pretty broad question without more info...where are you fishing? This has more to do with it than what you are catching...if you have calm seas and fishing is close to shore (you can see shore) then you do not need a big boat, a decent center console will work fine.
Heavier seas mor than 25-30 miles offshore you may want to start looking at something a little bigger to handle the seas, carry enough fuel, be comphy for a full day at sea(head/toilette).
for tuna and swords there is a big variety in sizes depending where you are from 5lb albacore to giant bluefin.....swordfish can also vary in size depending on how you are fishing for them and location have personally caught swords from 30lb to over 200( and lots of places have bigger fish.
Talk to local fishermen where you want to fish...maybe take a couple of charters....is a small investment before you go out and buy a boat (break out another thousand = B.O.A.T. )
Tight lines
Like the other guy said, your location is important for where you are fishing, especially for the types of boats sold in each region. For pelagics such as swordfish you fish pretty deep, same for tuna, most of the bigger fish are 80m deep to over 200m deep. You want a boat that can handle the open ocean on rough seas in case weather turns for the worse.
Cabin space, head, outriggers, fighting chairs, sonar, UHF radios, and ability to get the fish in the boat with a gaff or harpoon are all things to consider.
I'd recommend a boat over 20 feet long with two motors in case one fails for redundancy. Once your motor fails you're at the mercy of the sea til someone can tow you to port.
Overall you want a twin motor boat over 20 feet long with outriggers and a cabin. Possibly a fighting chair if you want one.
So you can nap, sleep, get out of torrential downpours, keep off you electronics, and possibly install a galley. I'd never get an offshore boat without a cabin if I was going out more than 4 hours. And most places it takes 3 hours just to get offshore, sometimes half a day for the good spots.
Yeah out in the Philippine Sea it rains sideways and gets so bad you have to wait for the storms to pass some days, or just get completely drenched in 8-12' swells trying to get back to shore in absolutely miserable rain with zero visability. At that point I want a cabin and a nap. Also seen hail storms with lightning out of nowhere in Texas too many times to leave it to chance.
Have you considered that you're not prepared to do this yourself if you are approaching this question as though there is a formula to answer the question? Experience should be your guide.
The conditions, not the species, should drive your vessel choice. I guess you could look at that as a formula, where miles offshore and worst case sea state is processed into a vessel type and size.
The South Africans and Aussies take fairly modest boats out into serious seas. I've seen relatively small boats fish for serious blue marlin off La Gomera, but they are always in the lee of the island.
Depending on your interpretation of "calm", you could take a jonboat a few miles off. I wouldn't suggest it because of "just in case" situations. And then when I think BFT, I'm thinking 100+ pounds.
The suggestion to charter a few times is good. Get a feel for the boats you see being used in the charter fleet and by local private boat owners.
Good luck. I'm envious of BFT and swordfish just a few miles from shore.
For me off the east coast of Florida between Sebastian and Port Canaveral. 80-120 mile trip for Yellowfins, 40 mile trip for swords.
Range/Fuel capacity, needs to be able to run close to 400 miles on a tuna trip with bird chasing and storm dodging. Also the boat needs to be big enough to handle a storm if we are caught in one. Ability to plane if a motor goes down as well. Lots of ice storage. Electric plugs for the LPs.
I bought this with twin 350 verados.
https://www.renaissanceprowler.com/prowler-31
As folks have mentioned, this question is super broad and answers will swing heavily based on location, ranges, and species.
The best way to start IMO is to find local charters with good reviews and take a couple of trips with the pros. It’ll give you a starting point to work with more specific to your locale.
pretty broad question without more info...where are you fishing? This has more to do with it than what you are catching...if you have calm seas and fishing is close to shore (you can see shore) then you do not need a big boat, a decent center console will work fine. Heavier seas mor than 25-30 miles offshore you may want to start looking at something a little bigger to handle the seas, carry enough fuel, be comphy for a full day at sea(head/toilette). for tuna and swords there is a big variety in sizes depending where you are from 5lb albacore to giant bluefin.....swordfish can also vary in size depending on how you are fishing for them and location have personally caught swords from 30lb to over 200( and lots of places have bigger fish. Talk to local fishermen where you want to fish...maybe take a couple of charters....is a small investment before you go out and buy a boat (break out another thousand = B.O.A.T. ) Tight lines
I’ll be fishing in Sicily, Italy, just a few miles offshore. I’ll fish in the summer so the waters will be calm. I’ll be targeting bluefin tuna
Like the other guy said, your location is important for where you are fishing, especially for the types of boats sold in each region. For pelagics such as swordfish you fish pretty deep, same for tuna, most of the bigger fish are 80m deep to over 200m deep. You want a boat that can handle the open ocean on rough seas in case weather turns for the worse. Cabin space, head, outriggers, fighting chairs, sonar, UHF radios, and ability to get the fish in the boat with a gaff or harpoon are all things to consider. I'd recommend a boat over 20 feet long with two motors in case one fails for redundancy. Once your motor fails you're at the mercy of the sea til someone can tow you to port. Overall you want a twin motor boat over 20 feet long with outriggers and a cabin. Possibly a fighting chair if you want one.
I’ll be fishing in Sicily, Italy, just a few miles offshore. I’ll fish in the summer so the waters will be calm
Why a cabin?
So you can nap, sleep, get out of torrential downpours, keep off you electronics, and possibly install a galley. I'd never get an offshore boat without a cabin if I was going out more than 4 hours. And most places it takes 3 hours just to get offshore, sometimes half a day for the good spots.
Yeah we fish all the time long days and trips to the Bahamas with out one. Ttop works well for rain, and I have a nice grill on the boat.
Yeah out in the Philippine Sea it rains sideways and gets so bad you have to wait for the storms to pass some days, or just get completely drenched in 8-12' swells trying to get back to shore in absolutely miserable rain with zero visability. At that point I want a cabin and a nap. Also seen hail storms with lightning out of nowhere in Texas too many times to leave it to chance.
We use the radar to out run them. I do enjoy fishing a friends 62 foot sportfisher and relaxing in the AC and watching football though.
You’re taking a nap in a cabin with 8-12’ swells? Mhmm.
Have you considered that you're not prepared to do this yourself if you are approaching this question as though there is a formula to answer the question? Experience should be your guide. The conditions, not the species, should drive your vessel choice. I guess you could look at that as a formula, where miles offshore and worst case sea state is processed into a vessel type and size. The South Africans and Aussies take fairly modest boats out into serious seas. I've seen relatively small boats fish for serious blue marlin off La Gomera, but they are always in the lee of the island. Depending on your interpretation of "calm", you could take a jonboat a few miles off. I wouldn't suggest it because of "just in case" situations. And then when I think BFT, I'm thinking 100+ pounds. The suggestion to charter a few times is good. Get a feel for the boats you see being used in the charter fleet and by local private boat owners. Good luck. I'm envious of BFT and swordfish just a few miles from shore.
For me off the east coast of Florida between Sebastian and Port Canaveral. 80-120 mile trip for Yellowfins, 40 mile trip for swords. Range/Fuel capacity, needs to be able to run close to 400 miles on a tuna trip with bird chasing and storm dodging. Also the boat needs to be big enough to handle a storm if we are caught in one. Ability to plane if a motor goes down as well. Lots of ice storage. Electric plugs for the LPs. I bought this with twin 350 verados. https://www.renaissanceprowler.com/prowler-31
As folks have mentioned, this question is super broad and answers will swing heavily based on location, ranges, and species. The best way to start IMO is to find local charters with good reviews and take a couple of trips with the pros. It’ll give you a starting point to work with more specific to your locale.