Discussion about Flat Bottomed Hulls

As I expected, publication of an opinion on the subject of flat bottomed boats has resulted in some discussion. Graham brought up the matter of adequate rocker and running the forefoot above the waterline (see my previous post), and he used Phil Bolger's Black Skimmer as an example. Black Skimmer is one of my all-time favourite designs, by the way - I think she is a superb example of sophisticated simplicity, and even after thirty-one years of studying her shape and construction, I still find her to be awe-inspiring.

Here is a photo of Black Skimmer, copied from Woodenboat Magazine - I hope I haven't infringed any copyright. She is close to being my absolute favourite design.

Dennis has written in with the following: -

Ross, I love it that you are discussing flat bottomed skiffs. I love 'em. My first sailboat was a Bolger Featherwind which exhibited the design principles you mention in your post. I am not so sure I agree about the curvature in bottom and sides. W/L length gets shortened, more of the bottom is exposed to waves and the pounding is obnoxious (this is not to say that the boat was not a blast to sail). How would John Atkin's Lark (14'3" sailing skiff) stack up to your discussion? It has a fine entry, but the stem is immersed. It has good flare and I would bet that it does not pound the fillings out of your teeth while sailing in a chop. While I have not built and sailed another flat bottomed skiff since owning the Featherwind, I will probably do so one day because the nostalgia of that first boat has a powerful draw. Best, Dennis

I know what Dennis is talking about, and it is a good demonstration of how everything in boat design requires compromise. The shape which provides the excellent sailing behaviour (adequate rocker, matched curvature of the topsides and the bottom panel, forefoot run above the waterline) is very likely to pound badly when floating level.


There are plenty of flat-bottomed skiff designs around which have the forefoot immersed, and scores of William and John Atkin boats provide excellent examples to study. One of my favourite Atkin designs is Ration and she shows exactly what we are talking about.

Lines of Ration - courtesy of Motor Boating's Ideal Series - Chapman and Horenburger

Ration shows a rowboat which is less likely to pound in a small chop - immersed forefoot and very fine entry at the waterline and bottom - but she will still pound as soon as the waves get large enough to make the forefoot clear the water. The main problem is that the shape of the chine-line is such that there will be turbulence formed as the water running around the sides at the bow inevitably runs down and across the chine, and subsequently runs back across the chine in the aft sections (although this will be less of a problem than at the bow). A mitigating factor in this design is that the boat is relatively slim. It is in wide, flat boats that the problem is at its worst, causing excess drag and wild, unpredictable steering - particularly downwind.


To get an appreciation of what I'm trying to describe, compare the shapes I show below: -

A typical flat-bottomed skiff with the heel of the stem immersed, and the bottom of the transom coming to above the waterline. I'm only showing the body plan here, but the boat I've drawn as the example is fairly long and slim.

The same boat heeled 25 degrees. In reality, the stern would probably be forced a bit higher and the bow lower than I've shown here, which would make matters even worse.

A clearer view of the same boat. See how the chine line will generate turbulence and drag, and will tend to force the boat to round up.

For comparision, here is the underwater shape of the example I drew for the previous posting. While this is not a perfect shape by any means, it is vastly superior to the example shown above - but it will pound more when flat in a ripple.

It is all a matter of degree - the Featherwind mentioned by Dennis shows close to the ultimate in matching the curve of the topsides and the bottom - at least for a sharp-bowed boat - but she is best used as a sailing boat, or a rowing boat on flat water.

Sharpie Hulls and Fine Sections

Graham has written a very interesting comment following-up on the last two postings. As it happens, it touches on a subject which is of great interest to me, as I'm in the process of designing a boat which incorporates the very characteristics he mentions. This boat has come about because of a request from a customer, but she has been in my head for many years and some of the reasons that I am so interested in this hull type are illustrated by Graham's comment.

