Hull Test Data

Footy Class General Discussion (Footy)
41

With regard to the Fast Frank/Ljungstrom yoke, my setup is similar, except that it has also been calibrated for angular deflection of the tow bar. So far, I have been using the angular deflection, since results can be gotten from a single tow, to a sufficient accuracy. It also has the capability to slide the attachment points (using rubber grommets) for more accurate measurements.

Brett has mentioned towing in a stream. This may be a good idea, depending upon the uniformity of the flow. I was looking at a small local stream this morning, and watching the bubbles. It appeared that the bubbles were traveling at a constant velocity across a reasonable width of stream. But I was wondering whether this is really the same as towing in still water, since the stream has turbulence induced from friction with the bottom, as well as the previous history of the flow. Of course, the “still” water that we sail in has also been strirred up by the wind, so this may be an irrelevant concern.

A question for Brett: have you found that the Hullform program is reasonably valid at the very low Reynolds numbers where we are working? I would suspect that it works for skin friction and wave drag, but not for form drag. Of course, I could be completely wrong.

42

Ljungstom achieved ease of observtion by making the pivot ajust able in a slot along the line of the yoke. A long pointer on the yoke could be lined up visually with the line of the ‘tow’ rope. He adjusted the system until the yoke was normal to the line of the tow-rope. Relative frag was then the ratio of the length of one rm to the other.

Hope this helps.
:graduate::zbeer:

PS As you my hve gthered this keybord hs very sticky A (got the bstrd) key)!

43

Hi Walt,
No the hullform data as it is presented for small models such as what we are discussing here is completely wrong when compared to actual data measured,But there are similarities and trends that can be identified by comparing the two data types and thus comparisons can be made once the correlations are understood!! I think! told you it was crude!

44

I feel that Walt is correct when pointing out that ,due to the fact footy are sailing at very low reynolds number , hull drag as calculated using a software like “hullform” ( just ot mention one ) is not fully reliable.

Brett is also correct - as always, due to his so deep experimental background - when pointing out that ( using hullform ) “there are similarities and trends that can be identified”

To put it simple, wave drag is “almost” correct, frictional drag is a bit more than it should be.

For this reason, using the same method of hullform ( the well know delft regression ) I wrote my own model drag spreadsheet using a specific laminar flow frictional coefficient ( blasius ).

Entering main hull parameters ( LWL, BWL, draft, displacement and so on ) I can plot drag vs speed data

To take in account the fact that many footys are flat bottomed and not round bilged, I have also envisaged an additional margin for suche extra drag.

I will be happy to compare my numbers with experimental results from Brett , Walt or other testers.

Flavio

Folgore ITA 5

45

Flavio,
Don’t underestimate the slippery shape of flat bottomed hulls when heeled,also there could be side benefits from reduced keel area needed due to the heeled chine acting as a “lifting foil”
The hard chine/flat bottomed shape is much better on these small boats than others,no other shape allows such small draft or beam for a particular volume.

46

Today I did some more testing on Bob-About 2. A trip was added about 2 cm behind the prow. It appeared to have little effect, probably because the hull shape is more streamlined than the bottle boats. Then the weight was moved to create some heel. This appeared to reduce the drag by about 6%. However, the hull has no deck, to allow playing with the placement of the weights. It also appears the the weight moved and increased the angle of heel. The hull took in water and sank before I was able to make any more detailed tests. So I need to do some work on this. Fortunately, the hull was retrieved.

Summary of data to date: Bob-About 2 has about 7% less drag than razor when both are level, and improves another 6% when heeled (Razor was not heeled).

A question for Brett:
My version of Bob-About 2 has a blunt prow, matching the shape of the deck and bottom pieces (a narrow pram prow). Some pictures appear in a previous post on this thread. Is this what you intended, or should it be a piece of rounded balsa? If so, it can be added.

47

I have not done any systematic estimation, let alone anything more elaborate, but a little mental arithmetic suggests that the Footy rule may have hidden subtleties.

Since sail area is untaxed, the upright drag figures relative to a Razor do not matter. However, I guess that Bobabout 2’s 13% improvement elative to Razor cannot be very far away from the difference in their righting moments IF BUILT TO THE SAME STANDARD. In other words, Razor has 13% more drag but can also generate something like 13% more power.

Please do not, anyone, understand this s knocking B 2. Bret is a very clever designer. If he has done many of the things that he and I have talked about (And that is not supposed to be code for ‘which I told him about,’ either!) the result should be an excellent boat. My comment is merely a reflection that, whereas I can produce a set of theoreticl calculations that suggest hat narrow boats are better, I can also produce one that suggests the reverse. These figures appear to be showing that there is little in it in practice.

