How to Replace Trailer Bunk Boards

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.

BEFORE - hanging on by a Zip-Tie
Don’t wait until your bunk boards are this bad to replace them!

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:

  1. Confirm the bow is held tightly by the trailer winch.
  2. If you have a swing keel, raise the keel until you’re finished replacing the bunk boards.
  3. Jack the entire trailer up one corner at a time and support the entire frame on jack stands to keep it stable.
  4. 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.
  5. 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.
  6. 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.

BEFORE - top of one bunk, bottom of the other
BEFORE – top of one bunk, bottom of the other showing what was left of the carpet and that venerable Puget Sound patina (moss)

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:

  1. 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.
  2. Drill the hole in only one end now.
  3. 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.
  4. Install a fastener in the first hole.
  5. 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.
  6. 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.
  7. 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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10 Comments Add yours

  1. Curtis from Alaska says:

    “I simply used three jacks, one on the frame next to each bolster and jacked them up a little at a time in turn until the hull was about 4″ above the bunks.” I don’t understand this sentence. Where did you place the jacks? It seems like you would have to lift on the hull to get it above the bunks. But you write that you placed the jacks next to the frame. Could you clarify this for me. Thanks.

    1. Hi, Curtis

      I placed the bottom of the jacks on the frame next to the vertical frame members where the bunk boards are attached but inboard from them. Any convenient place on your trailer frame that will support the jack base safely and will lift the hull near the bunk boards should work. I placed 2×8 blocks on top of the jacks to protect the hull. So yes, I jacked the hull up on one side, replaced that board, then repeated the process on the other side to replace that board.

      If you need a picture, let me know and I’ll take some.

  2. Curtis from Alaska says:

    This makes sense now. I was thinking originally that you placed the jacks on the ground and pushed up under the trailer frame. But literally you placed the jacks ‘on’ the frame and pushed against them upwards towards your boat hull. Thank you for the clarification.

    I have a bunk board replacement this summer to do. I also have to do some repairs on the outside of the hull up in the keel trunk area so I need to take the whole boat off the trailer to drop the keel so I can gain access up inside. While doing that I was going to investigate the keel lifting mechanism and confirm the pivot point. Probably will just replace it all since I don’t know when the last time it was done and I don’t want to do this project again. I enjoy your site.

  3. Curtis from Alaska says:

    I raised my boat up and placed it on cribbing enough that I was able to pull the trailer out tonight. I wish I had set up a camera on time lapse. That would have made for an interesting video to put on your website. I will send a photo later as I ended in the dark. I did it without any troubles and it only took 2 hours.

    Next I will lower the keel from the boat using threaded rod. The threaded rod (5/16-18) will screw into the weldments to help lower and raise the keel to do this work. I plan to do the following work this spring:

    -Keel pivot assembly (check condition of pivot pin, add keel centering washers, new SS bolts in weldments)
    -Lifting hardware (enlarge diameter of winch drum, replace cable, check condition of turning ball, replace keel eye bolt, replace keel cable hose and clamps, add reinforcement of bridge deck where winch mounts)
    -seal the leak in the fiberglass from the outside up inside the keel trunk
    -replace the trailer bunkboards
    -construct guide on trailer to help align the keel on it when pulling the boat out of the water

    Any other suggestions for work I should do while the boat is off the trailer? Not interested in refinishing the keel this summer. That might be a project for another year.

    Curtis

    1. Hi, Curtis

      Congratulations on getting your boat cribbed up. That’s the hard part and now you’ll be able to work underneath it a lot easier. Just be careful climbing in and out that you don’t knock it over. Especially with the keel detached. The center of gravity is much higher without it.

      I hope the threaded rod trick works for you. In my situation, I couldn’t see it helping and just slowing the job down. I found a hydraulic jack worked better. But others have had good luck with it. Maybe have a plan B in mind in case things don’t work out like you’d hoped.

      Is your trunk leak caused by the keel scraping when it’s lowered or is it from an accident? If from scraping, you might need to use thicker or more washers on one side than the other to better center the keel in the trunk and prevent future scraping like I describe in part 5 of the keel refinishing series. Also consider centering shims on the keel like in part 4. And check for the three defects I describe in part 5.

      If your keel eye bolt is in good condition, I wouldn’t bother replacing it. It might take a lot of work to remove due to galvanic corrosion and cause more damage than good. Reinforcing the bridge deck is a good idea. I’ve seen a lot of them cracked. It’s a fairly weak spot in the design.

      The only other thing that comes to mind is to consider moving the trailer axle back a few inches if your tongue weight is low and you have trouble with the trailer tracking straight at highway speeds. Now’s the time for that while you have the boat off and you’re working on the trailer. You should have at least 10% of the gross weight on the tongue. Many owner’s setups are lighter due to so much gear in the back of the boat.

      Good luck with the work and I hope you can get lots of airtime this summer!

      1. Curtis from Alaska says:

        My boat is a ’76 model. I purchased it three years ago from the third owner. That owner had let the boat sit uncovered unused for 5+ years. I didn’t know the condition or maintenance schedule of the previous owners. I wanted to replace the major structural parts of the boat. One of the first things to do was replace the keel bolts. Three changed out easily enough. The forward starboard bolt did not budge and eventually I sheared off the head. I tried removing the stud. no such luck. So I dug out the weldment from inside the cabin. While doing this, I accidently stuck my oscillating tool blade through the keel trunk.

        I replaced the weldment and went to glass it again. As you know, for resin to cure, it needs to be protected from air. The glass that I shoved in the hole is not cured where it exits out into the keel trunk. I tried shoving plastic coated cardboard up in between the keel and the trunk to aid in curing. But obviously not enough. Once I drop the keel I will redo that fiberglass from outside up in the keel trunk.

        I used the winch tonight to lower the aft end of the keel down to the ground. The forward end of the keel is still up hanging by the hanger and bolts. the metal of the keel is all intact around the eye bolt up to the shoulder. Do I replace the eye bolt still with the larger diameter one? i want to replace the cable, increase the diameter of the winch and reinforce the bridge that supports the winch. The eye bolt would then be the weakest link.

        I am going to add some washers to better center the keel as it is able to clunk from side to side.

      2. If I were you, as long as the existing bolt doesn’t show signs of excessive wear or cracks, I’d leave it. The standard dry torque for a 3/8″-16 (coarse) 316 SS bolt is about 15.5 ft. lbs. If there’s Loctite or galvanic corrosion in the threads after all these years and you have to force the bolt to loosen, it could shear off like your hanger bolt. Then you’d have an expensive welding repair job on your hands. And for what?

        I suspect that the switch to 1/2″ eyebolts coincided with the switch to 1/2″ chain plate bolts so that they could use the same bolts in both places and not have to stock one odd bolt per boat. They look identical. The 3/8″ bolt is easily capable of lifting the few hundred pounds of one end of the keel in water (buoyancy).

        It’s interesting to me that my ’81 keel has the older 3/8″ bolt.

  4. Don Grantham says:

    Stingy, I have just picked up a 40 year old trailerable sailboat and need to rehab my trailer. I would sure appreciate pictures of “how you used the jacks to lift the boat safely. Thanks for your great website.

    1. Hi, Don

      Unfortunately, the last time I jacked my boat up, I didn’t take pictures. But you can find a good description of a technique similar to what I used at this forum thread: http://forums.catalina.sailboatowners.com/showthread.php?t=171536. The description is in a PDF file attached to the first post of the thread so that you can print it out and take it with you out to the boat while you work.

      Good luck rehabbing that trailer. Hope to see you around here more in the future!

  5. If you lived in Maine we call that Yankee ingenuity!

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