply:

    Sorry Steve, no I don’t, I guess there must be people out there though that do?.

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    Peter Reply:

    Yanmar do them also. http://www.smp-ltd.co.uk/product/productid/1795/productname/Yanmar-D27/D36-Marine-Outboard-Diesel-Engines/

    Steve Dashew Reply:

    Are these new, Peter, or rebuilt? And, if rebuilt, do you know anything of their reliability?

  11. Scott Says:
    I like the idea of a small inboard diesel and a jet! One fuel type, reliable, ease of servicing (make it accessable. clean fuel, clean air, and away she goes! Jet gives great manuveability, right onto the beach, and off! I have also run twin 25hp mercurysvery successfully on small boats and as previously mentioned, modern technology has made the new ones even better. Clean fuel, clean air, and away she goes.

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  12. Paige Says:
    I’ve spent a lot of time thinking about the ideal small(ish) tender for a cruising boat and like you I’ve come to the conclusion that aluminium is the way to go. I’m not so sure about the hull form though. I’ve used a fibreglass catamaran dinghy to good effect but aluminium would be much better weight wise, even if built to the exacting FPB standards. I can’t say that I have a preference for twin engines or a main and auxiliary.

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  13. david Says:
    Hi Steve, This is looking an awful lot like a SOLAS rescue (MOB) boat missing the righting float. If you and Circa are up to tweaking the design and approving it for SOLAS there is definitely a market. This could be a requirement for your largest boats for some commercial applications. Most have some type of foam or air/foam hybrid collar rather than tubes like a RIB. These boats usually have sealed decks with buoyancy foam or chambers so they’ll stay afloat even if the collar is damaged. I agree that new outboards are extremely reliable. I’m buying a 60hp 4-stroke or 2-stroke E-tec outboard which will move my 4.2M SOLAS boat 25+ kts with a full load. Also very interesting are the small waterjets available. One example is the FNM HPE110JD, 110hp Diesel waterjet package which looks really cool. http://www.fnm-marine.com/ Keep the posts coming. Always Keen to see what you’ve got going on.

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    Steve Dashew Reply:

    We would normally have a double bottom with the sole above the waterline. Then, between the fender around the perimeter and a boxing of the coaming on the inside, there will be floatation and a degree of stability if swamped. Once we get the basic concept worked up, we’ll have to come back with a weight calc and see what is required.

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  14. Gerhard J. Says:
    asking Google .. other thread found on http://www.boatdesign.net/forums/outboards/diesel-outboard-motors-43056.html Military and Navy likes it …

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  15. John Says:
    As a design consideration, you might consider a hull that has a very sharp V that turns into a truncated shallower V, sort of nested Vs. Hodgdon utilized this concept as a way of reducing shock impact in their MAKO patrol boat. I think Douald Blount suggested the concept.

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  16. Daryl L. Says:
    My dream dingy: http://www.atkinboatplans.com/Utilities/HappyClam.html Not designed for sheet construction but could be done with the English wheel or strips of aluminum. Very sea worthy boat. Use your gunwale and protection details and floatation. Second choice: http://www.atkinboatplans.com/Utilities/RescueMinor.html Designed for sheet construction. Same gunwale and protection details and floatation. This would solve the different fuel issue but would be a little slower than your outboard design. If these don’t appeal to you consider a jet river/ocean sled with diesel engine. The diesel outboards don’t seem to be “ready for prime time” yet.

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  17. Patrick Lasswell Says:
    As a hull that will endure for the service of the FPB, this design makes a lot of sense. I’d want to see how you handle cargo tiedowns, because balancing gear security with maintenance ease is a serious challenge. As much as I love flight deck padeyes, I never had to maintain those attractors of all things corrosive and hard to clean. “Lie-flat” D-rings are great, until a piece of small stuff gets jammed into the hinge and turns them into a toe/finger traps. In some ways I favor an ablative cargo containment panel that you plan on replacing every five years or so. Instead of pretending that this tricky piece is going to last forever, secure the cargo rig with big bolts, and replace the whole rig when it starts to wear. Thick marine plywood is not cheap, but it costs less than dents in the hull or re-welding fittings.

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  18. Stedem Wood Says:
    Looks as if the diesel outboard is going to cost about 50 pounds, or 23% more than a 40hp four stroke. May not be an issue in the bigger dinghy, but seems like a lot of extra weight for the smaller version used on the 64. That outweighs (so to speak) the advantages of using diesel fuel for me.

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    Steve Dashew Reply:

    Hi Stedem: The other issue is reliability, about which we know little at this stage. Hopefully we will learn more on this subject from SetSail readers.

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  19. Daryl L. Says:
    I seem to remember you mentioning at some point that you liked the Symmons Sea Skiff type of motor well. I have often thought that would be nice for the protection of the motor when the dingy is tied to busy docks. One could run the rub rail all the way around the boat.

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    Daryl Reply:

    If you go with the twin outboards would there be room for transom extensions? If so you could run your gunn’l guard around aft of the engines. This would give added protection to the motors. Both from a getting banged on things and from a security standpoint. They would certainly look harder to steal…..

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  20. Michael Jones Says:
    A small version of the Voith Schneider Propeller would be interesting…

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    Ben Woodford Reply:

    The V-S prop is a very slick piece of work, looks like it has all the benefits of pod drives. The only issue I see is if it breaks, you need to be pretty close to a 1st world supply line to get it fixed. A screw type prop is more easily found.

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    Michael Jones Reply:

    No doubt about that. It’s basically a “vertical helicopter” — if you have a sense of cyclic and collective then the way it works will make immediate sense. One of the fun things about it is that (hypothetically) the blades can be raised and lowered. Your kW/HP will scale with the blade depth, but even a 6″ depth gives some shallow maneuvering power. I like it because it is clever. No doubt Steve would not be willing to bet “getting back to the ship” on a little-known Star Trek propulsion device.

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  21. Victor Raymond Says:
    This is probably blasphemy but I like the newer aluminum RIBs. They are very light weight in comparison to fiberglass and will take hard beach landings better too.

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Comments or Questions?