3 blade to 4 blade prop- what pitch?

[malbert73]

I am replacing my Michigan MP3 fixed 3 blade 14x14 prop this winter with a 15 inch diameter 4 blade variprop feathering prop.

Does anyone have a prop equivalency chart they can help me with? I want to know what pitch to set the variprop at prior to launch. Can always tweak in the water but my aperture will make it tough to adjust pitch.
The 14x14 was the perfect pitch for my 33 foot, 13000 lb displacement with a Yanmar 3YM30 and standard reduction gearbox, so I am looking for best estimate of pitch given I am increasing 1 inch in diameter, and adding an extra blade! Of course the variprop blades are flat and less efficient I imagine, but not sure if this should be factored in.


Thanks in advance-

[S/V Alchemy]

Your Variprop will have a "nominal" setting as expressed on the markings: I for instance have a 19 x 15 four-blader pushing a steel cutter of some 40 feet.

You'll have to experiment or consult the Variprop person who sold you the prop. Obviously, if the aperature is too small for the blades to rotate flat, you've bought the wrong thing, as that's the point of them underway...even less drag than a folder. So clearly they can't be pitched via the Allen key settings to something that is going to hit the hull or the rudder post.

[malbert73]

I get what you are saying, but trust me that I already worked through the details of clearance and what it will take to enlarge my aperture to fit the prop. Thanks for the thoughts, though!

The short question I have is what equivalent pitch in a 4 blade 15 inch prop will match a 3 blade 14 inch diameter x 14 inch pitch prop?

I think I have heard that you reduce pitch by 1-2 inches for every 1 inch increase in diameter, but not sure how to calculate additional blades.


Incidentally, variprop is the only feathering prop that will fit in a tight aperture like my Luders 33, albeit with some grinding and re-glassing work to remove "excess" fiberglass surrounding the opening. I was lucky to find someone selling a new spare at used prices, which had been installed in a sistership Seabreeze 35.

[S/V Alchemy]

I think if you can't contact a Variprop dealer (and they should answer this in the interest of good public relations), you should experiment. Four blades have more area than three, but it's the profile of the blade ends that really matter most. You also have to consider if your engine was operating in its most efficient part of its power band and was working as you expected. This is important in terms of wear and tear and fuel efficiency.

To give a brief example, on my smaller sailboat, I went from a two-blade 12 x 6 fixed prop on a direct-drive Atomic 4 to a 11.5 x 8 Gori two-blade folding prop.

I lost about a third of a knot off top-end speed, down to 6.1. I rarely run full throttle, however, as the fuel cost is not worth the last 3/4 of a knot.

I lowered my "dead slow" speed to 1.7 knots from 2.1. This is better for tight places/docking.

Shifting from forward to reverse takes a couple of seconds longer.

Stopping and reversing are enhanced, due to the squared off tips of the Gori. A big plus.

I sail faster due to the reversed drag. How much faster in terms of acceleration impressed me: I harden up and shoot away as opposed to gradually coming up to speed. It's improved my sailing experience far beyond the minor penalty in speed under power. I even use (slightly) less gas, possibly because I rarely exceed half-throttle, which gives me 5 knots on a 33 footer.

So there are a lot of variables. If 14 x 14 with your engine seemed good, I would try 15 x 12 and make adjustments. But first I would Google "prop area and pitch calculations" and see if you can test-fly some numbers. If I was a daysailer in a crowded marina, stopping and backing down power would matter more to me than cruising speed. On passage, the opposite.

You know that with a Variprop you can set different "pitch stops" for forward and reverse, right? Efficient and slippy forward and torquey as hell in reverse for slow, massive water thrust.

[malbert73]

Thanks- super helpful. That had been my idea as well- try 12-13 inches of pitch in forward. Yes, I am looking forward to the improved reverse with a low pitch setting! The 14x14 felt perfect in power range- hull speed at 2600 RPM, easily able to run wide open.

Glad to hear about the drag reduction. I have let my prop freewheel for drag reduction and due to Yanmar recs, and I also hate feeling my tiller flutter from the prop wash, especially at 6+ knots

[S/V Alchemy]

You may consider as part of your repropping either a shaft break (you stop at a splash of paint corresponding to the prop feathered like this: +) or simply leaving the prop in forward when you switch off.

A feathered prop will generally spin very little even in neutral.

Good luck.