[Public-List] propeller

[Don Campbell via Public-List]

For those interested in playing around with the engineering  and the way you 
fit out your boat try this: rbbi.com/folders/prop/propcalc.htm
It is an excel spread sheet that allows you to calculate what you need for 
the motor you have and the mass of the boat,  considering waterline length 
too. What you will find is that you cannot use more than 16 HP with an AL30 
because of the bow wave. The biggest opening is 13" available for a prop, so 
you can play with the pitch and rpms with that as a limit. There are a 
couple of things that do matter and the first is that engineers allow for 
50% slip in all of their calculations. I can cope with slip at slow speeds 
but dislike that at higher speeds so I have a 13-8 prop which slips less at 
faster speeds than slower speeds. It moves the boat well. Michigan Wheel 
also has a prop calculator if you go to their web site. The other thing that 
is considered is the tunnel a prop works in and the engineering suggests 15% 
of the tunnel to be without prop so that the venturi and laminar flow of 
liquids around the hull and prop works well.  Since the prop is below the 
hull and behind the keel, the tunnel is not quite so well defined on our 
boat as some, and the 13" prop does not seem to be a problem for this. 
Janski had a Michigan MP3 prop he liked. It has much more blade area than 
some other designs, but is still a 2 blade prop.
Don

-----Original Message----- 
From: charles pleisse via Public-List
Sent: Tuesday, February 23, 2016 8:49 AM
To: public-list at lists.alberg30.org
Subject: [Public-List] propeller

I have the engine (original A4) out this winter so I can replace shaft and
associated hardware and I'm thinking of trying a two blade prop for better
sailing performance.  The current prop is a three blade 12x5.  What was
spec'ed originally on these boats and should any consideration be given to
the age of the engine when determining propeller pitch?

Charles Pleisse
Sookie #221
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