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Old 24-12-2009, 08:26   #1
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Repowering Formula


Hi Everyone, I am expecting to repower my 42 foot (LWL =35 13.6 beam 2600 lbs draft 4.5) I presently have 2 props 18 x 13 X L and 18X15XL I am looking at Beta marine 60 HP 2:1 PRN2 hyd. transmission
I am sort of in the fog as to what I am trying to achieve top RPM and HP is at 2700 RPM and max torque around 1600RPM
Here are some questions am I trying to get 8 Knots Hull speed, at full throttle of 2700RPM which I would prefer to run at @2000 RPM (Less wear and tare less fuel etc. also should not be continues run at 100% but around 80% Though I have lots of torque at 2000RPM the tranny out put only at 1000 RPM may not give full speed etc. I also understand there is slippage in prop so you cannot go sol on pitch. I would like to stick with my 2 LH props so if I work backwards then the pitch should dictate my speed at certain RPM??
This gets confusing. I have heard on this forum you should run Diesel at max RPM (is this to keep engine Hot or to achieve hull speed) How do you figure all this out? Help me understand
If it was a car I would rather run engine at low RPM and higher gear but seeing we are in a fixed transmission the only way to increase speed is to increase RPM which costs more fuel and wear??
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Old 24-12-2009, 10:08   #2
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Diesels are different beasts from gasoline engines. Diesels will last longer if they are run closer to their maximum output. Do not lug a diesel, meaning making it so that it can never reach its maximum RPM. In fact, most diesel manufacturers that I know of will not honor a warranty on an installation where the engine cannot reach its maximum governed RPM. The other factor is that a diesel must be able to reach its maximum RPM in order to eliminate carbon accumulation in the combustion chamber.

Its a flawed thought to think that a Diesel runs best if you gear it like the fifth gear in a car. Doing this will significantly shorten the life of your diesel. Its difficult for some people to understand this concept and therefore don't agree with it. But all you have to do is check with any diesel engine manufacturer to confirm this.

This book was helpful when I repowered a twin screw workboat. It goes in to hull shapes, engines, transmissions, gear ratio's, propeller sizes, desired speeds etc. The formulas in the book allowed me to nail the correct size propellers the first time.

The Propeller Handbook: The Complete Reference for Choosing, Installing, and Understanding Boat Propellers

Also, you really do want to have a right and left hand outboard turning propellers. Its more efficient and it makes maneuvering your boat in tight quarters much easier. Many modern transmissions can be run backwards or forwards, its just a matter of which way you attach the shift linkage.
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Old 24-12-2009, 11:02   #3
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Thanks David I must of given you the wrong impression I have a sail boat only one prop and a spare prop in case I was in Mex. and hit a rock and damaged the main prop." run closer to their maximum output. Do not lug a diesel, meaning making it so that it can never reach its maximum RPM"
a continus duty engine may be ok at max RPM but most engins for my application would void the warrinty if ran at max output for more than 80% of the time so 2160 RPM for an engine rated at 2700 RPM .So I would expect we would reach hull speed at 2160RPM and have some spair hp in rugh weather , current etc.
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Old 24-12-2009, 11:19   #4
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That's correct. You never run a diesel at 100% RPM for long periods. You can and should run one for short periods at max RPM to burn out the carbon. You though always size the gearing and propeller so it can reach maximum RPM....irrespective of what speed through the water it gets you.
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Old 24-12-2009, 12:45   #5
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Boatdiesel.com has a very useful calculator that you can use to get an idea of where you stand. Stanley at Beta USA (Don't know about Canada) is a very knowlegable guy and will also give you some guidance. Pleasure ratings vary by Mfg But it usually means that the engine can be run at Max listed RPM for 1 Hr in 10 or 24 hr. The Beta's are Marinized high speed lightweight modern diesels and it is critical not to over prop them.
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