It's the water........and a lot more

[Dr. Jet]

Some time ago, I took my Key West up to Legg Lake. Prior to that, I had it running really well at Santee. It was loose and “flew” quite well. At Legg, it was just a dog. I was completely embarrassed by the way it ran. I had to skulk home with my head held down in shame.

What’s up with that? I know the guys that run IC engines have to worry about mixture control at different altitudes, whereas electrics don’t have that density altitude problem when producing power. So why did my KW run like crap at Legg?

Some discussion around this topic started on the RRR forum, and I thought I would bring that topic here as well.

The density (weight per volume) of a fluid is described by its specific gravity. Pure water has a specific gravity of 1.0000 at 60º F (by definition). The specific gravity of a fluid is determined by dividing the weight of a unit volume of that fluid by the weight of the same unit volume of pure water. Any change in temperature or composition of the water will change the specific gravity of that sample. Everybody knows that salt water has a lower specific gravity than does pure water. Could it be that the quality of water in different ponds results in markedly different performance results with the same setup? I have heard people say that at “X” pond, you need to prop down to keep temperatures in line, or that at “Y” pond the water was “faster”. Some ponds have a lot of suspended silt that may increase the density of the water (the specific gravity). Some may have soap or other chemical residue that may impact the way water “sticks to” or “slides off” of the boat’s planning surfaces (surface tension).

I found a website that sells highly accurate hydrometers (devices for measuring the specific gravity of a liquid). I may invest in a couple of these and see if there are significant variations in different ponds, and if these variations can be used to predict performance based on the specific gravity of the water in a particular pond.

For example: Your dialed-in setup runs an X642 prop with temperatures at 130º in your pond that has an S.G. of 1.0500. You travel to a neighboring pond that has an S.G. of 0.9500. Based on your research, you know that you can push a bigger prop in the less dense water, so you put on the X645. A quick test run confirms temperatures are “in the green arc”.

Voilá, you have a new tool for dialing-in your setups at different locations, just like your trusty thermometer.

[Steven Vaccaro]

Interesting topic. Not sure if it means anything, but my carburated full size boat ran much better in salt than the lake. I always thought it was the salt water, the cooler water temperature or the water conditions.

With rc boats in different lakes and ponds I have noticed the same things you are noticing. If you figure out some testing methods you would like to use I will be in for some testing. Let me know?

[Ub Hauled]

Doc, that's a great thing if one can proof it... I heard many people say that this pond is faster then that pond... it'd be nice to see numbers on that. Lemme know if I can help, I live 20 minutes from Legg Lake.

[Dr. Jet]

Ub,

I'm going to get two to start with. One that goes from about .94000 to 1.0100, and another that goes from 1.0000 to something like 1.0700. Once I determine the S.G. of Santee (I may have to get another higher or lower one if I'm not within range), I can send them to you for a reading at Legg. This woud be the first step to see if there is any appreciable difference in the two bodies of water.

This could start a number of comparisons. Don't most full-sized competition boats have different props/setups for fresh water versus saltwater? This specific gravity testing may be yet another tool for the fiercely competetive guys to stay one step ahead of the competition.

I would think the best way to collect data would be to take one particular hull, equipped with data recorders and gps, to several different bodies of water; then record all the relevant data. Nothing on the boat would be changed as it goes from one pond to another. The same prop and angles. Compare speed, current draw, temperatures, and rpm, to see if you can find any correlation between or more of those values and the pond's S.G.

We can postulate all day long on weather or not it will or will not have an impact, we should really do some testing.

Who here goes to MANY different ponds over a short period of time?

[Ub Hauled]

Ok.. we can start by measuring the waters and then go for a small boat from one pond to another with the same setup. So far we know that denser water has more bite then less dense water, for obvious reasons, I guess we need to determine if there is difference enough when the difference is in the "millies", right?
I usually am at Legg... maybe this coming year I'll be going to the winter warm ups, not sure yet.

[Dr. Jet]

This is pure conjecture on my part, and something I wanted to avoid, but I suspect the biggest correlation will be the "millies" and everything associated with millies: Denser water = More bite. More bite = Less slip = More speed.
Denser water = More bite = More force = More power = More amps = More heat.
Denser water = More drag = Less speed

[Bill-SOCAL]

Now I have two threads to keep track of. So let's start off with a few corrections:

The density (weight per volume) of a fluid is described by its specific gravity.

Not quite. Density is a physical parameter of all materials. It is expressed in units of mass per unit volume. Grams per cubic centimeter are the usual IU (International Units) mode of expressing density.

Pure water at 20 deg. C has a density of 1 gram per cubic centimeter (1 gr/cm**3).
Saltwater has a density of 1.025 gr/cm**3.
Saltwater is denser than fresh water.
Fresh water will "float" on saltwater.

We see this phenomenon when dealing with saltwater intrusion into fresh water aquifers in coastal ares like in Orange County.

Specific Gravity, which is unitless, is a comparison of the density of one material relative to water. It is a ratio obtained by dividing the density of the material by the density of water under the same temperature & pressure conditions.

