Zero-drag passive water inlet - RC Groups
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Aug 12, 2017, 10:41 AM
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22KV's Avatar
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Zero-drag passive water inlet


As we know, sailboats go much faster in heavy wind if you can keep them more upright. When the boat heels over, the leeward side of the hull drops deeper into the water and the windward side lifts out of the water into the air. A lot of the Round the World racing boats have ballast tanks that allow them to pump water into the tank on the windward side to reduce heel.

In the model I am building, I want to have a water inlet on the submerged (leeward) side of the hull that will use the forward speed of the boat to 'pump' in water that will flow via a hose to the windward side of the boat where it will fill a ballast tank and help to pull the boat upright, reducing the heel angle and increasing speed. When the boat changes to the other tack, the full tank will empty from a drain hole and the inlet system on the new leeward side will start pumping water into the ballast tank on the new windward side.

When the boat is more or less upright, both tanks will drain to empty.

I don't want to use an underwater scoop, because it will cause too much drag when the wind is light. I want something like the surface-flush NACA duct used on aircraft and high-speed cars, but a NACA duct does not work in water. What water inlet system is there that will not disturb the surface of the hull or the laminar flow of the water, but will allow water to be forced into the system once the boat picks up speed?

Obviously I could use a battery, electric motor and impeller, but I do not want to add weight to the boat and want to rather utilise the 'free' energy of the water flowing over the hull to do the work.
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Aug 12, 2017, 11:22 AM
Registered User
Ballast transfer is an "offshore" technique for stability and comfort. It takes energy and time because you need to lift water in the air on the up side and you need energy to pump the lower tank as it is often below the water level so your drain become a fill. I will try and post a illustration.

What you can do is completely illegal in radio sailing racing circles and have a weigh on a servo arm or pull a weight along a track. This is faster and more responsive than moving water.
Aug 12, 2017, 11:30 AM
The wind is free, go sailing!
Scratchy101's Avatar
Quote:
Originally Posted by 22KV
Obviously I could use a battery, electric motor and impeller, but I do not want to add weight to the boat and want to rather utilise the 'free' energy of the water flowing over the hull to do the work.
I applaud your creativity 22KV.
However, how ironic that you don't want to add weight to the boat, but you are adding the weight of the water.
Have you calculated the waterline of the boat with this extra weight when heeled?
Will the extra weight and drag be offset by the reduced heel and implied additional speed?

I see this as a benefit only if you can increase the size of the rig for improved light air performance.
The water ballast might allow you to keep the up-sized rig on longer in heavier air.
Aug 12, 2017, 11:37 AM
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hiljoball's Avatar
There is no 'free lunch' - water does not flow up hill. It will require energy to push the water uphill and that energy will come from the forward motion of the boat - ie will create drag.

Normally moveable ballast is not permitted by the RRS, (see R 51), however some classes or events may specifically allow it.

But if you want to experiment for you own enjoyment, go right ahead.

John
Aug 12, 2017, 11:38 AM
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22KV's Avatar
I am not bothered with "completely illegal in radio sailing circles'. I can build and sail whatever I want to, at least in the democratic country in which I live.

;-)

I think you are assuming I want to modify a one-class design, which is not the case. This is a scratch-built model.

As I said, I don't want to add weight to the boat with more batteries and motors, and your moving ballast suggestion permanently adds the ballast to the boat, instead of only adding it when needed. I want to try and utilise what is already freely available - water, the energy of the boat, etc. - and only when necessary.
Aug 12, 2017, 11:41 AM
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22KV's Avatar
Quote:
Originally Posted by hiljoball
There is no 'free lunch' - water does not flow up hill. It will require energy to push the water uphill and that energy will come from the forward motion of the boat - ie will create drag.
Yes of course that is true, but when the heel angle is reduced the boat will accelerate, so I will be trading off some drag for increased speed and hopefully the latter will exceed the former.
Aug 12, 2017, 12:31 PM
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hiljoball's Avatar
The other factor is whetted surface area and its related drag. The offshore boats that use water ballast tend to be very beamy to get the ballast as far outboard as possible. As a result, they are relatively flat bottomed, and have a lot of surface area.

John
Aug 12, 2017, 03:48 PM
Registered User
Quote:
Originally Posted by 22KV
As I said, I don't want to add weight to the boat with more batteries and motors, and your moving ballast suggestion permanently adds the ballast to the boat, instead of only adding it when needed. I want to try and utilise what is already freely available - water, the energy of the boat, etc. - and only when necessary.
I think you have experience sailing boats of many sizes which is something I have not been able to do. I just build toys. Sometimes I take insiration from sailing boats but other times the limits of minuturization crash in and require innovative solutions. For example most RC boats don't have a way to furl sails because it's easy enough to just pull the mast to swap a suit.

