Tumblehome

[SuperPiper]

I have read the Volvo Ocean 60 rule. It is very specific that the widest part of the hull must be at the hull-deck joint. Therefore, tumblehome is not permitted. I assumed that this was for safety concerns and would aid in self-fighting a capsized boat, etc.

However, the boats in the current Around-Alone race feature very significant tumblehome. Their hulls resemble kayaks more than canoes. So the question is:

What is tumblehome? What does it do? Why would it not be allowed on a fully-crewed ocean racer but allowed in a solo race? Does it have a hydrodynamic advantage? Does it merely reduce windage?

What do you know?

[gonzo]

Tumblehome alone is not enough to define the characteristics of a design. It has to be taken within context. Usually, the handicapp or class rules affect the shape of the hull more than common sense. Boat designers, in their effort to beat the rules, often come up with bizarre solutions. Stability is affected by many factors and how they are related. For example a narrow deep boat can have adequate stability, but so can a wide shallow one. However, narrow and shallow or wide and deep are bad combinations. Do you have a problem or question with a particular design?

[SuperPiper]

If the Volvo Ocean 60 rule had allowed tumblehome, do you think that Bruce Farr would have incorporated tumblehome into his designs?

Not allowing tumblehome must have been for 1 of 2 reasons:
1 - safety; or,
2 - to ensure that no yacht could achieve an unfair advantage.

What is the advantage of tumblehome?

The radical designs of the open 60's, 50's and 40's in the current Around-Alone race seem to have embraced the concept of curling the hull back inboard at the topsides. For why?

What is the technical explanation for such a feature?

[gonzo]

One of the advantages of tumblehome is that the center of gravity is lower than in a design where the maximum beam is at the deck. This, of course, is assuming the depths are similar. Tumblehome also has a more even stability curve. The early radical "dish boats" of the IOR formula had the problem of floating upside down. The flotation reserve of a deck and hull above the maximum beam helps a boat right itself after a complete capsize. On the other hand, a lot of flare with the maximum beam at the deck can allow the boat to carry more sail because of the higher initial stability. The argument of safety versus sail carrying ability is what makes some rules forbid such extreme beams at the deck. I hope this help you.

There are several advantages to tumblehome: as already mentioned, it reduces inverse stability; it allows outboard shrouds to be moved in relative to overall beam and thereby reduces sheeting angles; since a curved panel is stronger than a flat one for a given scantling, one can achieve greater strength with less displacement. The disadvantages are reduction in side deck area and a slight increase in tooling costs. Since offshore racing boats do not utilize outboard shrouds, one can only assume that the reduction in side deck area was a safety factor considered more important by the developers of a rule intended for crewed vessels. Brad

[SuperPiper]

The common theme of the last 2 responses is that a boat with tumblehome is more likely to remain upright and/or return to upright.

I think that I will need to draw a cartoon to prove this to myself.

Thanks for the help, guys.

[gonzo]

Try this experiment to see how tumblehome works as opposed to a beamy boat with extreme flare. Take a ball and tape a weigh on one spot. Now float the ball and try to make the ballast stay on top. Next take a piece of board, foam or anything flat and put the same wieght on one side. It will float with either side up. These reperesent both extremes.