View Full Version : Stabilizers
08-01-2003, 02:21 PM
As some of you know, I'm converting a salmon troller. This weekend, I'm putting the mast back up. Before you comment about a mast on a power boat, it will be used for stabilizers and a boom. Now, my question. Where does one find guidelines for the design of a stabilizer system. I plan on using Kolstrand stabilizers, but what are the loads? Good designs for the deployment and retrieval, rigging?
08-01-2003, 02:31 PM
Seems to me that Beebe's book had some good info on different types of stabilizers. I'll have to go and look for it.
08-01-2003, 02:34 PM
Here we go.
Voyaging Under Power
by Captain Robert P. Beebe
Mine is 3rd edition, edited by James F Leishman
08-01-2003, 02:45 PM
...and if you think the load on a sailboat spar izz ruffff, water izz 800 times as dintz and at 10 knotz is one helluva load. If you can't get the info...I'll copy it and email it if you like...as I am sure Scott will do the same....
08-01-2003, 02:47 PM
Or you could fully gimball key cabins.
08-01-2003, 07:30 PM
I don't know just how one figures out the size of 'floppy stoppers' needed on a troller but is there noone down at the docks to give you some ideas and how about Kolstrand the maker/supplier no info from them?
I don't think there is much science involved. The best way is to go down to the commercial docks and ask the "good old boyz" the best way.
It's amazing what you learn from some of them who have been doing it 40 odd years.
1. Make sure they are long enough so that if a following sea pushes the stern around the rigging and stabilizers pass astern and don't foul in your prop.
2. A lot of the boyz run more than one set of rigging depending on the type of seas they are working in. I know they change depth for inside and outside water.
3. Make provision for locking the poles down. It's acually an OSHA / WCB regulation on commercial fishboats.
4. Wire rope runs deeper than chain, but chain is easier to pull up by hand and wire rope "sings"and can be annoying.
I run 3 fathoms below sea level. I also pack longer ones.
What you don't want to happen is to have one come out of the water when you roll. They change from being stabilizers and become WOMD (weapons of mass destruction)
For deployment, lower poles, chuck stabilizers overboard.
For recovery, raise poles, haul them in by hand.
Oh, I gotta jump in here. Probably the libations shared with the good folk at the boat show has loosened my lips beyond what is appropriate, but a complex subject is being treat wa-a-y too cavalierly here.
Firstly, apologies to LIMc up front. Sir, I do not know you nor your depth of knowledge on this subject, and if I tread on your toes I do not do so maliciously, but with intent to enter into friendly debate.
I take issue with your statement that submerged hydrofoil dynamic stabilizers (flopper-stoppers) "don't have much science involved". It is true that "the good ol' boys" on the wharf have accrued a considerable amount of seat-of-the-pants knowledge on the subject, and I would be terribly remiss by discounting their experience. But to presume that it is a simple subject, and that the experiences of a group of people involved in a specific size and type of boat in a limited region are able to be extrapolated to a wider range of boat types and differing ambient conditions is seriously short-sighted. To draw an analogy to boats in general, just about anyone can build a boat that will float; and a local boatman can build a boat well-suited to the ambient conditions in the area that he is familiar with. But to expect this local boatbuilder to create a boat that can deal with conditions that he is unfamiliar with and further, to have him predict accurately the performance of the boat, is to court failure. Creating devices whose performance can be accurately predicted for conditions that are foreseen but as yet not experienced is the purview of scientists, engineers, and in this case, naval architects and hydrodynamicists.
In the same way, the performance of underwater towed bodies can be determined with trial-and-error to good ends when the conditions of use are static, but to predict what is the optimum shape and size of the towed body for a differing set of conditions - which includes hull shape, hull dynamic stability, anticipated sea states to be operated within, and operational speed - is a topic of considerable scholarly study. Angle of attack, centre of gravity, wing form to minimize tip vortices, and dynamic planing surface areas are all factors to be considered when designing a flopper-stopper for a specific boat. Considerable time has been spent in tank-testing facilities on this very problem.
Is tank testing and engineering studies required for every flopper-stopper application? Of course not. But to suggest that the "guys on the dock" possess the requisite level of expertise is equally inaccurate. I bet if you asked the guys on the dock who had the best flopper-stoppers and then went to talk to that guy, he would at some point allow that the devices were based on good hydrodynamic research and engineering design, and not just local trial-and-error development.
In an area such as the Pacific North-West, where flopper-stoppers are commonly used, I would be willing to bet that a check with the local University engineering faculty or with a naval architecture office who specializes in fishing boat design would yield a plethora of scholarly papers and studies related to the design and engineering of flopper-stoppers. Much like baseball, the hydrodynamics of towed bodies only looks simple to the uninitiated - closer study will reveal a bewildering array of subtleties, all of which have an effect on whether you get to play in the big leagues or stay on the sand lot.
