Design project - model tow test

[Brands01]

I have been working through a design for a 10 metre displacement powerboat. Here is an earlier thread on the subject ...

http://www.boatdesign.net/forums/showthread.php?t=14836

(the boat has grown by a metre since I started the project)

A friend and I made a model of the hull to 1:16 scale from model aircraft plywood - primarily to check that the panel developments from FREE!Ship worked properly. This model is 62.5cm LOA.

We decided on the scale of 1:16 so we could tow test it as well.

The designed displacement is 5.9 tons. We loaded up the model with 1.25 kgs of sand to bring it down to its waterline (at this scale, I believe equivalent of 5.12 tons) and towed it behind a dinghy at various speeds.

Here are a few pictures of the test. (NB: the waterline on this model is painted a little too high at the stern)

http://www.boatdesign.net/gallery/data/500/medium/at_speed.jpg

http://www.boatdesign.net/gallery/data/500/medium/side_view.jpg

http://www.boatdesign.net/gallery/data/500/from_stern.jpg

The simulated speed of the model is 4X the actual speed of the towing vehicle at this scale.

It was fun to tow the model and weigh it down to its lines, but I don't know enough to recognise if there is anything to be learned from its bow wave or wake.

Someone suggested to me that I should disrupt the waterflow at the bow with a few pins to stop the water sticking unrealistically to the side of the hull as it moves through the water. They also suggested that the towing point should be at the point of propulsion (ie, the location of the prop) rather than the bow.

Any comments or thoughts on the test, the photos or accuracy of the comments made to me would be greatly appreciated.

[Leo Lazauskas]

I have been working through a design for a 10 metre displacement powerboat. Here is an earlier thread on the subject ...

http://www.boatdesign.net/forums/showthread.php?t=14836

(the boat has grown by a metre since I started the project)

A friend and I made a model of the hull to 1:16 scale from model aircraft plywood - primarily to check that the panel developments from FREE!Ship worked properly. This model is 62.5cm LOA.

We decided on the scale of 1:16 so we could tow test it as well.

The designed displacement is 5.9 tons. We loaded up the model with 1.25 kgs of sand to bring it down to its waterline (at this scale, I believe equivalent of 5.12 tons) and towed it behind a dinghy at various speeds.

SNIP

It was fun to tow the model and weigh it down to its lines, but I don't know enough to recognise if there is anything to be learned from its bow wave or wake.

Someone suggested to me that I should disrupt the waterflow at the bow with a few pins to stop the water sticking unrealistically to the side of the hull as it moves through the water. They also suggested that the towing point should be at the point of propulsion (ie, the location of the prop) rather than the bow.

Any comments or thoughts on the test, the photos or accuracy of the comments made to me would be greatly appreciated.

Sorry to be the bearer of bad news but it is very
unlikely that you will find much of hydrodynamic
significance from these tests.
For hulls shorter than about 2m surface tension
effects are important as are laminar effects
near the bow.

The suggestion to add pins (or some other turbulence
promotion devices) is fraught with difficulty.
Bertorello et al found differences as large as 18%
in the residuary resistance between 1.8m and 4.7m
hulls at low Froude numbers. At Froude numbers greater
than 0.6 the differences were less.

"A small model (L<2m) seems inadequate for the model
ship correlation because of the appreciable influence
of laminar flow on the total resistance with the bare
hull and of the uncertainty about the value of the
parasitic drag due to the applied device in the tests
with the turbulence stimulators".

Remember that these tests were conducted under
laboratory conditions. Your tests are likely to be much
less reliable. And you haven't said anything about
scaling the propulsor which adds many further
uncertainties.

"From model scale to full size. Investigation
on turbulence stimulation in resistance model tests
of high speed craft",
Bertorello, C., Bruzzone, D., Caldarella, S.,
Cassella, P. and Zotti, I.,
FAST 2003, Ischia, Italy, 2003.

All the best,
Leo.

[Brands01]

Thanks Leo,

I expected no more - certainly this was a very uncontrolled "experiment", and you've removed any doubt about its usefulness (or lack thereof!).

It was still useful to build the model to check the plywood plates developed from FREE!Ship had no compound curvature, not to mention an excuse to get out on the water and play with a toy boat :)

[yipster]

the good news is you can use the freeship file in michlet
nice to see a model build from the plates

[LP]

Here is a link to someone elses tow testing. Thought you might find it interesting if you haven't read it before.

http://bluejacketboats.com/designing_liz.htm

[alpamis34]

Almost In all professional towing tanks,the thrusting arm is located at a point close to amidships'axis.From whichever point you tow it, the Correction Factor has to be applied unless you do it from its real propulsion point.

[Brands01]

I never did figure out how to use michelet, and I thought my design would be too beamy.