First things first. The speedometer in 98% of all boats, unless they are the "Smart Craft" type gauge based on gps are at best, "horribly optimistic." The only thing I believe now when someone talks how fast their boat will run is if they are talking gps or radar speed. The water pressure pitot gauges are terribly inaccurate. On Wednesday, while running the eight miles back to the landing to get in before dark, my in dash speedo told me I was running almost 80 mph. If I were the type to believe it, I would have been the happiest guy on the planet to get that kind of speed with a Triton TR-21 and a 225 hp outboard. However I don't believe everything I see and looked to my gps. It told me that I was really running only 69.8. While that speed is more than adequate for most people, it simply illustrates how far off the in dash gauges can be.
The thing to remember here is that people still believe that aluminum boats are lighter than fiberglass boats, and so can get by with lower horsepower. That might have been true when aluminum boats had three or four bench seats, a six gallon gas tank and an anchor for standard equipment.
However, hull weights have slowly and steadily crept up on aluminum boats and fiberglass boats have reduced their weights. I had a 20' FastCat with an 8'6" beam that weighed 300 pounds LESS than a 17.2' Smokercraft Pro-Angler! (1,100 lb hull weight vs 1,400)
I realize that not everyone wants to go 80 mph in a boat, but take this into consideration.
Take that 1,400 pound boat and add three batteries at 60 pounds each, 168 pounds of fuel, another 175 pounds of water in the livewell, another 100 pounds of tackle and accessories, a 60 pound trolling motor and I'm leaving a lot of stuff out here. Suddenly you're at 2,083 pounds before YOU ever step into the boat. Figure 200 pounds for you, a 100 pounds for your kid and 250 for your beer drinking neighbor and now you're asking for adequate performance from a 75 hp motor when it's expected to push 2,633 pounds across the water?
This boat is going to be slow to plane, sluggish in handling and not an enjoyable ride at all. Back off the throttle a little bit and the boat is likely to fall off plane. AND, you'll be using more fuel in the process than the same boat with a larger motor.
Yes, it is more expensive to max out the horsepower on a boat up front, but it's by far cheaper to do it up front than it is to do it down the road. You'll be paying for motor installation twice, and you'll never (or VERY rarely) get a trade-in value that's anything close to what you paid initially for the motor. And what's it going to add to the payments over the life of the loan? $25.00 a month? Heck, most guys here pi$$ that away on beer anyway.
Someone else mentioned disatisfaction with an underpowered boat, what is the dollar value on that when you're trying to get back to the ramp ahead of the lightning or hail storm? What's the value of the frustration when the kids want to tube or water ski on that family vacation and the boat doesn't cut it?
You're also going to spend more in fuel with an underpowered boat. Most outboards have a preferred powerband where they run at optimum efficiency and minimal fuel burn. Yep, that 75 hp
might be able to push the boat to 32 mph, but that's with the throttle jammed against the stops. But now that engine is trying to reach maximum rpms and it's burning fuel at an alarming rate. My 17' Tuffy, with a 75 hp outboard, my gear and two guide clients went 32 mph at WOT and I burned eleven gallons of gas each outing. My 20' FastCat with a 250 RACING motor, with even more gear, three clients on board went 55 mph while cruising easily at 3,800 rpms and I burned ten gallons of gas on an outing. And the best thing? We got to more places to fish in a day, spending less time running between spots.
Yes, it is true that a larger outboard won't troll down as slow as a smaller outboard, but buying a kicker motor isn't always the only solution. A single drift sock can be hung off the stern, or two smaller ones from the bow and will slow a boat to a crawl. Heck, even 5 gallon buckets with holes drilled in the bottom will often suffice. A trolling plate can be added to the cavitation plate to slow the trolling speed. All of these options work quite well and are considerably less expensive than a kicker motor.
The old 20% of maximum horsepower is still a valid argument and one I live by.
Formerly Steve @ G & S
Glad I did it. If you can max it out do it, you will not be sorry. 
