Mercury 115 or 150 for a 20 SF?

[deerfly]

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.