Trailer bunk replacement - materials?

[MISkier]

I need to replace my trailer bunks.  I've lost several bolts, several more are loose, and the wood seemed soft when I attempted a temporary fix to install some lag bolts into a new location on the bunk.

I have 4 bunks that are 2"x6"x8' and two bunks that are 2"x6"x4'.  Is #2 pressure treated deck wood a good enough material or are there other preferred wood options?  Are there sources for this (it's a Boatmate trailer)?

I don't think I want to go with the plastic or composite bunk, due to the potential for the boat to slide off too easily and unexpectedly.

I'm surprised at the condition of the bunks, as the trailer is only 14 years old and spends a lot of time in the garage.

I'll probably get some bunk carpeting from the local boat dealer and I'll be looking for stainless steel lag bolts, as I believe the corrosion on the original bolts may have been a contributing factor to their loosening.

Edited August 7, 2023 by MISkier

[Hockdog]

Bunk carpeting is available on Amazon in rolls, easy-peasy.

I’ve always replaced damaged/worn out bunks with just plain old fir, no pressure treated because I’m not really sure if the preservative chemicals will leach through the carpet and stain the hull.  Stainless or hot dip galvanized lag bolts will work just fine.

[HMan66]

I just did mine and used pressure treated lumber with hot dipped galvanized lags. I glued the carpet on the hull side of the bunk, then wrapped around and stapled. 

[dave2ball]

First are the bunks affixed to the frame?  That may make it a bit more labor intensive. You may have to drill the old screwed out and retap. If the bunks are like a normal trailer  it s pretty easy. I wood use pressure treated 4X12 and I would through bolt the bunks with stainless bolts and self locking nuts. Make sure the bolt heads are counter sunk about 1;2 into the board and seal with a rubber seal. 

[RichardDoane]

we don't use pressure treated because it seems to split/crack more than just good old douglas fir.  use stove headed galvanize bolts, and counter sink the  board so the crowned head is a touch lower than the surface.  trailer bunk carpet comes in a number of grades, spend a few more dollars on better carpet.  i like to through bolt instead of lag screws from underneath.  fit your uncarpeted bunks, install the bolts, then a first layer of carpet on top with just a few staples to hold it in place, then another layer that wraps the entire board (just like wrapping a christmas present) then attach to the trailer, with washers and double nuts (all galvanized).  jam nuts prevent loosening when that trailer bounces down the road.  buy an inexpensive electric stapler and you'll be set.

[MISkier]

@RichardDoane, how do you tighten the nuts on the stove bolts without spinning them?  I would think the countersink won't necessarily insure that the bolt head and hex portion beneath will bite into the wood enough to be held in place once some torque is applied to the lower nuts.  Or, are you using something with a slotted head?  Also, is it the pressure treating that causes the cracks and splits or is it the grade of wood (with more knots, etc.) that contributes to that?  That is why I asked if #2 grade was good enough.  I was thinking that #1 or select grade might be needed for better integrity.  I don't think douglas fir is prevalent in our neck of the woods, so I'm not sure I can get it.  It seems to be more available out West.  I'm not sure I can get anything above #2 grade, either.

 

@dave2ball, I shouldn't have to drill or re-tap anything.  I'll just need to remove the old lags, many of which are already loosening.

[BraceMaker]

@MISkier

Carriage bolt washers, they're like prong nut washers with a square hole to secure the carriage bolt.

Overdrill the through hole for the bolts and use something like a dremel or a flap disc in an angle grinder and carve a shallow channel from the through hole to the side of the bunk for drainage this helps water get out of there and prevents the wood soaking around the hardware so the wood won't rot.  Also the wood swells around the hardware and makes it split.

Skip nyloc nuts and just use plain nuts and washers with some locktite this seals the threads and will come off easier.  You may occasionally lose a couple of nuts so just keep a few spares around but this is far better than trying to remove nylocs at some point in the future.

 

 

[Gale K]

16 hours ago, MISkier said:

I'm surprised at the condition of the bunks, as the trailer is only 14 years old and spends a lot of time in the garage.

I'll probably get some bunk carpeting from the local boat dealer and I'll be looking for stainless steel lag bolts, as I believe the corrosion on the original bolts may have been a contributing factor to their loosening.

My trailer (and boat) came from my buddy (he lives in the Seattle area), and he had the boat on a lift. He left the trailer outside uncovered. We changed the bunk wood out twice when he owned it. I figure the wood is a maintenance item that in the right (wrong?) environment will need to be replaced at certain intervals. Last go-around, we did not wrap the bunks fully with bunk carpet. Left the underside open. Lets the wood dry out better than when it was cocooned all the way around! 

Edited August 8, 2023 by Gale K
mistype

[MISkier]

@RichardDoane (and others), what do you think of cedar as a bunk option versus douglas fir or pressure treated?  I am not having much luck locating the douglas fir, but can get cedar.

Edited August 8, 2023 by MISkier

[Drago]

Cedar is very soft

[MISkier]

@Drago, I looked at the Janka hardness, which does indicate it can be dented easily.  But, I didn't think it was an indicator of strength, which is what I was concerned about.  They also say it is rot-resistant and less prone to cracking/splitting than pine.  I guess I am wondering if some have used it structurally, which would be a similar application to bunks.

[BlueSki]

@MISkier I just did my bunks and used Cuda Powersports carpet.  I was impressed with the quality and it was nicer than the piece of leftover carpet from my old boat and trailer. 
https://cudapowersports.com/collections/bunk-carpet

[MISkier]

@BlueSki, I found that carpet yesterday and ordered it.  Thanks for the feedback.

