When we were looking at the Hunter 41DS that we eventually bought, I assumed that the rigid boom vang it had was fully operational. It turns out that this boat, and a lot of other Hunters I gather, had a boom vang without a gas spring making it only marginally more useful than a traditional block and tackle boom vang.
I checked into the gas spring kit, part number 308-072-04 for a Selden RodKicker 20, which costs upwards of $650. The kit consists of a gas spring, a lower block and a spacer tube. Some searches turned up the specifications for the gas spring that I needed and I found a very close match at McMaster-Carr, p/n 4175T36 (stainless steel) or p/n 9416K52 (painted steel). Both are available with extension forces ranging from 200# to 550#. For my boat, a 2500 Newton gas spring is specified and the 550# unit is equivalent to about 2200 Newtons. The painted steel one in the 550# version wasn't in stock so I opted for the stainless version at a cost of just under $150 including shipping.
I decided to make my own lower block using some scrap UHMW that I had from a previous project instead of buying the Selden part for $24. However, I wanted to know for certain what the configuration of the spacer tube was so I bought the 40mm dia. x 580mm Selden part for $21.00. It turns out that the spacer tube isn't anything special - just some 40mm diameter aluminum tubing with a 1.5mm (about 0.060") wall. However, I didn't like the idea of the thin raw ends of the spacer tube transferring the extension force of the gas spring to the lower end casting and the lower block so I made some plugs out of aluminum that were pressed into both ends. You can see the modified spacer tube in one of the images below.
One of the issues that arises when fitting the gas spring is to calculate the appropriate length of the spacer tube. I wanted the upper tube to bottom out on the "buffer bearing" at the top of the lower tube before the gas spring reached the end of its travel under compression. In one of the photos below you can see the parts laid out as they would stack up when assembled. I calculated that a 20" spacer tube length would allow 10-1/2" of the 12" travel of the gas spring to be used. This allows some additional compression in case the vang is slightly over-compressed.
Normally, you wouldn't need to drill out the rivets and remove the lower end casting. In my case, however, I wanted to see exactly what the inner end of that casting looked like and, further, I needed to replace the buffer bearing that was damaged due to over-compression by a previous owner. Since the wire rope goes through the buffer bearing, I needed to disconnect the end of the wire rope from the casting to allow installation of the new buffer bearing.
The photo below shows the parts layout of the RodKicker 20. The casting at the left attaches to a swivel on the mast near the base and the casting on the right attaches to the boom. The block and tackle (not shown) attaches to the lower casting and the eye of the wire rope. The lower casting is fastened to the lower tube by three 1/4" blind rivets (two stainless, one aluminum). The black plastic piece on the right end of the larger tube is called a "buffer bearing". Its purpose is to act as a spring when the vang is fully compressed and also as a bearing for the inner tube to ride on.
The photo below shows the lower block that I made out of a block of UHMW. Both ends of the gas spring have M10-1.5 threads and the upper end threads into the end plug of the inner (upper) tube while the lower end (the piston) normally threads into a second end plug oriented in the same direction as the upper one. The substitute lower block that I made is about 1-1/2" thick and shaped for a slip fit in the outer tube. It has a threaded insert pressed into it made of aluminum bronze. This insert has a T shaped profile so that the insert can't be pulled out of the block.
Edit: the material used for this block was actually Delrin. I had some left over from when I made new bow rollers for the anchor. I think that either UHMW or Delrin (or even aluminum) would work fine for this.
This photo shows the finished spacer tube. The end plugs extend into the tube about 1/2" with an overall length of 3/4". The were turned so that the larger O.D. is slightly larger than the 40mm tube while the smaller O.D. was turned to be a 0.001" press fit into the I.D. of the tube. The spacer tubes for the RodKicker 10, 20, and 30 are 31mm, 40mm and 50mm diameter, respectively. You could use some 1-1/2" diameter aluminum tube (about 38 mm) for a RodKicker 20 if you also add the end plugs like I did. A 1-1/4" tube would probably work for the RodKicker 10 while the RodKicker 30 would need a 2" tube.
The photo below, which I got from one of the suppliers' websites, shows how the gas spring kit is installed inside the boom vang.
My total cost for installing the gas spring was under $200 (compared to $650 for the kit). If I hadn't had the materials in my scrap bin to make the lower block and the spacer tube end plugs it would have been more but still less than $250. I installed it today and it works great.
Below, I've listed the Selden part numbers for the various pieces you would need as well as a few possible sources of those parts. The biggest issue will be determining the strength of gas spring you need. In my case, a 41DS with a furling main, the 2500 Newton size was recommended. You might be able to find out from one of the suppliers which gas spring kit you should buy for your boat and from that you can determine the stength of gas spring you need.
