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Old 08-07-2008, 18:50   #121
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Hydraulic steering recommendations, rudder pressure

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One note though, Hydrive have a form you can fill out with your boat specs and rudder design, from which they will advise on which system to use. But they recommend systems that are way beyond what is required, at least for a multi. I guess they are just covering themselves, but the system they were advising me to use could generate over 300kg/m of torque!! I wonder if my home made rudders could cope with that! And frankly, on rudders of only around 1/2 a square metre area I can't see why they would see that as being neccessary.
Kobelt, of Canada, sugggested a pump that produces 1000 psi and two cylinders that can take 1500 psi each-for a catamaran (my BigCat 65) that has 2 x 8 sq. ft. balanced rudders. Each cylinder piston is 4.12 sq. inches, and the tiller arms are about 9.5 inches, so the power of the system would be about 3000 foot pounds.
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Old 08-07-2008, 22:07   #122
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Rudder pressure calculation

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Kobelt, of Canada, sugggested a pump that produces 1000 psi and two cylinders that can take 1500 psi each-for a catamaran (my BigCat 65) that has 2 x 8 sq. ft. balanced rudders. Each cylinder piston is 4.12 sq. inches, and the tiller arms are about 9.5 inches, so the power of the system would be about 3000 foot pounds.
Professional BoatBuilder - February 2006/March 2006 by N.A. Dave Gerr has a formula for rudder pressure. At 20 knots, you get 7,402 pounds of pressure on a 9 sq. ft. rudder, and it jumps to 11,567 for 25 knots. The lever arm isn't very long. If I understand him correctly, the lever arm on my LAR rudders is about 1/2 of a foot.
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Old 09-07-2008, 01:52   #123
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Quote:
Originally Posted by 44'cruisingcat View Post
One note though, Hydrive have a form you can fill out with your boat specs and rudder design, from which they will advise on which system to use. But they recommend systems that are way beyond what is required, at least for a multi. I guess they are just covering themselves, but the system they were advising me to use could generate over 300kg/m of torque!! I wonder if my home made rudders could cope with that! And frankly, on rudders of only around 1/2 a square metre area I can't see why they would see that as being neccessary.
One other thought.
How many turns lock to lock is required with what they suggest.
I went with 2x 213c12 cylinders and a 105c helm pump. this combo gives 2 turns kock to lock.

Mike
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Old 09-07-2008, 02:01   #124
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Quote:
Originally Posted by BigCat View Post
Professional BoatBuilder - February 2006/March 2006 by N.A. Dave Gerr has a formula for rudder pressure. At 20 knots, you get 7,402 pounds of pressure on a 9 sq. ft. rudder, and it jumps to 11,567 for 25 knots. The lever arm isn't very long. If I understand him correctly, the lever arm on my LAR rudders is about 1/2 of a foot.
I would think the load would be highly dependent on how much of the area is used as balance.
On second look those numbers seem HUUUUUUUUGE. 5 ton at 25 knots.
Even with the rudder turned fast to hard over it sounds extreme as a load on the rudder stock let alone as a load on the tiller.

Mike
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Old 09-07-2008, 02:13   #125
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Might be worth having a look at Ultraflex over Hydrive

This bundle of joy cost about $800 (not that long ago) and far more powerfull than required for my boat.



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Old 09-07-2008, 04:27   #126
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I would think the load would be highly dependent on how much of the area is used as balance.
On second look those numbers seem HUUUUUUUUGE. 5 ton at 25 knots.
Even with the rudder turned fast to hard over it sounds extreme as a load on the rudder stock let alone as a load on the tiller.

Mike

Yer, the load required to turn the rudder (with a balanced spade type) is pretty much dependant on the rudder balance. The load on the rudder shaft can certainly be quite large when the boats travelling fast and getting belted around by waves. I suspect the shafts have got to be strong enough to be able to pick the boat up by the rudder blades, and give the whole shooting match a good shake.
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Old 09-07-2008, 09:39   #127
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I haven't chimed in because I really like everything you are doing, except the biplane rig. My only reservation about that is mostly because its different, and seems to introduce complications at certain points of sail. I HAVE been know to change my mind in the past though.

