I'm presenting this in the interest of providing observations and information for folks to chew over whilst making the hard choices about which direction to go...it is not intended to offend anyone comes to a different conclusion than I did or has contrary opinions to mine; it's your boat!.  However, I can't really share my thoughts here without taking some risk of criticism so I'll just have to accept that it may happen.

Let me address the most controversial subject first since it's one that gets asked of me a lot...the diesel.  I'll tell you up front that my opinion is that a diesel only belongs in a displacement hull (or one used exclusively in that mode); a long-distance, high-hour cruiser; a commercial boat; or when gas engines can not produce enough horsepower for the performance requirements.  With but a singular exception in the above list the Rosborough RF-246 meets none of the criteria.

The first thing you need to realize is that none of the current high speed diesel engines used in modern planing hulls have much resemblance to the old heavy-iron slow turning diesels most folks think about.  The modern lightweight diesel tends to last about 2500 to 3000 hours (though with extraordinary care it could go longer) and NOT the 10,000 hours diesel advocates tout.  The diesel available in the RF-246 at the moment is the Volvo Penta D3 of 190 hp with a Duoprop outdrive.  The ad copy reads:

Volvo Penta´s new 5-cylinder D3-190 is developed from the latest design in modern diesel technology. The engine is extremely lightweight and compact.

Combined with features like common rail fuel injection system, double overhead camshafts, 4 valves per cylinder, variable geometry turbocharger, after cooler and the EVC system (Electronic Vessel Control) a world-class diesel performance, combined with low emissions, is achieved.

That is NOT your grand dad's old-timey lugger, it turns up to 4,000 rpm.  And look at the list of technology gizmoes!

Then there's the noise...or NOISE!  It's quite difficult and expensive to really quiet down a diesel in a small fiberglass boat which has all the acoustic characteristics of a hollow body guitar.  I've been aboard several boats in this category with the Volvo Penta, and many with Yanmar inboards, and they're all noisy.  It's hard to have normal conversations in the cabin at cruise speeds in most installations.

Since the above unit is an I/O (inboard/outboard) set up there's going to be a large engine box right in the middle of the cockpit; not good for socializing and it makes the cockpit on the Sedan Cruiser miniscule.  And since we can't have the Power Hull Extension it means we have a lot of weight right in the stern of the boat (there's weight right on the Power Hull Extension too but there's more buoyancy there than what the engine weighs).  Not to mention all the mess and mechanicals that go along with any inboard engine putting components, hoses, fuel lines, and such in the bilge.  Working on it means being on your hands and knees or laying down.

So what are the reasons folks ask for diesel power?

1) Longevity.  Well that would be true for the old heavy low rpm diesels but not the new lightweight models.  I personally know of Honda BF90's with 8,000-plus hours on them.  I'd never tell someone they'd go that long but even being conservative there's no indication anywhere than a modern high speed diesel will outlast a modern outboard.

2) Economy.  Not as much as you think.  It's not like the optimized power of a perfectly matched slow rpm diesel in a displacement hull.  Volvo Penta lists the D3-190 as consuming 6 gph at 3200 (about the rpm needed for a 15 knot cruise).  By comparison a pair of BF90 Hondas (or BF135/150's) is consuming about 7.2 gph at that same boat speed.  If you put lots and lots of hours on the boat per season this may finally pay off but certainly not if you're in the average range of 100 hours per year or so (that's 2 hours for 50 weekends in a row; every time you skip a weekend you've got to do 4 the next one, or 6 if you skip two.  That's really a hard number to hit for a lot of casual boaters.).

And economy from a purchase standpoint is even worse.  The diesel option adds over $32,000 to the price of the RF-246 while a pair of Honda 90's (equivalent power) comes in under $20,000.  You'll have to cover a tremendous number of miles to make up the difference.

3) Safety.  If we were comparing a gas I/O with the diesel I/O it would be somewhat a consideration but when comparing the diesel I/O against the same boat with outboards there just isn't any evidence whatsoever that the diesel I/O is safer.  How could it be really?  With outboard engines absolutely everything mechanical is outside the hull; only the fuel tank itself is located inboard.  No fuel, oil or other gunk gets in the bilge and there are no worries about the bilge pump discharging offensive (and I might add fine-able) stuff overboard.

