[Alberg30] props and blades
Bill Ydel
Bydel at bellsouth.net
Mon Apr 14 12:46:42 PDT 2003
Mike,
Where are you going to throw that prop and when???????
Bill
SallyB #441
----- Original Message -----
From: "Mike Lehman" <sail_505 at hotmail.com>
To: <public-list at alberg30.org>
Sent: Monday, April 14, 2003 8:44 AM
Subject: Re: [Alberg30] props and blades
> 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
>
> +---------------------------------------------------------------+
> | This Old Boat by Don Casey |
> | http://www.amazon.com/exec/obidos/ASIN/0071579931/alberg30-20 |
> +---------------------------------------------------------------+
>
> Public-list mailing list -- Public-list at alberg30.org
> http://www.alberg30.org/mailman/listinfo/public-list
> To unsubscribe: email to Public-list-request at alberg30.org
> Include command "unsubscribe <password>" in subject or body.
> Use command "help" for more options.
>
>
> _________________________________________________________________
> Tired of spam? Get advanced junk mail protection with MSN 8.
> http://join.msn.com/?page=features/junkmail
>
> +---------------------------------------------------------------+
> | This Old Boat by Don Casey |
> | http://www.amazon.com/exec/obidos/ASIN/0071579931/alberg30-20 |
> +---------------------------------------------------------------+
>
> Public-list mailing list -- Public-list at alberg30.org
> http://www.alberg30.org/mailman/listinfo/public-list
> To unsubscribe: email to Public-list-request at alberg30.org
> Include command "unsubscribe <password>" in subject or body.
> Use command "help" for more options.
> +---------------------------------------------------------------+
> | This Old Boat by Don Casey |
> | http://www.amazon.com/exec/obidos/ASIN/0071579931/alberg30-20 |
> +---------------------------------------------------------------+
>
> Public-list mailing list -- Public-list at alberg30.org
> http://www.alberg30.org/mailman/listinfo/public-list
> To unsubscribe: email to Public-list-request at alberg30.org
> Include command "unsubscribe <password>" in subject or body.
> Use command "help" for more options.
>
>
> _________________________________________________________________
> Help STOP SPAM with the new MSN 8 and get 2 months FREE*
> http://join.msn.com/?page=features/junkmail
>
> +---------------------------------------------------------------+
> | This Old Boat by Don Casey |
> | http://www.amazon.com/exec/obidos/ASIN/0071579931/alberg30-20 |
> +---------------------------------------------------------------+
>
> Public-list mailing list -- Public-list at alberg30.org
> http://www.alberg30.org/mailman/listinfo/public-list
> To unsubscribe: email to Public-list-request at alberg30.org
> Include command "unsubscribe <password>" in subject or body.
> Use command "help" for more options.
>
+---------------------------------------------------------------+
| This Old Boat by Don Casey |
| http://www.amazon.com/exec/obidos/ASIN/0071579931/alberg30-20 |
+---------------------------------------------------------------+
Public-list mailing list -- Public-list at alberg30.org
http://www.alberg30.org/mailman/listinfo/public-list
To unsubscribe: email to Public-list-request at alberg30.org
Include command "unsubscribe <password>" in subject or body.
Use command "help" for more options.
1050349602.0
More information about the Public-List
mailing list