Saturday, February 23, 2019

Flint and Fleet - Different Hulls for Different Jobs

My introduction to “reading” the shape of a boat’s hull by viewing a lines drawing came from watching over Dad’s shoulder as he read copies of “The Rudder”, Motor Boating’s “Ideal Series”, Howard Chapelle’s “American Small Sailing Craft” and other such documents. I guess I was about five years old at the time, and it wasn’t too long before I had a practical introduction to a table-of-offsets when Dad started to loft the lines of William Atkin’s Nancy, a 15ft 6in LOA modified sailing dory of 5ft breadth, and 13ft 6in length on the waterline. My part was to read out the offsets as Dad marked everything on his full-size lofting. I didn’t understand what the numbers meant, but I knew they were somehow very important.

Full-size lofting taking place in my workshop around 2003. Basically, expanding a set of lines from the scaled paper drafting to full-sized pencil and ballpoint drawing on white-painted plywood.

Full-size lofting taking place in my workshop around 2003. Basically, expanding a set of lines from the scaled paper drafting to full-sized pencil and ballpoint drawing on white-painted plywood.
As years went by my free days were spent sailing a Charles MacGregor-designed Sabot at the local sailing club and generally living a salt-water life, enjoying the freedom afforded to kids in the early sixties. But in the evenings I found myself drawn more and more frequently to the stash of books about boats which lived in a particular part of the wall of bookshelves in our TV-deprived house.

The drawings which attracted my attention most were the outboard profiles and the sail-plans. To me it appeared that the lines drawings were only for higher beings than myself, and they meant little to me.

Moving on fifty or sixty years, I now know that a boat drawing without a set of lines is like music without notation. Yes, music can be played by ear, and a boat can be built by eye or half-model – but for most of us, a set of lines is the key to visualising and understanding the shape and behaviour of a particular boat.

A typical lines drawing, showing waterlines, buttock lines, and hull sections. I also draw diagonals, but they are not shown in this particular drawing
But just as I know people who are amazed that I can’t ‘hear’ a piece of music by looking at the notes on paper, I find myself puzzled when someone who has a set of plans for one of my small sailing or rowing designs asks me if she will take their 15hp outboard!

Back in about 2003 I was asked to draw a design for a fifteen-foot rowing boat which was to be used by a very experienced waterman when he needed to row to the mainland from his island home on Moreton Bay, Queensland, Australia. Moreton Bay is a wonderful and varied paradise for boat people, but one of its notable characteristics is the ability to generate a particularly short and steep wave pattern. This new rowing boat design needed to be shaped to handle such conditions.

The resulting design, called Flint, has turned out to be very popular with home-builders. In fact, I use a Flint as my recreational boat-of-choice at the moment. She is 14ft 10in x 4ft 3ins and weighs in at as little as 40kg (88lbs). The boat is arranged to carry a small outboard for those who for whatever reason need that facility, and shortly after the plans were released, I started to receive letters and emails about using motors larger than the 2 to 2.5 horsepower units I recommended in the plans and instructions. Much time was taken up in writing emails to potential builders, explaining why excess power was counter-productive

In order to be an efficient rowing boat, and to be able to sail effectively with the optional sailing rig, the lines drawing for Flint shows considerable keel rocker, and the buttock lines (i.e. the bottom of the boat in profile) sweep upward at a significant angle to the waterline, terminating above the water at the stern transom.

In order to satisfy the many people who still insisted on using larger outboards, I drew another design - based on Flint - which has almost zero keel rocker aft of amidships – she is called Fleet, and will run happily at speeds of up to 20 knots (23 mph) in good conditions. More importantly, Fleet will run quietly and smoothly at her sweet-spot of about 12 knots (14 mph). At such a speed she is covering ground to windward at nearly four times that of a similar sized sailing boat. A potent and efficient magic carpet for an explorer who has limited available time.

Flint is 14ft 10in x a fraction over 4ft at the outside of the hull planking – Fleet is 15 ft x 4ft  - and both are made from the same pile of plywood and can carry similar loads. But from there the similarity ends. Flint is sweet as a rowing boat, close-winded and fast under sail, and cruises nicely with a two horse-power motor throttled back to the point where noise is not intrusive – under such outboard power she sits in proper trim at about 5.5 knots (6.3 mph).

