Wednesday, August 10, 2011

More on External Chine Logs

Phil Bolger's Otter II design, showing her external chine logs
The contributor I referred to in my previous post, Graeme, has written back with with some more comments. I don't want to bore people with an on-going debate, but I think Graeme's comment is worth printing: - 

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...

Having read what Graeme has written in this piece and in his previous comment, I have to agree that my statement about Phil Bolger having a theory that external chine logs may reduce drag on a properly designed sharpie hull was overstretching the mark. However, he did imply that the increase in drag, if any, was minor - certainly far less than most people would think intuitively. If you look at my previous post, you will see a list of practical advantages to be gained from using external chine logs.

All of this has been generated by a post I wrote about a Jim Michalak-designed boat. My aim was to inform those people who seem to have a prejudice against external chine logs. Perhaps I was not careful enough with my phrasing, but the practical advantages remain. It is interesting that the people who are horrified by external chine logs don't raise any objections to external keel battens, bilge runners, or lapstrake planking...

Regarding Graeme's discussion about dates of design and of publication, I have this quote from a re-publication of the books, "Small Boats" and "The Folding Schooner".

The combination book was called "Bolger Boats" I think (I don't have a copy) and in the preface (Dec 1982) he said, "...What I wrote in these two books, as opposed to what I drew, leaves me fairly contented. Draw you own conclusions from that."

Regardless of the resistance argument, Phil Bolger continued using external chine logs on designs for a very long time.
Birdwatcher with her external chine logs (sorry, I don't have a picture credit and will remove it if anybody is concerned)

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