MPX-11 Very Small High Power Trimaran

Variations on a Theme

The concept of using two lifting foils on the main hull of a small trimaran has loads of benefits not the least of which is that it allows the boat to use a square or over square configuration thereby utilizing close to the maximum RM possible with a trimaran. Without the lifting foils the boat would have so much RM that it would not want to fly the main hull until the wind was around 15 knots or so.
The concept is not limited to using planing amas but could just as well use a small ama with a "DSS" type foil(if it can be kept low enough) or a small ama with a t-foil + wand. In other words, the concept is open to being "foil assist" or full flying foiling. The MPX-11 and 12 are "foil assist".
If the concept was adapted to a three foil system it could have significant advantages over other three foil foilers:
1) The Rave uses three identical foils to generate lift and RM but has relatively poor light air foiling performance mainly due to its weight but also to having three foils(and three surface penetrations). The three foils are in the water from the time the boat is launched and because they have dual independent altitude control systems(wands) on the main foils as the boat begins to heel the windward foil pulls down and the leeward foil pulls up creating virtually unlimited RM in strong wind-limited only by the structure of the boat.
2) Version 1: A boat using the MPX bi-foiler system(two foils on the main hull with an altitude control system on the main foil) could also have a retractable small third foil for use only at speeds over about 15 knots. This would potentially allow the boat to be sailed like a Moth in relatively light air with a lot of sail area and just the drag of two foils. When the wind picks up and the boat speed begins to exceed 15 knots the leeward foil(about one half the area of the main foil) could be deployed and would gradually support more and more of the boats weight while the two main foils are UNLOADED to a large degree except for their pitch control responsibilities. This would allow for a boat that took off very early(5 knot wind) but that could use its tremendous RM in heavy air for very high speed. The boat would still require a "tilting" main foil as does the MPX 11 & 12 for optimum upwind performance.
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3) Version 2: The same as Version 1 EXCEPT that the boat would be heeled to windward(veal heel like a Moth) and the deployed foil would pull down
over a certain boat speed. This has advantages in generating around 20% more RM as well as not requiring a special daggerboard since the main hydrofoil would be oriented as it is on a Moth and would not only unload the vertical strut but would generate lateral resistance for the boat. The sail would also contribute a small amount to vertical lift in this configuration.
See the rough sketch below showing how "Veal Heel" on a small trimaran would work and you can read more about how it works on a Moth in the paper below.
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4) Both Versions 1 & 2 would have advantages over other three foil foilers in light air. Version 2 would have those advantages PLUS the advantages of "Veal Heel" in all conditions with an approximate increase in RM of around 20% "for free". It is interesting to note that these advantages of Veal Heel are not possible in a beach cat design-only in a monohull foiler or a trimaran foiler. And the combination of the MPX system-very wide trimaran with two foils on the main hull+ retractable "power foil"-gives the trimaran a potential edge over the monofoiler.

On rough sketch below,click on image and then again on resulting image for greatest detail:

 

Basic Specifications Page-updated thru Version 5 of the Design Specifications
11/13/10


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MPX-12 Specs

-- LOA-12' 7"

-- LWL-12' 7"

-- Main hull beam-5.29'-

-- Main hull beam at the waterline -3'

-- Overall beam- 17.5'*(15' cl ama to cl ama) updated and corrected 8/19/10--NOTE: approx the same overall Beam/overall Length as Hydroptere.(not including gantry on MPX-12) *tentative as of 11/10/10

-- LOA- ama-10'

-- Ama LWL(F&A dimension of planing surface)-1.5'

-- Ama beam- 2.5(max and max at waterline-tentative)

-- Ama Buoyancy - 3.25 cu.ft/ 208lb / 53% of sailing weight

-- Sail Area- 178 sq.ft /25.5' mast length. Slightly taller than a scaled down A Class Cat.

-- Main foil planform area-1.6 sq.ft

-- Rudder foil planform area-1.3 sq.ft.

