keithw said...

It seems a bit quiet on forum so I thought I would ask about the pros and cons of saildrives? in particular Volvo penta makes.

Easy installation and quiet. Bit more effort required when slipping, they need hard antifouling if your using ablative for the rest. Anodes have to be kept up to scratch and they are expensive[Volvo]. Can cast your own though.

I am not fussed on them as mounting them usually relies on the strength of the skin of the vessel rather than main frames.

Also when the drive shaft has two 90 degree changes in axis there must be a certain amount of horsepower loss.

One advantage would have to be the better angle of attack of the prop in the water. Many large commercial vessels are powered in a similar fashion these days with rotating nozzles.

This allows them to dock without the aid of tug boats and turn 180 degrees in their own length.

What exactly are you talking about when you say "Can cast your own though." Ramona.

We recently replaced our yanmar sd20 anode, about$60. I love Hobart!

An advantage of sail drive is they are more compact. No drive shaft through vessel. But the engine is aft of the vessel centerline :-(

Poods

Poodle said...
We recently replaced our yanmar sd20 anode, about$60. I love Hobart!

An advantage of sail drive is they are more compact. No drive shaft through vessel. But the engine is aft of the vessel centerline :-(

Poods

I'm guessing the $60 included the taxi fare! The other advantage is turning circle. With the drive well forward from the blade rudder the boats turns very sharp.

Thanks for the info, The boat I'm looking at NS38 the engine is mounted under the stairs wheather it is a shaft or sail drive just the 1991 model has saildrive.

I believe the cooling is very different between the two ?

G'day Keith. You are right, sail drive don't normally have an engine inlet cock.
With respect to prop walk, this is caused by the bottom of the prop cutting through denser water than the top. A main factor is where the prop is located, near the middle there is little effect but if the prop is well aft there can be a lot. If the prop is shallow that can also increase the effect.
Used well prop walk can be very useful. I have little walk but it can help bring the stern in in reverse port side to.
Let me know if you want to come out. I will be having Fridays off in future.

Not sure that article is overly correct and would like other references.
Prop effect on a single engine plane, the articles example, is largely the effect of the prop wash on the rudder, not blade angle of attack. It is strong at high power at low speeds e.g. on the runway at take off and reduces as speed increases. You do get some effect at high angles of attack such as approaching the stall if you apply power when it can causes some yaw. Gets you into a spin if not careful, a cause of crashes in powerful single engine aircraft wave offs from an aircraft carrier.
However with boats the effect of the prop pointing down is real but I would think quite minimal. There is very little leverage on the boat of the different angles of attack of the up going and down going prop. Think of a 15" prop, you might have only 10" between the centres of thrust which wouldn't have much turning effect on the boat. The different angle of attack of the ip and down going blade requires forward, or astern, motion. As prop walk is evident at very low boat speeds ie stopped or nearly so, when the difference in angle of attack between the up and down blades is very low it is hard to imagine it is a major contributor to prop walk.
Prop angle will affect wash on the rudder as it does on aircraft. My boat has no wash on the rudder because the prop is a long way forward and angled down, I only get the walk from prop rotation through the different water densities.
I'm not an expert on this stuff. I just apply my Navy and private flying training/experience and my limited Navy seamanship and boat handling training. Happy to be further educated.

I do like a technical discussion.

With over 16,000 flying hours, it's been a very long time since I've flown a single engine prop, and the theory is a bit rusty, but I'll offer the following:

During takeoff, a little single prop driven aircraft will yaw to the left and require right rudder to keep it tracking down the runway. There are 4 factors that cause the yaw.

1. Slipstream - the airflow corkscrews down the fuselage hitting the rudder on the left side and causing left yaw, this is worse at low speed.

2. P-factor, - the down going prop (right side) has a greater angle of attack and generates more thrust, causing left yaw - for a taildragger.

3. Torque - the prop rotates right, the aircraft wants to rotate left. Normally aileron is used to counteract this but rudder is used on the ground and at low speeds.

