Paravanes or Towed Passive Stabilizers (TPS)
Updated June 3, 2009
Jenny has some uniquely designed passive stabilizers. The birds are stored in shoes that are mounted on the booms. To deploy the birds, you first lower and tie down the booms on both sides to ensure that the down-load is carried up to the top of the mast and then back down the other side to the hull. You do not want to put only one boom out, because that puts a tremendous side load on the mast. Each bird has a retrieval line attached to the tail of the bird's fin. This line is also used to pull the bird out of the shoe when you are ready to drop it. I describe my experiences with this rig fairly extensively in my book Jenny's Journey here and on Amazon.com.
The retrieval line that is attached to the tail of each bird is now:
This retrieval line runs from the bird through a block that is about 5 feet from the boat when the boom is lowered. When you want to deploy the bird, you pull the retrieval line, the bird falls out of the shoe (about a 2 foot drop) and hangs from the block about 5 feet from the boat. Thus, there is no danger to the boat from the bird. My retrieval lines are 75 feet long. The shackle that attaches the retrieval line to the bird should have a breaking strength of around 500 pounds. This is the break point if something big and heavy gets snagged between the retrieval line and the bird. Carry a few extra shackles.
The bitter end of the retrieval line runs from its block on the boom to a ratchet block located at the base of the boom. Thus, the line at the bird does not move when the boom is raised and I can control the drop of the bird because the block does not rotate out. The block acts as a friction break as you lower the bird. I run the line from these ratchet blocks to jam cleats by the stairs on each side of the bridge. This way, I can deploy or retrieve the birds without going any further than the stairs. I never have to go up top once the booms are out and I always put the booms out as soon as I leave a slip, or before I raise the anchor. That way I am only minutes from having the birds in the water.
On the west coast I found that kelp builds up on the chains that went down to the birds. I also have had to cut a log loose from one. Because the trip line is about 4 feet out from the boat, I can haul the bird in, get it on plane along side the cockpit and have it dragging about 4 feet off the side of the boat. I can then easily reach it with a boat hook. I also carry a tree trimming pole that has a blade at the end for cutting stuff off.
When we had to deal with the log, we also had the dingy boom and lift line set up off to the starboard side and hooked close to the door in case we had to haul someone back on board (we were on a 600 mile ocean run down the coast from Canada). It was a safety precaution. Luckily, the log picked that side to snare our bird. So, we brought the bird up even with the cockpit using the retrieval line, pulled it over with a boat hook and hooked it to the dinghy’s electric winch. We winched the log up onto the cockpit railing, and cut it free. It had about a dozen other ropes and bits of cloth on it, and was only floating due to the amount of poly-propylene wrapped around it. It sank after we cut the snarling lines off of it. This was in heavy seas, but the setup provided lots of safety, including the fact that everyone was tethered.
Retrieval is simple. You just pull the retrieval line in. As you do so, the bird rises to the surface and at the surface flips backwards and surfs along the water. You bring both birds up to this stage first so there is a minimum of unbalanced forces on the gear. You pull one bird in up to where it is hanging 6 inches from the drop block. You can then raise the boom and lower the bird into the shoe. This is an easy one person job, except…
The exception is the chain. It causes significant drag, and hauling the bird in takes a lot of strength. The chain drag also decreases fuel efficiency. Both are annoying. So, in 2005 I switched the chain for 1/8 inch 7x19 wire. The 1/8 inch size proved to be inadequate.
WIRE V. CHAIN
During the summer of 2005 I experimented with replacing chain with wire rope on my towed passive stabilizers (TPS). My concerns were drag, kelp buildup, retrieval weight and the bird’s running depth. My hypothesis was that replacing the chain with wire would improve all of these characteristics. So far, this has proved correct.
My TPS are now set up with 5/8 inch rope from the tip of the boom. With the snubber in place the overall length is 15 feet which also brings the rope down to the water when the booms are out and the boat is stationary.
This is long enough to enable me to reach the connection between the rope and the wire from the upper deck when the booms are up. In the event of a break in the wire, this allows me to change out the broken part. Another advantage with the wire is that I can carry spare sections in minimum space and weight.
I started with a straight replacement of wire for chain. I chose 1/8 inch 7x19 with a break strength of 1750 lbs. This is approximately what the chain break strength was, and I wanted this part to be the weak link. I then matched the other parts to this break strength. When I crossed the Caribbean Sea with its large steep chop I found that 1/8 inch wire was not strong enough. The wires broke several times. I have since switched to 20 feet of 3/16 inch 7/19 wire:
I chose the following parts with Marine West the numbers that I used.:
On my first sea trial, I ran chain on the starboard side and wire on the other. We ran from SFO Bay down to Pillar Point on Half Moon Bay. The photo below is a composite of one picture taken of the chain on the starboard side and another taken of the 1/8 inch wire on the port side. The picture shows the significant difference the wire makes. The red lines are the wire / chain that holds the bird. The forward line is the wire, running almost vertically. The blue lines are the wire trip lines, same in both cases. The other evidence was my speed. I normally was experiencing a drop in speed from 7 knots to 6.4 knots with the birds down. This is a typical 10% loss. On this trip I was only getting 5.6 knots at my normal 1600 RPM setting. It took me a while to figure out that the chain side was causing Jenny to crab through the water.
I was warned about vibration and the warnings were accurate. The wire vibrates significantly under a normal load. It quiets as the load increases to block a roll. The vibration is about 100 Hz and if left unchecked would damage the TPS and Mast. So, the next challenge was to remove the vibration. After considerable thinking, I decided to try putting a dock snubber in the rope part of the rig. Originally I bought a pair of Taylor Made snubbers for 5/8 inch line and made up new, longer lines to accommodate them and still be able to reach the wire / rope connection from the deck. At the end of the summer of 2005, I made one trial run with this rig and there was no vibration and no noise. The birds are running silent and deep. Big smile for a while. Then I discovered that the Taylor Made snubbers are not up to this kind of duty cycle.
In 2009 I switched to go-cart tires. You can see them in the picture below. I use a bowline knot above and below the tire and enough line between to allow the tire to work, but the rope takes the load as soon as the tire is stretched a few inches. I plan to replace this with a better design.
I now have 4 years of experience with this rig and am satisfied it will take a beating and really keep Jenny vertical.
One final change I made was to run 3 part block and tackle from the end of the booms to the hull. I felt the two part setup was rigged wrong and not stiff enough. The bitter end of this tackle runs from the end of the boom to a cleat on the boom at the boat. The block at the hull has a snap shackle attachment. With this setup I can unsnap the block from the hull and use it two other ways. First, I can attach a boson's chair to it and have someone haul me up to the end of the boom for maintenance. Second, I can attach a true flopper stopper to it at anchor! Just keep a trip line on the flopper stopper running back to the boat and you can pull the block that snaps to the hull back nice and easy. Big smiles.