calculating loadings on rudder

Alastairdent

Alastairdent

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I need to replace the rudder on a steel 28ft sloop. To replace it, I need to fix new rudder fittings to the transom.

I'm thinking of an interim solution using blind fixings such as these

http://www.screwfix.com/prods/76466/Scre...5mm-Pack-of-100

to fix a plate to the transom. It's not possible to get to both sides of the transom without butchering a lot of woodwork.

The fastening would just need to be up to pootling round the harbour, which is enough to get us to a slipway. Then I could get the plate welded on.

I want to work out how strong the rudder fixings need to be, so can someone guide me to a way of calculating the loading on the rudder and hence fixings?
 
For that "trip" why do you want a rudder? An oar over the transome would probably do or even a piece of ply nailed to a length of 4 x 2 lashed to a stauchion.
 
It's a commercial harbour, with oil-rig supply vessels and fishing boats.

My hairy spheres would be nailed to a plank for trying to get by with a lash-up.
 
I think I'd go for an alongside tow with a RIB (or two!).
The load will depend a lot on speed, depth of rudder etc.
A deep dinghy rudder is usually held on by a few M5 screws at the bottom gudgeon.
What thickness of steel do you have? can you tap it M6?
If you think in terms of 'can I break this with the leverage of the tiller?' You might be near enough for a temporary job?
 
I'm more worried about shear loads.

I'm very constrained by time and organisation, as the marina is a long long way from home.

As for steel thickness . . that's a good question, prob something between 4-6mm. I've tried tapping in the past, but not got a thread that's any stronger than the hold that the tekscrews get.

The constraint on time is why I'm after getting half-way to a finished job. Attachment by tekscrews then welding seems a progressive route. Heck, I think I should just go for it.

the boat has a windvane rudder, which gives a little steerage control.
 
Two things will govern it - speed squared and inertia. If you go slowly (slooooowly), as slow as you can to maintain steerage in calm water then the drag loads will be low but as soon as you start accelerating the drag loads will increase by the square of the speed. This is rapidly bad!! Inertia comes into it when you suddenly slam the rudder over and the boat has way on which it then wants to get rid of by using the rudder as a brake. Move the tiller sloooowly.
 
Wow it's a long time since I did anything like this and I havene't got my old noteboos.

Basically you need to work out the pressure on the surface area of the rudder, that typicaly is applied 1/3 back from the leading edge at the centre of the height.

Bending etc forces on the stock are then worked out from there.

Remember forces are bigger astern because the trailing edge is now the leading edge.

The force is related to spead so doing the job slow reduces all the forces.


I would suspect but with no warranty that those bolts would do as long as you kept the speed down to just above stearage way
 
Would definitely go for a workboat or RIB lashed alongside. We use this method in our club marina to move boats to and from their berths all the time. Much better for a one off than trying an unknown bodge.

Bet the harbour master would be happier too!
 
Alistair, its jury rig not lash up! But do what ever you are comfortable with. I would point out that the Vikings crossed oceans with my suggested solution called it a steer board.
 
On my little boat with transom mounted rudder the bottom pintle gave way. 3 6mm screws were sheared off. (fatigue) Possibly the fact that it has side pressure on swing mooring as boat sheers around had something to do with it.
However the rudder has a depth of about 1 metre but the pintles are only 35cms apart near the top. So when I move the tiller and especially when going fast with a lot of weather helm caused by heel the bottom of the blade exerts a large sideways force on the pintles. On occasions I reckon I have seen the rudder bend.
So as Whiteoks7 says it is water speed that really increases side load ion the rudder. In my case with a lot of rudder balance this does not translate to tiller load.

On my f/g boat I put 2 layers of carbon fibre over the outside of the transom in a 25cm square to increase the area of load of the bottom pintles. The inside was to awkward to add the reinforcement. I now have 4 6mm screws for the bottom pintle. (essentially dinghy fittings). Certainly now the transom doesn't flex so much.

So yes you are right to ask about the strength needed for rudder fixings even for a temporary job. But it is the speed that kills. olewill
 
If you really want to know?
Just a rough formula to check your loading:
Torque (in kgm)= [surface area in sq metres] x ([0.4 x total chord of rudder ]-[ length of unbalanced part ]) x [ velocity squared in knots ] x 15.89.
The formula is really for forward calculations- if you wanna see something scary, change the balance area for the trailing area's values and prepare to be scared! But then try backing hard up under motor and moving the helm- it's SCARY how much torque there is!
It's all a bit nerdy, but it's a good approximation. If you then do some schoolboy leverage calculations on first- and second-order levers you will be able to get the shear-strangths of your fastenings.
(Sorry if it's a bit nerdy-wordy!)
 
Isn't this one of the basic "What if ..." questions we are meant to be prepared for?
What if --- my rudder broke at sea? So of course you already have the necessary bits and pieces on board, and an idea of how you would devise a jury rudder. /forums/images/graemlins/ooo.gif
 
s I said above it is a very long time since I did these calculations and cannot find that particular notebook, but should this part of your formula
([0.4 x total chord of rudder ]-[ length of unbalanced part ])
not be ([0.4 x total chord of rudder ]-[ length chord forward of the stock])

Of course this only really works for a rectangular rudder, more complex shapes needing more work to determine the location of the centre of pressure. The basic formula works well though and has stood the test of time having been devised by Froud in the late 19th C.
 
Well, if I was sailing, would I really need a rudder at sea? Even on a sloop I should be able to balance the boat with the sails.

Oh, and I'm fully aware of the increase in loading when going astern; I used to have a 12t dutch barge and had occasion to do an emergency reverse. It's not fun being pinned against the rails by your own tiller, particularly when you can't reach the throttle from that position.

thanks very much for the formula, that will help a lot.

My concern is mostly over low-speed maneuvering, which seems to require a large rudder with this boat. She's pretty light on the tiller tho' despite not having a balanced rudder. Maybe it is the transom shape.
 
As I said at slow speed the forces will be low and if you keep just around steerage way I suspect your plan will ork without doing any calculations.

Your rudder may wll feel light either because it is a high aspect ration, ie tall and thin, or because the tiller is relatively long, or a combination of both
 
Re rudder balance. If the transom ius vertical then you can picture or calculate the balance. However if the transom slopes toward the bow at the water line and the rudder pintles follow this line you get a built in balance by the angle of the rudder. ie the bottom of the rudder is ahead of the pintles. So balance. olewill
 
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