Transom Build

The transom is built from 3/4" Okoume plywood. The layout lines were transferred to the plywood with carbon paper to show outer contours and added inside frame locations. For some of the curved areas, the added frames allowed using them as a template for final cut of the transom plywood with a router and trimming bit. I angled the motorboard just because I thought it looked better. I also raised the height of the transom at the motorboard to 16.5" from the plans 15". The "Mustang" boat plans show different dimensions up to 17" depending on make of old outboards and Im told that performance can be enhanced by tuning the height upwards and the transom can always be cut down if too high. Since I dont have a motor yet, I moved it up, but 17" didnt look good, so I settled on 16.5". The Okoume 3/4" plywood is very dense and heavy and after bending a couple of silicon bronze nails when trying to glue and nail the transom knee together, I increased the diameter of the pre-drilled holes. Bent nails were broke off and another hole drilled adjacent for another nail. Close inspection shows a couple of curious bronze spots in the transom knee....

The bottom of the transom has a shallow "V" cut at a 12 degree bevel. I used the method in the "Boatbuilding with Plywood" book by Glen L. Witt. I added an auxilliary fence suspended about 1/8" higher than the transom thickness off the table saw surface, and then screwed down a straight board to the transom along the line to be cut (one side of the "V" at a time). With the blade angled 12 degrees and the outside of the blade set just below the edge of the aux fence, the cut turned out super. I think the above mentioned book is a "must read" for a new boat builder like me as it answered many questions that I had and hopefully prevented me from making poor choices for materials, assembly and building techniques. I also enjoyed learning about something I knew little about and gave me the confidence to begin and avoid many mistakes that would have happened otherwise.

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Discussion about Transom Bows Whimbrel related

Back in December 2011 there was some animated discussion about the design of boats with a transom bow - in this instance the discussion related to my Whimbrel design - with a number of people expressing their concern about Whimbrels bow transom being too wide and too close to the waterline. The feeling seemed to be that the bow would dig in on some points of sail, and would bash into the wave in a steep, choppy head sea, therefore slowing the boat, throwing spray into the air, and maybe causing wild steering running downwind.

This perspective shows Whimbrels hull viewed from the starboard bow - you can see that the base of the bow transom is close to the waterline when the boat is heavily loaded.

In this drawing there are three different waterlines drawn - the deepest one with her at close to one ton of displacement, which would not be likely to occur. But even at that heavy displacement the heel of the transom is just above the static waterline.

My argument was (and is) that the boat was designed to be only seventeen feet long, and that she would be faster, roomier, and more stable - for the load-carrying ability specified - with the large bow transom. For some background discussion on this matter have a look at this post  . In fact, if I had been brave enough to withstand the negative comments, I would have made Whimbrels bow transom vertical, as I am convinced that her performance would be superior if drawn that way. I must say the raked transom looks better in profile, but Im sure the performance is degraded.

I often quote Phil Bolger in my discussions, and at the time of the Whimbrel debate I used one of his remarks about bow transoms.

Fieldmouse - scanned from a photocopy sent to a customer by Phil Bolger in October 1971. Note how the waterline is at the heel of the vertical bow and stern transoms.

Here is Phils quote when discussing his Supermouse and Fieldmouse designs: -

"Raked transoms make a faster boat less badly stopped by waves if they are raked out from the given bottom length, but in that case the boat would be better still if the waterline were carried out to plumb transoms at the new overall length...." (Boats with an Open Mind, International Marine 1994)
The logic in this remark seems to be lost on most people, but the technical importance of this comment, and others, has obviously been understood by commenter, Graeme: -

"No corners painted by Bolger here - even on those square boats! ;-)

Whats not to understand? Drawing the bottom (and waterline) out to be as long as the deck of an otherwise raked ends hull makes for a narrower stern and narrower bow at the waterline. The finer entry is less stopped by waves and less wet. The drawn out waterline on the same beam has higher theoretical speed. Importantly (never forget the several intricate ways Bolger stressed the virtues of "shallow", this is but one) it also has less draft, and so less resistance, thereby being more likely to achieve or exceed that theoretical speed.

It is in no way all about the bottom though. What is often passed by in what Bolger said is that its the decks that raise hull cog, and Bolger viewed decks, rightly, as a penalty. Other considerations not withstanding, he may just have stuck to raked ends, clipper bows, and such, if hed been able to design decks of weightless unobtainium. As it is, plumb ends (and sides) impose a smaller deck weight penalty on a given bottom, or on stability, or on ability to stand up to sail, or on overall performance.

Flow on efficiencies there, which may also continue to run right into the pocket.

No corners then, painted or otherwise, rather Bolgers genious yet again seen remarkably squaring a circle unlooked for. A virtuous circle spiralling on with yet more angles round each turn.

Graeme"

You can read other very interesting comments by Graeme and Rick at the end of this post.

Now, I am not saying that bow transoms are the answer to everything. Most of my designs have unusually fine, sharp sections up forard. In fact, I sometimes wonder whether I may overdo it in an attempt to cut down pounding in a short head sea.

The very sharp entry on Flint

Periwinkle has a long, fine entry.

What I am saying is that for certain applications, a hull with a bow transom may be the most appropriate way to design a hull to perform a particular function and if one of the design constraints is overall length, then a wide, vertical transom extending almost to the waterline should be considered.

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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 Ive 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 dont 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. Ive 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 rivers 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 youd 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 Im 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 cant stop fantasising about building boats for my own use, and Im 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.

20x 6

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Transom

27th Nov 2007

The Transom was fitted in just a few hours, First it was tacked on the flat truck as it is in 3 pieces.


Dimensions checked and its good to go.


Lifting the transom with lighting stands allows exact positioning.
I have to say that what looked to be hard, turned out to be very easy with the stands.
They can lift 150kg each. We will probably use 3 stands to lift the hull side plating when fitting.



Transom fitted and tacked on to the floor plates and the upright supports inside.

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