Wind Tunnel Test

Footy Class General Discussion (Footy)
1

Introduction

Last July, I had been playing around with a test hull featuring a very long bowsprit and a single sail, which was a jib with a large rake angle (see thread entitled “Slighty Unconventional”. The objective was to get enough lift from the jib to avoid nose-diving when running. There were several questions that needed to be answered. First, could it be balanced sufficiently to sail to windward without a lee helm, which it could, by moving the keel far forward. Secondly, would this scheme prevent nose-diving. It helped a little, but not enough to be worthwhile. It appeared that any further improvement would need a longer bowsprit, which would prevent balancing the rig. But recently someone has mentioned building a ketch. This made me realize that the bowsprit could be lengthened if a small mizzensail was added, hoisted on a second mast mounted on the transom. Therefore I did some testing to see how long a bowsprit is needed.

Test Set-up

The test set-up consists of 2 picnic benches, a pencil, a string, and a large fan. The picnic benches are facing each other, with a gap of about an inch between them. This allows dropping the keel of the test hull between the benches. The test hull is balanced on a pencil which spans the gap between the benches. The fan is placed on the benches and blows wind into the sails. The string is tied to the rudder to keep the hull from sliding forward from the wind pressure.

Test Results

The initial testing showed that the balance point moved forward about an inch when the fan was turned on. However, this was not a very useful measure, since this was outdoors, and there was always a slight wind blowing, which made it difficult to get an accurate calibration. So I moved the keel weight fore and aft, varying the position of the balance point, until I found a balance point where tthe wind made no difference, and the fan made no difference, the hull would just sit there. The end result was that the bowsprit needed to be lengthened significantly (started at 9", increased to 13").

Correlation to expected results

I had made some calculations, using a free body diagram as in Physics 101, which indicated that a bowsprit of 11" would be needed. So the results are at least in the general ballpark

Tested sail

The tested sail has a 45 deg luff angle, with a 17.5" leech and a 17.5" foot. It is mounted to a club which has a swivel attachment to the end of the bowsprit about 25% aft of the tack. The club is counterweighted so it will swing out easily.

Comments

This is not a very accurate procedure, for several reasons enumerated below, however it may be sufficient for the intended purpose.

  1. The wind created by the fan is probably quite turbulent, but the flow across a stalled sail going downwind is also probably turbulent, so it may not matter too much.

  2. The wind created by the fan is not uniform across the sail, which will cause some inaccuracy. An enclosed tunnel, with a screen at the input end and the fan at the output end, would be better, but would be quite large.

  3. On a boat in the water, the wind flow off the bottom of the sail will be blocked by the water. This is approximated by the existence of the picnic benches under the boat.

2

Hi Walt,

I have watched a few Footys sail at the 2009 NCR in Orlando, and noticed that many of them have rather large bow waves, probably caused by plumb stems and vertical sides created in an effort to keep the waterline length to a maximum. Also, wider hulls seem to have more of a bow wave than skinny ones. In order to maximize keel depth under the rules some had pretty low freeboard, which meant the deck had less distance to go before it was underwater. Many have flat decks, too, perhaps because they are easier to build than ones that are crowned. When the bow goes under, that flat deck really puts the brakes on.

I read somewhere that many full sized sailboats that have plumb stems also have a similar tendency to bury their bows, while ones with more rake and flare tend to ride up and over their bow waves, rather than dive into them. With all this in mind, when I built a new 1/64" plywood hull to replace my original water-soaked 1/16" balsa one, I changed the bow from a rounded & plumb stem to a sharp one with some rake (sacrificing a tiny bit of WL length), and this also added a bit more flare at the bow. I also raised the sheer line at the bow by about 5/8", and changed the deck from flat to a crown of about 1/2". Initial sailing trials indicate that all of this reduced the bow wave considerably, allowing the extra flare and slightly higher sheer to help keep the bow from submerging as much as the old hull. The boat will still bury it’s bow in stronger winds, but not as soon or as deep as it did with the old hull (the keel, rudder, and sails stayed the same, although I did remove some of the rake in the mast so that it is now closer to vertical instead of tilted slightly to the stern). Now I’m wondering if it would have been even better had I added a bit more rake to the stem and a bit more flare to the bow, raised the sheer by 1" instead of 5/8", and made the deck with even more of a crown…

Regards,
Bill

3

Hi Bill, you actually want to rake your mast forward of vertical a couple of degrees. When you have the mast raked aft gravity holds it on center when the wind dies and makes it harder for the sails to buck gravity and react to the wind when it fills in again. In light winds, raking the mast slightly forward uses gravity to make the boat more responsive and accelerate better in the shifty stuff because the weight of the sail and mainboom want to go out. Don’t overdue the rake angle though because too much will deaden response just the way aft rake does. There is a “sweet spot” somewhere between 2 to 3 degrees of forward rake that helps your sails become “lighter” in action and encourage flow over the sails when responding to a new wind.

