How strong is a zip-tie? Could one hold the end of a bunk board carrying a 2,200# sailboat for 250 miles at highway speeds, including over a mountain pass? Would you do it with your boat? How about two, one on each end with a rusted bolt in the middle of the bunk?
Pretty strong as it turns out. The picture below shows the aft end of the starboard bunk just as the previous owner had tied it before I purchased the boat and towed it home. The other end was like it. They held the whole way. Notice that there is no bolt holding the bunk to the bracket. Don’t try this at home, kids.
The port side bunk was in the same condition, more or less: rotted bolt holes, loose and rusted bolts, rotted bunk boards, no carpet. No wonder the bottom paint had chipped and peeled away in places. Those bunk boards needed replaced, pronto.
Raising a boat on its trailer
Making new bunk boards is the easy part. The hardest part about this project is raising the boat off of the bunk boards if your boat isn’t already in the water. It’s actually easier than you think if you raise one side at a time.
To raise each side:
- Confirm the bow is held tightly by the trailer winch.
- If you have a swing keel, raise the keel until you’re finished replacing the bunk boards.
- Jack the entire trailer up one corner at a time and support the entire frame on jack stands to keep it stable.
- Place two (or three if you have them) bottle or scissor jacks on top of the frame next to each bolster bracket that supports the bunk.
- Place one or two short blocks of framing lumber (2″ x 6″ x 8″ min.) between the jack and the hull to spread the load and prevent oil-canning.
- Alternately raise each jack an inch or two at a time until the hull is about 4″ above the bunks.
Removing the old bunks
When you have one side jacked up, remove the bolts that fasten the bunk to the bolster brackets and remove the bunk. If the fasteners are rusted and uncooperative, use a reciprocating saw with a metal blade to cut them off. In any case, discard the old fasteners and replace them with new galvanized or stainless steel fasteners so they won’t be a problem the next time or for the next owner.
Making new bunks
Purchase new, pressure treated 2×6 boards a little longer than the distance between the two end bolster brackets. For example, if your bolster brackets are over 9′ apart and less than 12′ apart, purchase 12′ long 2x6s instead of 10′ long. It’s better that the bunks are a little too long than barely long enough. The extra wood will help prevent splitting as the bunks age. If necessary, cut the boards down to 1′ longer on each end than the distance between the bolster brackets.
Many trailers have their bunks attached to the bolster brackets with carriage bolts countersunk into the top surfaces of the bunks (so the heads don’t damage the hull) and held by lock washers and nuts on the undersides of the bolster brackets.
My preference is lag bolts (5/16″) screwed into (but not through) the bunks from underneath the bolster brackets. This method eliminates the countersinks in the tops of the bunks that hold water and accelerate softening and rotting. Besides, the fasteners don’t need to provide compression strength; the weight of the boat does that. All they need to provide is lateral stability and even a zip-tie can do that.
You can use just about any material to pad the bunks. Carpet remnants, artificial grass cloth, and discarded fire hose are popular choices. I prefer to buy traditional bunk carpet by the foot from an online source like trailerpartsdepot.com. It works well, lasts a long time, and looks better than dumpster filler. I buy it only wide enough to wrap around the bunk edges and to leave the bottom of the bunks open to the air to dry faster like in the first photo. Attach it to the bunk boards with stainless steel staples that won’t rust and fall out.
Getting the bends
Mounting the boards on the bolster brackets correctly means putting just the right amount of bend in the boards to conform to the shape of the hull before you install all the fasteners. You can use the sailboat itself to get perfect bends.
To mount each board:
- Measure the distance between the farthest two bolt holes in the old bunk boards. Center that distance on the midpoint of the new bunk board and mark the hole locations.
- Drill the hole in only one end now.
- Place the board on the bolster brackets with the drilled hole aligned with the hole in the bracket. Don’t worry that the other marks don’t line up for now or that the board isn’t bent enough. You may need to set the drilled end of the board on its bolster bracket first and then manually lift the other end of the board to get it to bend enough to slide onto the other bracket. If necessary, jack the boat up a little higher until you can center the board width evenly on both ends.
- Install a fastener in the first hole.
- Alternately lower the jacks completely so that the hull presses the bunk board into the right shape and down to the middle bolster bracket if your trailer has one.
- Drill the remaining two holes using the bolster brackets as guides. This ensures the holes are in the right spots. If you will install lag bolts from underneath, drill pilot holes. If you will install carriage bolts down from above, also drill pilot holes now, then you’ll have to jack the boat up again to complete drilling and countersinking the holes.
- Install the two remaining bolts and you’re done with that side.
Replacing bunk boards certainly isn’t a very fun project, but considering the cost and trouble of a bottom paint job, it’s the wise choice if your boards aren’t doing their job.
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