08-02-2003, 02:53 AM
I'm a relative newcomer here. I read most the post, and understand little. smile.gif I say that to qualify (or rather dis-qualify) my opinion on this subject. ;)
Living in S.E. Alaska, I see lot's of trollers. But have actual hands-on experience on only one boat with stabilzers.
Anyhow, I tend to lean towards Dave Flemings, and LIMc's viewpoint on this subject. You could spend many hours and lot's of money researching this sort of thing with the experts, and the universities. Or, just go down to the dock and talk with the folks who actually use them. Some things seem to work better in real life than they do on paper.
BTW, LIMc's point on deploying the stabies, "lower poles, chuck stabilizers overboard" made me laugh, as the first time I deployed a set, I did it the wrong way. smile.gif
08-02-2003, 10:17 AM
Ed Monk Sr., N.A. wrote several articles on the use of stabilizers on fishboats and yachts. At least, one of them is upstairs in the library at the Center for Wooden Boats.
MMD - I'm was realitively new to this forum. I've tried to offer some experience that I've aquired over 30+ years. I've learned a lot from this forum but I've also learned a lot by asking a question, keeping my mouth shut and listening to the good old boyz. Go ahead jump right in, all your gonna do is get wet. We are not trying to build a rocket and precision isn't the target. Not like "Oh, it's just a few little tiles that got knocked off we scientists / engineers don't think it's gonna cause a problem". By the way how long are the poles you are running, what kind of stabilizers do you have. How long is your shock rope, and how deep do you run your birds? Rather than be high handed your response would be much better recieved if you just offered your information rather than slam someone else's to get your point across.
I've got better things to do with my time. I'll be on my way now. I'm gonna go talk to the old boyz.
Well, LIMc, it was not my intent to insult you or your friends on the waterfront. If I have, I apologise. I had hoped that I had given adequate recognition to those many working sailors and boatbuilders who have successfully created many wonderful things, but that I had also pointed out that technology and engineering plays a significant role as well. Apparently I wasn't very tactful in the way I presented my case, and for that I'm sorry.
If you wish to be angry with me for that, so be it. But don't go away in a huff because one jerk (me) disagreed with you. There are plenty of good people here to have a pleasant yarn with. I will refrain from opening debate with you in the future so we won't rekindle this dispute.
08-03-2003, 10:51 PM
To clear up a few things, I've still got the rigging from the stabies. I'm just going to replace the shock line with a new one. It's got fairly new SS cable. So, that will get reused. The stabies that came with the boat were home made. They were a piece of plywood attached to what looked like 1/2 of a 30 pound trolling ball. I'm going to get a new set from Kolstrand. To keep the poles from coming back up, they have an over center mechanism to lock them down. The mast is in the same location as when the boat was a troller. The poles are going to be about 15 feet long. I plan on keeping the angle pretty close to what it was as a troller.
Now, on to what I want to change. The pole were held in place fore and aft with cables. I want to use tubing mounted forward. After adding a cabin, the existing single pin joint at the bottom of pole wouldn't work. I'll be needing a new one. To insure the load at the base of the pole is properly reacted, I'd like to know what pressure a stabilizer sees. I'm guessing I'll need some type of partial bulkhead at the base of the pole to tie things together.
Terry, the original pole was obviously strong enough for commercial fishing and you are unlikely to exceed the conditions that fishermen would work in, so I think you can be reasonably confident in the strength of the original structures. To determine the worst load condition of your outrigger pole, look at its weakest point. The pin or bolt that it pivots on is most likely to be this point. Find out what it’s shear strength is and that would be a reasonable load figure to work with. As for replacing the slewing wire with a tube, determine what the breaking strength of the original wire size is, and make sure that all the components (pipe, welds, pins, etc.) are as strong as the wire was and you’ll most likely be fine.
But, being the curmudgeon that I am, :rolleyes: I’ll take this opportunity to explain below just how complicated it sometimes is to answer such a simple question as “what pressure a stabilizer (paravane) sees”.
The dynamic force on a paravane stabilizer is a combination of the force of gravity acting on the paravane, the force of the water acting on its surface as it moves forward through the water, and the resistance force created when it is being pulled up through the water column when the boat rolls.
The gravitational force is the effective mass of the paravane times the gravitational constant. The effective mass is the absolute mass of the paravane less the mass of the water that it displaces.
The force of the water acting on the paravane as it travels forward through the water is the effective area of the paravane presented to the flow of water times the density of the water times the speed of advance through the water. The effective area of the paravane in this case is a function of the angle of attack of the paravane relative to the direction of the flow of water over it, times an efficiency factor to compensate for paravane shape and appendages. The speed of advance through the water is a combination of the horizontal velocity of the towing vessel plus any additional velocity difference between the paravane and the hull imparted by wave-induced surge, which varies depending on the relative positions of the paravane and hull in the wave train. The drag force of the tow cable or chain is calculated in a similar manner and added to the force imparted by the paravane.
