Designing the Great Alaskan 26  
by Brian Dixon - Fairbanks, Alaska - USA
www.glacierboats.com

Part 2 - Detailed Designs

part 1 - Initial Design Considerations

As mentioned in part 1 of this article series, the Great Alaskan 25-28 was designed with the following goals in mind:

  • Seaworthy
  • Efficient to Operate
  • Suitable for slow-speed operation, e.g. sight-seeing, camping, fishing, etcetera
  • Larger than other similar boats yet still legal to trailer without a special permit

For those that did not get a chance to read part 1 of this series, you will have missed some of the philosophies of design that went into this boat. Since it has been a couple of years since that last article, each of the sections below will provide a short review of what went into meeting the goals that I have listed above along with some tutorials on the concepts underlying the decisions made while designing this boat.

Seaworthiness and Stability

First and most important is the goal of seaworthiness. Although there are many terms bandied about when it comes to discussions of stability, e.g. initial stability, ultimate stability, etcetera, there are really only two primary types of stability (or stability factors) that all boats are subject to. The first is weight stability. If a boat’s vertical center of gravity (VCG) is below, preferably well below, the vertical center of buoyancy (VCB, the centroid of the submerged volume) then the boat is said to have weight stability. If the VCG is equivalent in height to the VCB, then the boat has neutral weight stability. That means the boat’s center of gravity isn’t going to help, or hurt, a boat in righting itself once heeled. If the VCG is above the VCB however, the boat has negative stability and the boat’s center of gravity is detrimental to the boat’s ability to self-right when it heels.

The hull on the left has positive weight stability, the center has neutral, and the right has negative weight stability. Boats with negative weight stability depend on form stability and hydrodynamics to overwhelm the negative stability.

Only displacement hulls such as sailboats and slow cruisers have the opportunity to keep the VCG below the VCB however, so planing hulls are out of luck in this respect. Planing hulls must, since they ride on top of the water, have a VCG above the VCB when planing but the boat’s hydrodynamics and form stability far overwhelm the negative weight stability. When planing hulls are not planing however, a good design goal is still to produce a VCG as low as possible so at worst, it has only a minimal negative effect on the boat’s ability to right itself when it has been heeled over. The Great Alaskan utilizes lighter construction up high, and heavier-than-required construction low in the hull, and we recommend locating heavy components such as fuel tanks in the belly of the boat in order to keep the VCG as low as possible.

The second component to a boat’s stability is form stability. Consider for a moment a cylinder of Styrofoam floating on the water, and beside it, a flat raft of Styrofoam that is wide versus its thickness. The cylinder can easily be spun or rolled on the water and it shows no tendency whatsoever to maintain any particular orientation to the surface of the water. The flat raft of Styrofoam however is tough to flip over. You can push one edge down quite far and it always self-rights when you let go. The difference between the two chunks of Styrofoam is how far the center of buoyancy moves to one side when the foam is ‘heeled’ or rotate on the water. As the cylinder spins, the center of submerged volume, the CB, stays where it started. With the flat raft of Styrofoam, the center of submerged volume, the CB, moves far off to one side and exerts a higher righting moment (or force.) In mathematical terms you are observing the difference in the two pieces of Styrofoam’s Transverse Metacentric Height (GMt), a quantitative measure of the floating body’s ability to self-right when heeled. The lesson here is that wide boats with shallow deadrise (or even flat-bottomed) self-right easier than boats that more or less float like a cylinder in the water, e.g. heavy deep-V hulls:

If the center of buoyancy does not move when a vessel heels (top), then the vessel has zero form stability. If the center of buoyancy moves to one side when a vessel heels, then it does have form stability. Narrow hulls with greater deadrise often have lowered form stability, while wider hulls with lower deadrise most often have greater form stability.

