Speed, Sex, Rules, & Dinghies: Size Does Matter

FPB 78 111 2

We want to talk about a subject often avoided: Size. It is important for comfort, for aesthetics, and for speed. This has been much on our mind of late as we reach the “hard point” in the build cycle for the FPB 78, after which changes are not allowed. If you study the renderings in this post you will note the Dream Machine has a different look.

She has had a bit of sculpting aft on her topsides, and the swim step lengthened. The impact, in terms of aesthetics and performance is substantial.

FPB 78 209 ext profiles 103

The logic behind this, and the tradeoffs, may be of interest.

First is our desire to have an official length less than 24 meters, as above this a yacht is considered a ship in traffic separation areas and required to stay with the big guys, a prospect we find unpleasant. Measurement rules vary with the flag flown, but often start at the rudder post or corner of the deck, and exclude the swim step.

The FPB 78 until recently had an official length under Lloyd’s Rules of 22.8m or 74.8 feet. Adding in the swim step brought her to 23.8m/78 feet LOD. Which meant we had some “official” length left with which to play if it looked favorable, while still staying below the 24m rule.

Many dimensions in yachts can be compressed or made smaller without impacting their function. Not so waterline. Comfort, speed, range, and as a result safety, are directly related to waterline length. Longer waterline means more boat speed, plain and simple. Still, we don’t increase the canoe body length purely for speed – at least not often. There is docking to consider, swinging room at anchor, maintenance, and a desire to have sufficient length to get the job done and no more. Also, excess longitudinal stability leads to an unpleasant ride heading into the waves. To the extent the shape of the hull results in a hull form with less beam aft, the pitching issue is mitigated.

FPB 781 211 VST 18RIB

It was our search for a dinghy which would be good in chop that drove extending the hull. Having come to the conclusion that what we were after was a long skinny RIB, we needed a place for it to sit.

EVI0682 resize

Adding a bit to the waterline aft, and then going back to the type of lockers used on the swim steps of the FPB 64s and Wind Horse (FPB 83), provided a place to park a 5.75m/18.6ft RIB as shown here. This is a VSR5.8C (www.vsrlab.com) that is  just 2.22m/7.23’ wide. These are reportedly very good in a chop. If you have ridden in one or seen them in action, we’d love to hear about it.

Aside from the big primary dink, there is added capacity in the stern lockers. The starboard flammable stores locker will hold 14-to-18 20 liter containers of outboard fuel. Another benefit is the port side fishing station, where you can fight your fish close to the action, and coordinate with the person on the swim step trying to boat the thrashing monster.

There is so much freeboard – 2.2m/7+ft from waterline to the deck edge at the stern – that coming alongside and holding the dink by hanging onto the deck edge, belting or stanchion, is not practical. Hence the topside pipe rail aft, situated where it can be easily grabbed or tied onto.

FPB 781 211 VST 18RIB 2

The swim step extension itself opens up some opportunities for unloading groceries and passengers. It also makes hauling the smaller dink at night a very simple affair.

FPB 781 211 Swim step dink

Rather than leave it floating, or hauling it up on deck with the booms, we can now drag it up onto the swim step with the aft deck winch. We would not carry the dinghy here at sea, but for protected between-anchorage trips, when at anchor, and in emergency if we have to quickly exit an open anchorage that has become a lee shore, this will work great.

Which brings us full circle back to the ultimate benefit of this longer waterline: speed and efficiency.  Waterline is the single most powerful ingredient you can employ in the war against drag. Adding an extension like we have shown, a modest cost and minimum weight gain, yields a substantial improvement in efficiency. The fact that this added canoe body occurs all the way aft gives us a chance at better controlling prop flow, which can lead to an even more favorable outcome.

You can use this efficiency for better range or more boat speed. In our case, with sufficient range for the longest passages, we will most probably add a bit of boat speed. Where 11 knots/264NM per day might have been the previous sweet spot for the mix of ambiance, fuel burn, and weather risk factors, we are probably now cruising at 11.5 to maybe 12 knots  (276/288 NM per day).

Posted by Steve Dashew  (January 31, 2014)

25 Responses to “Speed, Sex, Rules, & Dinghies: Size Does Matter”

  1. Andrew Says:

    Hi Steve,

    No rowing gig harbor rowing dingy this time around?

  2. Steve Dashew Says:

    No decision yet on second dink. We are just looking at possibilities. There is a good chance we will still carry a sliding seat rowing dink as in the past.