-- Draft(max) -3.5' tentative

-- Draft @ 10 degrees -2.2' tentative

-- Weight-159lb all up,ready to fly minus crew

-- Total sailing weight(displacement)- 395lb-

-- Max crew weight-236lb

-- Minimum crew weight(at max power)-120lb (boat can sail in same windstrength with minimum or maximum crew weight(!)

-- Max Pressure/w/o reefing 1.8 lb/sq.ft( 1.8 for F18 cat)

-- Designed Sailing Angle- 10 degrees from 5 knot wind. Maintained by wand surface sensor in conjunction with main hydrofoil and rudder hydrofoil.
System allows hydrofoil to lift up or pull down automatically-regardless of wind(up to 1.8lb. per sq.ft) or crew weight(120-240lb.)

-- SA/WS:
a. not flying main hull-5/1
b. flying main hull-13.8/1(moth on foils=13.65/1)

-- SA/D= 47.47/1

-- W/SA= 2.42 (With a 236lb crew on MPX this ratio is still better than Moth w/Veal or Payne!) (Weight/ Sail Area="sail loading"-quick and dirty comparative ratio for low resistance boats-particularly foilers. 26' Mirabaud and 11' Moth about the same.

-- SCP/total weight= 72.9% (according to Bethwaite above 30%=capable of upwind planing)

Notes:

---The crew will sit on a very comfortable sliding seat with a backrest.

---The seat will slide a maximum of 2' .

---The boat will have a simple robust folding system-nothing to take apart-ready to go in 5 min.

---This boat will use just two lifting hydrofoils which are critical to its operation.


For more detail and comparisions to other boats see the Version 5 specifications sheet- post 186, page 13.

 

Has anyone done a study of the drag difference between inverted T or Y foils (with the junction connection being the big culprit) to those of a curved or a J? My thoughts are the T and Y are more draggy and create more disturbance - but have no evidence.
That is why I question all your IT foils, Doug. The outer foils could be straight and do the same work, yet with less inefficiency.

 

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I haven't seen such a study and I like "J" foils-the only problem being(for now)
how to make a flap move.(maybe a flexible shaft?) Got any ideas?

PS- heres a paper where Tom Speer looks at T-foils, V foils, and ladder foils:

 

A J foil could have a single flap with a torsion cable to drive it (like a speedometer cable). However, I am not sure how workable such an arrangement would be as it might not provide enough torsion and it might be too springy. It would probably only be effective with very small flap loads.

Of course the method used by Longshot & the trifoiler was to rotate the entire foil up topside to change the angle of attack. A J foil used in this manner has a clear advantage that the foil can be an optimum profile without any flap joint and without losses due to flap deflection. The disadvantage is probably the required operating force. However, for a manually controlled system with two J foils up front, a single Joystick could be used to "fly" a small to medium sized boat with full control of both roll and craft height. This may not work out for top performance, but it kind of sounds like fun (jumping on command, etc.)

I would like to see some honest J foil studies & comparison work for another reason. I am not sure, but it seems that there may be an advantage to limit the number of foil tips. There are not many example of top performing T foils designs with 3 or 4 foils in the water. The J foil actually has only one tip and no strut to foil junction per foil. When you look at how much better most T foils are aspect ratio wise, is seems amazing how well J foils have done.

 

MPX Foil System

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I'll look more into flex shafts and write Greg Ketterman about the idea. Its worth considering if there is zero backlash/springiness.

 

J-Foil / flexible shaft / flap

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Mr. Flados, received an e-mail response from Greg Ketterman and he said that he had considered using a flexible shaft to move a flap on a J-foil but never actually tried it. He thought it would be worthwhile to experiment with it.

PS-if you want to read his response e-mail me and I'll send it to you.

 

Doug, those shafts may work if you have light loads, however they only have any significant strength in one direction (of rotation), if you load them the wrong way they unwind themselves and snap

 

J foil flap control

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Yeah,Cheesy-I've used them on UAV heli's and for other control applications. There are some available that can load both ways ,if I remember correctly.
And for a flap application they would only have to turn about 20-30 degrees absolute max each way-if that.