So to summarize the diesel:

	It's noisier
	It costs more
	It's only slightly more fuel efficient in lightweight planing hulls
	It's heavier
	It's not safer
	It takes up room in the boat
	It requires more expensive maintenance
	It's not as tolerant of layups

Is it always a bad choice?  Not at all.  Some situations just fit the diesel better (like the only fuel readily available in the area of operation is diesel) and some folks regardless of the list above just like it and doggone it that's their prerogative, after all it's their boat.  For the rest of us it's generally not as good a choice as the outboards.  Not the least of which is the fact that we can opt for the Power Hull Extension and gain over 9 square feet of cockpit space (and get the transom seat if we want it) by utilizing outboard power.

With the diesel discussion out of the way that leaves the outboards.  Obviously this is my motive power of choice for the RF-246.  They're lighter, quieter, and less expensive than a diesel I/O and you gain a tremendous amount of cockpit space (that alone would do it for me).

The questions that usually follow are 1) how much power and 2) single or twins.  I'll answer both questions at length below.

1) How much power? 

Hmmm...how fast to you want to go?  Really?  The hull characteristics of the RF-246 that make it so special also make it viable for a very wide range of power.  I could literally envision a pair of 50's on the stern, a single 90, or at the opposite extreme the max horsepower of 300 (which we've done).  In fact if someone told me they wanted to install a single 50-horsepower engine it wouldn't worry me overly much as long as the folks understood exactly what they were getting.

So really the first thing to figure out is realistically how fast do you want to cruise on a more or less routine basis?  I usually power boats based on that and not top speed since we're not running sports or bass boats here.  Tell me your required cruise speed and I can tell you how much horsepower is required to meet that specification.  It's really as simple as that.  The thing I can tell you with absolute certainty is that speed costs.  I do get folks that tell me they want a reasonable cruise speed (let's call it 15 knots) and then add that they'd really like to be able to kick the speed up to say 25 when "needed".  Well, that's ok but it takes a lot more horsepower to run 25 knots as opposed to 15 knots.  With modern fuel injected engines we can overpower (that is, more horsepower than we need for cruise speed) to get the higher speeds and not give up much fuel economy at slower speeds.  It still costs though as those larger engines are more expensive and if the extra power is used they go though more fuel.

I like to break it down to four general speed ranges...150 hp for a 10 to 12 knot cruise (15 knot fast cruise), 175 to 200 hp for a 15 knot cruise boat (20 knot fast cruise), or 250 to 275 hp for a 20 knot cruise (25 knot fast cruise), and the max 300 hp for a 25 knot cruise boat (at which point I'd wonder if the folks really picked the right hull for their use!).  You could certainly choose less power for slower speeds but we're here in the Pacific Northwest where high currents are a reality (over 7 to 8 knots in places) and snotty weather happens (with few harbors to run to).  The slower your anticipated cruise speed the more the currents and wind affect you.  That is, a 2 knot current against a 6.5 knot boat knocks the heck out of ground speed, at 12 knots it's less of a factor, and at 20 knots it's not very noticeable.  So most folks don't want to limit themselves to slow speeds by choosing engines that won't let them travel faster (even if it's moderately so).  If they get into heavy currents or bad weather is coming they want to be able to move out.  Also, we do not run at night in our area due to the large amount of debris in the water (read that as whole trees sometimes) so getting in before dark is important.

It used to be that large engines with carburetors optimized for mid-range performance did quite poorly at slow speeds but that's all changed with fuel injection which can be optimized (by the computer in the Electronic Control Module) for all speeds.  So now if your engines are just a bit larger than absolutely necessary you don't pay the high price of inefficiency when running them at low speed.  For instance, at 12 knots there is very little difference in fuel burn between a single BF150, a pair of Honda BF90 engines, or a pair of Honda BF135 engines.  On the other hand, the BF135's are more efficient at 20 knots since the BF150 or BF90's are working hard at that speed.

2) Single or twins? There really is no one answer but read on...

[Edit 9/10/08: I'm adding a bit more info here based on two things: 1) the price of fuel at the moment and 2) more experience with a single engine RF-246.]

