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Quick Tip from Blue Collar Man
Fuel Filtration Sizing

Selecting the right size or flow rating for a filter housing is as important as the filtration capacity of the filter, which is typically termed in Microns (one Micron is equal to 1/1000 of a millimeter) Following the KISS (Keep It Simple Stupid) principle of engineering, the lower the Micron rating the finer the filtration i.e. a one (1) Micron filter has better or finer filtering capabilities than a thirty (30) Micron filter, hence the filter rating is inversely proportional to its effectiveness meaning the lower the numerical value the higher the filtration or Low Number = Greater Filtration.

All filter housings have a flow rating termed in GPH (gallons per hour) or GPM (gallons per minute). Sizing is important, if too small and you will restrict fuel flow to your power plant starving the combustion process. Too large and you will be wasting resources, i.e. more expensive filter cartridges, larger physical size and over all greater initial capital investment.

Here are some handy Blue Collar Man rules of thumb for selection of your filter housing. Keeping in mind the KISS principle as none of us are as smart as we think we are. A typical four (4) cycle diesel engine returns 2 to 4 time as much fuel as it burns and two (2) cycle diesel engines return 5 to 6 times. Thus to calculate the flow through a injection pump we would multiply peak engine fuel consummation by 3.5 or 4 then divide by 60 to obtain the gallon per minute flow rate of fuel feeding a typical 4 cycle diesel engine and hence the fuel through put of the filter. The second and simpler rule of thumb is simply brake horsepower X 0.18 = gallons per minute feeding the fuel injection pump.

Practical Example: Cummins 6CTA rated @ 430 BHP burns 24.2 Gal/HR at WOT (wide open throttle)

• WOT rule of thumb
24.2 Gal/Hr x 3.5 = 84.7 GPH
84.7 GPH / 60 Min per Hour = 1.41 GPM

• Percentage rule of thumb
430 BHP x 0.18 = 77.4 GPH
77.4 GPH / 60 Min per Hour = 1.29 GPM

• Available Racor Models:
500FH rated 60 GPH
900FH rated 90GPH
1000FH rated 180 GPH

For this installation a Racor 900FH housing would be a smart selection as this housing is rated at 90 GPH.

Racor is an industry standard for diesel fuel filter housings, there are many other selection available but Racor was used as a representative example.


Bugs in your fuel or microbial degradation is the venereal disease of fuel systems and the ramifications of water in fuel were ever it may come from should be understood by all who operate and maintain Marine vessels. Water not only destroys expensive equipment on engines such as injectors and fuel pumps but causes a condition in common terms called "Bug in the fuel" It can consume a marina (progression is very noticeable) as well as line the pockets of vendors and in the end put life at risk. This is an area of my expertise, I am not a professor but my training education and experience in this area is very documented.

The scientific names for the most common types of organisms that live in petroleum products are Cladosporium resinae and Pseudomonas aeruginosa. There are three (3) elements or conditions required for this nuance to occur A) water B) fuel, diesel C) heat. These slimy bugs live and multiply in the fuel/water interface. They exist in the water and feed off the hydrocarbons in the fuel. They are referred to as Hydrocarbon Utilizing Micro-organisms, commonly known as H.U.M. Bugs. As they grow, they form mats that are dark in color and appear gel-like. Their waste produces water, sludge, acids and other harmful by-products. Micro-organisms will consume rubber gaskets, O-rings, hoses, tank linings and coatings in an effort to obtain their mineral content.

What this all means is algae and slime is formed and there is continuing fouling of filters.
In the heavy duty marine environment fuel tanks are constantly begin clarified and purified via Sharples or Delaval type purifies removing the water and condition which cause this phenomenon and we learn to treat for microbial derogation with Bibor .


First we must determine what the proper diameter propeller for your boat is, typically it is desirable to swing the biggest dia. propeller you can fit. There is a rule a of thumb the prop should be as square as possible in order to achieve good performance, By square we are speak of the ratio between diameter and pitch. i.e. 20” dia. x 20” pitch. To determine how large a wheel you can swing we must look at prop aperture.

•The clearance between the tip of the prop and the hull is "a". You can go down to 10% if you're stuck.
•The clearance between the rudder and the nearest point of the propeller is "b".
•The clearance between a full keel and the leading edge of the propeller, measured half way out along the blade from the center of the shaft, is "c".
•The clearance between the prop strut bearing and the propeller hub is "d".

The main concern typically is how close the tip of the propeller can be to the hull of the vessel before to much turbulence is developed. From the above diagram we can see this is dimension “a” so for a 20 in dia wheel the closest the blade tip can be is 20 in x .15 = 3 in.

