You get one of these tandem sliders to match the length of your springs, either 20" or 25 1/4" springs.
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http://www.sturdybuiltonline.com/Boa...gs_p_1413.html
Plus don't get hung up on the number of leafs in the spring. For example the 1750# 25 1/4" spring has four leafs or three, the 2000# has three and the 2500# has four. Buy the capacity you need. A 14" "C" rated tire has a 1760# capacity and if you match that to the 1750# springs you are at a tandem trailer with a 7000# capacity. More than enough for your boat with added safety built in, that was how my original trailer was for my 23'er. Good luck!