Hull Test Data

Footy Class General Discussion (Footy)
21

Brett,

Thank you for your offer. I have a cheap CAD program called TurcoCAD 7. I have not used it recently, and therefore am not very fluent in its use. Apparently it can can handle the following file formats:
.TCW
.DGN
.DWG
.DXF
.3DS

However, it appears that your design has vertical sides, in which case I can use the water line beam and depth dimensions at stations spaced perhaps a centimeter apart starting at the prow. I have been using an Excel spreadsheet to convert these numbers into panels. Your pictures show that this data has been plotted directly on your hull.

22

Walt,
Send me a e mail with your physical address and I will mail you full size paper panel shapes.
That should be the easiest method of all.

23

Brett,

I have sent you my e-mail address as a “private message”

24

Walt,
Have sent you an e mail.

25

Today some more data was taken at the pond, using variations on the basic 3-liter bottle. Three things were examined:

  1. The laminar flow trip was tested, and found to actually work. However, the effect is small. It reduced drag about 4% at low speeds, but only 1% at high speeds. The device consisted of 3 layers of electrical tape across the hull, just aft of the water line entry, using 3/4" tape. The total thickness is about 0.020"

  2. Testing was done to determine the sensitivity to pitch attitude (nose-up vs nose-down). It was found that 1" nose down did not increase or decrease drag. However, 1" nose-up increased drag by about 10%.

  3. A new nose cone was tried, using a plastic bowl to get a more streamlined conical shape below the water line, but with a blunt shape above the water line (like a pram). It was not successful in reducing drag. It actually increased drag by about 4%.

Today’s activities were hampered by the large amount of goose droppings, making it nearly impossible to avoid stepping or kneeling in them. Our country has been invaded by these creatures, who no longer migrate north to Canada in the summer. It has become a health hazard, and our politicians are too wimpy to to do anything about it. Please forgive the off-topic rant.

26

Hi Walt,
The panel shapes are in the mail to you.

The laminar flow trip is really interesting,something to try for sure.

I have noticed that the traditional approach in model yacht design has been to increase the volume in the bow region to help counter nosediving.
Nose diving gets worse as models get shorter and overhangs at bow and stern less.
This puts the Footy class near the worst possible case for diving due to both short length and small overhangs to gain max waterline length.

I did a small series of hull testing a few years back,I towed 2 models at a time on a yoke arrangement and noted which hulls yielded the least drag.Quite obviously the hulls with low displacements and fine entries faired the best.

Light displacement seems the obvious way to improve the boat speed but experience with models as light as 320g total weight show that you can lack the inertia to tack in strong winds and waves and the increased amount of skill required to handle very small trimming tasks and generally sail the boat at optimum speed around the course. despite these shortcomings several models showed tremendous ability downwind. But of course we spend much more time sailing upwind and the need to have a boat that can sail itself most of the time was felt to be to important to ignore.

With all this in mind I reverted back to my first guess at optimum displacement which is in the 450- 550g range.
Our first boats nose dived quite a bit and it seems this is true for most footys,boats with very wide bows also dived but could carry on downwind “nose dancing” as Walt puts it.
The wide bowed boats have more drag(Walts testing shows this also,Bottle v Razor) so my answer to the problem was to make the bows very narrow to reduce drag and accept that all footys nosedive.

What I found though was that the narrow bowed boats didn’t nosedive as much!! when a puff hits them they have less resistance so accelerate instead of going bow down. Of course when hit by strong and violent puffs they still dive but we reduced this tendency by increasing the height of the bows.
This has lead to where I am now ,long bows with narrow entries,high freeboard at the bow,medium displacement.
The by product of this is making the boat longer since a narrow boat can fit into the box on an angle.

Comet is my current"best boat" beautifully balanced in all conditions we have only had the rudder in the air once!! the high narrow bow does its job fantastically…the boat just goes and stays remarkably level.
Just a few of my thoughts that may help someone.There may of course be better answers than mine,most likely there is and I look forward to learning more by hanging out in these forums and listening to what others have to say,Walts "laminar tripper " seems like it has potential!

27

The laminar trip is most famously used on golf balls, which are very blunt objects, and there is data from various sources which indicate that it reduces drag by 50%. The questions arise:

Is this useful on a Footy?
What is the best implementation?
How much can be gained?

