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

[DennisRB]

Oh yeah. You did. I am particularly eager to see the self righting aspect of the build.

 

[Doug Lord]

MPX /SRT Test Model

First assembed pictures since it was painted-keep in mind the thing is 7' wide:

Click--

 

[Doug Lord]

MPX / SRT Test Model

In these shots ,I mocked up the bucket seats that the boat would(might) use full size.(remember, this is a mostly scale model of an 18' high performance trimaran) They won't be on the test model. Only two seats would be used and each can slide fore and aft and fold over for transport. No tramps on the full size version though there may be crossarm fairings on the model and full size-down the line.
The seat back is tailored for the position of my head(!) but it may be too high-we'll see.
click-

 

[Doug Lord]

MPX/ SRT Test Model

A different perspective:

click--

 

[Doug Lord]

MPX Test Model

I wrote this in answer to a question on RC Multihulls(SA) about why the main hull was so wide and didn't I think that would cause drag in light air:
----

As to my boat, the L/B ratio of the main hull is wider than a "normal" tri specifically to allow the use of a full size hull I already have to build a prototype, at some point, should the model prove my concept. A secondary reason for using this hull type is the bow volume which, including the deck, is much greater than a "normal" trimaran main hull and plays into the self-righting concept I want to test as well(as do the "curved pieces" on the amas).

The boat has the power and the foil design to allow it to take off at 5mph(4.35knots) of boat speed achievable in 5-6 knots of wind. At that point ,of course, the main hull has no more water drag. But there is another potential benefit in tough conditions: the main hull will be much better in resisting a pitchpole than would a normal main hull. I have designed and built a 14' and 20' tri using planing main hulls and they have to have more power than a normal L/B ratio hull to do well in light air but were great in heavy air. As in all design, this is a compromise to satisfy a number of competing requirements. The oversquare beam, hydrofoils and powerfull rig should allow it to perform very well in very light air and heavy air and that is the reason for building a test model of the concept.

If I was designing a fast tri, with no other requirement than speed ,I'd probably use a high L/B ratio main hull and eliminate the "curved pieces" that are part of the amas on this boat. I would, however, maintain the high dihedral of the crossarms, the planing + foil ama , and the wand controlled main foil / rudder foil on the center hull.

PS-I'm considering fairings on the crossarms to reduce aero drag-after she flies the first time. Cost ,in terms of weight will be offset by the better aerodynamics.

 

[boradicus]

Doug, this is a fantastic looking model. Have you tested it on the water? Can you explain how it is supposed to self-right again? I saw something early on in the thread about the righting moment both lateral and vertical if it were to pitch pole. I was trying understand the mechanics of what would actually happen. Are the extended foils heavy enough versus the bouyancy of the tops of the amas? What would cause the capsized model to right itself?? I am new to the forums but I am excited by all of the designs I am seeing!

 

[Doug Lord]

MPX Test Model

Doug, this is a fantastic looking model. Have you tested it on the water? Can you explain how it is supposed to self-right again? I saw something early on in the thread about the righting moment both lateral and vertical if it were to pitch pole. I was trying understand the mechanics of what would actually happen. Are the extended foils heavy enough versus the bouyancy of the tops of the amas? What would cause the capsized model to right itself?? I am new to the forums but I am excited by all of the designs I am seeing!

===================
Thanks! There are pictures a ways back of the float test but since it's not complete yet it hasn't been tested under sail. To self-right the daggerboard would have ballast on it at the board/ hydrofoil juncture.
It's probably unlikely(but not impossible) that the boat would capsize-pitchpole is more likely but the planing main hull would reduce that chance a bit. Tripping over the ama is the most likely scenario-if any scenario could be said to be "likely". The volume of the forward part of the main hull plus a sealed mast and the crossarm dihedral+ the curved pieces should all help right the boat.
Welcome to the forum-hope you have a great experience!

 

[boradicus]

Doug - thanks for the hearty welcome! And thanks for your explanation! Sounds like there is a lot going on with this design! As long as you are using the mast's bouyancy to help right the trimaran, do you think perhaps making say the last foot of mast its own bulkhead with a manual release airbag would be of any help? I think this is a great idea, because it helps to tackle the one issue other than hull separation that would most likely create an issue out on the water. I also like the idea of the deployable hydrofoils, as I have heard that in the shallows or in fishing areas you can get caught on lobster traps, trash, and such that could rip your hulls up before you are up and planing. Great job on the design so far! I hope to see more!