So for instance, fresh water has a SG of 1 since 1 divided by 1 is 1.

Saltwater would have a SG = 1.025/1.00 = 1.025.

Specific gravity numbers greater than 1 indicate materials that are "heavier" than water. Numbers less than 1 are lighter than water. Gasoline has a SG of ~ 0.739 or so, it is "lighter" than water and will float.

http://en.wikipedia.org/wiki/Specific_gravity

http://www.engineeringtoolbox.com/specific-gravity-liquids-d_336.html

In general the range of densities and SG that we deal with in the water we run is likely not that important.

Now, relative to our interests, the viscosity of the water is far more important than the density or SG. Viscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or extensional stress. In general terms it is the resistance of a liquid to flow, or its "thickness". Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction.

http://en.wikipedia.org/wiki/Viscosity

Viscosity is not directly related to density.

But we have an additional problem, and that is our size, which affects the Reynolds Numbers for our craft. Due to our size, the effective density and viscosity are much greater than they would be for a full size vessel. This effect is most dramatically seen with the bumble bee. The only way a bee can fly is due to the fact that its small size makes it like it is flying in a material as thick as water. Just like we cannot float on air, but we can float on water, and float easier on saltwater.

This is why boats are faster in saltwater. Despite being denser than fresh water the boat "floats" higher, with less resistance to movement. This is not really a factor in big boats, but is very apparent in smaller boats.

What I am getting at is this is by no means an easy thing to characterize and it is certainly far more complex than the simple SG of the water.

[Bill-SOCAL]

To continue on. As I said, viscosity is a far more important parameter. It effects the prop and also the surface tension that shows up as friction against the hull.

I think "faster" water is water where the viscosity is lower than "slow" water which would have a higher viscosity. Some of the things we have talked about would potentially effect this.

Colder water is less viscous than warmer water. It is also less dense so it may be a wash when it comes to the effects on the speed of a boat. Less dense water will allow the hull to sit deeper, and therefore have more friction.

What we need to understand are the effects of varying viscosity and density (SG will be helpful here) on how out boats run.

Things like dissolved mineral content, total dissolved solids, and suspended solids will cause differences in both the density and viscosity of the water.

We see it more directly than our IC cousins since we have a more sensitive tool, the amp draw and resultant heat. But even they struggle with this since they all know that props perform differently at different lakes. Through trial and error most learn which props work best under what conditions and go from there.

In the end, I am not sure that we will ever be able to have a measurement that will allow us to predict how the boat will run on any given pond under a given set of conditions.

Fun to think about however.

[Rex R]

ah bill, about your hot v cold cold water is denser, cold s/w is denser yet thats why freighters have various markings for summer/winter, fresh/sw. cool sw you have less hull in the water too.

[marker]

The maximum density of water is at 3.98 degrees C below that it expands again, ie ice floats.
Mark

[Bill-SOCAL]

I guess I should have been more specific, I was talking in the larger trend. Also making the point that the density variation is not as critical.

For instance here are the densities of water in grams per cubic centimeter at various temperatures in degrees Fahrenheit:

80 - 0.9966
70 - 0.9979
60 - 0.9990
50 - 0.9997
40 - 0.9999

The density variation over a span of 40 degrees is 0.0033 g/cm**3 or about 0.3%. I have a hard time thinking that this small of a variation is significant for our purposes.

Looking at viscosity over the same range it varies from 0.000798 @ 80 deg. to 0.00131 at 50 degrees. So in a 30 degree change in temperature we'll get nearly an order of magnitude (10 fold) change in the viscosity.

Again, for our purposes I think it is clear that viscosity is the more important variable. It is also clear that viscosity is not dependent upon the change in density since density changes only 0.33% over a range of temperatures in which viscosity changes by an order of magnitude.

Think of it this way. If you had a $100 to start a 0.33% change would drop that $100 to $99.67. While if it were viscosity over the same range $100 would become $10. So the same variable, temperature, causes a significant change in viscosity and a relatively small change in density.

And viscosity is really the thing that determines how "sticky" the water is.

Ultimately we really need to step back and obtain actual data as opposed to subjective observations about the relative "speediness" of the water. I am not sure how that could even be done and get some kind of reliable data set if it was collected under real world conditions. We need a Ph.D. candidate looking for a cool project to look at this problem. Dr. Michael Selig did it for model aircraft airfoils and his work on Reynolds numbers and efficiency, so why not model boats??

And ultimately is it significant enough to worry about under race conditions?

Beats me.

[properchopper]

Great thread !

