Scale down weight to test a model

Thanks for all the help. The river boat is a centre console and I am trying to determine where to place the centre console to balance out the weight a 250 lb 50 hp outboard motor

 

The model will be a great way to do it. Gravity scales !!

Petrol, Water, gear and passengers are important too.

You have no idea how hard it is to buy a scale passenger that articulates properly to test leg room and seating positions.

 

If it is a boat that is going to be used on smooth water, place your console well forward. Planing boats will tolerate weight forward quite well, as a rule. Reason being that the leading edge is where dynamic lift is greatest, shift weight forward and the leading edge goes forward too. Think of the "hang ten" move on the malibu board.

 

The simple calculation for scale weight is to use the cube of the scale as a divisor for the weight of the full sized boat. In this case a quarter scale boat uses the cube of four = 64. DIvide 1200 by 64 to get 18.75 pounds for the model. This does not take into consideration the relative positions of the various weights but it is sufficiently accurate for the total displacement figure.

Not to start a hassle here but.........Tansl, help me argue with RWatson about scale speed. I have thought that scale speed was a function of the square of the scale. That makes it an exponential, not a linear, calculation. Perhaps this applies only at displacement velocities? Or not at all? ???

 

Messabout - you may not have spotted the distinction I was trying carefully to make.

There is a difference between the pure, mathematical scale speed formulae

and

the formulae for trying to establish the performance of a model compared to a full size craft.

Velocity is linear because it is 1 dimensional. Weight and volume are cubed because they occupy 3 dimensions.

 

messabout, I'd like to help but it is clear that we lost the game.

 

Worthwhile reading the entire thread which rwatson referenced. Turns out the post he choose to partially quote is related to scaling speed for "realistic appearing" operation, not about scaling speed for equivalent hydrodynamics.

Speed for similar waves (deep water) scales with the square root of the length. This is represented by the Froude number.

Speed for similar viscous phenomena such as boundary layers scales inversely with the length. This is represented by the Reynolds number.

The result is that there is combination of speed thatr two geometrically similar boats of different sizes will have the exact same combinations of waves and viscous phenomena. "Froude's hypothesis" is the basis of the usual ways to deal with this in tow tank testing.

 

Area scales as length squared.

Volume scales as length cubed.

The volume a hull displaces is the volume of the hull below the waterline. Displaced volume equals weight of the boat divided by density of the water. So if both the full size and model boats will operate in fresh water with the same density, the weight scales as the length cubed.

 

In relation to the OP's boat, it is all a bit academic, because in such a boat the potential alterations in weight distribution are such that a minor discrepancy in where the static waterline is, can easily be adjusted for. The proportion of the total mass, loaded, that is free to be moved, could be 50% or more. Doesn't really matter if it sits slightly off-level, empty.

 

Excellent discussion and the timing couldn't be better. I am interested in building a 1/10 scale version of my dream for testing in the local lake.

I'm sorry to say I am still a bit lost on model speed. If I want to replicate the performance of a boat going 12 knots how fast should a 1/10 scale model go?

By replicate performance, I will be measuring and logging exactly how much power is being used at all speeds for the model. I would be nice if this could provide an estimate of full size power requirements. I am concerned it is a waste of time because a model props performance won't scale.

So the question is will the data collected from a model be of any use at all?

I have resistance data from Maxsurf. Any opinions on which is a more accurate representation of power required, model testing or Maxsurf?

 

haha - dont we love typos

 

Will you be measuring and logging the electric power going into the motor? Without very careful and accurate calibration of the electric motor and propeller it will be a waste of time to try to get a better estimate of power than a good empirical method will provide.

A 1/10 scale model in a tow tank would be run at 3.79 knots to correspond to a full size speed of 12 knots. Then calculations would be made to convert the measured resistance to an estimate of equivalent full size resistance. Look for references on tow tank testing for more information on the calculations.

 

What typo are you referring to?

 

Rather than go to all the trouble of building and trialling models, it might be reasonable to put the lines of a known, established similar design into the program, and compare what it calculates for that, with your design. Then look at the power and performance figures published for the established design.

 

In a few threads on this site, it discusses the need to do models to at least 1:5 scale for any useful data.

speedestimate=modelspeed*sqrt(modelscale)

Probably not.

Maxsurf will provide much better info. Mr E 's suggestion of looking at similar boats is the way to go.

You may find this thread helpfull

http://www.boatdesign.net/forums/boat-design/scale-model-power-calcs-26955.html