Rudder balance

[TrentePieds]

Hm... If your stock has negative rake and the rudder bangs against the stops when you are dead in the water, it implies that the rudder has POSITIVE FLOTATION, i.e. that it is hollow. That mustn't be! A rudder should have NEGATIVE flotation, so it "hangs down" from its hinge line when the boat is stationary.

It is inevitable that in a solid rudder the centre gravity (which has nothing to do with "balance") will lie just about at the geometric centre of the rudder blade. It is also inevitable if the stock (and therefore the hinge line) has negative rake, that the CG will fall below and behind the hinge line and the rudder will therefore "hang down".

There is something very weird here! I gather you've just bought the boat. What do you know, if anything, about an OP having "repaired" the rudder?

Colvin's built several types of boats of widely varying specifications. Some were just fine. Some not so much. It'd still be good if we could have a pic so we can identify the design positively.

TP

[MrChris]

Eventually found the brochure photos.
I bought this boat about 8 months ago, sails and motors etc no other problem than the rudder so far. I loaded her to the stern more and put a 60 kilo drum of water right aft to test if changing the fore and aft trim helped and it does, but not much and certainly not enough.

[MrChris]

It is inevitable that in a solid rudder the centre gravity (which has nothing to do with "balance") will lie just about at the geometric centre of the rudder blade. It is also inevitable if the stock (and therefore the hinge line) has negative rake, that the CG will fall below and behind the hinge line and the rudder will therefore "hang down".

Not totally accurate maybe, the centre of gravity will alter with mass differences, eg if the leading edge is a solid spar of wood and the rudder body is a foam filled ply box.
And the vertical line through that centre of gravity is the point where the fore and aft are in balance (not confusing the balance of the rudder with a balanced rudder which are different things)
But this illuminates another point, the further over the helm goes the heavier it gets. This surely means that the centre of gravity is moving forward as the rudder rotates and so increasing the weight of the rudder in section,
That this is not too much leading edge too forward because it is the same when motionless.
I had not thought of the flotation issue though

[MrChris]

Originally Posted by MrChris

It is inevitable that in a solid rudder the centre gravity (which has nothing to do with "balance") will lie just about at the geometric centre of the rudder blade. It is also inevitable if the stock (and therefore the hinge line) has negative rake, that the CG will fall below and behind the hinge line and the rudder will therefore "hang down".

Not totally accurate maybe, the centre of gravity will alter with mass differences, eg if the leading edge is a solid spar of wood and the rudder body is a foam filled ply box.
And the vertical line through that centre of gravity is the point where the fore and aft are in balance (not confusing the balance of the rudder with a balanced rudder which are different things)
But this illuminates another point, the further over the helm goes the heavier it gets. This surely means that the centre of gravity is moving forward as the rudder rotates and so increasing the weight of the rudder in section,
That this is not too much leading edge too forward because it is the same when motionless.
I had not thought of the flotation issue though

I am moving toward the floating rudder idea now: because the rudder is basically hanging from the rake, then buoyancy would try to float the rudder more toward the transom the more it was laid over.
I can test this just by attaching weight to the rudder as far down as I can.
Watch this space!

[GrowleyMonster]

Remove your tiller and see what happens. If there is improvement, then make a new, lighter tiller.

When you are sailing at say 5kts does it still do that? Is the effect greater, or less, when sailing the boat hard, or motoring at full throttle?

Adding to the trailing edge could help. Maybe. Both weight and area.

[TrentePieds]

Growley is on to something.

You've said that the rudder deflects fully when you are dead in the water. That would then be true even when you are tied up in your slip.

Remove the tiller while you are in your slip. Then heel the boat by some convenient means. If the rudder then turns toward the low side, which you'd obviously be able to tell by observing the head of it, the problem is with the relative weights of the tiller and the blade.

Judging by your first photo, I'd say you need to add a copule of square feet to the bottom of the blade, beginning sort of where the axis of the prop shaft intersects the trailing edge of the blade. If, in doing that, you add some weight in that new area, I think your problem would be solved.

In days of yore, before radio control of models was in everyman's ken, "weighting" the trailing edge of the rudder of model sailing boats was a standard means of counteracting the boats tendency to "gripe" (turn to weather) as heeling increased.

TP

[MrChris]

Originally Posted by GrowleyMonster

Remove your tiller and see what happens. If there is improvement, then make a new, lighter tiller.

When you are sailing at say 5kts does it still do that? Is the effect greater, or less, when sailing the boat hard, or motoring at full throttle?

Adding to the trailing edge could help. Maybe. Both weight and area.

Thanks for suggestion but already ruled out those things, current thinking is that the rudder is buoyant due to rebuild by previous owner, a rough calculation shows that the rudder displaces very approx 90kilo and the construction of underwater part around 45kilo. I am currently scouting for a 50kilo weight to hang on lower parts to test the theory. If proved then I will try lesser weights until I find neutral buoyancy.

[TrentePieds]

Quote: " I am currently scouting for a 50kilo weight to hang on lower parts to test the theory. '

Good! I'd though of suggesting just that, but hesitated since I don't know how easy that would be for you. I have a coupla lead ingots lying about. They were made by melting discarded wheel balancing weights and casting them in a standard bread pan set in a box of sand. they weigh about 80 lbs each as I recall.

It seems you are in England. If you are anywhere in the south, you should be able to let 'er dry out on a low tide while you attach the weights.

TP

[MrChris]

Originally Posted by TrentePieds

Quote: " I am currently scouting for a 50kilo weight to hang on lower parts to test the theory. '

Good! I'd though of suggesting just that, but hesitated since I don't know how easy that would be for you. I have a coupla lead ingots lying about. They were made by melting discarded wheel balancing weights and casting them in a standard bread pan set in a box of sand. they weigh about 80 lbs each as I recall.

It seems you are in England. If you are anywhere in the south, you should be able to let 'er dry out on a low tide while you attach the weights.

TP

I will first prove the theory, then adjust the weights until I find the sweet spot weight, which I am guessing will be a couple kilos over neutral bouyancy maybe, then do the work when I lift out in the winter. I would prefer to put the weight in the rudder rather than on it.

[Cheechako]

Originally Posted by MrChris

Thanks for suggestion but already ruled out those things, current thinking is that the rudder is buoyant due to rebuild by previous owner, a rough calculation shows that the rudder displaces very approx 90kilo and the construction of underwater part around 45kilo. I am currently scouting for a 50kilo weight to hang on lower parts to test the theory. If proved then I will try lesser weights until I find neutral buoyancy.

If your problem was just sitting in the marina then yes, but with the boat moving? no, bouyancy not an issue, water force is way beyond that issue even at 1 knot.

Also, Depending on where you put the weight it may solve your issue in a test but have nothing to do with bouyancy, rather the force of the weight.
I really wonder if the prop/aperture in the rudder is your issue, even when sailing only from turbulence...?

[malbert73]

I bet it’s buoyancy. That would be proven if the rudder hangs straight with the boat hauled out or drying out on a low tide. Easily fixed

[TrentePieds]

Quote: "...then do the work when I lift out in the winter. I would prefer to put the weight in the rudder rather than on it."

Agree entirely. If you're gonna have 'er out, work on this kinda rudder is really easy. Once you are satisfied that you've identified the problem and decided what's required to correct it, the rest is just donkey-labour.

On this coast - if it were me - I'd buy in some 2x4 Red Cedar and laminate up a blade with a shape to to suit me. I have a bandsaw so the final outline would be cut on that, but with some patience a "sawzall" would do the job. The foil (the section) can then be worked in with a grinder if you don't have an electric plane.

Weight can be introduced by cutting a hole right through the blade and inserting into it a slug of lead. A square hole of 6” x 6” (15 x 15 cm) in a blade 2” (5 cm) thick will accommodate about 30 lbs of lead. From there you can calculate any required hole size yourself :-)

The whole issue should, of course, then be covered in glass and painted. Poly-resin will do if you are sensitive to epoxy.

TP

[MrChris]

Originally Posted by TrentePieds

Quote: "...then do the work when I lift out in the winter. I would prefer to put the weight in the rudder rather than on it."

Agree entirely. If you're gonna have 'er out, work on this kinda rudder is really easy. Once you are satisfied that you've identified the problem and decided what's required to correct it, the rest is just donkey-labour.

On this coast - if it were me - I'd buy in some 2x4 Red Cedar and laminate up a blade with a shape to to suit me. I have a bandsaw so the final outline would be cut on that, but with some patience a "sawzall" would do the job. The foil (the section) can then be worked in with a grinder if you don't have an electric plane.

Weight can be introduced by cutting a hole right through the blade and inserting into it a slug of lead. A square hole of 6” x 6” (15 x 15 cm) in a blade 2” (5 cm) thick will accommodate about 30 lbs of lead. From there you can calculate any required hole size yourself :-)

The whole issue should, of course, then be covered in glass and painted. Poly-resin will do if you are sensitive to epoxy.

TP

My thoughts exactly. after I have proven the issue

[MrChris]

Originally Posted by Cheechako

If your problem was just sitting in the marina then yes, but with the boat moving? no, bouyancy not an issue, water force is way beyond that issue even at 1 knot.

Also, Depending on where you put the weight it may solve your issue in a test but have nothing to do with bouyancy, rather the force of the weight.
I really wonder if the prop/aperture in the rudder is your issue, even when sailing only from turbulence...?

I hear what you say, but it swings over at anything under 2.5 knots. I am pretty sure it is a bouyancy issue, a rough calculation finds the underwater section displaces 80 to 90 kg and the construction weight around 45kg all roughly estimated but clearly there is substantial uplift which would increase either side of vertical. I am beginning to wonder if the thing was designed to allow it to fill with water because that would give it a couple of kg weight in water.
But we will see when I get to test it. I am currently waiting for a weightlifting neighbour to come back so I can borrow a selection of his dumbell weights [emoji3]

[Cheechako]

Originally Posted by MrChris

I hear what you say, but it swings over at anything under 2.5 knots. I am pretty sure it is a bouyancy issue, a rough calculation finds the underwater section displaces 80 to 90 kg and the construction weight around 45kg all roughly estimated but clearly there is substantial uplift which would increase either side of vertical. I am beginning to wonder if the thing was designed to allow it to fill with water because that would give it a couple of kg weight in water.
But we will see when I get to test it. I am currently waiting for a weightlifting neighbour to come back so I can borrow a selection of his dumbell weights [emoji3]

Any weight you try just added anywhere will just "leverage" the rudder. You wont know if the bouyancy is the issue unless you know exactly where to add that weight so it's neutral balance, but heavier than it was. Hard to explain!

One cubic foot of air is 64 lbs displacement. Are you sure 90KG (198 lbs) is correct? That would be over 3 cubic feet of rudder (if it was all air).

Still ...at 2 knots, water pressure will overwhelm any unbalance in the rudder I think. Water creates a lot of force. Try dragging a thin board through the water on edge, then with the broad side against the water.
JMHO!