  3. Nigel Says:

    Think you have made a good choice of rib – I have settled on a 5.8m ribtek rib which has proved big enough to cope for 6/8 people and yet small enough to push off a beach. VSR ribs are being used a lot by olympic and national coaching squads and have a high bow and cut through chop really well.

    Is that the exhaust pipe you show forward of the new grab rails on the topsides. Is it dry or wet exhaust? Will you not get smoke or worse blowing over your new rib?

  4. Steve Dashew Says:

    Yes, Nigel, that is the exhaust. For a variety of reasons we think this will be the best location for the exhausts. Engines are typically not on when doing dink operations, the exhaust is wet, and there s both aqualift sand dual stage muffler. The latter remixes water and gas which will reduce any tendency toward messing up the topsides or dinghy.

  5. Louis Says:

    Definitely one of these on the long term Christmas list. I noticed that in the first render there are two inverted staples (or ‘D’ shapes) in the side windows of the matrix deck. They aren’t present in any other renderings, are they a new feature?
    I love the mindset behind the whole FPB series, please keep the great work going.

  6. Steve Dashew Says:

    Those Matrix deck window shapes are a start at the design of the enclosing window system. They are small openings to allow s ome ar flow without having to open the entire window.

  7. Chris Says:

    Very curious about the decision to move the exhaust outlets up there, vs. the underwater outlets on WH and transom outlets on the 64’s.

  8. Steve Dashew Says:

    Moving the exhausts keeps the plumbing out of the aft end, and having them high on the topsides means less (or no) burble when a sea runs by compared to a transom installation. There will be less backpressure, and we t hink less of a station wagon effect. The negative is having the exhast noise source closer to the outdoor areas, which is why we have gone with an aqualift and dual stage muffler.

  9. David Says:

    Sorry, this is a bit off topic, but one of the first posts you made regarding the 97 (AKA Wicked) was the forward dorade boxes that increased airflow and thus reduced the a/c needs. I have just realized that these are not included on this design. I thought those were really innovative! Just wondering what changed in this design regarding that feature.

    Also, thank you so much for these detailed posts. I know the praise flows frequently from readers, but it is all well deserved. We all enjoy them and learn so much.

  10. Steve Dashew Says:

    There are indeed large dorades integrated into the forward facing structure on the FPB 78. One set under the roof overhang and another in the forward face of the Matrix deck coaming. The 97 is the saame.

  11. Scott Evangelista Says:


    What would be the downside to lowering the exhaust outlet to just above the boot stripe? Then any possible blackening would be on the black bottom paint?

    always a pleasure to see the updates!


  12. Steve Dashew Says:

    Low exhaust burble when the waves run by. That change in frequency is highly annoying to some, in particular ourselves.

  13. mark heeley Says:


    Ref VSR RIB . The picture you show is a 5.8R model .I have spent a week using a 5.8C and as the brochure says they are pretty remarkable in a chop for their size. Essentially rides like a much longer boat. I assume it’s the narrow design but there could be more to it. As well as a great ride they run fast enough (25 knots ) on smaller than expected engines (50 hp). Only issue for some will be that they have low/no hull form stability. So pootling around at less than 5 knots ,the boat tends to rock from one buoyancy chamber to the next. Fine by me but annoying for some. If length on deck is sensitive you could look at the smaller sister the VSR 5.4C. Proportionally wider than her bigger brethren but still narrow by conventional standards. Overall the best small RIB I have driven by some margin.

  14. Steve Dashew Says:

    Thanks Mark:
    One has to decide decide between initial stability and sea going comfort. You know where we will come down on that! We have room for the 5.8/18.5′ model, and almost certainly that is the direction we will go. But we hope to work a bit with them to come up with a yacht tender model. Way better tham doing it ourselves.
    One question for you – how are these in terms of spray wetting occupants when they are other than dead upwind?

  15. Mark Heeley Says:

    I would say (spray protection) better than average. You sit/stand relatively low in the boat as it has relatively high sides (tubes mounted high) – the very factor that allows the hull to rock to one side until the a tube contacts the water providing buoyancy. As you would expect their internal volume is smaller than a normal 5.8m RIB. Draining surface water in the hull is worth thinking about – unless you could ask then to build in a double bottom. Current system is the usual transom sock drainage solution. Not great for cruising in cooler climes.

    I prefer the plumb bow style of the “C” model. Looks great in grey (grey tubes/grey console). I am sure a stand-up console & T Top could be fashioned. Might be better for Circa to provide those finishing touches.

