[Alberg30] props and blades
chrishardy
chrishardy at tds.net
Mon Apr 14 14:15:21 PDT 2003
I'm gunna keep my prop but I'm gunna call the pointy end of my boat the
bow and when motoring towards the direction of the bow I'll call that
forward and when motoring towards the blunt end I'll call that reverse.
I like to go forward the most, reverse is only fun when there is
something bigger than my boat looming ahead of the bow(pointy end). I
only worry about my prop when there is something that it won't cut
wrapped around it.
Mike Lehman wrote:
> Whoa! This is getting way too complicated for me. I've decided to take
> my prop off and throw it away and just sail the boat from now on. Wind
> and aerodynamics is much easier to understand.
>
>
>
> Mike Lehman
> "Gilleleje" #505
>
>
>
>
>
> ----Original Message Follows----
> From: <quest433 at msn.com>
> Reply-To: Alberg 30 public list <public-list at alberg30.org>
> To: "Alberg 30 public list" <public-list at alberg30.org>
> Subject: Re: [Alberg30] props and blades
> Date: Fri, 11 Apr 2003 22:22:41 -0400
>
> 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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