Which sub 37 ft yacht to cross the North Atlantic in?
- Thread starter thecommander
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geem
Well-known member
Haha, if you thinks its fairly strong what would you consider strong? This is a 44ft grp yacht with a rudder shaft that is 100mm diameter at the middle bearing. The shaft is solid 316s/s. The total shaft length from bottom of the rudder to the deck is 3.6metres. There is a bearing at each end and the middle. The bearings are bronze. The main central bearing has a grease gun attached permanently. The ‘lightly engineered’ skeg is built like the rest of the boat, super strong?. What the drawing doesnt show is that the top of the skeg is part of the hull not an added appendage. Where the skeg meets the hull the glass is thick. Imgine a tree trunk where is meets the ground and thickens.
Buck Turgidson
Well-known member
Looks to me like the keg is effectively a crumple zone and the rudder shaft is the load bearing device. Like a well designed spade where the leading edge does the same thing.
geem
Well-known member
The section through skeg and rudder is taken right at the bottom where the rudder and skeg is at its narrowest. You can see this if you look at the shaft diameter in the section. The skeg certainly is not a crumple zone but I guess anything becomes a crumple zone if you hit it hard enough?
dom
Well-known member
That's certainly the more modern approach. My balanced spade rudder has a 130mm carbon shaft. Similar rudders I have seen following a big impact have suffered serious damage to their leading edge without compromising the overall integrity of the system.
By comparison, really fast boats seem to still find kick-up rudders the optimal choice.
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Laminar Flow
Well-known member
I have seen this on several instances, but, depending how good your German is : Report of the sinking of the German yacht "Allmin" in the Baltic after running aground off Ruegen and with the loss of two lives. ... the hull showed a triangular hole to the aft of the root of the rudder stock ... the edges were bent inward ... the top of the rudder showed corresponding crush marks.
Laminar Flow
Well-known member
Hydrodymaically speaking, transom mounted rudders, i.e. rudders that are surface piercing, are much less effective than a rudder mounted under the hull and with a minimum gap. The hull acts as an end plate, effectively "doubling" or "mirroring" the span of the profile; same as for a keel as compared to a leeboard.
Sometimes, it seems, we just have to sacrifice out and out efficiency for practicality and safety.
Buck Turgidson
Well-known member
and yet all the fastest boats use them.
Laminar Flow
Well-known member
So one might rightfully assume then that none of it, skeg, spade, transom mounted with or without kick-up, makes, in practical terms, any bloody difference and everyone is free to make his or her own personal choice.
Buck Turgidson
Well-known member
or that all design is compromise.
dom
Well-known member
The key point here is that engineering issues are gradually being solved over time, materials are improving, and boats are gradually getting safer. Hence the increasing anecdotal vessels of old MAB favourites sailing in far flung storms finding themselves rescued by sometimes surprisingly lightweight modern AWBs.
Turning to the question of ultimate safety, any vessel can be catastrophically holed - the Titanic taught us that. Which is why the majority of offshore sailing race rules specify stringent watertight bulkhead requirements. This is in some sense the final frontier of safety and yet relatively few modern designs have gone this route, partly owing to the rarity of catastrophic damage outside of the bow area - many modern vessels have a crash bulkhead - and beyond that the aesthetic and cost implications of watertight bulkheads.
As Buck Turgison says, "all design is compromise". To which I would add, and boats are generally getting better and safer. Nothing to be afraid of - it's called progress
doug748
Well-known member
I think I have spotted a syndrome which we might call the Technological Death Roll - where each design shortcoming is fixed with a sticking plaster of escalating complication:
You design a big fat hull, around accommodation and it broaches when pressed
So you fit a small rig and inordinately big, long rudder which makes it a better family prospect but also slows it up, it still broaches when overpressed and really needs a lively crew.
So you fit two rudders which stops it rounding up on it's 7 ft keel but makes it even slower in light airs and you find it's difficult to handle under power.
So you fit a bow thruster. At the same time filling out the hull and taking the beam back even further. The windage is huge, I'ts still a handful in a marina
So you design in a stern thruster.....Your boat that is now very big, has huge wetted area, underwater carbuncles, is heavy, unrewarding to sail, you can't see the ends and need a stepladder to get on and off it
Next, doubtless; folding transom flap things, retracting bow thrusters, furling mains, video cams on the bow, parking sensors, steering engine legs.........
In the bar we spout about sailing to see nature and get away from the modern technological world and it's fripperies. ; -)
Exactly the same scenario is going on the other place, concerning electronic instruments. I tell you - Technological Death Roll
.
You design a big fat hull, around accommodation and it broaches when pressed
So you fit a small rig and inordinately big, long rudder which makes it a better family prospect but also slows it up, it still broaches when overpressed and really needs a lively crew.
So you fit two rudders which stops it rounding up on it's 7 ft keel but makes it even slower in light airs and you find it's difficult to handle under power.
