Water Viscosity Effects on Boats

[David Santistevan]

A few weeks ago my idle mind was trying to solve a puzzle for one of my fellow boaters. He had and FE Tunnel and FE Rigger that he ran in a race at the end of March. During the race both boats melted power leads to the motors. Now this was a warm day but nothing like we have during the summer so I am not sure that the ambient temp was a large factor. To add to the mystery these are setups he ran all summer last year with no problem. As I thought about it, it began to occur to me that water viscosity may be the culprit. Here in Colorado when we start racing in the spring the water is about 50 degrees F, in the dead of summer water temps at the lake are around 70 degrees F. This made a lot of sense to me since a prop running through more viscous water would be much more efficient (less slippage) and therefore create more load on the motor. This also made sense to me since several other boaters (at the same race) were complaining that their engines seemed to be loaded and not pulling the prop. In doing some research I found that the difference in water viscosity from 50 degree F to 70 degrees F was about 30% (see that attached chart). So it stood to reason that the prop was somewhere around 30% more efficient. As I continued my research it appears that is not the only factor. Because the water is now more viscous it also generates +/- 30% more drag on the hull as it moves through or on the water. Just reasoning this out, another drop of 20 degrees F and the water is now a solid (as we know all too well here in Colorado).

It stands to reason then that when we are running in colder water we have to take into account the added viscosity. This would suggest that we would have to "prop down". Although we would be pulling a smaller prop the prop would be more efficient. This is also a big consideration for the nitro and gas boats in maintaining the proper RPM for the pipe's power band.

I have search the forums and Internet at large looking for more information about model boating and this dynamic but cannot find much. Am I missing something here? It would appear to me that water temp is a huge variable to consider with the boats. I usually carry an IR thermometer to the lake and have been checking the temps so I know my lake temps are good.

D

Water Viscosity by Temprature.pdf

 

[Zack matson]

Sounds logical from a scientific standpoint . Just like cold air is far more dense than warm air .

 

[MrHammerHead]

Interesting subject. I have never thought about temperature affecting water viscosity but it seems feasible. I do know that sediment in the water can have a huge effect on performance and loads. I have seen this at time trial events where we ran one day and then it rained and washed a lot of soil into the lake. Running the exact same setups as the day before I saw as much as a 4mph slow down in speed. At the SAW in Valdosta Ga last year the water was very murky when it is usually very clear. The water was very slow and draggy.

If an FE setup is already on the edge amp draw wise bad water quality and warmer temperatures meaning less effective cooling can put them over that edge.

Chris

 

[MrHammerHead]

Interesting subject. I have never thought about temperature affecting water viscosity but it seems feasible. I do know that sediment in the water can have a huge effect on performance and loads. I have seen this at time trial events where we ran one day and then it rained and washed a lot of soil into the lake. Running the exact same setups as the day before I saw as much as a 4mph slow down in speed. At the SAW in Valdosta Ga last year the water was very murky when it is usually very clear. The water was very slow and draggy.

If an FE setup is already on the edge amp draw wise bad water quality and warmer temperatures meaning less effective cooling can put them over that edge.

Chris

 

[Ronald van Essen]

Don't know if this is relevant information but I tell it anyhow:

In the 1990's when I did a lot of waterskiing, slalom, I noticed the differences in watertemperature, water depth and different lakes/other water. The waterski behaved different in different conditions.

So it is possible that waterviscosity has affect on our boats.

 

[David Santistevan]

I have done some more research and this is a fact. There are some other factors because of fluid dynamics drag on liquids is different from drag on solids. I will have more to follow on this. It appears that the hydros would see much less effects from hull drag increase but they would still see the increase in prop efficiency. There are documented data in performance of crew boats and sailing boats where they talk specifically about the effects of temp on speed. More dense water molicules (colder water) will move slower. Because of fluid dynamics the boat actually drags water along with it as it travels through the water. If the molicules are moving more slowly because of water density and viscosity then 1) the boat will be slower, 2) the drag will create additional load on the drive system.

D

 

[Robert Elder]

Thanks for sharing your research, David. Very interesting.

 

[Martin Hamilton]

I know that years ago we use to run on salt water so our boats were set up on our lake, if we went to different lakes to race & they were fresh water the boats seem to be running slower but reving higher as if underpropped, probably due to more slip in the fresh water.

 

[Joe Wiebelhaus]

Living in Florida and Michigan, all my boats ran different from north to south, after running and setting up for Florida, I had to make strut adjustments on most my hulls, not to mention change props, I had a hop in my sport 40's and scale boats , this had me confused, so I think temperature and density of the water has allot to do with it.

 

[Andy Greene]

Did you notice the speeds where faster or slower down here Joe ? I would think we have heavier water ?

Andy

 

[John Knight]

Did you notice the speeds where faster or slower down here Joe ? I would think we have heavier water ?

Andy

Having raced several times at Brandon, I know it is slower, due to the amount of sediment, than what we have in Charleston. Our lake is so clear grass grows on the bottom more than 20' deep. Makes it easier to find boats and boat parts on the bottom here too!

Heavy or thick water, that which has a lot of sediment in it, tends to be slower for the boats as they are having to push much harder to get through the dense water. Essentially it is a higher and denser water composition. When I get to a lake to run (not my own) the first thing I do is to look closely at the water makeup and measure the water temp. Those 2 things can tell you a lot of what to expect for the race.