Hi Ross, Have really enjoyed the last couple of posts. I like thinking about the laps as spray rails, it added another dimension to what I thought of as simply a nod to tradition. On the matter of fine entry + flare I would like to make an observation - is this discussion of fine entry and flare dependent on hull type? I am thinking of flat bottomed sharpie hulls in particular. These hulls often have nearly plum or vertical sides, but are designed to be sailed with a fair amount of heel, thus introducing a degree of flare. Also, the most successful sharpies have a fair amount of rocker in their flat bottoms, this means that more often than not they carry their stems at or above the waterline. I think that Bolger's Black Skimmer is an excellent example. Are the needs for flare and fine entry different between say, displacement hulls versus planing hulls, or between boats that are designed to be sailed fairly flat versus those with a little heel? cheers, Graham

Yes, the combination of a fine entry (i.e. sharp waterlines in the forward sections, with a half-angle of less than 19 degrees when viewed from above) a flat bottom, adequate rocker, and a boat which sails at an appreciable angle of heel will produce a soft and dry ride in a chop. I lack suitable photos to demonstrate this in decent wind conditions but here are a couple which may help.



Green Island 15 in light conditions. You can see the chine rising above the waterline, and gently slicing through the water. If the boat was heeling more, the effect would be greater, as the angle between the bottom and the topsides would make a pronounced "V"



Martin Kortlucke's Folding Schooner designed by Phil Bolger. Once again the wind conditions are light, but you can see the way the chine would work if the boat was heeling more - it would cut like a knife.

There are two primary reasons why so many flat-bottomed sailing boats have a bad name: -

• bottom too wide; and
• not enough bottom rocker

The idea is to have the curve of the topsides (in plan view) match the curve of the bottom (in profile view). This over-simplification really only applies if the boat has no flare i.e. vertical sides. However, with normal flare of between 10 and 12 degrees, it works pretty well. In theory, such a chine line would slice through the water without the formation of eddies associated with the normal flow of water across the chine.

Here are two drawings to give some idea of what I'm talking about. These show the forward hull sections of the design I'm working on, drawn at an angle-of-heel of 25 degrees. For reasons of practicality, the curve of the bottom and the curve of the topsides are not perfectly matched, but you will get some idea of the process involved.

Forward sections of sharpie heeled at 25 degrees

Same drawing, but with only the underwater sections shown. This gives a better visualisation of why properly designed sharpies can work so well, and be so fast.

What should be obvious from the above illustrations is that (within reason) a narrow hull with the chine line running high will perform better than a wide hull with the chine line running low.

Maybe I'll be able to come up with some better way of describing what I mean, but this will have to do for the moment.

Just to get you thinking, I believe that the fastest sharpie would actually be a scow hull, in which case the limitation on breadth of hull is removed. Look at the work of Phil Bolger and Jim Michalak for some clues...

More About Fine Lines and Lapstrake Planking

Dennis has written a comment about my last posting which is quite interesting: -

Ross, this is precisely the problem I face in my own region sailing in the Great Lakes USA. The water never gets warm, really, and getting a dousing is not a lot of fun, especially as one ages and gets "thin blood." Your first picture illustrates perfectly the trouble my 15 ft sailboat gives me. And while she sails very well, I increasingly feel the need to replace her with a drier boat.

I figured the problem was that there was not enough flare in the top sides of my boat's fwd section. Does flare contribute to a fine entry? The relationship between the two is not clear to me and the picture of Phoenix III seems to challenge that supposition. Can one have a beamier boat than the Phoenix III, say 6 ft, on a 15-15.5 ft length and still attain a fine entry to the bow you mention in your blog?

Best, Dennis

Well, this is a very good selection of questions, and the answer to all is, "Yes and No".

Yes, flare can contribute to a fine entry angle at the waterline and below, but it is only one of the design elements involved. To get a combination of flare, adequate breadth (or beam, as it is commonly called), and fine entry angles, you usually need to have a construction method which allows for compound curves in the structure. I will describe an exception shortly, but compound curves normally eliminate sheet material (for us that means plywood) as a building material.

The forward sections of Phil Bolger's Harbinger design

In the above photo, you can see a hull which is very wide (7' 1" on a 15' LOA) and yet has extremely fine entry lines and substantial flare.This boat was designed to be built using bent-frame carvel planking - a system which allows one to build a hull which contains compound curvature. Other options would be strip planking, cold molded using muliple layers of diagonal veneer, or, as I did with this boat, strip/diagonal, which is a combination of stip planking and double diagonal.

