TUMBLEHOME

[Praxinoscope]

Whilst working on my boat (Sadler 25) earlier today started talking to a couple of visitors, we got around to discussing the tumblehome on the 25,
it certainly gives the boat good visual appearance and provides an extra degree of beam in the cabin, but I have never really seen any discussions from yacht designers/builders on any advantages or disadvantages on the performance. or stability that a tumblehome creates.
None of the books in my sailing library really have much to say on the subject, so would be interested to hear any thoughts on tumblehomes.
I should add that I find the Sadler 25 a beautiful little boat to sail so David Sadler in my opinion definitely got it right.

 

[MikeBz]

Extreme tumblehome was useful IOR rating trick back in the day - I wouldn't call this attractive though:

 

[Praxinoscope]

MikeBz, I think that woukd be classified as extreme and certainly not attractive, but if I remember correctly Heaths ‘Morning Cloud’ had a tumblehome and the linex looked good.
But did these extreme tumblehomes inprove or reduce the sailing qualities?

 

[KevinV]

IIRC (and I probably don't) it lengthens the waterline as the boat heels, while also increasing dynamic stability by pushing more volume down - hence IOR boats sailing fastest with more heel than modern boats. All without lengthening waterline at rest, or widening the deck, which were both punished under IOR.

The increase in stability was limited - once you got beyond the "round" of the tumbledown it worked the other way.

All as per what I think I remember from what I understood of what I've read - I could be very mistaken!

 

[srm]

Way back in the 80's I had a McGruer designed 8 Metre Cruiser Racer. A 1950s development of the 8 Metre class to try and keep it relevant with fixed berths, base of keel parallel to waterline etc.
Design LWL 26ft, LOA 42 ft so impressive overhangs. She also had a nice tumblehome on 10ft max beam. The idea was to reduce weted area in light winds but as the wind increased she would initially heel, then stiffen with lee rail just clear of the water and the overhangs extending the waterline length (and speed potential). I suspect that the tumblehome was to keep the immersed lines fair and reduce drag when heeled going to windward.

 

[Laser310]

Is this tumblehome?

 

[srm]

Is this tumblehome?

Possibly the hard chine equivalent. I have always associated tumblehome with round bilge/hull designs rather than those with hard edges. There is a big Pogo in our marina with a narrow hull on the waterline that extends out to a much wider hard chine well clear of the static waterline. But then the topsides are more or less vertical, so just a hard chine hull intended for fast down wind work. Your pic looks a more interesting shape.

An early example is HMS Victory, (and other long gone warships) where the tumblehome provided stability and space on the gun decks.

 

[wombat88]

A grp boat with tumblehome needs to be made in two haves...otherwise it won't come out of the mould...I think the Sadler 25 is a good looking boat that goes as well as it looks.

 

[LittleSister]

The key question with tumblehome is not the advantage that extending the breadth of the boat has - every boat has that feature - but why that additional beam is not then carried up to the deck.

(Which would give more space inside, a stronger righting moment, more deck space, and fewer dings in the hull.)

I am only aware of
(a) racing rules;
(b) the cultural influence of boats built to those rules; and
(c) aesthetics (it looks very nice (e.g Sadler 25) if not done to extreme as is, to my eyes, the boat in MikeBz's Post #2).

I'm interested to know of any other advantages of tumblehome.

Do any traditional workboats have it? And if so why?

 

[srm]

Do any traditional workboats have it? And if so why?

The only ones I can think of are the Yorkshire and Northumberland Cobbles. Open fishing boats evolved for launching and recovery off open beaches. Not a full tumblehome but the bow with deep forefoot that would hold the beach and keep the bow in to the waves transformed to a wide shallow stern with raked transom and definite tumblehome. Sorry no pics.

 

[Praxinoscope]

A grp boat with tumblehome needs to be made in two haves...otherwise it won't come out of the mould...I think the Sadler 25 is a good looking boat that goes as well as it looks.

In fact it was the mould that was in two halves, not the hull. laying up the grp was exactly the same as any other boat of that period and then the two halves of the mould were separated to allow the hull to be removed.

 

[KevinV]

An early example is HMS Victory, (and other long gone warships) where the tumblehome provided stability and space on the gun decks.

I wonder if curved sides (vs slab sides) would also be stronger against cannonball and hull impacts - like an arch is stronger than a lintel over a door opening? Might increase the chance of the cannonball being deflected too?

 

[dunedin]

The key question with tumblehome is not the advantage that extending the breadth of the boat has - every boat has that feature - but why that additional beam is not then carried up to the deck.

(Which would give more space inside, a stronger righting moment, more deck space, and fewer dings in the hull.)

I am only aware of
(a) racing rules;
(b) the cultural influence of boats built to those rules; and
(c) aesthetics (it looks very nice (e.g Sadler 25) if not done to extreme as is, to my eyes, the boat in MikeBz's Post #2).

I'm interested to know of any other advantages of tumblehome.

Do any traditional workboats have it? And if so why?

