Calculating Boom Modulus...

[davef]

Thanks so much for looking! I really appreciate it.

I have long been suspect of my righting moment which seems too high but I cannot find an error.

I did not create a stability curve. I generated the righting moment by computing the moment with the boat heeled at 30 degrees. Chapter 10 of Principles of Yacht Design (pg 183)


You can see the attached to follow my equations. I can convert to metric if that would help.


“The starting point when dimensioning the rig is to calculate the righting moment. It is commonly agreed that a heel angle of 30 degrees is a good design angle”

Using my CAD program I rotated the hull by 30 degrees and then moved the water line until I found where the boat would trim out at 30 degrees. From there I can compute the heeled center of buoyancy (as the CG of the area displaced by the boat heeled at 30 degrees I assume).

From there I calculate the transverse righting moment as being the ship displacement X the distance to that center of buoyancy. In my calculations, this is RM30.

I then added the righting moment due to the passengers by multiplying their weight (3 x 175 lbs) by the distance between the passengers and the heeled center of buoyancy.

If it would help, I could can and send you copies of the section of the book I used for this calculation.

Do you see a mistake?

Is there a better way to create the righting moment?

Thanks once again!

Dave

[davef]

Thanks so much for looking! I really appreciate it.

I have long been suspect of my righting moment which seems too high but I cannot find an error.

I did not create a stability curve. I generated the righting moment by computing the moment with the boat heeled at 30 degrees. Chapter 10 of Principles of Yacht Design (pg 183)

“The starting point when dimensioning the rig is to calculate the righting moment. It is commonly agreed that a heel angle of 30 degrees is a good design angle”

Using my CAD program I rotated the hull by 30 degrees and then moved the water line until I found where the boat would trim out at 30 degrees. From there I can compute the heeled center of buoyancy (as the CG of the area displaced by the boat heeled at 30 degrees I assume).

From there I calculate the transverse righting moment as being the ship displacement X the distance to that center of buoyancy. In my calculations, this is RM30.

I then added the righting moment due to the passengers by multiplying their weight (3 x 175 lbs) by the distance between the passengers and the heeled center of buoyancy.

If it would help, I could can and send you copies of the section of the book I used for this calculation.

Do you see a mistake?

Is there a better way to create the righting moment?

Thanks once again!

Dave

[Dutch Peter]

Originally Posted by davef

Using my CAD program I rotated the hull by 30 degrees and then moved the water line until I found where the boat would trim out at 30 degrees. From there I can compute the heeled center of buoyancy (as the CG of the area displaced by the boat heeled at 30 degrees I assume).

From there I calculate the transverse righting moment as being the ship displacement X the distance to that center of buoyancy. In my calculations, this is RM30.

Very well done!

Originally Posted by davef

I then added the righting moment due to the passengers by multiplying their weight (3 x 175 lbs) by the distance between the passengers and the heeled center of buoyancy.

This is the mistake! You can add the weight of the crew to determine the Centre of Gravity of the boat+crew, but place their weight on the centre line of the vessel!

Originally Posted by davef

If it would help, I could can and send you copies of the section of the book I used for this calculation.

No need, I have it myself! But thanks.

Good luck with recalculating, I'll post a formula from an other book tonight.