Late last year my boating friend, Ian Hamilton, and I had a wonderful opportunity to carry out a series of tests, comparing the speed and behaviour of a Flint and a Fleet while using a range of motor sizes on both boats. The conditions were perfect, and because there were two of us, we were able to photograph and video the action, as well as recording the GPS-derived results in a note book. What a tough job – but I guess that someone has to spend a couple of days on the water playing with boats in the interests of science…

Although we tried a range of engines, I’m only going to discuss in detail the results from the two engines which I consider to be a good match – in efficiency and utility – for each of the designs. These engines were a 2hp Honda four-stroke; a 4hp Suzuki four-stroke; and an old 4hp Yamaha two-stroke. Fleet has a planing hull appropriate for higher speed operation, and Flint has a displacement hull as is appropriate for her row/sailing hull-form.

The reason that I was particularly interested in discovering the performance characteristics of Fleet with a 4hp motor, was because that is the size of motor I had in mind at the time of drawing the boat – but it was one size I had no performance reports about. One of the reasons for my curiosity is that 4hp/2.9kW is the maximum size motor one can use in my State without having to register the boat.

Flint – one adult
Flint – two adults
Fleet – one adult
Fleet – two adults
2 hp Honda BF2
6.8 kts/7.8 mph
6.2 kts/7.1 mph
7.9 kts/9.1 mph
7.1 kts/8.2 mph
4 hp Yamaha 2-st
9.7 kts/11.2 mph
8.3 kts/9.4 mph
11.2 kts/12.9 mph
9.9 kts/11.4 mph
9.8 hp Tohatsu 2-st
21.3 kts/24.5 mph
18.5 kts/21.3 mph

The speed data shown in the table is exciting to me, because it demonstrates just how efficient a lean, light-weight outboard utility can be, and how the law of diminishing returns works in reverse when the size and weight of power plant is reduced.

A spin-off which came from our testing is that we had two boats of the same length, breadth, and weight, being powered by identical engines, with the only significant difference being the bottom shapes from amidships aft to the stern. This allowed a photographic comparison of the longitudinal trim of the displacement and planing hulls under power.

Fleet above and Flint below

The drawing above shows the profile view of both hulls. Length, breadth and depth are just about the same, but the bottom shape from the midsection aft indicates major variations in performance.

Fleet at 8.5 knots (9.8 mph)

Above - Fleet running under the urge of a 4hp Suzuki four-stroke, making 8.5 knots (9.8 mph) at part-throttle – the engine being new and still in the early stages of the running-in process. Trim is good, but in a small boat like this, crew placement is critical.

Fleet at 8.5 knots (9.8 mph)
Above - another shot of Fleet running at 8.5 knots (9.8 mph). In both photos the boat is running cleanly, and is probably consuming around 2.5hp. Not much energy being wasted making spray. 

Fleet running at 9.8 knots (11.3 mph)
In the photo above, I'm at the tiller of an old 4hp Yamaha which is pushing Fleet at a GPS-measured 9.8 knots (11.3 mph). The boat is carrying two men weighing a total of 171 kg (376 lbs) plus anchor, chain and a few gallons of drinking water. You can see just how clean and flat the wake is, and I find it amazing that the boat is doing this on 4 horse-power.

Now things changed! We put the exact same motor on Flint to see what would happen with a displacement hull. Most people would not think a  4hp engine to be excessive power, but read on...

Flint being pushed by an old 4hp Yamaha

Here we see Flint being also being driven by a 4hp motor – in this case an old Yamaha. Note the completely different fore-and-aft trim, caused not by crew placement, but by hydrodynamics. A substantial amount of the engine’s power is being converted into spray and wave-making. This is borne out when you look at my speed and power tabulation - Fleet is fully 1.5 knots (1.7 mph) faster than Flint with the exact same engine and the same skipper – a 15.5% difference.

