We can't scale the viscosity of water, but the displacement directly follows a scale^3 rule. 8000lbs of mass => 8lbs at one-tenth scale.
Andy
Although your quite right in regards to displacement in scale i can't argue to facts....
That is only based of the model being made of the same material as the full size, which is why i stated that a model will behave differently in terms of displacement.
A model made of
scaled ''2.5mm'' thick steel ( from 2.5cm" full size) compared to being made of lighter materials such as 2.5mm thick wood will naturally displace less... which is why ballast is added later which only then the displacement becomes true to scale with less needed for a heavier boat compared to a lighter machine where more is needed.
To which is why i made my original statement.
In this case, with the construction of this vessel in order to hold more weight, a reduction of dead weight ballast can be used allowing the model to hold more 'cargo' physical weight, as dead weight ballast can be substituted for weight in cargo.
This however dose leave an issues when using the model unloaded, to which suggestions can be made to counter, such as water tight chambers which could be flooded when the model is 'empty'.
This dose not however resolve issues of the counter balance when cargo is loaded as weight will be unevenly distributed, but i do believe that with a bit of fiddle enough ballast could be fixed in place as ''trim weight'' for when the vessel is loaded.
Tweaking the waterline is also another trick to help in allowing for more cargo, although with this project not necessarily suitable given the fronts ramp which for many landing craft models can be a bit of a headache keeping water from flooding in should it sit lower in the water.
The argument is pedantic but its still valid. As for motors, props and shafts...Shafts are simple enough, i wouldn't use anything less then an M4 shaft, the length is determined by the space you have in the working section of your boat. so it needs to be long enough to exit the hull correctly, and hold a coupling of a length (again variable depending on the coupling), and the motor itself, so it all fits within the working space.The prop should be if keeping to scale one of similar pitch and blade amount, which is why i suggested using an M4 shaft as prop ranges are higher. However thats not always right for a model the prop is dictated by the boat, you may not get a 40mm 3 blade prop running at a set RPM from the motor to drive the boat adequately, to which you will have to 'experiment' which may be looking at things as 2 blade plain 40 or X 40 or larger sizes. ( i used 40mm props here as an example only) The prop physical size is determined by the room under the hull, if you only have 15mm of clearance between shaft center and the base of the hull then you can only use a maximum of a 30mm prop ! if you have 25mm from shaft center then you can fit up to 50mm prop.Prop thrust is also dictated by the motor RPM. different props act differently even if the RPM is the same between an 3 blade and a 2 blade which is mostly the blade pitch & blade size. ( i kept it simple here someone else can fill in the boring science) So selecting the right motor and RPM is critical. 16,000 speed boat rpm is not necessarily going to thrust your model to hell, but then again 16,000 rpm maybe what you need to get the model of that size moving at a scale speed.
Until we know exactly what size prop you can fit and if your planning to keep with a scale version or the originals a motor suggestion at this point can only be pure guess work.
At a
very rough guess i would think a standard 540 size motor ( 1 per shaft ) would be adequate running on either a 2 or 3s lipo,( upping the voltage from 7.4 to 11.1 will increase the motors rpm)or a 6 or 12V Lead acid battery ( which can be used more for all day runs due to their MHA size and their weight can be used as part of the dead weight ballast.
Run off a Mtronics 15amp esc (one per motor) or should there be no rudders for this model tank steering would be needed to control the model then i would suggest ACTions P94 unit would be perfect for the job and possibly the ACTion's power distribution board?, google '' The component shop'' for those.
Whoo fricking essay!