Where I do think that a narrow boat scores is in ease of getting really good helm balance without hobby horsing and a light boat scores on acceleration. If you think about it, the ‘inertia of a steam-roller’ argument does not actually work very well.

It is ironic that, in many ways the Mac rig lets the Muscle Footy off the hook - it’s intrinsic handling problems are at least partly overcome by abendy rig.

48

Angus, you are obviously correct in saying that the fatter hull can carry more sail. I went through some quick geometry, and if you make the simplification that the Razor, when heeled 45 deg, carries its center of flotation at the max chine (2.5" off center), with an 8" deep bulb, then make the same simplification for the Bob-About 2 at about 2", then the Razor has about a 6% advantage in righting moment. The Razor’s advantage will improve further at less extreme roll angles. But the righting moment of the Footy is probably less sensitive to beam than other classes, because of the relatively deep bulb.

Please note that I have no data on the Razor when heeled, it was just being used as a tare. It may also have reduced drag when heeled. The objective of today’s testing was just to see if a flat bottomed hull has less drag when heeled. This makes sense because, when heeled, the flow doesn’t have to change direction so sharply at the chines.

49

Walt,
Yes a balsa fashion piece can be added at the bow.

Angus,
Hope you factored in the increased stability due to length in your Bob2 vs Razor analysis?
I think you may find there respective righting moments are much closer together than what is calculated using the simple CB comparison.
Longer boats are more stable than shorter ones.

50

Flavio,

This sounds interesting. How does your spreadsheet results compare with the test data that appears in my early posts on this thread? Do you need additional information to make the comparison? I am referring to the drag vs speed tests of the bottle hull, and a drag test of a Razor at a single speed. None of my drag vs speed data is very accurate, nevertheless the tested drag is very much higher than the blasius equations would predict.

I would be happy to supply any additional information that you might need regarding the hull shapes and test results.

51

Hi walt,

in order to carry out my calculation there are several different “depth” of analisys.

The simplest one require basic dimensions only :

waterline lenght
beam on waterline
hull draft
displacement
wetted surface ( if not available , I can make an estimation )

further data for an “advanced” study are

hull coefficients

prismatic
midship section
waterplan coefficient
position of center of buoyancy

appendages

basic side area ( keel + rudder )
or average span and average chord of each blade

lenght and diameter of ballast bulb , if any

broadly speaking I have already done a basic estimation of drag of existing designs, based on published informations :

bob about
bob about 2
cobra
kittiwake

as well, most obviously my own boat - folgore ITA 5 -

During next days I will very busy in order to arrange all details of my trip to liverpool, on my come back I will be happy to publish my informations on the forum in order to make a comparison with your experimental data

By the way I have received from Brett the plans of his bob2 so to have more detailed informations about his hull

I will ask permission from all designers to publish my data

Otherwise I will show just “virtual” comparison of boat named Ann, Karen, Lisa, and so on

All the best to you

Flavio

ps - let me know the dimensions of the additional skegs you added to models in order to improve their course keeping

52

Flavio,

I will gather the data and send it to you in an e-mail, so it will be easier to use attachments. I will be away for a few days, so expect it in about a week.

53

Some of my past data appeared incompatible, and I have been struggling to find a reason. In particular, I had taken a bunch of data on drag vs speed for the 3-liter bottle hull. The data was taken by towing it with a calibrated fishing rod at various speeds, then developing a performance curve of drag vs speed. Then I set up another test, in the bathtub, using a pendulous weight to tow the same hull. It was very puzzling to find that the bathtub test showed no measurable drag. The answer has now surfaced (I think).

All of the original data was taken at speeds above 0.5 knots, and the numbers were far above the expectations of skin friction, so were primarily caused by “residual drag”, the combination of wave drag and form drag. All of the bathtub data was taken at speeds below 0.5 knots. If the data followed a smooth parabolic curve, there should have been plenty of drag to measure in the bathtub test.

However, I have recently come across an interesting factoid. According to Wikipedia, a moving hull can not create a bow wave unless the speed exceeds 0.44 knots, because the bow wave will be suppressed by surface tension. If this is true, the absence of measurable drag in the bathtub test implies that most of the drag in the fishing rod tests was caused by wave drag, which disappeared in the low speed bathtub test.