You are right. A lot of boats with tumblehome were driven by rating rules, and for most purposes continuing the beam up to deck level has more advantages, particularly for a cruising yacht.
There are some other potential advantages of tumblehome for modern racing yachts - they reduce the windage slightly, and with boats like Ran and IMOCA 60s going at very high speeds, windage starts to become more important.
Also depending on construction techniques, can slightly reduce weight. On the other hand, for fully crewed race yachts, narrower beam at deck level would reduce the leverage for crew on the rail. Hence more common on single handed boats.

 

[Praxinoscope]

The renowned William Fife was noted for designing yachts with a tumblehome an. example being the 8m ’Ierne’ built in 1914,
and has been described as good as a yacht could be at that time.

 

[LittleSister]

Do any traditional workboats have it? And if so why?

When I was writing that question I thought I remembered that Galway Hookers had quite pronounced tumblehome, but checking a picture of one it appeared it didn't. Now I've seen a picture of another that definitely does.

I wonder why they (or at least some of them) had it.

 

[srm]

The renowned William Fife was noted for designing yachts with a tumblehome an. example being the 8m ’Ierne’ built in 1914,
and has been described as good as a yacht could be at that time.

As I noted in my earlier post, these boats were designed to sail with their topsides immersed to increase the waterline length. A good designer, like Fife or McGruer would draw the hull lines to give a smooth water flow when heeled, resulting in the tumblehome. A case of Le Corbusier's (a building architect) dictum that "form follows function".

 

[Tranona]

The tumblehome from the early IOR days was indeed to play the rules which penalised stability (look at the short light ballast keels with the ballast high up) so relied more on form stability. Reducing waterline beam was advantageous to reduce wetted area so the combination of narrow waterline beam, hard turn of the bilge and wide immersed beam for stability was obvious. Narrower deck beam allowed narrower sheeting angles with the large genoas of the time. Also allowed pilot berths in the saloon so that you could have a crew of 6 or 8 necessary to keep the boats upright on a beat and still have berths for them. some designers like Kim Holman loved the type because building that shape in GRP meant you could have all curves and no straight lines!

Split moulds were less of a problem as the boats had relatively flat bottoms and bolt on keels unlike earlier boats that had moulded in keels with internal ballast where the two halves had to be moulded individually then joined together. Completely different type of boat with no tumblehome, my GH was built like that to ensure the laminate was properly consolidated in the keel trough. You can still make out the join particularly on the transom!

 

[Bajansailor]

The only ones I can think of are the Yorkshire and Northumberland Cobbles. Open fishing boats evolved for launching and recovery off open beaches. Not a full tumblehome but the bow with deep forefoot that would hold the beach and keep the bow in to the waves transformed to a wide shallow stern with raked transom and definite tumblehome. Sorry no pics.

Here is a nice description of the Yorkshire cobles, along with a drawing showing the very pronounced forefoot -
The Yorkshire Coble

Re tumblehome, I crewed on an S & S 34 once on a long passage, and she was an absolute joy to sail - as the wind picked up she would heel and dig herself in a bit, and just start trucking along, all while being beautifully balanced on the tiller.

 

[DownWest]

I remember that our 27ft hard chine cruiser rated badly compared to a rounded hull. It was the girth measurment that suffered. Tumblehome could have worked on keeping the deck narrower, but I don't know how exactly it was measured.

 

[flaming]

The tumblehome from the early IOR days was indeed to play the rules which penalised stability (look at the short light ballast keels with the ballast high up) so relied more on form stability. Reducing waterline beam was advantageous to reduce wetted area so the combination of narrow waterline beam, hard turn of the bilge and wide immersed beam for stability was obvious. Narrower deck beam allowed narrower sheeting angles with the large genoas of the time. Also allowed pilot berths in the saloon so that you could have a crew of 6 or 8 necessary to keep the boats upright on a beat and still have berths for them. some designers like Kim Holman loved the type because building that shape in GRP meant you could have all curves and no straight lines!

Split moulds were less of a problem as the boats had relatively flat bottoms and bolt on keels unlike earlier boats that had moulded in keels with internal ballast where the two halves had to be moulded individually then joined together. Completely different type of boat with no tumblehome, my GH was built like that to ensure the laminate was properly consolidated in the keel trough. You can still make out the join particularly on the transom!

Don't follow how narrow decks allow narrow sheeting angles. That's all in the width of the shrouds and where you bolt the tracks to the deck...

My understanding was that there was a loophole in the IOR regs where beam was only measured at deck level, so you could get some useful form stability from a wider boat without actually rating for it. In latter iterations of the rule they changed it to also measure max beam and tumblehome largely disappeared before the end of the IOR rule.

The more modern designs with things that at starting to look a little like tumblehome, well obviously they can't call it that, far too old school IOR. "Chamfered deck" is the new phrase... And is normally only applied to the bow sections. The idea is both aero advantages, encouraging wind that hits the bow to accelerate over the deck and into the jib, and also wave shedding. The other advantage is supposed to be weight reduction, the class 40 pictured above is almost certainly a way of getting the required strength into the hull/deck join using less weight.