1/4 sized - Plywood - Hull Model Testing

Hi There,


I'm planning on building and testing a 1/4 sized model of a 12' power boat.
The model, and ultimately the boat, will be built with sheet plywood.

The final 12ft boat will be powered by a 6hp outboard motor.

My basic plan is to 'videotape' the hull under different load conditions while I drag/tow it, behind another powerboat.

This is my first ever model test so I'm looking for tips and tricks to make the test meaningful.

Thanks advance for your kind suggestions

 

I will include a reply I put in a previous thread on the same topic that might be of interest.

The model performance of scale model boats is not as significant as you may like.

Interesting discussion including comments like "I have no objection to whatever any modeler wants to do with his boat or his clock. It's a hobby, and you are free to pursue it anyway you please. My only objection is when someone tries to take towtank procedures and prove that they are somehow "real" and should be followed by all "right-thinking" modelers Tow tank procedures were designed to deal with a specific problem, and that problem is only real to the towtank. btw, towtanks are not held in the high regard they were held 100 years ago ... "

at
http://www.rcgroups.com/forums/showthread.php?t=1113269

Based on my own experience - it may be just as easy and not much less expensive to build the full size boat at 12', especially as plywood that would suit a 1/4 scale model is usually quite a bit more expensive.

 

It's fun to build your own design.

If the path is your journey then enjoy, and accept the outcome, good or bad.

But if you're looking at the destination and a good outcome, then build a proven design.

 

To get anything meaningful from your observations of the model, you will need to load the model to scale weight. In this case you can take the cube of the scale factor 4^3 = 64 and apply it to the model weight. Example; If the full sized boat is estimated to displace 384 pounds when underway, then divide 384 by 64 to get 6 pounds model weight. 515/64 = 8.04 pounds ,etc............

rwatson has given you the straight dope about relying on model observation to predict full sized boat performance. There are many factors that influence performance that do not scale without a lot of complex number fiddling.

Do not let that discourage you. Build the model and see what it does. That will be a fun project regardless of the outcome.

 

Thanks your your reply Messabout. I think just messing about with the model will be fun.

 

What can you learn from a 48" model of a 12' boat' and more importantly what about this design is so questionable, as to warrant these tests?

 

That would be 36"

duh...? it is my first design. You tell me what I might learn/not learn.

Since posting, I found this...

http://marsh-design.com/?q=content/model-testing-almaguin-part-5-detailed-behaviour

The method and results seem reasonable. Do you have a different opinion?

 

Unless you are trying to achieve something special or significant, model testing isn't necessary. If looking for maximum efficiency, testing out odd configurations for comparative purposes or maybe just checking some speculative theory, then sure you might consider a model. Other wise, 12' plane mode, semi plane mode and displacement mode hull forms, are relatively easy to understand and acquire. What is the purpose of the tests? Nothing about Almaguin's hull form suggests it would behave outside of predictable patterns, so the tests were for demonstration I would think.

 

http://www.devlinboat.com/gallery/main.php?g2_itemId=4020

Disclaimer: I'm most definitely not picking on Sam.

The above is an exchange between Sam and a client (on Sam Devlin's web site) regarding a 16' Sam Devlin design.

Now, there is no way to know how much chop and wind there was on client's maiden voyage but Sam seemed to think some spay rails might help.

My question is, wouldn't model testing help determine if spay rails are needed, would be effective... under certain given conditions.

Regards

 

At your model size, you won't really notice any spray but rather a continuous, jelly-like wall of water climbing up the hull. It takes high speeds and sharp edges to break water into spray, pretty hard to scale down to a model size. So it will be difficult to get some conclusion about the spray behaviour from the model testing.

As for the rest,

I would suggest you to not tow it behind a powerboat, because it's wake and irregular prop wash will strongly influence your observations. I'd suggest you to rather tie it to a side-protruding pole, sufficiently long to avoid any interference with waves produced by the powerboat. In that way your scale model will run in calm water and it will also be easier for you to make photos/videos of the side view.

 

The only way you could model the effectiveness of spray reductions methods with a scale model would be to test the model in a liquid with scale properties equivalent to the scale you're working at. At the least this would mean scaling density and viscosity, which isn't practical.

It's the reason that scale model testing is very limited in application. As daiquiri rightly says, water will be not break away and form spray at scale dimensions and speed, it will tend to just stick to the model hull, as the Reynolds number will be massively different (for a quarter scale model you'll be running at an Re of around 1/4, far less if you scale speed down to get the right Froude number).

Re is key here, because it determines when flow trips from laminar (i.e. stuck to the moving surface) to turbulent. This happens around the critical Re, which is a function of the length of the surface and the velocity. To get the right Froude number you have to run the model more slowly than the full size boat, yet doing this means you can't get to the critical Re to create spray from the transition to fully turbulent flow.