Ok,
This is going to be a long post, but I'll try to break it up into bits.
Had a bit of a breakthrough today, and one of Santa's presents has been a huge multi-part e-mail with a ton of attachments from my mole within the EarthRace group. Can't discuss the full contents, as I'm under an NDA for parts of it, but will discuss what I can.
OK, working through your posts in some from of order:
Pressure on the outriggers:
This is interesting. The 1:1 sized boat takes a different approach than we have been discussing. They have an amazingly complex piece of bent carbon "tube", roughly circular in cross section, which forms am "M" shape. Around this, they have built up a lightweight structure with carbon rods and sheet, and then clad it in carbon to give the outside shape.
To give you some idea of the engineering that they have had to use, the inner tube is designed to support stresses of up to 72.5 GIGA pascals - which my maths works out to be approximately 4000 ton-yards ( rather than foot lb's). This is obviously over-engenieered, but remember that they don't have an unlimited budget and only spend where they have to.
OK, to explain my 200lb figure first.
I'm reckoning on my model being able to hit 25knots. ( which has become realistic given new information about power systems - of which more below ).
I reckon that each outrigger will be placed under the following forces at this speed:
lateral force ( i.e forward motion pots pressure on the floats, and they want to band backwards) : 15lb
Torsional force (i.e floats want to bend upwards or outward/inwards - a twisting force ) 45lb
lift when submerged (as the outriggers are lifting planes - they're essentially funny shaped wings ) 40lb ( ish - assuming running fully submerged at this speed through a wave that is at least 1.5x length of boat )
approx 100lb per outrigger - 200lb in total.
All these figures have NOT been scaled from the full sized boat, but are based on a model a friend of mine pulled up for me based on information available from the area race web site, some calc's from the general arragement diagrams on there, and some back of the envelope calcs from me.
Currently I have a REALLY rough (becuase it's the first I've ever done ) CFD ( computational fluid dynamics ) model to cross check my friends model, and I'm satisfied that we're definately within an order of Mag. and probably 1/4 an order of mag. of accuracy, as the numbers do sort of relate to each other.
2) having a detachable top section:
As above, the outriggers are designed to act as "wings". When the boat is submerged as it travels through a wave, it's tendency is to dive. The "wings" apply a lot of upward force. This is the same force, when used in conjuntion with the dynamic ballasting, as the diving force. The result is that the boat stays on a level, and goes in one side of the wave, and out of the other at the same vertical height.
The force needed to do this is considerable, although totally dependant on speed ( just like a planes wing - apart from stall speed = 0). If the boat hits a wave at it's designed 25knot speed, it will produce approx a lift approx 5x the displacement of water from the outrigger arm and the float + half the hull. ( rough figures - found a roughly similar NACA areofoil and made some back of envelope calcs ).
As you can see, there will be a significant upwards force acting on the top section when the boat is submerged and at speed ( and remember it's specifically designed to run through waves of any height, so I can't really avoid it ( and wouldn't want to either)
This comes back to Tigers point about "am I trying to design out the 100 year wave. The answer is "maybe". I'm certainly being very cautious, becuase this is going to be horribly expensive to build, so I want it to survive intact, and would rather pay to over-engineer than not pay and under-engineer.
But, there is a "but". It's important, when thinking about this kind of hull, when we talk about waves, not to think about the "Height" of the wave, but the "length" of it. This boat goes straight through waves, and, at scale sizes, this means it will be submerged fully by any wave more than about 6" high, and that is height from peak to trough, rather than peak to average height, so really if you think about a 3" wave causing pretty much full submersion, your on the right track.
The bigger issue is how long, or deep the wave is. if it's a wave from a pebble being thrown in, you might have relatively high waves but each wave is very thin. However, I will be regularly sailing this boat on a tidal river when the waves aren't very high, but are usually at least a meter in length ( peak to peak distance).
If I pierce a thin wave, I am carrying the weight of, and generating lift against, a small total volume of water. When I pierce a LONG wave, I'm displacing the entire volume of the boat in water, and generating a lift between 3x and 5x that in the outriggers. In all this could easily amount to 1/3 tonne ( metric ) of water, so you can see why I'm being careful.
Therefore, this is the debate I'm having with myself at the moment:
On the one hand I *want* a detachable top section, as it makes transport and fitting of the electrics that much easier. However, there is no point in having it if it will be forced off when it goes through a wave.
If I can come up with a way of having it VERY strongly attached to the hull, but still be detachable, I will jump at that option, but, what ever that answer is, it can't compromise the scale appearance of the model, so no bolts etc are allowed.
I LIKE the sound of Hasting's idea of the sub-assembly in the main hull which can be bolted onto. I guess it comes down to finding a way to hide the bolts from the outside. Current idea is to have the bolts built into the bottom of the detachable section permantly, and then drop them through the sub assembly, and bolt them up using wing nuts. Might be a bit fiddly, and would still mean having a removable "back door" section, but I can live with that.
Rudders: I have "come accross" the proper designs for the rudders. Tiger was partly right, they are tilted, but they are not really a balanced rudder design. They are effectively 2 rectangles, with an aerofoil shape, and the tiller tube is sent into the rudders at a 10 degrees off centre. They look similar to tigers design on page 4, but not quite. There is a very slight balancing action, but it is only a few pounds on the full sized boat.
Power train: The boat has two props!!!!! This makes me happy, as I can apply a lot more power to it now and will be able to get it to a good speed. diagrams to follow when I've converted them into something I can post.
Props: We guessed a two blade cleaver prop. Actual boat initially used 2x 3-blade sumberged cleavers, 1x left handed, and 1x right handed. The prop blades kept tearing off the bosses reasons un-known. They went through a series of makes and models with the same effect, often with a couple of miles of sailing. Supposition is that something to do with the boats hydrodymanics or design of stern places a massive amount of back pressure on the prop blades. They changed to a less agressively pitched 4 bladed semi-cleaver, and, while there have been a big drop in performance, they can actually keep the blades on the bosses now.
Having had a look tonight, I can't find a source of 4 bladed cleaver props, so will use a 4 bladed "warship" prop for displace, and will probably use a 1 ( one) bladed cleaver for running about. If there IS a hydrodynamic issue, it will scale, as water is self similar at scale. A 1 bladed prop should avoid any issues as there is less area for a pressure wave to build against. Scale size will be 35mm.
Mixing props: Long way from this yet, but will probably use my JJC prop/rudder mixer for different levels of deflection for each side ( will just wire it backwards - instead of having motors change speed and rudder deflection as standard, I'llkeep the engine speed standard and vary the rudder deflections.
However, this IS another point of failure, of which I seem to have a lot of already. I'm very tempted to take Circlip's lower tech, more fiddly but ultimately more reliable approach on this.
For the "deflect less at speed" - I can do this with my 9 chan Multiplex aero radio, but never seen a similar feature on a marine Tx ( or the Spektrum 2.4Ghz set that will control this boat. . MTroniks do stand alone hardware mixers, so might use one of those.
Attached you should see a picture that has been a swine to get hold of, bu is finally in my grubby mitts - a rear view of the EarthRace out of the water. Note that this is an old picture showing three bladed props. The rudders are also an older design ( or at least are not the same as the diagrams that I was sent )
once again guys, thank you SO much for all you help.
I've said it before, and I've said it again, without you chaps, this thing just wouldn't be being build.
Hope you've had a great christmas.
Steve
Author
Topic: Advice required - how to build a very complicated shaped boat? (Read 25214 times)