1986 20' SF with a Merc 150 4S performance numbers
 

I looked around a lot for numbers like this when I was looking for new power, found a lot of people asking similar questions but a lot less that got a detailed answer to it. Maybe this will help someone with their decision making down the line. I know lighter power is preferred on these hulls and I would tend to agree, but since I was doing all the work myself I tried to take a lot of steps with rebalancing weight try minimize the change in CG. A little information about the setup in general:

- 1986 20' Seacraft SF
- 2019 Mercury 150 4 stroke (25" shaft)
- Engine mounted in middle hole
- Mercury black max 13 1/4" x 17p 3 blade prop (p/n 48-77344A45)
- Original floor height
- Fuel tank reduced to 52 gallons, rear edge of tank slid 30" forward of front edge of in floor livewell compared to original tank (70 gallons)
- Factory center console replaced with a 10" narrower custom one with a cooler in front instead of cooler/storage seat molded to front of original console.
- Rear edge of console moved 6" forward of rear edge of original console location
- 2 batteries moved inside console from rear hatches.

Boat sits in water with water level touching deck scupper outlets in transom. I have check valves installed inline for the deck as well as the hoses crossed port to starboard so it minimizes water on the deck if someone is standing in the rear corners.

RPM/GPH/MPH
2500 3.5 15.8
3000 4.3 23.7
3500 6. 28.8
4000 8.2 32.4
4500 11.2. 37.2
5000 41.0
5300-5400ish 44.8

all the numbers up to 4500 are with 2 adults and 3 kids on board, 40 gallons of fuel. 5000 and up were just me, half tank of fuel. I have to scroll through my smart craft gauge to get to GPH, I didn?’t want to take a hand from throttle and steering to do that at that at those speeds.

Those are pretty good numbers with a 4S

My written performance numbers are long gone, but the speed/rpm numbers look very close to my old 20 Seafari's performance with a 150 Black Max 2 stroke. I bet your fuel economy is far better than mine.

Did a 143 mile round trip for an overnighter a few weeks ago up the ICW, burned 30.2 gallons. Took 10 hours total between slow zones and areas we could run faster. Very happy with that.

I would recommend trying other propellers. I believe you are leaving a ton of performance on the table with that existing propeller.

I would recommend trying either

17 Pitch Enertia
or
17 Pitch Mirage Plus

When you see the size difference between you existing prop and one those I suggested, you will be surprised.

I?ll give one of these a try for sure, thanks for the recommendation. I was getting close to the end of the build, I?m sure most of you know how at that point my budget was blown and just about doubled! This was what got me on the water in my budget at the time and I have been so happy with it I haven?t revisited yet.

Been wanting to keep a spare prop/hub/nut on the boat, when I can move to something like you suggested my current one will become just that

"I?m sure most of you know how at that point my budget was blown and just about doubled!"

Welcome!! There are many of us that have lived in that world!! :)

I agree but 2 adults and 3 kids on a 20 w/full fuel is a big load.