"Simple" hydrofoil addition to a laser II

In order to experiment with hydrofoils I was willing to construct some structure to attach to my Laser II dinghy.

The structure I have in mind would include 4 surface piercing foils, one on each corner, 45 degree in the water, similarly to hydroptere but 4 instead of 2 plus rudder.

The reason to have 4 put in this way is to "auto-stabilize" the boat, meaning the section (front, back, left or right) that will go more in the water will be the one with more push from the foils, so trying to revert this movement.

Moreover (again similarly to hydroptere) I was thinking of a decreasing section from the top to the bottom, in order to have at the top a powerful lift in slow speed while at the bottom minimizing the drag to increase speed when lifted. Again this should also increase the auto-stabilizing mechanism (the more in the water the more the lifting force).

As last design point I would maintain also the vertical end (45 more degree from the foil) to increase efficiency of foil (minimize parasite drag as I understood) and to use it as a rudder/daggerboard surrogate help. But I would discuss this point after the initial design.

Considering that Laser 2 all equipped and with 2 people on board should weight around 250-300kg, each foil (fully inside the water) should give an initial lift of 100kg (300/4*sqrt(2)) at let's say 4-5 knots of speed.

Said that, to more easily build them I was thinking to use the ones with profile that will have one of the side flat (like e.g. ag35-il), so that shaping would be required mainly on one side only.

The questions for the experts are: which ones would be the best top and bottom sections? and what proportions to keep between chord and top-bottom distance, and which approximately will be then overall dimensions?

Thank you!

 

470

I'm no expert but one of the things that helps Hydroptere to work is her oversquare beam -getting the forward foils as far apart as possible. Also seems going to four foils is just adding drag-a rudder T-foil ought to be plenty with two widely spaced forward foils and a moving crew. Good Luck!

470 on foils years ago:



Note distance between main foils:

 

Also might want to consider retractable surface piercing foils similar to those developed by Hugh Welbourn for the Quant 23 and Flo 1. The unique thing about them is that they do not provide lateral resistance* for the boat like most other hydrofoils-but they do provide vertical lift and substantial righting moment.
* a boat using these foils needs a keel or a daggerboard/centerboard.

Flo 1


Quant 23 with (I think) Benoit Marie sailing her:

 

You might want to look at Hanno Smits hydrofoils for a Flying Dutchman. He went with a single pair of surface piercing foils and an inverted T rudder. He has tow tank test data on his site, too. I fit some simple relationships to his data and was able to get a pretty good match. You could start with them for your initial sizing studies.

The most important thing for foiling performance is to get the sizing of the foils right. This includes span, area, and (for surface piercing foils) the running angle of attack. The angle of attack is important for surface piercing foils because their performance is a strong function of the amount of submergence.

A fully submerged stern foil, like the inverted T, is a good match for a surface piercing main foil because it naturally leads to an increasing bow-down attitude as the speed increases. This is helpful to keep too much of the main foil from coming out of the water, which increases the drag due to insufficient wetted span.

 

You might want to try it with a model first rather than spending a lot on something that may not work. Most foilers balance the bulk of the boat's weight over a single foil or pair of foils and then use the horizontal foil on the rudder to control boat pitch (in addition to moving crew weight fore and aft). With a foil on each corner you may need to have an automated system for controling their pitch - if they're set at a fixed angle you'll likely suffer from porpoising with the boat making sudden jumps upwards or crashing down violently, and that will probably happen because any change in boat pitch will lead to all foils generating more or less lift at the same time. Experiments suggest that having 80% of the lift generated by the main foils and the rest coming from the rudder foils is as far as you can go without running up against this porpoising problem.

To get round the problem, you may need to use wands to measure the height of the front and back of the boat over the water and feed that mechanically into a system that increases the pitch of all four foils if the boat tips forwards or decreases the pitch if the boat tips back so that the lift remains fairly constant. An alternative to wands might be to use a damped pendulum or go for electronics and accelerometers, though keeping the boat level in waves may not be as good a solution as keeping it parallel to the water's surface.

There is a small motor-powered foiler with four main foils which is very stable, so they must have solved the problem in some way (no sign of wands), but I have yet to see any explanation of how it works. It may be that there's a simple solution, but you don't want to spend a lot of money building something that doesn't fit with any available solution, so you want to be very sure that it'll work before you build it. Models are the way to go.

 

Thanks for the comments! I'll try to reply to them:

The front foils are aligned to what I have in mind!

Pretty interesting, I'm trying to grasp all the physics and math inside, but I do not see foil section.

I printed the appendix. I'll try to study it!

This was this thread for, trying to determine the approximate size (and shape). In my opinion T submerged back foil is for sure more performant, but then should be properly dimensioned in order to work well in the system. Not only, with 3 foils (back T) you need also to properly position the front foils, while with 4 foils as I described (with increasing power with the submersion) the boat should "self-regulate" itself... I wanted to try this.