I do have a special concern about new designs in general, and this is not directed at your beautiful design in any way beyond generalities.

Ian Farrier studied how his designs would float inverted. Perhaps others do too.

I believe that attention should be given in the early design stage toward a boat's habitability inverted, and towards survivability of crew inside the vessel during an inversion, whether a lateral roll over or a pitchpole.

While there has been some discussion on this site about re-righting an inverted cat or tri, I believe that the realities render this idea only remotely possible. I think these fertile imaginations should explore more mundane ideas, such as:

Dry berths with fresh air while upside down.

control fuel spills automatically, and preserve battery function for radios and navigation.

Safety lines and non-skid surfaces under the bridge deck.

Emergency equipment storage accessible from both right-side up and upside down.

Accessible fire extinguisher, dry flares, food, first aid kit and water-maker. Portable radio, epirb and GPS in Farraday Cage with rechargeable batteries, solar recharger or hand cranked recharger.

And every one of these features should be easy and simple to use; victims are ALWAYS disoriented, bruised or injured, hypothermic, and shocky. They need to use them at the height of the storm, in violent waves and wind, with broken parts lashing around uncontrolably.

My previous lifetime as an accident investigator with the NTSB has inflicted me with a concern for safety beyond what other sailors want to think about, and an occasionally ghoulish sense of humor. That might be because so few of my clients were in a position to explain what really happened, and I had to spend a lot of time guessing. Having a few more Survivors around could really help.
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Old 09-07-2008, 11:06   #128
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The world upside down

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Nordic Cat
I haven't chimed in because I really like everything you are doing, except the biplane rig. My only reservation about that is mostly because its different, and seems to introduce complications at certain points of sail. I HAVE been know to change my mind in the past though.

I do have a special concern about new designs in general, and this is not directed at your beautiful design in any way beyond generalities.

Ian Farrier studied how his designs would float inverted. Perhaps others do too.
Chris White has a lot about this in his book. My bridgedeck and connector beams have are filled with more foam than needed to float the boat, so even if the very large tanks are full, my BigCat 65 will float high. This was all discussed at length on another couple of threads on this site a month or two ago. I think it was the thread about life rafts that had a lot of this discussion.

As far as complications on some point of sail go, if you have two or more sails you have them. The biplane rig just gives you the interference on a different point of sail. You can either bear of or head up, or alternately you can slack the weather sail and harden the lee sail. You can read about trimming a biplane rig on articles about the Radical Bay design.
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Old 09-07-2008, 12:07   #129
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Water pressure on rudders.

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I would think the load would be highly dependent on how much of the area is used as balance.
On second look those numbers seem HUUUUUUUUGE. 5 ton at 25 knots.
Even with the rudder turned fast to hard over it sounds extreme as a load on the rudder stock let alone as a load on the tiller.

Mike
The calculation assumes 35 degrees of rudder, at which point you should hit a rudder stop on a good installation. Water is 300 times denser than air, and pressure increases by the square of the velocity (this is true of wind, too.) A half square meter = about 5 sq. feet, so 2.5 tons at 25 knots for a half sq. meter rudder.

The Kobelt engineer knew that I had balanced rudders when he made his recommendations for my rudders. 17% balance is the usual recommendation for the amount of rudder balance. The center of pressure of a foil is forward of the center of the rudder, because a well shaped rudder is foil shaped, and develops lift. Gerr's article uses 1/3 of the chord of the rudder as the center of lift, and his calculation measures the distance between the center of lift and the geometric center of rudder, as the lever arm for the rudder torque. On my rudders, you multiply the pounds of pressure by .5 to get the foot pounds of torque.

