Per Merriam-Webster Dictionary : 

\ ˈhəl  \

Definition of hull (Entry 1 of 7)
1a : the outer covering of a fruit or seed b : the persistent calyx or involucre that subtends some fruits (such as a strawberry)

2a : the frame or body of a ship or boat exclusive of masts, yards, sails, and rigging b : the main body of a usually large or heavy craft or vehicle (such as an airship or tank)

3 : covering, casing

So in my opinion, I would say the prod, either fixed or retractable are not part of the hull. 

Please See World Sailing Equipment Rules of Sailing (ERS) in the current book it defines Bowsprit as Hull appendage so it is separate from hull.   Hull definition is D.1.1    Bowsprit is F.1.4 (c)(i)
it is better to use the ERS than a dictionary when looking at the Rules.  (unless ERS are not in effect for that boat)

Submission 143-18 Proposal 2 which result in change of definition Finish and Start is pretty clear https://www.sailing.org/tools/documents/14318RacingRulesofSailingDefinitionStartDefinitionFinish-[24325].pdf

In the Equipment Rules of Sailing D.1.1 we can find that fittings associated with the hull shell are part of the hull. If here is maybe not clear if bowsprit is a fitting we have to read further. In paragraph F.1.4.(c)(i) bowsprit is defined as hull spar. So bowsprit fixed or retractable is a spar and not part of the hull.

 David Sprague  wrote:

 Sue Reilly is quite correct in referencing the dictionary definition with regards this question.  The term "hull" in the definition of Finish is not in italics and falls into the category of "other words" in the Terminology section of the Introduction.

[Edited]
@Murray, And yet the quote on other words and terms says "ordinarily understood in nautical or general use", and I would think that the ERS my be a better source for the nautical use portion than MW.

This is something that more clarification is needed on and I hope that it will be coming from World Sailing.

I have gotten clarification from a very reliable source. the dictionary definition of "HULL" should be used, not the ERS.  A bowsprit, whether extendable or fixed is NOT part of the hull.

Yes, I think in both general nautical use and in the ERS, a sprit is a spar and not part of the hull.

Where does the hull begin on Wild Oats (with this prod configuration)?

My point is that this change isn't always obvious. More clarity from world sailing would be welcome. 

1- The dictionary definition of "hull' should be used unless the ERS definition of hull is invoked.

2- ERS have to be invoked fully or in part by a number of mechanisms (see ERS intro):

Applicability
The ERS are rules only if they are invoked by:
(a) Class Rules.
(b) Adoption in the notice of race and sailing instructions.
(c) Prescriptions of an MNA for races under its jurisdiction.
(d) World Sailing Regulations, or
(e) Other documents that govern an event.

Yesterday I attended a good part of this event: https://www.youtube.com/watch?v=K98O2dxEjtU
This same question came up and John Doerr was very clear on it: "use the ERS"

@Cxema Pico - what is your reliable source?

@Thomas,
I was quoting the reliable source text from Charley a couple of posts above and trying to add some explanation as to why hull in its normal sense has to be used, unless the ERS definition is explicitly invoked.

As for the rules talk from yesterday the key part is in minute 59:00 where John explains "that some work has been done linking the ERS and the RSS, but that you will notice that "HULL" is not in italics so it is not a defined term, so you have to use hull in its normal use in language [...] the ERS is useful, but there is no direct link between this term and the RRS".

I went nearly directly to the source and asked Dick Rose for an interpretation. He is my source, and this is his reply:
All,

I can see that this is far from settled, and as I said originally, I think more official clarification will be needed, but for now, I'm going with Dick's interpretation.

@Cxema Pico - I stand corrected - thanks for the time reference, I clearly missed that.
John Doerr clearly explained about "hul"l not being in italics in the RRS...

As regards John Porter's question about the WOXI sprit, I don't think John Doerr was definitive but recall him saying that an extension of the stem line might be used to determine where the hull ends and the sprit begins.

Interestingly the submission which triggered the change states:

‘This submission will establish consistency between events, and the ERS have a clear definition of ‘hull’, so there is no doubt what that is.’

They submitters possibly intended ‘hull’ to be in italics when they made the submission?

I personally disagree that this change is an improvement. Are we allowed to do that here? In the past, race officials had to sight down the line, if they had un-impeaded sight of the far end of the line, all clear. Now they must ignore say, the two sport boats with 2.5m fixed prods protruding across the line. What about a down wind start with gennakers flying off said prod, the RC haven’t a hope of seeing anyone further down the line. My own boat has the headsail tack out on a fixed portion of prod, which would mean I would block sight past me down the line even on an upwind start, sure, not by much, but once the pin end is obscured, by how much becomes a bit irrelevant. 

