Russian Alfa Class

[He Who Shall Not Be Named]

Can anyone who has set up a boat tell us if this sound right.

I sliced the model at the waterline and got a volume of 5.782 cu in (94747.16 cu mm). This does not include the rudder or bulkheads.

Randy

Include ALL structures above water line open to sea.

David

[RanSan]

Can anyone who has set up a boat tell us if this sound right.

I sliced the model at the waterline and got a volume of 5.782 cu in (94747.16 cu mm). This does not include the rudder or bulkheads.

Randy

[Subculture]

You don't need to guess anything.

Displacement is key when sizing ballast tanks, not weight. Water weighs 1kg-1000g for every 1L-1000ml of displacement, and will vary by less than 1% depending on temperature and mineral content.

We size ballast tanks based on the amount of water displaced. Most unreinforced plastics e.g. ABS, polystyrene, pvc tend to have the same mass as water, but filled plastics and composites like glassfibre reinforced hulls are denser. If you use weight then you end up with a tank which is too big. PLA is a pretty dense thermoplastic.

With conventional precast hulls, we don't tend to know the volume above the waterline, and with hand laid fibreglass, they can vary from hull to hull, so it's best to use empirical methods to find out the hull displacement. The method for doing that is straightforward, trim the boat for neutral buoyancy (or close to it), then use foam blocks to bring the boat to the required waterline. The volume of foam blocks will equal the displacement of the hull- or close enough, the weight of the foam itself skews things a little.

With a 3d printed file we have no such worries, you can split the boat at the intended waterline in CAD, then check the properties to find out the volume of the structure above the waterline- easy peasy lemon squeezy .

[type7]

Remember, you want the top of the cylinder under the water line or it is working against you instead of helping. Also, it is nice to have room around it for foam at the water line.

[trout]

Will what you are lifting out of the water weigh more than 200 grams? Metric is so cool for us here, 1cc of water equals 1 gram of weight. 200 grams is around 7 ounces.

[rwtdiver]