A lot of Footy guys set up their boats with aft raked masts under the false assumption that aft rake will help prevent diving. If you take the lesson of the McRig, with its shock absorbing Z-Bend wire, the concept of dumping the excess wind power that the boat can’t handle is a valid one for Footies. I’m not saying that McRiged boats don’t dive, but my experience with them have shown that the rig’s flexibility gives me a few split seconds to take corrective measures before a full blown dive takes place.

So, fine for McRigs, but what about conventionally rigged boats. Well, here is where forward raked masts help as well. The same forward rake that encourages the sails to take shape in lulls helps spill extra wind power in gusty conditions downwind. Visualize an aft raked mast on a downwind leg. The top of the sail (the part that exerts the most leverage pushing the bow down) stays in full exposure to wind pressure until the mast passes vertical when some of the wind will start to spill or deflect over the leach. If the aft rake angle is, say, 3 degrees, and the wind doesn’t start spilling off the top of the main to relieve pressure until a couple of degrees forward of vertical then the boat’s static waterline will be aimed 5 or 6 degrees nose down, in a full dive. Starting with a forward rake the mainsail starts to release excess wind as soon as the boat shows any nose down attitude and as the bow dives further the mast’s angle to the wind becomes more oblique and reduces its leverage on the bow.

In diving, all of you who are reading this may have noticed that a boat accelerates sharply as it’s angle of dive increases just before the point where the boat buries it’s bow. In my opinion the increase in speed is due to the change in attitude of the sails. Dead downwind sails act as a WWII parachute does, catching the wind as it blows by. When you rake the mast forward slightly wind flows somewhat diagonally over the sail imparting lift, more like the modern airfoil parachutes which are more controllable and operate better in windy conditions. It is the change from drag to lift that accounts for the increase in speed as the dive angle becomes more severe in boats with a vertical mast or one with an aft rake. Setting a boat up with forward rake to the rig gives the boat that bit of a boost derived from lift on downwind legs where others are still just bagging the wind.

4

Walt I noticed that my delta rigged skinny v-12 does not nose dive nearly as much on the down wind runs… I think the angle of attack of the sail has a lot to do with this…my sails have an angle similar to that of a Victoria’s jib…

5

Hi Niel,

Thanks for the information on raking the mast. I had accidentally built in the slight aft rake on my previous boat, I was trying to get it vertical, but ended up with a rake due to poor “eyeball technique” during the construction. After sailing it, I had begun to reach the same conclusion that the aft rake might be contributing to it’s nose-diving tendencies, so I tried to get things closer to plumb on hull #2, but I hadn’t even thought about raking it forward - Thanks! I’ll have to give that a try. Now I wish I had gone to the trouble of setting the mast into a box with a removable block as I had originally planned, rather than just epoxying some 1/8" aluminum tubes into the deck, but I had gotten behind schedule, and was in a hurry because the NCR was fast approaching. Haste makes waste! Next time…

Thanks again,
Bill

6

Mast Rake Adjustment

There is an easy way to play with the mast rake. I have been using a velcro pad on the bottom of the hull, which mates with a velcro pad on a “mast step adapter”, which contains a 2-56 screw facing upwards. The mast tube goes through a hole in the deck, and slides over the screw, which holds it in place very nicely. The “mast step adapter” is a small piece of sheet aluminum, about 3/4" x 1/2", with a tapped hole for the screw, and velcro on the botom. You can move the bottom piece around to adjust rake. The screw needs to be about 3/4" long, so the tube doesn’t hop out. Seal the mast hole with nail polish, which removes more easily than epoxy, so you can pull the tube out when moving the adapter (although my mast hole is a tight fit, and doesn’t leak withoiut any sealant).

This method arose out of necessity, since I coudn’t reliably glue anything to the plastic of my bottle boats, but the velcro tape worked very well, and is still working after several years.

A question for Marc: what is a “skinny V-12”? Do you modify the V-12 hull in a crusher?

7

I visited Georges shop in CA a few years ago he was working on quite a few different footy hulls, and he had what I call a skinny version of his v-12 similar hull lines but about 1/2 as wide… bit more freeboard…so after buying some odds-n-ends i talked him into doing a hull for me…

I think this is a good hull but the immersed transom has got be slow. the boat will fit on the diagonal which means I may be able to “canoe” the stern a bit more and make the transom smaller, or go for the double diagonal and go radical with it… I did make a mold of the hull so I have a master to at least play with in the off season…