    For reference – there is a Chinese made copy of the VSR, they also copy other popular types. http://www.lingbo-marine.com.
    There is one of these as well as two real VSR’s in regular use at my local sailing club here in Sydney http://www.woollahrasailingclub.org.au/


  16. Anthony Kolliou Says:

    Steve , would you consider heating the forward windows in a similar fashion that aircraft do (bird strike for them) , in order to strengthen the glass (solid impact by large wave in the PFB case ) when cruising the higher latitudes?

  17. Steve Dashew Says:

    Hi Anthony:
    Impact on the forward windows is not a major concern given the strengthn of the materials being used.

  18. Hunter Lipscomb Says:

    Steve, I know the discussion came up in the early part of dinghy decisions about diesel outboards. Do not have any experience nor have I heard any reports, but has any thought been given to the Evinrude 55HP Multi-Fuel Engine? That would allow you to utilize the already polished diesel on-board, without needing to tote around volatile gasoline? I see the recommended HP for the 5.8R is 60HP, 55HP doesn’t seem far off from the recommended (maybe knowing there is an “infinite” supply of fuel back at the “mothership”, one could reduce the amount of fuel stored onboard the dinghy, reducing weight to make up for the lack of HP). 240 pounds is heavy in comparison to the 2-stroke motors you usually like for the dinghy, I would think having the booms would somewhat negate this as the motor wouldn’t have to be hefted around if the need to remove the engine came about. I also do not know about parts availability for Evinrude motors on a global scale, nor do I know what the reliability is like for these motors.

  19. Steve Dashew Says:

    Hunter, we have no knowledge of these “multi-fuel” engines. Do they really work with diesel and gas? Sounds wonderful, if they are not too heavy. Anybody have real world experience with them?

  20. Louis Says:

    Looking at the website it seems that they were originally spec’d and sourced for the US military, so reliability shouldn’t be an issue. What is an issue is the important not below making them not great for long term diesel use at least based on MFG notes.

    “2008 Evinrude 55MFE In new condition, Model E55MRLSCC, Special E-Tec, 2 cyl. Outboard Engine. 55 HP(41KW) – 5500-6000RPM , First Multi-Fuel engine capable of running on heavy fuels such as JP4, 5, 8, JetA, JetB, Keresene, gasoline based fuels: Gasoline, Gasohol, 87AKI (R+M)/2 or 90RON minimum. Emergency Fuel: Diesel DFM-F76, BioDiesel.

    Important: If outboard must be run on diesel, it should then be run on gasoline to clean internal components. Originally designed for use by Military Special Forces. Currently available for civilian use.”

  21. Louis Says:

    Reading more details around the web it seems that the key difference between marine diesel, JP-5 and JP-8 are the additives, sulfur levels an lube oil amounts. With the polishing that occurs prior to the day tank. It appears that the key is to determine exactly what kind of fuel is being used. The large discussion can be found here.


  22. Walt Says:

    Only available to the U.S. Department of Defense, Evinrude’s Multi-Fuel Engine (MFE) technology was designed for jet-A but will run on anything that burns though not as efficiently.

  23. Shannon Says:

    I don’t know about the multi-fuel outboards but the technology isn’t new. They have been running military trucks with multi-fuel engines for many years. Gasoline, diesel, kerosene, ethanol, basically anything that will burn & can be pumped through a fuel system.

  24. Ed Says:

    VSRs are narrow, wet in chop, with very limited payload compared to more standard RIBS. For support services for sailing – particularly dinghies – the advantages of the narrow hull form for lowering the strains on coaches’ backs and low HP engines can give reasonable turns of speed with good fuel economy (with a payload penalty) outweigh the disadvantages. Not sure the general RIB market feels the same – as shown by the vast majority of other builders’ boats being of a beamier shape.

  25. Colin Stone Says:

    TSS – I think the small/normal ship cut off is 20m, not 24m Certainly is in the Channel and in Rule 10d:

    (i) Inshore traffic zones shall not normally be used by through traffic which can safely use the appropriate traffic lane within the adjacent traffic separation scheme. However, vessels of less than 20m in length, sailing vessels and vessels engaged in fishing may under all circumstances use inshore traffic zones.

    (ii) Notwithstanding subparagraph (d) (i), a vessel may use an inshore traffic zone when en route to or from a port, offshore installation or structure, pilot station or any other place situated within the inshore traffic zone, or to avoid immediate danger.