The Engine
The standard options for a Selene 59 are Cummins QSM11 or John Deere 6125.  Coming from a sailing background, my knowledge about diesel engines was limited, therefore I set about a survey via passagemaker and owner web sites.  Both brands seemed to be very well regarded, but Cummins recieved universal accolades for their support, particularly in our region.  So the first decision was made, Cummins it was.

The Cummins QSM11 range is an 11 litre engine available in 300HP, 350HP, 400HP, 450HP for trawler type applications and also 610HP, 640HP, 670HP & 710HP primarily for sportfishing applications.  Basically they are all the same engine block but with different electronic configurations.  More information on Cummins marine diesels is available at:
 
http://www.cmdmarine.com/recreationalinboards.html    

The next decision was what sort of horse power did I think I need, the standard offering was 450HP for the Cummins, but did I need this much?  Considering I am looking at long ocean passages where fuel burn is critical, could I get away with a smaller horse power and improve fuel burn?  Or did I need more power?  I had inputs from reputable sources advising anything from 285HP to 700HP for a vessel like the Selene 59, so that didn't work!!  

After reseaching this point 'ad nauseam' from; books, magazine, engine data sheets, web sites, standard engine & propeller calculators and more postings on passagemaking sites, the conculsion I came to (rightly or wrongly) is that fuel burn is approximately 1 US gallon (3.8 litres) for every 20HP used and maximum engine horse power has little affect at cruising revs.  At maximum revs it is a whole different matter, but that is of little importance to me.

Given that the designed top speed of the Selene 59 was stated as 11.5 knots (12 knots in some places but I think that is a bit optimistic), I ran some calculations based on maximum displacement of 126,760lbs and for a slow semi displacement hull.  I used slow demi-displacement because if you use a straight displacement hull, the calculations won't allow anything higher than hull speed which for a 56.5 ft waterline length is 10.1 knots (1.34 * square root of waterline length).  The result was that the power required for maximum speed (11.5 was used) was 355HP and for a cruising speed of 8 knots it was 130HP both expressed at the flywheel.  This equates to approximately 25 liters per hour at cruising speed (130/20 = 6.5 US gals/hr * 3.7878 = 24.6 lts / hr).

Therefore although I could go with a smaller HP, I made the decision to stay with the QSM11 450HP configuration as it didn't appear to make much difference to fuel burn at cruising speeds and a bit of extra power if you have it available, doesn't hurt.  Also by way of additional information, Cummins in Australia advised staying with the 450HP configuration as they would re-configure in downwards to 405HP at no cost if I required it, but if the engine was supplied at say 405HP, there would be a charge for re-configuring upwards.  However in practice, if the engine HP is re-configured, the combination of transmission ratio and propeller design may need to be revisited so this step should not be taken lightly.

Transmission & Propeller
I don't intend to go into this topic too deeply as the whole area of transmission ratio's and propeller design is a very specialized area and appears to have some degree of 'balck art' associated with it.  However, if like me you have "the need to know", a bit of research to understand some of the basic principles is very useful.  It should also be made clear that transmission ratio and propeller design are integrated, one depends on the other. 

So here goes.

My objective was to ensure that the design was biased towards the cruising end of performance rather than achieving a desired top end speed.  A top end speed of 11.5 knots is not particularly important to me, but I did want to achieve my desired cruising speed of 8 knots at the lowest revs practical so that noise and vibration was kept to a minimum.

Using a standard calculator, one good example is available on the Boat Diesel site http://www.boatdiesel.com/, it is possible to gain some insight into what is required by trying a few different configurations.  From there, I suggest that you put your case forward as forcefully as possible but in the end go with what the 'Professional' designers say.

Basically, it is generally accepted that the larger the propeller diameter, the more efficient (less slip) it is.  In practice of course, the maximum diameter is already established by the general hull design and draft of the boat.  It is also necessary to allow a clearance from the hull of 20% of the diameter of the propeller.  This ensures efficiency and noise / vibration minimization.  Also in general, for a displacement hull design, the propeller pitch should be between 0.6 and 0.8 of the diameter.

So at least we have a starting point.  On the Selene 59, the maximum propeller diameter is 42" and from the general guidelines, the pitch should be somewhere between 25.2" and 33.6".  All we need now, is to establish the transmission ratio and the pitch to do the job without over loading the engine.

So it was back to the calculators.

I had calculated that with a 3:1 transmission ratio, which was decided upon after many calculations and some discussion with propeller designers and manufacturers the most prominent being Hundested in Denmark, I could use a 33" pitch if I manually limited the maximum rev range of the engine to 1900rpm rather than it's maximum of 2100rpm.  This seemed sensible to me (again I emphasise, not necessarily right, just sensible to me!!) as the Cummins QSM11 450 according to the rated output curve reached 446HP @ 1900rpm and only an additional 4HP to 450HP @2100rpm.  The QSM11 450 also reaches peak torque at about 1400rpm, which is where I wanted my designed cruising speed.

A 3:1 transmission ratio was also convenient as it was a standard ratio available from Twin Disc, which appeared to be the tranmission of choice from JTM.

This proposition was put to JTM, who in turn submitted it to their propeller supplier and it was accepted, so a 42" diameter propeller with a 33" pitch and a 0.70 DAR it was.  More informative reading can be found at:
 
http://www.amartech.nl/English/propellersalgemeenE.html and

http://www.teignbridge.co.uk/propeller_questions_and_answers.html 

Fuel Polishing

Speak with just about anybody associated with boats and they will tell you that the most common problem that you are likely to encounter with diesel engines, is a fuel problem.  Therefore it is pretty important that you do what you can to eliminate this as a problem.

One way is to have a good Fuel Polishing system.  As well as having a Fuel Polishing system that works, it is also important to have one that is sized large enough to handle your fuel capacity of your boat, you don't want one that takes two days to cycle through your full tanks!!

The system fitted on Buena Vista is the ESI CFS-1000FRE, which has a capacity of nearly 700 litres per hour (180 USG/hour).  This means about 6 hours per tank if they are full, although if in an area where you are nervous about the fuel, allow for two passes.  The system is 230v and uses approximately 1.2A at full load

The ESI system, which can be used as a fuel transfer system as well a fuel polishing, consists of:

 A continuos duty high capacity pump specifically designed for marine fuel applications.

• A Racor 1000FG filter and water separator with a 2 micron element and water separation of 100% of free water and 99.95% of emulsified water.
• A De-bug L-1000 Microbial Decontamination Unit, which achieves a near 100% elimination in a single pass. eliminating the need for chemical biocides.
• A control box that allows variable run time up to 12 hours with automatic shut off and a liquid filled vacuum gauge.

More information on ESI products can be obtained at www.fuelmanagement.com