 

Variations on a theme

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In addition to the other advantages of the MPX control system on a full flying version using two foils on the main hull is the ability to use manual altitude control-particularly for racing applications. The flap on the main foil can be controlled by a twist grip on the tiller extension. It's simple and it is fun....

And this is something that seems like it may be worth testing on a model in both wand controlled and manually controlled versions:

From the F18 thread:

 

MPX-12-design focus: Sail Area to Wetted Surface Ratio

This is an out-take from the Main Spec sheet focusing on the SA/WS ratio of the MPX-12 compared with a Moth. This comparison is done with the modified planing area discussed earlier. This comparison is done with the Moth flying and with the MPX-12 flying the main hull.
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The changes to the boat to facilitate a one off(and on which the model is based) shown in the post above reflect a very high SA/ws and a low W/SA.
A) MPX-12 Sail Area/wetted surface ratio:

--
Main foil-(1.9 sq.ft. planform area)--------------3.8 sq.ft.
Rudder foil-(.8 sq.ft planform area)--------------1.6 sq.ft.
Vertical Fin/dggrbd(1.76' immersion)------------ 2.1 sq.ft.
Rudder----------------------------------------1.4 sq.ft.
planing surface(ama)approx. as of 11/8/10------4 sq.ft.
____________
TOTAL WETTED SURFACE---------------------12.90sq.ft.
MXP-11(12) SA=178 sq.ft.

SA/ws= 178/12.90= 13.8/1


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B ) Comparison to Moth:
--------------
Main foil (planform area 1.1 sq.ft.)-------------2.2 sq.ft.
Rudder foil( planform area .88 sq.ft.)-----------1.76 sq.ft.
Vertical Fin/dggrbd(18" immersion)-------------1.17 sq.ft.
Rudder---------------------------------------1.17sq.ft.
_________
TOTAL WETTED SURFACE-------------------- 6.3 sq.ft.
Moth SA= 86 sq.ft.

SA/ws-86/6.3= 13.65/1



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MPX-12 Foil Loading-comparison with Moth

A) Main foil loading(assuming that the main foil supports 80% of the weight). On the MPX-12 the foil lifts both up and down.

1)---Because of the nature of the foil system on this boat the highest foil loading occurs in the lightest air(.3lb. per sq.ft. wind pressure) at takeoff. In this case the main foil loading is 252lb. of vertical lift( 157.5 lbs. per sq.ft. main foil area).
2)---The next highest loading is with the minimum weight crew in maximum conditions(1.8lb. per sq.ft. SA pressure) where the the load is 187 lb. down force ( 116.8 lb. per sq.ft. main foil area).

3) Summary:

Heavy Crew-Max vertical lift: 252lb(157.5 lbs per sq.ft.)

Lite Crew-Max downforce: 187lb( 116.8 lbs per sq.ft.)
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B)---Comparison to a Moth(vertical lift only-main foil @ 80% ) :

1)---Lite crew(154lb)=160 lb/per sq.ft.

2)---Heavy crew(180lb)=178.9 lb. per sq.ft.

 

MPX-12----design comparisons

Interesting A Class & C Class Cat comparisons with Moth and MPX-12--ALL WITH 175 lb* crew:
*(2 X 175lb. for C Class)

1) W/SA(weight in pounds divided by sail area in sq.ft.) smaller better :
a. Moth(66+175=241lb) 241/86= 2.8
b. A Class Cat( 150+175= 325) 325/150= 2.16
c. MPX-12( 155+175= 330) 330/178= 1.85
d. C Class Cat(approx.)-680/300= 2.27


2) SA/WS(sail area in sq.ft. divided by wetted surface in sq.ft.)
Note: A & C Class calculated with one board, one rudder and flying one hull(A only). In addition 10% of its wetted surface is deducted to account for lift from the new curved boards.

