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Old 02-05-2018, 01:38 AM   #1
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Paravanes boom position?

Hi, all. I've been reading some of the threads here regarding paravane stabilizers with great interest. It seems that most boats have the booms installed near cg(?) - or around 1/3 of the boats length from the stern.

What exactly are the negative effects of having the booms and vanes closer to the bow - apart from perhaps reduced rudder response ?

I'm asking because I found pics and illustrations of boats where the booms are closer to the bow - and also because I plan to dampen the roll on my own former FV. The easiest for me would be to install the booms near the front mast, but I realize it may not work so well




Thanks, V.
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Old 02-05-2018, 02:47 AM   #2
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My fishing boat (55') had the poles about 1/3 of the boat length back from the bow. I've seen them rigged everywhere between the 1st 1/3 to the last 1/3 of the boat. The poles should be fine where they are in the picture. The angle when out was about 45į. I used a commercial paravane similar to the picture.
If you're going to cruise with them in the water, use a hole forward of center. I only used mine when trolling or anchored, so used a hole toward the back that also slowed me down to trolling speed. Rigging needs to be stout. The poles aren't just lowered, they're locked in position. I had brackets on the pole and mast that held a stout wood brace when in position. The pole was lowered, a little too far, the brace inserted into the brackets and the the pole pulled up until the brace was locked in position. Some people had an eye bolt near the waterline and used that to hold down the pole.
In the ocean, 45į seems to work well although I had my poles in the water a few times. Pics - one poles forward and one poles aft. Both line up with the mast. 2nd boat has the poles off, but the bracket is on the rail. Doesn't really make a difference where the poles are.
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Old 02-05-2018, 03:58 AM   #3
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Welcome to the forum, Cold-Smoked. Its nice to have a Norwegian on board. Both myself and my boat are of Norwegian heritage.

Sorry but I don't have much experience with paravanes, but I'm interested in learning more so I'll be watching this thread.
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Old 02-05-2018, 10:29 AM   #4
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Thanks, Lepke and AusCan - I appreciate it.

Bilge keels are what is commonly used up here, but not very effective from what I 've heard. So what I know about paravanes, is what I've read here.

Regarding the position of the poles, I believe someone here referred to Beebe's book - saying that ~28% from the stern is the best location. I can only imagine that this has to do with rudder control, I guess the effect is similar to that of daggerboards location on a sailboat. Also, I think it's possible that the boat will be pulled a bit down by the vanes when going over wave tops -if the poles are too far from CG.

My boats front mast is around 1/3 length from the bow, would be great if that works.


Thanks, V.
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Old 02-05-2018, 10:51 AM   #5
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We have found that when the seas get more extreme, the autopilot has more difficulty maintaining course when the paravanes are in the water. This has happened on a couple of occassions and I had to reset the AP. I think you might find that the vessel will hunt more if the forces are too far forward. It’s a good idea to consult with a naval architect prior to installation. I am aware of a couple of cases where the install failed when in use on KK42’s.
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Old 02-05-2018, 11:08 AM   #6
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CS, interesting topic.
Dragging paravanes causes drag which is resistance. To overcome resistance requires effort.
The center of effort is not at the paravanes as most people would think. Nor is it on the outrigger poles. It travels through the rigging and ends up being at the head of the mast that supports the poles. This is a fixed position and cannot change.
The net effect is to raise the vertical center of gravity (VCG) which will in turn reduce the righting moment (GM). Because the masthead is high above the vessel center of gravity (G), a small amount of force (resistance) has a large effect because the moment is calculated as force x distance. So the relatively low resistance is multiplied, causing a disproportionate affect on intact stability, reducing the righting moment and slowing the roll period.
Concurrently, the position of the mast horizontally affects the position of the longitudinal center of gravity (LCG). In your case, it will move it forward. Again, this a moment calculated as f x d. Force remains as resistance. Distance in this case is the distance from the vessels original LCG to the mast base. That lever would be applied to the overall calculation of the position of the LCG in a total moments calculation and the net effect will be to move the LCG forward, but not as much as you might think.
The vessel rotates or turns around the LCG. Pushing it forward INCREASES the distance from the rudder post so the effect the rudder makes on turning INCREASES, again due to the turning moment is f x d where f=rudder force. Some people may consider that this new steering effect makes the boat squirrelly.

In your case, the boat was designed to drag nets which have much more resistance than the paravanes might make. My opinion is this will be a non issue. If you have any real doubts, you can consult a naval architect to make a calculation using real numbers.
Best of luck with your boat. We would all love to see some more pics and details.
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Old 02-05-2018, 12:38 PM   #7
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McGillicuddy describes some the forces more eloquently than I did.

