Designing “Liz”

Designing “LIZ”

By Tom Lathrop 

Getting The Bug

Except for a small runabout in the early fifties, I have only owned or built sailboats.  The advantages offered by powerboats for cruising became more attractive as Liz and I sailed past middle age.  For Intracoastal Waterway and inland cruising, the tall mast and keel of a sailboat restrict access to many areas and the speed limitation of 6 knots under power can become boring. We have also experienced the advantages of trailerable boats that can get to desired cruising grounds at highway speeds, so it was natural to look at the possibility of a trailerable power cruiser.  For financial reasons and the urge to design my own boat, I decided to do it myself.  Also, none of the available boats on the market suited my requirements.  An association with local naval architect Graham Byrnes of B&B Yachts in teaching boatbuilding at the local Community College and building numerous small boats gave me a valuable introduction to the various aspects of design and construction, so I felt enabled to begin this project.

Design Objectives

The desired speed range gave the most concern.  I wanted a cruising speed of about twice that possible with a sailboat, or 12 to 14mph.  A preliminary study of available powerboats showed that this was a never-never land for efficient cruising.  Planing hulls are prone to bog down well before getting down to that speed and semi-displacement hulls only reach it by the application of lots of power.  Long, slender hulls can achieve this speed but are not easily trailerable unless accommodations are compromised.  Much study of Lindsay Lord’s “Naval Architecture of Planing Hulls” offered hope that, with careful design and attention to weight control, reliable planing in a hull of normal aspect ratio (about 3.5 : 1) might be achieved in this range.  Other books by Farmer, Gerr, Edmonds and Brewer were also useful but Lord’s book is the only comprehensive study of planing hulls that I was able to locate.  I could not find a copy of Savitsky’s book and some of the work by him that I saw was highly technical. Note: Since this article was written, I have found several other sources such as “High Speed Small Craft” by Peter Du Cane that would have been helpful but they would not have changed the design in any significant way.

There are varying opinions on what is acceptable trailering weight, but our past experience dictated that, for performance, lighter is better and a limit of 2200 lbs for the boat looked like a reasonable objective.  The weight limitation imposed by both trailering and performance objectives rule out inboard power, so a four-stroke outboard seemed the logical choice.

The list of design objectives began to take shape and looked something like this:

1.  Small and light enough to be readily trailerable

2.  Capable of economical cruising at about 12 to 14mph with a top speed at least 50% higher

3.  Seaworthy in inshore conditions

4.  Economical operation

5.  Sitting headroom in the sleeping cabin and standing headroom at steering station & pilothouse

6.  Good looking, classic appearance

7.  Reasonably simple to build in plywood and epoxy

These are not listed in order of importance, I wanted them all.  As we shall see, other requirements came along later.

Preliminary Design

In my undoubtedly biased opinion, the classic powerboats of the early twentieth century are the most aesthetically pleasing.  I had long admired the little 17’ Weston Farmer classic design, “Sundance”, with its nearly plumb bow, flush foredeck and broken sheer, so that became the starting point.   In the heyday of custom yacht building between the world wars, these designs were the undisputed queens of any fleet.  One is occasionally seen today and always seems to make the surrounding spaceship inspired creations look like cheap hot tubs by comparison.  I hoped to be able to capture a whiff of the essence of these handsome craft in my boat.

To get adequate sitting headroom in the cabin and have the profile look right, my eye determined an overall length of at least 22 feet was needed.  I had driven one of Sam Devlin’s 22 foot Surf Scoters at Port Townsend, WA, and it helped to give me some reference points. Sam’s boats are clearly derived from the same classic designs that appealed to me.  While I found it a very handsome boat and beautifully built by Sam and his crew, it did not really fit the concept that I had in mind.  It was a heavier, semi-displacement hull that required a 90hp outboard to give the speed range I wanted, and it did not have sitting headroom (for me) over the bunks in the forecabin.  Necessary headroom is a subjective matter, but spending rainy days on a boat without it tends to bring on “cabin fever” much more quickly.

As the requirements and ideas coalesced into a mental picture, I began to sketch the complete boat.  As anyone knows who has ever followed this path, some ideas prove to be incompatible or unworkable.  It’s a humbling experience as it becomes clear that we are not quite as knowledgeable as we thought when just daydreaming or kibitzing someone else’s work.  After the initial drawing was completed, it was time to take a serious look at the waterplane.  I concluded that the monohedron hull, as proposed by Lord, will best suit my purpose and the next task is to decide on the degree of deadrise.  One school holds that a flat bottom is not only simple to build but preferred from a performance view.  My own experience is that such boats usually stick their nose in the air while planing or attempting to plane and pound in any kind of chop.  The deep “V” seems to be the style du’jour, but is not suitable for this boat. I suspect the conventional wisdom of the higher wetted surface of the deep-V requiring high power may be in error.  It seems more likely that the high deadrise hull shape requires a heavy displacement to immerse the chines and make it acceptably stable and that the resultant high bottom loading is a major factor in causing the high drag and high power requirements associated with these hulls.

