[Alberg30] props and blades

[George]

hello there,  my last dyno sheet shows 43.1ft lbs  max torque on my a-4 at
2200 rpm at that point 19hp adv. George K                  Linda Ann #259
----- Original Message -----
From: "Don Campbell" <dk.campbell at sympatico.ca>
To: "public list, Alberg" <public-list at alberg30.org>
Sent: Thursday, April 10, 2003 10:14 AM
Subject: [Alberg30] props and blades


> 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                     |
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 |                This Old Boat by Don Casey                     |
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