Calculating Wt & Bal on a new restoration boat

[ChuckCallahan]

Restoring an 18 ft Seacraft and trying to determine the boat's arm for CG purposes. The boat is empty of gear. Everything installed, minus the engine, basically as it would come off the assembly line. The weight I can get. If the datum is say the tip of the bow, how would I determine the boat's arm?

[mhd]

Restoring an 18 ft Seacraft and trying to determine the boat's arm for CG purposes. The boat is empty of gear. Everything installed, minus the engine, basically as it would come off the assembly line. The weight I can get. If the datum is say the tip of the bow, how would I determine the boat's arm?

You're looking for the righting arm to calculate GM? I'm assuming you don't have the lines of the boat to do the full calculation? This link gives you a quick summary of what I think you are looking for and how to get it via an inclination experiment.

https://www.practical-sailor.com/safety ... lity-check

You could also contat and ask Seacraft and explain why you need it - might be easier!

Good luck.

Mick

[ChuckCallahan]

Hey Mick

Thanks but this is for a 1975 Seacraft 18ft center console.

I am curious about how to measure/determine the arm for the boat as I described it.

Thanks

[mhd]

Hi,

Okay, sorry - didn't explain properly - but same thing applies to all hulls - sail boats and power boats. There are only two ways that I know to get the righting arm GZ:

1. Use the lines of the boat and calculate long-hand. If you don't have the lines, then they have to be taken from the hull via measurement. I did this for my own build - the displacement I calculated was less than 1% different than specified by the designer based on his designed waterline (DWL. A naval architect or someone else could do the same for your boat. (Divide the boat up into stations, measure the lines, calculate displacement, etc.). It takes time. I'm currently in the middle of generating the full stability curve for my boat. And that takes even longer.

2. Put the boat in the water and perform an incline test.

Sadly, I'm not aware of any other method. The inclining test is used for large newly built boats/ships to check they match up to the designer's calculations of GZ, but the same test can be used for any boat when the original lines are not available for any hull.

There is more discussion here:

https://www.boatdesign.net/threads/info ... est.50964/

I'm no expert. If anyone knows an easier method I'd love to here. Maybe something simpler would satisfy the CG?

Sorry I can't help more.

Mick

[ChuckCallahan]

Mick

That is a lot of useful information. Thanks for taking the time and sharing it with me.

[fallguy1000]

For a center console; you only need lcb and lcg, unless you plan a high tuna tower or something wild like that.

And LCB and LCG must be the same for the boat to trim level at static rest.

Most planing vessels are designed for them to be about 1/3rd of the way from the stern, I believe with the engine. But this is a crude generalization.

Righting arms are not generally given much consideration for a vee hull. Okay, maybe a well engineered boat subject to CG review for retail sale.


But lcg is real important. Finding the boat's center of gravity can be done by making a 2x4 jig with feet. Keep the boat well supported and move the jig until the boat finds stasis on the 1.5" timbers.

Then weigh the trailer at a commercial scale and weigh the trailer and boat.

Then determine the distance and mass of the engine you plan and calculate that moment and adjust with math to find the lcg with engine.

Then all additional moment arms work to about there.

The designer may have used half a fuel tank in the calcs as well or other lightship values.

But all you need to do is keep offsetting.

Or you can also just use 1/3rd from the stern.

Another way would be to measure and layout the waterplane on paper. The original lcb was the original lcg, with some certainty. Only some go fast boats are not on design lines at static rest afaik. Where 50% of the area forward equals 50% of the area aft is lcb.

[ChuckCallahan]

Fallguy1000,

This is the information I was looking for, thank you! I am leaning towards the 6 ft rule of thumb you describe, but would you have more info on the jig you mention? Being involved in aviation over the years, weight and balance were ingrained as to its importance for safety and performance. The boat is at the stage where the final placement of under-deck hardware, storage locker, fuel, and then water tank could be tweaked. Then above deck, the console's placement.

Regards

[mhd]

Ah, now I see what you were looking for. Thanks FG. I learned something also.

[fallguy1000]

You'd make a plywood or timber jig.


start by laying two long timbers like 2x4 or 2x8s parallel to the right side up boat and even with dwl or so on each side...

Then screw another 2x across those two. These are the feet.

Then you need some uprights. They will be screwed to the crossmember and will go up to the hull or just under at again dwl. Maybe 4 of them.

Then a couple more 2x4s will go under the hull and be screwed to the uprights. You will want to tie them together under the vee somehow.

If this is not clear, let me know.