A Footy operates in the same range of Reynolds numbers as a golf ball, so some improvement might be expected. My recent testing has shown that it can be useful, but this testing is limited to a fairly blunt hull, and it only provided a 4% advantage. It may be less useful on a more pointy hull.

With regard to the best implementation, a golf ball is covered with dimples. Perhaps we need more trips? However, about a year ago I found some data on-line, from a term paper of a student at MIT, who did some drag testing of spheres with a single trip wire. This showed a 50% reduction in drag, the same as the dimples. So no further improvement should be expected from more trip wires. The exact placement, as well as the thickness and width, may be another issue. However, I had found an article on another website which gave information on trip wires, and desired dimensions, which led to the selection of the 0.02" thickness. I have found nothing to suggest a particular width.

Wire vs tape? - It is difficult to get a wire to lay in contact with the hull, so I have used tape. A 3/4" width was selected because it was easy to implement at the pond without any real effort. It might be better to cut it into thinner widths. Most available data seems to use wires.

The objective of the trip is to convert laminar flow into turbulent flow, preventing separation of the flow from the body. Laminar flow separates very easily. Turbulent flow has more drag, but separates less easily. Separated flow has the most drag.

On a blunt object, the flow separates very soon, so the trip needs to be near the front. On a more pointy hull, the flow wil not separate until much later, perhaps around amidships (just a guess), so it is possible that a pointy hull may gain some drag reduction by placing the trip amidships, or even futher aft, to facilitate flow around a dragging stern. This is of interest because the pitch stabiility drops drastically as soon as the stern comes out of the water, so it may be desirable to have a deeper stern, if its drag can be minimized.

Pictures of the flow around a golf ball show separation occuring before the mid-point without dimples, but well after the midpoint with dimples.

The above is based on very limited knowledge, with a little bit of test data. Any comments or suggestions from those more knowledgable in this area would be appreciated.

28

Hi Walt,
still no expert but I can come at this from an RC sailplane point of view. Tape V Wire… as you said wire is difficult, a thread can be glued to the surface but is then rather permanent. Popular on glider wings would be car trim tape about 1/8" (3mm) wide, extra thickness can be built up layer by layer. I do wonder if the narrow tape might give more of a trip effect because the up and down sides are closer together?

Trip locations tends to be around the maximum change of curve, or just ahead of it on wings. Say 25% to 30% on a top surface and on flying wings which have a reflexed wing section you might see trips on the underside just where the reflex starts (not exact locations you understand).

Quote, “On a blunt object, the flow separates very soon, so the trip needs to be near the front. On a more pointy hull, the flow wil not separate until much later, perhaps around amidships (just a guess), so it is possible that a pointy hull may gain some drag reduction by placing the trip amidships, or even futher aft, to facilitate flow around a dragging stern.”

This I find really interesting and I even wonder if it could be the other way around Walt? What I am thinking is that on a blunt bowed medium beam hull the hull curves are more gentle. So unless the flow seperates in the first 1/2" (12mm) or so then it is presented with a flatter curve and is maybe less likely to seperate. On the sharper bowed hull with the same max. beam the curve presented to the flow will be of smaller radius and more likely to induce seperation, maybe? I have no idea in practical terms I admit so hopefully your testing will continue to shed some light.

all the best, Graham

29

Did a wee bit of research into laminar trippers.

Tank testing is one area where laminar trippers are used to more closely simulate full size conditions.
Here are some guidelines I found for attaching laminar trippers to models.

The model should be fitted with a recog-
nised turbulence stimulator which should be
clearly described in the model documentation
and the report on the experiments. Suitable hull
turbulence stimulators include studs, wires and
sand grain strips. Figure 1, from Hughes and
Allan (1951) and NPL Report 10/59 (1960),
gives guidelines for the dimensions of studs
and the location of the studs as turbulence
stimulators on a raked stem of conventional
type.

Wires used for turbulence stimulation will
be typically between 0.5 mm and 1.0 mm di-
ameter, depending on position and model
speed, and be situated about 5% L
PP
aft of the
FP.
Sand strips used for turbulence stimulation
will typically comprise backing strips/adhesive
of 5 mm to 10 mm width covered with sharp
edged sand with grain size around 0.50 mm,
with its leading edge situated about 5% L
PP
aft
of the FP.