Quote:
Originally Posted by McGillicuddy View Post
CS, interesting topic.
Dragging paravanes causes drag which is resistance. To overcome resistance requires effort.
The center of effort is not at the paravanes as most people would think. Nor is it on the outrigger poles. It travels through the rigging and ends up being at the head of the mast that supports the poles...
...but also to the bow of the boat through the forestays on either side, which will serve to apply rotational forces at the bow. At least I think it does. And that force may not be even (port and starboard) through the wave period if the direction of travel is at angles to the beam sea.

Be aware that the forces are considerable and failure of the system typically comes when you need it the most. Some people tend to underestimate the forces on the rigging and under-build their setup. Don't make that mistake. Also, it's important to replace the stays regularly. It's cheap to do so (perhaps $200-300 for the 4 stays). The consequences of rigging failure are considerably more expensive.

The position of your mast can't be changed and you might be challenged by that. I think the best advice is to consult with a naval architect. I know of instances where people have relied solely on the advice of fabricators who are less familiar with the forces involved and the systems failed.

Jim
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Old 02-05-2018, 02:35 PM   #8
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[QUOTE=JDCAVE;633463]McGillicuddy describes some the forces more eloquently than I did.



ď...but also to the bow of the boat through the forestays on either side, which will serve to apply rotational forces at the bow. At least I think it does. And that force may not be even (port and starboard) through the wave period if the direction of travel is at angles to the beam sea.Ē



Sorry Jim, thatís not quite right. Its the opposite, stays do dissipate the stress on the head of the mast but donít do anything to move the center of effort.
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Old 02-05-2018, 03:00 PM   #9
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This is from Beebe's first edition (1975). It gives you an idea of the loading for paravanes. He's was convinced that the fish can generate resisting forces up to 10 lbs. per square inch. Our fish are ~220 square inches each. Proper design is critical.


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Old 02-05-2018, 03:52 PM   #10
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Paravanes boom position?

[QUOTE=McGillicuddy;633482]
Quote:
Originally Posted by JDCAVE View Post
McGillicuddy describes some the forces more eloquently than I did.



ď...but also to the bow of the boat through the forestays on either side, which will serve to apply rotational forces at the bow. At least I think it does. And that force may not be even (port and starboard) through the wave period if the direction of travel is at angles to the beam sea.Ē



Sorry Jim, thatís not quite right. Its the opposite, stays do dissipate the stress on the head of the mast but donít do anything to move the center of effort.


Perhaps Iím missing something. Itís still somewhat logical to me there would be a rotational force about the mast, exerted on the tip of the boom by the forestay. Pardon my schematic drawing of a birdseye view of a vessel with paravanes in the water. If you only had the port side paravane in the water, would there not be a rotational torque to port?
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Old 02-05-2018, 04:04 PM   #11
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Another photo of the attachment of stays to the tip of the boom.
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Old 02-05-2018, 09:23 PM   #12
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[QUOTE=JDCAVE;633499]
Quote:
Originally Posted by McGillicuddy View Post



Perhaps Iím missing something. Itís still somewhat logical to me there would be a rotational force about the mast, exerted on the tip of the boom by the forestay. Pardon my schematic drawing of a birdseye view of a vessel with paravanes in the water. If you only had the port side paravane in the water, would there not be a rotational torque to port?
Attachment 72716


Actually a plenty good diagram. But the forestay only resists the load pulling aft on the pole. The net effect on the boat will be that aft pull x distance off C/L, centered around the mast location (just like the diagram shows). The forestays could lead anywhere forward; doesnít really matter.

With 2 fish pulling aft equally, no net effect on steering. But since the drag is a component of the total pull from the fish, itís rarely going to be even from side to side; there will be some effect on steering forces. Also, donít forget the downward pull. Thatís acting thru the mast location as well. Thatís going to have an effect on pitching.

Will it work to stabilize against rolling if itís forward? Sure. But what seems to have been true on Beebeís boats and many others is that the typical center of motion overall is ~1/4 - 1/3 of the LWL forward of the stern. If you center the mast & stabilizer forces at that location, you wonít change much except the rolling motion. If you move the stabilizer forces forward or aft, youíre going to change something - pitching and / or yawing motions - and that will almost surely increase resistance and maybe make the steering Ďsquirrellyí. Even assuming the rudder can counteract it, itís creating more drag in doing so.

Now all that said, an NA much more familiar with your particular type of boat should have some valuable input. The comments about dragging fishing gear from there are quite valid, but remember that fishing may not have been done at your cruising speed. And Beebe was hunting for the Ďbestí location; the difference between that best and your actual location may not be a big deal. Giving up an extra fraction of a knot to get stabilization vs not having any is an option many would enjoy having.
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Old 02-05-2018, 09:40 PM   #13
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From my memory, the location for the paravane poles is to mount them 27% forward of the stern, using LWL as your 100% number.

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Old 02-06-2018, 11:41 AM   #14
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Thank you guys, all good and useful information.

