Wednesday, December 21, 2011

Bow Transom and Cabin Top on Whimbrel

The matter of the bow transom on Whimbrel has stirred up some discussion over the last couple of days - see this post for information.

In the drawing above, you may be able to see that I've drawn in three separate waterlines with accompanying displacement number attached. The upper waterline shows the displacement (952kg/2094lbs) with the water just touching the lowest point of the bow transom.

Here is an interesting comment from Rick Hayhoe: -

Phil Bolger sometimes said or wrote the strangest things. I have to point out the fact that you don't make the boat faster in any practical sense by drawing out the waterlines to the full LOA, leaving the boat just whacked off in a plumb scow bow that touches the water, or nearly so, at rest. What you do is create a monster that will misbehave in the slightest seaway and almost any wind.

I very much like the concept Ross is promoting in Whimbrel. I've admired the design since first seeing it almost a year ago, but I certainly would make the boat eighteen inches or two feet longer at the forward end without changing anything else, getting that scow bow up, well out of the water and raked a bit more. Just imagine for a moment the danger of broaching on a downwind point of sail with following sea and opposing chop set up by a river's outflow agains tide and wind, with that scow bow slamming into the chop and the following waves trying to push your stern ends around. Those are conditions one can expect to encounter routinely when entering many estuaries, let alone the conditions encountered while struggling in a squall or making for home in suddenly rising stormy weather.

Living as I have for many years in Asia, I have had the opportunity to look closely at a variety of scow bowed Asian sailing and motoring craft. Generally, you find that either the bow transom is raised well above the LWL or it is raked well aft. Some are built with a profile like a garvey, that is, raked and rounded, cross planked, to recede as they approach the LWL. In most cases, some compromise has been made in order to reduce the impact of the transom or scow bow against wave and chop.

Bolger was a brilliant designer, but he had a tendency to paint himself into the damnedest corners, and he was stubborn enough to stick to his pet theses no matter what. You have to admire most of his work, perhaps even his stubbornness when he was right, but you'd better take some of his philosophizing with a grain of salt. Boats move through a constantly troubled and dynamic pair of fluids that make up one of the least forgiving environments on our planet. You flout at your own risk the limits and dangers they pose.

Rick Hayhoe

I estimate that a hull built from 600kg/cu.m plywood (a conservatively heavy estimate based on the weight of all the ply in the boat and then doubled to account for timber components, glass, epoxy and rig etc) would weigh about 314kg/691lbs. Add to that 240kg/528lbs for crew and gear and you end up with a displacement of 554kg/1219lbs fully loaded displacement. This is a conservative figure which I believe would be reduced significantly in reality.

Below is a perspective and a lines plan showing Whimbrel at the maximum displacement in salt water, and I'm reasonably happy that the forrard sections are buoyant enough and shaped well enough (in terms of rocker, distribution of volume, and chine shape) to prevent the disasterous behaviour that Rick predicts. I may well be proven to be wrong, but after having modelled a number of other hull shapes at 18' 6" length on deck and with a raised, smaller bow transom, I have elected to stay with the design as drawn at 17' length with the big bow. Storage and building space is a real issue for many people and I was trying to get a roomy and capable boat without excessive outside dimensions. There is no such thing as a perfect boat!

By the way, the bow transom is set at 65 degrees to the waterline, which is a long way from being plumb.

A perspective of Whimbrel showing the waterline at maximum displacement of 554kg/1219lbs in salt water

Whimbrel lines plan at 554kg/1219lbs displacement

Robert relied to the post yesterday regarding his question about sleeping space: -

Ross, I can see why you were taken with the concept. It sounds as if much thought has been given the design. I appreciate the amount of interior room designed in. Is there to be a slot top or more enclosed cabin? If designed to be totally roofed over, could the cabin be modified to add a pass through slot? It is very hot in Florida much of the year and the slot top cabin seems to offer greater forward access.

This is an interesting subject, and I had been giving it a lot of thought myself. You see, I can't stop fantasising about building boats for my own use, and I'm quite taken by Whimbrel. While dreaming of the boat, I had decided that I would modify the cabin top if I built her for my own use, and that I would use a Birdwatcher-style slot-top. However, thinking that the majority of the buying public would prefer a conventional companionway with a sliding hatch, that is what I drew.
My opinion is that the slot-top is a superior option for this size of boat, allowing for better ventilation, unlimited headroom, and secure footing right up to the mast.

Just for interest - no plans available in any form - here is a twenty-foot boat I drew up for fun, to visualise a full Birdwatcher cabin on a nicely shaped conventional hull. She has got very firm bilges so that a person can sit comfortably out at the sides.

20'x 6'


  1. Ross, since you see the utility inherent in the slot-top option and go so far as to state you would thus modify Whimbtrl 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...

  2. 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