 

[Doug Lord]

MPX /SRT Test Model

Doug - thanks for the hearty welcome! And thanks for your explanation! Sounds like there is a lot going on with this design! As long as you are using the mast's bouyancy to help right the trimaran, do you think perhaps making say the last foot of mast its own bulkhead with a manual release airbag would be of any help? I think this is a great idea, because it helps to tackle the one issue other than hull separation that would most likely create an issue out on the water. I also like the idea of the deployable hydrofoils, as I have heard that in the shallows or in fishing areas you can get caught on lobster traps, trash, and such that could rip your hulls up before you are up and planing. Great job on the design so far! I hope to see more!

----------------
There are inflatable air bag systems for full size boats for the top of the mast.

 

[Doug Lord]

Scale
Here is how scale works:
This model is scaled down from my 18 foot design(preliminary specs below-see comparisions with an F18 catamaran)
--The Rule: sail area varies as the square of length, weight(displ.) varies as the cube of length, RM varies as the 4th power of length.

---FULL SIZE DESIGN= LOA 18'----squared=324,---cubed=5832,---4th power 104976
---MODEL DESIGN=LOA 5.0625' ---squared=25.63,--- cubed=129.75,---4th power 656.84
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so for weight, going from full size to model size:

---60.75"= 5.0625' ; 5.0625 cubed = 129.75/ 5832 = .02225 ; 750 X .02225= 16.688lb all up weight. Estimate now is that the model will weigh between 15.5 and 16.5lb when complete. Sail area is less on the model but only slightly because of the drastic difference in RM. The full size boat is 3.56 times as long as the model, the sail area(nominally) of the full size boat is about 12.65 times that of the model, the weight of the full size boat is about 45 times the model and RM of the full size boat is about 160 times that of the model. The model sail area is close to scale but is designed to reef whereas the full size boat can carry its full SA in 2+ lb/sq.ft. pressure(1.8 for an F18 before depowering). The model can only carry its full SA in less than 1lb/sq.ft. pressure which is a direct result of the change in RM as a boat is scaled down.


UPDATE- I forgot that I changed the scale(1/10/11) from 3.33"=1' to 3.375"=1' .



======================
MPX /Self-Righting Trimaran(SRT)-Full Size / Preliminary Specifications as of-3/27/11(original in post #1):

The main hull design is based on a planing hull I have. The planing main hull is beneficial for it's resistance to pitch pole-in fact a pitch pole caused by the main hull is extremely unlikely since at speed it develops tremendous lift forward. A skinny hull does not have the resistance to pitchpole that this hull has but after it pitchpoles would probably recover more quickly. I've designed, built and sailed for years two different trimarans using planing main hulls and never pitchpoled once due to the main hull-in fact the main hull resists a pitchpole started by tripping on the lee ama. The hull that I have now on which this model was based was used as a monofoiler as a test and though there were some problems with the foiler design the boat did foil and never once pitchpoled in 15 hours of sailing in different conditions.


SEE THE MPX TRIMARAN COMPARED WITH AN F18 CATAMARAN BELOW:

===============

Length: 18' / 5.49m

Beam: 22' / 6.7m (changed to 23.1' at least for model testing-6.54')

Ama LOA: 10'

100% ama buoyancy: 337lb.

Ama attachment(curved piece ) buoyancy: 320lb

Total Buoyancy for ama + curved piece attachment: 657lb

Draft w/boards Up: 6.4"

Draft, dagger/daggerkeel down: 4' 8"(deeper on model)

Mast Length: 28' / 8.3mm

Sail Area:
--284 sq.ft. upwind SA(changed down slightly for model)
--575 ft2 / 53.4 m2 downwind SA

Total Boat Weight incl. ballast and crew : 750 lbs ( same as F18 cat with two crew @175lb each)

Crew weight: wide range-for these numbers 175lb.

Ballast if used: (at the juncture of the daggerboard and main foil)- 175lb.

