Join Date: Feb 2000
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Tumblehome is a complex issue to explain in detail. It existed historically for a wide variety of reasons. As mentioned, the case could be made the timber ships of war had tumblehome in order to keep the weight of the heavy guns within the limits of the waterline, to allow the guns to be rolled out and fired even when ships we grappled together in close combat, and due to issues related to timber ship construction (the convex surfaces associated with tumble home meant that the seams were compressed rather than stretched open when exposed to high loads.)
In more modern designs it was often about cheating some racing rule. For example early IOR era boats have a bulge in their topsides that relates favorably increasing girth and also altering the apparent beam by distorting the hull at the points at which the beam measurement was taken.
Like so many things in yacht design, tumblehome isn't inherently good or bad. It all comes down to how the specific boat is modeled. There are several factors at play when you try to determine whether it’s a good or bad thing in a particular case.
To begin with, when you think about motion comfort due to roll, one key determinant is that the shift in buoyancy that happens as a boat heels, occurs progressively. Since the center of gravity does not move, this in effect means that a plot of the stability curve changes shape gently and without humps as the boat is rotated through a full cycle.
Steep spots in the curve (rapidly increasing stability) typically mean that somewhere there is a flat spot (a place where stability levels off or decreases rapidly). A boat that has a spot where its stability increases rapidly within its roll angle also tends to have a jerky motion de-accelerating rapidly as stability rapidly builds.
When you talk about a stability curve for a boat with moderate tumblehome, the modeling of the hull below the waterline and in particular area just below the maximum beam becomes very critical as this controls whether the boat builds stability progressively or whether the boat simply flops over until fetching up against the bulge in the curve lurching to a halt as the stability builds.
Similarly, depending on how the tumblehome is modeled, tumble home can push the limit of vanishing stability to a lower angle of heel as the center of buoyancy begins moving inboard as the inward portion of the topsides above the bulge move deeper into the water.
By the same token, the narrow deck line associated with tumblehome can reduce the initial force needed to start to right an inverted boat however, depending on how the tumblehome is shaped, it can also increase less significant ultimate force required to right the boat.
A less obvious case where tumble home comes into play is 'roll out' and 'roll down' (AKA 'roll in'). This is an area of hull dynamics that is rarely discussed, but dependent on the shape of the topsides, as a boat heels the vertical center of gravity moves both vertically and horizontally relative to the center of buoyancy at any given heel angle. The horizontal movement is where stability is generated, but the vertical angle does come into play with regards to motion comfort and the impact of rolling on stability.
Depending on the shape of the hull, some boats actually move lower into the water and are said to roll down. While others that rise out of the water are said to roll out. Ideally, a boat does not change trim, or roll down or roll out as it heels. Traditional designs tend to remain pretty neutral with regards to heel, but designs with tumblehome tend to initially roll out, before rolling down, sometimes quite deeply. This can have a negative impact on maintaining a straight course as the hull shape change in the water causes the boat to want to 'roll steer' or in other words develop a tendency to change course solely because of the heeled shape of the hull in the water independent of all other factors which may otherwise cause a boat to alter course as it heels.
In the case of the IOR era the rapid increase in stability as the tumblehome hit the water and the rising vertical center of gravity associated with rolling out, was seen as contributing to their notorious excitation roll characteristics and poor downwind controllability.