I read on a topic today that some-one is has used vortex generators on a r/c boat. Where would the value be in this?
They would help to slow you down!
Unless the model was well over 2 meters in length, the numbers say that anything you add to the hull or appendages will only cause drag.
We played with the idea for almost a year before giving up on it. Any hull shorter than 8 feet showed significant losses in speed. On the larger hulls they were quite helpful to expel the boundary layer after it broke the laminar flow, and also keep the flow more constant around the keel, bulb and rudder joints.
I read a similar article in Multihulls Magazine which seems to also suggest the longer the hull the better. The article further stated there didn’t seem to be any “scale effects” by adding a generator. For the benefit of the reader, they were specifically talking a multihull with a sharp cornered piece of wood added the length of the keel (in the case of the article) and about 3" deep.
Peter ® - care to elaborate on what you guys tried? Similar concept?
Our objective was #1, to try and increase the length of the laminar flow along the hull, and #2 to clean up the drag created at the keel/hull, keel/bulb, and hull/rudder joints.
We used several different ideas. One was simply small bumps (pin head sized) added to the surface to break up the boundary layer in specific spots. Another idea was actual foil vortex generators (small fins) added to direct and funnel water flow around the hull.
We also played a bit with thin strips (thick tape) that ran across and along the hull and bulb.
None of the ideas increased speed, although some did change handling characteristics.
In reality, Reynolds numbers show that any foil we would ever use on a boat of less than 2 meters in length would have no more effect than painting it a different color would.
Of course you could increase the foils ratio, but we all know that fat foils are already slow, so there was no point in pursuing the idea further.
We spent just over a year and hundreds of hours (in test tanks and real world scenarios) to find out we were no better off than where we started.
We did learn a lot though.
The MG30 Kit is a product of that testing. Probably the fastest documented 30-inch monohull available to date at over 7 knots with a GPS on board.
Thanks, appreciate the further details.
I beg to differ with the “fat foils” point. Over here we have two multi’s very similar in speed. On this day we made sure that both boats were set up correctly then they sailed the course against each other and finished seconds(5-6) apart.
The boat that lost was then fitted with a centreboard that was nearly twice as thick in chord but the same length and width. The two boats sailed the course again but this time the boat fitted with the “fat” foil won by 15 seconds (approx). Under this configuration the two boats went three more times for the same approx. result.
The boats then switched (thin to thick and viceversa) again the “fat” foil won out.
Maybe on monohulls the idea doesn’t work, but it sure did on the multi’s. At least over here.
Peter R: Interesting stuff. In a few conversations I’ve had with some well known yacht designers, they agree with your thoughts about vortex generators on the hull and keel, but have a different opinion about the rudder. Essentially, it was suggested that having some turbulent flow around the rudder might actually increase its effectiveness. Last year I tried it by using tape dots on an IOM rudder and while I couldn’t tell if the rudder was performing better, it certainly didn’t slow the boat down, and I finished in the top five of a regional race. Might try it again this year.
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by Peter _Birch
Maybe on monohulls the idea doesn’t work, but it sure did on the multi’s. At least over here.
<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
Peter & Peter -
Is it possible the difference in actual boat speed and/or weight might be a factor in thin vs. thick boards?
Was there a patten to the dots - or just random to break up the flow. We are talking like adhesive type paper dots - not 3 dimensional - correct?
Re: Dots-- Three vinyl adhevise dots on each side, about an eight of an inch in diameter, in a straight line, an equal distance apart, about a third of the way back from the leading edge of the rudder.