[RichardDoane]

Cedar would work, it’s just more expensive. As for wrapping the entire board, it just looks better. The important part is the contact surface and padding feature of the carpet. While you’re updating your trailer, add the new style ratchet tie downs to the rear. So much better that old school straps 

[MISkier]

@RichardDoane (and others), Do you have a recommendation on the self-retracting ratchet strap transom tie downs?  Some I am seeing are Rhino USA, BoatBuckle, Fulton, Reese.  I see some complaints about mounting hardware, but that is less of a concern (I'll just get better stuff).  I am more interested in durability and construction quality of the ratchet/retractor.

[Dano]

@MISkier I recently bought Reese which quit ratcheting first day.  Returned them replacement with same result. I then bought Fulton and have not had an issue with them in over a year. 

[RAWSki]

@MISkier we put these on our CP trailer in 2017 and they are still working well.   On amazon for about $100

STAR BRITE Heavy Duty Retractable Tie-down with Quick Release Buckle (060428) 2 x 4 Black Steel https://a.co/d/chvMm6b

[BlueSki]

My BoatBuckles are on year 11 and going strong, but I plan to replace them this year just to be safe. 

Edited August 16, 2023 by BlueSki

[JDskiNECA]

I will add my $0.02 to this topic. I prefer pressure treated wood and in the west that is usually Douglas Fir. Douglas Fir is structurally much stronger than pine, and fir and especially Ceder. Cedar although will withstand weather better, Cedar is not used structurally. I would not use Cedar to support your boat.

Bunk carpet comes in sizes to wrap three sides of the boards. This is to allow air to have access to dry out the boards and carpet. Wrapping the entire board does look better but will hold moisture causing mold and premature wood rot. All of your staples should go on the side of the boards not on the surface in contact with your boat. Wrap the ends as you would with wrapping paper on a package and staple in place. The carpet should be installed after you have fitted the lumbar to the trailer frame.

I would use lag bolts and finder washers stainless or galvanized. The bolts must not go through the boards into the surface where the boat will sit. Pre-drill your holes about 1/2 the thickness of the board with a drill bit smaller in diameter than the bolts you choose to use I would guess 1/2 inch diameter bolts.

Now here is something that has not been discussed and that is fitting the boards properly to the trailer. This takes a bit of planning to be done at a boat ramp/dock. I have done it and this depends if some or all of your attachment points are welded to the trailer or you have some that are adjustable. The bottom of our ski boats are fairly flat but there is some curvature toward the front. The boards need to curve to fit the bottom of the boat prior to bolting the boards fully in place. Other wise when the boards bend into place some of the bolts will be ripped out of place. I have use a floor jack and spare lumber to aid in this process of proper fitting the boards to the bottom of the boat and mounting location..

Put the boat in the water at a non-busy ramp, dock it, anchor or send the family out for fun. Pull the trailer out and find a spot you can park and work on the trailer.

Measure the old bunks and make your new boards the same length or longer as the curving of the boards will change the bolt location when the boards flex. So a little longer is better as you can cut to final length after the boards are in place.

Remove the old boards and set the new boards one by one in place, start at the front of the trailer and pre-drill one hole in the boards and place a bolt and washer in place snug it. If you are on the flat portion of the boat bottom you can do this at all of the brackets. When it comes to the curves/angled section towards the front of the boat continue to place one bolt in place and then bolt loosely in the second bracket from the front to help hold the boards in place. Do this with all of the bunks.

Place the carpet on the bunks wrapping the front ends of the trailer boards and leaving the rears unwrapped until the boards are bent into the proper shape for proper length.

Then put he boat on the trailer gently making sure the boards stay in place as you slowly pull the boat out of the water with friends monitoring this for the driver.

Go back to your working area hopefully very close to the ramp, no bumps in the road surface. Now the boards will be conforming to the bottom of your boat.

Crawl under the trailer and finish pre-drilling the holes in the bunks and bolting the boards in place. Sometimes a jack and some persuasion is needed to get the board angled properly as you are bolting the boards in place.

Finish stapling the carpet in place and wrap the back end of the boards.

To some, this process may seem overkill but when you bolt the bunks in place without allowing for the curvature of the bottom of the boat to bend the board properly. The bolts then under the weight of the boat get pulled out of their position and now you have destroyed your surface to mount the bolt to.

So this is the process I have used to put new bunks under a boat. I have one boat I did this to with 20 years ago and one from the factory that is 23 years old and they are both in great shape today.

Side note: West Marine has many tutorials on their web page on do it your self boat projects with reason why you do certain things a certain way such as bunks and anchoring.

 

 

 

[MDB1056]

Making this way too hard. PT is fine. Carpet with quality indoor/outdoor. Done. How much time does your trailer spend in the water? .00001%

Want a lifetime solution - make your bunks - sister joint the wood like building a stringer  (4 pieces to make 1). Glue and screw the sections together, than wrap in quality boating cloth (fiberglass) and then finish with 3 coats of quality epoxy resin. Drill your mounting holes - seal those with epoxy also (2x). Then add your carpet. Glue it on - no staples or nails.  Will be 3X stronger than any wood alone, will not let a microbe of water in and will out live you by 30 years................      

   

Edited August 16, 2023 by MDB1056

[MISkier]

@MDB1056, you have a point.  My trailer does not do many in/out cycles per year.  Maybe 10.  There was a stretch of a few years where it was 30 or more per season, but not now.  In fact, if the manufacturer had just through-bolted the originals, I may not have needed to replace these - at least not now.  But, the lags were apparently shifting and getting stressed to the point they chewed up the wood and fell out.  The softness of the old wood is a slight problem, but likely would not have been as much of an issue than what was caused by the lags.

i like the laminated bunk idea.  Maybe next time, if I’m still around then.