Sure will be better a model, but anyway if I can have a sections with one surface flat also target ones should not be very difficult nor very expensive to build, and while I do have a laser 2, I do not have a model boat!

There are examples of this "porpoising" effects on 4 corner foils as I described?

The foil increasing power (or decreasing going bottom) should also minimize this effect!...

I want to try to avoid this pitch controlling mechanisms and have just 4 "simple" foils in the corners to "auto-balance" themselves... The small motor boat you're describing might use this principle and no other sensors/actuators...

 

Why would you need a model boat to test model foils? You want to test the stability with the hull out of the water, so you don't need a hull. Just build a model of the foils on a frame and lower it into a fast-flowing stream (or over the side of a boat that's moving along over smooth water).

I don't know, but with boats where they try to put more lift on the rudder foils and less on the main foils, they report problems if they go significantly beyond 20%:80% lift, and what they've reported is sudden violent crashes downwards or leaps upwards. It may be that if they kept going towards 50%:50% they would find it getting stable again, but there's no guarantee of that, which is why it would be wise for you to test a model first in order to find out. Whatever you do though, there are a lot of people would like to hear how it goes, so please let us know.

It may not make enough difference quickly enough to stop a sudden violent movement upwards or downwards. That's why it would be worth testing this with models first, and you can change the design many times for low cost.

Hopefully no active control mechanisms are required, but don't be surprised if it turns out that they're essential. Test a model - you will almost certainly save money that way and end up with a better set of foils on your boat.

 

Test model

Fully got your points!!

Like this model will be just reduced size foils attacched to a simple structure (some tubes) with a boat dragging it!

Now need just to understand what to prototype!

 

When towing the frame, you want the line attached to the middle rather than the front, so leave the front of the frame open (no crossbeam there). The other end of the line can be attached to a pole sticking out sideways from near the bow of a boat so that the model is to the side in relatively smooth water, though if it works as well as you hope it will, it should work in the wake of the boat too. I'd also think about putting a handle in the middle of the frame so that it can be held with that and lowered into the water without being towed, and then you can apply a twisting force through the handle to push either the front or rear foils down to see how well they handle the pitch changes, but if the model's too big for that, you can still push either end down with a stick while towing it.

 

Testing

I think if you test the four foil concept as a model it should include the hull and be carefully done and as large as possible. The model should be towed with a line attached at the Center of Effort of the sail plan(to simulate the pitching moment of the rig) with the line led at an angle from the model to the towboat(to simulate leeway). This requires a controllable rudder(done via RC).
Or just sail the model via RC.
See the video below at 22:50 AND 28min in to see how TNZ rigged their large(1/3rd scale) test model:
https://www.youtube.com/watch?v=XQoNYe2jFP8

 

For later testing that's good advice (which I'll make use of myself), but if there's a major problem with the idea it should show up before you go to the trouble of building a model hull. If the thing has to float, I'd just use plastic bottles to begin with because the most important thing to test is how stable it is when up on the foils. A fixed rudder on a pole some way out the back should keep it on course. If its behaviour is too wild, there's no point in building anything more complex.

 

I recall a video of Tom Speer presenting the mathematical conditions required for stable foiling. I think it was at foiling week a year or two ago. At any rate, I would review all the papers on his site -as aerodynamics go they are very 'accessible'. Another good one I recall compared height control -T foils vs V foils.

There is one topic I think we all need that I have never found -characterization of the lift and drag of hydrofoils near the surface (less than 1 cord deep).

 

Here's a good place to start:
http://ntrs.nasa.gov/search.jsp?R=19930092993

 

Design Build Fly

One of the best guides around for the designer of small hydrofoils is Ray Vellingas book "Hydrofoils Design Build Fly".A small amount of stuff on the Moth as well. Lots of good reference material.
I think it's still available from Amazon.

 

foils and top speed

I have a dilemma concept that troubles me also. How to get up above 30 knots. Using intuition I have a design concept.

Absolute square footage of foil is a top speed issue. Something that rises up on foils at 10 knots is going too have more square footage than wanted at 30 knots. A vessel mass to square foot of foils might be a critical design variable. An all sail vessel/just nets is the obvious design maximizing this variable.

The concept of low end foiling as needed is backward in my thinking. I would use a step system of foiling. Get the vessel up to planning speed, about 20 knots and engage the foils. Size the foils for this start speed and then the drag of foils will not have a 30 knot barrier.

The Americas Cup catamaran foils are tipped to rise out of the water as speed increases. On a monohull it would be an inverted U foil. Or an inverted T foil sized to 50 knots plus. Foil drag is this dramatic.

The chord length of a foil is a personal choice I consider. An aspect ratio of foil length to chord could be varied. The goal would be a foiling sailboat heeling dynamics, intuitionally.

Low speed foiling is like using a flat plate representation of the laser in the first place. If the foils are angled and sized flat plate equal the top end is lost.