Two comments here surprise me:
- Peter R’s comments about Reynolds numbers. Peter, you state that for model yacht reynolds numbers it would not matter what foil you put under the boat… I think you should double check your numbers. Model yacht keels operate in the 40,000 to 120,000 Reynolds number range. This very low Reynolds number range is very sensitive to laminar separation and is one of the most difficult regimes to design airfoils for. I designed several RC glider wings for this Reynolds number range and the leading edge curvature was a critical design aspect that needed to be considered. You need to be careful not to have too sharp of a pressure recovery over the aft portion of the foil or you will surely stall early. Turbulators (or vortex generators as they are referred to here) can be very effective in this Reynolds number regime to re-energize the boundary layer and postpone separation and stall. I’m surprised that your scientific studies found the contrary…
It is possible, I suppose that the loading on model keels is quite low (low lift coefficient) such that stall is not really that critical of an issue). Model yacht keels have a very large surface area compared to the lift that they need to produce, so I that could be the reason here.
- The fat foils comment - Fat is bad for low Reynolds numbers. However, it might be that the thin foils had too sharp of a leading edge and were excessively turbulent for their thickness. But in general, foils at about 4-6% thickness with a properly shaped leading edge and a gentle pressure recovery should have a 20 - 30% better drag coefficient compared to a similarly shaped foil at 8-10% thickness.
You comments about rudders are spot on. Rudders operate at very high lift coefficients when they are deflected. So rudders are prone to early stall unless care is taken to keep the boundary layer from separating. Turbulators are an excellent way to do this. They generally will produce slightly higher drag when the boat is straight line sailing, but will produce noticably less drag during tacking and other maneuvers. Is this consistent with what you noticed?
On the human powered hydrofoil project we used a needle to punch small holes in an index card in a neat row with the holes separated by about 1/8" each. Then we placed the card at the critical chordwise location on our wing and painted 5 minute epoxy over the card so that it bled through the holes. When the card was removed, small dots of epoxy were left on the wing in a nice neat pattern which was then allowed to cure. The critical thing is to find the correct chordwise placement of the turbulators. We had CFD to tell us where to place them. But I would recommend using oil film and look for the place where the oil gets washed away by the turbulence. The forward edge of that turbulent zone is where you want to re-energize the flow with the turbulators.
Re: Dots–In hindsight, I think the boat showed better control with the “dots” on the rudder than without. I think I’ll put them back on again this season and see what happens. I didn’t notice any decline in straight line speed, but then again that was never one of the hull’s strong points anyway.
I can’t agree more on how critical the shape of the leading edge of the fin can be. Rounder seems to be a little slower in a straight line but has a much wider groove. As to thin vs. fat foils, generally thin is much faster. However, we saw at the IOM Worlds that an ultra thin fin in choppy conditions was subject to such severe flow seperation problems that it kept the boat constantly stalled. Very, very slow.
Will & Roy -
now you have me confused regarding low Reynolds and “fat” foils!
Seems looking at some very slow speed aircraft wings, for indoor lite electrics, the wings seem much more thicker and with greater camber for slow flying.
Is there a great difference in the R numbers between water foils and slo-speed aircraft foils?
I had been under the thought that thicker required/produced faster speed to stay laminar, thus it had less propencity to stall. Was I Misguided?
Drag off the “thick” foils is way too high.
An Airship/Zeppelin is a case for Laminar flow, or the nose of a Jumbo jet, sharp just wouldnt work.
I too have built Gliders and made wings with very sharp LE s, sods to fly, even worse to land because of a sharp stall.
Rounding the LE improved flying a lot.
Again I beg to differ in relation to “fat” foils. The foil we experimented with was the centreboard only. I fully agree that thin rudders are required. The boat with the “fat” centreboard actual had/held better height than the “thin” centreboard.
Maybe it was a multihull thing, as I have never tried a similar experiment with a monohull.
This is becoming a very interesting topic.
I hope it doesn’t go like the rest do.
I’ve been looking for further information about the vortex generators, and i found a lot of on the airplanes wings sites.
Do somebody try with VGs on the mast? and (maybe it’s a stuppid idea [^]) a semi-rigid jib?
Aaaallways look on theee briiiight siiiide of liifeeeee
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by Roy Langbord
Re: Dots – Three vinyl adhevise dots on each side, about an eight of an inch in diameter, in a straight line, an equal distance apart, about a third of the way back from the leading edge of the rudder.<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
And how high or thick, please? How much did they project above the rudder surface into the stream?