4. Gyroscopic precession - for a taildragger on takeoff, as the tail lifts up, precession causes a left yaw.

Of all the factors, Prop wash is considered the most significant, this article pretty much dismisses p-factor:

www.qmfc.org/school/asym.htm

Back to boats. The factors are no different except precession is excluded due to the fixed angle of drive.

Prop walk is most apparent on a long keeled yacht in reverse, simply because of the slipstream effect over the big long keel with the prop usually very close to the keel. You can play around with all different kinds of props to try and reduce the spiraling slipstream, but it cannot be eliminated. Going forwards, if you have a great big slab of a rudder very close to the prop, again you are going to get prop walk.

For an angled drive shaft, the prop walk effect is mostly again due to the wash slipstream up against the hull as well . If you have a right handed prop, in reverse the wash spiral will want to push the stern to port.

As in the article, p-factor would be negligible as prop walk is most apparent at first with very slow/no speed. P-factor increases with speed.

As for the change in water density explaining prop wash, that's one for the boffins but I think the effect would be negligible as compared to slipstream.

Billyboy026 said...

Good Luck, every vessel will behave differently

Exactly right.

My usual berthing technique is to disengage the drive and use the way of the vessel to carry me to the berth. Rarely do I need reverse thrust, but if I do it is usually with revs.

Prop walk usually only happens with low revs and large pitch propellers. A large pitch propeller acts like a paddle wheel at low revs.

Come in slow, nice and gentle, and if your fenders are set and berthing lines cleated aboard, you can usually just step off the yacht, onto the dock, throw a turn, twist and a hitch with your stern line on the dock cleat then walk forward, grab your bow line and secure it.

No fuss, no yelling and shouting and nobody else needed.

Practice makes perfect. There is not much more satisfying than a good berthing.

Charriot said...
Yes Cisco, you describing how berthing real yachthies.
Watch what happening in reality. You agree it's far from it.
By the way I am in Hobart for a few days and was watching
old cray boat going strait to the jetty, very slowly than
about 6 m he put engine in reverse and just swing the stern alongside.
He tight the starboard side to the jetty. Impressive.
Is it what we talking about here?

This is easy with a fishing vessel. Large, slow prop, lots of prop walk astern one way. Only problem is if your forced to tie up at a wharf the other way and your boat kicks away from the wharf! That keeps the pensioners amused.

Wow - I thought it was going to have a simple answer

Glage I choose Gliders all I had to worry about was secondary effects when banking!

Thanks all great discussion.

How luck are we novices surrounded by all this greatness, I am sure Cisco and Ramona & The Free One have diplomas hanging out of there back sides...? or in some dark corner of there cabins, OK maybe Cisco smoked his but damn fine job done explaining the prop walk effect. I personally love prop walk, it gone from enemy to friend, it never fails me and some times it makes me look good.

I think propwalk is a bit of a carry over myth from older days when most yachts had long keels and relatively smaller rudder areas. Also older diesels were slower revving as were propellers that had long pitches and therefore would act as paddle wheels when the yacht had little or no way on in berthing situations.

With today's design of yacht which is usually fin keel and skeg hung or spade rudder and the proportion of rudder area to keel area being considerably higher, the result is a hull that is highly manouverable be it under sail or power.

The analogy is the 300 mm rule in the bath tub. Run it through the water horizontally and it will maintain direction (or be harder to turn). Run it through the water vertically and it is directionally unstable (or easy to turn).

Add to this factor that modern diesels are higher revving and can be slammed into full power instantly without damage to the engine. This is handy if you are heading into a disaster or a bad berthing. Cavitation needs to be accounted for.

So logically with a higher revving engine, matched to it will be a shorter pitched propeller which will have less of a paddle wheel effect, or propwalk.

In summary, if you have a modern design fin keeled yacht with a high speed diesel engine, propwalk should be a minimal consideration in your berthing manouvres.