Estimating required propulsive power - How do you estimate power requirements?

[mermaid]

Smaller scale nukes and most WWII boats are relatively easy to power because the props are small end the speeds relatively low. How does one estimate the propulsive power requirements for a large Nuke? Case in point. Say you want to build a 1/41 scale long hulled Permit. The hull is about 87" x 9.3". The prop is about 4.4" in diameter. Scaled displacements (meaning the weight of the sub plus all the water contained in the wet hull) are 130 lb surfaced and 145 lb submerged. The scaled power @ 15,000 shp FS comes to 0.218 shp. (seems way too high) Scaled top speed @ 30 knots FS comes to a disappointing 0.732 knots or 1.23 ft/sec. Certainly, a slow walking speed does not reflect the speed and power of an SSN! So, one might want the model to achieve about 3 to 3.5 mph at full throttle. How does one estimate how much shp will be required?

[feet wet]

Have you remembered that while

Have you remembered that while scaling all the propulsive numbers , we continue to operate our boats in non-scale full density medium? Take that into account and up your power and see where it brings you.
Cheers

[mermaid]

Dear Feet Wet]http://www.subcommittee.com/forum/icon_question.gif



Edited By Mermaid

Dear Feet Wet]http://www.subcommittee.com/forum/icon_question.gif[/img]



Edited By Mermaid on 1148837373

[warpatroller]

Scaling the horsepower of real

Scaling the horsepower of real boat down to the model size does give you a figure that is very high in relation to the model's size and in most cases even unachievably high as a motor large enough to generate that power wouldn't even fit inside the model.

From what I've seen for advertised speeds of large sub kits out there, they end up having a top speed of 3.5 to 6.5 mph. I personally probably wouldn't even want a pocket sub to be slower in top speed than 1.5 mph (which is still a lot faster than the scale speed if just dividing the real boat's figure by the scale of the model).

Feet Wet is right about the water density and such being the same as what the real boat travels in of course. That is another reason why model boats NEVER look like real boats when cruising on the surface. A model sub would have to be going EXTREMELY fast to produce the white froth around it and trail behind it like a real boat does. The exception to this would be if the model was so large that it was the size of a jet ski or something, and would still have to be moving at a good pace. A typical model sub makes a wake similar to a swimming duck...not realistic at all. This is the one aspect of model submarines I don't like...no white froth and spray.

Steve

[carcharadon]

“The hull is about 87"

“The hull is about 87" x 9.3". The prop is about 4.4" in diameter. Scaled displacements (meaning the weight of the sub plus all the water contained in the wet hull) are 130 lb surfaced and 145 lb submerged.”

Not sure what you mean by scaled displacement. Displacement is not the weight of the Sub, or the internal weight with water.

Displacement is the weight of the water that the shape of the hull displaces when submerged.

It doesn’t matter what the sub is made of, it has nothing to do with displacement weight, if the weight of the sub, however, exceeds the displacement weight, it sinks.


I would think that a hull 87" x 9.3" would displace more than 145 pounds of water.

you might be interested

http://groups.yahoo.com/group/vulcania_submarine/

discussing the same, might have to go back a few weeks

[mermaid]

Thanks Warpatroler and Carcharadon.


Thanks Warpatroler and Carcharadon.
To obtain scaled power for a 1/41 scale model, I divided the FS shp of 15,000 hp by 41 cubed. Likewise, the surfaced and dived displacements (in long tons) were converted to lbs and similarly divided by the same factor. I am aware of the definition of displacement. I have found, however, that displacement combined with loaded WL length are parameters in some calculations in that they help define the wetted area of the hull and hence the frictional and hydrodynamic drag. This in turn can be factored in to estimate the required shp for a given speed. Unfortunately, the equations that work for small surface craft are next to impossible to apply to a model sub. I too would wish for some impressive wakes. In a reasonable size, I fear it is not to be. I find larger models fascinating because there is so much more operational potential. They are much more prototypical in operation and can incorporate many features that are near impossible in smaller scale models.



Edited By Mermaid on 1148874851

[carcharadon]

The following data points were

The following data points were developed in an effort to come up with some reasonable estimates as to what Pat Regan could expect in speed from the adaptation of a jet ski pump to his fantastic functional Nautilus replica.

For anyone interested in these experiments or how a model sub could relate to a real sub here is the link,

http://groups.yahoo.com/group/vulcania_submarine/

As best as I can come up with I've managed to generate two data points (that I'm reasonably comfortable with). You could plot these on graph paper using thrust/displacement weight ratio as the X axis, and speed in mph as the y-axis. This curve should be the same for any size Nautilus.

