O ne of the questions I hear regularly from readers is whether an X horsepower engine will be enough for a Y length boat.
Naturally, to get to the answer you need to dig quite a bit deeper. What is the hull made of, how much does it weigh, and what design is it? How will the boat commonly be loaded? What are your personal requirements (in other words, how much do you like to go really, really fast)? What are some of the other advantages and disadvantages of having more or less power, that are important to you?
We can, of course, lay down a few parameters. Any boat’s power plant must be sufficient to get the boat up onto plane, in a reasonable amount of time. It needs to have enough oomph to do so in fully loaded conditions, too.
On the flip side you don’t want to over-power a boat. Coast Guard maximum power capacity plates pretty much resolve that issue, but it’s still possible to get more power than is ideal for your purposes, which usually has the net effect of hitting you in the wallet in initial cost, maintenance costs, and operational costs.
The Hull Story
Let’s consider two 18-foot center consoles with more or less identical hull designs, but one is aluminum and the other is fiberglass. The fiberglass boat is going to weigh significantly more, and as a result, there will be a power band that will work for the aluminum boat, but which would be woefully inadequate for the fiberglass boat. A 40 horse can get a relatively light 18-foot aluminum rig up on plane without a problem, even with several people aboard. It can run at a cruising speed in the 20 mph range.
That same outboard on a heavily built 18-foot fiberglass boat might not even be able to break a plane. A 60-horse outboard might be great on the aluminum rig and just barely sufficient for the glass boat.
A 90-horse outboard begins to feel like a LOT of juice on that aluminum rig, which can now scream across the water’s surface and may feel a bit flighty at full throttle. But on the fiberglass boat that very same motor may feel ideal.
Now, hull design comes into play. A flat-bottom boat has oodles of planning surface, and is the easiest design to get up and running. That’s one of the reasons some small flat-bottom jon boats can get up and plane with an engine as small as five or six horses.
On a V-hull design, however, more power is usually necessary to get a boat up onto plane. The deeper the V (transom deadrise) the more power you’ll need.
Power catamarans and tunnel boats throw another twist into the mix. Contrary to popular belief, some don’t really gain any efficiency from their design. Many others (usually those with “compression” tunnels that are designed to pack air between the two hulls) do generate extra lift. All other factors being equal, these designs can get by with smaller power plants than might be necessary on either flat-bottoms or V-hulls.
Then, there are a number of less-impactful variables to consider. Steps, chines, strakes, and unique hull tweaks that may be found in one boat or another. All can contribute to just how much power it needs, in order to do exactly what you want it to do.
Take a Load Off
Load is a huge determining factor when deciding how much power you’ll need, especially for those who like to use their boat for waterfowl hunting. Decoys, blinds, shotguns, other gear, and retrievers can combine to quickly sap an engine’s effectiveness, and that’s before we even account for a handful of hunters.
For anglers, load weight may be less of an issue, but it still rates consideration. In most cases, the determining factor here will simply be how many people you usually like to take fishing. If you’re a loner don’t worry about it one bit.
If you sea-trial a boat all by your lonesome and decide it has just enough power, you’re headed for a future of disappointment if you bring a half-dozen friends with you.
On an average 20-footer a couple people more or less can have a rather dramatic effect on performance, especially when a boat has minimal horsepower.
The Need for Speed
Beyond needs we have wants, and many of us want to go fast on our boats—it’s just plain fun. Some other people simply can’t stand to miss one minute of fishing-time.
Whatever your reason, if you want to feel the wind ripping your hair from your scalp, obviously, you’ll want the most horsepower feasible.
There are some fringe benefits to having excessive horses, too. In general, boats that are powered at or near the top of their horsepower range tend to be easier to re-sell, and retain their value better. In many cases the engine works less than a smaller engine would, since it can spin fewer rpm to attain an equivalent amount of speed. If your engine turns 3000 rpm all day instead of 4500 rpm, this can have a significant impact on the life span of the engine and can even lower maintenance fees.
Yes, there is such a thing as too much horsepower. But like we said earlier, the biggest issue here has been more or less eliminated by power caps put on boats by regulation. So these days when someone asks me if X engine is going to have enough horsepower for a Y length boat, I answer: Yes. No. Maybe. But if you can afford a bigger engine, why not get it?
Email Lenny Rudow at
Email Lenny Rudow at [email protected]