There are other options. Take as an example this lobster boat - also a Phil Bolger design, which I built using glued-lapstrake planking.

Planty of flare, but lots of planks

Phil Bolger Hope showing her fine lines

Once again, there is plenty of flare and a fine entry angle - all in a boat which is wide for her length. In this case I was only able to achieve the required shape by using a lot of narrow planks. In therory, if I had used an infinite number of planks, I could have achieved any shape I wanted, but the building process would by infinitely time-consuming and infinitely heavy - so a compromise had to be reached.

When I was designing Phoeinx III and Periwinkle, I was determined to use the minimum number of planks which was consistent with an nice hull shape. This allowed me to capitalise on the wide planks which could be cut from standard sheets of plywood, while at the same time reducing the marking, cutting, and gluing labour time for the builder. Once again a compromise. The individual planks were wide enough to need to be 'developable' shapes, but I still had more latitude with hull shape than would be the case with a sheet plywood boat.

The first Periwinkle awaiting her launching

 One of the design elements which allowed me to get a nice shape with so few planks was that the boats were relatively slender for their length. There are other good examples in the work of Joel White and Iain Oughtred and others.

Periwinkle, showing that she is relatively long and narrow.

There is another option, and that is to take sheet plywood construction to the limits of plywood's ability to twist and bend at the same time. This approach requires very careful design to ensure that all planks (or hull panels in this case) are what is called "developable". What this means is that the hull shape is designed so that the hull panels, which are cut from flat sheets of plywood, steel, or aluminium, are never required to bend in more than one plane at a time.

To visualise this, consider taking a sheet of card, bending it into a section of a cylinder, and then trying to bend it in another plane - it just won't work. If you bend a section of sheet material, it must take up a shape which is a segment of a cone or a cylinder. Therefore, you will always find a place on the curved surface where you will be able to lay a straight edge, and have it contact the surface along its length.

Now, it is possible to draw, using geometry, a surface which is composed of a number of adjacent cones and cylinders, but it is very time-consuming, and the resulting hull shapes are limited. Many thousands of stringer/frame plywood boats have been built this way over the decades, but they are generally fairly blunt and full in their forward sections. The resulting design options are limited.

Here is a good example of a plywood boat with developable panels taking up the normal sort of shape common to this design method. A nice boat, but fairly blunt up forward.

These days, the combination of computer modelling and stitch-and-glue construction has allowed developable shapes to enter new realms of shaping. The computer's number-crunching ability has allowed thousands of calculations to be carried out per second, and the stitch-and-glue method of construction has facilitated the un-folding of bottom panels which have been cut from pre-computed shapes and sewn along their edges.

Bottom panels of Flint cut to shape.....

...unfolded.....

...and made into a boat....

...which goes nicely with a beautiful, flared and fine entry - all from sheet plywood.

The final answer to Dennis' question is that I think it is possible to design a wide, flared, plywood boat which has fine entry lines - but the main problem is the "wide" bit. Perhaps glued-lapstrake or multi-chine plywood is the best approach. One day soon I'll have a go at a dingy hull of 15.5' x 6' in stitch-and-glue to see what I can achieve - but it will be a while before I get around to it.

Fine Lines and Lapstrake (Clinker) Hulls

I was having a discussion recently with one of my sons about which hobby activity was the most fun. We have far too many hobbies, but flying and sailing are close to the top. Well, we both decided that there was no contest - dinghy sailing came out as number one.

Where I live, the biggest problem with sailing is exposure to the savage sun, but running a close second is being continually doused by water as you drive to windward. Yes, I know that we have a nice climate on the east coast of Australia (at least as far north as we are), but for a large part of the year the water is frequently very cold when it hits a hot body!

Most of the boats that I have sailed have had relatively blunt forward sections, and they tend to bash water into the air when butting into a wave, and the wind blows it back directly over the crew.

Hard to windward with a deep reef tied in - relatively smooth water as we exit the harbour, but we are still getting wet!

