Rudder behaviour while motoring - sail drive

[cmedsailor]

Just curious...I have noticed that while turning the rudder (one tiller and the other with wheel) of two boats having a sail drive while motoring, both rudders were stiff. During sailing both rudders where just fine. On the contrary I have never noticed such thing with shaft driven boats. Is this common to boats with sail drive? Does it have to do with greater distance of the propeller from the rudder?
Thanks

 

[Seajet]

I think you've already answered it, the saildrive prop' is just ahead of the rudder; does the tiller vibrate too ?

 

[cmedsailor]

does the tiller vibrate too ?

Oh yes, it did!

 

[maby]

I think you've already answered it, the saildrive prop' is just ahead of the rudder; does the tiller vibrate too ?

Surely the prop is further from the rudder on a saildrive than it is for a shaft drive?

 

[Twister_Ken]

Not a problem I've ever noticed on saildrive boats. There was some vibration on the tiller of my Twister, but that had a shaft-drive prop running in a cut-out in the rudder, so was explicable.

 

[Tranona]

Surely the prop is further from the rudder on a saildrive than it is for a shaft drive?

Yes, usually it is - on some boats it is negative because of the lack of propwash over the rudder. Generally saildrives are deeper in the water than shaft drives and the thrust is parallel to the waterline rather than 10 degrees down. Can't say I have noticed the effect on my boat.

 

[DaveS]

My boat has a saildrive which is about 1.5m forward of the spade rudder. The tiller steering works equally well sailing or motoring.

 

[Vanilla]

Just curious...I have noticed that while turning the rudder (one tiller and the other with wheel) of two boats having a sail drive while motoring, both rudders were stiff. During sailing both rudders where just fine.

Yes on my tiller steered boat with sail drive, the tiller definitely feels much heavier under engine than sail when moving through the water at similar speeds. No idea why but very noticeable.

 

[Davegriff]

Could it possibly be that if the rudder lacks a forward 'balancing' section, the extra force of the water flow driven backwards by the prop could give the impression of heavier steering?

 

[JohnGC]

Turbulence?

When motoring, there is considerable turbulence behind the prop and the rudder is in its path.

Crossing the Bristol Channel last year on a windless passage, I noticed the Dolphins liked swimming in the propwash just behind the boat. Our Etap32s has a Volvo 120S saildrive and at 6ish knots there is a cone of turbulence trailing the prop which is about 1/2m diameter at the rudder. We have the Etap vertical steering system which has a similar feel to a tiller. I have noticed the vibration when under power and it is heavier to steer (at speed) than when sailing.

 

[winsbury]

When under power the water speed over the rudder is usually faster than when under sail because the thrust from the prop is concentrated into a relatively small cone which is moving faster than the hull. Depending on the position of the prop in relation to the rudder and the rudder design ( skeg versus transom hung and the relative fore/aft pivot point ) the amount of pressure on the leading portion of the rudder will vary the effect observed but particularly with saildrives, which push the water much more horizontally than a shaft drive and hence more driven flow goes over the rudder, the effect of the thrust on the rudder will be more pronounced so inevitably will feel as though the boat is moving faster and hence have a heavier action than sailing at the same speed through the water. How noticeable this is is very much down to the overall design.

 

[oldsaltoz]

+ 1.