So you fit a bow thruster. At the same time filling out the hull and taking the beam back even further. The windage is huge, I'ts still a handful in a marina
So you design in a stern thruster.....Your boat that is now very big, has huge wetted area, underwater carbuncles, is heavy, unrewarding to sail, you can't see the ends and need a stepladder to get on and off it
Next, doubtless; folding transom flap things, retracting bow thrusters, furling mains, video cams on the bow, parking sensors, steering engine legs.........
In the bar we spout about sailing to see nature and get away from the modern technological world and it's fripperies. ; -)
Exactly the same scenario is going on the other place, concerning electronic instruments. I tell you - Technological Death Roll
.
newtothis
Well-known member
To be fair, it's all been going wrong since they started putting sails on proper boats
dom
Well-known member
No, that is an early AWB !
Just look at those generous stern sections, shallow draft, reliance on form stability.....
It even sports a mini-Transat scow bow.....
All it needs is a carbon mast and Alex Thomson will jump in
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eddystone
Well-known member
Further to earlier post suggesting crossings oceans in a Contessa 32/Sadler 32 I can’t comment on C32 but IMHO a few mods would be needed to the Sadler before putting it in a situation where it might be upside down. Specifically to stop the bathtub/cockpit sole falling out and also making sure cockpit/anchor locker lids stay in place.
tyce
Well-known member
I don't understand all the harping on about skeg hung rudders etc etc that are a must before attempting an ocean crossing of any kind.
I would guess that lots of the forumites that say they would never set sail for far lands without the most bombproof of underwater appendages are probably the same forumites that are happy whizzing themselves and their family's around at 70mph in an older non 5 star rated Ncap car. Nothing wrong with that of course but why be happy to take the risk in the much more likely to have an accident car but would consider nothing but the most bombproof boat.
Id bet you spend a lot more time in your car than you do doing Ocean crossings.
Surely if you cant entertain any risk then you would be driving around in a Sherpa Tank never going above 30mph. Why would you drive anything less if risk of a collision on the road could total you and your family.
Of course there may be some forumites whom have both - a safe new car with the highest of safety features and an ancient solid boat but I bet there arnt many.
Ocean crossings take approx 3 weeks cars are used all year round, why accept one and not the other.
I would guess that lots of the forumites that say they would never set sail for far lands without the most bombproof of underwater appendages are probably the same forumites that are happy whizzing themselves and their family's around at 70mph in an older non 5 star rated Ncap car. Nothing wrong with that of course but why be happy to take the risk in the much more likely to have an accident car but would consider nothing but the most bombproof boat.
Id bet you spend a lot more time in your car than you do doing Ocean crossings.
Surely if you cant entertain any risk then you would be driving around in a Sherpa Tank never going above 30mph. Why would you drive anything less if risk of a collision on the road could total you and your family.
Of course there may be some forumites whom have both - a safe new car with the highest of safety features and an ancient solid boat but I bet there arnt many.
Ocean crossings take approx 3 weeks cars are used all year round, why accept one and not the other.
flaming
Well-known member
Don't really disagree with a lot of what you write, but the highlighted section is simply incorrect. If you think modern designers are designing inherently unbalanced hulls as shorthanded offshore race boats you're simply wrong.
Twin rudders are normally used to reduce rudder loads, especially on boats with wide, flat, stern sections. If you fit a single central rudder to a boat with stern sections like this you have to mount it quite far forward in order that when the boat is heeled the rudder stays immersed. This picture is not an especially extreme example, but you can see how far forward the centreline is out of the water when heeled.
Obviously the further forward the rudder is, the closer to the pivot point of the boat it is, so the larger it has to be to obtain the same turning effect. And therefore the larger the loads are. You then also see other handling/performance advantages of twin rudders. Principally that by mounting them toe out they become vertical when the boat is at it's ideal heel angle and can then also be smaller still. And the ability to keep control when pressed is far, far greater with twin rudders than single rudders. Again because as you heel a single rudder boat at some point the top of the rudder comes out of the water. This is an extreme example, but you see the point, the leeward rudder of a twin setup would still be immersed.
I've recently swapped driving a boat with 1 rudder for 2, and whilst there is more to it than just this, I am completely sold on the idea of twin rudders simply making sailing more fun, especially in wideish, lightish boats.
Obviously not all boats suit this approach, yours definitely not...
From a cruising context there are other, more fringe, benefits. But one that I think could be quite reassuring is the ability to inspect a rudder simply by heeling the boat. (See picture for example) and this is also a great way of clearing anything caught on the rudder. Just tack and most times whatever is caught will just fall off. If not, heave too with a bit of heel on, and the rudder you want to clear is out of the water and probably can be reached from the deck. Certianly we can clear weed from our rudders just by lying in the cockpit and reaching over the stern.
flaming
Well-known member
Oh, and on the vulnerability side, there are numerous transatlantic races for shorthanded boats that principally have twin rudders, and there is precious little evidence that they have a bigger incidence of rudder failure than single rudder boats on the ARC etc.
