Goin' Yachtin'

This weekend Markla and I will be tagging along with Woo and Princess Fiona on their blazer 23 trailer sailer, Shrek (it's big and green, not unlike the hulk (but some ppl didn't like that name)), for the Bay to Bay race.

Starting at Tin Can Bay (A) and finishing in Hervey Bay (B), it is a two day race/cruise/piss-up with a stopover behind Fraser Island. Hopefully the mosquitoes don't eat us. Apparently it can also get a little shallow at times, it is after all the Great Sandy Strait. Lots of tacking and or gybing. Wish us luck.

Winglets

A failed gantry lets me continue my development of my new foilset this weekend. Did a little research and thinking about winglets. Only an overview here, and only the stuff I consider of relevant and of significant effect.

Winglet

What is the effect of adding winglets? The major effect is to reduce downwash, make it harder for the high pressure side to leak around to the low pressure side.

This increases apparent aspect ratio of wing reducing induced drag. But it comes with the penalty of adding parasitic drag without adding any extra lift.

As a consiquence of the above you should always add wing span in preference to winglets.

The induced drag effect is much more significant at high lift coefficients (slow speeds, when your foils are struggling to provide enough lift) so you could see the effect of adding winglets (or extra span) as helping you take off sooner, but it will reduce your top speed.

Wingtip Fence

When are winglets used? Always when wing span is limited
ie by structural considerations
ie by outside influences, such as 15m class in gliders or aerobridges for airliners.

So should winglets be added to your moth?
Only if you need to take off at a slower speed and
1. You are using the same production hydrofoil as everyone else.
2. Your proto foil design is aready solid carbon and at its structural limit.
3. If your moth doesn't look enough like the bat boat yet (but I suggest adding winglets in a more visible spot).

In all other cases, add span, not wingtips, and add more reinforcing if necessary.

Spiroid Wingtip

Name Me

Only 4 days left to get your vote in for a name. Perverted Moth is out in the lead, but remember that write-ins are being counted so "the boner pirate" and "Leeroy Jenkins" still have a small chance!

FOURTH SAIL THIRD FAIL

Another sail today. SE 15kts 28deg. Holleywell.

I and I as usual, but today I invited Doug and his thundercat too. Flight more stable but still often ended in a big crash. N4rkla got the top speed 16.66. I got longest ride approx 40 secs. Best photos were of Doug (on his first sail of a moth).



N4rkla ended the day by destroying another of my gantries. A youtube of him below doing the speed of the day.

3

Third sail on sunday at Runaway Bay. Started at 12knots before falling to 9kts, SSE, 24.5deg.
Testing new jury rigged wand arrangment.

I went out first and could only manage a few launch+crashes before noticing the paddle had come off. Replacing it with 10mm rope did the trick and it was flying for longer before crashing, but still reluctant to take off.

N4arkla the had a go and almost immediately had it up and going. Flying level. I was on the shore and could easily see the spray peeling off the centreboard. I don't know who was more extatic -him flying it or me seeing it works. He clocked 15kts a few times before the wind fell away and the tide rocketting upwind was too frustrating

(I was too excited to take any photos of him, and he hid the camera)

We suspected the wand was turning off the lift too early (before hull cleared the water) and the linkages were not stiff enough to hold the flap down so after dinner we worked this out:

Unfortunately there was less wind on monday, and rain all day, so I will need to wait before I can test this arrangement.

The weather wrote off the rest of the weekend of sailing, but I and I did have a chance for some mothematics.

• I and I have had a look over a resistance study by Olav of a systematic variation of Flashearts.
  
• I have developed a steady state foil assessment program the determines liftoff speed and resistance at 30kts while allowing you to vary span, chord and lift distribution between rudder and CB.
  
• N4rkla added SLino to his resistance study and simulated the response of double linkage wand mechanism.

I and I shall post once once we have cleaned it up a bit.

Hull Resistance

For those of you that don't know, Markla and I work in the marine industry as engineers. Naval architects to be precise. We have at our disposal some pretty neat tools and it is always cool to put them to the test.

For this reason we decided to compare the full scale tow tank resistance data of Bill Beaver and John (better copy and paste this) Zseleczky with results from one of the tools in our kit. It is a program that predicts resistance of ships using a number of methods, one of which is called the Slender Body Method. Basically it uses an analytical method to calculate the resistance of narrow hulls rather than using traditional statistical/regression methods. blah, blah, blah, that's enough background.

Firstly I produced a hull model traced off the Hungry Beaver linesplan in the report (Bill has since sent me a file of his hull which I haven't had a chance to run with yet.). Static drafts for each of the towed displacements were calculated and then run through the program. Neither the dynamic trim nor squat that was present in the tank tests was modelled, for better or worse.

Ok, raw data comparison graphs attached...


