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HULL, PROPELLER, ENGINE COMBINATIONS
Many "problems" can be traced back to a mis-match between these three basic components. This mainly concerns cabin cruisers, but a mis-match on any type of boat must raise questions over adequacy of basic design or maintenance.
Your lecturer would categorise propellers into four basic types :-
TURBINE
EQUIPOISE
SPECIAL
TURBINE
TYPE
This is the "typical" propeller. Most displacement craft will use this unless there is a very good reason not to.
High torque (Twisting force) transmissions would use a four bladed version.
EQUIPOISE TYPE
The blades have a larger surface than those on a turbine type and therefore the pressure difference across the blade is less than on a turbine type.
The lower difference in pressure helps prevent cavitation.
This type is normally found on high speed planing craft, although it should not be detrimental on a displacement (low speed) boat.
WEED SLINGER TYPE
This particular picture runs anti-clockwise (from the rear) when running in ahead.
The shape of the blade is not as efficient as the turbine type but if something catches on its leading edge that item should be thrown off by the water resistance and rotation.
If you pick something up in astern – hard luck!
SAILING TYPE
This two bladed propeller is designed to be braked in a vertical position behind the keel of a "long keel" sailing boat.
All sailing craft need to stop the propeller turning when under sail because a "windmilling" propeller absorbs energy that would otherwise be used to increase boat speed.
Stopping it behind the keel shields it from water flow and helps prevent it creating drag.
OTHER SPECIAL PROPS
This category includes various folding propellers, both automatic and manual via a lever down the centre of the prop shaft and variable pitch/reversing propellers.
These all have "mechanisms" of varying complexity thus must be more likely to give trouble than one of those illustrated.
It is reasonable for a racing yacht to try to minimise drag by fitting a complex propeller and/or a light, but complex, engine/transmission, but this would be questionable practice on a cruising, family yacht of similar design.
There is nothing inherently wrong with a well-designed prop of one of these types, but the owner must recognise the weak spots and plan maintenance accordingly.
Lessons
A turbine or an equipoise prop on a displacement boat should not be a major concern.
A Turbine propeller on a planing boat should raise questions and lead to further inspection and tests for cavitation (a growling noise under power, especially when turning).
A weed slinger on an estuary cruiser should raise questions as to why. There might be a very good reason or it might be because that is what was hanging around at the time it was needed.
A weed slinger on a narrow boat should also raise questions because of the amount of time narrow boats spend in reverse and also because of the amount of rubbish found in the water.
No cruiser or narrow boat should ever have a two bladed propeller fitted because it will cost the owner fuel and engine life (lack of efficiency = higher revs).
A two blade propeller on a fin keeled yacht with P or A bracket should also raise questions because it will not be close enough to the keel to be shielded from water flow.
Summary
Comparing the hull, engine, & propeller gives clues as to the soundness of design and maintenance which can then be used to inform questions of fitness for use, survey, and maintenance.
This is very much a "black art". There are computer programs that are supposed to do it, there are prop sizing graphs, and there are the experts at propeller suppliers who appear to ask a few questions and make a wild guess. Of the three I have found the experts to give the most consistent results.
The amount of variables within the engine, transmission, hull and propeller matching all conspire to cause problems – they even messed up on the QE2!
Some general pointers to help check the match of prop to boat.
SMALL PROPELLERS employ a small cone of high-speed water to get the thrust. This makes them suitable for high speed craft and less suitable for low speed displacement craft.
LARGE PROPELLERS employ a large cone of low speed water, this makes them more suitable for low speed craft.
There should be between 1" & 2" clearance between the tip of any propeller blade and the nearest obstruction. Much less than this is likely to cause cavitation.
1" on the pitch is worth about 2" on the diameter – only any use if you are trying to solve propeller matching or cavitation problems.
CHECKING PROP SIZE (Practical)
You need to know the engine speed at which it develops maximum power. You also need to know the hull design speed and the maximum continuous engine revs.
Now find a nice clear stretch of deep wide water where no one is going to point a radar gun at you or shout "watch your wash".
Gradually increase the throttle opening whilst noting any acceleration of the boat.
The boat should keep accelerating until it reaches its hull design speed, and that speed should not be more than at the engine revs giving maximum power. There should be no black smoke.
Finding Hull Design Speed
Work it out if you know the constant for your hull, or if near enough will do for you.
Gradually increase the engine revs until the bow wave is so long the stern drops into its trough. This is then the hull design speed.
If the speed stops rising or the engine starts to make black smoke then the boat is likely to be over propped.
Summary
Do not have unreasonable expectations of maximum speed.
Test the boat to ensure it will produce the speed you require without excess smoke.
Check that the engine sounds comfortable and reaches running temperature at your normal cruising speed.
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