[Public-list] Props and prop shafts

[Meinhold, Michael J]

Countering wind and waves, towing a dinghy or towing another boat are
all added resistance, requiring more horsepower at a given speed. Extra
horsepower can help here, but only within the limits of your propeller's
design. Hopefully it's been optimized for the RPM, forward speed and
resistance at your expected cruising speed. If you add a lot of required
thrust you may be driving it into very low efficiency, and as J says,
you will hit a limit of deliverable power. I've been in nasty Chesapeake
chop generated by 25 knot winds from the north and made only 2 knots
with my 30 hp A4. 

Countering current is largely (not entirely) subtracting the current
from your attainable speed at a given RPM. Extra horsepower helps you
little with this. An A30 that can go 6.5 knots in slack current should
be able to make about .5 knots in a 6 knot adverse current.


Michael J. Meinhold / SAIC
2111 Eisenhower Ave, Suite 303, Alexandria, VA 22314
703 842 2606 / michael.j.meinhold at saic.com


-----Original Message-----
From: public-list-bounces at alberg30.org
[mailto:public-list-bounces at alberg30.org] On Behalf Of J Bergquist
Sent: Tuesday, July 18, 2006 9:14 AM
To: Alberg 30 Public List -- open to all
Subject: Re: [Public-list] Props and prop shafts

Funny I had exactly the opposite experience. My dealer talked me OUT of
the
20 hp model in favor of the 16 hp because he said that with a 2 blade
prop
there was no way I would be able to put 20 hp in the water and that most
people overpower their boats. Truth be told he said I could probably use
the
13.5 hp model and be just fine. He said he had plenty of very happy
customers with that motor in boats similar to mine. Since I haven't been
in
the situation of running against a 7 knot current and head sea yet, I
don't
know whether I will have trouble with that, but 7 knots is more than
hull
speed for our boats, so I'm not sure I understand how you managed to go
through the water at much more than that without a LOT more than 20
hp...

It doesn't matter how much power you have between your bilge boards. It
only
matters how much you can put in the water. Spending extra $$$ on
horsepower
that you can't put into the water is a waste. That's just my opinion.

I have a 13x8 2 blade prop and a 2 cylinder 16 hp beta. Vibration is not
a
problem. It's quieter than my A4. It's a great, compact little motor.

I also don't see why you would change shaft diameter. that seems like a
big
and needless pain in the butt to me. 7/8 shaft is plenty big, I think,
especially if you use stainless instead of bronze.

J

On 7/18/06, FINNUS505 at aol.com <FINNUS505 at aol.com> wrote:
>
>
> In a message dated 7/17/2006 4:46:45 PM Eastern Standard Time,
> whittle at shaw.ca writes:
>
> Dear  Lee,
> Comments:
> 1. I,also, was attracted to the 16 HP Sole version.  The
> dealer/engineer talked me out of it for the reason that the boat will
do
> fine with the smaller Horsepower until you run into adverse
> conditions  and
> then you will possibly regret not having ponied up for the extra
reserve
> which is there just when you need it. Cost was my initial  influence,
of
> course, but I got over it! I do not regret one bit the extra
horsepower.
> We
> hit Dodd Narrows an hour early and needed every ounce of  energy to
combat
> a
> 6 or 7 knot current and get through, which we did with  some in
reserve as
> it
> turned out. I don't think the smaller engine would  have made it.
> Secondly,
> three cylinders are better than two from a  vibration standpoint.
> 2. I went with a  two blade wide blade and it works just fine. I
> don't care about reversing  issues since I have basically given up any
> reliance on being able to  predict the behaviour of my boat and have
found
> other ways of landing  safely at and leaving the dock.
> 3. Why  would you change the shaft size?
> 4. I  have no knowledge about plastic props.
> Regards  Jonathan
> ----- Original Message -----
>
>
>
> thx, Jonathan :)
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