High Performance MPX Foil/Self-righting Trimaran-The Test Model

Hello Curtis. My experience is that wind speed and sail area also need to be linked to the efficiency of the foils. I launched a new trimaran this weekend. It has a lower aspect ratio rig, the rig is further forward, the foils are further forward, the foils are a new shape, and it foiled in much lower wind speeds than I have managed in the past.
You can see the boat sailing and foiling here:
http://www.youtube.com/watch?v=O3PJO8iX_8U&feature=youtu.be

Its designed and built to the Mini40 rules. I started building the boat in October last year. The current rig is made up of old 10rater sails cut down, but I will have a new set of rigs made up soon. The foils are made by eye, and are thicker than anything I have seen form professional builders, but my experience is that thick foils work well on rc models

 

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Welcome to the forum, PM40! Richard Woods posted your video yesterday on the previous page and it is terrific!
My experience with foil thickness is just the opposite of yours-thin foils seemed to have worked better than thick foils. What is the thickness/chord ratio of your foils? That would be really helpfull in understanding what is going on. My foils are 9% on the F3 and 10% on the new boat.
Congratulations on a first class boat!

PS- I hope you can share some of your specs like the above for the F3, MPX and Curtis's boat. These details can help everyone building RC foilers and would be a great contribution to the data base:
1) thickness of a main foil divided by its chord,
2) foil area of the main foil and rudder foil
3) total weight
4) sail area for lightest wind takeoff.

 

logically (hydrodynamically) thicker foils will generate more lift but that comes at the price of more drag.

So if you have enough power to foil, then a thicker foil will get you foiling faster than a thinner foil. But a thicker foil - because it generates more drag, will limit your upper speed range.


This is the game that the Mothies are currently engaged in: the top guys are starting to bring "quivers" of foils to any major regatta, and choose the foil they will use as close to the event as possible based on what they think the forecast is. Because if it is at the bottom end of the foiling range, then a thicker, higher lift foil, that keeps you foiling longer will pay off.

but if the wind builds to where foiling is consistently achievable and Fybes and Facks are possible, then a thinner foil works better.

But then again - most of those sailing Moths at the top level are engineers. I wouldn't take their views that seriously in the face of the expertise demonstrated in this thread.

 

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Hello Curtis-your numbers are fine. There is probably a way to calculate the windspeed necessary to foil but I'm not familiar with it. The best way is to compare with a boat of proven performance like the F3.
The F3 would take off in very light air(see specs above) and would take off at a boat speed of 5 mph(same as windspeed). It's top measured speed was 20mph(17.4 knots) and it was capable of more. It did this speed fully reefed in a 15-18 mph breeze( 13- 16 knots).
Good luck with your test sail!

 

Hi BB,
Your reasoning works fine in the world of full-scale boats, but not in the world of RC models. The main difference is made by the Reynolds number (Re). At the very low Re range (where R/C models normally operate), funny things happen to the "hydrodynamic" logic, and the thin airfoil sections are indeed known to perform better than the thick ones.
In the Re regimes characteristic of full-size boats like Moths, the thick sections perform better than thin ones.

A small digression into the aeronautical world - this different behaviour of airfoils at very-low and high Reynolds numbers is one of main reasons why early airplanes were all designed with very thin wings. All the available wind-tunnel data of that time (early years of the 20th century) indicated that thin sections were the optimum ones. What those brave pioneers of aviation were ignoring was the difference between low-speed flows in the wind tunnels of their time and high-speed airflow around full-size wings. This difference is expressed mainly through Re number, for incompressible flows. Once the aerodynamic research started to investigate by properly distinguishing various airspeed regimes, the airplanes started to grow fatter wings, which had the additional benefit of being structurally strong enough to allow the introduction of monoplanes and dismissal of bi- and tri-planes.
Cheers

 

There are even more tricky details with thickness.

For example, if you need a long foil for a surface piercing application, thicker may be the only way to get required strength without going totally oversized.

I also did some some evaluation efforts for a reversible camber wing (hard sail application) that found that overall thickness did not seem to drive the high drag as much as how thick the back 30% percent was (discussed in: http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/what-significance-wing-thickness-45383-2.html#post619208.)

And yes, those pointing out the effect of Re on foils are absolutely right. Most models operate in a totally different range than applicable to a full sized boat. At a very very low Re, a simple thin plate does a lot better than many would think.