So, what deadrise is needed to satisfy the contradictory requirements of quick planing, economic operation and rough water capability?  For stability, the aft chine needs to be immersed at the design displacement and the forward sections need to be fine enough to not pound excessively but not so fine as to create downwind steering problems or drop buoyancy and interior room below acceptable levels.  I calculated that 10 degrees for the aft monohedron sections, forward to near station 6, then rising to 25 degrees at station 2.5 (recommended by Lord along with convex sections near the bow) as the most reasonable compromise to satisfy the design objectives.  As a point of reference, the C Dory, a popular small power cruiser, has a deadrise in the aft sections that varies from about 4 degrees near station 6, to 3 degrees at the transom.  This is a very shallow “V” and, as a result, the C Dory will pound when running into a chop.

The pronounced knuckle in the bottom bow sections, made necessary by the nearly plumb bow shape, introduces a great amount of twist in the bottom panels.  I attempted to locate the apex of the conic sections by the methods described in various texts, but I simply could not find the beast.  Therefore, I resorted to the empirical method of fitting cardboard to a half model to determine if the shapes could be formed from flat plywood. Two half models were carved before I was satisfied that the lines were pleasing to the eye, developable and also satisfied the hydrodynamic requirements.  The lack of a complete mathematical foundation for the bow shape was tempered by the experience of visits to the “Hoi toider” boatbuilders of Harker’s Island and Marshallberg, NC.  When asked how they developed the sometimes-extreme bow shapes for which they are famous, would reply; “aww, oie just oie ball it.”

When I showed my half model to one of these old timers, he took a look at the convex forward sections of the bottom and with a shake of his head and rolling motions of his hands indicated that there should be concavity in this part of the hull.  We had owned a small fiberglass shrimp boat with such forward sections and found it to be bloody awful.  Aside from a lack of forward buoyancy, it had a nasty habit of digging into any off-angle oncoming waves and trying to throw the occupants overboard.  Such concave forward sections create an exponential buoyancy increase as the bow enters a wave and result in pounding as the downward motion comes to a sudden stop.  Many of the WW II PT boats with such bows had the distressing habit of tearing their bottoms out in waves at high speed.  Convex sections allow a much easier entry and should give a smoother ride to windward.  Fortunately, when plywood is bent and twisted to meet the chine and keel in the forward sections of a boat, it naturally creates a convex shape.

By this time the design was becoming more fixed and other requirements and ideas were taking form that changed the direction a bit.  It had proved very difficult to include a suitable galley and head in the convertible version for extended cruises and a weatherproof pilothouse began to look a lot more comfortable.  I had originally been attracted to the outboard motor in a partial well in front of the transom.  This is a very common feature of many traditional workboats here on the Carolina coast and Sam Devlin uses it in his Surf Scoter designs.  It would have been primarily an aesthetic item although local fishermen use it to allow unobstructed working of nets over the transom without the risk of tangling fishing gear in the propeller.  In the end I decided that it would rob some stern buoyancy, adversely affect low speed steering control and, most of all, take up cockpit room.  Anyway, I moved the motor to the transom and gave the boat two more feet of length to make the pilothouse more spacious and allow a private stand-up head.  Another plus resulting from mounting the engine on the transom is that it reduces the transverse width necessary for the engine space and allows for comfortable permanent seats on each side of the engine.  Changes to the length of the boat were made by increasing the spacing of the 11 stations which allows the section shapes and most of the hydrodynamic factors to remain unchanged.  Of course, I was beginning to push my original size and weight objectives but went ahead anyway.

All along the way, I had been using the formulas of Crouch, Keith and others to make performance projections of speed and power requirements.  It looked like an engine of 50hp might be adequate to get the cruising and top speeds that I wanted to reach, although almost all local boaters offered the opinion that it would take 100hp to meet my expectations. They have big, heavy motors on boats with higher bottom load factors that would be less efficient than “Liz”, so I went ahead.  I also had contact with owners of Bolger designed 22 foot power cruisers that were supposed to perform with 50hp but failed to plane and had to be increased to 90hp or more to reach similar objectives.  This was somewhat disquieting but these boats were heavier (possibly heavier than Bolger intended) and I’m somewhat skeptical of the effectiveness of Bolger’s box-keel, flat bottom hull shape, especially in the transition from displacement to planing mode.  Trying to visualize the flow of water around this hull in the semi-displacement mode gave me a headache.  This is pure intuition since I have no experience with the type.