The explanations in the submission I reject also.  

‘Appendix C has already adopted this change in the current rulebook, and it has not caused any problems for race officers. Therefore, this submission will re-establish consistency between fleet racing and match racing.’

In Match racing, an OCS is either the boat in front of you, which you have a clear, full view of, or the other boat.  Simple. Not so in fleet racing. 

Also ‘. There is an increasing probability that tracking systems will soon be reliable and accurate enough for OCS calls. When such systems are introduced, the call of OCS will be on the hull (bow). Therefore, it would be useful to introduce the concept as soon as possible‘. 

Although this may be true For some, top end, maybe bigger boat racing, it will not be true for the absolute majority, as most boats, and most races, will not have such equipment. Surely the minority of races this will be true for, could continue using the appendix C alteration. 

Too late, I know. 

@Matt Mitchel,

I agree with you on every point, but there is another scenario that this change does fix which was not mentioned in the explanation.

How would you score the following boat? 

Discuss!

Another point that John Doerr brought up. In the not too distant future, particularly at major events, boats and courses are likely to be more fully instrumented and OCS will be called electronically rather than visually. That will eliminate the problem of the nearer OCS boat blocking from sight further OCS boats. You can attach a sensor to the hull, perhaps you could attach one to the sprit. But you can't attach the sensor to crew or equipment (i.e. sails). 

Matt, I agree that there are pluses and minuses to the change. Line sighting in a fleet race is hard, and I'm not sure whether the rule change makes it harder, easier or about the same. I also agree that the example of "it works fine in match racing" is a poor one - it's much different sighting the line with two boats than with 10 or more.

I don't think focusing the discussion on the start procedure (or finishing line) is the right thing to do. I think this change makes sense because it simplifies how the rules in general are applied. 

Fleet behavior at the start is a difficult thing to control for race officials whatever the definition of hull is... my take is that applying R29.2 (general recall) and Rule 30 (start retries & penalties) should be enough to discourage reckless start behavior. 

This rule change raises more issues than it solves. Agreed that accurately, and with absolute certainty, seeing a boat OCS at the start has never been guaranteed in any type of fleet when boats are masked by others. However, in the majority of starts (at all levels of racing) you still had a pretty good chance of sighting when any part of the BOAT, crew and equipment were crossing the line, (except again, when boats down the line are masked). In those races with a large fleet, an additional race committee boat at the pin end, can help in minimizing that problem. In any case, in most events, it's a small number of boats that are the offenders, and if it's an unidentifiable number, the RO already has tools to deal with it i.e. RRS 29 AND 30

Similarly, at a downwind finish, any form of masking makes it difficult to be certain. With many more boats flying asymmetrical spinnakers, they are even more likely to mask ALL  the hulls down the line, due to the lower cut of the foot, but not necessarily mask ALL leading edges of the spinnakers.

I find the reasons presented related to seeing OCS offenders, and accurately recording finishers, are weak and not going to be any better using the HULL. In a mixed fleet of sprit and non sprit boats, seeing the leading edge of the boat, crew and equipment is consistently easier than trying pick out or many mixed overlapped boats, where the hull is, in very short order.

The other issue is the definition. For the want of an italicized font, WS introduce conflict/inconsistency between the ERS (which provides a perfectly reasonable, explicit list, diagrams, and definition tailored for our sport), and the variable definitions of " hull" in common language. In one instance, WS say they want to reduce inconsistencies across rules, and in this case, introduce an issue with the existing ERS, with broad ramifications. I'd readily lend an italicized font for HULL if that helps, and/or simply refer to the ERS.

With respect to future sensing equipment, this has to be the weakest argument for change in this next cycle of changes. I would bet that 99% (approximately, and I use the reference loosely !) of boats within the next 4 years, or the next decade, will not have this technology. I'd bet that neither will all YC'S afford, or race committees have hardware and software that senses the bearing between RC and pin, and plots each boat relative to that "line". Perhaps at the Americas Cup level, but all the way down through racing at all levels?

In my opinion, this needs to go back for a re-think. Let's be realistic and practical.

That is okay, Tim.
RC cannot see "the course side" of the inflatable pin mark in any kind of chop, anyway.
Nor will the hardware and software.