[Alberg30] props and blades
quest433 at msn.com
quest433 at msn.com
Fri Apr 11 19:22:41 PDT 2003
If your gonna go into the real esoterics about prop design you need to talk about Kt's and Kq's, design J's, wake fractions and thrust deduction. The solution to the problem begins with a wake survey done on the bare hull in a tow tank or wind tunnel. The wake survey will give you the 3-axis velocity profiles which have more to do with how the flow is shaped by the hull, propeller aperture, shafts and things like that. These measurements give you the inlet conditions. The velocity parameters are then put into various computer codes using, today that's lifting line theory. Out of this comes optimum pitch, rake and skew. Rpm and torque at the design J or speed. Of course these computer codes are based on potential flow which does not factor in viscosity. While your in the tow tank you'll also measure the drag on the body and from that derive the effective horse power. the ehp. The ehp is the horse power you put into the water. It's the product of the measured drag, in pounds force, times the velocity in feet per second divided by 550. Drag comes in many forms and they're all additive, there's skin drag, which is a function skin roughness which is related to the Reynolds number and the wetted surface area. There's what they call pressure drag which comes from things like the rudder on which the water impinges. There's drag due to lift. Drag from the wind on everything topsides. Heck, there's people who've gone sailing with me that were real drags.
That's how they design propellers. And its not a closed form problem. Unfortunately its the same when picking the right prop for a boat that's already in the water. I haven't reviewed the program yous guys have be talking about but to the layman I think you can get lost real easy in the thing and end up with the wrong answer--so beware. One of the beauties of this organization is that you can find someone that has the same engine and find out what they're using and how it works and that's the best way. And I have to tell you everything I wrote here is bullshit, plain and simple. I wrote it all from memory in about 5 minutes, none of it is thought out or logically developed.
Jim Mennucci
----- Original Message -----
From: Mike Lehman
Sent: Thursday, April 10, 2003 12:27 PM
To: public-list at alberg30.org
Subject: Re: [Alberg30] props and blades
WOW - Somebody put this in the mainsheet!
Mike Lehman
"Gilleleje" #505
----Original Message Follows----
From: Don Campbell <dk.campbell at sympatico.ca>
Reply-To: dk.campbell at sympatico.ca, Alberg 30 public
list<public-list at alberg30.org>
To: "public list, Alberg" <public-list at alberg30.org>
Subject: [Alberg30] props and blades
Date: Thu, 10 Apr 2003 11:14:18 -0400
Mike has suggested that the number of blades will make a difference on
the attitude of blades to the water, and this may be so. Momentum and
resistance have to be considered in the case of A30's and so does gear
ratio of the transmission. These are really complicating and complicated
considerations, especially if one considers the differences in roughness
of antifouling surfaces. I suggested earlier that the Michigan Wheel
MP-2 2 blade prop had twice the surface area of most 2 bladed props, so
that may also be a consideration for you if you want to max out thrust
with the ability to have it behind the keel when sailing.
However, the main consideration for motoring a boat should be motor
performance, and there are 3 factors to consider. The first is horse
power (HP) and RPM, the second is Torque and RPM and the third is lbs
of thrust vs RPM. One may also want to consider fuel efficiency but
that is usually an extra. If your motor is not operating at or near the
maximum efficiency of the torque curve, the life of that unit is
compromised because you are lugging the engine, applying too much fuel
or are not applying the thrust correctly. For most engines, the torque
curve peaks just below wide open throttle. The specs on an Atomic 4 gas
motor for wide open throttle is 3600 RPMs, at which the motor develops
36 HP. Max torque occurs at about 3300 RPM. Plug in the values for your
own motor, as these will not apply to any other. Diesels are usually
much lower RPMs.
The maximum Hull speed for an A30, regardless of motor, is 1.34 x
sq rt of the waterline or 1.34 x sr rt 22.67 = 6.38 Knots. That
translates down to about 129.22 inches per second. If we get 1800 rpm on
the tach, then that is 30 rps. Thus the distance the prop needs to move
is 129.22/30 = 4.31 inches. That is the theoretical pitch of the prop.
Add in a 55% slip and the pitch is 7.8. Why the slip is as high as that,
I do not know, but that is the figure that is in common usage. The disc
area is determined by the thrust available from the motor, and so
determined from that graph. As I said earlier, Michigan Wheel always
allow 15% of the slot open for added waterflow, ie correct venturi
dynamics for Bernouilli's principal to work efficiently, so a max
diameter for the A30 is 10". The next problem is that one has to know
the HP curve to know if you need all that the motor is capable of
producing. At 1900 rpm, the Atomic 4 gas will produce 19 HP. Thus there
is a cushion of 17 HP and 1700 RPM with this motor that should allow for
more available power if motoring into waves. If that is restricted by
the rules of thumb that 1" of pitch decreases RPM by 200 and 1" of
diameter is equivalent to 2.5" of pitch, consider your options, and how
they compromise motor performance, even though we are supposed to be
sailors. (A 13" prop adds the equivalent 7.5" of pitch over a 10"
prop, which would decrease motor RPMs by about 1500! That limits a 36
HP motor to about 20 HP, but will be far more critical on a 20HP diesel
that has a WOT of 2000 rpm. At less than about 800 RPM, the oil pump
will not generate enough pressure to lubricate that motor.) If you try
to increase RPMs by increasing throttle and the tach does not change,
then you have not gained more HP, more thrust or more efficiency, but
have only put a greater load on the motor with increased fuel in the
cylinders. It is the prop that is the stopper! It is back to high
school Physics of work and motion, and you thought that would never be
applicable to you life, especially the fun parts of it!
I suppose that I, for one, appreciate the capabilities of racers,
whether sailing or motoring, and you would not expect to win any race if
the power plant were not at its peak efficiency. There is not one
Formula I or NASCAR racer who aims to operate at less than top motor
efficiency. There is not one speed boat operator who aims for less
either. Mercury Kiekhaufer is the best help for consideration of
performance boat motors and they don't recommend anything less than wide
open throttle for performance and engine life of their motors either.
We, as sailors, are far too content to accept a motor without the
torque curve, the HP curve or the thrust curve, and so never know what
performance might be. In too many cases we have put on the wrong prop,
because we can still push the boat through the water at Hull speed,
however inefficiently we do it. The maximum that the prop on an A30 can
ever move through the water with no slippage is less than 5 inches in
almost any case you want to choose because the hull speed is limited to
129 inches per second. Gear reductions reduce pitch value, by the way.
If you are able to spin the prop so fast that increased slippage occurs,
you will boil the water on the face of the blades and that will produce
steam which is compressible so you will slow down! It also shortens the
life of the prop. If the forces against the hull from wind, waves or
current are greater than that required to generate 129 inches per second
forward, it is time to put up the sails, because hull speed limits the
relative forward motion and the anchor is not likely to hold in those
waves or current.
Just as an aside, if you think this is a low pitch, the A22 is
limited to 108 " per second and try to find an outboard motor that will
give the option of a prop with pitch below 7!!
Don Campbell
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