But do I really believe this? Visually, looking at a moving object in the water, I think it makes a small wake even at low speed. But is the wake caused by a true bow wave, or is it just evidence of turbulance at the stern, or perhaps separation of laminar flow at convex surfaces further forward? And all of this may change if the water surface already has ripples.

We are working in an area of Reynolds number which has not been heavily studied, where all kinds of unanticipated effects may occur.

All of the referenced test data can be found in previous posts to this thread.

54

That is fascinating. It is the first really sensible argument (I’m not saying it’s right, just sensible) tht the very small size of Footys does actually make them ‘different’ rather than just pushing the scale factors to extremes.

It also encourages me more than a little. I am still reasonably convinced that our immersed transom designs work quite well, for other reasons than the general disposition of volumes and areas that this type of hull allows. However, Flavio has pointed out there is a well-developed theory of immersed transoms in mainstream naval architecture and that the sterns of Moonshadow and Voortrekker break almost very rule in that particular book. Howver, the boats do seem to be very efficient at high speeds and there are certain characteristics of the wake that would not be expected ‘by the book’. In particular, a deeply immersed transom generally makes virtually no wake whereas a wide, shallow one kicks up all the turbulenc you might expect. It is nice to know that there are other things that defy easy explanation.

55

Gentlemen, I would like to say that I have found this thread absolutely fascinating ~ thank you for your continued efforts

56

I have taken some more data, comparing a V-12 hull with a Razor hull. The method and results are presented here. Carl Hansen assisted in this effort, and supplied the naked V-12 hull. The conditions at the pond were a little more windy than desired, but we wanted to get this done so he could finish building the hull.

The razor hull was weighted to 16 oz, similar to my previous tests, so results could be compared to other tested hulls, which were tested against a 16 oz razor.

The V-12 towing dummy was weighted to agree with a completed V-12 that Carl had already built, which was 20.5 oz.

Comparative drag was measured with a tow bar, similar to my previous tests.

At low speed, both hulls had similar drag, within about 2%, but varying by that amount over time, perhaps because of wind or wave action.

At high speed, the V-12 had more drag, by at least 9% (the set-up saturates at that level).

The day was a little windy, with wavelets probably affecting the measurements, so we did not go any further.

One could argue that I was comparing apples and oranges, due to the weight difference. But a Razor can be built to 16 oz, whereas a V-12 probably can not (now I have set myself up for getting nasty comments from people who have built 12 oz V-12’s). Obviously, if we want to compare hull shape only, the weights should be the same on both hulls.

The shape of the V-12 is probably better for pitch stability (nose-diving) than the Razor, which may allow it to carry more sail, but we did not test that parameter on the V-12.

57

Hi Walt - The displacement weights that you should use are those specified by the designer, or in the case of the V-12 the displacement (or range thereof) recommended in the building instructions. Since a builder’s skills are a variable one can’t control the weight of Mr. Hansen’s built V-12 may not correlate with the displacements given by the designer (that is my suspicion only, I have no experience with this kit boat).

Also, “if we want to compare hull shape only, the weights should be the same on both hulls”, is not correct, you would not achieve your stated goal for the reasons above. Different hull shapes should be evaluated for what they are and how they are intended to float. Changing displacement weights for different hulls might indicate how they might be improved but won’t yield the test results that you are after.

I think that the tests you are conducting are very interesting and with enough data on many different boats a picture of Footy performance characteristics will present itself. I follow your posts with keen attention. Thanks for your efforts.

58

I too have been following this with great interest. I have wondered why you haven’t built a test tank? My bro-in-law is a hot tub repair guy and the thing they end up with is surplus pumps. He’s sure that you could get a pump for nothing or next to nothing. They put out between 50 and 75 gal per min. Just thought I’d suggest it in case it was pump expense that was holding you back.
Don

59

Don,

Your suggestion to build a test tank has been considered, but the ambition to actually start working on it has been lacking. Flavio has kindly sent me some good information on measurement methods for a simple small towing tank. Brett has suggested using the current in a river. There are a lot of things I would like to learn about hull drag on these small hulls. I had considered (and am still considering) a towing tank in the back yard. It is important to get reasonably accurate measurements to get good comparisons, so I have not considered methods that use moving water, since it is difficult to get a non-turbulent flow with constant known velocity without having a very large trough. Swimming pools also have this problem, due to circulating pumps etc.

There is also the matter of negotiating these things with my wife, who is very understanding, but doesn’t like a big mess.

60

Walt ~ ever the diplomat