There are plenty of advantages in having full sections forward, such as adequate buoyancy when running down waves, and the ability to move forward in the boat while still retaining reasonable trim. In some boats where load-carrying is critical, a very broad bow is essential - for example a praam dinghy

My Alby design - lots of carrying capacity in a short hull

The problem is that the disturbed water pushed up by a blunt boat - at least in a dinghy - consumes power, and as previously mentioned, bashes water up into the wind so that it can wet the crew efficiently.

This is a nice design I built a few years back, but you can see that she pushes up a lot of water, even in light conditions

Here is another good example - a really nice design, but pushing up lots of water.

In both of the above photos, the boats were drawn by exceptionally good designers, and the shape ended up the way it did for important reasons. But it does demonstrate the point I'm making about the water.

When I was asked to design Phoenix III, I really wanted to keep the bow fine. In fact, it has become a bit of an obsession with me, although I do change my attitude when necessary (e.g. Alby and Whimbrel).

Phoenix III showing her fine forward sections

My reasons for going down the path of fine forward sections include: -

• Dryness when pushing to windward;
• Reduced pounding - something which is very important in a small and light-weight boat;
• Easily driven hull - particularly when under oars

Paul Hernes in his Phoenix III under oars - he is too busy smiling, and should be rowing faster!

One of my favourite methods of construction is glued-lapstrake (or glued-clinker). This system is only possible if the planks are made of plywood, because natural timber planks lack cross-grain strength and will crack where the thickness of the planks changes from double to single. That is why traditional clinker hulls have lots of closely-spaced bent ribs, and have their plank laps riveted or clench-nailed - the ribs provide cross-grain strength, and the mechanical fastenings allow the planks to move relative to each other as the boat shrinks and swells in changing conditions. Plywood has almost equal strength in all directions, and can be glued at the lap - this produces a strong, stress-skin hull which requires few, if any, internal frames. The glued laps form an integral "stringer" which further strengthens the hull.

A clean and open interior

A wonderful side-effect of the lapstrake construction method is that the laps produce a series of "spray rails" on the external surface of the hull.

Fine forward sections and prominent laps on my Periwinkle design

The combination of the fine lines and the overlapping planks makes for a dry ride in most conditions.

Here is a good example of the effectiveness of the sharp forward lines and the "spray rail" action of the laps. This is another photo of Periwinkle.

As with all things, there are drawbacks. Any boat with fine lines forward (or aft for that matter) needs to be properly trimmed, as they are sensitive to weight distribution.

Here, the crew of Periwinkle is a little too far forward for the conditions. In gentle winds their position would be ok, but I think the big mainsail was driving her down somewhat at the bow. However, she is still going nicely!

Every boat is a compromise, and there is a place for all different sorts of hull shapes. The more more you understand about hull-forms and construction methods, the easier it will be to make a decision about what is right for your circumstances.

More about "Bird's Mouth" Spars

Last night I wrote a short piece about the process of assembling a "Bird's Mouth" mast. Since then, I've dug out some photos from my achives to show some of the steps in the production of the "Bird's Mouth" cut-outs.

The equipment required is basic, with the primary component being a table saw with a tilting arbor. This does not need to be expensive - mine is a second-hand Makita which cost me $120. I do use an old thicknesser and a cheap (and old) bandsaw - but these are luxury items, and the job can be done without them - it just takes a little longer.

This is the blade of my table saw tilted at 45 degrees, and set to cut to the required depth. On the right you can see the fence of the saw, and on the left is my home-made feather-board which will hold the sections of timber firmly against the fence while I make the cuts.





This shows one of the mast staves being pushed through the saw, with the home-made finger board holding the piece firmly against the fence. Accurate cutting is essential if you hope to get a mast with the correct diameter at each location.

Note the use of a 'push-stick' at the end of the run - be careful with power tools, as they can be very unforgiving and dangerous. If in doubt, get an experienced person to do the job for you.

First cut completed on one stave. After this, turn the stave end-for-end and run it through again.

This is how it should look after the second pass - now just do the same thing seven times more!

After all of the staves have had the 'Bird's Mouth' cut-out completed, I mark the pre-calculated taper on the side opposite to the bird's mouth and cut to the line with a bandsaw. This is a fairly tedious job, as there are eight very long cuts to be made, but it can be done with a plane and/or a jigsaw if you don't have access to a bandsaw.