A) FLYING: (higher=better)
a. Moth(from above) SA/ws= 13.65/1
b. A Class Cat(wetted surface=21.53 sq.ft) SA/ws= 6.96/1(corrected)
c. MPX-12(from above) SA/ws= 13.8/1
d. C Class Cat (wetted surface incl 1 board,1 rudder=35.63sq.ft./approx.) SA/ws= 8.42/1

B) NOT FLYING : (higher=better)
a. Moth- SA/ws= 3.51/1
b. A Class- SA/ws= 5.56/1
c. MPX-12- SA/ws= 6.73/1
d. C Class Cat(approx.)- SA/ws= 7.5/1
==========================
Note: these comparisons using these ratios are a good way to get a ball park feel for the design of the MPX-12. They relate the design targets to actual, proven designs which is as close as you can get until a prototype is built or additional design testing is undertaken. At this point, the 12 looks very promising(and very exciting) and its control system seems to have wide application to other boats. The planing amas will be interesting to test-whether they work well or not. Lots of potential for the whole design. More to come....

 

MPX-12 UPDATED: How it works

The Theory behind the MPX-12(based on proven performance) :


1) The hydrofoils' principal job is pitch control and sailing angle(heel) control. Pitch control is critical as is a well controlled heel angle for the planing ama to be effective(optimum running angle). The sailing angle is controlled by the surface sensor(wand) setting and is adjustable. The ama
may use a small fully submerged foil set at a fixed angle to assist the planing ama in certain conditions.

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2) The foils are designed to lift the boat off between 6 and 8 knots of boat speed-about 5 knots of wind. With the 16.5' beam the thing wouldn't fly the main hull until over 15 knots of wind or so without the lifting hydrofoils on the main hull. Most people sail in 10 or less so it is essential to have the boat perform exceedingly well in those conditions. In up to 10 knots of wind there is no need for the amas to touch the water since the crew has the ability to move to keep them clear-after that, the leeward ama gradually loads up until it is carrying most of the weight. As that is happening the foils unload reducing their drag considerably. The reserve "power" of the foils is always there for pitch control. This allows the wetted surface of the main hull to disappear early and the drag of the foils to drastically diminish as speed picks up. The beam is necessary to generate the tremendous righting moment required to sail fast in a breeze.

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3) A side benefit of a wand controlled main foil is that not only will it lift vertically but once the boat starts to heel beyond the "set" altitude(heel angle of 10 degrees) the wand causes the flap on the main foil to go up generating downforce(extra RM) as necessary up until the maximum recommended speed where the sails should be depowered and/or reefed. This allows a very wide crew range since a 120lb kid would be able to sail with the same maximum wind pressure as the heavy crew because of the extra RM from the downforce of the foil.
Response of the altitude control system is virtually instantaneous. Some of the early posts in this thread discuss other boats that use an altitude control system for VERTICAL LIFT and DOWNFORCE-like the Rave and Hobie Trifoiler.

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4) This system is critical for the performance of the boat-without it in light air or heavy air it would be a dog. All of it works together and is inter-related-without one part of the system the whole thing is useless.

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5) The MPX system is not just about performance: it is about performance that is easy on the crew. No other boat this length anywhere that I have seen has the combination of high performance potential, based on comparative design analysis, and crew comfort as does the MPX 12(or any boat using this system). The crew sits on a sliding bench seat that is exceedingly comfortable at the design sailing angle of 10 degrees. The seat allows the crew to move up to 2' athwartship and 3' fore and aft but the movement is only necessary when racing. For just daysailing the crew can sit in one position should they choose to.
The foil system is completely automatic requiring only that the crew engage it. The foils are retractable and the boat can easily be sailed off a beach and trailered with the foils in their retracted position. The boat folds so that it is within the legal limit for trailering anywhere. The system will be very simple and very quick.

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MPX-12 rig design

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After looking carefully at this idea I've decided against it-primarily because I can't get the weight low enough. But it remains something to look at again.
With the news from the Moth guys about the poor performance of the wing in marginal conditions and downwind and looking at Ben Hall's results in the A Class with his wing I think its fair to say the the modern soft sails in those two classes are extremely refined-and all things considered-the best way to go.
Surely the best way to go with this prototype should I build it.


actual rig the same configuration as this but a bit taller:
(click on image)