I would now say that a very attractive setup if one is looking for the most "bang for the buck" would be an RF-246 with a single Honda BF150 on the factory Armstrong bracket (this bracket has a full width swim platform and a built-in kicker mount); in round numbers this choice saves about $14,000 over the BF135 twins.  This choice is going to give you a boat with the least up front investment and great fuel economy.  It's also going to be a very simple boat to run and maneuver.  With the full width swim platform and the single engine there is a lot more room on the platform (as opposed to twin engines) for ingress/egress to a dinghy or the ladder, a generator (usually Honda EU2000i), or a cooler.  The helm station and electrical system are also much simplified (which saves money).

What's the downside?  That totally depends on your perception of the hull.  It's a semi-displacement hull optimized to run in the 9 to 12 knot range.  Now, semi-displacement does not mean slow and it doesn't mean that the hull won't plane (that assumption is totally incorrect); it simply means the hull will operate in the speed range where most "full planing" hulls (like deep-v's will not).  As we've shown with our twin Honda BF150 installation, if you put horsepower on the transom the boat will move (the BF150's top out in the mid 30 knot range).  But...if you want a 20 to 30 knot cruise I'd suggest that the RF-246 is not the correct choice to start with; anymore than a minivan would be the choice for sports car speeds on mountain roads.  What the RF-246 does best, and what makes it fairly unique (especially on the west coast), is that it can slow down to 1) increase economy and 2) make the boat more comfortable.  With a single BF150 the Rosborough RF-246 has an operating envelope of idle speeds to about 23 knots at wide open throttle.  I'd suggest it's best performance will be up to 12 knots with a "fast" cruise of 15 knots if you need to get somewhere a bit quicker (still well within the comfortable cruising rpm of the engine).  When things are kicked up and you're comfortably covering ground at 9 knots (usually burning about 3 gallons per hour) in rough conditions without having to hold on for dear life you're doing exactly what a deep-v hull can not do; it would be horribly uncomfortable at 9 knots and wallowing all over the place.

I always power for the intended cruise speed unless someone tells me they "need" more power for their application (and that's totally their choice).  However, if you stay true to the design of the boat and operate it in its best range then there's no need for more than 150 hp on the stern as that's a perfect choice for the 12 to 15 knot range.

Is a single Honda BF150 always the "best" choice?  Absolutely not.  First and foremost you might just want twin engines and this is the best reason to install them.  Too many of our customers try to justify their choices and I think that's just not always appropriate.  I mean really, how do you justify the boat in the first place other than as a recreational activity for your enjoyment?  So why go through the "justification" process on choosing power for the boat?  Just pick the setup up that will make you the happiest every time you look at the transom and don't worry about why; just accept it's what feels good.  We've had folks choose the twins just because they look the best to them and I think that's a great reason.

Oft times the "safety" issue of a single versus twins comes up but it's really a situational answer at best.  If one engine conks out 100 miles from the nearest repair facility you might be quite happy that there are twins on the transom.  On the other hand if you go scooting around a rocky point, hit bottom, and damage the lower units you might have been happier with a single engine since the kicker (back up motor) would have been up out of the water and undamaged.

Folks worry about contaminated fuel but that's going to affect two engines as easily as one (there's one fuel tank).

Another situational item is handling.  Yes, you can do things with twin engines that you can't do with a single but I've never not been able to get an RF-246 where I wanted it because it didn't have twins.  For a lot of folks twins are more hassle and more confusing so they definitely aren't an advantage for everyone and underway you're definitely messing with the throttles a lot more with twins in order to keep the engines "synched".

Personally, I think it boils down to this...if you like the KISS approach and want the most boat for the least amount of money with the lowest operating costs the RF-246 with a single Honda BF150 is the boat of choice.  If you just have to have more power available (the RF-246 will cruise nicely at 20 knots; it just takes more power (and therefore fuel)), or you're used to twins and you really want that aspect for maneuvering, or you will just feel better with two large engines on the transom then I'd choose an RF-246 with twin Honda BF-135s on the transom and that's a great combo; you'll still have good economy if you choose to slow down.