Discussion in 'Ocean Yacht' started by sv211nautique, Jun 2, 2016.
What props to run on a 1989 48 SS with J&T 671 TIB
I think the best way to determine what wheels you need is to see what the boat will turn at WOT with the current wheels .
You are going to want to run the load WOT test with clean bottom, full water and fuel and healthy engines that MAX no load.
Then go to a prop calculator and reputable prop shop to tune and add or take pitch out.
Its good to have the actual weight of the boat if you have a yard that can give you the actual sling weight.
Finding out what other guys run on the same boat and power is a good place to start, but dont chase their numbers.
My guess would be something in the 24x26 range, but that is just a seat of the pants guess
Thanks good info
Ideally the revs produced with WOT at your dock with a well warmed engine and out of gear, should be the same as those produced at WOT at sea......., if it is, and the boat's bottom is clean and she is fully laden.....then the prop being used is ideal,
If it doesn't rev freely to WOT, you will overload and can damage the engine(s)
How did you come to this conclusion?
the engines i am familiar have different No Load WOT and different Load WOT
both are important determine proper engine operation and proper wheels especially in a plaining boat like a 48 Ocean
This is an incredibly misunderstood reality, quite frankly very difficult to explain in a few sentences and by me in particular.
I assure you, however, if your engines cannot rev to their makers WOT, they are overloaded and you are shortening their lives, using xs fuel, and goodness knows what else.
One must assume the engines are operating in peak condition, ergo can rev to the makers WOT at the dock and out of gear. If they cannot meet this parameter, to attempt to determine optimum prop dimension is futile.
BTW....what constitutes a Senior member of this forum, does it mean how long one has been a member, or does it refer to one's age......as I am knocking on the door of being an octogenarian, do I quality as a senior?
Brief notes as how to confirm an existing prop is correctly matched to the engine and boat and to alter it if not.
I...Rev engine to WOT at the dock, out of gear, checking revolutions with a laser handheld counter. The stock rev counters are notoriously inaccurate .......their reading will invariably produce incorrect data. The revs should reach the number presented by the engine manufacturer.
You may wish to have friend check the revs.......it's a bit unnerving to set the throttle for max revs, then go below into the engine room, leaving the throttles to their own devices.
2...with the boat fully laden with fuel, water, crew and with a clean bottom......she should, with the throttle wide open, rev to the same number, or even better, a few more.
3. In this fortuitous event you can consider your props have the correct dimensions and most importantly your engines are not being overloaded, thus they are not over stressed and should give a long trouble free life,...... all things being equal, ergo, provided they are fed clean fuel, clean oil and clean air.
Should the engines not rev to the makers number, then the pitch should be reduced, obviously by an educated quessed amount, you can allow 1" reduction to give about 50 to 100 rpm increase for an average 24"dia prop.
One of main misconceptions is that the original manufacturer of the boat will have used the correct prop(s). .....
Not so, ....in order to produce good speeds and sales, the boat is invariably tested with minimal fuel, little water and possibly a single crew......thus as soon as the new owner loads his gear, fills the tanks, adds clothes, bedding, anchor chain, scuba gear, you name it..... the engines are quickly overloaded and under stress thus the engines lives are limited.
This problem was rather ignored during the days of the old fashioned diesel engines....the Jimmies and Cats.....but in this modern age where high speed diesels, weighing half and producing twice the horse power of their ancestors, it is a different kettle of fish......many of these engines fail with very few hours.....not unknown for catastrophic failure with engines less than 500 hours.......
There are many reasons for engine failure, but why add to their problems by swinging the wrong wheels.
Mod Edit: redirect removed.