Mark Smaalders Yacht Designs

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

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KAHUNA
Questions and Answers

 

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?

I've kept Kahuna's draft moderate as I've found -- after sailing many thousands of miles on a boat with a raft of 6'+ -- that there were many times that I wished for less draft.

The question regarding whether she's tender, and how much she will heel, 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: 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 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 ratios for 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.

Boat & Designer

LOA

LWL

BEAM

DRAFT

DISP

BALLAST

SAIL AREA

SA/DISP

DISP/LWL

COMFORT RATIO

CAPSIZE RISK

ROLL ACCELERATION

PERIOD

Kahuna

32.33

28.75

10.625

4.5

15550

6000

624

16.0

291

34.2

1.64

.044506

3.8


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.  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.

Q. Kahuna can be built framed, non-framed with epoxy sheathing, or coldmoulded overlay with veneers.

Which is the strongest ? What is the advanage of veneers over epoxy glass? Surely laminated frames give more strength than non- framed method? I would appreciate some insight in this area.

As designed Kahuna uses either
1) strip planking w/ veneer overlays,
2) strip planking w/ heavy cloth+epoxy sheathing (inside and out), or
3) cold molded veneers (no strip planking at all). None of these have traditional framing, but all use the bulkheads, locker dividers etc as framing (this is done by securing these to the hull with biaxial tape and coved epoxy). Another option (but this is not detailed on the plans) is to use more traditional framing (either steamed or laminated frames) in conjunction with strip planking (with or without veneer overlays, or cloth sheathing).

What is strongest? Probably 1), the strip + veneer overlays. This produces an extremely strong hull, and one that is least likely to experience problems as a result of movement in the strips. You could use laminated frames with this method, but bonding in bulkheads etc will also result in a very strong hull. The biggest difference is probably in the flexibility you get in terms of the arrangement: laminated frames can be set up at each station, and then leave you free to adjust the interior arrangement to a greater degree than you could if building with bulkhead framing. One advantage of using strips + veneer overlay (with or without frames) is that you can build in as much of the interior as you wish while the boat is not yet planked, which can much simplify the process. That can't be done with method 2 above (a hull that must be sheathed with cloth on the interior).


Q. I am very very attracted to the gaff rigged version, I want to try to avoid having to buy tons of custom rigging ( Lewmar, Harken etc).I also want the rig to be maintainable in third world countries. I also guess the mast section is stronger because of its shorter length and lack of spreaders, not to mention lower centre of gravity. I have never in my sailing life sailed a gaff rigger and my only concern is windward ability, as I am considering engineless, and also ease of handling for a single hander ( reefing in particular). Also which would be better balanced for self steering?

A. Gaff rigs win out over Bermudian hands down when it comes to cost, maintenance (esp. in remote places), simplicity, lower CG, etc. If properly designed and set up they don't give up much at all in the way of windward performance, at least compared to cruising boats (racers are something else), and of course are much more efficient downwind. Handling a gaff rig is somewhat more complicated, because of the gaff and dual halyards, but careful set up can usually deal with this. I've sailed many miles with both and would encourage you to try the gaff rig.


Q. I was wondering if a quarter berth, aft of the chart table, underneath the stbd cockpit seat, would be a possibility? I am very fond of this arrangement as a sea going berth away from the main saloon area, I am willing to sacrifice cockpit storage for this.

A. A quarter berth could be fitted in. The chart table would end up just aft of the STB settee, and then the berth.