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Propellers - a work in progress (updated October 17, 2011)  

March 2, 2010. There are few, if any, more complex pieces of the sailing game than selecting the right propeller. The mathematical formulas are incomprehensible and the subjective variables require painful choices. Some consider this subject an art as well as a science.

Recently some Dolphin owners have been dealing with this matter and it seemed we should have a page on this subject. There are good resources online that explain the technical issues, and a list of these is below. Its a good place to start.

This page is, hopefully, just a start on this subject with, hopefully, input coming from readers. As with many of our Dolphin decisions the right prop selection depends on the particular needs of the owner - and they differ substantially. Much of what follows applies to inboards, but outboards have their prop issues as well. Here a partial list, not in any priority order.

* Horsepower at x rpm, and gear reduction ratio of motor -The difference between the theoretical distance the propeller should travel in one revolution and the actual distance the vessel travels is called slip. For example if you cruise at engine RPM of 2000, and your vessel has a 2:1 reduction gear, and a prop with an 8" pitch, your theoretical speed through the water should be 6.58 knots (the distance an 8 " prop should move in one hour). If, In reality, your vessel only does 5 knots at 2000 rpm on a calm day with no current, the difference is slip.

Generally, continous operation (max cruising speed) maximum RPM is 80% of max/redline RPM for the motor - although this should be checked for each motor.

* Size of existing aperture - typically you need about 2" of clearance between the tips of the prop blades and the edges of the aperture, and about 3" fore and aft.


* Right hand or left hand - direction of rotation. Right hand propeller rotates clockwise when viewed from astern facing forward. Left hand propeller rotates counter clockwise when viewed from astern facing forward

*Diameter and pitch - Diameter is the diameter of the circle created by the prop blade tips when turning. Pitch is the 'distance' the prop moves forward in one complete revolution. ie., a 10 x 8 would be a 10" diameter by 8" pitch.

* loaded displacement and WL length - for a Dolphin these are 5000 - 5500lbs and about 19'6" repectively. For more on displacement click here; for more on waterline length, click here

* Need for maneuvering, slow speed control in forward and reverse

* Cruising, racing, both? Important rules of thumb

a) a cruising sailboat spending about 50% of its time under power. Our Dolphin 24 has a theoretical hull speed of 5.7 knots. Ideally, when cruising under power you want to reach 90% of that speed (5.1 knots) in flat water at an efficient engine RPM, and still have reserve power to reach that speed with a headwind and chop. Achieving only 70 or 80% means a 10-20% longer time getting there. Needing high RPMs to reach cruising speed can double and triple your fuel consumption, and can be harmful to your motor.

b) The PHRF rating for a Dolphin 24 without a prop in the water is decreased by 6 seconds a mile. On a 252 rating (spinnaker) this is about 2%. But, my experience with my outboard down in the well is that my sailing boat speed is down about .3 knots - that's closer to 5%

* cost - the prop itself is one cost, changing an aperture size is another,

For motors that have their prop in the water all the time performance tradeoffs are inevitable - compared to 2 bladed props, larger, 3 bladed props have better under power performance and slower under sail performance.

Variable pitch, feathering and folding props are other approachs but tend to need larger diameters, can be problematic in reverse and close quarter maneuvering, and are expensive. None of these alternatives are usually high on the Dolphin owners list of choices. BUT, they have their advocates! Check out the links below and we will have more on these at a future date

Typically, we choose between 2 and 3 bladed props, of a higher or lower 'pitch', and a diameter of 9-10", consider the above list of variables, listen to our local supplier/expert, make a choice, and live with what we get.

Above left is a picture of the 10" diameter, 3 bladed prop on Fred Croft's Flipper, O'Day #41. The numbers stamped are 11(pitch?), LH (left Hand?), 1(?)

Fred recently installed a 'new' old motor into Flipper - a Yanmar YSM8 7.5 hp, 3200 rpm diesel with a 2/1 reduction gear. He sent us an email about the boat's performance that is illustrative, not only about the motor but also his 3 blade prop..

The new motor is running like a well oiled Swiss watch. The thing is I have no tachometer so it's impossible to calibrate speed to rpm's. By ear I'm guessing about 2500 rpm to about 4.8 knots towing a six foot hard shell dinghy. I did hit 5.1 today in the river but there could be tidal factors. There was definitely a moderate headwind. Any input I can give you is very un-scientific but I must say this older YSM8 is pretty smooth for a one-lunger. Apparently the newer Yanmars have a smaller and lighter flywheel and vibrate more..

The above left picture is of the 2 bladed prop from Passage, O'Day #10. The numbers stamped are 010 (10" diameter?), P8 (Pitch 8"?) L (left hand) The original inboard was a Palmer 27 - left hand wheel, direct gear, 7.5hp at 2800 rpm. The Palmer 27 manual recommends ”left hand wheel, 10 x 6, 3 blade or any wheel that limits top RPM to 2800. Don’t permit too large wheel to pull RPM below 1200 at full throttle

Some online links - have fun!








to be continued, revised, updated, etc....







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