I'm glad to see some informed discussion of this ; It's probably been all of our minds from time to time. And while waiting for my parts to fix last week's stupidity, what with holiday shipping delays, I'll jump in at great risk of being flamed by the better informed [ I'd be an Engineer today, except for two things : Math & Science ].
I believe it boils down to two interrelated factors : propeller traction and hull resistance [ coefficient of kinetic friction ?].
I think that we can all agree that propeller design and configuration has the most potential effect on overall speed [ given equal hull parameters, of course]. The most pitch and least cavitation will result in the most forward movement [ again given equal prop speed.] The differences in water resistance to the prop will obviously create the differences under discussion here. Changing the prop configuration is the simple fix here, of course keeping amp draw in mind. O.K., so I've just stated the obvious, so I'll go to the next consideration : Hull resistance.
Here's where any "lab" talk may fail in the real world. Consider the ever-changing "contact patch" between the hull and water. Instantaneously changing in size and speed [ does water have the same speed/frictional effect over flat or curved surfaces {{ any airfoil designers here, Bill ? }}]. Is this frictional resistance a linear function relative to speed ?If one could actually calculate the net hull/water contact cumulatively during a heat race, I believe there would be a huge difference between two similar hulls on the race course. The line around the course with other boats on the same course would also determine how much hull is immersed or airborne as well ! Prepping the hull bottom for minimum frictional resistance has always been the simple, practical answer to this. Scuffing has been promoted by many as the answer to this. Chris Fine told me some time ago to push my mono across the pool & see how far it drifted, then spray the bottom with WD 40 and observe a further coast-distance. My landlady came out and chased me away before I could finish this one ! I decided that I'd have better sucess if I sprayed WD-40 on my driving ability and that's that.
Alright I've wasted enough of your time; Where's that Damn UPS truck !!

[10gauge]

Formula For Calculating Speed - revisited

It's all about fluid mechanics or dynamics and viscosity. Very good points have been made by everyone!

Dr. Jet, specific gravity is a creative way, but rather difficult to ensure accuracy to 4 significant digits or the nearest +/- 0.0001 as is necessary for the relatively small fluctuations that we're dealing with. Very precise analytical techniques are required.

You're also right Bill, water viscosity relates very closely to RC boats. The frictional characteristics of hull and prop design are all dynamics that will influence speed through the water medium. Yes, it is a form of the Reynolds number that can be applied to most liquids and their viscosity be it water or air. Changes in physical parameters - temperature, S.G., ionic content, can amount to significant changes in speed in RC boats as it would for airplanes. However, size matters in that the larger the vessel or airplane, more frictional surface, larger the Reynold's number, and hence more measurable difference in speed.

In most cases, physical parameters are always changing by the hour, so ensuring accuracy would be difficult at best by any method.

One could also do quantitative measurements with test boats under variety of conditions and apply derivations of classical mechanics such as F=ma, F=mv*2, and F=kx, where F, m, a, and v are all measurable quantities. One could then derive the frictional coefficient of a particular body of water under certain temperatures, ionic content, and or S.G. Again, accuracy could be difficult to achieve due to inconsistencies in the test boat - fluctuations in power, speed, ripples, etc. This frictional coefficient will be different for varying temperatures, ionic content, etc., but it could be ultimately applied as a factor to calculate speed (v), or the coefficient could be used to determine how positively or negatively the speed will deviate from a previously known standard condition.

I posted a thread on how to calculate speed of LiPo powered catamarans in Feb. 2008. I used similar quantitative methods and basic classical mechanics by correlating prop pitch, rpm, and a constant that I derived by averaging the results of many actual setups of a particular hull design (catamarans). It was fairly accurate for most setups as was confirmed by Jay Turner, ReddyWatts. Overall, the thread didn't generate much interest, so I never posted my calculations for monos, and riggers. However, measuring and finding a frictional coefficient for a particular body of water under certain conditions, and adding it to same equation might yield a valid result.

http://forums.offshoreelectrics.com/...ead.php?t=2695

Here's a mathematical formula I derived in Feb 2008 to calculate speed (mph) for a cat only. Basically, it gives you an average figure for speed, based on a collection of 30 actual catamaran setups I've come across online. It takes into account the prop, kv, volts, rpm. This formula works for LiPo power only. Differences in prop slippage, friction losses, cat hull design, weight, electrical/mechanical issues are all moot since this is not a theoretical equation. This is a statistical equation based on samples under their actual running conditions. Here is the formula for catamarans:

mph = [Prop Pitch] x [Prop dia] x [kv motor] x [LiPo Volts] x 0.000687
or
mph = [Prop Pitch] x [Prop dia] x [Prop RPM] x 0.000687


In any case, my guess is that the fluctuations in fluid dynamics of a particular body of water might be within the margin of error, so the argument of varying temperature, ionic content, S.G., viscosity might be a moot point after all...

[Steven Vaccaro]

WOW, my head has spun more times than Linda Blair's!!!

But somehow I understand, thanks for putting this stuff in layman's terms.

Tony its Saturday, don't wait to long for the UPS truck, I think he may not show until Monday.

[10gauge]

Properchopper, I've been writing my thread for the past hour, inspired by all the discussion. I didn't see your post until after I posted mine. I see that you also are also proposing a coefficient of friction theory to this discussion... I just wanted to properly acknowledge your input to the discussion too.