purpose of tumblehome in runabouts

I have long wondered why present day aluminum runabouts (and classic wooden powerboats) have tumblehome in the stern cross-sections. Then I might know if it is a desirable feature in (wooden) planing sailing skiffs that I am interested in (www.econologica.com) Please, does anyone know a technical naval architecture reason for this ubiquitous tumblehome?

 

Humm, good question. Mostly I hear, and agree, it just darn nice... Then there are points like it reduces weight up above, improved aerodynamics, and the barrel shape should be stronger than flat sides. I can't think that there is any 'working boat' history in this design, as it makes access to the water much less practical.

I found this explaination: 1. The American-Built Clipper Ship 1850-1856, Characteristics, Construction, Details. W.L.Crothers, McGraw Hill (1997). p54. Tumblehome has the effect of making the top deck (weatherdeck) smaller. The claim is that this was introduced during the galleon era when large numbers of canons could make the hull top-heavy. A smaller top deck lowers the centre of gravity, but it also reduces the righting moment quite significantly. Another reason for tumblehome might be to keep the lee rail out of the water when heeling. A more sensible excuse is the strengthening effect of a barrel shaped hull, approaching the cylindrical shape of a submarine or pressure vessel. Significant tumblehome is virtually absent on modern ships, the sides are more likely to be vertical or even slightly outward sloping (flare). at www.worldwideflood.com/ark/design_draft/midship_section.htm (this in its elf is worth another thread)

 

Tumblehome is for stability & comfort.
It is a "buffer" to sudden & drastic shifts in heel angle. It provides for sudden turns (runabouts), sudden load shifts (canoes) and wind loads (sailboats).
Built in sponcins.

 

As usual ask 10 designers get 10 different answers...
Without going back to 1800 and as I knew it Tumblehome just makes it easier to pull things up on the boat (didn't say lobsters!!)
I disagree with Ted when he says it helps stability...it actually affects stability the other way around removing immersed volume when the boat heels->reduced righting moment (as also stated by Grant Nelson)but I do agree with Ted when he says it helps comfort,reduced righting moment would also make the boat roll smoother.
As far as i see it today Thumble home is used on boats for the very same reason suddenly steps are used everywhere including 30/40 knots hulls...pure fashion...I do agree both look very intriguing though

 

Thanks for the replies. Ted touched on what I expected as an answer without explainning it. That is that somehow tumblehome on runabouts is related to their turning and presumably heeling in a turn. But how? Can Ted or someone else please explain in detail?

 

On a sailboat without tumblehome, max righting moment usually comes when the hull is rail down. With tumblehome, as in the Ranger 28 with max beam being about halfway up the topsides, righting moment gets a boost earlier at lower angles of heel. Better or worse overall, I don't know.

I put tumblehome on powerboats because I like it.

 

In a high speed turn, centrifical force acts as a load shift to the inside of the turn. The reserve bouyancy in the tumblehome acts to resist the tendency to lean further.
A kind of shock absorber for boats.

 

does anyone know a technical naval architecture reason for this ubiquitous tumblehome?

1. Style
2. Style
3. Style

 

Look at small boats built in the fifties. They all had tumble home. Now you rarely see it except on small aluminum boats. Why, Because the design of the little aluminum boats has never changed. They are still building exactly the same boats. They just add fancier gadgets and gilhickies to make them look modern.

My family had a 14 foot norseman open outboard boat back in the late 50's early 60's. Strictly small O/B's not more than 10 hp. This thing had at least 6 inches of tumble home on both sides of the transom. It sure made it easy to haul the salmon in. But it was a pain in following seas, really wet. I would prefer flare at the transom.

As someonje said it's all style. I suppose someone had a reason for it back in the 30's and 40's but that is no longer valid.

 

Having said style, style, style, I must admit that in a small aluminum boat the thin plating would gain a lot of stiffness from the curvature, as Grant mentioned, and that's probably reason enough to do it in boats like that.

 

Tumblehome was always thought to help reduce the area of negative air pressure that exists at the transom of a boat at speed. This tends to trap spray and exhausts fumes around the back of the boat.
Oh - and Style, Style, Style!

 

I think this is the definitive answer.

 

Tumble home

Can someone post a picture or describe tumble home. Interesting thread, but I have no idea of what you are talking about.

 

The piroque VS the canoe. Stand or stumble to the edge of a canoe & after a brief "tip" the canoe will recover. Do the same in the Piroque & the gunnel goes under water & you sink. People fall "out" of canoes, they "sink" in piroques.
Canoes have a tumblehome. Dats all I know!

 

Tumblehome...
http://images.google.com/images?q=tumblehome&hl=en&btnG=Search Images
Flare...
http://images.google.com/images?svnum=10&hl=en&lr=&q=flare+boats