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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Whimbrel Questions

Mike has written a comment asking a question about Whimbrel:-

Ross - quick question on whimbrel. Is she to be a pram/scow bow craft? If not, could the plans be fairly easily amended for a pram bow option? A pram bow, coupled with the leeboards would sure make a roomy little craft for the overall length

Yes, Mike, Whimbrel is a pram, and was designed that way deliberately for the following reasons _

• Greater internal volume and stability for a given length. Alternatively, you could say that for a given volume you get a much shorter and more easily stowed boat. If she was faired up as a sharp-bowed boat of the same volume and stability, Whimbrel would need to be about 21 feet long;
• Better hull shape to take leeboards - the use of leeboards has many advantages, such as a completely uncluttered interior, and freedom from the worry of having a centreboard jambed in its case by sand, shellgrit and stones - a constant worry when beach-cruising;
• Greater reserve buoyancy up forward. This is important when running off in a big sea, and is helpful considering the large sail area and forward mounting of the main sail and mast;
• In flat water at least, better speed potential due to the chine shape allowed by the pram configuration. See previous post

Whimbrel viewed from the starboard bow

Unfinished working drawing showing inboard profile of Whimbrel. Dont be alarmed that one of the figures has his feet through the bottom of the boat - it is the only suitable figure I had to check head room!

Whimbrel sail plan, also showing the mast (dashed lines) in the lowered position, which is made possible by the use of a tabernacle. A tent can be rigged over (or hanging from) the lowered mast, so as to cover the cockpit and to provide shade to the cabin top.

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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 havent 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.
Im 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 Ive had with a well credentialed Westlawn graduate, Dave Gerrs scanting rules are conservative in comparison with ABS standards - in other words, if you build to Daves 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 Rosss Whimbrel design, have since I first saw it, and I understand how the original clients 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, Id 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. Its 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 dont feel the need to reply further. However, I hold Ricks 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 - Im 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 cant get it all done, Im afraid), that the initial modelling attempts need more time devoted to the detail.

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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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Whimbrel Developments

My plans for Whimbrel have been sitting 90% finished for a long time. The drawings which have been finished are nicely detailed, and I really like the boat. But for some reason the final step of completing the plans for publication has been very difficult to complete.

Outboard Profile of Whimbrel, showing her with the original balance lug rig and free-standing mast. Note how the heel of the mast has to swing down and up through the foredack. This arrangement is catered for by the installation of a slot-type box - similar to a centreboard case, but upside-down.

Whimbrel was designed with a balance lug mainsail, and to capitalise on some of the advantages of this rig (no headsails, no sail-track or lacing, self-vanging boom, simple and effective reefing) the design was drawn with free-standing masts. An important component of the rig was to be a main-mast set in a tabernacle, so as to allow quick and easy rigging and un-rigging for trailer transport, and for reducing windage when on a mooring in bad weather. You can see the lowered mast position in dotted lines on the drawing above.

Tabernacles can be bulky affairs, and I dislike having a tabernacle which extends a long way above the deck line. However, a free-standing mast benefits from substantial "bury" of the mast within an arrangement of mast step and mast partners, or, as in this case, within a tabernacle. My approach was to have the base of the mast swing up and down through a slot in the foredeck, with sides to the box which extend all the way to the level of the mast step. That way, water can drain down the mast, or come on deck as spray, and it simply runs down to the bottom on the mast case and is then drained over the sides through scuppers - making the mast case self-draining.

The design of the slot and case was fairly straight forward, and I was very pleased with how the structural members support each other and go together in a logical fashion. The trouble is that although the mast case (i.e. a structure similar to an upside-down centreboard case) is simple to construct, it was and is very difficult for me to explain on paper. The drawings of all the components are complete in a 2D form, but I do not have the skills to operate the 3D CAD program I have on hand (TurboCAD 17). I do all of my CAD work in an AutoCAD 2D program (AutoSketch 9).

Several times Ive tried to do a simple diagram as an "exploded" isometric drawing, but nothing satisfying has eventuated. I thought that the simplest thing would be to build a prototype, and photograph the case construction in detail, as I was/am sure that as soon as people see it, they will appreciate the simplicity and structural elegance.

In the meantime, one of my sons has been experimenting with a tiny jib (set flying) on a small clinker/lapstrake dinghy he built years ago. The small jib has boosted the windward performance of the boat enormously, even though the sail area of the jib is only 11sq. ft (from memory). This boat sailed quite badly with the free-standing rig as originally built, using a carbon windsurfer mast. The mast was just too soft, even with the addition of substantial alloy sleeve at the lower end.

Here you can see Daves boat sailing with the original free-standing mast. She had initially sailed with the sail made according to the sail-plan on the plans, but that sail had been awful. Perhaps it would have been ok on a stiffer mast instead of the windsurfer stick. We then changed to an old Laser sail as shown in the pictures, but the mast was still far too soft, and the boat was a very poor performer to windward.

Ive told this story before on the blog, but I suggested to Dave that we put some stays on the rig, with the point of attachment (hounds) being just a fraction over halfway up from the deck to the masthead. He was reluctant, but I eventually convinced him to give it a go and we made up a forestay and two shrouds using Dyneema/Spectra. This also allowed us to set an old jib, as a "flying jib" i.e. a jib with is not attached to the forestay with hanks.