There are some significant differences between gliders and sailboats. Gliders are manly concerned with sink rate. Without going into all the math, the sink rate is related to lift and drag as:
Cl^1.5/Cd (Cl is lift coefficient and Cd is drag coefficient)
Sailboats are mainly concerned with the minimum drag for a given amount of lift. This would of course be expressed by:
For sink rate, the lift coefficient is more important relative to drag (as indicated by the 1.5 power Cl). Of course the other thing that gliders have is that they only need to generate lift in one direction (up). Thus, glider wings are cambered and the camber and thickness needed to get to low sink rates is more than you would want for the best lift to drag ratio. But even so, you generally have airfoils that are 8% to 10% thick - no more. The most popular airfoils in RC glider competitions these days are generally in the 7.5% to 9.5% thickness category. such as the Eppler 387 (8.7% thick) the Selig SD7037 (9.2% thick) although Mark Drela has some really nice foils out there that are gaining popularity that are in the 9.5% to 10.2% range.
For sailboats, however, the lift to drag ratio is the important measure - this puts more emphasis on drag and a little less on lift than the sink rate measure. And given the large surface area of keels for model boats, the lift coefficients are going to be relativly low. So you need to aim for a low drag coefficent at low lifts. In addition, since the sailboat has to tack, the airfoil needs to be equally adept at generating lift in both directions. This means that the airfoil will need to be symmetric (unless you are really clever and the rules allow things like cambering keels). For a well designed symmetric airfoil, lower drag will generally be achieved at slightly lower thickness due to the form drag reduction. If you make the airfoil too thick, then the pressure recovery zone (the aft portion of the airfoil where the flow has to recover from the low pressure required for lift back to the ambient pressure) will get too short. Since fluid wants to flow from high pressure to low pressure, the steeper the pressure recovery, the more the flow wants to go backwards and the more likely it is to seperate.
There is a limit to the thinness of the foil. I would say that a well designed foil with a nice round leading edge and smooth curvature in the pressure recover zone will be optimal around 6%-7% thickness. I have seen some airfoils that appear to be better around 4% or 5% for some reason and others that reach their max L/D at 8% or 9% thickness. Quite a bit depends on the lift coefficient range that you are looking for and the amount of time that you spend at higher lifts (upwind) versus lower lifts (downwind). If you were only to use a foil for upwind, higher lift regimes, then a thicker foil might be better. But at low to no lift conditions such as down wind, the thicker foil will hurt you and your overall performance around the course will suffer.
Peter, can you give us an idea of how thick your “fat” foil was and how thin you thin foil was? Also, do you have any idea how sharp the leading edge of the thinner foil was. I’m guessing that your results were perhaps skewed due to a foil with a very sharp leading edge that was causing laminar seperation very far forward on the airfoil. Just a guess…
Jose, the idea of putting vortex generators on your mast is not new. However, keep in mind that unless you have a rotating mast, the angle of attack on the mast will change from +30 degrees to -30 degrees or more when sailing upwind. Reaching and downwind will be even worse. So positioning vortex generators so that they work well for both of these extremes is nearly impossible.
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by Dick Lemke
Is there a great difference in the R numbers between water foils and slo-speed aircraft foils?<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
Always useful to be able to calculate the Rn, so here is the formula – multiply the velocity of the foil, by the length of the chord, by the magic number 6,400 for air or 81,000 for water assuming the (imperial) FPS measurement system, or by 68,500 for air or 880,000 for water when using the (metric) MKS system.
Let’s have your indoor aircraft have a wing chord of 6 inches flying at 1 ft per sec, so
Rn = 1 * 0.5 * 6,400 = 3,200
Let’s have an IOM sailing at 3 ft per sec, and a rudder with an average chord of 2 inches (for example it tapers from 3 inches at root to 1 inch at tip), so
Rn = 3 * 0.167 * 81,000 = 40,500
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by jose
I’ve been looking for further information about the vortex generators<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
I have a page which mentions turbulators: