Trailer Tongue weight Loading Calculations

pcarew

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Oct 13, 2015
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I wanted to figure out the correct maths based calculations for adjusting my tongue weight.
I have a trailer that has an axle in a fixed position, so moving the boat is my option.

After deriving the various formulas, I thought that others might find this useful.

There have been many posts about various guidelines and methods for adjusting tongue weight, but I hadn't seen the mathematics anywhere in one place, so I thought that I would add it here.

The salient formula is: d = twm * TD / BW This gives the amount of distance to shift the boat weight to obtain the desired tongue weight change.

'd' is the distance that the boat must be moved to change the tongue weight by the desired amount
'twm' is the amount of tongue weight to be added or removed (moved from/to the wheels)
'TD' is the total distance between the tongue and the wheels
'BW' is the weight of the boat that will be moved to achieve the tongue weight.

If you would like to see the derivation of this and other formulas, see below....

Paul.



Trailer Calculations


A trailer of course is a 'balanced lever'.

Picture of balanced lever model:
<---------------------------------TD-------------------------------->
<---------- x -------------------><------------ y ---------------->
A================CG================B <--- Balanced Lever (trailer)


A = Weight at point A (the Tongue Weight)
B = Weight at point B (the Wheels)
CG = Centre of Gravity
BW = Boat Weight portion of total weight (this is the movable weight)
W = Weight of Trailer and boat
TD = Total distance between A and B (x+y)

General Centre of gravity formulas

Center of Gravity = Total Moments / W
If CG Datum is assumed to be at point A, then
CG = (A *0 + B*TD) / W
CG = B * TD / W

Change in CG due to weight shift (cargo moved),

Wm = Weight moved
d = Distance weight moved
CGm= Distance CG moved
W = Total weight

CGm = Wm * d / W --- (1)

Calculating distance to shift cargo to achieve desired tongue weight

IF one wishes to move/transfer some tongue weight (point A) to the wheels (point B)
1st Calculate what the shift in CG would be for that movement and then calculate how much the boat would need to move to generate that CG shift
IE, CG change due to to tongue weight moved = CG change due to Boat Weight moved

twm = amount of weight to shift from tongue to wheels
bwm = Boat Weight moved

CGtwm = twm * TD / W
(CG movement due to tongue weight movement)

CGbwm = BW * d / W
(CG movement due to boat weight movement)

therefore as we're saying that we will move the boat to achieve the desired CG movement.....
twm * TD / W = BW * d / W

'W' cancels out from both sides leaving
twm * TD = BW * d

Rearrange to get 'd' which is how much we need to move the boat.

d = twm * TD / BW (3) The amount of distance to shift the boat weight to obtain the desired tongue weight change.


Trailer weighing (Lever) formulas


===================^====== <-- ( Balanced Lever)
A (scales) <------- x -------> <-- y -->B

'^' The point to be weighed (Fulcrum)

To determine either tongue or wheels weight, a lever can be used if direct measurement (ie by bathroom scales) is not possible
W = A + B
(i) (Total weight = weight at point A + weight at point B. For this calculation, A and B are the two endpoints of the lever to be used)

A*x = B * y
(ii) Balanced Lever moments,
Weight at point A times the distance to the fulcrum = weight at point B times it's distance from the fulcrum

B=A * x / y
(iii) from (ii)

W = A + (A * x / y)
sub iii into i and then group the 'A' together

W = A(1 + x / y) ---- (2) Can be used for calculating a "Point Weight" using a lever.
If 'y' is made to be 1 foot then the calculation is easy (1+ 'x' feet) times the weight read at point A
 
Last edited:

smokeonthewater

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The fly in your ointment is that you should not be moving the boat at all to adjust tongue weight... You should be moving the axle(s).
 

pcarew

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I have a 'Sunfish' boat trailer that has a fixed axle point, moving the wheels is not possible.
 

smokeonthewater

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Oh it's possible.... Just not easy.

The boat should be positioned on the trailer correctly to properly support it and then tongue weight adjusted via axle location.

If yours is welded in place then cutting and welding are in order which, yeah, stinks for the average joe.
 

robert graham

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Supposed to be about 10% of boat/trailer weight....maybe get a scale and check tongue weight.....you're probably close to correct now, and it ain't brain surgery....
 

Fed

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Because of your interest pcarew here's a little thing I found on the net years ago to work out your boat only weight.
You can weigh the boat with a regular household scale using simple math and physics.

1. First put the scale under the tongue with the boat pulled all the way up on the trailer. For me the scale read 150lbs so W1=150

2. scoot the boat back on the trailer EXACTLY 10" so 10=X

3. reweight the tongue. For me it was 100lbs so W2=100

4. measure the distance in inches from the axle to the tip of the tongue. For me it was 180". so 180=C

5. Plug all 4 of your numbers into the equation. C(W1-W2)/X


Mine looked like this... 180(150-100)/10

6. do the equation
a. first you subtract 100 from 150 which = 50
b. then you multiply 50 by 180 which= 9000
c. finally you divide 9000 by 10 which = 900

So my boat weighs 900 lbs. It will only take you about 5-10 minutes to do.

I also found a site where a guy suggested you could find your total rig weight (BMT) by measuring the footprint area of the tyres then calculating off the tyre PSI.
Probably work best with half flat tyres rather than flat or rock hard. ;)
 

thumpar

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It doesn't have to be that complicated. The tongue weight should be about 10-15% of total weight. I took calc in highschool and found that sometimes it is easier to do and try than to work on the problem all day and get it wrong.
 

batman99

Petty Officer 1st Class
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Sep 13, 2012
Messages
393
I agree as well. Way too much math. And, way too much over thinking.

May I suggest one "average loads" their boat & trailer as if going on a towing trip. If their boat trailer weighs 500 lbs and their "guesstimate" loaded boat weighs 2,500 lbs, then the combo is 3,000 lbs. Again. This is simple math. Try to lift the trailer's tongue. If it weighs around 300 lbs (or a little more), then connect to the Tow Vehicle. Tow average loaded combo to nearest weight scales (like hiway scale or gravel pit scale) and professional weigh it. Based on real weight numbers, make adjustments were needed. For example, move more heavy weight stuff (like loaded portable gas tanks) to front of boat. Or, perhaps to the rear of boat - if trailer tongue is too heavy. For my boat trailer, I simply move its bow stop beam forward or backwards - to change weigh position on the trailer. Its axle and steel fenders remain fixed - to the boat's frame. If one moves the boat all the way forward (as much as possible) and still don't have 10%-13% tongue weight, then its the wrong trailer (for the boat). Get a larger size trailer. This works for me. And, it's that simple.
 
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pcarew

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Because of your interest pcarew here's a little thing I found on the net years ago to work out your boat only weight.


I also found a site where a guy suggested you could find your total rig weight (BMT) by measuring the footprint area of the tyres then calculating off the tyre PSI.
Probably work best with half flat tyres rather than flat or rock hard. ;)


Thanks for posting. I hadn't seen those calc. before.
P.
 

Silvertip

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Sep 22, 2003
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Or if you uncouple the trailer and the toungue launches you into the air you know there is negative tongue weight.
 

bruceb58

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You are also missing an important variable. A sunfish might be a uniform weight but a boat with an engine is not. A power boat with an engine that weighs exactly the same as a sailboat will need the axle further back.
 

WIMUSKY

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Good on the OP. But if I had to sit and figure all that out I'd never make it to the lake...
 
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