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vacendak · 20-07-2008, 12:12

does anybody know how to choose propeller shaft of right diameter?
could not find anything certain at producers' (Vetus, Sleipner)
websites

**Alan Wheeler** · 20-07-2008, 12:36

What are the technical details of the boat please???

vacendak · 20-07-2008, 13:32

it's a 45" cruiser by DixDesign

Vickers 45 radius chine steel or GRP cruising cutter

with electrical main drive (DE-shaft 28kW / 216Nm / 1200 RPM)

(BTW these 28kW are another problem - producers give me undersized propeller calculations

)

**Alan Wheeler** · 20-07-2008, 22:27

Ooops, being DE, I am not going to jump in to this one. Surely the suppliers of the DE system should be able to give you advice. Certainly the prop guys should anyway. I would be asking either of them. This is a problem that requires a great deal more information than just simple boat size and Hp of the engine.
Why on earth do you want to go DE in such a large mono. 28Kw is not going to give you much drive for a boat of that displacement.

vacendak · 21-07-2008, 00:16

I hope it will; at least they are successfully driving
Bavaria 49er with 35 kW
http://fp01.fischerpanda.de/public/F...propDE_eng.pdf
and Jeanneau 37 with 12 (!) kW
DE-Whisperprop

it is hard to deal with FischerPanda itself - they send you with all questions
to local dealer, who (in our case) does not care about one-off customers

Steve Pope · 21-07-2008, 01:04

Fischerpanda don't always get rave reviews, there are some seriously unhappy customers out there.

vacendak · 21-07-2008, 02:03

Quote:

there are some seriously unhappy customers out there

I agree - being not very happy, too

when buying a generator never thought such a 'world famous' producer
would have customer service of such a low level

well, what I know is: there are simply very few people in central office (Padeborn),
they physically cannot answer to each client's questions - this should be done by local dealers;
and our dealer is selling FP generators to the militaries,
so does not care about civilian or private boaters

a remarkable vicious circle, isn't it ?

BTW: a subject for a separate discussion (complaints ?) board -
DEALERS & THEIR ATTITUDE TOWARDS CUSTOMERS
Administrators & Gurus, what do you say ?

**GordMay** · 21-07-2008, 02:55

You might want to peruse ABYC Standard P-6 “PROPELLER SHAFTING SYSTEMS”
Goto: http://www.abycinc.org/committees/P-06.pdf

Specifically paragraph 6.5.3 (page 1), and Figures 1 - 8 (pages 7 - 10), and etc.

**Pblais** · 21-07-2008, 04:32

Quote:

BTW: a subject for a separate discussion (complaints ?) board -

If you look at the thread you will see Gord's response. The rule was partially developed because of such threads.

We have had Fischer Panda threads before and you'll find some in the archives. Currently CF is not a powerful force in the international marine market. Industry leaders don't shudder when we (or our members) speak. Campaigns designed to embarrass products, people, and companies through public threads on CF is thus a prohibited activity.

It becomes a distraction to members providing help to other members. Sorting out customer service problems is complicated and not a one sided debate. Members are better served by direct communication with companies about their problems and using all legal methods to seek redress. We choose not to be a platform of last resort for those problems. This would conflict with our goals. We are not unsympathetic but we must accept our limitations.

vacendak · 21-07-2008, 05:40

a very useful document - but my problem is that their calculations are based on horsepowers & RPMs
our engine has ~40 HPs & 1200 RPMs, which gives shaft Ø ~20 mm - IMHO too thin

but it's torque, regardless of RPMs, is 216 Nm - more than 80 HP VetusDeutz

might there be any known algorythm for such case ?

**Alan Wheeler** · 21-07-2008, 12:45

Quote:

but it's torque, regardless of RPMs,

Correct! The power loading is completely different and in some or maybe all respect, Hp is difficult to use in the equation. Now in saying that, it is not that the there is a safety aspect here. It is simply that the shaft is going to be, or at least could be different to that of a standard combustion installation situation.
Torque could be said to be sum of power times speed of rotation. But energy is imparted to the rotation of an electric system in a different way. Thus torque can be produced low down in the RPM range. However, unless you have the ability to feather the prop, the prop places no extra load on the motor when spinning slower. So although the torque is available, it is not used. You need to spec the shaft for the highest load point. That will be the force seen on the shaft at the greatest point being full speed rotation.
However, the diameter of the shaft is not really spec'd for Hp or Torque. It is spec'd for propeller diameter. The further out the tips are from the centre line of the shaft, the more lateral forces come into play in trying to bend the shaft. So wipping can take place. Shaft size, Hp and prop diameter, prop placement on the shaft all come in to the equation..... this is getting long winded.
So.... firstly you need to work backwards. The biggest prop you can afford to fit to the boat is that which allows the proper tip clearance to hull measurement. Then follow the information in Gords link (I assume that info is in there) to spec shaft diameter for the prop.

vacendak · 21-07-2008, 13:44

2 AlanWheeler

Quote:

The biggest prop you can afford to fit to the boat is that which allows the proper tip clearance to hull measurement

with such approach - can't I oversize the propeller ?

**Alan Wheeler** · 21-07-2008, 22:37

Good question. It depends on the installation. Normally the limiting factor is the appture the prop fits into and normally this is small enough to restrict the ability to fit too big a prop. So some detail on what surrounds the prop would be helpful.
Diameter does not have any affect on the condition we often name around here as "over-proping". It is pitch that is critical to that issue. Diameter and No. of blades equate to surface area. More surface area, the greater the friction, thus the more energy that goes into just turning the prop and not being put into thrust. However, more surface area equates to less slippage and thus more efficient at producing thrust. So it comes down to experience in a lot of situations and I have found not all experiences have the same outcome.
Diameter also predicts speed of rotation to some point. A ruff rule of thumb in most of our common boat applications (in other words not powered racing craft) the tips of the blades should be traveling at a max of about 160Km/hr. So a large blade turning slower should have a tip speed ruffly equal to a smaller blade turning at a higher speed, if that makes sense. However, the reality of this can be far from it in most of our make do type applications.
So as you see, there are several variables and nothing really nailed down.
If Aparture dictates prop size enough, then work from there and work backward as i suggested earlier. And once you settle on a Prop diameter, then the prop manufacturer should be able to help you with the rest.
There is also a spec sheet available (I think) I put it in the Study Hall above.
Gord I am sure can place a link to it. It shows sizes in relation to hull and skeg distance.