As far as rudder shaft loading goes from bending, yes, there are huge loads on the commonest type, the cantilevered spade rudder. Eliminating this load is one reason I used a skeg. The second is to protect the rudder, prop, and shaft from grounding. The third is to reduce the liklihood of a line wrap around the prop shaft. The fourth is to provide an efficient daggerboard like protrusion, in order to reduce the boat's draft while still maintaining good resistance to leeway. The fifth is to fair the propeller, shaft, and support assembly in order to reduce resistance.
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Old 09-07-2008, 12:46   #130
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Speaking of balanced spade rudders, a close observer can see that my BigCat 65 wingsail rig is essentially a balanced spade rudder sticking upright. Turn one of those high latitude surfing monohulls upside down and there you have it-two side by side balanced elliptical foils. It is amusing that people think that the idea is so outre in a rig but that it looks so modern and sleek in rudders.
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Old 09-07-2008, 17:40   #131
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The calculation assumes 35 degrees of rudder, at which point you should hit a rudder stop on a good installation. Water is 300 times denser than air, and pressure increases by the square of the velocity (this is true of wind, too.) A half square meter = about 5 sq. feet, so 2.5 tons at 25 knots for a half sq. meter rudder.

Is 35 degrees of rudder at 25 knots a likely scenario?
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Old 09-07-2008, 17:56   #132
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Might be worth having a look at Ultraflex over Hydrive

This bundle of joy cost about $800 (not that long ago) and far more powerfull than required for my boat.



Dave
Where from Dave?
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Old 09-07-2008, 19:01   #133
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Ultraflex Hydraulic Marine Boat Steering Kit

http://www.uflexusa.com/hydraulic_steer_sys.html

hydraulic
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Old 09-07-2008, 19:05   #134
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Thanks for that Mike. I'm actually in Aus, I've found the distributor here, was wondering if Dave had bought direct from them or from somewhere else.

But with the dollar the way it is, importing from the US could be a viable alternative, so thanks for the link.
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Old 09-07-2008, 20:36   #135
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25 knots?

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Is 35 degrees of rudder at 25 knots a likely scenario?
35 degrees is the standard used by naval architects. I don't know about the 25 knots, but I wouldn't be surprised if a sleek Outremer 50 / BigCat 65 type catamaran hit 25 knots under sail while surfing down big seas from time to time under really good conditions. By this I don't mean 25 knots steady, but rather, say 14 or 15 knots climbing the wave and picking up speed to 25 knots on the way to the bottom of the wave.

See:


Such a boat should easily sail 50 percent faster than a typical cat like a Lagoon 44, between the narrower waterline beam, lighter weight, (lighter weight proportionally, that is D/L ratio,) and longer waterline.

(See All about Fountain Pajots. for some statistics on Lagoon / FP cats, and see my website for BigCat 65 statistics.)

The BigCat 65 I am building will be able to motor at up to 20 knots with two 205 hp - 6 cylinder John Deere engines. I wouldn't be surprised if it hit 25 knots occasionally, motoring down swell through channels between trade wind islands in places like Hawaii. If that seems like a lot, well, that's the magic of having a long waterline, a narrow hull beam at the waterline, and not much accommodations for a 65 foot boat.

All of that for $300,000 USD seems like a pretty good deal to me. This is why I like 'little big boats.' By using vinylester / balsa/ uni glass with resin infusion, I keep the weight quite moderate (this is the voyaging weight, mind, with large tanks full and plenty of goodies on board,) and the budget moderate too.

Derek Kelsall's formula for predicting speed under power is:

The square root of (< DWL in meters x horsepower > / displacement in tons)

In BigCat's case that's 19.84 m x 410 hp / 20 tons = 8314 / 20 = the square root of 415 = @20.35 knots.

His speed under sail formula predicts a speed of 9 knots in 10 knots of wind for the BigCat 65, without taking the rather narrow 12 to 1 hull beam into account. All of this with no cranking of gennies or setting of spinnakers. I don't like worrying, though, so I expect to loaf along doing lots of 200 NM days in the trades.

For Derek's formulas, see: http://kelsall.com/images/articles/A...ormulas_KC.pdf
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