The force of the water on the paravane as it is pulled up through the water column when the boat rolls is the effective area of the paravane presented to the water column times the density of the water times the speed of advance through the water column. The effective area of the paravane in this case is a function of the angle of attack of the paravane relative to the direction of force pulling it to the surface of the water times an efficiency factor to compensate for paravane shape and appendages. The direction of the force pulling the paravane to the surface is a function of the position of the paravane relative to the tip of the outrigger pulley. The density of the water acting on the effective paravane area is a function of the depth of the paravane in the water column. The speed of advance through the water column is a combination of the roll period of the boat hull times the angle of roll times the distance from the hull metacentre to the outrigger pulley, plus any additional velocity imparted due to the velocity difference between the paravane and the hull imparted by wave-induced heave, which varies depending on the relative positions of the paravane and hull in the wave train.
The aggregate force imparted by the paravane is then multiplied by some reasonable factor of safety to account for corrosion, fatigue, and unforeseen conditions.
With the forces imparted on the paravane now calculated, it is relatively simple to resolve the forces acting on the outrigger structures. Through vector analysis the paravane force acting on the outrigger pulley can be resolved into compressive and tensile forces in the outrigger pole, the topping lift, and the slewing cable. To these values one adds the forces imparted to the load points by the mass of the outrigger structure, the dynamic loads placed upon it by the motion of the hull in a seaway, and (if a likely event in the vessel’s area of operation) the static and dynamic loads created by ice accumulation. The aggregate force imparted on the outrigger load points are then multiplied by some reasonable factor of safety to account for corrosion, fatigue, and unforeseen conditions.
With the forces acting upon the hull by the paravane and outrigger structure now resolved, the structure required to support the outrigger can now be designed using standard strength-of-materials calculations to determine number and size of bolts, area of foot- and backing-plates, sizes of decking, doubler plates, and underlying beams or bulkheads.
Or you can rely on past practice and guesswork. ;) :D
08-04-2003, 10:44 AM
The nices paravanes are like an upsidedown wing, so they make downward lift. If you don't do that, you need a little bow down trim, which increases the paravane's drag, reducing your overall efficiency more than a good foil shape would do.
But that aside, the verticl stress on the paravane may be larger than you might calculate from squareinches surface area against a watercolum. Whether by trim or foil, the paravane in motion provides more resistance than it will if you're just on the drift.
Fortunatly, as so often happens in science, you can do the calculation backwards. Figure an acceptable roll speed. Brace you paravane poles upright, well stayed to one side, and pull on the other. A neighbor's power capstan will be nice here. A really big spring scale part way up the line is nice. Heel her over and let her back, with some nimble fellow trying to read the scale.
Don't break out the beer till the job is done and the capstan shut down.
Make sure everyone is clear of potential flying objects and whipping lines. Figure if you break anything, this is a good time and place to do it!
Real engineers love breaking stuff!
Geneological aside. When Grandfather (Mom's side) went off to fly in WWI, he fancied he'd have a crack at the Red Baron.
Planes would come out of assembly and be tested.
The theory was take it up and shake it till it broke.
Not surprisingly, there was a very high mortality rate among test pilots. Grandfather had the radical idea of shaking it till just before it broke. When he actually lived for more than a week of test flying, they made him permanent.
He was never reconciled to the fact that he never got to shoot down or be shot down by the Red Baron and the drawer full of medals they gave him for that dangerous unglamorous job was of no consolation.
He did regail us as children with his photos, like the one where he's still in a Spad tangled in a power line. Had to wait for the line to be de-energized before rapelling to ground.
But that family legacy of caution not withstanding, it's still sometimes fun to break stuff.
Even if you don't break anything - and chances are you won't as this is not a really high stress thing - it will give you lots of real life info not readily available from calculations. Stability diagrams are calculated from many assumptions and often the actual metacentric is different from calculated.
At some point, do a rough static incline test. Just a slow pull noting the increasing stability and then the angle of maximum stability and how sharply the stability declines. This will give you an excellent idea of when to get scared in real life.
08-04-2003, 01:29 PM
Jim Leishman is my boss here at nordhavn. We've put out approximately 200 Nordhavns with paravanes. Use chain to connect the paravanes, not cable. Cable whines and makes way too much noise. The chain should be about 3/16" so that it will break if you snag something and not pull the whole rig down.
We also use Kolstrand vanes.
08-05-2003, 10:45 AM
Well, the airframe structural engineer in me wanted a pressure. Just like at work, I let the aero guys worry about how they got the pressure. I followed Ken's advice. Beebe said in his book, that he found 10 psi to be the pressure on a paravane. Now, if his hull shape is similar to mine, I'll use that number.
Also, by following Adam's advice from Nordhavn, and using a "fuse chain", I can size my mast, poles, and rigging.
Maybe, one day in the distant future, I'll mount some strain gages, load cells, accelerometers, and pressure transducers on my stabilizers, mast, and rigging. Na, on second thought, I'd rather do heat transfer experiments. Maybe, see how fast I can chill my beer on a sunny day while running up the inside passage.
Powered by vBulletin® Version 4.2.1 Copyright © 2013 vBulletin Solutions, Inc. All rights reserved.