Mercury 115 or 150 for a 20 SF?
 

Hi All,

I am considering repowering my 1975 20 SF with a new Mercury four stroke.
I'm currently running a 1993 150 Mercury Black Max and am happy with the performance overall but it chokes out a lot at lower idle which is why I'm considering a repower.

I was looking at the Mecury 115 with a big gear case because it's closest in weight to my current motor until my local mechanic suggested going with a 150 Mercury four stroke. The batteries have been moved under the console and I will be raising the transom with the repower so it can hold a 25" shaft. Everything else is original.

Any suggestions on which way to go?

The new 150 with a 20" shaft is 455 lbs
The new 115 with a 20" shaft is 359 lbs

200 hp

I have a SC 20 MA which I repowered with a 150 Etec - It has more power then needed.
The majority of the time I comfortable cruise around 28 - 32 mph, in bumpy seas 14 - 18mph. I rarely use full power and speed.
The less weight on the transom the better - weight savings between 115 and 150 engines are around 80 lbs plus or minus - less water coming up thru the scuppers - the ability to get up on and stay on plane in the slop for a comfortable ride is more important to me than the rare occasion when the wind whistles around my ears at a higher top speed.
My marine mechanic and others tell me the 115 hp engine is a great fit for the SC 20 hull - sips fuel.

I would go with the 115 hp outboard next time around.

Bushwacker ran a Evinrude 115 for years on RESTLESS LADY, a 20’ Seafari, including trips from WPB to the Abacos. He eventually replaced it with an ETEC 150, but believe he was pretty satisfied with the 115.
I have an ETEC 150 HO on my 20’ Seafari used mostly for watersports and tow a friend at 41 for barefooting. At 418#, I think it was the liightest 150 in 2012.

I agree on the 115hp. My 02 Johnson 115Hp is plenty fast, and pretty light. With the big gearcase and 25 Inch lower, it comes in around 365 lbs, which balances just right. Of course, as a 2 stroke I have the weight of an oil tank in the stern locker, but that is offset by what is likely an extra 10 or so HP at the prop. Go with the 115hp and enjoy!

I can see up north where the wave action my be different, but down in South Florida and the Keys we enjoy mostly flat seas which let's us run at higher speeds. Plus in my mind I can always slow down and not use the horses but it sure is nice to have them

Can vouch for the newer merc 115 command thrust, pushes my boat very well and is very fuel efficient.

The 150 is quite a step up in weight, but the newer mercs 200 to 225 are only about 20lbs more than the 150

I recall reading a thread here about 115 command thrust performance on a 20. As I recall, top end over 40 and exception fuel economy. I tried to find it using the search function but I struck out. Maybe you will have better luck finding it.
I know it's been done before on this forum!

May have been my boat. Nothing in the boat except me and a few gallons of gas, no T top I was touching 47mph in ideal conditions. It was scary lol, 3 blade 21pitch prop

It'll do 40ish now, running a powertech 4 blade

I decided to put a merc 115 c/t on my 77 MA based on a number of performance bulletins from merc and yamaha for similar size/weight boats and 72potter's reported numbers weighed heavily too, probably put me over the top. :)

Anyway, very happy and impressed with the power of the 115 on this hull. Mine tops out around 40-41 with engine trim, no trim tabs on my hull. 25" shaft on 27" transom (cavitation plate about 2" above keel). I have the pro-xs model and turning 5900ish wot with 13.8x20 merc enertia prop. Engine is rated to 6300 so dropping pitch a bit may yield slightly better top end and maybe better hole shot too. SS props were hard to come by last year. The one I have I found on ebay at some small dealer in MI I believe. Not motivated to experiment with props any further, but if i did need to replace it I'd test something a little smaller.

Cruises comfortably in mid to upper 20's at 4000-4200. That's with a full tank(apprx 65 gallons in 70 gal tank), two adult males (200lbs +/-), safety and fishing gear (no live well weight). I have a bimini too, no t-top, but its up when getting these numbers.

Comes up plenty strong on plane too. Doesn't leap out of the water like it did with 175 ocean pro 2strk that was on it when I bought it, but i'm not pulling slalom skiers out of the water either, so I don't need that kind of hole shot. Minimum planing speed (no trim tabs on my hull) is about 17-18mph.

Fuel economy is pretty good I think for a relatively deep v hull. Having a 70 gallon tank that I top off before every trip I haven't been anal about tracking fuel mileage, but at cruise speeds it burns about 6 mpg.

On one 84 mile round trip into the gulf of mexico I burned 18-19 gallons, which works out to about 4 mpg. It was a pretty calm day so able to settle in on cruise for most of the trip as opposed to battling the seas the whole time.

Not that I would, but theoretically under those conditions I could have done that trip 3 times on one fill up. Also means I could drop down to a 50 gallon tank which would shed about 90lbs of fuel loading and still have more than enough practical range for a typical day or weekend on the water. Similar as with the prop, if I needed to replace the fuel tank I would go smaller too, 45-50.

Bottom line i think 115, esp the merc with c/t, is plenty of power for this hull with moderate loading and use. If you typically load the boat with people and gear and want high performance too then go for more hp. For my needs the 115 is perfect.