 

Foils sails ect

Hi, thanks for that. Foils, similar to Dougs are 9 and 8.75% thickness. (Moths are at about 7%)

At the Reynolds Numbers my boat is working in ( 60000-100000) the drag coefficient is about 1/3 of the way up the drag bucket (0.55) and the lift coefficient is 0.6. when foiling the lift coefficient is 0.2 as is the drag. this is for the main foil with a servo controlled flap.this is a symetrical section- NACA0010 at 9%.

The AMA foils are (will be) NACA 63-412 with a thickness of 8.75%. these have a lift coefficient of 0.5 constantly and a drag coefficient of 0.3 (1/5-1/4 up the drag bucket)

In essence my foils are fairly efficient but far more efficient after takeoff because my boat is designed to take off at a boat speed of 1m/s!! so there is a bit of unnecessary area there....

How much did you change the aspect ratio by on your sails like what percentage is the top of the headsail at/ how much did you lower the mast?

My boat is 90cm long and has a mast of 1m and the headsail comes 82% up the mast. Total sail area is 0.35m^2 roughly.

I've seen a lot of your videos, your boats look great. I'm in Australia too but over in Brisbane....

Its great to see another Foiler out there!!! mine should be done in 3-4 weeks!!

 

Hi Doug thanks for that. Did the F3 get to 5mph fairly quickly or was it close to the boat's 'in water' top speed? 5mph is about 2m/s, double my design takeoff speed.

The F3 was 1.4m in length(?) compared to 0.9 so In theory my boat should get to 1m/s fairly easily, so do you think I should reduce foil area and drag and change the takeoff speed to something like 1.5-1.7m/s? (3.4mph)

what are you thoughts? thanks so much for all the help!

-Curtis

 

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Curtis, the F3 got up to speed very quickly. It had enough power that even in light air the boat could be pitched up(by sliding the battery back) and it would "jump" onto foils in just a couple or less boatlengths.
I don't think you should make a single change until you test sail! Just make sure you can accurately measure windspeed and boat speed(lay out a small course with makeshift buoys and an accurate distance between them and time it if necessary).
And after sailing, then analyze what happened and why and make one small change at a time if you need to.
As a suggestion, make sure you have some help there for your first test sail-it will help you and assure you get all the info you're after.
Best of luck!

 

MPX Test Model

There has been some discussion about the correct positioning of the main foils on a foiler on a European forum and elsewhere. The F3 had its two mainfoils positioned so that the quarter chord was ahead of the CG by the amount necessary to allow both mainfoils to support 80% of the boats weight.
That is a rule of thumb that has proved to work. The MPX is set up the same way.
The picture below shows a side view of the F3 illustrating the relatively forward position of the main foils and rig:

 

Here is the noggle with the 30 degree plates in place(pictures 3-6). The noggle automatically keeps the center line of the boom 30 degrees off the centerline of the wing mast. Its not much more work to make each plate adjustable. For now, 30 degrees seems like a good starting point with a 7% main foot camber to start(fully adjustable).

click--

 

MPX Test Model

I've had a little time to work on the boat. Working on detailing the mast:
adding the boom(detailed), vang, mast heel,mast step pin and plastic ball, noggle/goosneck, noggle rail, gaff control device, gaff goosneck, gaff(detailed), mast tang for forestay and shrouds, and the luff wire tubes. Much of it complete and pictures before too long.

 

MPX Test Model

Here are three pictures of the mast-boom-vang-noggle-noggle plate-noggle rail assembly, mast step pin and rotator ball. The rough parts are the carbonated(epoxy/carbon tow) straps that retain the noggle rail-may still get a dose of cosmetization*. The vang is monumentally powerful and should serve the rig well:
*I will spend no time on the cosmetics before she sails.
click for best view:

 

What are those black blobs on the carbon tubes?

 

MPX Test Model

A couple of notes:
--Next is installation of the gaff, gaff gooseneck, gaff control device, gaff control line and the mast tang for forestay and shrouds (led to the front of the mast) and main.
--Both the gaff and boom will use a method from previous models to attach control lines, outhauls(upper at gaff and lower) and mainsheet: a grommet is wraped with line and a loop is created for the various attachments. The grommet slides over the boom or gaff very tightly but adjustable by hand-works good.
--Luff wire for main and retaining tubes are already attached to the mast-just a matter of making small rectangular(one sided) cuts to the luff and threading the wire down the luff, and attaching the throat, peak, tack and clew to the rig and the rig is very close to being ready to sail.