Prop drag

wxt8981 · Nov 12, 2005

Hi Steve;

keep the prop locked- but you wont go any faster- it will
slow you down (but I don't mind that)

Wilhelm, V-257

claudiomty · Nov 12, 2005

Steve
I make a living delivering yachts (power & sail) and the rest of the time I
run towboats and my tugboat. It has been with a 100% track record that
locked shafts slow me down. Example I towed a 70ft hatteras (salvaged
sinker) to hatteras North Carolina from Rhode Island. WHen she had her
engines in place and the shafts could spin I could drag her 9.2 - 9.5 knots
day and night. When we passed through New York the salvage company pulled
the engines and transmissions (for rebuild) and I towed the boat to NC for
hull repair and rebuild.

When the engines were pulled they left the shafts and wheels (almost 1
meter wheels) in place and built a bracket to bolt between them to keep them
them in the boat. For the remainder of the trip even though I was now over 3
tons lighter 5.5 - 6.2 knots was my top speed.

Whether they are 2, 3 or 4 bladed wheels regardless of what I'm towing when
locked I slow down. I also used to try to lock them to tow faster and then 2
years ago well into a 3 hour debate my deckhand boarded the huge Beneteau we
were dragging and freed the wheel, 1 knot increase... After trying it
several more times in the following tows 100% of the time letting it spin
gave me a 10% speed increase across the board.

I'm no engineer, but I can attest to the practical side of things actually
in use, not theory. For what it's worth...

Claude

pjacobs55 · Nov 12, 2005

It sounds like a field test, under ideal conditions and with a good
GPS, is called for.

Peter
'Sin Tacha' #1331

alanderjakob · Nov 13, 2005

There is really no scientific need for field tests, although it migt
be both fun and interesting. Tests performed in a custom built
propeller test tunnel at Chalmers University of Technology shows the
CD to be 1.2 for fixed blades and a locked propeller, 0.3 for a fixed
blades free to rotate and 0.06 for a folding one.

/Jakob Alander

eagervega · Nov 13, 2005

Hi Steve,

My engine, shaft and prop are the same as yours, so for what its worth, my findings after exhaustive testing in the most uncontrolled and unscientific environments are as follows:

Locking the prop slows the Vega by approx 0.2 Kts.!

A lot of people would not agree.

A simple test to prove this is to tow your dinghy with outboard attached . Check what speed the engine lifts at, both with the engine in gear and in neutral (Free to spin).

I like most others always lock the prop while sailing to save wear .

Best of luck, and give Southern Comfort loads of TLC !

Nigel.
From: Steve Birch
To: AlbinVega@yahoogroups.com
Sent: Saturday, November 12, 2005 9:39 PM
Subject: [AlbinVega] Prop drag Hi All

I am still very confused. I have seen answers saying exactly the opposite.
All I wanted to know is:

Is it best to keep a 3 blade fixed prop locked whilst sailing to increase
speed or is it best to let it rotate freely?

C'mon you eggheads, there must be a Physics Professor out there somewhere!

Cheers

Steve Birch

SPONSORED LINKS Boating safety Boating supply Gsi outdoors
The great outdoors Albin vega

brockhaus77 · Nov 14, 2005

Hi Steve,

I hate to admit it, I AM a Physics professor, but I can't claim
particular knowledge on the drag problem. This falls clearly in
the realm of fluid dynamics (which is a rather complicated subject)
while my specialty is Nuclear Physics (now THERE is an idea for
repowering, instead of those Diesels...;-).
Well, I'll try anyway:
For a simple picture, consider that the surfaces of the prop are
tilted, so that if the prop is locked, water streaming against them will
be diverted sideways. This costs energy and will undoubtedly contribute
to drag. If the propellor were rotating exactly at the right speed, it would
"slice" through the water without diverting its flow - saving that energy.
However, it all depends on how much energy goes into friction (and how
effective this "slicing" will be) if you simply let the prop rotate "freely".
I actually have a different reason to keep it in "neutral": This will allow me to
start the engine more quickly in a sudden emergency, not having to fiddle with
my power/transmission control (single handle for both). Now of course there
is the competing concern about wear and tear - I certainly hear the
"whining" but so far have considered it an added benefit - a "poor man's
audible log". So the real issue is which parts might get impacted adversely
if one lets the prop spin freely over the long run?

- Sebastian (Vega Lyra 1060)

zippydoda_captain · Nov 15, 2005

Anybody with a Beta replacement engine should check their gearbox
manual. My gearbox is a TMC40 and it specifically states that it should
be in neutral when sailing. I.e. left to spin freely.

