Wednesday, June 22, 2011

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 I've 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 isn't 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 I've 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 I've 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 I'm planning on finishing it off with a Jonesport Cleat.
So, there you have a nice little boat. But you don't 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


  1. Hi Ross,

    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 don't 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.



  2. Benefits of an external leeboard: Makes the boat easier to build and boat is more likely to be completed. I like my Mayfly 14 very much!

  3. Making a wooden boat is really a hardworking job. You did it very well. The boat which you made is awesome. she looks so pretty. By the way, it is very nice post.

  4. Thanks again for the comments. It makes the time spent putting up the posts worthwhile. Best wishes.

  5. "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."

    Ross, Bolger had a theory of water flow and how that applied to sharpies. With respect, what he wrote about exterior chine logs of relevance here was: "The external chine log... adds a minute amount to the stability, which certainly needs anything it can get (Light Dory Type V); I don't think it increases the resistance but I can't prove it yet." He worried that it may increase resistance, not reduce it.

  6. Yes, and he also wrote, regarding Poohsticks, on page eight of "Small Boats",

    "This was the first design I made using the outside chine log, which I've since adopted quite often in constant-deadrise boats: it saves some labor in fitting the log, and gives better bearing and fastening for the size of the log. It also gains a bit of stability, and I have an idea, just conjecture, that it reduces eddying along under the chine by carrying the side flow aft......"

  7. Yes but... Poohsticks is design 10-5-57, ie from the early numbering sytem Bolger used (most of that series originals and other early independent Bolger work destroyed in an unfortunate office fire.) It's almost certain the Poohsticks design was completed in 1957. The date of the write-up about it is not clear to me, but the thinking behind it may well be contemporaneous with its design.

    The Light Dory Type V is from the later adopted numbering system, post fire, and denoted as design #265 [there are eight types of Bolger light dory (counting the larger ones #526, #555) as modified from #140 (types -1 to -6) and on; #140-4 being the Orrell famed "Gloucester Gull" version, and #140-6 Payson's "Gloucester Light Dory"].

    Dynamite Payson first built a #140-4 in 1967 (Bolger drew #140-6 for Payson's plans sales business much later in the 1970's due to issues with Orrell and proprietry rights. However, what goes around... later, upon Orrell's death, Payson successfully purchased all Orrell plan rights, and passed most of the Bolger blueprints/rights back to Bolger!). The Type V, #265, the only one with external chine logs was certainly designed around 1973 specifically for publication in "Small Boats."

    Now, to our points of difference: if, as you seem to imply, the 1957 Bolger conjecture (with his obvious caveat) that a chine log "reduces eddying along under the chine by carrying the side flow aft......" is evidence of, as you say, a "Phil Bolger theory that if the hull was properly shaped, an external chine log may reduce drag", then, repectfully, I must disagree for the reasons that follow.

    First, in my view it's a bit too much of a stretch to turn a Bolger conjecture into a Bolger theory. Bolger raised so many conjectures throughout his career, sometimes with wry humour, often with a caveat as here, yet when he stated his theory in any way he was quite serious, and unreservedly adamant (a good example is at page 50 of "Small Boats" "My flow theory accounts for this...")

    Second, sixteen years elapse between Poohsticks and the Light Dory Type V, by which time clearly Bolger is not stating an exterior chine log reduces drag, rather that he merely thinks it may not add to it.

    Third, a further nine years later still, in 1982, of the projecting edge of the bottom of Lions Paw #404 ("30-ODD BOATS", p97), which is an excrescence effectively the same as an external chine log, Bolger stated that it "doesn't seem to create much added drag in a hull of these proportions". Unlike Poohsticks, or LDT V, the Lions Paw hull is certainly shaped according to his theory as applicable to sharpies to minimise eddying flow at the chine, ie to minimise drag, yet overall the chine excrescence does result in added drag despite it somewhat fencing cross chine flow (in this design it is tolerated for other reasons).

    Fourth, well, admittedly not a reason carrying much weight, but allegedly the LDT Type VI actually is a bit faster than the LDT Type V. There is more going on in the way of design modifications to produce the Type V than just the external chine logs, but perhaps the fact is that they don't help either? (Type VIs have been built with external chine logs too. As far as I know there's been no comparison made with a standard VI.)

    best to you

    interrogate the bolger chart,
    every phrase, every mark...

  8. Graeme,

    Thanks very much for writing back with your well reasoned comment. I have already started a new post on this subject, and I'll re-print your reply and give my further opinions there.

    For me, the most important part of what you have written is contained with in the little verse after your sign-off. It has astonished me just how much information Phil Bolger was able to convey in his brief essays.

    I find that the books (and other writings) are yeilding up information even though I've read many of them for twenty or thirty years. However, that may just mean that I am a very slow learner....

    Ross Lillistone

  9. hi..Im college student, thanks for sharing :)

  10. The Mayfly is my goal, having experimented first with a smaller skiff. My own struggle with external chine logs ended after my first cartop and launch. Handling the boat out of water was much easier with external chine log "handles". It mayn't fly but it will take me where I want to camp.

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