What's with this semi-displacement stuff anyway?

I'm often asked about the speed range, efficiency, and such regarding the RF-246 and RF-22 so I thought it was time for a bit of an article to answer some of those questions.

First, we have three major speed ranges under consideration; displacement, semi-displacement (also referred to as semi-planing on occasion), and planing.  One should be aware that these descriptors reference the speed range the boat operates in and not the boat per se.  That is, a boat that's (say) designed to be able to run in the semi-displacement speed range may be fitted with a smaller engine for economy (maybe due to initial purchase and operation-wise) or because the mission profile for the boat at the time of manufacture didn't require a larger engine.  From a design standpoint, even though the boat is operated in the displacement speed range (due to the choice of engine) the boat is not a "displacement" model.  This will make more sense later in this article.

Things are not always black and white in the boat world but the following describes the speed ranges as they're often accepted.  As users we tend to think of speed in terms of knots (shame on you if you use miles-per-hour; that's for cars and bass boats) but designers think in terms of speed-length ratios.  This is simply a factor (multiplier) times the square root of the waterline length of the boat in question.  Displacement speeds are from dead idle in gear to a speed-length (SL) ratio of 1.4, semi-displacement speeds are from a SL ratio of 1.4 to 2.5, and planing speeds are SL ratios of 2.5 and above.  "Hull" speed is often defined as that speed when the bow does start to climb over the bow wave and it's generally accepted to be about a 1.34 speed-length ratio.

Let's use the RF-246 with a waterline length of about 25' with the Power Hull Extension.  The square root of 25 (the length of the waterline) is 5.  In this example our displacement speed range is up to 7 knots (1.4 times 5), our semi-displacement speeds are 7 knots to 12.5 knots (1.4 to 2.5 times 5), and our planing speeds are 12.5 knots and above.

The only two speed ranges where the boat is planing are the semi-displacement speeds and the planning speeds; at displacement speeds the boat is down "in the water"; that is, it is not trying to climb over its bow wave.  So, for the purposes of the following discussion lets throw out displacement speeds since any boat (any normally available consumer boat at any rate) can always operate at displacement speeds.  What we have left are the semi-displacement and planing speed ranges to discuss.

Now, let me bend your mind a little bit here.  The designation of a boat as being semi-displacement or planing does not refer to the highest speed the boat can attain but rather the lowest speed at which it can efficiently operate.  Uh?  Put another way, a semi-displacement boat is a semi-displacement boat because it can operate in the 1.4 to 2.5 speed-length ratio range, not because it can't go faster than a SL ratio of 2.5.  That's a big deal because what's necessary to understand here is seeing a boat labeled "semi-displacement" does not mean "slow pig".  What it should mean is relative to a like sized deep-v hull (which has a lower efficient operating speed-length ratio of about 3.0) the semi-displacement boat (at 25 feet) can operate at 12 knots efficiently where the 25-foot deep-v hulled boat can not (it's minimum is 15 knots).  Too many times folks that I talk with think because I call the Rosborough a semi-displacement boat that higher speeds (say over 20 knots) are not possible; that's just not the case at all.

Think in terms of an SUV (or minivan, or family sedan) and a sports car.  Both are able to run at 90 mph but which one do you think is going to do better at high speeds?  Especially in the twisties?  Obviously the sports car is designed for high speeds and twisty roads so it's going to do better in that situation than an SUV is going to.  Out on the freeway it doesn't make that much difference; it's generally straight and smooth and both will do fine.  Ditto for the Rosborough compared to a deep-v hull.  Out on smooth water (or just slop) the Rosborough can hang with a deep-v it's own size with equivalent power although most deep-v hulls have much more power installed though (just as a sports car would) and therefore it's usually the faster boat.  As an example, an RF-246 with twin Honda BF150 outboards on the transom is an over 30-knot boat; not exactly a slug.  However, when things get rougher the Rosborough is going to have to slow down sooner because it isn't designed to run at high speeds in rough conditions.

Now let's change things up a bit; now lets go to town where speeds are moderate and there are more potholes and railroad tracks.  Would you rather be in a comfortable SUV or in the sports car?  All of a sudden that stiff suspension and precise steering of the sports car becomes a liability rather than an asset.  Wow!  Those potholes are big!  Let's make this more interesting and take away third gear from the sports car to simulate the effect of not being able to run our 25-foot deep-v hull in the 1.5 to 2.5 SL ratio range.  Now, not only do you have to put up with potholes and lousy street surfaces but you've got to do it at a higher speed unless you want to shift way down.  This is exactly what happens when the waves build to a point where running the deep-v boat above its SL ratio 3.0 lower threshold is just not comfortable; there isn't a way to slow down just a little, it's pretty much all or nothing.

The previous paragraph might make it sound like I'm "pro" semi-displacement and "anti" deep-v but that isn't the case at all.  I do believe one has to choose the characteristics of a boat that will most suit the requirements and that it's important to understand what they are.  I mean, we all go to the car dealership and see some sexy sports car sitting there and we start to drool.  But, how many times do folks drive home in an economy car, a minivan, an SUV, or a family sedan compared to how many drive home in a sports car?  The sports car is a fairly defined set of wheels and isn't as versatile so unless you really know that's what's going to scratch the itch it likely isn't the best choice.  The deep-v hull as been so over-hyped for so long in the U.S. boat market I feel the buying public has been brain-washed to some degree and isn't aware of other choices.  A comfortable semi-displacement cruiser just isn't a sexy as a fast boat with a good looking model aboard in ads in the glossy boating magazines.  But!  If you want a sports car by all means buy a sports car; nothing runs quite the same.

So where does all that leave us?  Bottom line...if you want to be able to run a boat efficiently at semi-displacement speeds (that's a speed-length ratio of 1.5 to 2.5) you need to purchase a boat designed for semi-displacement speeds.  A deep-v hull with a lower SL ratio of 3 just isn't going to do that well.  A modified-v hull (less "V" in the transom) with a lower SL ratio limit of around 2.5 is going to do better but still not as well as the semi-displacement boat.  If you want to run a boat a high speeds in rough conditions you need to purchase a deep-v hull; you're not going to be happy with the ride from a semi-displacement hull at the same speeds in rough conditions (you won't break a Rosborough but I can't guarantee your body parts).