Ray Wilton

Ship and Field Support Manager

School of Ocean Science

University of Wales (Bangor)

Menai Bridge

Angelsey

LL59 5AB

Tel 01248 382614

Fax 01248 716367

eagervega · Nov 15, 2005

Ray,

Thanks for info on TMC gearbox, I never got a manual with engine.
I wonder why they advise to leave in neutral.

Cheers,

Nigel . Vega 3314 " Eager Vega"
Kilmore Quay, Ireland.

zippydoda_captain · Nov 15, 2005

Nigel.

Im not a gearbox expert, but I think the reason is due to the clutch
mechanism in the TMC40 gear box. (The Hurth gearbox may be different)

If you put it in gear and try to turn the prop shaft by hand (engine
stopped of course) you will find that in one direction youre trying to
turn the engine over (very hard) and in the other the clutches slip.
I.e. The engine will drive the prop but the prop wont drive the engine
(the clutches slip). This will therefore wear the gearbox clutches if
left in gear while sailing.

Leaving the prop to rotate (engine in neutral) while sailing causes very
little wear on the stern gear as negligable power is being transmitted.
I got 5 years service out of my last stern bearing (Nyoil from Steve)
and it only takes half an hour to change and cost £12. Much cheaper than
a gearbox rebuild.

Regards,

Ray.

Ray Wilton

Ship and Field Support Manager

School of Ocean Science

University of Wales (Bangor)

Menai Bridge

Angelsey

LL59 5AB

Tel 01248 382614

Fax 01248 716367

eagervega · Nov 15, 2005

Steve and Ray,

I must be going stupid , I do have a Hurth box.
Will pass on the info to others who have the technodrive.

Thanks,

Nigel.

zippydoda_captain · Nov 15, 2005

Steve/Nigel.

After a quick search on the net I found the following regarding the
HURTH(ZF) gearbox.

"CAUTION When sailing or in tow NEVER leave the Transmission in FORWARD
position as damage may result.

To Lock the propeller put shift lever in REVERSE or otherwise in
NEUTRAL"

Ray.

Ray Wilton

Ship and Field Support Manager

School of Ocean Science

University of Wales (Bangor)

Menai Bridge

Angelsey

LL59 5AB

Tel 01248 382614

Fax 01248 716367

eagervega · Nov 15, 2005

Thanks Ray,

Nigel.

wxt8981 · Nov 15, 2005

Any one with a mechanical gearbox can stop the prop from
turning (use reverse gear), but any one with a hydraulic
gearbox MUST leave the gear in neutral!

Wilhelm, V-257

zippydoda_captain · Nov 15, 2005

Hi Steve.

I guess so, Hurth seems to have been bought out by ZF.

Ray Wilton

Ship and Field Support Manager

School of Ocean Science

University of Wales (Bangor)

Menai Bridge

Angelsey

LL59 5AB

Tel 01248 382614

Fax 01248 716367

imbissfrank · Nov 15, 2005

Steve you really opened up a hornets nest with this one !!!

Let me quote what my manual says about the Hurth transmission on my engine:

HBW Short Profile Sailing Gear:
Operation without a load:
Rotation of the propeller without load (freewheeling), e.g. while the boat is sailing, being towed, or anchored in a river, as well as idling of the engine with the propeller stopped, will have no detrimental effects on the gearbox.
Locking of the propeller shaft by an additional brake is not required, since locking is possible by engaging the reverse gear. DO NOT sail while engaged in forward.

Frank Gallardo Jr
V-2184, Cin Cin

Ray Wilton r.j.wilton@... wrote: Hi Steve.
I guess so, Hurth seems to have been bought out by ZF.
Ray Wilton

Ship and Field Support Manager

School of Ocean Science

University of Wales (Bangor)

Menai Bridge

Angelsey

LL59 5AB
Tel 01248 382614

Fax 01248 716367

imbissfrank · Nov 15, 2005

er...... I mean't can of worms

Frank Gallardo Jr imbissfrank@... wrote: Steve you really opened up a hornets nest with this one !!!

Let me quote what my manual says about the Hurth transmission on my engine:

HBW Short Profile Sailing Gear:
Operation without a load:
Rotation of the propeller without load (freewheeling), e.g. while the boat is sailing, being towed, or anchored in a river, as well as idling of the engine with the propeller stopped, will have no detrimental effects on the gearbox.
Locking of the propeller shaft by an additional brake is not required, since locking is possible by engaging the reverse gear. DO NOT sail while engaged in forward.

Frank Gallardo Jr
V-2184, Cin Cin

Ray Wilton r.j.wilton@... wrote: Hi Steve.
I guess so, Hurth seems to have been bought out by ZF.
Ray Wilton

Ship and Field Support Manager

School of Ocean Science

University of Wales (Bangor)

Menai Bridge

Angelsey

LL59 5AB
Tel 01248 382614

Fax 01248 716367