In summary, planing hulls without deep keels (rare!) can not generally take advantage of weight stability, but can take full advantage of form stability. Heavy deep-V hulls must then have higher freeboard in order to prevent down-flooding when waves or swells climb up the side of the boat or when the boat is heeled far over. But lighter weight, shallower deadrise, hulls can get away with lower freeboard. In trade, they must have a higher transverse metacentric height (GMt) instead. GMt’s for heavier deep-V hulls in the 24 to 28 foot category tend to run from 32” to 44”. Don’t worry too much about what the number of inches means, but note that higher inches means a stiffer boat, one that rolls with the water rather than resisting it. There is no magic number that the GMt should be, but you should compare it to what other boats in a similar class of boats have, boats that are proven successful. If the GMt is too low, the boat will roll late in comparison to when the wave or swell rises under the boat (a sickly motion) or will generate an extra-large roll now and then (an odd whipping back and forth.) If the GMt is too high, then the boat snap-rolls with the changing surface of the water and is both uncomfortable to be in and high mechanical stresses are placed on the boat instead. When a GMt is selected as a target for a particular design, it should be similar in size to the example boats with which you wish to compare, but with an adjustment up or down depending on whether you wish to optimize the boat differently than the example boats. The Great Alaskan has a GMt of 49” to 50-1/2”, depending on loading. This is slightly higher than you see for heavy fiberglass boats and the boat will roll with the water in a snappier fashion as well. Noting that the number one reason for a boat capsizing when drifting or moving slowly is the taking of a wave over the side (or stern) of a boat, it is important that boats designed for slow or drifting usage are able to bob up and over waves rather than allowing them to climb up the sides (or stern) of the boat too highly. The goal is to be safe even if not on plane.

There are a few other minor factors that have been designed into the Great Alaskan in the name of seaworthiness or safety at sea as well. The boat utilizes a semi-dory sea skiff hull form which has a reasonably significant amount of flare to the boat’s sides. This means that the boat’s pounds per inch immersion (PPI) dramatically increases as the boat is momentarily pressed down into the water as the ocean around it heaves (moves vertically upwards.) Having flared sides also means the boat’s righting arm (GZ) increases faster than you would see with a similar boat without flared sides. The righting moment is the GZ times the boat’s displacement, so (obviously, I hope) the boat’s righting moment is more powerful for any particular degree of heel than it would be otherwise. The Great Alaskan’s “curve of areas”, which represents the boat’s fore and aft underwater volumes was designed to be as balanced as possible for a planing hull. What this really means is that the bow sections are as healthy as possible without making the boat into a ‘pounder’, and the aft sections are slightly reduced. The stern of the Great Alaskan is a little narrower than the amidships beam. This helps the boat respond properly to both maneuvering efforts and to swells as the boat runs down one and up the next. If a boat is too fine in the bow, it will plow or dive into the next swell and generate a sudden upward pitching moment in addition to resisting appropriate steerage. Noting that the local water speed in a wave is higher at the peak than in the trough, that this occurring in a following sea will tend to drive the stern to one side, a dangerous situation called ‘broaching’ that can lead to capsize as a boat is subjected to a large swell or wave sideways. If a boat is more balanced in its fore and aft sections, then this effect is minimized. The Great Alaskan was also designed so that its center of lateral resistance is not too far aft of the center of buoyancy since this can also result in a boat’s tendency to broach.

The final factor to consider is the boat’s freeboard. The distance from the water surface to the sheer, since this is how high water must go before it can down-flood into the boat. Rather than utilize a classic sheer line that dips lowest in the amidships region, common in work boats, the Great Alaskan has a sheer that rises constantly from stern to stem in a sweeping arch. The boat is over 5 feet deep at the bow and the amidships freeboard is higher than similar vessels by 2 to 4 inches. The Prince Rupert version of the design utilizes a 7” bulwark forward and a 2” coaming aft that further increases how far the boat can heel before down-flooding occurs. Even without the coaming and bulwark though, the Great Alaskan can heel 60 degrees before down-flooding occurs, and the Prince Rupert model can heel several more. This is 10 to 15 degrees more than what similar commercially-designed boats can absorb.

