[Alberg30] props and blades
George
george at intraworldcom.net
Thu Apr 10 11:55:06 PDT 2003
hello again fyi Moyers dyno graph shows 25 max hp at 3000 rpm , on our
boat with Indigo prop 6.2 k at 2050 rpm, I can't see turning the old girl
above 2500 (two main bearings and all) with Original Message -----
From: "George" <george at intraworldcom.net>
To: <dk.campbell at sympatico.ca>; "Alberg 30 public list"
<public-list at alberg30.org>
Sent: Thursday, April 10, 2003 1:24 PM
Subject: Re: [Alberg30] props and blades
> 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 |
> > | http://www.amazon.com/exec/obidos/ASIN/0071579931/alberg30-20 |
> > +---------------------------------------------------------------+
> >
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> +---------------------------------------------------------------+
> | This Old Boat by Don Casey |
> | http://www.amazon.com/exec/obidos/ASIN/0071579931/alberg30-20 |
> +---------------------------------------------------------------+
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| http://www.amazon.com/exec/obidos/ASIN/0071579931/alberg30-20 |
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