Making the trailer bunks

[Kern O]

If you've been to a boat manufacturer, you'll notice that the boats sit it cradles while they are being built. Those cradles have crosswise supports, not longways like a boat trailer. Cross bunks self center very well are are great for a trailer, but they are very labor intensive to make. They are also not adjustable and fit only one specific hull. I have spent several days doing the work, but the first time I go to load the boat on the trailer in a cross wind, I'll be glad I did it.

The first part of the process involved jacking the boat up off the trailer and putting it on blocks and stands. Then, the original bunks were removed from the trailer frame. Then the trailer has to be jacked up so it centered and level with the boat. If you ever have to line a boat up with a trailer to position the bunks, make four blocks of wood. Two of them are exactly the same length and go at the stern. Two others (usually a bit longer) go at the bow. If you then jack the trailer up so the four block are touching, the trailer should be level with the hull.

Once the trailer is positioned, I cut up a bunch of tubing and 1/2 thick aluminum bar to make the bunk supports. This trailer took 12 of them. Three of the supports sit directly on the torsion axle tubes, so they are pretty rigid. The aft pair and the forward two pair are supported by the trailer frame and are a bit more flexible. When the weight of the boat is on the bunks, the outer ones will flex downward, while the ones on the axles will not. As such, I preloaded the outer ones about 3/16". I set the suports up so they were about 4 inches from the hull. The plates are straight but the hull is not, so the 4 inches is an average.

I picked up a bunch of southern cypress in Louisiana. It was rough sawn pieces 6" x 6" x 7 feet long. I then bandsawed 'em down to about 4 1/2 square and ran them through a planer to finish them to size. The comes the fun part. Having done this a few times, I've learned to make a cardboard template for each station so you can position the notches for the chines and get the height right without scrapping a lot of wood. Heres a shot of one of the templates up against a piece of the wood:



Then the layout is transfered from the template to the wood and the shape is sawed out. A bit of belt sanding and a router to round the corners and they are ready to bolt on:



Here they are all lined up as bare wood, before being carpeted.:




You can easily tell when you're done. The trash bin looks like this:

[welder]

Kern O , wish I had the scrap wood .

Have you taken her down to the Marina yet and tried her out yet ?

Lester

Man , You do some purdy work.

[Kern O]

I've got about 10 more days of work before the boat goes back in the ocean for sea trial. The really big question is how long it will take me to do the 10 days worth of work.

[welder]

If you go psycho it should take about 5 days , give or take a few beers.

Lester

[Capt. Fred]

Man that is some sweet looking work there Kern.

[Garett]

Obviously some very nice looking work , but, one question. How do the chines line up to the notches in the bunks when the boat is being loaded? Will the chines ride the flat portion of the bunks until the boat is basically loaded or do the chines line up all the way along?

[jenkinsph]

Looking Good KernO, that trailer ought to fit like a glove when your done.

It is easy to see that a trailer built this way would enable you to launch and
retreive the boat many times with less chance of damage. I am thinking that
a trailer along these lines would enable me to deal with a 22' boat with few
problems.


Keep it up man




Steve

[Kern O]

Garrett: I've made the chine notches oversize so they have a fair amount of clearance. The whole idea here is that the 14 cross bunks make the whole thing self aligning and the boat has little choice but to fall in place. As you'd expect, that is a theory and can fail in the world of reality if the wind is blowing too hard. But since everywhere the boat touches the trailer it finds a "V" and those "V's" tend to be self centering, it does seem to work. Here's something thats hard to get your head around..... The boat has 12 fixed bunks that are about 4 inches wide by about 36 inches long. So, each bunk has an area of about 144 sq inches. Let's subtract 44 square inches for the chine notches and to make the math easier.. That means the 12 bunks each have an area of 100 square inches. Since there are 12 of them, there are then 1200 sq inches total of bunks supporting about 10,000 pounds of boat. If the load is evenly distributed, the bunks are only supporting about 8 pounds per square inch. That does not sound like much. But every time I put jacks under the boat to lift it, I am reminded just how much 10,000 pounds really is. That thing is heavy and 10,000 pounds is very unforgiving.

[jenkinsph]

KernO, 8 psi, about the same as a typical bulldozer track. Something to think about.


Man I like your bandaw must be about a 30" throat. I was proud of mine and it is only a 20" Delta.

Gotta get my post count up


Steve