And theoretically you can find the COG of the boat by pushing it back on the trailer 1 inch at a time until the tongue weight is at or near 0. Then the COG of boat is basically right on top of axle (or a little behind assuming trailer COG is in front of its own axle).
In reply to Duke :
It doesn't work for two arbitrary points; d1 in Sparkydog's drawing is specifically the distance from the axle to the center of gravity.
I'll borrow his sketch to illustrate. We'll have two trailers, each with an 8' distance from axle to tongue, each with a 1,000 lb tongue weight.
Trailer 1:
Trailer 2:
The upshot is that we get the same tongue weight AND the same weight at other locations than the tongue for a system whose mass and CoG together provide a given tongue weight.
In reply to Ransom :
Ah, thank you for the explanation. It's been 30 years since my last statics courses, and my structural courses didn't really get into CG etc because it isn't excessively relevant in structural engineering.
Well said! Since the OP's original goal was to estimate total weight using tongue weight, if the CG of the boat+trailer is directly over the axle centerline then you don't have a tongue weight and therefore can't estimate anything.
Put another way - if all someone is trying to do is get a somewhat reliable DIY weight then they should intentionally shift their boat far enough forward on the trailer so that there's a definite distance separating axle CL and CG location. The larger the value of d1 the more accurate the tongue weight technique will work.
Shifting the boat will help you get its CG, but not the trailer's. That will still give an incorrect weight for the whole system.
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