# Surface piercing foils, why set at 45 degrees

Discussion in 'Multihulls' started by waynemarlow, Oct 24, 2009.

1. Joined: Nov 2006
Posts: 430
Likes: 44, Points: 28, Legacy Rep: 134
Location: UK

### waynemarlowSenior Member

OK looked and read the very long threads on foils, in particular the fully submerged type on the moth. With the multihull however it seems Bruce foils are still the babies to do the job as per hydropetre.

My question which I can't seem to find anywhere though is why we are not moving to a hybrid foil of surface piercing but at a much reduced angle of say 25 degrees to get the benefits of the fully submerged foil ie lots of controllable lift ( flaps ) with minimum surface area ( induced drag ) ?

Let the debate begin, open minded discussion please and personal insults are not welcome as so often foiling threads have become.

2. Joined: May 2009
Posts: 16,672
Likes: 339, Points: 93, Legacy Rep: 1362
Location: Cocoa, Florida

This is relevant to the use of flaps on surface piercing hydrofoils:
By Tom Speer:

"A wand simply measures the distance between the boat and the water. You can have any relationship you want between that measurement and the controls. Other factors besides height can be added to the feedback to the foils. Even when pure height feedback is used, it will depend on the gain between the wand and the lift on the foils (determined by the linkage, size of the flaps, etc.) as to whether the change in area or the feedback from the wand is more powerful.

For example, say the main foil is a V with constant chord. The area is proportional to height, and therefore varies with speed-squared to maintain constant lift if the foil is operated at a constant lift coefficient (approximately constant pitch attitude). The problem is, the span will also shrink with speed-squared. This will make the induced drag from the lift on the foil go up. (On a fully submerged foil, induced drag goes down with the square of speed - that's the principal reason for using foils in the first place.) So you may not want the SP foil to go up as quickly with speed. If instead of a constant lift coefficient, you operate it at progressively lower lift coefficients as the speed increases, meaning the boat is rising but also trimming more bow-down, you can tune it so the foil has essentially constant drag. The parasite drag goes up with speed-squared but on a shrinking area, while the induced drag goes down with speed-squared but on a shrinking span. The two trends can be made to balance each other to get nearly constant drag. The key is in controlling the height to keep from rising up too quickly.

Feedback from a wand would be one way to inhibit the rise. The faster the boat went, the more the SP foil would rise out of the water, but it would meet with increasing resistance to the rise from the feedback. In control terms, there would be increasing steady-state error between the height command (set position of the wand for neutral control) and the flying height. That may be exactly what you need for best performance. If the feedback is very strong, the SP foil will operate with a near-constant flying height and its performance curve will look like that of a fully submerged foil. But the feedback doesn't have to be that strong.

Now consider what happens when the SP foil hits a wave crest. There's an increase in lift due to increased immersion of the SP foil. But the feedback from the wand partially counters the lift increase. The same thing goes in the trough - the wand will deflect the flap down and reduce the loss of lift. The result is a smoother ride with less disturbance from the waves compared to the uncontrolled SP foil. Not as smooth as a fully submerged foil platforming the waves, but quite possibly comparable to a fully submerged foil contouring the waves. "

3. Joined: Nov 2006
Posts: 430
Likes: 44, Points: 28, Legacy Rep: 134
Location: UK

### waynemarlowSenior Member

Not sure what the control of the flaps has to do with my original question Doug.

What i'm getting at is by having the angled foil at a more horizontal angle it will create a greater lifting moment than at say 45 degrees. More lift then equates in my book to a smaller foil being required which then is less surface drag.

I do wonder if this has become the norm for no other reason than historical reasons of being able to build strong enough foils to handle the leverage loads, but a smaller foil is always going to be able to handle upward loads better.

4. Joined: May 2009
Posts: 16,672
Likes: 339, Points: 93, Legacy Rep: 1362
Location: Cocoa, Florida

------------------
I guess that's what it has to do with it..... In the book "Hydrofoil Sailing" by Alan Alexander,James Grogono and Donald Nigg on page 36 they say:
" Thus the practical range of dihedral for this type[surface piercing] of foil configuration narrows to about 30 degrees to 50 degrees. A figure of 40 degrees is a good compromise. A flying hydrofoil with a dihedral angle in this range will have a calculated yaw angle of about 1/2 to 1/3 that of a typical centerboarder. This means that yaw angles under foil-borne conditions will be in the order of 2 degrees. With such a low angle, there is very little incentive for improvement at the sacrifice of lifting efficiency."
This is a good book to read on surface piercing foils. Another is the just released book by Ray Vellinga.
You should probably PM Tom Speer who'll probably answer you and or contribute to this thread. And Gary Baigent....