Note: Some discussions concerning stability mention terms such as initial stability and ultimate stability. Ultimate stability has to do with the maximum number of degrees of heel that a boat can absorb before either down-flooding or negative stability occurs. Open-cockpit planing hulls can not be effectively sealed up to prevent down-flooding, so the point of negative stability (the boat’s preference to remain upside down) is never reached and down-flooding is instead the limit. Initial stability is the boat’s resistance to heeling the first 10 degrees or so. The chart below shows the Great Alaskan’s righting moment versus degree of heel and clearly demonstrates that the boat has been designed to be more stable over this initial ‘comfort zone’ in heel angles:

Notice: a) the curve is steeper over the first 10 degrees, b) the righting moment grows continuously clear up to 50 degrees of heeling, and c) the boat can heel to 60 degrees before down-flooding will occur (for the Newport …the Prince Rupert can roll further before down-flooding). This curve was generated in a free-float condition, which means “as the real boat would perform” rather than just by rotating the boat about its longitudinal axis. See the full hydrostatics report at: http://www.glacierboats.com/hydrostatics.html

Efficiency

The number one factor that either hinders or helps a boat to be efficient is the boat’s weight or displacement. The second most important factor is the hull’s overall resistance, whether planing or operating in displacement mode or somewhere between. The overall resistance is determined by factors such as loading (pounds per square foot) and the form or shape of the hull. The last of the factors worth listing that can hurt efficiency is the parasitic resistance of the finished boat. For example, things such as surface rust and appendages rudders, outboards, or other hardware that extend into the water increase the boat’s parasitic resistance. Note: The term ‘residual resistance’ which you will see in some literature refers to the resistance associated with wave making (water pushed aside by the boat) and should not be confused with the term “parasitic resistance.”

The most efficient class of boats are slow displacement boats. The second most efficient class of boats are planing hulls. The third and least efficient of all boat classes are the semi-displacement boats that operate in a mode that is not quite on plane yet is still beginning to lift out of the water enough to not be in displacement mode either. (More info on this topic below.)

Weight or Displacement:
Wood-composite or stitch-n-tape construction weighs, on the average, about 40 pounds per cubic foot of material. Some say 35 pounds, but my calculations differ and with some wood selections I find that wood-composite construction can actually reach densities as high as 45 pounds per cubic foot. Aluminum weighs about 170 pounds per cubic foot and requires about half the volume of materials as compared to wood-composite. Four times heavier but half the volume means aluminum boats of similar size weigh about twice as much as a wood-composite boat. Polyester fiberglass layups weigh about 100 pounds per cubic foot (but varies quite a lot depending on total composite density and core materials utilized in the layup.) Polyester fiberglass boats may use more or less physical volume than wood-composite constructed boats depending a lot upon what type of cores and structure is utilized to produce the required strength levels. Because polyester fiberglass is not in itself very strong and must depend on additional thickness, structure, and cores to produce the strength required for a boat however, polyester boats do tend to weigh quite a lot …often more than equivalent aluminum boats, especially when comparing deep-V offshore hulls.

The bottom line here is that the wood-composite method of boat building is by far the lightest way of building mid- to large-sized boats and has the most potential for producing a high-efficiency hull in this size range and displacement capability. The primary reason that commercial makers don’t use this method for building their boats is cost, and the primary factor of which are the required man-hours, not the cost of materials. You can’t spray a wood-composite boat into a mold, and forming composite seams with fiberglass and epoxy results in fewer inches of seam produced per hour of work compared to standard weld rates. But there is no reason that a home builder (who works for ‘free’) cannot take advantage of what wood-composite construction has to offer. Because efficiency is of prime importance to me as a designer, especially in light of increasing oil prices, wood-composite is my natural go-to first choice when I’m deciding what material and building method that I’d like to use.