The Model 

It was time to build a scale model and start some towing tests to observe the reaction of the hull to varying speed and water conditions.  Several designers report that a model needs to be at least four feet long in order to give good results. At 24 feet, “Liz” translates to four feet with a scale ratio of 6:1. The model could also be built with 1/8-inch plywood and still meet the designed displacement with adequate margin to allow added lead weights for balance and displacement changes.   For simplicity, the model does not include any structure above the sheerline.  A set of lead weights was cast which allow varying the scale displacement, from –10% to +80% from nominal, for testing the hull at varying loads.  At a scale of 6:1, the design displacement of 2400lb translates by the cube of 6 (216) to 11.1lb for the model.  A major departure was made from normal practice in that the aft portion of the hull bottom was designed so that it could be varied from a regular monohedron of constant 10 degrees deadrise to a warped plane, varying from 10 degrees at station 6 to values from 10 degrees down to 0 degrees at the transom in 2 ½ degree increments.  Replaceable sections for the keel and transom set the warp angle and plastic tape on the chine and keel held everything together just fine.

All towing setups I’ve seen attach the towline to the bow but I feel that this can dampen and hide possible instabilities and so I made a towing bridle that attaches to the approximate CG of the model.  A “Y” yoke of fishline is attached to the lateral projection of the CG on both sides of the hull.  In practice it proved difficult to get the model started in the proper direction without running over the yoke, so after some problems that are explained later, I eventually attached a third line between the bow and the apex of the “Y.”  This line is normally slack and has no effect unless the model tries to veer off course. A towing rig was made from a Sunfish boom that projected to the side of my skiff so that the model could be run alongside the towboat, in full view, and out of any wake action.  The ability to watch the model up close from the side allowed observation of any antics in all conditions.  The towline ran through turning blocks to a resistance-measuring device made from a postal scale.  It seems a bit crude but it worked fine, although the operator had to mentally integrate the bounce of the scale pointer to arrive at a drag force.  Speed was measured with a digital paddlewheel knotmeter on the towboat.  Model speed varies as the square root of the scale ratio of 6:1, that is, one knot of model speed equals 2.45 knots in the full size boat..

Time and weather conditions curtailed the test sessions somewhat and all desired tests were not completed, but I did get useful information about performance.  In its monohedron form, the model performed well with no visible bad habits and moved smoothly from slow to max scale speed of about 13.5mph (33mph full scale) in all sea states.  Drag vs. speed data agreed pretty well with predicted values derived from Lord.  It tracked true in scale wave conditions in which I never intend to run the full sized boat as it would be an extremely rough ride on both boat and crew.    Towing tests were repeated with the model configured for displacement values ranging from –10% up to +80% of the nominal design displacement of 2400 lbs.  The model performance exhibited no major changes other than increased drag as the loading was increased.  Due to design changes outlined earlier, the nominal maximum cruising displacement has been upped to 2800 lbs.  This is still a light boat, relative to other power cruisers of comparable size.  The model skipped along happily in scale wave conditions that the full sized boat will never see at the high speeds at which it was towed.

Adding weight to the model up to a value of 80% over nominal displacement showed no observable bad habits.  The towing drag increased proportionately to the weight increase throughout the speed range and the hull made more wake.  The added weight was placed on centerline in a longitudinal position such that the static trim of the model was unchanged.

In the second phase of the tests, the model was reconfigured to a warped hull form.  The general consensus is that a warped hull form will have slightly lower resistance at lower planing speeds and that the monohedron type is superior at higher speeds.  This may well be true but the difference was not significant enough to show up on my drag resistance tests. The hull forms appeared to be about equal in towing resistance at the speeds at which these tests were run although I did not carry out any tests for angles of transom deadrise less than 5 degrees.  Lindsay Lord reported that when the warped hull model was towed at higher speeds, it had a tendency to instability that took the form of rapid sideways movement or lateral oscillations.  It may be very presumptuous of me to second guess his work, but I think it possible he may have been misled due to shortcomings in his towing rig.  When I decided to use a bridle attached to the center of gravity of the hull instead of towing from the bow, I fortuitously made it possible to view this instability in a different light.  Instead of a sideways movement, my warped hull model exhibited definite yaw instability.  That is, the longitudinal axis of the model oscillated horizontally about a vertical axis located at or near the Center of Gravity.  At a differential deadrise of 5 degrees between the transom and amidships, the instability could be first noticed above 10mph model speed, and became progressively worse up to the maximum speed at which I was able to tow of a little over 13mph.  During the fastest tow, the oscillations became so great that the model actually completely swapped ends and destroyed the towing rig.  It was at this point that the third line was added between the yoke apex and the bow, which would allow the instability to be seen but would limit the yaw oscillation amplitude below disaster levels.