A rough drawing to show the material to be cut away from the side opposite the 'Bird's Mouth'. This produces the taper in the finished, assembled spar. I usually give all the required dimensions in my more recently-completed plans.

Two masts (a main mast and a mizzen mast for a cat-ketch) given a trial (or "dry") assembly

Here they are, loosely assembled.

These are the solid inserts I put inside the bases of my masts. The 'V' cut-out is so there is no abrupt transition in stiffness between the solid and the hollow sections of the mast.

The section cut out of the solid plug. If you look carefully at the apex of the cut-out on the right of the picture you will see that I have finished the cut-out with a round hole. This is to prevent splitting of the solid plug.

Testing the fit prior to gluing.

A nice fit!

A glued-up mast.

Have a look at the previous post to see some pictures of the assembly process.

Assembling a "Bird's Mouth" Hollow Mast

Most of the masts that I make are built hollow using the so called "Bird's Mouth" technique. This is just a development of a vast range of similar methods used over many hundreds of years, but it is particularly suited to this day and age - mainly because of tilting-arbor table saws and power routers being readily available.

In this series of photos, I'll just show some basic steps in the assembly process. The marking and cutting of the staves which make up the spar need an article of their own.

Because of the large surface area which needs to be covered with epoxy, time is critical, and you need to be organised!

First four staves laid up dry to test fit. I am using 'U'-shaped plywood forms to hold the staves loosely together and in alignment. The forms are clamped to a long piece of timber acting as a strongback.

There are only about four forms for this entire mast, which is 23 feet long. They are simply there to prevent the eight staves getting out of control during the assembly. Once together, the structure is self-supporting, but it does need to be straightend very carefully before the glue goes off. 

I always make solid inserts for the bottom of the mast. It usually extends up to above the level of the mast partners. The internal 'V' cut-out is there so that there is no abrupt transition in stiffness between the solid and hollow sections.

I carefully shape the solid inserts so that they fit the internal shape of the mast without holding the staves apart. A slightly loose fit is good, as it allows a rich glue-line between the insert and the inner walls of the mast.

Another view of the insert

All eight staves are lined up in order, and at a convenient working height. Here I have a helper, as we need to be able to get epoxy out fast. We are applying the sealing coats to what will eventually be the inside of the mast, as it will not be accessible after assembly.

I use foam rollers to apply the epoxy, but you need to be careful to avoid pressing too hard and causing the wet epoxy to foam like whipped cream. After each coat is applied, I run over it lightly with a paint brush to smooth the surface and break any bubbles.

These are the staves at the masthead end. As you can see, these are almost at the minimum size limit for this method - I had done careful calculations ahead of time to make sure it would be feasible. You can see here that the epoxy coatings are building up.





This shows me working on another mast. We had already applied the glue to the gluing surfaces using various techniques, and at this point I was rapidly adding extra where required, and was smoothing the glue on all mating surfaces



The assembly just after putting together and placing a few cable ties around it to keep everything in place. Note the squeeze-out beginning to occur. It is important to have the glue fairly runny, as excess viscosity makes it very hard to clamp the total assembly tightly enough so that the mast assumes the correct diameter.

Start with a few, evenly-spaced cable ties, and then put on more in-between.

I use high-quality cable-ties and tighten them with the aid of pliers.

As you can see, I'm serious about getting a good and even clamping! Although you can't see it well in the photo, I use multiple windings of packaging tape to apply extra pressure where needed. There are many different methods of clamping, but this system works for me.

Once the clamping has been completed, I then spend time making sure that the mast is straight. While the glue is still wet, you can fairly easily push, pull, and hammer the thing until it is straight. Put some effort into this, as when the glue goes off, the shape is fixed forever! You will find that the human eye is a very effective tool for determining straightness.

Slices sawn off the top and bottom ends of the mast shown in the construction  photos. You will note that when I cut the bird's mouth cutouts, I run the saw blade just a little deeper than required, so that I get a little resevoir for epoxy, and that the staves fit in without hang-ups.

Here is a finished mast on another boat I built a couple of years ago.

Be open-minded about this process - it may look intimidating, but in fact it is quite easy - as long as you are organised!