We recently purchased a 1977 Typhoon. When we examined the boat, we noticed a minor hump in the deck, just inside of the shroud chain plates. We found, due to the stiffness of the cabin structure, tensioning of the standing rig causes the cabin to sink lower and cause buckling of the deck, not the normal depression of the cabin under the mast step as in most sailboats.
To counteract this effect, I designed a stainless steel adjustable compression post. The upper compression post plate is through-bolted through the cabin top to the mast step to provide maximum suport. The lower plate is 1/4" angle through bolted to the upper aft edge of the porta-potty/V-birth cut-out, reinforced from behind with 1'x 2" x 2" teak bedded in proseal.
The adjustment mechanism is constructed from 3/4" SS all-thread, with a single tab welded to one end, which mates with the two ears welded to the lower plate mounted to the V-birth and is bolted together with a 3/8" bolt. Two nuts are added to the rod as an adjustment and a second as a jam nut. The All-thread slides up into a schedule 40 SS tube, the end of which has been flatteded, rounded and drilled to accept a 3/8" bolt, which becomes the upper tang which mates with the two ears on upper plate, which inturn, through-bolted to the mast step.
We designed the upper tube as long as possible to over lap the adjustment screw (all-thread) so as to provide a smooth surface to grab on to, leaving very little exposed thread when fully adjusted. The all-thread rod is inserted up into the upper tube, runs 2" short of the tubes full length, this added length, adds stiffness to the union of the rod and tube assembly.
Installing the upper mount plate: The upper mount plate is drilled to match the hole pattern of the mast step, however, the origional mast step were installed at the factory by hand drilling, and the exit holes in the interior of the cabin will not match the exact hole spacing of the mast step. We placed one bolt in the mast step,through the cabin top and through the upper mount plate. We examined the remaining holes for alignment from inside the cabin in relation to the compression post upper mount plate. By placing a drill bit up through the mast step mounting holes inside the cabin and slightly canting the drill, we were able to correct the sight misalignment.
This is a slow going process, all holes must be realigned to minimize stress when the fasteners are tightened. A thin rubber sheet is placed between the upper mount plate and the fiberglass cabin liner and between the lower plate and the liner before final installation.
WORD OF WARNING!: This compression post design, if over tensioned, can exert tons of vertical force to the hull and deck structure. If used improperly, could result in hull deck seperaton!!!! We only intended to add minor tension with the compression post to hold it in place while positioning, drilling and mounting the lower mount plate. We made no attempt to intentionally jack up the cabin top as we felt this would place undue stress on the structure. Even with minor pressure, there was an aparent lifting of of the cabin top.
At the same time as we built the compression post, we sent the old standing rig to the rigging shop to have new rigging made. When installing the new rigging, we came up a little short on the turnbuckles, with the turnbuckles screws near the end of their adjustment when the rigging was properly tensioned.
As mentioned above, since we made no attempt to raise the cabin top up to it's origional height, there was not much drop to begin with, we suspect the old rigging was made after the deck had sagged a little, the rigging was made shorter to accomodate. With the compression post in place, the cabin nolonger drops when the rig is properly tensioned, which explains why the turnbuckles are fully extended. Although the turnbuckles are extended to there maximum length, we were able to insert a cotter key and feel they should function properly, next time we order new rigging we will add an inch to their length.
In retrospect, I would have perfered to have installed the compression post with the old rigging to determine the proper length of the rigging before ordering new standing rigging.
All in all, we are extreamly pleased the new compression post design, she sails stiff to weather with a 150 jib in under 15 kts and has yet to burry a rail in the San Francisco Bay.
If you are interested in a compression post for your boat, my fabricator said he can make them for $325.00 plus shipping. Drop me an e-mail and I will put you in touch with him.
Happy sailing.
Marty
baymarinediesel@earthlink.net
Typhoon compression post source
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