Selden RodKicker Parts
RodKicker 30
------------
319-588 End Plug
512-141 Spacer Tube, 50mm dia. by 640mm
512-138 Spacer Tube, 50mm dia. by 760mm
RodKicker 20
------------
319-587 End Plug
512-142 Spacer Tube, 40mm dia. by 350mm
512-139 Spacer Tube, 40mm dia. by 460mm
512-137 Spacer Tube, 40mm dia. by 580mm
RodKicker 10
------------
319-586 End Plug
512-140 Spacer Tube, 31mm dia. by 220mm
512-136 Spacer Tube, 31mm dia. by 335mm
Possible Sources:
-----------------
Vela Sailing Supply
Fisheries Supply
Toad Marine Supply
I checked into the gas spring kit, part number 308-072-04 for a Selden RodKicker 20, which costs upwards of $650. The kit consists of a gas spring, a lower block and a spacer tube. Some searches turned up the specifications for the gas spring that I needed and I found a very close match at McMaster-Carr, p/n 4175T36 (stainless steel) or p/n 9416K52 (painted steel). Both are available with extension forces ranging from 200# to 550#. For my boat, a 2500 Newton gas spring is specified and the 550# unit is equivalent to about 2200 Newtons. The painted steel one in the 550# version wasn't in stock so I opted for the stainless version at a cost of just under $150 including shipping.
I decided to make my own lower block using some scrap UHMW that I had from a previous project instead of buying the Selden part for $24. However, I wanted to know for certain what the configuration of the spacer tube was so I bought the 40mm dia. x 580mm Selden part for $21.00. It turns out that the spacer tube isn't anything special - just some 40mm diameter aluminum tubing with a 1.5mm (about 0.060") wall. However, I didn't like the idea of the thin raw ends of the spacer tube transferring the extension force of the gas spring to the lower end casting and the lower block so I made some plugs out of aluminum that were pressed into both ends. You can see the modified spacer tube in one of the images below.
One of the issues that arises when fitting the gas spring is to calculate the appropriate length of the spacer tube. I wanted the upper tube to bottom out on the "buffer bearing" at the top of the lower tube before the gas spring reached the end of its travel under compression. In one of the photos below you can see the parts laid out as they would stack up when assembled. I calculated that a 20" spacer tube length would allow 10-1/2" of the 12" travel of the gas spring to be used. This allows some additional compression in case the vang is slightly over-compressed.
Normally, you wouldn't need to drill out the rivets and remove the lower end casting. In my case, however, I wanted to see exactly what the inner end of that casting looked like and, further, I needed to replace the buffer bearing that was damaged due to over-compression by a previous owner. Since the wire rope goes through the buffer bearing, I needed to disconnect the end of the wire rope from the casting to allow installation of the new buffer bearing.
The photo below shows the parts layout of the RodKicker 20. The casting at the left attaches to a swivel on the mast near the base and the casting on the right attaches to the boom. The block and tackle (not shown) attaches to the lower casting and the eye of the wire rope. The lower casting is fastened to the lower tube by three 1/4" blind rivets (two stainless, one aluminum). The black plastic piece on the right end of the larger tube is called a "buffer bearing". Its purpose is to act as a spring when the vang is fully compressed and also as a bearing for the inner tube to ride on.
The photo below shows the lower block that I made out of a block of UHMW. Both ends of the gas spring have M10-1.5 threads and the upper end threads into the end plug of the inner (upper) tube while the lower end (the piston) normally threads into a second end plug oriented in the same direction as the upper one. The substitute lower block that I made is about 1-1/2" thick and shaped for a slip fit in the outer tube. It has a threaded insert pressed into it made of aluminum bronze. This insert has a T shaped profile so that the insert can't be pulled out of the block.
Edit: the material used for this block was actually Delrin. I had some left over from when I made new bow rollers for the anchor. I think that either UHMW or Delrin (or even aluminum) would work fine for this.
This photo shows the finished spacer tube. The end plugs extend into the tube about 1/2" with an overall length of 3/4". The were turned so that the larger O.D. is slightly larger than the 40mm tube while the smaller O.D. was turned to be a 0.001" press fit into the I.D. of the tube. The spacer tubes for the RodKicker 10, 20, and 30 are 31mm, 40mm and 50mm diameter, respectively. You could use some 1-1/2" diameter aluminum tube (about 38 mm) for a RodKicker 20 if you also add the end plugs like I did. A 1-1/4" tube would probably work for the RodKicker 10 while the RodKicker 30 would need a 2" tube.
The photo below, which I got from one of the suppliers' websites, shows how the gas spring kit is installed inside the boom vang.
My total cost for installing the gas spring was under $200 (compared to $650 for the kit). If I hadn't had the materials in my scrap bin to make the lower block and the spacer tube end plugs it would have been more but still less than $250. I installed it today and it works great.
Below, I've listed the Selden part numbers for the various pieces you would need as well as a few possible sources of those parts. The biggest issue will be determining the strength of gas spring you need. In my case, a 41DS with a furling main, the 2500 Newton size was recommended. You might be able to find out from one of the suppliers which gas spring kit you should buy for your boat and from that you can determine the stength of gas spring you need.
Selden RodKicker Parts
RodKicker 30
------------
319-588 End Plug
512-141 Spacer Tube, 50mm dia. by 640mm
512-138 Spacer Tube, 50mm dia. by 760mm
RodKicker 20
------------
319-587 End Plug
512-142 Spacer Tube, 40mm dia. by 350mm
512-139 Spacer Tube, 40mm dia. by 460mm
512-137 Spacer Tube, 40mm dia. by 580mm
RodKicker 10
------------
319-586 End Plug
512-140 Spacer Tube, 31mm dia. by 220mm
512-136 Spacer Tube, 31mm dia. by 335mm
Possible Sources:
-----------------
Vela Sailing Supply
Fisheries Supply
Toad Marine Supply
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