When heeled even a little bit I think you'd be surprised at how little sideways the rudder is from the keel on the track that the boat is actually making through the water. Certainly the window a UFO would have to miss the keel but still hit the rudder rather than missing the boat completely would be be a couple of feet. Not a huge increase in risk.
When heeled even a little bit I think you'd be surprised at how little sideways the rudder is from the keel on the track that the boat is actually making through the water. Certainly the window a UFO would have to miss the keel but still hit the rudder rather than missing the boat completely would be be a couple of feet. Not a huge increase in risk.
geem
Well-known member
There are thousands of spade rudders in use like my friends Dufour that was lost in the Atlantic when the rudder was bent backwards and jammed hard against the hull. Major manufacturers are still constructing rudders in the same way as far as I know. Whilst in the Azores this summer on my way back across the Atlantic two Jeaneau yachts were lifted out in Horta due to spade rudder failure. Fortunately nobody lost their life in the incidents and compared to the many yachts crossing the Atlantic the proportion of failure is small. Is this move to spade rudders progress or a cost saving measure by manufacturers? There is no doubt that spade rudders need more maintenance that a skeg hung rudders. Is this progress? Most car manufacturers manage to increase the mileage between services. It appears this isnt the case with spade rudder design.The key point here is that engineering issues are gradually being solved over time, materials are improving, and boats are gradually getting safer. Hence the increasing anecdotal vessels of old MAB favourites sailing in far flung storms finding themselves rescued by sometimes surprisingly lightweight modern AWBs.
Turning to the question of ultimate safety, any vessel can be catastrophically holed - the Titanic taught us that. Which is why the majority of offshore sailing race rules specify stringent watertight bulkhead requirements. This is in some sense the final frontier of safety and yet relatively few modern designs have gone this route, partly owing to the rarity of catastrophic damage outside of the bow area - many modern vessels have a crash bulkhead - and beyond that the aesthetic and cost implications of watertight bulkheads.
As Buck Turgison says, "all design is compromise". To which I would add, and boats are generally getting better and safer. Nothing to be afraid of - it's called progress
I have done 4000 miles in my car in four years. In the same time I have done 15,000nm in my boat?
I dont have a problem with spade rudders. Quite happy they are on somebody else's boat though?. I want a skeg hung rudder in the ocean and a spade rudder when reversing in a marina?. To be a fair, rudder design is just one aspect of boat design. If I had a boat for UK coastal sailing, marina based and I used it at weekends and summer holidays then an AWB would be fine. Thats not my kind of sailing so I have a boat optimised for what I do. My boat would be fairly rubbish as a UK based weekender. Its not got a bow thruster and its hard to steer in reverse. It does sail great though and its comfy at sea in bumpy conditions. Its set up for self sufficiency with huge storage, tankage and great accomodation for an almost fulltime liveaboard. We can spend time arguing about the merits of certain types of boats but we all look at boats with our own requirements in mind. Different designs suit different uses. One design cant possible tick all the boxes of potential uses.
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Laminar Flow
Well-known member
I do sincerely like the discussions we have.
When I am talking about imbalance in a design, I am referring to the longitudinal shift of buoyancy that occurs when a boat heels; unless a hull were absolutely symmetrical fore and aft that is of course simply unavoidable. A symmetrical hull is for hydrodynamic reasons not optimal. That said, the shapes we have today are quite extreme and it is easy to see in the current cheese wedge variant that are not only shallow and beamy, but have a high buoyancy gradient fore and aft.
You are certainly right that great beam makes centre rudders inefficient. For as the boat heels, not only does the bow with its lower buoyancy become more deeply immersed and, shifting CLR forward, increases weatherhelm and hence rudder load, the root of the rudder (as you correctly observed) becomes exposed causing aeration and loss of end plate effect and a significant loss in rudder efficiency. The result is nicely illustrated in you second pic.
There is no doubt to my mind that these hulls are, within the parametres as I describe them, imbalanced. How can they not be.
What I am not saying, is that designers have not taken measures to address and mitigate. To whit, the fact that the twin rudders are toed out to accommodate the angle of yaw caused by the shift in buoyancy/balance. Even though this feature, when the boat is sailing upright probably does to some degree increase drag.
The above phenomena is further illustrated by, as you pointed out, that, in spite of a considerable lateral separation between the rudders, in a heeled aspect, keel and rudder are in close alignment.
Twin rudders are hardly new (no, I'm not referring to the venerable tradition of Roman grain ships). Great lake scows have been using them for near on a century and for much the same reasons.
Ironically the Mini Transat scows are probably a more balanced form than their pointy end brethren, but that had likely more to do with increasing planing surface & lift within a restricted length.