Eduardo, these issues are usually spelled-out in the specific rules for each one design class.  In general, the idea that competitors in an OD class are able to use multiple hulls is pretty common as chartering a boat when out of town is very, very common in OD fleets (as long as each hull meets the OD specifications).  In the J/105 fleet for instance, we allow owners to charter boats when out of town.  In the J/30 fleet they have a rule which allows an owner to switch hulls within a single regatta if their boat is damaged ... "as if it were their own yacht".

If my memory serves, this is an example that should help shed light on the situation. During the Beijing Olympics, a 49er broke their mast on the way to the medal race. When they got to shore, another team that wasn't in the medal race handed them a fully rigged boat. They barely started within the 4 minute window after the starting signal. They finished drifting through the downwind finish capsized to win a medal. The hearing to determine whether this was a legal substitution took days to settle. Among the issues discussed were:

1. The boats had been sequestered and measured 24 hours before the medal race. The substitute boat hadn't been sequestered and measured.
2. The other boats in the race were all carrying onboard camera gear that added weight. The boat they raced had none of this gear.
3. The country code on the sail was wrong.

In spite of all of this, they International Jury decided that it was ok to sail the other boat and they won an Olympic medal. On point 2, they decided that the extra weight didn't matter in the prevailing conditions (survival). If that was ok, then your friend changing his hull during an 8 week series is probably OK unless the NOR, SI's, or Class Rules prohibit that kind of change.

Here is a formal write up. https://www.sailing.org/news/4233.php#.Xbig0ZpKgaY 

JP

Thank you for the help. Just to explain a bit more, the competition is held in eight weekends in home waters, one team has participated in first 4 weekends with one boat and than changed with another owner in the same competition and want to keep the points. Neither boat was damaged and they had 33 points in the first 4 weekends and only seven in the number 5&6. The class rules does not cover this situation

Regardless of the reason, there isn't a prohibition on changing hulls in the normal racing rules. 

This is an interesting question.  Certainly, chartering a boat for a regatta, as is done by many when attending an out of town regatta, is quite common.  The J30 example above is clearly meant as a clarification by the class to address this issue.  As to the 49er example, the link to the jury's explanation of their decision is broken so I couldn't read their logic so I can't comment on the why's of their decision.  It may have to do with what is considered an 'entry' in the Olympics, i.e. it is the country that is entered and not the individual boat?

In looking at Part 6 Entry and Qualification, RRS 75.1 states that "To enter a race, a boat shall comply..." [emphasis added].  RRS A2.1 states that "Each boat's series score shall..." [emphasis added].  This seems to say that it is the boat and not the competitor that is entered and scored in a race or series.  This makes some sense as, over the period of a long series, the crew and even the skipper can change but the scores are still the boat's.  There are no limitations in the RRS about changing crew or skipper during a series.  For example, on the boat I sail on regularly I am the substitute skipper when the owner is out of town and yet the boat only gets one set of scores for the series.  To score the boat based on each unique crew combination would be totally impractical.

It sounds like in your case the crews switched boats for some reason (to try out a different layout/setup?).

The only way that I can think to accomplish what it seems like you want to do (having the scores go with the crew) would be for the RC to score the series based on the boat (which is what the RRS seem to require), the skipper to request a scoring change, the RC to do it (if they want to score based on the crew and not boat), and then the other boats in the series can request redress if they care.  You could also skip the first two steps.  The conditions for redress can then be met and a PC can sort it out if needed. :-)

Eduardo
 

I think this goes back to the RC under the circumstances outlined.  As John C points out, RRS 75  .. a boat is entered by someone and then it goes on to differentiate between "boats" and "competitor".  

Questions ..  (rewritten after I first posted)

I think John Christman is correct in that it is the boat that enters.
Therefore, unless there are changes in the Class rules or NoR then each combination of boat and skipper is a 'new' entry.
It is also a good idea to have a requirement in a NoR for say a Club Championship series over a season to prevent changes of skippers. You do not want a series decided by a skipper being unable to sail his boat in all the races hiring in a gun skipper to win the series for him. This might no be illegal under the rules but it will almost certainly upset the fleet.
In summary make sure all your requirements are in the NoR!

Thank you all, it was very helpful. Angelo the second boat was already in the regatta and they have sailed under different sail#

Given the above (and the apparent absence of definitive  SI, NOR or CR’s)  I think the RC has what they need to score this correctly. 

FWIW as a data-point, here in then Chesapeake Bay we have a season-long award named “High Point” which is calculated across all the sanctioned events. It includes drop-mark races and distance races alike and is calculated with a formula based upon ‘# of boats beaten’.  

High Point is scored based upon the “skipper” (who enters the boat, not necessarily the helmsman but usually is) and the boat, therefore the same skipper can have different scores for different skipper/boat combos. 

We J/105’s have owner-driver CR’s, which also allow immediate family and “long term shipmates” to also helm.   We established for the 105’s that up to 15% of the season’s races could be driven by someone other than the owner and still be scored under the single owner-boat total. That way if the Owner is unavail, one of their normal crew can take the boat out and compete and still have the race count toward the season total. If there are more than 15%, the owner has the option to either drop the the most recent races driven by someone else from their total or be scored as a team (dropping then skipper’s name). 

All this above is just to show you what can be in place when the desire is for the score to track the competitor across multiple races. 

Say you are relatively new to your local one-design fleet. You have purchased an entry-level used boat.
As your confidence in your selection of the particular class and fleet (and competitors) strengthens, you decide in mid-season that you should further commit by purchasing a better, perhaps even a new, boat in the class. You thus make the entry level boat available for further growth in the local fleet.
Should your upgrade be punished?

I have not changed boats. Its not a question of being punished, its a question of being fair to the other competitors. In the present case was a used boat for another used boat

My question, a hypothetical, should be resolved by the same rule, if you can find one, as governs your situation.
Indeed, I am suggesting a situation wherein a far better boat is substituted.
Answer my question.
Should the fleet member be punished for advancing himself and the status of the fleet?

Philip, I believe your situation is answered by RRS 75.1.  If you switch boats it is a new and different entry and you are scored accordingly.  It isn't a "punishment", it's applying the existing rules.

Thank you all for the very helpful contribution but I think that the discussion is getting way off the matter, again thank you all but I am leaving the discussion on this subject, rgds