5. Joined: Nov 2006
Posts: 430
Likes: 44, Points: 28, Legacy Rep: 134
Location: UK

### waynemarlowSenior Member

James Grogono is way back in the past and things have moved on ( as much as I do admire his work ), I do remember him breaking foils left right and centre ( actually it was a cat so can't exactly say that ) and I do remember talking to his foil builder that welded aluminium probably wasn't the way to go.

Interesting as my next question was one Gary is probably the one to answer but then anyone knowledgible will do. My proposal is to turn the foils outward and run them at a relatively shallow angle. My requirements are for a very lightweight boat and this may have some bearing on the matter. What are the pitfalls of outer facing foils ?

6. Joined: Jul 2005
Posts: 3,007
Likes: 121, Points: 63, Legacy Rep: 509
Location: auckland nz

### Gary BaigentSenior Member

angle out?

Wayne, I've posted my thoughts here in the past - but on the point of angled out foils, the only disadvantage that I noticed was greater overall beam (and it's underwater, important point) - meaning required increased observance and excellent hand/eye coordination whilst rounding ... marks, mooring lines, anchored boats, the odd log, etc - otherwise you can end up dragging an empty powerboat plus dragging anchor at full sail power (yours), the powerboat diagonally across your stern and chewing away your epoxy, with absolutely no steering control (you again) - until the horrible tangle rounds up which allows you to leap adeptly aboard the power boat, cast off its anchor, re-leap aboard your foiler ... watch the power boat drift away astern to smash satisfactorily on lee shore rocks ... while you continue obliviously seawards. Okay, this is beyond apocryphal ..... but you know what I mean .... with a little imagination.
Returning to required seriousness (actually the canting keelers could also reproduce the above nightmare scenario, except they could never cast away lines, warps etc, because of stupid bulbs holding firm)? Anyway, another point: altered Buccaneer Miranda (to 28 feet) had her foils mounted (conventionally inwards) but at around 25 degrees - so they worked as compromised daggerboards as well as providing lift. Miranda is a fast boat and does everything well ... so that is not a bad solution. On angled out foils I believe the angled out sections dig against leeway and also work as the French say, anti derive. This is not scientific, just my seat of pants assessment (not accurate). If you angle outwards at say, 10 degrees, I can see no problems - but you will require a conventional dagger as well .... you need one anyway on whatever configuration you have.

7. Joined: Jul 2005
Posts: 3,007
Likes: 121, Points: 63, Legacy Rep: 509
Location: auckland nz

### Gary BaigentSenior Member

Correction: At 10 to 30 degrees float/foil angle, you would NOT need a dagger in the main hull. Remember angled outwards with the platform heeling a few degrees, the foil is more vertical. At 45 degrees the supporting case area depth is reduced and also the foil is exiting closer to the waterline - but again the boat is heeled so cavitation is not a problem. However lift is going to bring the float and case exit point clear of water and the float hull shape is going to have a tendency to drag air down the foil - (angle the foil forward to slow this) - better to have foil exit at the float base to reduce aeration - but set in this position, the foil case is insufficient to carry the lifting loads - because it is lying very close to the inner float side. Everything is a compromise.
The alternative angled in conventional setup, the more the platform heels, more lift is generated but there is no, or very little, anti-leeway produced (platform wants to slip sideways) - therefore you'll require a dagger in the mainhull.

#### Attached Files:

• ###### IMG_1682.JPG
File size:
233.1 KB
Views:
5,495
8. Joined: Nov 2006
Posts: 430
Likes: 44, Points: 28, Legacy Rep: 134
Location: UK

### waynemarlowSenior Member

Thanks Gary for the info.

In my intended use there are no outer Amas ( this is a 4 metre long boat ), simply an extended cross bar ending in foils, intention is to make the cross bar a curved arch with wing shaped section right across so the more the cross bar dips into the water the more lift created, drag would be horrendous but it should stop a capsize ??????.

Its probably not a very good air wing section but the foil section may even be in the " ground effect " zone and may create some lift on it own.

