You most probably won't be able to ship them as checked-in baggage, even as oversized baggage, but will instead need to ship them as cargo, either directly with the airline, or using a carrier such as UPS.
A couple examples:
Any items larger than 190cm x 75cm x 65cm (75in x 29.5in x 25.5in)
require separate shipping as cargo.
How to ship bulky items as cargo
The maximum length per piece on jet aircraft is 2 metres however this
may be reduced depending on the width and height of the item. Sporting
items exceeding 2 metres but less than 2.5 metres in length may still
be accepted and are subject to oversized item charges.
If you wish to travel with an
item that exceeds the allowable package dimensions, you will need to
consider making alternative freighting arrangements for this item.
Air New Zealand Cargo, or your local freight forwarder, can advise you
on alternative arrangements. Applicable rates will apply and items
sent via Air New Zealand Cargo or a local freight forwarder may not
necessarily travel on the same aircraft as you due to space
availability. Please remember it is important to prearrange this with
Air New Zealand Cargo or a local freight forwarder at least 5 days in
advance of your travel. There may be occasions when Air New Zealand
Cargo is unable to assist you with the carriage of your oversized item
due to limited cargo space availability to your destination.
Items this length may simply not fit in aircraft holds, or be impossible to load/unload due to the relatively small cargo doors on passenger aircraft.
I recommend you:
- contact your supplier, which must have experience shipping the boats internationally, and should have good advice
- contact the event organisers, who should have experience as well with this, and may have specific deals with carriers.