[alberg30] Prop size and tip clearance.

[sunstone]

From: sunstone <sunstone at idirect.com>

Bob:
Your right about Skene's, a couple of questions come to mind. How old is
the prop on the boat, I assume it is a two blade and is there any sign
of tip erosion from cavitation? 

I had some on my boat and I assume the prop was original but it was a 13
x 9 therefore I assume you are running an engine other than an Atomic
4.    I had the tip erosion welded and remachined at a prop shop and the
results were satisfactory.

You can repitch up or down by 2" (once) so you could up your revs to
3000 but I'm not sure you really want to do that.   Both of my Albergs
never got out (and couldn't) of the teens in RPM and there engines were
old but in fairly good shape.

I don't push the boat under power so fuel economy is my primary
objective as the engines don't seem to lug.   I had an Atomic 4 in the
A-30 and have a Westerbeke 4-107 in the A-37.   Both boats do 5.5 - 6
knots in flat water at 1300 RPM.

I wouldn't change the prop unless it needed changing (or is a 3 blade)
if it has given satisfactory performance, but were I to, I would follow
Skene's advice. 

By the way we have always used a two blade in both boats and the loss of
sailing speed to a three blade is demonstrable and for me unacceptable. 
Since the prop can be turned to vertical minimizing drag 
behind the deadwood with the shaft marked, a two blade fits the bill.

Fair winds,

John Birch, Sunstone KC-65
Bob Lincoln wrote:
> 
> From: "Bob Lincoln" <lincoln at cc.UManitoba.CA>
> 
> On checking my prop there is less than 1/4 inch tip
> clearance at top, and about 1/2 at bottom of aperture.  The
> prop is stamped 13 x 11.  According to Skene and Nicholson,
> the best fit is an 11 inch prop which gives proper clearance
> at top and a bit more than recommended at bottom.  The 13
> inch is quite oversize and probably cavitates, giving me
> 2500 max rpm versus design 3000.  The shaft is not centered
> in the 14" vertical space, something like 1/4" off.  Without
> raising the question of diesel vs. gas, or shaft
> revolutions, have people found any success with an 11 inch
> diameter prop rather than a 12 or 13?  I have received
> conflicting advice--e.g. "get the largest you can fit in the
> aperture" versus Skene (Elements of Yacht Design) and
> Nicholson (Boat Data Book).  Any comments?
> Bob Lincoln
> #590
> 
> 
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