Prop Expert Required

[mickp]

Changed my prop over winter from a 12 x 8 fixed 2 blade to a 12 x 8 folding "radice" prop. Both came with boat.
Its early days but so far the folder gives me greater acceleration and Max hull speed at 1600 revs and I have no slow speed control as i had before. Also going astern there is a considerable delay before the prop bites. ( Frightening when approaching berth )
Now my engine manual Mitsibushi / Thorneycroft T95 39Hp says min revs on load 1500, so I now have an operational window of only 1000 revs.
Clearly this is not right so i need to know what size prop would give me the correct revs and greater / finer control.
Info: Boat is Moody 33 Mk 1, 28'6" LWL, 11' Beam, 4'6" draft. Displacement 10,500 lbs, Ballast 3,500 lbs. Gear Box ratio is 1:1.
Engine continuos rating is 3000 revs and according to Thorneycroft should balance engine / hull / prop to give Hull speed at around 2200 revs.
I also believe a deisel should be run nearer full revs and worked hard.
Am thinking of going back to a 2 blade fixed prop as prefered control and stopping power. But did not like prop walk although did get used to it. Old prop 12 x 8 give Hull speed at 1800 revs.
I have room for adjusting size to get greater range of control over more revs. ( Note max prop dia is 12" )
Any advice ? What size should i be looking at ?
If i stick to 12" dia but increase or decrease pitch what is the likely outcome.
Someone must have been down this road before ?
Thanks in anticipation


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[oldharry]

It sounds as though the folding prop has a greater pitch than the original fixed 12x8 even though it is supposed to be the same. Perhaps it is opening further than it should on load and giving too coarse a pitch? The difference between 8 and say 10 inch pitch is not very great in terms of the angle of the blades.

The prop speed should not exceed 2200rpm anyway, as cavitation will set in any faster. Starting from the 12x8 that was originally fitted, if you increased the pitch you will get precisely the effect the folder is giving - lower full throttle RPM, and higher intermediate speeds for given throttle settings.

Decreasing the pitch, or the diameter, will allow the engine to rev faster at full throttle, thus developing more available power, and giving more precise speed control over the throttle range.

Unfortunately experimenting with different size props is an expensive business, so the easy way out is to speak to someone like Lancing Marine who have tables and experience which will enable them to identify the correct size propellor to solve your problems, and sell you the correct size in either a fixed or folding type.

Prop walk is a total pain, and changing the pitch/diameter is unlikely to cure it in itself, as it is more a function of the design of the prop than the size, but seems to be more pronounced on 2 bladers as a rule.



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