OK long post, but has the experience, the reasons why and hows that I would have like to know when going thru this exercise... and assume others have the same thoughts/ questions
1st the Commander is 5.5m or 18' 2"
And they are an awesome salt water hulls at rest and on the plane, but like all hulls, cars, tractors , trucks to be any good for the end use, need the correct power.
With about 80L fuel under foot, guys ave around 90kg, fishing gear, 95L bin, bait burly bin, spare props anchors, battery etc andwith full bimini up.. Its always up.. comes in around the 1250kg total gross weight on water.
We originally had 115 hp on the back fishing the Bottom end waikeke, Corro furth, Ahh ahhs etc
Good but min good power WoT around the 40 mph cruise 32/34 mph around the 3900/ 4000 rpms.
Get in bit ave chop would cruise thru well, and run on an oilly rag. Chop get up a little more and then start to work the throttle and once that starts, gas goes up huge.
Then where thinking about moving home, to West coast. Had time as next few months was hard out me doing complete reverbish the old old house and gardens etc. I had crossed bars many times over the yrs, and knew the 115 if something happens like wave climbs in front etc, or need to move quick, need that bottom end power.
Along with crotch straps and actual life jackets..
I worked out (calculated) and also being on several other commanders and similar hulls that confirmed the equations around the min 130 to 140 hp would be the go.
Now came the 20" transom...and looking for a later model , good service record, 140hp with 20" leg.
I was just tossing up to re build transom, but is still solid as, so
thinking make up a simple jack plate or a full plate across the transom... cost to buy is stupid price.
Eventually a V6 150 20" came up , ex lake , low hrs, serviced winterised from new.
Loading the engine bay with the 115, Extra weight would set the stern down at anchor about 1/2 to 1" and nil diff once boat moving.
Swapped engines was a breeze, over and done in couple 3 or 4 hrs and 1 other person to hold transom bolts from inside... and engine hoist.
Result :
Get in serious chop on top of westcoast swell or a good westerly down the bottom of Tamaki straight, white caps at near shoulder height, set throttle and chug home just on the plane and not falling off it... off Bottom end Waiheke good stherly comes up and tide changes..
Have had 1 instance in the commander a wave stand up in front on the bar...got to it just as started to break, over, then there is always the next last swell of the set, big period, get to the next before started to break.....the 115 the 1st would have broken, have the power to power thru.. I dont know. I did the 2nd would have been on us, and bigger..
Couple instances of breaking bar waves coming in from the side going in.. yes the 115 would have done it fine, the 150 instance move lot heaps area and time around us.
Now cruise at the old cruise speed of 32/34 mph fuel consumption drops WELL over 10% better economy.. If travel at around 3900 rpms and 38/39 mph very similar as previous 115 32/34 mph economy....
WoT is around the 48/49 mph now as gone from a 80L (which is all that fits in the std commander 120L tank due to air vents) to a full 150L fuel tank... Take lighter guys, big fish bins out, 1/2 tank fuel goes around the 52 mph ...starts to chine walk a little... not my cuppa tea.
Down side, well only 1... With tthe 150, I have to say every time we take off, " Holding on?" And always turn and check everyone is.. there is always some 'hero' who wants to fall backwards.. Its put into gear, boat moving, move throttle gently forward a little... wait to go over the bow wake start trim up, and now at cruise speed.. no throttle adjustment.
It is my understanding that these boats in general, made around the 70s /80s where rated to around 140/150 hp..
So 2 stroke 115hp good for general harbour stuff. 4 stroke 120/130 will do your bars , good chop etc.
The 115 and the 150 are/ where both correct height within 1/2 adjustment notch (bars on drops slightly) propped to correct WoT and Cruise slips, and pitch..