[Public-List] Another reason to avoid big ships

Sat Mar 22 08:31:01 PDT 2008

/ /Hi Pat.
    Jim might have sent it too  you too but propcalc is really easy to 
use. Save the file to one of your folders  on your hard drive rather 
than open it every time from the e-mail. Then open excel and open 
propcalc. There are several tabs in the bottom left corner. start with 
the most left one, "How To" and read that page. (Use the left arrow to 
get there if it is not showing).
    The main thing is that  where there is a green box, you can or need 
to add information there.
    Then go to Torque and shaft HP and fill in the 4 green boxes from 
your owner's manual or spec sheets on the new motor. There is usually 
just one bearing on the Alberg between the gearbox output and the prop. 
Use all the decimal places on the gear reduction number.
     For Power required, this is where you can explore what you want to 
get by changing the displacement mass and required velocity or speed. 
The AL30 is quoted as 9000 lbs but that is dry and without gear 
aboard.10,000lbs is more realistic with oil in the motor, water in the 
tank and fuel not to mention bodies, ice, booze, sails, food and gear. 
And if you are going for an extended cruise, it may get to 11,000 lbs 
(with only a very slight increase in waterline. Standard WL is 21'8" to 
get around a CCA rule at 22'). This is also the page that shows you that 
you have too big a motor and that you can never get any of the reserve 
HP into the propulsion of the boat because there can be no more than 16 
HP required to move the mass chosen at 1.3 x Square root of the 
waterline, which is max velocity possible for displacement vessels. So 
if you want a generator to work at the same time, then you can use all 
of the 30 HP. that you have. The trick here for longevity of a diesel is 
to match the prop pitch to the point on the torque curve where you start 
to level that out for HP at a given RPM and I suspect that it will be at 
about the 23 -25 HP level. You need to run the engine at that rpm to 
make the diesel work well enough for a long life, - oil flow and 
injectors and enough heat generated to avoid carboning up. So a bit of 
fuel wasted but less repairs and maintenance. 
    The prop pitch page has the dreaded information that there is a 52% 
slippage factor built in at maximum speed and so you can find how to 
juggle that by changing pitch. Lowering the pitch increases slip, and 
increasing pitch decreases slip at max speed.  But by increasing pitch, 
you increase slip at lower rpms. Thus the choice is control without slip 
or speed without slip when it comes to pitch. You cannot get both 
because you cannot compress a liquid with an open topped surface. (Pitch 
is the measurement of the number of inches travelled in one complete 
revolution so if you travel exactly the pitch at low RPMS with no slip, 
then you will have slip at high RPS because there mass will be slow to 
react due to inertia to the fast cut of the blades and the fact that 
water flows away to the top surface and is not easy to push against.
    There is one other factor they consider and that is the tunnel 
effect of the prop pulling water through the circle of teh blades. They 
say that 15% of that tunnel needs to left without any wheel in it for 
efficient through put of water- essentially leaving an undisturbed bit 
of water for a boundary layer for Bernouilli's principle to work. That 
means that on an Alberg, you are limited to a 12" prop. (I have a 13" 
two blade and it works fine, but I have chosen power at top speed rather 
than control at low rpms and so I have a larger pitch than normally 
specified for an Atomic 4).
    You might want to go back over some high school physics because that 
is just what this is.
    Mike's idea of Michigan Wheel doing the calculation too will be a 
confirmation of the above. However, it is still your boat and you make 
the choice for the waters you are in.
    As you might guess, if you have a current to navigate, and it is in 
the opposite direction of your travel plans, you need to consider what 
will happen there. Your velocity is relative to the water that you are 
riding in. You will see differences between your velocity in the water 
(knotmeter) and velocity over the ground on your GPS. The GPS considers 
slippage and currents and waves!  You cannot get over a current flow 
greater than 1.3 x the sq root of the waterline, so if you are trying to 
go against a 7 knot current, you must take that obliquely to try to get 
the resolved vector greater than the current. However, in all this, the 
HP requirement does not change, because HP is a measure of work which is 
the application of a force through a distance (w = F x d ) and force  is 
mass times acceleration ( F = m x a) with the limit of waterline as a max.
    Sometimes it helps to start by figuring out where you want to end up 
and work backwards to size things like engines and save some weight and 
money by the time you are finished.
Don

pat nolan wrote:
> Jim,thanks for the prop program . I will see if I can make sense of it . Pat s/v Arcos
>
> Jim Davis <a30240 at earthlink.net> wrote:  http://www.baltimoresun.com/news/local/bal-ship0317,0,1405917.story
>
>
> Jim Davis
> S/V Isa Lei
> CT35
>
> "If everyone is thinking alike, someone isn't thinking."
> George S. Patton
>
>
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