Effective weight vs distance to the point of attachment of the weight

[MrNewton]

Hi,

I want to know how to calculate the effective weight if i have an object with the weight 'X' that is mounted from the side to a wall, and what happens with the weight if i increase the distance between the wall and the object.

The reason i ask is as follows for the interested.
I have a boat with outboard motor. Now i have ordered a jackplate. This is a bracket that mounts between the outboard motor and the boat (also called a setback or spacer). In other words, instead of being directly mounted on the boat, the motor moves further back from the boat (6 inches to be precise).

I want to know how this setback affects the pulling force on the boat and to calculate the effective weight of the motor when it is mounted on a setback 6 inches from the boat. What would the equivalent weight be approx. of the weight would be if the mass is directly mounted on the boat.

I know the weight of the motor stays the same offcourse and it is the torque that changes. But an object that is further away from the body is heavier then a object that is close to the body.

 

[Baluncore]

There will be a slight increase in torque or moment. It will be dependent on the distance of the motor from the centre of buoyancy of the hull.

When you introduce the spacer, the boat will drop at the stern, while it rises at the bow. The neutral point, that remains the same, is the centre of buoyancy.

 

[sophiecentaur]

Now i have ordered a jackplate.

What was your reason for adding the jackplate? Something to do with water flow to the propellor around the hull? Are the faces of the jackplate parallel (i.e. is there any change of angle of thrust from the screw?). Everything can make a difference and mods need to be justified (apart from the salesman's pitch)

 

[MrNewton]

A slight increase? My body saus That the increase in torque / force on the transom is going to be massive when adding a setback

But does anyone know what the equivalent Weight woud be? The reason i ask is because i know Thath with my old motor (140kg, directly mounted on the boat) i had aboat 5cm left before water woud entertainment. And now with a 6 inch setback i woud like to know the equivalent Weight so i can make a guess how far it Will sink in the water

 

[Lnewqban]

The equivalent weight you are looking for should be the current actual weight of the engine times the percentage increase of the distance of the actual engine center of gravity to the center of buoyancy (please, see post #2 above).
The weight of the other new elements should be added up as well, of course.

 

[sophiecentaur]

so i can make a guess how far it Will sink in the water

The amount it will sink will be governed by the shape of the hull near the stern. If the hull is wide and flat then extra downward force on the stern will make very little difference. If the hull is deep and narrow, it will sink a lot further before it displaces enough water to balance the added load. In fact, hull shape is everything if the boat is stationary. (Just think how safe a Dory or Catamaran hull is when you step on the side; a few cm of depression will cause loads of upthrust right where it's needed.)

I would suggest an experiment (but I nearly always do!). Just walk to the stern from amidships and see how the level changes. That's around 90kg, moving about.

PS you still haven't explained your reason for the jacmplate.

 

[MrNewton]

Thanks again for the responses!

The centre of buoncy is approx. 140 cm (1.4m) front the rear.
@sophiecentaur
That is exactly what i did. Last summer i stond with a Friend on the rear of the boat. Together we weigh aboat160 kg. That was right under the limit before the rear would go under.
That was with my old motor

Now i have a New motor, which weighs 30 -40kg more, and a jackplate that weighs 30kg. Plus the motor Will be 3xtra heavy because of the increases distantie.

The boat is relatively wide, 1.60m, as the chines but also had a sharp v.

 

[sophiecentaur]

That was right under the limit before the rear would go under.

Doesn't that imply that your added load will sink the boat? It seems your experiment was telling you something.

 

[Baluncore]

That is exactly what i did. Last summer i stond with a Friend on the rear of the boat. Together we weigh aboat160 kg. That was right under the limit before the rear would go under.
That was with my old motor

Can you confirm that the test was done while the old motor was also mounted on the boat?

That conflicts with this statement:

The reason i ask is because i know Thath with my old motor (140kg, directly mounted on the boat) i had aboat 5cm left before water woud entertainment.

Is the 5 cm margin measured while unspecified people are in the boat?
Depending on how I interpret the orthography, that 5 cm margin is unsafe. Increasing the weight of the motor by 40 kg, or introducing a 150 mm spacer, will each push the margin further from the safe operational envelope.

We are still missing any certainty of the distance from the transom to the centre of mass of the motor. I guess we are expected to assume zero for the old motor, since there is probably a motor tilt axis nearby.

 

[sophiecentaur]

that 5 cm margin is unsafe.

It's a very calm day when 5cm of chop is not present. There must be some extra 'experimental' factors that the OP hasn't counted into his evidence. It all sounds very marginal, the way he describes the situation. Of course, once the boat is under way there will probably be a lot more freeboard around the stern - so it'll be ok as long as he's on the move.

 

[MrNewton]

Correct, test was done with old motor and NO jackplate (so 60kg in Total lighter)

See here a image of the boat with old motor. I taken back my 5cm comment. Its more like 12 - 15cm

The distance from the COF of the motor to the planet where its mounted is approx 30 - 40 cm

Again, we can stand on the rear with 2 persons (approx. 160kg) and it would be fine
My theory is that i have 60kg more weight, + an extra "40 kg" because of the distantie of the motor (but that's the question i asked here). So that would be 100kg more at the end of the transom. But that's all based on assumings.

 

[sophiecentaur]

an extra "40 kg" because of the distantie of the motor

I think that can't be an accurate model. You're assuming that there's an actual fulcrum somewhere but as you push the hull deeper in, any 'effective' fulcrum will move backwards and the buoyancy force from the back of the hull being immersed will act nearer and nearer to the motor.

There's one possible source of reliable information for you and that would be the boat designer / maker. There is bound to be information about maximum safe motor weight - or even (the other way round) a list of hulls that your new motor is suited to.