trailer set up

[WillyC]

Hello guys,
I'm in need of some measurements for setting up a 20sf on a dual axle trailer. I am rebuilding an old double axle aluminum bunk trailer and will be remounting the whole axle and spring set up. What I am looking for is two measuremnts. One is from trailer hitch to the approximate center between the two axles. The second is the distance from the bow stop to the approximate center between the two axles. When I bought this boat and trailer it just seemed like there was way to much tongue weight. If anyone has a perfectly set up 20sf on a double axle trailer that could grab a few measurments for me I would greatly appreciate it. Thanks -will

[Fr. Frank]

Every boat and trailer setup is different. The are a number of factors to consider. But I'll bet we can get you close. I have set up dozens of trailers and boats over the years.
I have added photos for illustration.

Here are some preliminary considerations:[list][*]With a tandem axle trailer, you want your balance point to be very close to the front axle, but still in front of it. My trailer/boat setup is 3300 lbs. If I jackstand my front axle, I have exactly 160 lbs of tongue weight. My balance point is just 4" in front of the front axle. Very important to use a scale for this. Ideally, with tandem axles, you want 5-7% of your total towed-vehicle weight for tongue weight. I have 5%. (with single axles, you need 7-10% tongue weight.)



[*]You also want your towing angle to be level, or slightly negative (tongue downward) to minimize trailer sway at highway speeds. Hook the trailer to the tow vehicle, and set a level on the trailer frame for this. Choose a ball-mount with the appropriate rise or drop to get the right angle.



[*]You want the transom of the boat to be even with, or slightly forward of the back of the bunks. This gives the transom full support.



[*]You also want the bunks to extend forward enough that bunk extends beyond where the curve of the hull rises above the bunk. This prevents spot pressure on the hull at the bunk end. With mine, you can just get a pencil between the hull and bunk at the end.



[*]The bunk separation in width should be 3-4 inches wider that the distance between the outside of the inner hard-chine. You can use the outer hard-chine if you wish, but using the inner makes it easier to forklift the boat off the trailer if necessary. Close to the chine is also close to the stringer, and so stronger, and makes for better self-centering during drive-on loading.



[*]Make sure you have enough trailer tongue length to allow a 90 degree jackknife of trailer without the boat hitting the tow vehicle. This makes it easier to maneuver in tight spaces. I have 47" from coupler to forward point of winch stand. It also makes it easier to launch without getting your tow vehicle in the saltwater.




All of this should get you close.
If your axle position is fixed (welded hangers), you need to move the boat forward or back on the bunks to balance the trailer. Fit the bunks side to side first, though.

If you have a movable carriage for your axles, like me, get the boat situated right on the bunks side to side first, then front to back as described above, and only then worry about getting the tongue weight right. You can use four jackstands for this. Lower the tongue of the trailer as far as possible with the tongue jack, place jackstands under the aftermost section of the frame, near the after crossmember. Now raise the tongue jack to nearly lift the tires off the ground (but not quite). Place jackstands under the trailer frame near the forward crossmember. Loosen the carriage u-bolts that go over the frame. Very loose, but not removed is right. With a helper, roll the tires forward of backward to achieve new carriage position. The whole carriage will roll together. Measure the carriage position from the back of the trailer frame to make sure it's square. Tighten the u-bolts slightly, and then lower that trailer weight to the axles again. Measure the tongue weight. Repeat if necessary. When the tongue weight is right, retorque the u-bolts.

[65Bowrider]

Just wanted to say "thanks" for posting the info on the trailer set up. We just got a new-to-us dual axle alum trailer & setting our 21 on it. Good information to know.

[WillyC]

Fr. Frank,
Thank you very much for all the great info. Next week I will be setting up this trailer. My only hitch is that the moveable carrage that holds the springs has to be bolted to the I beam trailer. I guess I will just guestimate a good spot to bolt the carrage and adjust the boat forward or backward on the bunks to get the tongue weight right. Thanks again. Your insight and pictures will really make my job easier. -will