Frequently Asked Questions

Construction
Q. Can you do it?

Some prospective builders express concern about their building skills. My cruising boats feature round-bilge hulls that require lofting, careful setup, and attention to fairness throughout the building process. But one need not be a master craftsman, furniture builder, or a wizard with your hands to build one; any reasonably handy person can build a boat.

I've drawn my plans to simplify the building process as much as possible, and modern adhesives -- epoxy in particular -- make perfect fits less critical in many areas. Lots of builders learn as they go, and there are many good books available to help you through the hard parts. Building a boat -- and developing confidence in your abilities and skills -- is a very satisfying process.

Q. Will adding aditional veneers to those specified (or using thicker veneers) add too much weight to the hull?

The buoyancy gain depends really on the wood species you're using. Sea water weighs 64 lbs /cu.ft., and adding underwater volume with something that weighs less results in a net gain in buoyancy. For example, red cedar is about 23 lbs/cu ft, d. fir 32-36 lbs, etc. Naturally, you're also veneering the topsides, which is strictly a matter of adding weight (not offset by added buoyancy), and then there's the weight of the epoxy. The overall result is beneficial buoyancy-wise with lighter woods (such as red cedar), and moves into the negative column with woods above about 30 lbs/cu.ft. Red cedar is a fine choice and often available in veneer, yellow cedar (about 27 lbs/cu ft) is also good, or you may have a local wood that fits the bill.

If you're considering a thicker layer, you'll get most benefit by using two layers of thinner (1/8" or so) veneer, each set at 45 degrees to the strip planking (and 90 degrees offset from each other). That provides the modern equivalent of bronze strapping, and does an excellent job of resisting athwartship and twisting loads. The downside, of course, is that you have twice the work in applying it. You may also find that thicker (1/4") stock resists bending to some of the curves, needing many fasteners to hold it in place while the epoxy cures. Best to try with a piece (of the species you'll be
using) before going that route.

Q. I doubt if I can get anything except western red cedar for the veneers. Any problems with such a soft wood on the exterior?

No problem, especially if you sheath the hull with cloth. I prefer Dynel or Xynole polyester to fiberglass because of their higher flexibility and greater abrasion resistance.

Q. Do you suggest a bead & cove for the strip planking around the bilge. In strip planking would staples delivered from a pneumatic stapler work as well as nails?

Bead and cove strips can help in fitting the strips, though thickened epoxy will easily deal with small gaps. The strip planks could be stapled (in essence the fasteners are there only to keep things in place until the glue dries), though if the hull was ever damaged and water did penetrate the epoxy, you might have trouble with steel staples. I specify bronze ring nails because they'll be there for the life of the boat, no matter what happens. Also, they provide excellent holding power.

Q. How accurate does the loftng have to be?
Small errors (1/8" to 3/16") won't make any difference to performance. They may be noticable visually, but this can be dealt with (especially the topsides) if you're careful about fairing up the molds after setup, but before planking. There is also another chance to fair things after the hull is planked.
Paul,

Q. I am still having much trouble with the lofting of the transom in regards to planking thickness deductions. Have tried all methods. I have added waterlines to assist but still does not work out. Must I add extra buttocks in this region?
Extra buttocks can help with planking thickness, in that the more lines you have that are nearly normal to the hull surface, the better off you are. But extra butts and waterlines may be most helpful in making sure you have an accurate shape. Deduct planking thickness using whichever lines are most nearly normal to the raked transom.

Design

Q. Why is the draft of Kahuna only 4' 7"? Is the boat tender? At what angle of heel will she be at in 15, 25, 35 knots of wind?

This raises two important issues: ultimate stability and power to carry sail. Both power to carry sail and ultimate stability are determined by the boat's hull (and for ultimate stability, cabin) shape and the location of the center of gravity. But a boat that is very stiff (initially very stable) may not in fact have the type of stability curve that's desirable for a cruising boat. A barge and a catamaran are good examples of vessels that derive all of their stability from their hull form, and are very stiff initially, but are lacking in ultimate stability. In general beamy boats will show greater initial stability, but have a smaller range of total positive righting moment, than an otherwise equivalent narrower vessel. Draft is a good way to gain stability, and a narrow, deep boat can have a very good overall stability range (some are positive through 180 degrees), while being somewhat tender under sail. In addition, too much draft (or more accurately a very low CG), combined with a narrow hull, can produce a very quick motion that is uncomfortable at sea. In essence one does a balancing act between all the factors, striving to deliver a boat that will be stiff (but not excessively so), show a healthy range of positive stability (say 135 degrees or better), and have reasonable draft (to give access to a wider range of cruising grounds).

So to Kahuna: I've inserted a copy of the stability curve below; you'll see she has positive stability through about 140 degrees. I evaluate stiffness using two methods. Both serve to compare a design with others, rather than produce absolute figures of anticipated heel at given wind strengths (There are too many simplifying assumptions for accurate prediction) The most useful is probably the Wind Pressure Coefficient, which compares righting moment to heeling moment (righting arm x disp)/(sail area x heeling arm), at 20 degrees of heel. The ratio for Kahuna is 1.32; some 20-30 years ago this would have been considered quite stiff, while today this is more in the normal range. The other method ( Dellenbaugh Angle) produces a heel angle for the boat assuming a wind of 14 knots The Dellenbaugh angle for Kahuna is 16.8 degrees, which is again stiff by standards of some 20-30 years ago, and fairly typical today.

As to heel at 25 and 35 knots, that depends on many factors. I would expect to reef when the wind builds to around 20 knots, and to reef again when the wind exceeds 25-28 (remember that wind pressure varies as the square of the wind speed, so a 28 knot wind has about twice the force of a 20 knots). At 35 knots you'd be looking at carrying a double reefed main and a staysail.

Q. What is a suitable marine diesel for Kahuna?

Many engines are suitable. I've recently repowered my own boat (35', 15,000 lbs) with a marinized 21 hp Kubota, with excellent results. Many companies now offer marine diesels based on small Kubota engines, and both initial price and parts are very reasonable. Otherwise, many other small (20-25 hp) marine diesels will work fine (in terms of weight and space). I'd suggest discussing engine reliability and serviceability with a mechanic you trust.

Q. I am a bit uneasy ( and will be until the boat actually gets into some rough weather) of the comfort of ride and speed. One of the reasons I want a boat of this type design is to come through rough weather well, that is to say not bounced about like a cork or floundering and on it's ear becuase she's too tender. Could you compare Kahuna to any production boats?

The chart below lists (in addition to basic dimensions, D/L and SA/D ratios) the following for a number of boat, including Kahuna:

Comfort ratio: Developed by Ted Brewer. Larger numbers indicate a smoother, more comfortable motion in a sea way.

Capsize risk: the lower the better. Boats with values over 2 are not considered safe offshore.

Roll acceleration: Calculated in units of gravity (Gs), from a formula by C.A.Machaj, who suggests that malaise starts at .1 G. Lower is better.

Note that the boats that rate the best in terms of these factors are Kahuna, the Bristol Channel Cutter, and Westsail 32. Boats like the Calibre 35 and Island Packet 32 come out looking much worse.

In essence no small boat will be really comfortable in rough weather, but a relatively heavy, not too beamy boat like Kahuna will definitely fair better than lighter big volume hulls. The boats on the chart are all intended for cruising; I don't have the numbers for "cruiser racers" or common charter boats handy, but they would do that much worse. In bad weather Kahuna will behave as boats should: by taking care of the crew. She's really an attempt to combine traditional looks and motion with modern construction methods and improved performance.

Designs for seaworthy, affordable cruising boats -- specializing in wood/epoxy construction