purpose of tumblehome in runabouts

Tumblehome was used on olde tyme sailing ships before clipper hulls were adopted for speed; makes it easier to climb up the side. I like the "keep the rail out of the water" theory for dinghies but that has nothing to do with runabouts. It's mostly appearance I think, but it allows more area under the stern at speed and is probably drier; I suspect it reduces wash too. I have seen long skinny powerboats skid at speed and flip over, don't remember seeing a wide boat flip as easily. My tiny kayak has lots of T and seems to be unsinkable even when I put a sail on the poor thing; my piroque sinks if I sneeze hard. My canoe doesn't have T and I hate paddling the damn thing; a bit of T would improve it no end but that has nought to do with T on a runabout. I doubt its much use on a slow power boat.

 

It just looks purty

 

As I see it, tumblehome on wooden motorboats looks great, as on my 1949 42' Owens Flagship. As for sailboats, I have read that tumblehome (blisters?) may have been used to alter a designs rating without significantly altering its speed.

 

There are a couple other reason that tumblehome is a positive thing in a small motor craft. First it keeps people closer the center line of the boat, this will effect stability in any small powerboat. The old gentlemens runabouts of the 1920s '30s and 40's generally had a reasonable amount of tumblehome, this was mostly for astetic reasons. These boats have a remarkable resemblence to a wide sterned canoe. I personally believe that the advantages of a wide sterned canoe in carrying capacity, ability to take an outboard motor, better planning, and an increase in stability are what lead to the gentleman runabouts, the canoes simply got beamier.

Another positive quality of tumblehome is saftey. The most frequent way that a small skiff gets swamped is water coming over the one of the aft corners when somebody is working on the engine. By bringing those corners as far in as practical some the risk is lowered. In a narrow boat, like one of the old wide sterned canoes, I would recomend bringing the tumblehome all the way into leaving just enough room for the engine. On a boat this narrow it doesn't look ugly, In my opinion it looks great. By the way the problem above of swamping can be dealt with by adding a sufficiently large outboard well.

pax

 

I'd agree with your observation, but Im not too sure about your conclusions. Certainly, significant tumblehome will prevent people from getting to the outer corners of a boat. But whereas flare in the aft sections will deflect waves, significant tumblehome will allow them sweep up and over the sides.
And again on the subject of outboard wells, I'm not sure where you are coming from - a well of any consequence can trap water almost as effectively as the cockpit itself - at least for long enough that the added weight will allow following waves to enter the cockpit itself. Of course, if you are simply stating that having a well as opposed to a lowered transom that is open directly to the cockpit, then I'd agree whole heartedly.

 

I second that.

From what I've heard the historic reason for tumblehome (as seen on old sailing ships) is harbour duties and other taxes. These days it is the Gross Tonnage that defines these costs, but in the old days it used to be the surface area of the main deck. So people built ships with big holds, but small weather decks. The result: less stability (less reserve buoyancy).

While this sounds stupid, the current practice is just as bad. The GT is a measure for the interior volume. To minimize running costs (taxes, harbour duties etc), ships are now built for maximum payload with a minimum of gross tonnage. This also means: less reserve buoyancy. Void spaces are great for survivability.

When will they come up with a measure that doesn't compromise safety?
(Length is another bad one, as a long slender vessel is better than a short fat one)
Would the amount of toxic emissions be an option? It's also proportional to payload, and will motivate ship owners to build cleaner ships...

Bruno

 

I agree about the outboard well. A well with good drainage will certainly lower the risk of being swamped in such a event, and is preferable to the engine opening being directly connected to the the cockpit. I was simply implying that having the corners for the water to come over further inboard would be preferable to having them farther out. This way the boat has to heel more for water to come over the cockpit. However your right about water coming up over the sides of tumblehome hull. Maybe the best solution for saftey is the flaring sides with a deck. and a cockpit coaming to keep water where it belongs.

I think my point about tumblehome keeping people where they belong, in the middle of the boat is valid. The old alaskan dog ferries used to have somewhat tumblehome sides if I remeber the photographs correctly. This certainly would add to stability by keeping the dogs further inboard.