The proper size propeller will at WOT (wide open throttle) have the engine performing at its rated RPM anything below rated RPM will be overloading the engine decreasing the useful life of your power plant anything over the rate RPM is detrimental as we know over rev’d engines tend to go to pieces

To obtain the proper pitch many prop shops utilize a prop calculator you will need to know the rated RPM of your engine, the gear ratio of your transmission, diameter of the propeller you can usefully utilize. Below is a Prop calculator from Castle marine of the UK.

I snapped my shaft what's with all the various type of material and do I need to increase the size of the shaft diameter.
There are a myriad of shaft material to select form Monel , Carbon steel, Brass to Naval Bronze. The vast majority of modern shafting utilized today is of the specialty heat treated stainless steel. Not only is it used for boat shafting but in many industrial applications such as pump shafting. It is known by the various trade names Aqualloy 17, 19, 22 Aquamet 17, 19, 22 Nitonic etc.. Lets understand how stainless steel is made. It is a carbon steel base with chromium and nickel added to produce a durable highly corrosion resistant material our typical grade are rated numerically base on there content of chromium and nickel, various grades are 304 , 316, 404 and 416. 316 SS is utilize in many marine application as it is the most corrosion resistant but it is also one of the more expensive 416 is magnetic and is likely to be used for knife blades because it's properties enable it to hold an edge. Most boat shafting is made of a specially treated class 304 SS it is nitride or heat treated with nitrogen ruggedizing it properties giving it better tensional properties

Determining your shaft size is highly important as to heavy and you are wasting resources such as monetary and mass, to small and there is the risk of snapping your shaft leaving you dead in the water. Some prop twisters with years of experience judge the size of the required shaft shearly on the wheel or propeller size. They in turn look at the diameter and pitch and judge approximately the proper shaft size based on years of experience. This is a very seat of the pants calculation and I am sure it has worked for many years but with today's many different material grades there is a more technical arrival at proper shaft diameter. The following formula is used by most machinist’s, mechanic and engineers alike and is relatively easy to compute knowing the shaft horse power transmission gear ratio and wide open throttle speed. Generally, safety factors of 2 are considered adequate for boats intended for light pleasure service. For more severe service-such as racing craft, workboats, diesel powered and gas turbine powered boats – higher design coefficient should be based on experience. The safety factor has always been curious to me but let use trust in the powers that be and say the rule of thumb or acceptable practice or good engineer practice is a value of 5 (five).

With that said let use run through a calculation using the most wildly used shafting used for work boats being Aqualoy 17, as it is with all matters a of machining manufacturing and construction there are always trade off's Aqualloy 17 is a superb selection it is one of the most durable cost effective materials with the trade off of corrosion resistance. Corrosion tends to be a factor when boats sitting idle for a long time hence work boats are not concerned with this draw back of Aqualloy 17 as they have to keep moving in order to survive.

  S.F. = D3 X St X N
  321,000 X P

  D = Shaft Diameter, Inches
  P = Shaft Horsepower
  S = Safety Factor
  St = Yield Strength, tensional shear, PSIG
  N = Shaft Speed

  Example for a recent repower:

  S.F. = 1.5 X 135,000 X 3200
  321,000 X 420

  S.F. = 648,000,000

  S.F. = 4.81

  One Fourteenth rule:

  Propeller Diameter 20”

  20/14 = 1.43” Diameter shaft

The One – Fourteenth Diameter Rule
 The oldest rule of thumb for determining propeller shaft diameter is simply that it be one–fourteenth of the propeller diameter. In spite of its simplicity, this rule works surprisingly well. A 36 inch diameter propeller would require a 2.57 inch shaft, by this method.

The one-fourteenth does not take into account many of the variables in selecting the best propeller shaft and propeller combination. As it does not reflect differences in shaft materials ( St: Yield strength)

Surprisingly both the calculation and the rule of thumb method came out with the same results 1.5 inch diameter shaft so for this repower we were able to save significant dollars by not have to upgrade the shaft size and therefore struts and stuffing boxes during this repower!

By Captain Jim Mahoney
 Capt. Jim Mahoney is a licensed Marine Engineer of unlimited tonnage and horsepower and served his apprenticeship with the Calhoon Marine Engineering Beneficial Assoc. Graduating form their highly respected school located in Easton Md.


​Boat talk is a web page we are putting together at Blue Collar Sportfishing in an effort to disseminate information in areas of our expertise about boat repairs improvement and modification as well as the great sport of fishing. The information contained in this page are the opinion of the owner and are based on sound engineering principles and years on experience. The implementation of this information is strictly at the risk of the reader.