Boat weight minus crew and ballast: 400lb

=====================

COMPARITIVE RATIOS:

Bruce Number: SRT=1.86 F18=1.66

SA/WS:
-- not flying-SRT=5.73/1 F18=4.77/1

-- flying main hull-SRT=11.14/1 F18(flying one hull)=6.03/1

SA/D: SRT=55 F18=44.16

W/SA-(smaller better): SRT=2.64 F18=3.29

=====================

Pictures 3 & 4 are the 16'(128lbapprox) hull I have now on which the main hull of the model is based and which would be the main hull of the full size boat with 2' added to the bow:

 

[boradicus]

Bi-laminar flows

Doug, this is a very space age looking design! Does some of the lift come from the aerodynamic foils that span the beam just where they connect to the amas? Have you also run the model through a wind tunnel simulator? There is a free wind tunnel simulation software called Falcon. I don't remember who puts it out, but I think it is a joint effort between one of the CAD software companies and a large engineering university. I have not used it yet myself, but supposedly it can work with 3 models. I don't think it is set up to calculate hydrodynamics as well. I just got delftship, but am in the learning stages. I know it has hydrodynamic calculations on it but I am not certain if it can do wind tunnel type simulation. Maybe there is a software out there that does bi-laminar flow? IDK...

I just found Falcon:

Poject Falcon

 

[daiquiri]

Hats down for all that effort, Doug. Very neat work.

I'm considering fairings on the crossarms to reduce aero drag-after she flies the first time. Cost ,in terms of weight will be offset by the better aerodynamics.

The benefit imo will depend on how you intend to make these fairings, and how big. The risk (with a dihedral your crossbars have) is to create windward lifting surfaces set at an angle of attack to the wind, which will hence create lift and contribute to the heeling moment, especially in the event of a pitchpole.

Good luck for the testing!

 

[Doug Lord]

MPX Test Model

Doug, this is a very space age looking design! Does some of the lift come from the aerodynamic foils that span the beam just where they connect to the amas? Have you also run the model through a wind tunnel simulator? There is a free wind tunnel simulation software called Falcon. I don't remember who puts it out, but I think it is a joint effort between one of the CAD software companies and a large engineering university. I have not used it yet myself, but supposedly it can work with 3 models. I don't think it is set up to calculate hydrodynamics as well. I just got delftship, but am in the learning stages. I know it has hydrodynamic calculations on it but I am not certain if it can do wind tunnel type simulation. Maybe there is a software out there that does bi-laminar flow? IDK...

I just found Falcon:

Poject Falcon

----------------------
Thanks, boradicus. The "curved pieces" won't have any aerodynamic benefit as best I can tell. I'll just sail it and see what happens...

 

[Doug Lord]

Hats down for all that effort, Doug. Very neat work.



The benefit imo will depend on how you intend to make these fairings, and how big. The risk (with a dihedral your crossbars have) is to create windward lifting surfaces set at an angle of attack to the wind, which will hence create lift and contribute to the heeling moment, especially in the event of a pitchpole.

Good luck for the testing!

===============
Thanks, Slavi. There won't be any fairings until after it sails and until I carefully look at the benefits vs the weight. The full size version wouldn't use them since it's a high performance "sport" boat not a record setter.

 

[boradicus]

Heeling moment

Hats down for all that effort, Doug. Very neat work.



The benefit imo will depend on how you intend to make these fairings, and how big. The risk (with a dihedral your crossbars have) is to create windward lifting surfaces set at an angle of attack to the wind, which will hence create lift and contribute to the heeling moment, especially in the event of a pitchpole.

Good luck for the testing!

I am fairly new to the forum and design too! But in trying to understand this, and intuitively from the shape of the model, there will either be a positive or a negative lift where the aero-foils on the beams are located - is that correct? I mean, intuitively from the design, it does not appear that there is anywhere for the air to collect on a horizontal foil but either above or below, either creating a positive lift or a negative - please correct me if I am wrong here!

So, in looking at the design, I am assuming that when the bow of the boat is higher, that there will be a corresponding lift at the stern in terms of laminar air flow. What I am wondering is how much lift there will be at the stern when this happens (assuming that the bow is lifting first when the boat begins to hydrofoil), and if there will be a way to adjust this corresponding lift in order to avert the sort of heeling moment that could lift the stern possibly too high from a gust of wind, etc.

Would it be slightly safer to stick with planning out contingencies for only the hyrdronamic laminar flow, considering the variability between the two densities (air and water), and rather to streamline for one and calculate for the other (meaning to aerodynamically streamline the horizontal pieces of the craft that might be cause for positive/negative lift and to concentrate efforts on calculating the hydrofoil's operations)?

Are there any thoughts about this, or are the aerodynamic considerations for this vessel's operation so negligible in comparison as to not be deserving of taking a look at them?

Thanks