single vs dual propellers

[Brian Ferguson]

The St. Clair has 3 engines, so the decision to go with a single screw is related to that. There wasn’t a lot of forward thinking into the Blough’s design. She would have benefited fromtwin screws, as she is a very difficult boat to handle. When she was built, she suffered from sheet cavitation and vortex hull cavitation. The tunnel over the prop solved most of that problem and the skewed propeller installed later on made her ride very smooth.

I worked on her in 2003 and she still vibrated pretty good through the St. Marys, St Clair, and Detroit rivers even with the blades and shroud. From what I was told by the old timers on board she was essentially a very similar design to a 1950's laker but just up sized to a high degree, and for the most part with experiences on older ships she is really similar just wider, longer, and diesel.

[Guest]

They can be inward or outboard turning propellers. I have come across both these types.

[GuestfromEU]

in dual prop boats, do they counter rotate, as in small craft ?
There is no incline of the prop shaft so I'd expect loads on both ascending and descending blades to be equal.

Yes, they counter rotate.

[hobieone]

in dual prop boats, do they counter rotate, as in small craft ?
There is no incline of the prop shaft so I'd expect loads on both ascending and descending blades to be equal.

[Mr Link]

What is a skewed prop?

A skewed propeller is one where the individual blades are asymmetrical and "sweep back" in the prominent direction of rotation

Highly skewed propeller that the St. Clair has (or had in 2001):

http://www.boatnerd.com/pictures/fleet/ ... -01-ns.jpg

Non-skewed (or minimally skewed) propeller:

http://www.boatnerd.com/news/newsthumb ... -12-bv.jpg

[Brandon]

What is a skewed prop?

[Guest]

The St. Clair has 3 engines, so the decision to go with a single screw is related to that. There wasn’t a lot of forward thinking into the Blough’s design. She would have benefited fromtwin screws, as she is a very difficult boat to handle. When she was built, she suffered from sheet cavitation and vortex hull cavitation. The tunnel over the prop solved most of that problem and the skewed propeller installed later on made her ride very smooth.

[jtrainman400]

This is what several people have told me, I do not know how true it is:

It depends on how you define performance, but there are some advantages each way.

A single engine set up has
1) less stuff in the water and therefore reduces drag from the second engine.
2) Less installed weight for the same horsepower (
3) Half the maintenance (since half the number of engines)
4) Less dollars/hp (generally)


A twin set up
1) allows differential stearing (unequal thrust) greatly enhancing low speed maneuverability
2) Less weight and thrust at a specific point
3) Better hole shot (low end torque)
4) No Torque searing
5) Less prop vibration (Not on centerline)

[mccollochd]

Good to see the St. Clair out sailing this season. Of the wide-beam boats, only the Blough and St Clair have single propellers ... all the 1,000 footers have dual props. Why were the Blough and St. Clair singles?