Note here, as was pointed out in our discussion, scale speed, is a parameter for visual effect. Hence real speed is used in the experiments without regard to “visual effect”. Understand visual effect is important say for example in a movie replica for visual effect. However in trying to determine real thrust versus speed, scale is not important.

Note also, that the Goff Nautilus was a design on paper and replicated as a whole model only in the 11 foot hero model (or a few others) which was not a functional model. It was supported by cable for underwater scenes in the movie. There never was a functional model until some RC enthusiasts put one together years ago but long after the movie.

I doubt that anyone with a Nautilus has had as much field experience as I have. Those of you familiar with my efforts have seen plenty of videos. I’ve seen a few videos and even a few web sites on the Engel buildup and others. These are nice models. However any contribution to Nautilus Hydrodynamics is pretty limited except to mention the tendency of the shape to nosedive.

Back to the task at hand, relating model numbers to a real sub, (real as in capable of transporting the person).

Using this curve, the resistance or drag from the water should be constant and proportional for any size Nautilus and therefore this curve would take into account the shape of the Nautilus and the resistance in water from this particular shape. From this curve, given any thrust, you should be able to predict speed for any size Nautilus. Scale not important since use of the ratio of thrust to displacement wt equalizes all models. (Particle size of water molecule relative to scale considered ineffectual)

The sub (7 foot model) is calculated to weigh 246 pounds as displacement weight of fresh water.

Thrust/Displacement Wt ratio 7.5/246 = .03 = 4mph

Thrust/Displacement Wt ratio 1.5/246= .006=2.23mph

4 mph at 7.5 pounds pull a .03 (ratio) = 4mph

2.23 mph at 1.5 pounds a .006 (ratio) = 2.23mph

so when

X= .03, Y= 4mph

when

X= .006, Y= 2.23mph


The following are test videos used to develop the two data points. Detailed information and an explanation of each video can be found at Pat’s site. The tendency for the sub shape to nosedive is apparent in the first video. For the second video, to better calculate a speed additional buoyancy was added. This kept the sub up slightly longer. Even with additional buoyancy this shape sub, still, has a propensity to nosedive. However speed could be reliably estimated at 4 mph at 7.5 lbs thrust (conservatively).

http://home.comcast.net/~gustofur/May20aa.wmv

http://home.comcast.net/~max0007/May20c.wmv

Between the two sizes of the JetNautilus, the 7 foot and a 4 foot I have probably tested these versions hundreds of times.

So here’s what I have to say about the Nautilus. Or for that matter any functional Goff Nautilus. It takes approximately 5 times the energy to double the speed. I believe this is an accurate representation of the Goff Nautilus shape and how it responds Hydrodynamically. I have no idea how this applies to any other sub shape since all the information I have, comes from a reasonable facsimile of the Goff Nautilus in the shape of the JetNautilus, and attempts to relate this to a functional 18 foot replica.

What was most surprising to me is that at the lower speeds the impression is one of sleek responsiveness through the water.

At the higher speeds you might as well be pushing a box through the water.

Also, speed is deceptive, estimates are easy to toss around, but unless you actually measure, so-called estimates are just that. I think there’s a tendency to exaggerate.

[chips]

Scaled top speed @ 30

Scaled top speed @ 30 knots FS comes to a disappointing 0.732 knots or 1.23 ft/sec.

It looks like you divided the prototype design speed by the cube of the scale. The formula for model speed for scale wake is proto design speed divided by the square root of the scale; in this case a speed of 4.7 mph. 30 knots/6.403 = 4.7mph.

[mermaid]

Dear Carcharadon / Chips
It would

Dear Carcharadon / Chips
It would seem to me that the relationship between displacement, speed and thrust (required power) would be unique to each hull design. In your case, you have data for the Jules Vern's Nautilus at various scales. The Permit hulls would have a different drag/speed coefficient. To generate a curve relating displacement, thrust (power) and speed, it would be necessary to build and experiment with a minimum of two models at different scales (assuming that the prototype is the third point). It would be much easier to build the 1/41 scale hull and measure the hydrodynamic drag directly at various speeds. By the way, static thrust is roughly 60 lb/hp.
The scale top speed (assuming that the prototype was capable of 30 knots) is 30/41= 0.731707 knots = 0.842034 mph = 1.234983 ft/sec. I am not aware of the scale wake formula Chips. Please explain. Are you saying that the model will do 4.7 mph at the scale 0.22 hp ?



Edited By Mermaid on 1149012752

[mermaid]

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