I am not sure if movable ballast adds weight overall as it is balanced with the size of the keel. I think there is an RC "proa" multihull that has the minimum keel and a large sliding ballast for stability.

If any thing a movable ballast is more of a scale effect as the crew is kinda a crude water tank ballast that moves about the boat for trim. You can go for a more scale daggerboard arrangement and add performance back with a swing arm.

I think your boat is more "dagger board" than keel. You might get what you need by making the dagger RC where you can pull up for speed downwind but control/tune depth for all else. A longer keel acts like a heaver keel (righting moment) but causes drag.
Aug 12, 2017, 06:20 PM
Registered User
Hmmm, the idea may have some merit, there are "few" problems though, the main one is not so much filling the tank, that ca be done by the boat movement (also limited a rc model boat is not that fasts), but emptying it after you change tack, at that point the water tank is underwater or close .... and to pump the water out (while tho other is filling) would involve more power than the one provided by the boat motion .... you will need motors and "power" of some sort eventually.

One easier solution would be a canting keel, you achieve the same result with no real increase in boat weight and displacement, you will need motors and "power" though.
Aug 13, 2017, 08:35 AM
Don't lie to my dog.
Gregg28's Avatar
Quote:
Originally Posted by hiljoball
There is no 'free lunch' - water does not flow up hill. It will require energy to push the water uphill and that energy will come from the forward motion of the boat - ie will create drag.
John is exactly right. Thermodynamics will not allow it. "Zero drag" is impossible, but minimal drag is a worthy goal. The question becomes, is the drag of filling the tank worth the extra power from the added stability? If the leg is long enough, it probably is. But from a wild ass guess, from left sight line to right sight line is probably not enough.
Aug 13, 2017, 10:18 AM
Registered User
Getting water in, even raising it a little, might be simple enough, but getting the unwanted water out of the "wrong" side after a tack change is the big stumbling block. Or at least, getting it out quickly enough to make it worthwhile.
Aug 13, 2017, 10:24 AM
The wind is free, go sailing!
Scratchy101's Avatar
Since this is not used for "competition" here is another approach;
Using a simple ball valve, fill the windward tank before you tack by opening the valve on the leeward tank. Since this tank would be below the waterline, you would not need to use hydro-static pressure to push the water uphill. Once the tank is full, close the solenoid and tack the boat. Then just make sure to open the valve on the windward tank to drain it first, close the valve once drained and open the now leeward tank valve to fill it before tacking.

You could use something like this plastic garden hose Y adapter to control the water flow into/out of the tanks:
http://www.homedepot.com/p/Camco-Pla...0073/100193852
https://www.amazon.com/Orbit-Sunmate.../dp/B000FPAPN2

You can use a small servo connected to the levers to control the water flow to/from each tank with a single water inlet/outlet in the center of the hull.
This does add some weight, but much less than a pump or metal solenoid valve.
Aug 13, 2017, 11:34 AM
Boomer1
Boomer1's Avatar
Scratchy
Have you tried to operate one of those valves? They are very tight and it would take a mighty servo to operate one. At least the valves I've had at home would. It's hard to image the benefit would be enough to off set the added weight from all the equipment it'd take to make it work. Just sayin...................I try to manage the sails and rudder to minimize the roll/heeling over. Some boats are more prone to heel over than others. My Yamaha RTW is very well mannered in that regard.

I think this is an interesting dialog.
Boomer
Last edited by Boomer1; Aug 13, 2017 at 11:45 AM.
Aug 13, 2017, 02:43 PM
The wind is free, go sailing!
Scratchy101's Avatar
I agree Boomer, those valves are very stiff, but this is more about the concept of using free energy (gravity and wind power) to fill a ballast tank.
I'm sure you could disassemble one of these ball valves and use lapping compound to "loosen it up", since it is not subject to high pressure like a garden hose.
There may also be other types of valves more suitable for this purpose.
Some other points for discussion of this whole concept on another thread would be.
  1. Length/beam/design of boat
  2. Location/shape of water inlet(s)
  3. Righting moment generated related to weight/volume of water
Aug 13, 2017, 03:16 PM
Registered User
Shifting ballast inside the hull could always be done with the battery. A simple tray for the battery and slack in the connection cable would allow a mechanism to slide the battery side to side to shift its weight. While I don't think this would accomplish much, it is a way to use existing weight.

The servo driving the weight could be modified so the servo pot is external and mounted with a pendulum on the pot shaft. Rigged so as the boat heels the pendulum swings causing an error signal and forcing the servo to adjust the weight. Totally automatic. You'd just have to have that servo plugged into a channel you can center. i.e. not a 2 pos switch.

Just some random thoughts.


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