Hull Form v. Resistance:
The overall resistance of a planing hull is most directly a result of the boat’s deadrise, change in deadrise from aft to fore, and its fineness of entry. Deep-V hulls have a higher prismatic coefficient of lift in the forward sections that can result in a more aggressive pitching motion (upwards) when the hull enters larger waves. You definitely do not want a boat that has too fine of an entry versus the width of the rest of the boat’s hull, and this is doubly true for lighter boats that have a lower longitudinal moment of inertia (the tendency to resist pitching as a result of the boat’s weight and fore/aft distribution of weights.) On the other hand, Flat bottomed boats tend to be comfortable only on calm water since they tend to pound when in rougher conditions. Wider boats are more efficient on calm water, yet narrower boats that slice through waves more nicely tend to be more efficient in rough water. For any particular boat, the designer can optimize the boat’s average resistance by making assumptions on the speeds and sea states that the boat is intended to operate in. Failure to do this will result in a boat with disappointing performance. The Great Alaskan’s 26-1/2 degree half angle of entry, 14-1/2 degree amidships deadrise, and the Savitsky waterline ratio (at-rest waterline beam divided by mean waterline length) were selected to be a best compromise for a boat that is intended to operate in average choppy offshore conditions at speeds averaging about 19-20 knots. The Great Alaskan’s 5” wide chine flats are intended to be a best compromise between having a lot of horizontal lifting surface, yet not so much that the boat pounds.

The Great Alaskan also has a highly prismatic, monohedron, type hull below the waterline. What this means is that there is not much change in deadrise from the stern to the amidships region where most of the planing lift is generated. There is only about 1-1/2 degrees difference from stern to amidships. Other hulls tend to have 4 to 6 degrees difference in deadrise and consequently waste more energy sending water out sideways from the boat rather than downwards. Water sent sideways means that the boat spent energy in a direction that does not help the boat plane, e.g. lost efficiency. Since the Great Alaskan is optimized for 19-21 knots (top speeds in the upper-30 knot range), this hull form works fine for the speeds that most people are willing to travel when in offshore conditions. The Great Alaskan’s length of 25 to 28 feet allows the transition from the entry deadrise to the amidships deadrise to take place over a longer span than what would be available on shorter boats, and this is taken advantage of in this design to allow both a reasonably fine entry yet still maintain modest-deadrise prismatic hull from amidships aft for efficiency. I would not, for example, use this hull form on a boat only 22 feet long intended for similar speeds because the boat would simply not be long enough for the required rate of transition in the bow. But I certainly would when considering design parameters for a longer boat such as the Great Alaskan.

Finally, all V-hull boats tend to allow water to travel up the sides of the hull, turning into spray that blows into the boat as it comes off the sides of the boat. All V-hulls, if you want a dry boat that is, should include some type of water rejection that prevents this upward climbing of water on the boat’s sides. The heavier the boat and the higher the deadrise angles, the worse these problems become. Water rejection by the Great Alaskan is accomplished by the combination of the 5” wide chine flats, the auxiliary spray rails along the chines forward that actually drive the sheet of water coming off the bow back downwards, and by the main spray rails along the sides. The models of the Great Alaskan that have larger pilot houses add additional water rejection capability by draining water that does land on top of the boat to the sides, and on the Prince Rupert, by cockpit coaming that extends aft to alongside the drywell.

Accommodations and Boat Size

Trailers:
One of the goals for the Great Alaskan was to provide the maximum amount of room inside the boat as possible, yet still stay within federal limits for trailers on the United States interstate highways. According to federal law, all interstate highways have to allow up to 8’6” wide trailers or loads plus reasonable access to and from the interstate. Reasonable access includes, for a boat, the roads to and from waterways and storage facilities, whether it is your home or otherwise. The Great Alaskan, when built exactly to specifications, turns out 8’5” wide including the rubrails along the sheer. This allows a bit of room for error and/or some flexibility on the part of the user in choosing the type and style of the sheer line rubrails.