The performance of the boat was dramatically transformed for the better, and the boat is giving great service.

My point in re-telling parts of the rig story is to illustrate that sometimes a very modest increase in complication can result in huge gains in performance. Now, back to Whimbrel....

The revelations about performance brought about by the addition of the jib to Daves boat got me thinking a little outside the box I normally occupy. I tend to be a bit obsessive when it come to rig simplicity, because I hate clutter in a boat when rigging and un-rigging at the boat ramp. Our experiments with the dinghy rig above have brought me to the point where Im considering some different rigging arrangements for Whimbrel, and the design evolution continues - perhaps it was fortunate that I hung off from completing the original drawings.

In the above drawing you can see the proposed new rig for Whimbrel. The hull is unchanged, as is the mizzen, but the main part of the rig is now a lightly stayed, gaff-headed mainsail with a staysail set on the forestay. The staysail can either be hanked onto the forestay, or could be permanently attached to the forestay with a simple roller-furling tackle set at the tack.

Because the mainmast is now supported by stays in the form of a forestay and two shrouds, the mast no longer requires the support of a tabernacle which runs deeply into the hull. In this case the tabernacle is short and extends only to the level of the deck, and the foredeck is no longer cut up by a mast case and slot. So the mast is significantly shorter at the foot, and the diameter of the mast is reduced from 92mm (3-5/8") to 65mm (2-9/16").

Whimbrel set up for the night with a boom tent rigged and the mizzen sheeted flat to hold he head -to-wind.  I cant see the anchor rode - it must be some of that invisible rope I sometimes use!

Im continuing to consider some options for the cockpit, and will up-date soon.

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Whimbrel Two More Questions

Two comments have just come in regarding Whimbrel - one via the comments function and one by email.

First came from Robert: -

Ross- Im following Whimbrel developments with keen interest. I have long considered Jim Michalaks Frolic2 based on Gary Bs Everglades Challenge success with the design. I live 30 minutes from the start of the EC and hope to enter in the near future.

My only concern thus far is the length of the cockpit and cabin floor. Your drawings show the opportunity for one to stretch out in each spot, but I am 6 4" tall. Can you share approximate lengths of those spaces?

The cabin and cockpit arrangement on Whimbrel owes much to the brilliant design concept that Phil Bolger employed in his Micro. By that I refer to the cabin space and the cockpit "overlapping" longitudinally so that a person sitting or lying down at the forward end of the cockpit is actually positioned above the legs of a reclining person in the cuddy-cabin - in much the same way as happens with quarter berths in larger cruisers.

You can see here how the accommodation spaces overlap

The cockpit and cabin space is unusually large for such a short boat, due to the 6 8" beam which is carried well forward because of the pram bow. In fact, there is space aft for a substantial aft deck and associated buoyancy/storage compartment. This aft deck will also allow for the use of a self-draining outboard well similar to the ones Ive used on a number of my other designs.

Periwinkle on launching day showing the self-draining outboard well. The camera had a wide-angle lens making the well look a bit larger than it is in the flesh

Back to Roberts question...

The longitudinal space available in the cockpit, measured between the main bulkhead and the forward bulkhead of the aft deck i.e. the total length of uninterrupted sleeping/sitting space is 6ft 8-9/16". In the cabin, there is 6 6-5/8" of bunk-space with an additional 11-1/8" of clear space between the forward end of the bunk and the forward bulkhead of the cabin giving 7 5-3/4" total. There are compartments leading forward from the end of the cabin for stowage - two accessible from in the cabin, and two from on the deck which drain overboard - used for anchors etc.

Another comment came from Wayne Jorgensen: -

Ross
another quick question re Whimbrels pram bow. Ive spent some time watching Sabots trying to punch into a chop- to the detriment of their speed and handling. Wont there be the same issues with Whimbrels near vertical pram bow? Scandinavian prams lift their bows much higher so this is not really an issue with them but then they are not live aboard cruisers either.
Regards
Wayne

I agonised over this very matter when I was doing the initial hull modelling. The competing factors were;

• maximum volume and reserve buoyancy on a given LOA;
• maximum speed (in smooth water) from a given LOA;
• lack of annoying wavelet noise under the bow while at anchor;
• a chine-line (in body-plan view i.e. viewed from bow and stern) which would produce the minimum cross-flow and eddy-making.

I like the Scandinavian-style pram designs with their bow transom carried high and narrow - you see the same thing on some sampan designs. The problem is that the length of the boat would have to be increased significantly if one was to retain the required shape and buoyancy, just as would be the case with a sharp bow.

It was Phil Bolger who pointed out that a vertical transom gives the maximum waterline length for a given length on deck. He also said that a vertical transom can dig into waves and throw spray - but he then went on to say that by raking the stem forward it would reduce the spray and the digging tendency, but would make the boat longer. So why not pull the waterline length out to match the new length on deck with a vertical transom and get even better performance? Here is one of his several comments on the subject: -

"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)

What I could say to make myself feel justified is to say that Whimbrel was a 17 boat with a nicely raked bow transom, but I pulled out the bottom length to produce more volume and speed...... But no, I drew what I thought was a good compromise for a fellow who wanted a maximum LOA of 17. The customer went overseas and had to cancel the design, but I was so taken with it by that time that I decided to finish her off as a stock plan. That is why it has taken so long for me to finish, as I only work on the drawings occasionally.

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