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S&S Swan General - S411, Volvo Penta MD22 50hp, Max Prop (possible replacement)
18 April 2021 - 18:29
#1
Join Date: 20 July 2020
Posts: 13

S411, Volvo Penta MD22 50hp, Max Prop (possible replacement)

 

Dear Professor, I would once more need your knowledge on Max Prop size and pitch.

 

Backgound: have not given much of a though to the installed Max Prop, however this weekend when Bacchus was launched the gear on forward made horrible sound and gear on reverse stalled the engine. Boat will be lifted to root-cause the problem (seems to be propeller related).

 

Due to this I gave some attention to the installed propeller + engine:

Boat: S411

Engine: Volto Penta MD22 (50hp)

Max rpm: 3000

Drive: Velvet drive, 2.91:1

Propeller (installed 1998, some years after the engine): Max Prop 21", according to documents pitch set to 12"

If I recall right 2000-2200rpm gave us 5-6knots speed with fully loaded boat (if my memory servers, but I might remember wrong)

 

We have been quite happy with the propeller, it works great on reverse and ok when driving forward. The engine noise inside the boat (with kids) is the limiting factor to use high RPMs, around 2000rpm feels fine inside but we often cruise with even less RPM, and anyhow we rather sail slow than motor.

 

If we need to buy a new propeller I would like to have 2nd opinion what size it should be, and what pitch to set? Given that for now the point is smaller RPMs but in future it might be max speed+efficiency.

 

 

19 April 2021 - 06:10
#2
Join Date: 02 January 2008
Posts: 1489

Dear Tapio

There may also be other reasons for your problems.
Needs to be checked if the propeller shaft can be easily turned by hand. If not, the Cutlass bearing and the shaft seal need to be inspected.
Can the propeller blades be easily turned by hand? If not, the propeller needs to be greased.
The number of blades for the propeller is essential information, not well visible on your video. Looks like 3 blades, pls confirm
Max-Prop is adaptable as its pitch can be set to fit the actual shaft rpm and power, but the 12 degree setting is unsuitable, and results in very slow performance.
22 degree setting recommended here, the estimated top speed is then 7.8 knots at 3000 rpm.
Sounds like you should improve the sound insulation for the engine box. The first step is to close all openings and gaps, but combustion air must be provided to the engine. A sound trap may be needed.
Kind regards
Lars

19 April 2021 - 09:03
#3
Join Date: 20 July 2020
Posts: 13

Pardon, I forgot to mention that, it is 3 blade version.

 

I just got information (I'm not on the boat myself) that one of the blades end up into different position that two others, in both reverse and forward gear, when the propeller is turned by hand. This was not the case when the boat was launched, wondering how it happened.

 

There seems not to be any steps and it turns well, but ends up into wrong position.

 

I was able to turn the shaft by hand, even when the boat was on water.

19 April 2021 - 09:10
#4
Join Date: 20 July 2020
Posts: 13

Dear Professor,

 

One correction. The pitch is documented to be 12 inches (not 12 degrees), which in 21" 3 blade classic would result to 16 or 18 degrees based on the attaced diagram. Can you confirm?

Anyhow seems wrong, and the obtained speed might indicate same thing. We have never reached 7.8 knots.

 

 

19 April 2021 - 09:37
#5
Join Date: 02 January 2008
Posts: 1489

Tapio

Thank you for the information
The propeller needs to be opened, and the gears inside checked for damage.
12 inch pitch is insufficient, 16 inch needed, i.e. 22 degrees.
Lars
 

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