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Tips for Selecting Axles for a Home Made Boat Trailer

Question:

I am building a boat with a Beam of 8foot 1inch and I am fabricating a trailer for it as well. I am looking at the CONTINENTAL 94inch Galvanized Torsion Trailer Axle 3500 lb. #22-AXT80 The legal road size in Oregon is 8foot 6inch wide max. I am going to use 10 inch decromat disc rotors. The dexter kit is what I am thinking. What is your recommendation for a correct size of axle for a boat with a 8foot 1inch beam? The boat is 23foot long. I was going to make the trailer 25foot long and make it out of 3x4 inch 3/16inch rectangular tubing with galvanized 6 inch wide rims and tires 225/75R15 that are 10 ply. The picture is just a guideline, I have a shop that rivals most machine shops and can do about anything. I have never built a boat trailer for a boat this big. Im a bored Navy Vet the needs a hobby instead of drinking and since fabrication is my hobby... why not? What are your thoughts?

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Expert Reply:

The first thing you will want to consider is the length of the trailer. A 23 foot long boat would need a longer trailer than 25 foot. You have to consider turning radius and space at the tongue for things like a winch, bow stop, and a jack as well, especially for a boat this size.

A 23 foot long boat is decent in length so you might actually want to consider tandem axles instead of a single axle. Boats are heaver at the rear so a lot of the weight would be on that one axle. Two axles are going to reduce tire and suspension wear and provide more stability.

As far as capacity goes depending on how the boat is equipped and how heavy the trailer is a 3500 pound axle could work. Using two 3500 pound axles would likely be too much. Using two 2,200 pound axles for a capacity of 4,400 pounds seems like the sweet spot but again it depends on the weight of the boat and trailer. With the 2,200 pound axles you can use 15 inch wheels.

Based on your drawing half the width of the trailer is going to be 36 to 38 inches meaning the width of the frame will be about 6.3 feet wide or 76 inches. You will need to get an axle or axles with spring seat centers that are the same distance apart on center as the frame width outside to outside. (Although a torsion axle may not have spring seats so the center of the bracket it uses to mount to the bottoms of the frame rails would be the same thing.)

Then you will need to know the overhang amount needed. This is the distance from a spring or mounting seat center to the hub face, where the wheel attaches to the hub. The section width on a 225/75-15 tire is 8 inches. Assuming that the wheel has a zero offset (the mounting surface is dead center of the wheel) you would probably need around 7 to 8 inches from hub face to spring/mounting seat center.

So it is starting to sound like an axle 92 inches long at most (from hub face to hub face) will be needed.

Regarding the brakes it is going to depend ultimately on how many axles you go with and the gross trailer weight when loaded and ready to tow. If you go with a 3500 pound axle then you would want a set up to match but you don't want to go too high because the trailer may not have the momentum to actuate a heavier duty system if you go with surge brakes instead of electric over hydraulic.

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