[Public-List] Prop Question
Jim Mennucci via Public-List
public-list at lists.alberg30.org
Thu Apr 21 09:36:13 PDT 2016
I can’t sleep… so will dip my oar in this prop talk discussion
First, it’s just not worth farting around trying to learn and understand
how to pick a prop. If you’re buying a new one, talk to the prop supplier
and he’ll tell you the right one. They got a prop code that they can run
based on your motor/transmission parameters and boat characteristics that
gives the correct answer every time (close enough anyway). If you got prop
problems give them the parameters and they’ll tell you what you need (this
assumes you’re thinking somethings not right). The prop people will tell
you what parameters they need for the code. It’s that simple.
Second, there are no closed-formed solutions (equations) for picking a prop
period. Mike Meinhold, who’s got the background, knows the steps followed
in designing a propeller and alluded to this in his post. You can read the
book but my bet is that you already know the key parameters and how they
kind of interact, reading the book won’t take you much further than that.
Rich Hazlegrove told you a little bit about the prop on Quest. Rich bought
Quest from me and I want to finish that prop story. When I repowered Quest
I was young with not a lot of money so I tried the reuse the A4 prop—like
Rick said: the transmission on that motor was 2.7 to 1 reduction. So the
7” pitch prop barely moved the boat. That was 25 years ago and the prop
selection codes I talked about above didn’t exist so you were on your
own—so I had to take a swag. I knew my rpms to the prop were reduced by
roughly a 1/3 into the water, but you run a diesel at higher rpms, say
around 3000 which is roughly double the A4. So I thought doubling the
pitch would get me close to where I needed to be. I bought a square
propeller (Pitch/Diameter = I, actually I bought a 14” pitch prop because
that’s what they had in stock) and I got my speed back up to hull speed, at
least in smooth water, in heavy weather the best I could do was a lot less
which was a problem I had to live with since I raced and wanted the 2 blade
prop for obvious reasons. Some years later I had to get the prop
reconditioned because of tip corrosion. After reconditioning I couldn’t
get up to hull speed anymore. When I was selling Quest the surveyor noted
that it was only a 12” prop—the reconditioner had fixed the prop by
removing material. Propeller thrust is proportional to the diameter of the
prop to the 4th power so a 1” reduction in diameter significantly reduces
thrust. To be fair to Rick and since he was not going to race I decided to
put a new 3 bladed prop on. Since I knew that the 2.7 gear reducer
increases torque output by that ratio--I set out to put the highest torque
prop I could find. Michigan Wheels MP series is a Netherlands B series
propeller with a 70% area ratio which compared to a 2 blade requires
significantly higher torque to turn. I used to know the numbers, I had a
Netherlands Ship Model Basin Naval Architecture book, which I left at work
when I retired, that had prop curves that I could compare directly. When
me and Rick set out to move Quest from Annapolis to Mobjack Bay we headed
out into 20 knots of wind. At less than operating rpm the into heavy seas
the boat easily did hull speed at which point I excitedly concluded that my
prop problems had been solved after all those years. A 2.7:1 gear reducer
implies a big wheel but with an Alberg and the small prop aperture you
can’t get to the big wheel.
The point I want to make is that I got to where I needed to be in the end
by making a couple educated guesses. Today prop manufactures have the
codes that take guess work out of it so use them. Like Meinhold said: the
final solution even using the codes can be 20 to 30% of optimum because of
the effects of what they call wake fraction and thrust deduction which is
unique to each boat design—but it’s generally close enough.
I did do a propeller design from scratch one time. To do it we started in
a wind tunnel where we had a scaled model of the aft section of the boat.
We used a rotating rake with multi axis petot tubes and determine the exact
inflow conditions. This allows designing a propeller with optimum pitch
distribution to meet stated propeller requirements. We went on to do a 1/3
scaled powered model test in a tow tank where we achieved a propulsive
coefficient (pc)of .98 which is as close to perfection as your gonna get.
When we did the full scale tests after the boat was built we only achieved
a pc of .94. The boat as-built had higher drag than the 1/3 scale model
which is another reason why things can’t ever be done exactly—too many
variables. But you’re not gonna undertake this kind of development for a
sail boat, so ask the prop manufacturer what you need.
That said… I’m going back to bed.
On Sun, Apr 17, 2016 at 7:17 PM, Mike Meinhold via Public-List <
public-list at lists.alberg30.org> wrote:
> Pitch is the angle of the blades at 3/4 of the diameter. Higher pitch
> develops more thrust at the same RPM, or the same thrust at lower RPM,
> while demanding higher torque from the engine. Pitch is chosen so you
> operate the engine at the most efficient RPM and output power when the boat
> is moving at a desired speed.
> There is only so much you can get out of pitch before you are developing
> too low a pressure on the back side of the blade, and caveating. Then you
> have to add blade area.
> Say you are swinging a 13 x 12 (13 " diameter, 12 " pitch) and you are
> hitting optimum RPM and not quite getting to hull speed. Then you may
> change to a 13" pitch and get closer to the optimum.
> Conversely, if your 13 x 12 was getting to hull speed before it go to
> optimum engine RPM, you would pitch down
> The boats with Atomic 4's have 13 x 7 2-blades .
> I am trying to decide now what prop I will put on the other side of my new
> Beta 16! I am decided on a 2-blade for sure.
> Rinn Duin #272
> On Sun, Apr 17, 2016 at 6:55 PM, Jonathan Adams via Public-List <
> public-list at lists.alberg30.org> wrote:
> > ok so my prop is a 13 - 12, and talking with a few others recently they
> > have a 13 - 13
> > The boats in question have Yanmar 2GMs or 2QMs in them (~15 hp).
> > I believe the second number is for the pitch - does this make a
> > difference?
> > Jonathan197
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