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Old 06-25-2005
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Best Hull type?

I have been following this discussion and its strange evolvolution. Frankly as I read it back I am not sure that I am clear as to Vastbinder''s goals for this discussion. As I am reading this, Vastbinder is looking for assistance in picking out a 20-30 foot design that is suitable for quick amateur construction that would also be suitable for offshore cruising. That is a pretty tall order. I apologize that this is a very long post but to even touch on the basics of your question requires a long answer.

To start with the opening question, I will start with the basics, which is that it takes a minimum amount of displacement to successfully voyage under sail. Traditional sources typically recommended 2 1/2 to 5 long tons per person (roughly 5,500 to 11,000 lbs). All other things being equal, the longer sailing length of the boat (essentially the static waterline length) for a given displacement the more comfortable the motion, more burdensome, the more seaworthy, and the faster the boat. But like all things in yacht design moderation is important, too long and too narrow the boat will be tender and so unable to carry adequate sail area in a breeze.

Beamy boats tend to be a poor choice for offshore cruising. Beamy boats tend to have a lot of drag. It is harder to make a beamy boat track. They tend to do poorly in a chop where each wave collides with greater impact than is ideal. Beamy boats tend to result in a greater tendency to develop weather helm when heeled and to round up when heeled.

Long ends (by which I mean a short waterline length relative to the boats length) carries a lot of negatives in terms of decreased motion comfort, decreased ability to carry weight for a given length overall, and diminished seaworthiness and performance. A comparatively long waterline length and comparatively short overhangs are especially important on small cruisers.

Then there are sectional properties by which I mean the shape of the hull as seen in cross section. This is a tough area to speak in generalities when talking about a small cruising boat. Most sources seem to agree that vee’d bow sections rather than round or flat bottom sections in the forward section of the boat result in a smoother motion. But once you move aft of the first third or so the sections become a very delicate balancing act.

Cylindrical sections offer a minimum of wetted surface for a given carrying capacity. They mean a gentler motion. Cylindrical sections are less likely to cause the boat to change direction as it heels. As a down side, cylindrical sections do not offer as much initial stability. As a result a boat with cylindrical sections tends to rock through wider roll angles and tends to heel further. It also means a deeper canoe body (the portion of the hull of the boat in the water). Deeper hull sections mean a shorter span for the keel for a given draft, which generally means poorer windward performance.

In contrast with cylindrical sections, flatter sections tend to offer a lot of initial stability. Flatter sections tend to permit a boat to surf at higher speeds. It allows a boat to be lighter for its stability. Like everything else, pushes to the limits flatter sections comes with the price of minimal directional stability, and an uncomfortably quick motion. When coupled with a lot of beam, flat sections result in poor ultimate stability. As a flat bottomed boat heels it develops a lot of stability very quickly but as the boat approaches larger heel angles stability begins to erode quickly moving into a range of stability where the boat is more stable inverted. This can be somewhat mitigated with additional freeboard (like the Bolger boxes) but ultimately that carries a penalty in terms of poor ultimate stability.

So what is the answer? Balance. The current thinking for offshore cruising boats tends towards elliptical sections that balance between round sections with their gentle motion and flat sections with their performance, greater initial stability, and greater ability to absorb carrying capacity.

One of the big changes in thinking for offshore boats is a general migration of the center of buoyancy further aft on the boat. Early cruising boats tended to carry a lot of buoyancy forward. This worked well with the comparatively inefficient rigs and keels of the era. Moving the center of buoyancy aft results in a finer bow and that offers all kinds of advantages in terms of improved tracking ability, gentler motions upwind, and better pointing abilities. It also results in more powerful stern sections, which allows greater sail carrying capacity, performance and reduced pitching. Again if the center of buoyancy is moved too far aft the handling characteristics can deteriorate quickly. In the worse case, wipeouts and broaches become inevitable, but even if the buoyancy has been pushed even slightly too far aft the penalty is an uncomfortably quick roll motion.

All of that is independent of the keel type. Cruisers often lean toward full keels. Full keels theoretically form a long straight plane, which keeps a boat on course better (greater directional or longitudinal stability). If you run aground they spread out the load over a larger area reducing the likelihood of damage. Once really planted they keep the boat from tipping over fore and aft. They are easier to haul and work on in remote areas of the world. You can spread out the ballast over a longer distance and so they can be shallower for the same stability. You have a greater length to bolt on ballast so it is a theoretically sturdier and simpler connection.

