For a while now, I have been interested in the model boat racing series, such as OMRA and MPRA (sp??) but I have never entered due to the fact that I didn't want to spend money on what a hull/model that I thought was less than optimal. At the time, I had no idea *why* I thought that the current hull shapes were less than perfect, so I set out to teach myself boat design and mechanics with the gola of designing, even if it was just in my own head a model boat that was as fast as possible.
The following ( spread over multiple posts ) will be some of the thoughts and findings that I've come up with whle I researched ( and continue to research ) how to design the fastest possible model boat. A couple of friends and colleuges of mine are also feeding the thinking, so this is a collective effort.
The ultimate aim is that, after some thoughts have been put down in a single place ( i.e this thread ) I can pull everything together and get some pictures down as well. Maybe this will lead to an actual model ( although Martin13 will shout at me for abandoning EarthRace ) but probably I'll give up at that point.
This post, and the ones after it, as designed primarily for me to share some thoughts or findings. They are't posting a question or looking for advice - at least initially.
feel free to add comments thoughts etc etc etc as the concepts are added. Note that they are in no real order or sequence at the moment - just a stream of conciousness initially.
First Principles
Some thoughts to base my future posts on.
A model boat is different to a full sized boat.
This may sound obvious, but it's the fundemental issue here.
Enginnering, all engineering, is about comprimises. A full sized racing boat has a series of compromises built into it. It has to handle waves of up to 4ft in height. It has to cary passangers. It has to carry 200+ liters of fuel safely etc etc.
In a model boat, we *still* have to make compromises, but they are a different set. We don't have to carry a passanger, so there is nothing stopping us, if we choose, from mounting the engine near the front of the boat. Becuase it's easy to build something strong and light which is small, rather than if it is large, I can build a model which is far stronger than a full sized boat. So a deep V hull, which is a *significant* comprimise is no longer nessacary. I can build an ultra strong hull with a dead-rise of 10, rather than 30 and not only will the boat survive high speeds without me having to resort to titanium and metal matrix components, but I also have no passangers to worry about being smashed to bits inside a hull that is subjected to huge slam forces, but goes quicker.
What does a model racing boat have to do?
Not a lot;
1) produce high straight line speeds for a given engine size
2) be stable and fast in multiple conditions, from flat calm to significant chop
3) turn rapidly, and not loose much speed in the turn
4) be able to get to the end of a single race in one piece, with as few fuel stops as possible ( this is related, in part, to the "slippyness" - a function of all forms of drag on the hull - and the build quality and technique.
Boats travel through different substances
Another seemingly obvious thought, but one that becomes useful later:
A racing boat, various parts of the SAME racing boat, travel through different substances, each of which have different properties - some useful, some not useful.
The substances are:
1) "static" water - ie. water that has not been affected by the boat itself. This is essentially the water in front of the boat, be it flat calm or choppy. It also is the water at the edge of the wake caused by the hull moving through the water
2) "dynamic" water - this is water that the boat has imparted energy and/or turbulance into during it's passing. This is the water under, to the side of and behind the boat. The wake essentially. It has cost us - as the model maker/racer - energy to make this wake, and it would be useful if we could re-use some of this energy to add to additional forward speed or to reduce drag
3) air: this is essentially "very thin water". We can assume it's all fairly static, as even if the boat travels at the top speed of 40mph that the OMRA has recorded, the dymanic elements aren't really very useful, as the energy is very spread out.
4) Foam: foam is horribly dreadful stuff - the maths behind foams are mind boggling. However, it's a different state to water or air, so we need to keep it in mind. It's a horrible cheat, but we model foam as "sticky air" - it has most of the properties of air, but creates more drag ( actually large amounts indeed )
Foams occur in two places: Around the hull - which is nice, as it's less draggy than water, so we'll take as much hull foam as we can get, and around the prop do to cavitation, which is not as good. Cavertation ON or AFTER to prop is fine. On a model boat I can put a new prop on for a few quid, so wear on the prop isn't a bother. Equally, cavitation the prop has moved through that piece of water is not an issue, as I'm not trying to reduce the sonar signature of the boat. However, props work best going through as dense a medium as possible, so what we really want is the first half of each prop blade to be biting "pure" water.
We want different bits of the boat in different substances:
Another obvious one, but bare with me.
Ideally, we want as much of the hull as we can possibly get away with NOT in water. In a perfect world, we'll have it in air. if not, then foam is a good compromise
The prop we want in water - and water that is not turbulated. (NOTE: immediate argument here is "What about the super-cavitating props that all the racing guys currently use??" - this is a whole post by itself, but for now, assume that if the prop "bites" water rather than air it can impart more force. Tomorrow I'll have finished some sums on super-cavitating props and other designs and will post these up )
Bouyancy isn't very important
We want a boat that goes over the top of waves. So currently boats have lots of bouyancy - and so lots of "volume" up at the front of the boat. However, in a model, it doesn't really matter very much.
This is becuase there is more than one type of bouyancy.
"Static" bouyancy is the bouyancy of the boat at rest. If you drop the boat in the water it's the bouyancy that stops it sinking.
However, there is more than one type of boyancy ( although this is a bit problematic ).
Can we add devices onto the hull that make it more bouyant when it moves? if we can, then all of a sudden we can don't need as much volume at the front of the boat holding "static bouyancy" - which reduces aero drag and, more importantly, water drag.
Yes is the short answer. Imagine strapping two plastic sewerage pipes along the length of the hull of a canal boat. Most of these pipes is submerged, with just a bit above the surface. When the boat is at rest, these pipes add zero bouyancy. However, when the canal boat is travelling at full speed , the air will drive out the water in the pipes ( becuase of turbulance etc ) and so there will now be two air flled pipes strapped to the hull. As long as the boat is moving, the air is in the pipes, and adds bouyancy.
Note that this is NOT the same as lift (which is where the contencion lies ) our view is that "Lift equals drag causes at a normal angle" - the concept of "dynamic bouynacy" doesn't fill this definition, so we've come up with the description of "dymanic bouyancy". there will be a proper name for this, but I can't currently find it.
If we want a fast boat, we want it to have a small wetted surface and a small volume. A lot of this volume is taken up to make a bouyant boat ( look at a deep V racer - all that space up at the front ). if we "make our own" bouyancy, we can reduce the need for static volume, so allowing us to do the same job for less drag.
That's some first principles. Now my lunch hour is over, so I'll add the next bits this evening.
Steve
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Topic: Some thoghts on producing a very fast model boat (Read 7474 times)