Off center prop

[CharlesM]

Hello all

A wannabe cruiser seriously in the market... Thats me, and needs some advice.

I am viewing a vessel which I am seriously considering purchasing. She has an off center prop. The engine is centered, so the prop shaft does not run in line with the axis of the boat but sticks out at an angle, next to the keel.

My question. Does this matter much? Any opinions would be appreciated.

Thanks
Charles

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[Oldhand]

This is not uncommon. There are two reasons for such an installation:

1. Removal of the prop shaft without having to unship the rudder first
2. Counteracting the lateral thrust component of the propeller to give a neutral helm under motor.

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[CharlesM]

Hey Oldman (umm sorry Oldhand lol)

That is rather interesting. The prop sticks out to the starboard side. I seem to remember my mates boat kicks to port, and perhaps all motors generally rotate in the same direction... would starboard be the correct side for the prop to stick out then?

Charles

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[bedouin]

I also has a quarter-mounted prop, in my case alongside the skeg.

There is only one real disadvantage, and that is that without the flow from the prop passing directly over the rudder you can't just the trick of steering the boat using the engine and the rudder without having to have steerage way on the boat.

In my case having it mounted against the skeg reduces the affect of prop-wash - you could see that as an advantage or disadvantage.

On the positive front it improves the handling of the boat under sail because the propellor is not there disrupting the smooth flow of water over the skeg and rudder. That ought to make the rudder more efficient and in particular improve the lift you get from a moderate degree of weather helm. Whether that is noticable is a different matter.

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[jleaworthy]

The only boats I've sailed with this arrangement were boats originally built without an inboard engine where fitting one was difficult for space or, if a wooden boat, where the stern post would be dangerously weakened by boring through for a stern tube. Frankly, I found an offset prop to be a pain; diificult turning under power in one direction, generally poor rudder response at low speeds, vulnerable to ropes or other debris in the water, and high drag when under sail.

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[Aeolus_IV]

What type of keel/rudder configuration are you talking about here? If it is a long keel then I wouldn't have expected it to make a huge difference - the handling will be predictably poor in close quarters. If like Bedouin it is a fin and skeg then you'll have handling somewhere between a long keel boat and a "normal" boat. I.e. no prop wash to kick the stern port or starboard while the craft is stationary, but pretty good with a little way on.

I've found the an offset prop (having the same type of boat as Bedouin) presents few issues once its foibles are understood - and the benefits on the maintenance side do make some aspects of life easier. I wouldn't rule the boat out only on the basis of the position of its propellor.

Regards, Jeff.

<hr width=100% size=1><A target="_blank" HREF=http://users.swing.be/FDB/centurion/index2.html>Centurion 32 Web site</A>

 

[Oldhand]

Not all propshafts rotate in the same direction, the engines usually do but the gearboxes can produce different results. That's why there are left handed and right handed propellers. Therefore can't say if stb'd is the correct side for that boat.

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[tome]

Respectfully beg to differ on this. I've owned a long keel boat with wing prop, and regularly sailed another. It's a right pain and the only way to have any control going astern is if you have sufficient space to get some way on, then put the gear into neutral whilst attempting to steer. Unlikely to find the space to do this in a marina. Going ahead is also fraught until you get some way on. After several years ownership I never found her entirely predictable, and I was doing a lot of boat handling in those days. When I subsequently sailed a long keel boat with centred prop I found it a vast improvement (even though the owner complained that she was a handful).

I believe that this arrangement is mainly found on old boats (mine was 1924) which were not designed to take an engine.

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[spark]

I agree wholeheartedly with John (jleaworthy) and Tome. Having worked on old gaff cutters with single offset propellers I'd avoid them at all costs.

<hr width=100% size=1><A target="_blank" HREF=http://www.qei.co.uk/spark.html>http://www.qei.co.uk/spark.html</A>

 

[Kurrawong_Kid]

I agree with Aelous-don't just discard the boat because of an offset prop. I think the main factor is " did the designer plan the boat with an offset prop in the first place, or is the engine a later addition"? Some quite modern yachts, even some Oysters, were fitted with offset props because with a fin and skeg configuration you cant' get the shaft out without lifting the engine unless you have an offset prop!
With boats without a skeg you just drop the rudder and so the shaft can be on the centreline. Another reason for some offset installations was to keep the engine installation angle correct in the days before downward sloping gearboxes.
My boat has a lefthanded propellor on a shaft offset to Port. When I recently re-engined I phoned the designers to see whether I could install a right handed propellor. They strongly advised against, saying it would wreck the handling, especially astern so i stuck with a lefthander. I find she steers fine ahead, although turning with a larger circle to Port, but needs reasonable sternway before she will steer astern with any certainty but thereafter quite satisfactorily. Hope this helps.

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[mikesharp]

I also agree that an offset prop should not put you off. Ours is offset to starboard and a left hand screw. Motoring is difficult but not impossible. Best method is a quick burst to gain speed and then back into neutral to regain steerage. Then just tickle along with not enough revs to cause propwash or paddlewheeling. Sailing is a lot to do with gaining new skills and the satisfaction which follows.


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