I have decided to give it a try, and install the poles parallel to the front mast. I ordered galvanized steel pipe, seems all the hardware will cost me around Usd 1000 here.

I think for sure there will be implications with the steering, so I guess I'll have to modify the rudder or build a better one later on. I planned to do that anyway, fishtail or articulating - as some of you here have demonstrated success with.

If this first install don't work out to my satisfaction, I *could* build a new mast on top of the wheelhouse. Similar to that some of you here have, with supports going down to the deck/bulkheads/frames.

The front of the wheelhouse is around 1/3 of lwl from the stern, my only concern is changing the boats appearance. I would prefer to keep it looking 'traditional'. It's originally from 1943, btw.



Thanks again, V.
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Old 02-06-2018, 11:59 AM   #15
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Paravanes boom position?

Jim
With only one boom out, the turning moment (rotational force) would be calculated as the moment f x d where f = drag of one paravane and d = the horizontal distance from the centerline of the mast to the tip of the boom (not the length of the boom unless it is completely horizontal). It is the distance from the centerline that provides the multiplier in the calculation. The boat is not rotating about the forward connection point, it is rotating about the mast.

I can see why you think it does Ďpullí the bow around. It seems logical that it would. The calculation of force on the rigging is separate issue and would be used to correctly size the rigging for the loads applied. You would also think the forward stay would reduce the torque on the mast by alleviating the backward pull of the paravanes and it does do that to the extent of the load is applied to the forward stay. But this does not apply to the theoretical calculation of where the load is considered to affect the vessel.

By the way, all the calculations applied to the boat that make up both horizontal and longitudinal stability are theoretical positions. But they seem to have been working out just fine for the last hundred years or so. Guys working in finite math have refined and model tested the heck out of the calcs and would laugh out loud at how crudely I have described the math. But in the end the finite calculations would be within 0.5% of mine. So I am okay with that.


Cold Smoked:
Nice boat. If it worked well as a trawler since the 1940ís, is there an issue now that makes you think you need a different type of rudder? Is it underperforming in any sort of meaningful way? If not, I donít understand the urge to spend significant cash. If you think that the steering characteristics are going to change substantially by running paravanes from the forward mast, I would strongly suggest a sea trial first with the paravane installed forward to see how it handles. I suspect you will find the steering is still good. No need to change either mast position or rudder unless you are doing it for aesthetic reasons. One advantage of having the mast forward rather than over the wheelhouse is that the entire system is in view from the helm while I operation. I agree with trying to maintain the original character of the boat. I think that is a lovely idea..
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Old 02-06-2018, 12:22 PM   #16
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Quote:
Originally Posted by McGillicuddy View Post
Jim

Cold Smoked:
Nice boat. If it worked well as a trawler since the 1940ís, is there an issue now that makes you think you need a different type of rudder? Is it underperforming in any sort of meaningful way? If not, I donít understand the urge to spend significant cash. If you think that the steering characteristics are going to change substantially by running paravanes from the forward mast, I would strongly suggest a sea trial first with the paravane installed forward to see how it handles. I suspect you will find the steering is still good. No need to change either mast position or rudder unless you are doing it for aesthetic reasons. One advantage of having the mast forward rather than over the wheelhouse is that the entire system is in view from the helm while I operation. I agree with trying to maintain the original character of the boat. I think that is a lovely idea..

Thanks, and yes - my plan now is to try it first with the front mast. The rudder performs less than satisfactory at slow speeds and when going astern. I am capable of modifying the rudder myself, so I don't think the cost will be high at all.


Thanks, V.
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Old 02-06-2018, 09:59 PM   #17
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CS. You might want to look up Becker Rudder to help with your slow speed maneuvering issues. If you are handy with a welding stick you might be able to modify the rudder this way.
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Old 02-07-2018, 05:49 AM   #18
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Yes, but I plan to make it as simple as possible.

This is my boats very crude rudder. I suspect the rudder is a bit too short, and the shaft is too close to the prop - so it doesn't get that much effect from the prop-wash as a more balanced rudder would. I did move that zinc, btw.

But it's too much work to change the position of the rudderstock/shaft, so I think I will modify it to something similiar to that of the second pic. Simple, cheap and easy to fabricate. I may add some steel angle bars on top and bottom to help direct the prop-wash.
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Old 02-07-2018, 06:53 PM   #19
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Wow. Yes, I can see why you might want to modify that. Thatís a really poor design. I guess that the original prop was a fixed blade type and located a bit further forward?
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Old 02-07-2018, 08:08 PM   #20
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Yes, we agree. And judging by that sort of foil shaped/tapered piece of steel behind the prop, who ever made that rudder seems to have had a plan. I guess this was done in the 1980's.
And you are probably right about the prop originally being fixed blade and located a bit further forward.
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