Another shot of Flint displaying what happens when a hull optimised for displacement speeds is provided with more power than she requires. If you look carefully at the angle the aft gunwale makes with the water and then refer to my profile drawings, it is easy to deduce that even though the bow is cocked high in the air, the aft bottom of the boat is nearly parallel with the water’s surface.

Although Flint looked most inelegant when pushed by a 4hp motor with an open throttle, she should not be written-off as a power-boat.

Flint at 5.5 knots (6.3 mph) powered by a Honda 2 hp BF-2 at reduced throttle

Here she is looking efficient and capable, running at 5.5 knots (6.3 mph) pushed by an eighteen-year-old Honda 2hp air-cooled four-stroke. The engine is throttled well back to a comfortable level of sound and vibration, and the boat is running sweetly. I am not a powerboat person by inclination, but there is great pleasure to be gained by running along quietly like this, and you can cover a surprising distance in a short time – all on a cupful of fuel.

Flint at 6.8 knots (7.8 mph) - 2 hp Honda at full-throttle

But even with the 2 hp motor, the boat starts to look out-of-place when all two of those horses are let loose, getting the boat up to 6.8 knots (7.8 mph). One-and-a-half miles per hour faster than before, but the pleasure has gone out of the experience – noisy and lacking the slicing motion.

Flint looking purposeful and dignified under sail.....

....and nestled into her surroundings under the gentle power of oars.

So what are the lessons illustrated by this interesting little series of experiments?
·         Two boats of similar proportions, weight and displacement, but with differing bottom shape in profile, will operate in vastly different ways when pushed beyond displacement speeds;
·         It is folly to use a motor which is larger than recommended by the designer, if your boat has a displacement hull (unless she is a tug);
·         A long, lean planning hull can be extraordinarily efficient when driven by a motor which is unusually small by current power-boat standards. Even a confirmed sailboat person such as myself can derive lots of satisfaction from the effortless speed, economy, and low noise-level of such a boat. A gentle way of covering distance in a short time, without conspicuous consumption;
·         Theory is wonderful, but gaining proof through gently controlled experimentation is a powerful way to demonstrate results.

The two boats – similar size and weight, but very different performance.


  1. Fantastic Ross, especially your lessons illustrated, especially about inconspicuous consumption. Thanks to you and Ian for performing the thankless task of putting around the lake for the sake of science. It's almost enough to make another confirmed sailor like me think about getting a stinker to put on the back of the Mirror (appreciating that it won't be driven quite like Flint)!

    1. Thanks, Tony. Those sweet little four-strokes are almost allowable in my mind, although you have to find the sweet-spots in the rev range to avoid harmonic vibration. You are welcome to try out the Mirror with my 18 year-old Honda if you wish.

  2. This is a very interesting comparison. Thanks for posting. When I was young we used to watersky with a 5 HP 2 stroke. I was much lighter then.

    1. I look at boats with multiple 300hp outboards strapped to the transom and shake my head. There is something very satisfying about the magic of a boat sliding along quietly (relatively speaking) on minimum horse-power. A 2hp outboard puts out about 16-times the sustainable power of a human, so we should be happy with small motors! Thanks for the comment.

  3. Thanks Ross for taking on those tough jobs! We do the same with our fleet small boats but our "Marine Surveys" and "Sea Trials" are much less scientific :) Next could you test out speeds under oar and sail compared to a 2hp with a gummed up carburetor? Bring a sound meter to record decibel levels as well!
    Clark and Skipper
    Small Boat Restoration

    1. Yes, Clark, it is a heavy load to carry - spending a life fiddling around with small boats. Here is a link to a short clip showing Fleet under oars. This particular set were made by a friend, and he would excuse me for describing them as 'rough'

    2. Clark, here is a post which give a bit of information

  4. Great article Ross. Just love the intro about your introduction as a child to boats and boat building. Mine was at 9 when dad decided to build a sportsman 13 by Peter Fletcher. Glides across the water with the original pull start Johnson 40 all fully restored. Thanks again for a great article.

    1. Thank-you, Robert. It is nice to here your comments, and for me to be reminded that we all have fond memories of such times in life.