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  #1  
Old 09-18-2008
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Tumblehome, stability, and performance

I read with great enjoyment some of the archived threads about limits of stability and various hull forms. Dey be some smart pipples on this board. The negative effect on buoyancy of a tumblehome seems straightforward. So what are/were the benefits of this hull shape? Why were some boats even into the 1980s built with tumblehome, most pronounced aft, and why is it almost nonexistent in newer boats? Older warships had loads of it -- was that about gunnery, or sailing?

I ask this mostly as a pure intellectual query. But I also have a crazy affection for Albin sailboats, and they have a name for seaworthyness despite major tumblehome. The Vega 27 has a shallow keel and low freeboard, but it seems plenty stable.
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Old 09-18-2008
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Nice picture of a nice boat!..

I'm no eggs-pert on the topic (there are brighter than me!), but as I understand it, tumblehome was primarily about maximising beam at the waterline (and hence lateral stability) and secondly about looks (a bit of tumblehome looks waayy nicer than straight sides) but it's not going to help reduce drag..

Bigger warships (and some sailing craft possibly) tended to be rather top heavy, wot with all those big guns and all, so it stands to reason that any gain in beam at the waterline is going to make for a more stable (hence accurate) platform to shoot off of.. I find it interesting that even really old warships (like H.M.S Victory) had serious amounts of tumblehome (probably deflects cannonballs nicely also), so it's a technique that's been used for hundreds of years.

My $0.02 worth.

Cameron
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Last edited by Classic30; 09-18-2008 at 04:25 AM.
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Old 09-18-2008
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Tumble home does not result in a loss of buoyancy until the tumbled home section is immersed. Righting arm is reduced with increased immersion/increased heel. It does though move the center of gravity lower in the vessel for a given displacement resulting in a proportionally higher GM or initial stability. Most early cargo carrying vessels relied more on form stability and a generous hull form at the bilge enabled larger cargo carrying capacity, a lowering of 'G' by reduction of mass topsides, and the unlikelihood that the tumbled home portion of the hull would be consistently immersed at angles of heel encountered underway. Like every design question, it's a matter of trade-offs. I suspect that the more modern yacht has less imperative to reduce weight topsides due to the reduction of weight aloft made with modern materials for spar construction among other things. And I'm giving short shrift to the discussion of form stability versus ballast conditioned stability.
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Old 09-18-2008
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A lot of tumblehome does complicate dumping over the side... so if you're going to be sailing with Hog, Cam or Craigtoo, you might want to keep that in mind.
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Old 09-18-2008
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I seem to recall that for a brief time certain rating rules measured beam on deck, and tumblehome was a way to add 'unmeasured/unpenalized' beam.

That said, a nicely drawn tumblehome hull looks pretty good - the Ranger 28 being one good example.
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Old 09-18-2008
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Actually tumblehome was a means to strengthen the hull. That curvature made the hull stronger than what a slab side would. So some tumblehome would be a good thing. As long as you don't go overboard with it and end up looking like a beer can floating on its side. Or an adult toy I have noticed that some power boats do have that adult toy look about them (pallus shaped)
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Last edited by Boasun; 09-18-2008 at 02:21 PM.
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Old 09-18-2008
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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.

Jeff
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Old 09-18-2008
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True but historically Tumblehome was used before they had gun decks.
A properly designed vessel may have a slight to moderate tumblehome. This is used by the builders/designers to strengthen the hull. Reducing the racking of the seas on the hulls.
But like a lot of good features in design work.... people carry it to the extreme and ruin a good thing.
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Quote:
Originally Posted by Jeff_H View Post
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.
Interesting, in that the Albin Vega is famous for tracking beautifully downwind -- almost a downwind specialist. If fact, for awhile the Vega held the transatlantic record for small monohulls, 14 days and change. Singlehanded, IRRC. That's speedy. Given a PHRF rating of 240, it must be a real dog upwind!

Did the tumblehome act at all like a soft chine, resisting leeway? I can see that design almost presaging the modern crop of beamy, chined, flat-bottomed racers (Farr, TP, Open40), sacrificing ultimate stability for higher initial stability
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Old 09-18-2008
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Funny thing...I was never attracted to those hull shapes...

Now I look at them...they bring a certain "classic" look....I look at them now...some Swans from the late 70's and early 80's...

Strange how things change in life....I have "negative" tumblehome eheheheheh

starnge how design evolved, isn't it???
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