So far so good, despite not accounting for trim or squat with the computer prediction, there seems to be some good (but not perfect) correlation. The biggest problems are underprediction of resistance at higher speeds and higher displacements/drafts and the existence of a hollow in the computer prediction at about 4 knots. There isn't any tank data points around there though so it could be a resolution issue.

Well I'm interested in the correlation at the faster speeds (not really that fast) so here's a graph at 8 knots for varying drafts...


This shows a clear discrepancy at the deeper draft for this speed. The difference is nudging 10% of the towed result. Well there's a number of reasons this could be:

• Incorrect/lack of calculation of transom drag at greater drafts.
• Effects of static trim
• Effects of dynamic trim
• Effects of dynamic squat/heave

I have no knowledge of transom drag calculation so I'll forget about that for now; static trim may be the cause but apart from the general hydrostatics 0 deg trim I'm not sure what the trims were for each displacement; we have nice dynamic trim/pitch data so this is a possibility but requires many more computer runs to get more resistance data (cbf right now and isn't as straight forward as my lazy arse would like). So this leaves us with dynamic squat (or just plain squat because it only occurs dynamically and really refers to shallow water behaviour of moving ships).

When referring back to the beaver hungry paper, squat increases with increasing displacement/draft (as well as speed, I know, but one thing at a time) so let's interpolate the computer resistance at the squatted drafts obtained in the tank tests.


mmm, that didn't work so well. Maybe not, while the average error seems to have increased (from -1N to +4N), the standard deviation of the errors has decreased (from 3N to 1.5N). Some gains, some losses.

So next thing for me is to get some dinner. On the resistance topic though, I'm happy enough to move forward with assessing hull designs based on the raw computer data. The main reasons for this are that I am happy with the correlation we've seen so far and predicting squat and trim is too hard/impossible with the tools and knowledge I have available so they can't be accounted for in the prediction. I'll probably run the actual hull model through the program to iron out any errors in the results from that area, but I don't expect major changes.

I'd like to thank Bill for his correspondence and congratulate Bill and John on releasing some useful info to the mothing, and sailing, world.

ps - all results are in fresh water, made that mistake too many times with this validation to ever forget again.

Wave picture

SECOND SAIL

Took (tba) out for another sail on sunday. I survived about twice as long, but was a wreck when I got back into shore. (Holleywell, QLD 17kts SSE, 26.7 deg.)

Boat still wasn't working. Initially it was doing huge lowriding nosedives - standing right up on its ear. One of those I fell out the back. When I surfaced I noticed the boat hadn't capsized and I needed to go swimming after it. Managed to crawl back onto it without it capsizing. I gave the tillerex one turn, and nosedives was no longer an issue. I was rather surprised how effective the twist is.

The foils were consistantly supporting almost all of the required force, but it only took off completely once in 45mins. Then gave spectacular crash. Leg on sidestay = hurt. Will wear full wettie until i can sail it properly.

N4krkla then had a go. He was too scared of dropping the rig to pull on enuf vang and wasn't cracking off onto bogan reaches. He prolly would have gotten to it, but the tiller universal popped off and we swam it back to shore.

When we were derigging we noticed that my fibreglass rod that the wand pivots on has failed so wand wasn't actuating the foil. Prolly explains why it wasn't inclined to fly.

Interestingly N4rkla commented that the boat was a lot more docile than he was expecting. I am inclined to agree. Probably a combination of good bouancy in the wings, flat sail, low freeboard, and to a lesser extent, dampening in roll provided by the chines.

On a seperate topic, I and I have run the hungry beaver thru a Slender Body resistance package and the correlation to Bill and John's research is surreal. We are formalizing the calcs at the moment and will post the results later this week.

Reviewlet: Foiler Performance Calcs

Click here for Foiler Performance Calculations from Alan Smith

EXCITED EXCITED EXCITED!!!!!

Looks rather good.

Haven’t played with it tho.

I don’t think I have a need.

I am more interested in making my own time step simulation.

I think I would do my own foil calcs.

Then compare to beaver’s results.

That would give me time to take off and take off speed.

Then genetic algoritm or brute force parameters to improve existing designs.

The Fxs might be worth referencing/borrowing/stealing, but it is probably in Imperial measurements.

Be interested to see how the self destruct works.

Can't get my password cracker working with Excel07. Need to find an old computer.

In the calcs tab he has max performance criteria:

Fxs > drag (this should be =, unless you want to slow the world or travel forwards in time)

RM/HM > 20 (Should be equal to, unless you want to do barrel rolls, he mults the ratio by 20 for better presentation on graph)

(paraphrase)Induced Drag Rudder / CB Lifter Area = Induced Drag CB / Rudder Area approx equal (don’t know why, and I may be reading it wrong, a little unclear)

Pity he doesn’t find an optimal automated.

I would brute force parameters to find bestest.