Photos of a Good Flat Bottomed Skiff

Back in April I wrote two posts about flat-bottomed sailing and rowing boats (see Discussion About Flat-Bottomed Boats and Sharpie Hulls and Fine Sections ).

Things have been pretty hectic at our place since the disasterous flash-flooding which hit our area on January 10, 2011, but over the last few weeks Ive been getting back to work on some over-due jobs, one of which is a nice flat-bottomed skiff for a customer in Brisbane. She is a Jim Michalak-designed Mayfly 14 and is a good example of a well-designed flat-bottomed hull. The point to note is the relationship between the curve (or rocker) of the bottom when viewed in profile, and the curve of the topsides when viewed in plan-view. These two shapes, in combination, determine the chine-line, and the the flow of water around that chine. The aim is to reduce cross-flow to a minimum in order to minimise eddying.

A nice, simple boat, showing the pivoting leeboard, centreline hatches, and external chine log.

Many people find external chine logs difficult to accept, but they have some advantages. They are much easier to install on a boat which isnt built on a strongback, avoiding compound bevels and determination of length. In addition, they provide good protection to the chine, which is somewhat vulnerable on a flat-bottomed boat. Phil Bolger had a theory that if the hull was properly shaped, an external chine log may reduce drag, by delaying the point at which cross-flow occurred. In addition, I guess that they provide a bit of extra lateral resistance, which is good in a sailing boat.

Brutally simple pivoting rudder design which requires only one rudder cheek. The key is the amount of blade above the pivot point, as it provides support to the blade regardless of which tack the boat is on. The patch at the bottom of the blade is where Ive cast in a block of lead to sink the blade. It is covered with a small patch of 200gsm/6oz glass to prevent cracking between the lead and the plywood. The white is epoxy and sanding filler. 

Very simple, but rugged, mast step and mast partner, well braced by framing on the other side of the bulkhead.

Close-up of the mast step showing the very necessary drain hole to get rid of water in the step. This is all a bit rough at this stage, still waiting for some clean-up and fairing work.

Close-up of the external chine log, rounded over on both edges, and showing how Ive brought the epoxy/glass bottom sheathing around the bottom edge and up over the chine log. This is to protect the vulnerable edge grain of the 1/2" plywood bottom.

Outer stem made from a superb piece of Celery Top Pine from Tasmania. Hard, dense, and strong. Still waiting for finishing work and fairing.

Close-up of the external gunwale/sheer clamp, laminated from two beautiful pieces of Silver Quandong. Note how in a simple boat like this, there are plenty of places where the edge grain of plywood planking is vulnerable - this needs to be considered, with action taken to protect the edge. The edge of this plywood will be treated with three or four soaking coats of epoxy prior to being painted, but even then it will have to be watched in service.

Detail of the corner joints in the hatch framing. Once again, Silver Quandong

The single-sided pivoting leeboard allows for a clean and open interior. Although simple in concept, the leeboard design requires great attention from the builder when fabricating the leeboard upper and lower guards, so that the board is absolutely parallel to the centreline of the boat. Trickier than it looks!

A nice, simple, rugged boat. The external stem cap is yet to be trimmed, and at this stage Im planning on finishing it off with a Jonesport Cleat.

So, there you have a nice little boat. But you dont get something for nothing. These boats might be easy to build, but they consume large numbers of silicon bronze screws, and the structure is quite heavy. However, she should give long and trouble-free service, given that she is painted properly (she will be) and stored properly

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Discussion about Flat Bottomed Hulls

As I expected, publication of an opinion on the subject of flat bottomed boats has resulted in some discussion. Graham brought up the matter of adequate rocker and running the forefoot above the waterline (see my previous post), and he used Phil Bolgers Black Skimmer as an example. Black Skimmer is one of my all-time favourite designs, by the way - I think she is a superb example of sophisticated simplicity, and even after thirty-one years of studying her shape and construction, I still find her to be awe-inspiring.

Here is a photo of Black Skimmer, copied from Woodenboat Magazine - I hope I havent infringed any copyright. She is close to being my absolute favourite design.

Dennis has written in with the following: -

Ross, I love it that you are discussing flat bottomed skiffs. I love em. My first sailboat was a Bolger Featherwind which exhibited the design principles you mention in your post. I am not so sure I agree about the curvature in bottom and sides. W/L length gets shortened, more of the bottom is exposed to waves and the pounding is obnoxious (this is not to say that the boat was not a blast to sail). How would John Atkins Lark (143" sailing skiff) stack up to your discussion? It has a fine entry, but the stem is immersed. It has good flare and I would bet that it does not pound the fillings out of your teeth while sailing in a chop. While I have not built and sailed another flat bottomed skiff since owning the Featherwind, I will probably do so one day because the nostalgia of that first boat has a powerful draw. Best, Dennis

I know what Dennis is talking about, and it is a good demonstration of how everything in boat design requires compromise. The shape which provides the excellent sailing behaviour (adequate rocker, matched curvature of the topsides and the bottom panel, forefoot run above the waterline) is very likely to pound badly when floating level.