9. Joined: Jul 2005
Posts: 3,007
Likes: 121, Points: 63, Legacy Rep: 509
Location: auckland nz

### Gary BaigentSenior Member

Wayne, you need just a "little" flotation out there - but if you're not, then your foils are going to have to be large - creating enough lift to stop your wing beam digging and grinding you to a stop ... amidst horrible drag and spray. Wing-in-ground-affect is not going to occur until you're moving fast, real fast. If you're determined to have hyper-purist foiler, then consider swelling the upper foil sections to give you some buoyancy. Just IMO.

10. Joined: May 2009
Posts: 16,672
Likes: 339, Points: 93, Legacy Rep: 1362
Location: Cocoa, Florida

11. Joined: Nov 2006
Posts: 430
Likes: 44, Points: 28, Legacy Rep: 134
Location: UK

### waynemarlowSenior Member

I would agree hence my original question on whether I can cant the foils more horizontally to say 60 degrees but reduce the size of the foil. It makes for a much less bouyant foil but has less drag at speed when things go wrong.

This is really a speed machine and nothing else, I already have a boat which I can average just under 20 knots, there is no point in making a slower boat. To do that this boat has to become a more dedicated speed machine by ( as per the Mothies ) "amidst horrible drag and spray" getting it wrong more often. But as per the Trifoilers, when it does go right boy is it a lot of fun.

OK then what about these flippers. My guess each flipper is going to have to lift about 15 kilos of boat and 40 kilos of pilot plus a bit of safety margin to a height of about 200mm only, each, at takeoff speed of about 8 knots ( below that I'm better to sail my F16 cat ) with a proposed max of about 25 - 30 knots. What sort of size foil are we talking about.

Doug, you mentioned ages ago that you found the foil section that Hydropetre was using. Would this scale down to the sort of size I'm thinking about ? Do you know what exactly that foil section is and is it available to be used by the general public. ?

Any help on this would be appreciated as I think there is a place out there for a two front, 1 rear foiler, a sort of Mothie of the multihulls.

12. Joined: May 2009
Posts: 16,672
Likes: 339, Points: 93, Legacy Rep: 1362
Location: Cocoa, Florida

------------------------
I have no recollection of that at all. I doubt the Hydroptere people would make that publically available just yet. I do have the section used by Steve Killing on Rocker(Selig S3010).
You have to be sure that any section you use is operating at Reynolds Numbers advantageous to the section you want to use; a Hydroptere section probably wouldn't be a good choice even if you could get hold of it. The following section(for Rocker) is probably a good choice...

#### Attached Files:

File size:
12 KB
Views:
1,390
File size:
203.2 KB
Views:
957
File size:
231.2 KB
Views:
2,101
• ###### S_3010_C.jpg
File size:
192.7 KB
Views:
1,992
13. Joined: Nov 2006
Posts: 430
Likes: 44, Points: 28, Legacy Rep: 134
Location: UK

### waynemarlowSenior Member

I had a look sometime ago at this route of a conventional hull with foils but quickly dissed it for a number of reasons, the main one was lack of conveniance of launching straight off a beach due to the high weight ( well compared to a sailboard or Moth ) and the fact of high cost of components to that of using sailboard sails.

My ideal design ended up really as a modified windsurfing board with a hollow rear section on the board to hydroplane on very small rear rails and foil the front section of the board about 150mm out of the water. Choppy seas would be a nightmare but these beasties would really be sailed in pretty calm seas and reservoirs as per most windsurfers and Moths. I'm still pretty convinced of the merits of tail dragging the hull until enough foward speed gets up enough to really foil properly.

Doug my recollection was wrong, looking back through my web history and it was the C class foil you mentioned, another poster had been talking about Hydropetre in the previous post or two. Interesting looking foil as it could be manufactured relatively easily, would it be suitable as surface piercer as it was intended as a fully submerged foil ?

14. Joined: May 2009
Posts: 16,672
Likes: 339, Points: 93, Legacy Rep: 1362
Location: Cocoa, Florida

==============================
I would think so-but thats a good question for Tom Speer or Mark Drela.
Why don't you post some numbers and sketches of your idea?

15. Joined: Nov 2006
Posts: 430
Likes: 44, Points: 28, Legacy Rep: 134
Location: UK

### waynemarlowSenior Member

Yes certainly will publish a few drawings etc, I need to finalise my thoughts as a few ideas I have will be crucial to the overall weight which I think is pretty fundamental to the foil design. In thereoy with the Bruce style foils weight may not be as crucial as I perhaps think though.

One more question then before I finalise drawings, how important is it to cant the rig to weather, in windsurfing it is very important to get lift and speed, does this follow with the apparant winds I would expect at the speed range I hope to achieve.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.