They have some disadvantages; a larger portion of the keel operates near the surface and near the intersection of the hull and keel, which are both turbulent zones. They also have comparatively small leading edges, and the leading edge is the primary generator of lift preventing sideslip. Because of that they need a lot more surface area to generate the same lift. Surface area equates to drag so they need more sail area to achieve the same speed. Long keels tend to be less efficient in terms of lift to drag for other reasons as well. As a boat makes leeway water slips off of the high-pressure side of the keel to the low-pressure side of the keel and creates a turbulent swirl know as a tip vortex. This is drawn behind the boat creating drag in a number of ways. The longer the keel, the bigger the vortex, the greater the drag. So they need more sail area again to overcome this drag. To stand up to this greater sail area the boat needs more ballast and a stronger structure, which is why boats with long keels are often heavier, as well. (Of course, then the spiral starts again as more sail area is needed to overcome that additional weight as well. It is the classic weight breeding more weight design cycle) Full keels tend to be much less maneuverable.

Fin keels:
By the classic definition of a fin keel any keel whose bottom is less than 50% of the length of the boat is a fin keel. Fin keels came into being in an effort to reduce drag. Cut away the forefoot or rake the stem, as well as, move the rudderpost forward and rake it sharply and pretty soon you have a fin keel. Today we assume that fin keels mean a separated rudder (skeg hung or spade) but in fact early fin keels had the rudder attached in a worst of all worlds situation. They offer all of the disadvantages of both full and fin keels, but with none of the virtues. Unknowing or unscrupulous brokers will often refer to boats with fin (or near fin) keels as full keel if they have an attached rudder.

Fin keels with separate rudders seem to be the most commonly produced keel form in the US these days. (I could be wrong, there is a resurgence of full keels these days)

Fin keels have some advantages as well. They have less drag as explained above so they typically make less leeway and go faster. You can get the ballast down lower so in theory they are more stable for their weight. They are more maneuverable. They take better advantage of the high efficiency of modern sail plans and materials.

They have some disadvantages as well, many of these have been offset or worked around by modern technology but at some level they are still accurate critiques. They have less directional stability than long keel boats so the tend to wander more under sail. Since directional stability is also a product of the dynamic balance between the sail plan and underbody, in practice they may actually hold a course as well as a full keel. In general though you can expect to make more course adjustments with a fin keel. It is sometimes argued that the lower helm loads requires less energy to make these corrections so a fin keel may also require less energy to maintain course. This I think is a product of the individual boat and could lead to a debate harder to prove than the number of angels that can dance on the head of a pin.

Fin keels are harder to engineer to withstand a hard grounding and when aground they are more likely to flop over on their bow or stern. (Although in 37 years of sailing, I have never heard of anyone actually experiencing this.) Fins typically have deeper draft. They are easier to pivot around and get off in a simple grounding.


Bulb Keel:
A lot can be done to improve a fin keel. One way is to add a bulb. A bulb is a cast metal ballast attachment added to the bottom of the keel. They concentrate the ballast lower providing greater stability and sail carrying ability than a simple shallow keel. Traditionally bulbs were torpedo or teardrop shaped. They have been re-contoured to provide some hydrodynamic properties. Recalling the discussion on tip vortex from above. Shallow keels need to be longer horizontally than a deeper fin in order to get enough area to prevent leeway. This means that a shallow longer fin would generate more tip vortex and more drag than a deeper keel. The bulb creates a surface to turn the water aft and prevent it from slipping over the tip of the keel thereby reducing tip vortex. This does not come free since a bulb increases frontal area and surface area.

If time and money were no object, and I were building a small distance cruiser, I would be inclined towards using a liftable fin keel (daggerboard) with a bulb. This would allow a deeper draft offshore for better stability and motion comfort, but would allow the keel to be lifted to permit access into shallower venues. I would also use an outboard rudder that could be lifted as well. Again this is a matter of ease of maintenance and the ability to rig a simple trim tab type self-steering system.

Which brings us to the implied second part of your question. You have suggested that you plan to build this boat yourself and that you would like to be sailing by the end of a summer’s work. That somewhat suggests that some form of fiberglass over plywood construction would be your best option. If you were considering a larger boat then steel construction might be a reasonable option, but in the size range steel makes a poor option, resulting in too much weight in the hull and not enough in ballasting or carrying capacity. The problem with the quick build versions of either material is some compromise in hull form and motion comfort.

I would suggest that you look at some of Dudley Dix’s designs in particular the CW 975 http://www.dixdesign.com/cw975.htm or the Didi 34.

One last point here, if your goal is to get out sailing quickly and cheaply then you would be better off buying a good used boat and go sailing. In any event this should hopefully begin to give you some preliminary ideas to mull over.

Good Luck,
Jeff
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