Will earthing an aluminium boat reduce the rate of hull corrosion? First question here as I am having trouble getting a decent answer anywhere else, such as boat service workshops etc. 
I wish to reduce corrosion on an aluminium hull over its lifetime, hence this question. Especially since corrosion being the most common reason of hull degradation for aluminium boats. 
Hull fittings: All electrical equipment on the boat is not returned through the hull, but through a return wire to the negative on the battery. Also, all dissimilar metal fittings are insulated with rubber or the like from the the hull. 
Also, the battery isolation switch is left in the off state when not in use.
The hull sits on plastic rails on a trailer when not in use. 
Some aluminium boats develop corrosion spots, and others do not. I would rather the boat does not - and just develop a nice even oxidisation layer throughout. 
I met an old guy at the boat ramp years ago and he had the best condition aluminium hull I had ever seen. He recommended polishing the hull and then earthing the hull when not in use. 
If I earth the hull to a ground spike when not in use will it reduce or increase corrosion on the hull? 
I made up a ground spike out of 5ft of reinforcing bar and an old car jumpstart cable, put together with solder. 
With the earthing alligator clip not connected to the boat, when I measured the voltage between the hull and the ground spike I get about 4 volts. I did not measure the current. (Edit and my apologies, I confused this from the notes I took fault finding something draining the battery of my truck the same day)
Am I doing good or damage to my hull having it earthed? 
 A: If I understand you right, you want the hull to get the protective surface layer corrosion, but you want to prevent the layer unevenness/spots/discoloration or other forms of corrosion.
TLDR: grounding in the electrical sense by itself does very little to prevent corrosion, but given grounding means metal touches metal in practice, proper choice of grounding metals can help protect from further corrosion by the cathodic protection effect.
Long answer:
I don't have experience with aluminum boats, and this site is focused on explaining physics aspect of things, not on giving practical advice, but this is quite interesting physics so take the following more as a physicist's viewpoint on the corrosion and protection processes, not as an experience-based advice.
Aluminum corrosion
There are several different kinds of corrosion processes, but it seems corrosion due to aluminium reacting with oxygen and water from atmosphere is the most relevant for you. Available sources on aluminum corrosion (see e.g.[1]), say that if the aluminum surface is smooth, of uniform composition and exposed to close to neutral pH environment, a protective hydrated oxide layer of even thickness and appearance should form quite quickly. This will protect the aluminum inside from reacting with oxygen and will stop further corrosion due to atmosphere/neutral pH water. It may not protect it from corroding with other metals touching the hull, or reactive chemicals that it comes to contact with (from the air/rain). If the pH of the environment is too extreme, pitting or much deeper and faster aluminum corrosion may happen.
Getting proper protection layer
Polishing and thus removing scratches/small grooves should help - the more even the surface, the more even the corrosion process that creates the protective oxide layer and the more even the result is.
To achieve that uniform protective layer, removing any oils/salts/other dirt should also help - perhaps use ethanol and then deionized water to clean the hull.
Also slowing down the oxidization process should help evenness - after polishing and cleaning the surface, I would let the protective layer grow under a roof, or at least start the process there, preventing rain and dust from attacking the hull. Dust and rain may expose the hull to a soup of chemicals which could affect the corrosion process unevenly.
Grounding
Grounding makes potential of the hull the same as that of the ground. If the boat will spend a lot of time outdoors, grounding it seems like a good thing to do, to decrease likelihood of a thunder strike, and (by using thick grounding bar and thick cables) to make it more resilient in case it does get hit.
If grounding wires are connected at several places along the boat, it could help to prevent occurrence of strong electric currents across different metals there, but this seems unlikely to provide much benefit, especially if all different metals are already insulated from each other as they should be.
Problems with grounding
However, in general, grounding aluminum parts can be problematic and high maintenance, because of the oxide layer that can grow on its surface between the hull and the grounding cable clip. Aluminum oxide is a bad conductor (oxides usually are), and with time, electric resistance of the connection will grow and eventually it may disconnect electrically and the boat won't be well grounded. You should check resistance between the grounding bar and the hull while grounded - it will probably grow in time but you shouldn't let it get too high.
Corrosion and grounding
Another problem with grounding is that corrosion can actually be accelerated near the points where the clips bite into the aluminum, due to electrochemical cell effect. The clips should be either the same aluminum alloy as the hull, or a metal that is more prone to corrosion, so they will corrode but the hull corrosion won't be accelerated. 
There is so-called galvanic series that helps to choose the metal that can safely touch the protected body without causing its corrosion: for an aluminum boat, such metal could be magnesium-aluminum alloy (or zinc, if the hull alloy to be protected is low on zinc) [2]. 
There is a so-called cathodic protection practice [3], where metallic surface that is to be protected from corrosion is touching a so-called sacrificial anode on the same side, which is usually a compact piece of different, more corrosion-prone metal. The more of such anodes, the better the protection effect. The effect is strongest near the anodes.
You could try this practice with the boat - it should substantially slow down or almost stop the corrosion of the hull, if you getting it into good contact with many pieces of zinc or magnesium-aluminium alloy.
So, grounding the hull can either help or hurt its corrosion because of introducing different metals. The important circumstance is which metals are used. As far as I know, there is no global effect on corrosion due to hull's potential being the same as the ground.
[1] https://www.alfed.org.uk/files/Fact%20sheets/2-aluminium-and-corrosion.pdf
[2] https://metallurgyandmaterialscience.com/galvanic-corrosion/
[3] https://en.wikipedia.org/wiki/Cathodic_protection
