I would like to apologize in advance but much of this was cut and pasted from articles that I had written for other purposes and so may not perfectly fit the thread. I hope that it still may prove helpful to the discussion.
Displacement vs Planning vs semi-displacement boats
To begin with this is potentially a very interesting topic that has gotten partially derailed and misdirected by what appears to be a semantics problem contained within the way that premise of the question is being asked. I do not mean this to be critical of INetRoadKill. At some point in our lives we all have accidentally misspoken and/or we all have started out somewhere, not precisely understanding the terminology that we may be using. My point is that I think INetRoadKill probably misspoke when he chose the term ‘planning’ for his question.
To clarify, a planning boat requires very specific set of design parameters that are directed towards minimizing drag, developing an underbody that provides dynamic lift to the hull at speeds approaching and in excess of the theoretical hull speed, minimizing weight, and maximizing the power to weight (sail area/disp) and in the end this produces a boat that is capable of very high speeds but at a price. The original post included some basic ‘numbers’ for a proposed ‘planning’ boat that were well outside of range for a planning boat.
More realistically a planning boat would have numbers closer to: LOA= 32 feet, LWL=30.5 feet, Draft=7 feet, Beam Max= 10 feet, Displacement 3,775 lbs (that’s not a typo), and Sail Area (100% jib)= 700 square feet (and would look like a Melges 32 whose numbers from memory are pretty close to these).
Cutting to the chase, boats like these are almost always single purpose race boats. There is no point in discussing the potential of planning boats like these in the context of being used as an offshore cruiser, especially in terms of tracking or motion comfort.
But fortunately the choice is not simply between displacement and planning boats. As mentioned above there is a third type referred to as a ‘semi-displacement’ or ‘semi-planning’ that has become increasingly popular as a type form and which, when designed properly, potentially offers tremendous advantages over traditional heavy displacement types on almost all counts.
It is here that I respectfully disagree with the majority of the comments made above. Properly designed a semi-displacement boat offers the opportunity for greater seaworthiness, ease of handling, motion comfort, performance and carrying capacity for its displacement than pretty much any other typeform.
Of course I need to point out that there are two underlined, very key qualifying phases in that last statement; ‘properly designed’ and ‘for its displacement’. First of all, designing properly designing a semi-displacement boat with all of the attributes that I mention takes a lot more skill than designing a more traditional heavy displacement boat. I’ll get to that in a minute, but in many ways it’s the second phrase that is more important to the discussion so I will start there.
While within the sailing culture we tend to think of length as ‘the size of a boat’, more than any other single parameter, it is displacement that truly best describes the size of a boat. Because we tend to think of length as the size of the boat, there is a tendency to use terms like heavy or light displacement which of course is relative to length. But to me, especially as one is contemplating designing or buying a specific boat for a specific purpose, in almost all ways thinking of a boat as short or long for its displacement provides a more useful way of thinking of this issue.
I say this because I believe that ideally designing or buying a boat is a step by step process that starts with defining the problem that you are trying to solve. In designing a boat, the problem is defined by the sailing venue, anticipated crew, and the nature of the sailing anticipated. Obviously taste, and style factor in, but that is another story, related more to psychology than to this discussion.
If you are to start with the length of voyaging that you intend to do, and consider the necessities plus the level of comfort that the proposed crew will require, you pretty quickly can make a good guess at a rule of thumb for the displacement that is required per person. Historically you used to see a rule of thumb of 2 ½ to 5 (sometimes quoted as 6) long tons per person (which roughly translates to 5,700 to 11,000 lbs per person. Modern offshore cruisers will often push that number a bit higher. And so if we decide that we have two-crew and, if, for the sake of example, we decide that they need minimally 3 tons each, we end up with a boat in the general neighborhood of the 13,500 lb displacement range that INetRoadKill is proposing.
If at this point we begin to think about comparing a boat with a longer waterline to shorter boat that has an equal weight of roughly 13,500 lbs, within reason and with reasonably similar weight distributions, the longer the boat, the better its seaworthiness, motion comfort, ease of handling, motion comfort carrying capacity, performance and so on. We can go through the reasons for that one by one but in the interest of time, I suggest that you do a search for other discussions on these topics.
But as I said to achieve these admirable traits in a longer boat requires more design skill, especially when the goal is to produce a semi-planning type hull. Modern semi-planning hulls have their centers of buoyancy and gravity a bit further aft than on more traditional boats and have more beam than traditional boats. Off hand this would seemingly promote a boat that changed trim with heel angle (going bow down) and would seemingly have more form stability and therefore a less comfortable motion.
Here is where careful hull modeling comes into play. Careful modeling of the hull at various heel angles can minimize if not eliminate changes in for and aft trim. If you can visualize a cone sitting horizontally in the water and you rolled that cone about its axis, it would not change trim fore and aft. Obviously, a boat is not a cone shape, but if the heeled hull is carefully modeled so that the proportion of buoyancy aft relative forward of the center of gravity remains the same at all reasonable heel angles, the boat will not change trim due to heel. This series of calculations takes time and skill,
When we talk about motion comfort there is a tendency to focus on form stability as it relates to beam, and historically long boats for their weight (lighter weight boats) tended to have proportionately more form stability than more traditional heavy displacement cruisers. It was that proportionately greater form stability that gave light boats their bad rep motion comfort wise. Despite the oft quoted ‘common wisdom’ to the contrary, in and of itself, except possibly in the case of heave, weight does nothing good for motion comfort. (Even in the case of heave, increased weight may only act to shift the motion out of phase which can in some cases actually result in decreased motion comfort). When you really analyze the factors that control motion comfort the big factors are dampening, the rate of change in buoyancy (and therefore stability) with changes in direction, weight distribution as it relates to rotational inertia.
And so, if the semi-planning boat is carefully designed with these factors in mind, then the boat will not beat up its crew, but again that requires careful design and engineering to achieve.
Lastly like anything else in the design of a boat, the keel design is a part of a bigger system. There are pluses and minuses to both the IOR era 'Trapazoidal keel', some of the later small area keels and the 'bulb on a stick' as you call it. Again all can be equally sturdy, but obviously some keel forms are easier to engineer than other.
A couple other comments on the comments above; the Didi 38 is not especially light weight for its length. This is a very nice design optimized for amatuer, one-off construction, but it is not especially light. As a point of comparison, my own boat is a 38 footer with a design weight of 10,600 lbs. This was a boat that was specifically designed as a cruising boat, albeit a bit spartan one. The structural system was engineered for the abuses of being offshore for long periods of time. Much of the weight savings came from the design of the structural system, minimal liners, light weight interior components, small fuel capacity which is just one part of the spiral cycling that light weight breeds other weight savings. My boat was sailed in from South Africa and sisterships have done all kinds of offshore passages. I would favorably compare her motion comfort to any other 10,600 lb boat that I have ever sailed. But that's another story.
By the way, restricting to a 5 to 6 foot draft is a serious motion comfort/ seaworthiness compromise on a boat the size that you are proposing.
Last edited by Jeff_H; 01-06-2009 at 06:11 PM.