Most naval architects say that the twist in the bottom of the warped hull induces a rotation to the wake on either side due to the forward motion of the hull.  Any rocking of the hull (which is always present unless the water is glassy smooth) will impart more or less energy to either side depending on the momentary immersion of that side.  The reaction of the hull to this lateral imbalance of forces in the after part of the bottom is likely to include a steering moment imparted to the hull, which sets the observed yaw oscillation in motion.  If this is true then I understand why nearly all the warped hulls I see, have large skegs in the afterbody.  Such skegs will add considerable lateral resistance to the stern (like the feathers on an arrow) and tend to damp out the oscillations before they reach the disastrous level I observed and make the hull handle acceptably.  A bit of further corroboration is that monohedron  “V” hulls are seldom seen with large skegs. Time, weather conditions and a desire to get on with building the boat conspired to halt these tests, but I intend to get back to them later and take a more exhaustive look at this phenomenon.  This reasoning may prove to be incomplete or even inaccurate, but it seems to best explain the observed results.

Back To the Boat

The test towing of the monohedron model showed no bad habits and promised to fulfill the objectives, so I decided to go ahead with building the boat.  One further modification to the bottom that I had been working on, a chine flat, tapering in width from zero at the bow to 12 inches at the transom was added at this time. In addition to the normal spray-deflecting task of such flats, these are intended to provide additional lift to the stern sections. The longitudinal trim angle of the flats is one degree positive relative to the buttocks of the aft monohedron hull sections and the transverse trim angle is about three degrees negative for spray suppression.  It is my one original contribution to the hull design of this boat and is intended to optimize its low speed planing performance and induce longitudinal stability to help keep the boat level at all speeds.  It is designed to work much like a fixed transom trim tab but without the high drag associated with trim tabs. If these chines work as planned, they may limit the top speed, where they may tend to depress the bow, but the boat is not intended for high speed anyway.  Their purpose is to provide lift to the stern and thereby reduce the high-drag “hump” in transition from displacement to planing mode and lower the minimum speed at which the boat will maintain “reliable” planing.  By “reliable”, I mean that the boat must hold its trim angle and not bog down while maneuvering, encountering waves and wakes from other boats or changes in crew position.  Ideally, a planing boat would be able to run at any speed from idle to its maximum, in comfort, under perfect control, with little change in longitudinal trim angle and without excessive wake at any speed.

The First Run

Liz was launched in early May and the preliminary results are very encouraging.  Top speed with the 50hp Yamaha four-stroke is 23mph and drops to about 19mph with seven adults aboard.  Since this represents far more weight than the design displacement for cruising with a crew of two, we have no concern as to her performance loaded to normal limits.  There is no discernible planing “hump” and she runs level at all speeds.  When reducing speed, it is difficult to determine just when planing ceases.  She never squats but just slows down, holding an almost constant trim angle, until finally settling at about 7 or 8mph.  For people used to driving planing powerboats, this is always a great surprise.  Rough water handling is better than expected.  She will run dry and fairly quietly into a chop at 12mph and by slowing to about 7 or 8mph, quartering waves in a 20kt southwester were handled with aplomb.  We are very pleased with these results and look forward to some pleasant cruises in “Liz”.

After Ten Seasons 

Liz, now as the prototype Bluejacket 24, has satisfied our needs for a small cruising powerboat over a wide range of conditions.  While most of the miles under the keel have been here on the coastal sounds of North Carolina, South Carolina and the Chesapeake Bay, she has been as far from home as the Trent Severn Waterway and Georgian Bay of Ontario, Canada.  The Yamaha T50 has performed flawlessly and delivered fuel mileage averaging from 6 ½ to 8 ½ mpg depending on conditions and speeds.  On the high way, the aluminum tandem axle trailer is very stable and quickly damps out any sway resulting from rapid steering movements or crosswinds.  Several additions such as a table that fits either in the cockpit or forward cabin, chart rack, shelves for books and other loose items, towel racks, etc. make life aboard more pleasant.

I am well pleased with the performance in not so calm water, which we have in abundance on our sounds.  She has made her way across open water in a northeaster of 35 knots, although I would not choose to do so again.  Steering is very easy and predictable.   The hull bottom design looks like a good compromise between speed capability and rough water comfort and I can think of nothing about it I would change.  She is very dry in a chop unless run at an angle to oncoming waves that can blow up the spray on the windward side.

Launching and retrieving the Bluejacket is so simple that it lives on the trailer most of the time, which makes maintaining a clean bottom and other chores much easier.  When not cruising or moored at out dock, LIZ rests on the trailer in a boathouse I built just off the driveway.  Many of her offspring ranging from 24 feet to 28 feet are now cruising or under construction throughout the USA as well as overseas.


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