Another design constraint that must considered when designing a boat of this size is whether or not it will fit onto standard, non-customized, trailers. Nobody wants to spend money on custom trailers if they don’t have to. For this boat, any tandem-axle trailer with a maximum weight capability of 5000 to 7000 pounds and has 82” or more room between fenders will work for the Great Alaskan, noting that a typical trip will result in around 3800 pounds (boat, motors, gear, crew, fuel, etcetera) and the maximum capacity of the Great Alaskan is around 6000 pounds. For this boat, some manufacturers may require you to order a longer tongue on the trailer, but that’s fairly normal on trailer orders anyway and it is a low-cost change to ask for. The interior construction has already been designed to allow the use of your choice of roller or bunk type trailers. Brake requirements vary from state to state, but all trailers in this size category have all brake options available. Fortunately the Great Alaskan’s waterline beam, which would normally sit just about even with the tops of the fenders when the boat is on the trailer, is 79” wide. It should not be hard or expensive to find a trailer for this boat.

Accommodations:
As far as accommodations go, the Great Alaskan is superior to the commercial boats that I have reviewed and has more room throughout. Part of the reason for this is that aluminum boats are generally not designed to be campers or cruisers, and polyester fiberglass boats require additional thickness and structure in order to make them strong enough (as described previously.) While fiberglass boats may commonly be designed for camping and cruising, they invariably have several inches less space inside them when compared to a wood-composite boat such as the Great Alaskan. As an example, consider the Great Alaskan’s cuddy. The cuddy compartment has nearly 7’ sized bunks in it and enough room to design-in a drop-down table (queen-sized bed when lowered, moderately-sized bedroom dinette when raised.) And inside the pilot house, the boat has a full 6’3” of headroom above a deck that runs flush from transom to cuddy. There is room for a stand-up head and shower should you desire, a galley, dinette, and seating. The plans package takes an approach that shows you basic methods of building all the above, including framing and interior support, and then gives example layouts that you may wish to use. Or, you can design your own. Whatever you do, I am willing to assist with final placement of heavy items that will help your boat trim properly at the dock and to perform properly when not. I have full center-of-gravity spreadsheets already produced for the boat (required for design anyway) and can easily model your trim, hydrostatics, and hydrodynamics if you provide me with information on what you wish to put in the boat and where.

Typical accommodations for the Newport or Prince Rupert. In order to provide the most flexibility possible in arrangements and/or cuddy or pilot house sizes, the plans provide instructions for building all 3 models of the boat in sizes ranging from 25 to 28 feet long and also provide acceptable ranges of locations for the primary bulkheads (fore/aft house and cuddy bulkhead locations.)

Current and Future Status

As of the date of this article, there are 6 builders actively building and I expect that the first hull will launch sometime in mid-2008. Builders currently exist in Alaska, Washington, California, Nevada, Virginia, and Florida. Printed and download-only plans are available both here at Duckworks Magazine and at our own web site at http://www.glacierboats.com. (But say thank you to Chuck for publishing this article for me by buying your plans through him if you can.) Plans include 200+ pages in construction manuals (2 parts) and 29 drawings designed to print on 11x17 (Arch-B) paper. All documents are in pdf format and those that opt for printed plans are given access to the online repository of the pdf versions of the documents as well. Support will be provided via email and evening/weekend phone access whenever I’m available. If it seems like a good idea then I may set up a forum-type environment for builders to interact as well. The web site will be updated with photos from all builders as they are provided to me of course, and after the snow returns, I will be putting together some better 3D photo-realistic renderings that will also be made available at the web site. If you are interested in having your name added to the ‘interested parties’ distribution list, just email me (http://www.glacierboats.com/contact.html) and let me know.

Brian

back to part 1 - Initial Design Considerations

Plans for Great Alaskan are available at Duckworks

SAILS

EPOXY

GEAR