Upgrading From A Single Axle To A Tandem Axle Trailer

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Question:

Hello, I have quite a dilemma. I bought a used boat that came with a new custom aluminum I-beam trailer for the boat. But the owner of the boat underestimated the weight of the boat and had a single axle 3000lb max capacity trailer built for it, thinking the boat only weighed 2500lbs. When I recived the boat, I took boat and trailer to a weight station and it turns out that the empty boat with no gasoline or gear, along with the trailer weigh in at 3600lbs. I know by the vin sticker that the empty weight of the trailer is 600lbs. So that puts the boat at 3000lbs and the trailer at its max GVW of 3600lbs on an empty boat...... not good. Trailer manufacturer is in Washington state, I am in Texas so I cant just take it back to them. Next, I know that the frame used by the trailer manufacturer for the 3000lb model is also used buy them for the 4000lb model. So, I would like add an axle, to have the safety and redundancy of a tandem axle trailer and upgrade my 3000lb trailer to a 4000-4400lb trailer by doing so. Where I need help is when it comes to the formulas to determine how far apart each axle should be from each other and how far forward the front axle should be based on the length of the trailer and lenth and weight of the boat. So that 1 it will be leveled out correctly and pull correctly. And 2 I can have proper tounge weight. The total length of the trailer is 24 feet. Length of the boat is 20 feet. The total length of the rear square box section before the I-Beam frame starts to bend into the tounge is 65 inches. Of which I can only use 57 inches to place the tandem axles. That would place the rear edge of the rear tire 9 inches forward from the rear end of the trailer. It would place the front edge of the front tire 2 inches past the frame bend. With 3 inches being the space between both tandem tires. With this tire position along the square box of the frame, all the axle mounting points for both axles would still remain within the square box section of the trailer and be behind the bend on the frame ensuring that my axles ride square to the trailer and tow vehicle. This set up would place the center of the space between both tandem tires, 6 inches forward from where the center of my now single hub is. Would this work? Would it just mean that I would have to move the boat forward to maintain proper tounge weight? Or should I just upgrade to single 5200lb 6 lug axle and forget the tandem upgrade? Thank you for your help! Sorry for the long message. Just need help deciding between a tandem or single upgrade. Trying to be safe and responsible with the weight Ill be towing down the road. Thank you

asked by: Jerry

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Expert Reply:

First of all I'm sorry to hear the inconvenience you're going through here. A new trailer designed to accommodate a boat your size is really your best solution and is my recommendation.

To add a second axle to an existing single-axle trailer you could do so however the the frame of the trailer, coupler, suspension, etc all have individual weight ratings and you have to go with the lowest rated component as the overall weight capacity. The attached help article provides a solid starting point should you decide to this route.

To get your trailer to it's proper tongue weight, you need mock up everything and weigh the tongue, making adustments unti you get the proper tongue weight. You want it to fall between 10 and 15 percent of the total trailer's weight, and probably closer to the 15 percent since it's a boat trailer and the boat is heaver at the rear because of the engine.

Another consideration here is that if your trailer frame has a 4,000lb capacity and you're boat and trailer are already 3,600lbs this sounds like a lot of effort when 400lbs of additional weight can be pretty quickly exceeded in a boat.

The 5,200lb axle is the better option than adding an additional axle however it may not address your capacity issue either since again, it's just the axle but not all the other components being upgraded that are limited by the lowest rated component. We have parts you would need should you decide to go this route instead.

expert reply by: Patrick R

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Upgrading From A Single Axle To A Tandem Axle Trailer

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