A New Propeller

Out With the Old

When I bought Selene, she had a fixed 3-blade propeller. It was in pretty good shape, after I had it serviced; but, as with all fixed blade propellers, while underway there were two choices: let the prop rotate freely in neutral, with some noise - and arguably more wear and tear on the gearbox (as an aside, there is much debate about the veracity of this). Alternatively, shift into reverse, which silences the prop but increases the drag on the boat, and so makes it slower; it's like towing a bucket behind you!

It's not that I am mad keen racer (although I like winning from time to time), speed also effects range, and comfort, when cruising.

Anyway, an unfortunate incident involving the wife and a mooring ball chain resulted in a few nicks and bend in the prop, giving me the incentive (and justification!) to replace it.

Fold or Feather

There are many types of folding/feathering propellers available from multiple manufacturers; and, of course, at multiple price points. I decided on a feathering prop over a folding prop as it has a few advantages:

And one big disadvantage - complexity and, therefore, price. The Max-Prop in particular caught my eye, offering very little drag when sailing as the prop instantly feathers unless the engine is engaged; no freewheeling in neutral, adjustable pitch, and incredible stopping power in reverse. And as the blades rotate, when reversing you have the correct blade pitch and reduced propwalk; both key elements for a skeg-hung rudder, where control when reversing can be difficult to achieve.

Sizing the Propeller

The original Propeller was a LH prop. 3-bladed, marked 13 x 9 1/2. 1.25" on 1.125" shaft. As Selene was originaly launched with a diesel motor, the prop is offset to Starboard.

I was not convinced that the original prop was correctly sized, so more research was required.

Selene is powered by a Westerbeke 30b3, through a Hurth gearbox. This gives the following characteristics:



Hurth also provide the following:
MODEL RATIO POWER/RPM INPUT POWER CAPACITY MAX RPM WEIGHT BELL HSGS
AND NOTES
AA
Pos
AB
Pos
kW hp kW hp kW hp kW hp kg lb
2800 rpm 3000 rpm 3600 rpm
ZF 15 MIV

8 degrees
2.134
2.718
2.995
2.224
2.224
2.224
0.0152
0.0111
0.0204
0.0149
43
31
57
42
46
33
61
45
55
40
73
54
5000
5000
5000
20 44 SAE 4, 5, B/W, Yanmar JH Max input power 55 kW. Ratio 2.134 B Pos. max torque 120 Nm.

AA POS = continuous running position (normally AHEAD).
AB POS = reverse position.
B/W = Borg Warner adaptor.
Note = For all (Mechanical) transmission reduce power capacity by the following factors:
1 cylinder engine ÷ 1.25
2 cylinder engine ÷ 1.20
3 cylinder engine ÷ 1.15.

Based upon all of this information, I decided upon a bigger propeller than the original: 15 x 9.

As a feathering prop, LH/RH - makes no difference. I also measured the distance between the propeler blades and the hull, to convince myself there would be adequate clearance - not for the blade itself, but for the turbulence associated with the blade: get the blade too close to the hull and drag - and inefficiency - would result.

The propeller can be installed underwater; and based on discussions with PYI, the initial fitting key - ensuring the prop goes far enough back - is critical; so as a precaution I ordered a new key.

Installing The Prop

The prop itself was installed underwater. It took a total of three dives, each of around two hours. General installation video on YouTube.

Removing the old propeller

Removing the old propeller gave me a lot of anxiety, especially as it was to be done underwater. When I first bought the boat I removed the propeller for servicing/tuning by borrowing a propeller-puller from the boatyard; it was not easy, needing a lot of forbe on the tensioning nut on the puller to remove it. With a bang! In this case I was one my own, so I bought something similar from Harbor Freight, and was reluctantly prepared to use it (secured with many lines so I did not drop any key components)! Would it work? Or just break?

However, I came across an implausible-seeming tip online - which worked!

I removed the cotter pin and backed the castle nut out to the end of the thread, so that the nut was flush with the end of the shaft. I then tapped it a few times with a hammer.

And the propeller popped straight off!

The nut, of course, kept it on the shaft, but unscrewing the nut enables me to remove the propeller with no stress. It seems the vibrations from the hammer blow travel up the shaft, then get reflected back down - dislodging the propeller. Easy!

Settings



Was it worth it?

In a word: Yes!

The new prop is bigger than the old one so clearly more speed can be expected; the tables below show the difference under power. But I also see a difference when sailing, with Selene about 20% faster (so I had to adjust my PHRF), but the biggest - and perhaps most gratifying - difference is in cruising. For one of my favorite overnights - from Redwood City to Angel Island - the distance is around 25nm. Motoring this with the old prop used to take over six hours, now it takes around five.

Fixed 3-blade:
RPM Speed (knots)
1500 2.5
2000 3.4
2200 3.8
2400 4.4
2600 4.8
2800 5.0
3000 5.4
Feathering 3-blade (2" larger prop, more pitch)
RPM Speed (knots)
1500 3.9
2000 5.2
2200 5.5
2400 5.9
2600 6.4
2800 6.6
3000 6.8