A friend of mine is considering buying a 25ft Sportcraft.  Hes a walleye/slamon/muskie guy.  Hes wondering how big of a kicker hed need to push the boat at 5mph for muskies.  Anyone have experience with this?  Thanks in advance

I don't have experience with kicker motors, but a 9.9 seems like it would be overworked. Get a 20

I run a 16ft Sylvan and a 9.9 keeps it perfect around 5 up to 8mph.
I agree with Pens, best off getting a 20hp. 25Ft is long boat 

A boat that size i would look hard into using the main motor for musky trolling. Several years ago I switched to using main motor for most trolling applications and my kicker is used for a back up should the main go down. 
 
My experience was i had better boat control with the extra power and grip from the bigger prop would steer the boat much quicker with less over/understeer the kicker would do. The main motor was also much less noise. 
 
If he is dead set on kicker powering for trolling, 20hp like other have said. 

15 HP minimum, I would get a 20 or 25. I have a 21' and my 9.9 a WOT pushes the boat a little over 6 SOG.  

A higher HP would probably be better, as stated above but, there seems to be many Pro kicker 9.9 HP high thrust/high output options out there that what would probably work as well.  Probably comes to initial cost (if buying new), operating costs, thrust available, etc.  They would need to dig into weeds to flush all that out to see what would work functionally and what would be the most efficient from an operational and cost standpoint.  If both motors can maintain 5 hp consistently (9.9 HO and a 15/20 HP standard), does the 20 hp at 1/2 throttle burn less than the 9.9 HO at 3/4 throttle?  What is the weight difference, cost difference, etc.?  

CTKsnowman
15 HP minimum, I would get a 20 or 25. I have a 21' and my 9.9 a WOT pushes the boat a little over 6 SOG.  

I was curious if someone with a bigger boat would chime in. That 25’ StarCraft (at least what Google shows), looks like a really heavy boat. I was thinking 20HP might be low. I don’t know if the owner is looking to try and use that boat on HP restricted lakes? Might be hard to launch at most HP restricted launches that I have been to. My 17’ aluminum is probably half the weight and likely drafts a good bit less than the 25’er. I have had issues launching at a few, especially when water is low. My older 15HP only pushes me to 10MPH alone and struggles to go 8MPH with 2-3 others and gear. Might not really be putting out 15HP any longer though, I believe 1994. Highly recommend 4 stroke for a trolling kicker. I can only tolerate trolling with mine for about 10 minutes if the wind puts the fumes back at me, although not as bad at 5mph than 2mph.

Porktown

CTKsnowman
15 HP minimum, I would get a 20 or 25. I have a 21' and my 9.9 a WOT pushes the boat a little over 6 SOG.  

I was curious if someone with a bigger boat would chime in. That 25’ StarCraft (at least what Google shows), looks like a really heavy boat. I was thinking 20HP might be low. I don’t know if the owner is looking to try and use that boat on HP restricted lakes? Might be hard to launch at most HP restricted launches that I have been to. My 17’ aluminum is probably half the weight and likely drafts a good bit less than the 25’er. I have had issues launching at a few, especially when water is low. My older 15HP only pushes me to 10MPH alone and struggles to go 8MPH with 2-3 others and gear. Might not really be putting out 15HP any longer though, I believe 1994. Highly recommend 4 stroke for a trolling kicker. I can only tolerate trolling with mine for about 10 minutes if the wind puts the fumes back at me, although not as bad at 5mph than 2mph.

I googled it myself, and now I'm saying 20 HP minimum. if you don't fish restricted HP lakes get a 25. My father has a 24' Sea Ray and he has a Merc 25 long shaft as a kicker for Erie.   

Appreciate all of the thoughts/input.  Passing along to my buddy.