There are plenty of flat-bottomed skiff designs around which have the forefoot immersed, and scores of William and John Atkin boats provide excellent examples to study. One of my favourite Atkin designs is Ration and she shows exactly what we are talking about.

Lines of Ration - courtesy of Motor Boatings Ideal Series - Chapman and Horenburger

Ration shows a rowboat which is less likely to pound in a small chop - immersed forefoot and very fine entry at the waterline and bottom - but she will still pound as soon as the waves get large enough to make the forefoot clear the water. The main problem is that the shape of the chine-line is such that there will be turbulence formed as the water running around the sides at the bow inevitably runs down and across the chine, and subsequently runs back across the chine in the aft sections (although this will be less of a problem than at the bow). A mitigating factor in this design is that the boat is relatively slim. It is in wide, flat boats that the problem is at its worst, causing excess drag and wild, unpredictable steering - particularly downwind.


To get an appreciation of what Im trying to describe, compare the shapes I show below: -

A typical flat-bottomed skiff with the heel of the stem immersed, and the bottom of the transom coming to above the waterline. Im only showing the body plan here, but the boat Ive drawn as the example is fairly long and slim.

The same boat heeled 25 degrees. In reality, the stern would probably be forced a bit higher and the bow lower than Ive shown here, which would make matters even worse.

A clearer view of the same boat. See how the chine line will generate turbulence and drag, and will tend to force the boat to round up.

For comparision, here is the underwater shape of the example I drew for the previous posting. While this is not a perfect shape by any means, it is vastly superior to the example shown above - but it will pound more when flat in a ripple.

It is all a matter of degree - the Featherwind mentioned by Dennis shows close to the ultimate in matching the curve of the topsides and the bottom - at least for a sharp-bowed boat - but she is best used as a sailing boat, or a rowing boat on flat water.

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Comment regarding Photos of a good flat bottomed skiff

Graham has written with a comment about leeboards: -

Nice pics. My life seems to be one long series of overdue jobs, so I know what a relief it is to be back on task!

The external chine logs I dont mind so much, but the leeboard?? I know there are some really good practical reasons for going down that path, and I know that Phil Bolger was a great advocate of them, but they take some getting used to.

Apart from the increased internal space, do you think you could just remind me what the benefits of a leeboard are; especially when used in conjunction with a flat bottomed hull like this one.

Well, Im not an expert on the subjuect, but the most obvious advantage of leeboards is exactly what Graham has pointed out - less clutter inside the part of the boat in which people want to sit. Now, having a centreboard case in there with you is not necessarily a bad thing. Ive found a case to be a great way to brace ones feet; to bundle people away from each other when sleeping; and to use as a sort of improvised set of hiking straps by hooking the soles of your feet under the upper external framing of the case while hiking out.

Another advantage of leeboards is that they are providing lateral plane from the surface of the water down to their tips, rather than from the bottom of the centreboard case to the tip. The problem there is that I suspect that the "end plate" effect of the bottom of the hull over the centreboard more than makes up for the increased effective area of the leeboard.

But despite all of that (including arguements about better hull strength without a centreboard slot), there is one overwhelming practical advantage of leeboards for a cruising dinghy, and that is avoiding the plague of having sand and shell-grit pumped up into the centreboard slot while the boat is pulled onto the beach with wavelets surrounding her. That is the reason why my centreboard designs have a positive method of forcing the centreboard down, rather than just relying on a block of ballasting lead in the board - a jambed board is a potentially serious problem, particularly when leaving a lee shore - you must be able to get the board down quickly, and the combination of sand and shel-grit inside a case is a serious problem. It is also a very good arguement in favour of a dagger board rather than a pivoting centreboard.

Ive been having an interesting discussion with a fellow in Maine about the effectivness of long, shallow keels. He sails a Phil Bolger Oldshoe and I occasionally sail a Bolger Micro - both of which have long, shallow keels. Both of us have been pleasantly surprised by just how effective these keels are - despite what the theorists will tell you. If you get the chance, read Arthur Ramsomes books in the Swallows and Amazons series (I read my first one at about eight years of age, and Im still reading the twelve-book series on a regular basis - they are that good), where he describes Swallow, a 14 ft clinker sailing dinghy with a shallow keel.

Here is me sailing along happily in my leeboard-equipped Bolger Nymph. The boat is on a run, with the boards raised, but I can assure you that they worked very well indeed!

This jpg shows my Whimbrel design (17ft x 6ft 9in), which incorporates leeboards for many of the above reasons. These plans are quite detailed, and although close to being finished, will still be a month or so. Too much work on my plate, and only one person to do the jobs!

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