30

Very interesting topic. I have a question Gentlemen. Sailplanes and golf balls do not start up from a stationary position. Sailboats are often becalmed and must accelerate from a standing start. I would imagine that these drag reducing “trips” would increase drag as a boat tries to accelerate from a standstill, hardly the desired effect.

Acceleration is often the key to winning races in light, patchy winds, the kind of wind that predominates in the summer months here in NYC and many other parts of the US. I also doubt that laminar flow is ever truly established on our short boats. Footies sail in fits and starts with slight variations in windspeed or surface conditions exaggerating their trim and reactions in the water.

The designers that are experimenting with diagonal box placement are trying to smooth out the hull shapes not so much to reduce surface drag or induce laminar flow but to reduce wavemaking, a much larger issue when it comes to maximizing a short hull’s potential speed. At least that is my motivation for dabbling with diagonal design.

31

Brett,

Your letter arrived today, and is much appreciated. A quick look at your panels shows that you have really minimized the exit angles with your choice of placement in the box. It should be very slippery. And your relatively wide transom should help with pitch stability. I have ordered a bunch of 1/32 and 1/64 ply for building it. I will probably use 1/64 ply for the hull, and 1/32 ply for the deck.

The 1/64 ply has worked very nicely in building my own diagonal design. It is quite strong, bends easily, and doesn’t try to split when you cut it with a scissor. But it is a little heavy (probably the same weight as 1/16 balsa). It appears to be a hardwood. This extra weight doesn’t matter in a towing dummy.

My diagonal hull is almost finished, and I will continue with it to get some more experience in working with these thinner materials, as well as some initial drag testing. The bare hull and deck weighs about 1.3 oz (37 gm). It may also be useful as a trial horse for some ideas on sails that may reduce submarining. But your hull design will certainly be faster.

32

I am not sure it will be a hindrance at very low speeds Niel!
Rogers boats are coroplast with plastic insulation tape holding them together.
Bumps and grooves all over those hulls and not visibly slower than anyone else from all accounts.
Of course they maybe and we just can’t tell!
So many factors at work it is very differcult to say for sure anything is better or worse.
One thing is for sure though…with every boat built and tested little things are learned and the evolution of these little boats continues.

33

Laminar Flow Trip

Thanks to all who contributed ideas on the subject of laminar flow tripping devices. I have borrowed the idea that Graham presented regarding pin-stripe tape. A thickness of 4 layers (0.025" or 1 mm) of 1/8" wide tape was used in 2 locations. The first location was near the prow, and the second location was near the end of the straight section of the bottle at the stern, before it starts rounding off. The performance was similar to my previous tests (approximately 4% improvement in drag, at low speeds only, using 3/4" tape at a single location), but the improvement was not limited just to low speeds. Of course, I don’t know why (is it the thinner tape, or the additional location?). Testing to date is still limited to the blunt soda-bottle hull, so may not be as useful on a more streamlined shape.

Diagonal Hull

I had built a diagonal hull, using balsa and thin plywood, and compared it to a Razor hull. Testing was done with the hulls level. The result was surprising (or perhaps not, considering the non-optimality of this hull). The diagonal hull had 5% more drag than the Razor. Regarding the non-optimal diagonal hull, it was not as sophisticated as Brett’s hull, or the hull that Bill Hagerup used at the Needham races. It was 1/2" off the diagonal, which allowed a 1" wide transom, and a 13" total length. The sides were completely vertical. The bottom was flat with rocker, but the transom was allowed to extend 1/2" below the water line (this was done to improve pitch stability). The hull was also tested for pitch stabilty in the bathtub, and measured 26 oz-in/in. The stern did not come out of the water at 1" nose-down. The next step should be to test a better-designed diagonal hull, and I will shortly start construction of Brett’s hull. Note: the towing test was done with only 14 oz total weight on both hulls, and results may differ if the designed weight of 16 oz is used.

Obviously, this work is very incomplete, and there are many variables that have not been considered. It is presented as food for thought and discussion. It is also fun to learn this stuff, and makes the racing more interesting.

34

A towing dummy of Brett’s new Bob-About II has been completed, and was tested in the water today. It has approximately 7% less drag than a Razor towing dummy, which is consistent with expectations. Testing was done only in the level attitude. Pitch stability is about the same, which is also consistent with expectations.

Pictures of the towing dummy are attached. The funny thing on the stern is a removable skeg, which is attached with Velcro. The skeg is needed to make the dummy tow straight. It is removable so I can make a real Footy if desired.

