Rudder-in-a-drum

[Steve W]

Mike,from a practical standpoint the drum needs to be at 90 degrees to the hull bottom and i believe the blade needs to angle back.While you probably dont need balance with the power of hydraulics after all,many boats dont have balanced rudders,but the situation with the drum rudder is that if you dont angle the blade relative to the drum you have too much balance. Since the pivot point is the c/l of the drum you have 50% of the blade area ahead of the pivot point.I dont know if this matters with hydraulic steering but it sure does with tiller steering.
After re reading your earlier post i think if you were worried about hitting something with the rudder,while you can sail with a drum rudders blade partially retracted if you did hit something you would do damage that would not be easy to repair.The other type of cassette rudder even if it could be installed in the transom so it could kick up would still do damage if the rudder were not straight ahead.
Steve.

[mick_allen]

. . . kick up would still do damage if the rudder were not straight ahead

well with a drum (i know you were talking about a cassette), one could still mount the drum on the transom where the 'fixed' drum was hinged at its top with a breakaway halfdrum mount at the bottom - near the hull. That design is sort of mixing 2 approaches of a traditional kickup rudder, but the drum approach does allow direct inline kickups no matter what the rudder angle is deployed to - a possible advantage over the usual kickup, depending on the duty.

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[mick_allen]

sorry, should have added that in this case the outer drum could be inset into the 'transom' with the outer rear part of the drum shaped flush with the transom surface - as all it does is directly kick back. (as this 'transom' gets cut into for this, it likely should be superficial shaping on the back of a structural real transom.

mick

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[Eric Sponberg]

Originally Posted by mikereed100

A couple of questions. Since my cat has hydraulic steering, would I still need to angle the rudder blade back a bit for balance or would this not be an issue with hydraulics?

If I do need to angle the rudder blade, could I just angle the whole drum/rudder complex so that they are coaxial but set at a 6-7 degree angle in relation to the water surface, or does the rudder have to have a different axis than the drum?

Thanks,
Mike

Yes, you are right, with hydraulic steering, the rudder and the drum may be co-axial since the hydraulics, in one direction or the other, holds the rudder at its angle of attack. Some hydraulic systems have a feed-back loop which, however, might feed back the wrong "feel", and so it would be the same as having a tiller. With a tiller, one wants to have the rudder self-center, which means that the center of pressure must be aft of the drum axis.

The drum should be normal to the hull, which itself on most sailboats is at some sloping angle to the waterline. Water at the stern generally flows parallel to the hull surface, so following the upward sweep of the hull. This just simplifies the shape of the drum end at the hull for ease of construction and the best fairness to water flow over the drum end.

Eric

[ancient kayaker]

Originally Posted by mick_allen

... breakaway halfdrum mount at the bottom - near the hull ...

An alternative would be the regular rudder-in-a-drum arrangement but with a spring-loaded rudder, on a horizontal transverse pivot within the drum astern of the drum's axis of rotation. The rudder's trailed edge could be shaped to clear the bottom in the event it gets kicked up. This would give a great deal of surviveability.

[mikereed100]

I am intrigued by the rudder in a drum concept and think it would be a good fit for my cat. I am undecided as to how to do the bearings though. The simplest way that I have come up with would be to make the outer drum 1/8” larger in diameter than the inner drum (which will be about 18”). The surfaces would have a graphite powder added for hardness and decreased friction, with a 3/32” acetal or UHMW sheet in between to act as a bearing. My only concern with this method is that the large bearing surface may cause a lot of friction.

Another way would be to use a smaller, thicker strip of bearing material, maybe 2” x ¼”, housed in a rebate on the outer drum. This decreases friction but would be a bit more complicated to build due to the rebate.

A third method would be to use roller bearings as Steve W. has done. This would be no more difficult than the second method but maybe noisier. Also, it sounds like the folks at Santa Cruz had trouble with the roller bearings on the 37 and had to send them back to Harken for a mod. I worry that if Harken can’t get it right the first time then I may not either.

Any thoughts?

Mike

[ancient kayaker]

Whatever arrangement you have, it would be wise to have the high wear elements on the drum so they can be removed for maintenance, with no moving parts on the outer drum, just a hard track. The noise of rollers could be reduced with non-metallic rollers. Something like the wheels of roller blades might work if the bearings could be made water-rsistant or water-lubricated.

[mikereed100]

Originally Posted by ancient kayaker

Whatever arrangement you have, it would be wise to have the high wear elements on the drum so they can be removed for maintenance, with no moving parts on the outer drum, just a hard track. The noise of rollers could be reduced with non-metallic rollers. Something like the wheels of roller blades might work if the bearings could be made water-rsistant or water-lubricated.

Yes, that it what I would do. Concerning rollers, I was thinking more of using rod (acetal or UHMW) cut to 2-3" lengths laid vertically in races.

[Autodafe]

Ancient Kayaker, just a thought on your friction concerns:
Friction is calculated as the product of normal (perpendicular) force and friction coefficient of the running surfaces. Bearing area is not involved, so my feeling is that total friction will be completely independent of the system chosen, assuming that similar bearing materials are chosen, that the rudder A/R is similar and that distance between upper and lower bearings is the same.

[brian eiland]

Gunboat observation & rudders

Originally Posted by nero

Most stricking was the rudders. The are 35 cm wide and very long. Very high aspect. They are held in cassets and pivot at the front of the blade. They can be pulled up from the top. They are the same as a square sterned boat with rudders hanging off the back. Nothing to stop them from ventilating. Nothing to protect them from impact. Clearly they work well so perhaps rudder ventilation does not occur that easily?? This is the first time I have seen this thin and high of an aspect on a bridge deck catamaran. Are there other examples of this?

You probably want to look at the VARA rudder system
http://www.socasailboats.com/VARA_Sy...ra_system.html

[ancient kayaker]

Originally Posted by Autodafe

Ancient Kayaker, just a thought on your friction concerns:
Friction is calculated as the product of normal (perpendicular) force and friction coefficient of the running surfaces. Bearing area is not involved, so my feeling is that total friction will be completely independent of the system chosen, assuming that similar bearing materials are chosen, that the rudder A/R is similar and that distance between upper and lower bearings is the same.

That's correct. I am less worried about friction than I was earlier especially with hydraulic power.

There is another point but it's a bit obvious. The bearing normal load due to hydrodynamic forces on the rudder is inversely porportional to the distance between the upper and lower bearings, so this distance should be made as long as is practical.

I read the earlier posts again and they are a mine of info esp Erics'.

[mikereed100]

I ended up building and installing drum rudders on my cat after recieving inspiration and excellent advise from the contributors on this thread. So far, after one year they are performing as advertised. The rudders are in a cassette offset slightly aft of the center of the drum. The offset is not enough to balance the rudders but this is not noticeable with the hydraulic steering. I have built a second set of rudders that I will angle back at 6 degrees to be used if the hydraulics go out or the rudders are damaged by collision. Emergency steering is by sticking a spare stanchion through a 1" hole in the upper part of the rudder. The bearings are 2 strips of 1/4" x 2" UHMW PE set in races. I experimented with 1/2" acetal rollers but they were noisy and tended to jam a bit. The strips are very low friction and quiet. No issues at all with marine growth as yet.

Mike