Building the Glen-L HOT ROD - Inboard Installation Notes
 
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Propeller Selection

Propeller Diameter

Propeller diameter is chosen to be optimal for the boat’s particular combination of horsepower, RPM and speed.  Deviating significantly from the best diameter may result in slower acceleration, reduced cruise speeds and efficiencies, and more difficulty in slow speed maneuvering.

Diameter & Efficiency

At low to moderate speeds (35 knots or 40 MPH) the slower the shaft RPM and the larger the diameter the more efficient the propeller will be.  Smaller diameter, high pitch-ratio propellers are better suited to speeds above 35 knots.  (Pitch ratio = Pitch / Diameter

Propeller Tip Clearance

Propeller tip to hull clearance should be 2" minimum or 10% of propeller diameter.  See Chapter 11 - Shaft angles and layouts (Plate 33 - Installation Tolerances, Page 131).



Rudder Selection

Rudder Shaft Diameter

Rudder Shafts (1" or 1 1/8") must be matched to the rudder stuffing box, collar, and rudder post bracket.

Rudder Offset

Direction of offset is opposite of rotation of propeller e.g. to the right for left hand rotation etc.

Rudder Depth

To eliminate the effect of torque, the rudder often extends to propeller shaft centerline only.  The deeper the rudder, the more the hull will tend to bank or lean inboard in a turn. (See Inboard Motor Installations, Page 190)


Plate 59 from Inboard Motor Installations Chapter 16 - Rudders and Steering, Page 188



Propeller Shaft Selection

Propeller Shaft Diameter

The diameter required for a propeller shaft depends on many variables, including SHP, propeller shaft RPM, propeller diameter, as well as the strength of the material to be used for the shaft.

Here's a link to some guidelines :
Propeller Shafts (Webletter 43)
If you don't want to run larger than 1" diameter and have copious amounts of torque on hand, look into Aquamet 22 shafting.  This is very tough shafting as we've had one mishap which twisted a strut and snapped a rudder without damaging the shaft.



Inboard Installation Clearances

Standard dimensions and clearances for an inboard installation
Inboard Motor Installation : Shaft Angles & Layouts (WebLetter 12)



Selecting an inboard engine : Engine Torque Vs. Engine Horsepower

GENERAL

When it comes to engine horsepower versus engine torque in marine engine applications, most people make the common mistake of focusing on the marine engine horsepower rather than the marine engine torque. When it comes to both gas marine engines as well as marine diesel engines, in most cases focus should be directed to the torque more so than the horsepower. There is a common saying with in the OEM industry that "Horsepower sells a boat however Torque is what actually moves it". This could not be closer to the truth! One should realize that horsepower is really a measure of the torque over a given period of time. This taken into account by the rpm variable in the specification. The following equation may help to shed some light as well.

Torque = Hp x 5252 / Rpm (5252 is a constant)

It is interesting to note that the formula also verifies the typical torque bell curve when the torque trails off as rpm increases at the top end. One may consider that engines making torque at a lower rpm tend to work better in marine applications due to the fact that "most" boats tend to plane in the range of 2,000 - 3,000 rpm. It's not by chance that most marine engine manufacturers continue using the larger displacement - lower rpm, cast iron marine engines because of this fact. Many people wonder why these manufacturers haven't changed over to the high rpm engines the automotive industry has been converting to over the past 10 years; for the same reason the LT-5 Corvette engine and Lexus V8 engines didn't work very well in these marine applications --- nice Hp but at higher rpm's and therefore poorer low rpm torque characteristics.

Another interesting item to note is that since the proper method for propping a boat is to select the size prop that allows the engine to turn at it's maximum allowable rpm. It is required that a similar level of torque be produced at the top rpm condition as well as the proper planing rpm for a given boat hull (draw a straight line across the torque bell curve and see at what "lower rpm" this takes place. If this is not the case the selected prop will over-load the engine at the boats planing rpm and therefore yield very sluggish low rpm performance characteristics. For example a high revving engine that makes 400 hp at 5500 rpm would be making about 382 Lb-Ft torque (using the above formula) at 5500 rpm, since it would have a maximum torque output at probably 4,000 rpm it would be likely not to produce enough torque at 2500 rpm to make the boat plane very well since the prop was selected based on the 382 Lb-Ft value. Notice the higher the rpm an engine makes it's torque the worse this situation becomes.

In contrast to this; large engines that make significant horsepower at "very" low rpm's will therefore make a tremendous amount of torque, but at extremely low rpm. For example a diesel marine engine that makes 300 Hp at 2,000 is making 788 Lb-Ft at this same rpm. Noting that the torque curve is generated in "bell form", and therefore the maximum torque could be as high as 900 Lb-ft on this 300 Hp engine. Comparing this against a 300 Hp GM small V8 engine that makes 300 Hp at 5,000 and 375 Lb-Ft torque at 3200 rpm, this is a considerable difference. Very low rpm diesel engines typically make tremendous low rpm torque and therefore require specific gear ratios not supported by sterndrives, as well as requiring much larger diameter prop shaft's.

Source : Marine Engine Torque Vs. Marine Engine Horsepower (perfprotech.com)



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Building the Glen-L Hot Rod : www.boats.chelseacoachworks.com      Revised 13-APR-2013