One thing to remember is to flush out the water jackets and water systems well or deposits can form and block the flow of water.

 

[Andy Greene]

I'm sure Gabe would agree John. We ran Brandon today and it was pea soup for sure.

 

[Dan Kramer]

jim irwin and i discussed this a long time ago, i think the technical term is "tuberity" ?? of the water. the crispness of the water.

i guess it might be better to explain it in beer terms .

clean, clear crisp mountain water,

never yukky, sludgy, murky , stagnent pond water <_<

hope this helps!!

dan.

 

[Joe Warren]

A few weeks ago my idle mind was trying to solve a puzzle for one of my fellow boaters. He had and FE Tunnel and FE Rigger that he ran in a race at the end of March. During the race both boats melted power leads to the motors. Now this was a warm day but nothing like we have during the summer so I am not sure that the ambient temp was a large factor. To add to the mystery these are setups he ran all summer last year with no problem. As I thought about it, it began to occur to me that water viscosity may be the culprit. Here in Colorado when we start racing in the spring the water is about 50 degrees F, in the dead of summer water temps at the lake are around 70 degrees F. This made a lot of sense to me since a prop running through more viscous water would be much more efficient (less slippage) and therefore create more load on the motor. This also made sense to me since several other boaters (at the same race) were complaining that their engines seemed to be loaded and not pulling the prop. In doing some research I found that the difference in water viscosity from 50 degree F to 70 degrees F was about 30% (see that attached chart). So it stood to reason that the prop was somewhere around 30% more efficient. As I continued my research it appears that is not the only factor. Because the water is now more viscous it also generates +/- 30% more drag on the hull as it moves through or on the water. Just reasoning this out, another drop of 20 degrees F and the water is now a solid (as we know all too well here in Colorado).

It stands to reason then that when we are running in colder water we have to take into account the added viscosity. This would suggest that we would have to "prop down". Although we would be pulling a smaller prop the prop would be more efficient. This is also a big consideration for the nitro and gas boats in maintaining the proper RPM for the pipe's power band.

I have search the forums and Internet at large looking for more information about model boating and this dynamic but cannot find much. Am I missing something here? It would appear to me that water temp is a huge variable to consider with the boats. I usually carry an IR thermometer to the lake and have been checking the temps so I know my lake temps are good.

D

This is Correct and it very much applied to the Evansville Indiana Race we just had......... Now add BAD Air (Hot & Dry) and a density meter reading of 90. Now it gets interesting......

 

[Ron Olson]

Turbidity is probably the word you're looking for Dan. It seems like that could be anything that is mixed in with the water from salt to algae or sediment. Water has to have a density from steam to ice so there must be a lot of densities in between those two as temperatures change and content of what is floating in it.

It seems like at one time Andy Brown covered this a little.

 

[lohring]

Skin friction drag is a function of the drag coefficient times the dynamic pressure. In water the dynamic pressure is pretty close to 1 times the speed (in ft/sec) squared. It will be a little less for fresh water. The drag coefficient depends on the Reynolds number which is strongly influenced by viscosity. In water the viscosity changes a lot with temperature. There is a transition from laminar flow to turbulent flow in the range of Reynolds numbers between 100,000 and 10,000,000. This varies with surface and test conditions, but usually happens around a Reynolds number of 1,000,000. The drag coefficient (therefore the drag) can double when the flow becomes turbulent in this area. Let's pick a speed of 60 mph (88 ft/sec or 100 kph) and a wetted length of one inch (25 mm). At 40 degrees F (4 C) the Reynolds number will be about 400,000. At 75 degrees (24 C) it will be about 730, 000. Propellers are smaller but faster, so the Reynolds numbers are probably similar.

This is definitely in the area where the transition to turbulent flow happens. In tank test models, turbulent flow is stimulated deliberately because flow at full size will be turbulent. We want to avoid this which is why (usually) very smooth surfaces are faster. I suspect what people are experiencing is this transition increasing the drag.

Lohring Miller

 

[Joe Warren]

Daid started this thread about the Water temp and the Viscosity changes ... there always will be particals in the water that changes other properties of the water and its ability to cool the engine.. The Efficency of a Small prop in Bad water is correct. And you propped down because that was all the water would allow. The Same prop in clean warm water would act like,,,,, very much less prop.

 

[David Santistevan]

We had a race this weekend I and I checked the water temp several times and we were in the low 50s (F). I ran my JAE 21FE in a class with two others of the same boat. All three were running the same motors, speed controller and batteries. This is a new boat for me and I am still trying to get a handle on it. However, I was running an X447 prop on my boat and he was running an cupped-up 1450. It appeared to me that I was slightly faster than his boat although I was running a wheel considerably smaller and my boat was a little heavier, this was my first clue, Later he convinced me to try a 1450 like he was running. He suggested that the added lift would allow the ski to run lighter on the water from the additional lift. In reality my boat ran noteably slower. I started to think about all of this in the context of this discussion and once again some new thoughts ocurred to me. A prop with more lift running through colder water (more viscous again) is going to have an even higher lift factor as a result of the thicker water. This in turns causes the boat to lift even further in the back and force the nose of the boat deeper or ride heavier in the water on the nose. Beacuase of this the drag on the hull increases as does the drag on the prop.

I would love to be able to do some controlled tests on this somewhere with various water temps from 50 to 70 degrees F with an FE boat. I don't know of a way to change the lake temp so I will have to just wait for the water to get warmer.