Additional weight when setting an engine back???

[Treefish]

Mathematitian needed.

How much extra weight is added to a transom for each added inch of setback?  

Is there a calculation I can do to figure this out?

Why we're at it, does anyone know the calculation to do to figure out recommended horsepower for a boat?
Thanks,
Jim

[slippery73]

Mathematitian needed.

How much extra weight is added to a transom for each added inch of setback?  

Is there a calculation I can do to figure this out?

Why we're at it, does anyone know the calculation to do to figure out recommended horsepower for a boat?
Thanks,
Jim

You aren't technically adding any weight to the transom by moving the engine further back besides the weight of the bracket or other means to move engine rearward. You are shifting the fulcrum point of the boat however. I'm not sure of an exact calculation for this type of scenario since the boat is sitting in fluid and a lever calculation wouldn't be the same since there is no hard fulcrum. If you do plan on moving the engine back most people try to move the fuel tank forward to offset the weight shift. Not sure what to tell you other than that.

As far as the coast guard hp calculation goes here is a link explaining how you do it. http://newboatbuilders.com/pages/hp.html

[Treefish]

thanks for that link.  Length x Transom Width x 2)-90 = HP.  

I'd still appreciate some kind of calculation that explains how to calculate weight when setting an engine back off of the transom.

[gw204]

I believe the number you will end up with is actually torque (in/lbs in this case).  Torque = Force x Moment if I remember correctly.  Force is the weight of the engine and Moment is the length of the bracket....in a nutshell.  I'm sure it's more complicated than that due the dimension of the surface where the bracket mounts to the transom, but that's a start.

[Treefish]

I believe the number you will end up with is actually torque (in/lbs in this case).  Torque = Force x Moment if I remember correctly.  Force is the weight of the engine and Moment is the length of the bracket....in a nutshell.  I'm sure it's more complicated than that due the dimension of the surface where the bracket mounts to the transom, but that's a start.

GW, I used http://www.wikihow.com/Calculate-Center-of-Gravity, to calculate the approximate center of gravity (CG) of my hull.  I then used the same weights but changed the distance of the engine from the center of the boat and I came up with a rough estimate for how much weight is added onto my transom by setting an enging back 17". I used 17" because I really wanted a porta bracket.  

Here's a rundown of my calculations:
Weights: Seesaw or boat weight 1970#, kid 1 weight 470#, kid 2 0#(I used 0 because I'm not adding any weight to the bow.)  
I understand that the boat's weight is scattered randomly, unlike the seesaw's that is evenly distributed. Makes my estimate ROUGH.

The hull length is 234" (19'6").
In my case the centers of each object are: 0" at the transom (kid1/engine) and 234" at bow (kid 2).
Datum 1 is 0" since kid1/engine is centered at 0"
Datum 2 is 234" since kid2/nothing is centered at 234"

Moments:
0" x 470# = 0 in lb
117" x 1970# = 230490 in lb
234" x 0# = 0 in lb  
Total Moment: 0 + 230490 + 0 = 230490 in lb

Total weight:
1970# + 470# = 2440#

Point of CG = total moment / total weight:
230490 in lb / 2440# = 94.5"(from transom)

After assuming that my CG is at 94.5" I changed the datum of kid1/engine to negative(-)17" (to account for setback of a porta bracket). These calculations gave me: -17" x 470# = -7990 in lb
new total moment: 230490 - 7990 = 222500 in lb
new CG: 91.2"
Dividing the new CG into my original total moment gives me a new total weight: 230490 / 91.2" = 2527.3# **I think this is where my calculations possibly turn south, in turn yeilding an incorrect result since I'm not really adding any weight to the boat, rather just changing the Center of Gravity**
Subtracting old weight from new weight gives me the added weight by a 17" setback bracket: 2527.3# - 2440# = 87.3#

Assuming this is correct I'll be adding 87# to the transom if I setback a 470# rig 17".  I'll have to recalculate with more weight (porta brackets weigh more than I thought).

I'm sure my calculations aren't totally correct, but they seem legit.

any thoughts/corrections are greatly appreciated.

Jim

[fabuck71]

I always heard the simplest way is for every 6" you go back off the transom you add 1/2 the engine weight.  So if you go back 6" on a 200 lb motor, the actual weight would be 300 lbs.

[Capt. Bob]

I always heard the simplest way is for every 6" you go back off the transom you add 1/2 the engine weight.  So if you go back 6" on a 200 lb motor, the actual weight would be 300 lbs.

Whoa :o  
That would mean that I added 1001 lbs to my transom.  
They build them good in Hialeah.

[Capt. Bob]

Torque = Force x Moment if I remember correctly.  Force is the weight of the engine and Moment is the length of the bracket....in a nutshell.  

You are correct in that assumption, Jim.
And that leads to your recalulation of the CG. It's important to know for the prediction of how your hull will now handle as opposed to what the original designer had in mind. What is really happening (as your numbers show) is that the transom will now have forces acting on it that it may have never been designed for, by the engineer. This is the real issue here.

If we assume that the force (weight of the engine) acts downward on the transom (in reality it also cantilevers out over the top) the transom is resisting that force through compression. Since it also sticks out over the back, the transom must resist that twisting force(bottom pushed forward, top pulled rearward) also. By moving the motor back, you have greatly increased that twisting force on the transom (resisting the moment) by both the weight of the engine and the moment arm (bracket) times the increased offset distance.
So the really important factor IMHO is, will the transom resist the new forces acting on it. We haven't even touched the additional forces that act on all transoms generated by the running of the engine itself.
I've been pretty lucky in the fact that my stock transom has done just that. My engine resides a full 26" behind the transom so "knock on wood" literally.

 since I'm not really adding any weight to the boat, rather just changing the Center of Gravity

Right again, so now you can redistribute your loading to allow for that (and hopefully move the CG closer to its original position)  and help with the handling.
Nice website (CG) for dummies like me. Thanks and good luck.

Jim