The dummies were weighted at 15.5 oz.

35

Hi Walt,

your “hometowingtank” looks to be a very interesting experimental way to understand the behaviour of these small yachts.

well done !

quoting you " bob II…has approximately 7% less drag than a Razor "
question : at what speed ?

I feel that difference between two boats is speed-dependent, due to the fact the bobII il a bit longer on waterline her advantage should be greater at high speed , an something less at lower speeds.

Are you going to plot a drag vs speed of both boats ?

It would be nice to know more about it

fair wind to you

Flavio

Folgore ITA 5

36

With regard to towing speed, all my comparative testing has been done at two speeds, “slow” and “fast”. The drag difference percentage between the Razor and the Bob-About 2 was about the same at both speeds. An exact definition of these speeds is more difficult.

“Slow” means there was a moderate deflection of the 10-ft towing rod.

“Fast” means a much larger deflection.

What does this mean in knots? Unlike my original fishing rod tests, there is no calibration on the 10-ft pole.

However, a moderate deflection can be caused with a weight of 1 to 2 oz, and a large deflection by 4 oz. This is pulling 2 boats in the test.

Based on this approximate information, and looking at my original calibrated testing on a single soda bottle, I am guessing the “slow” is less than 1 kt, and “fast” is around hull speed or more.

37

Comparitive testing can be made on two boats by using a Fast Frank T-Rig. Simply take an 18" x 1" x 1/4" stick (we say yard stick in USA) and drill a pattern of holes near the center of the stick say on 1/4" centers with the initial hole exactly at the center. Drill holes at each end of the stick say 1/4" inch in from each end.

Use a simple floating object such as a block of wood with a verticle nail and place the center hole of the stick over the nail. Setup the FF T-Rig so that rotation of the stick is limited to say 20 degrees. Attach a boat to be tested on each end stick. As you pull at various speeds, the boat with the most drag will pull it’s end of the stick more than the other boat. Change the pivot point until the stick remains relatively balanced. Calculations based on moment arms can be made to calculate the relative drag of various boats. For more precise comparisons, place holes at 1/8" centers.

A motorized pulling device could be used to pull at various speeds. Using this process, it should be possible to rank the drag of all hulls as a function of speed. If you have two equal boats, changes could be made to one boat to see differences. For example, change position of batteries.

If I can figure out how to post drawings, I will try to submit a sketch of the FF Rig. I would appreciate any feedback that would refine this T-Rig.

At this point, I only have one Razor Footy “Gold Foot” aka 007 and am unable to make any tests. I have a B-2 under construction and should be completed in about a month.

Best Regards, Fast Frank
Somewhere in SE Michigan

38

The FF Towing rig is better known as a Ljungstrom yoke ( and I do’t think he thought of it. My best guess is Sigurd Greentooth in about 1005 AD - those beautiful shpes of Viking warships didn’t come about just by accident.

Joking apart, keep up the good work - there is a tremendous amount to learn and very often a simple device will do a lot of the things for which bg corporation would use several million dollars worth of electronics.

:zbeer:

39

Nice to meet you. I am fairly new to these forums.

My knowledge basis is that of whiffletrees. I saw them on the farm as a kid and at a test installation to distribute forces across the wing of a jet aircraft to do stress analysis and fatigue testing. Here is a definition found with Google. A search (whiffle tree photos) also shows a lot of photos with various applications, but never saw it being used in a boat towing test. At this point, the fixture is simply a paper design. Hope to hear someone put it to work. A nice test location would be at a smooth flowing river or creek as they were call in our part of the country,

“Thesaurus Legend: Synonyms Related Words Antonyms
Noun 1. swingletree - a crossbar that is attached to the traces of a draft horse and to the vehicle or implement that the horse is pulling
whiffletree, whippletree
crossbar - a horizontal bar that goes across something”

Happy Sailing, Frank

40

Frank,
A fews years back I did a barrage of tests on some of my collection of footy hulls.
I used the yoke method you describe and followed the example in Bethwaites “High performance sailing”.
I have since collaborated my test results with the results generated by the “hullform” hull design software so now have in essence a crude VPP to help when drawing any new design.
In the past few weeks I have found an easily accessible stream and intend to to another round of testing by placing the models in the current and measuring the drag with a rule and elastic band! I will then compare this data with the previous data and hull form software.
The evolution continues slowly!