Thursday, December 22, 2011

Whimbrel Yet Again - Two Comments

Robert has followed-up on the previous couple of comments: -
Ross, since you see the utility inherent in the slot-top option and go so far as to state you would thus modify Whimbrel for your own use, would it be feasible to offer an additional sheet in the plans to make this an option?

Then there is that magic number I was waiting to see... the hull weight. I know more than one person is eyeing Whimbrel as a possible Everglades Challenge option. I do hope the actual number is "significantly reduced" as you believe. The Core Sound 20 finishes out at between 500 and 600 pounds sans gear, no cabin of course. Time will tell...

Well, I have to admit that I haven't done a full "weights and moments" calculation on Whimbrel. What I did in the previous post (with full disclosure at the time) was to take the total volume of plywood in the boat, multiply the volume by 600 to determine the weight of the ply in kilograms, and then I simply doubled it to account for timber components, glass, epoxy, fasteners, rig etc. This is totally unscientific, but I believed it to be conservative. What is more the selection of 600 kg/cu.m was also conservative, as we are able to obtain good quality marine plywood here which has an actual density of 420 kg/cu.m. The reason that I used 600 kg/cu.m as the figure was to account for the variability in the actual weight density of plywood supplied by the retailers. There is a retailer in this country selling BS1088 plywood which is advertised as being 430kg/cu.m - but when tests were conducted locally on actual samples, the density proved to be around 600 kg/cu.m!!!
If one was sure of having 420kg/cu.m ply, and even using the conservative doubling system that I described, the weight of the boat (including decks and cabin) could be as low as 220kg/484lbs.
I'm sure that the Core Sound 20 is an excellent boat, but she is a different vessel from Whimbrel. Firstly, Whimbrel has a full cabin, lots of built-in compartments, a self-draining cockpit, and a tabernacle. In addition, I designed Whimbrel to comply with the scantlings rules as laid down by Dave Gerr N.A. who is currently the Principal of Westlawn. as far as I can make out from my own study and from discussions I've had with a well credentialed Westlawn graduate, Dave Gerr's scanting rules are conservative in comparison with ABS standards - in other words, if you build to Dave's rules, the boat will be stronger and slightly heavier than required. I am very happy with that!
One of the things I had in mind when I drew Whimbrel was that I wanted a solid little ship which would last a lifetime. As you are probably aware, I hold the work of Phil Bolger in very high regard, but I was mildly concerned to discover that a Micro built to specification was somewhat flimsy. For example, you can push the side panels in and out, and when walking around on the cabin top it is necessary to place feet near frames, bulkheads or other supported areas, because the 6mm/1/4" plywood (from which the entire hull is made) flexes alarmingly.

Whimbrel has 12mm/1/2" panels on the bottom, 9mm/3/8" ply on the topsides, 12mm/1/2" on the cockpit floor, cockpit seats and cabin sole, and the cabin top is made of 6mm/1/4" on 50mm x 20mm (2" x 3/4") longitudinal stringers on 200mm spacings or less. This is a rugged "tugboat-tough" boat and not a light-weight daysailer.

Yes, I will do a sheet covering the simple option of a "slot-top". One could easily stand against the forward end of the cabin and reach down into the anchor locker and deck hatch.

Rick Hayhoe has written back: -

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I went back over images I have and could access of scows among American watercraft, where they were used in the Gulf of Mexico fishery and for transport of timber, coal and other bulk cargo along the US west coast. I find, contrary to the opinion I expressed earlier regarding Whimbrel, that most or all of them have the bow transom terminating at or near the waterline. They were, of course, coastal and inshore craft, but they still would have met severe conditions at times. However, all of those were much larger vessels than Whimbrel, with displacement in the tens and hundreds of tons, with vastly more inertia in meeting chop or steep waves, able to punch through any but the biggest waves but hopefully able, while plying their trade of coasting, river transport or inshore fishing, to make shelter before such severe conditions arose. Their hullforms, like Whimbrel, were determined by their function, with priority over seakeeping, often for the benefit of operating in shallow water, taking the ground at low tide and carrying large volumes of deck cargo. In a boat the size of Whimbrel I would still find it safer to follow the design habits of one of the peoples with a long tradition of scow bowed small craft, either the Scandinavians, who invariably got their open water round bottomed prams' profile and buttock lines well up out of the water before they terminated them in bow boards, or the Asians, who on smaller craft either rolled the bow boards back to a very low angle of attack, like a garvey but much narrower, or built them with narrow stems flaring into transoms well above the waterline, or some of each. Still, I want to make it clear that I admire Ross's Whimbrel design, have since I first saw it, and I understand how the original client's parameters resulted in the outcome. The boat is meant to be used as a camp cruiser, not a passagemaker, so the design is defensible as Ross puts it. That said, I'd still make what I consider better use of scarphed whole and half-sheet lengths of marine ply and make its shout a bit longer, thus narrowing and raising it, while keeping all else the same. His rig, accommodation and use of leeboards are a winning combination on such a small craft, and the accommodation and applicability of leeboards are both facilitated by the scow bowed hullform. It's a very neat solution to a nettlesome set of design parameters, which I like a whole hell of a lot better than solutions to similar parameters by some others who shall remain unnamed.
Rick Hayhoe
I thank Rick for his comments, and given that I have already responded to his initial comment, and because Rick has written so well in his second piece, I don't feel the need to reply further. However, I hold Rick's opinion in high regard, and I have therefore been moved to continue with hull modelling experiments with longer hulls which carry their transoms higher and less wide. Unfortunately, my time is so limited (it is the old 'One-Man Band' thing again - I'm trying to be a boatbuilder, a designer, a blogger, an email respondent, a magazine writer, and a retailer of components all by myself - I just can't get it all done, I'm afraid), that the initial modelling attempts need more time devoted to the detail.

1 comment:

  1. Hi Ross,
    I think I just purchased the Eve 16 used in the 'Nim's Island' movie. Please check your email.
    Thanks. Andrew