[alberg30] Re: Prop / Skeene's - Tip Clearance

[sunstone]

From: sunstone <sunstone at idirect.com>

Jack Old Sport, Et Al:

According to Skeene's Elements of Yacht Design, out of print and if
anyone knows where a copy can be had please "E" me, the tip clearance on
a 2 blade prop is 10% of the diameter of said prop.  For a 3 blade the
tip clearance is 15% of the diameter.  These are minimum numbers which
if ignored can lead to tip cavitation and the erosion of the blade. 
Remember water does not readily compress and so at each rotation the
blade tip attempts to compress the incompressible resulting in a shock
load at the tip which wears away the tip of the blade and in extreme
cases shock loads the drive train.   The situation is exacerbated by the
type of aperature in an Alberg where there is a rapid change in the
hydrodynamic profile as the prop tip reaches the top/bottom.

If a 10" 3 blade is use for example it will need a minimum tip clearance
of 1.5" all round from any part of the hull, a 2 blade 10" needs 1"
minimum clearance all round.

In most cases, a 12" 3 blade is all you can reasonably site in the
aparature as 13" does not allow for a full 15% clearance (minimum).

I might add that A-37s regularly use 12" 3 blades when a 3 blade is used
and their displacement is twice that of the A-30.   

Personally I won't use a 3 blade as the performance loss, even on a 37,
under sail is unacceptable to me.  Having raced and sailed 30s I have
watched the horizon jobs done on 3 blade equipped boats and seen
personally how the same boat after changing the blade to a 2 became
competitive again.  Same crew, same sails, different prop.

Take care.

J Birch, Sunstone

Jack Vanderloo wrote:
> 
> From: Jack Vanderloo <jvdloo at cyberus.ca>
> 
> Ahoy:
> For the record, I'm running a Volvo Penta MD7A (14 SHP) with a 13 x 11
> 3-bladed prop, and while I have no tach, the old girl happily does 7 knots
> in calm water at near full throttle (and 6 - 6.5 knots into any sort of chop
> and wind).
> Had occasion this summer to run her 12 to 14 hours a day for 6 days -
> usually wide open - while transiting the Trent Severn canal system (42
> locks, 250 statute miles).  Checked oil daily; used no more than half a
> quart at that pace.  Never felt I was stressing the engine.
> 
> Did notice, however, at hau-out, that there was a noticeable (1" all round)
> "wear" mark in the fibreglass on the rudder (port side) aperture.  Thought
> it had been an occasion when the helm must have been thrown, and /or held,
> over particularly hard.  Now - as a result of this particular thread -
> believe it may have been the result of sustained cavitation of the
> LH-turning prop.
> 
> Anyone else with this phenomenon?  Comments?
> 
> Cheers
> Jack Vanderloo
> KC 641
> 
> "Gordon E. White" wrote:
> 
> > From: "Gordon E. White" <gewhite at crosslink.net>
> >
> > Lee:
> >     I think your prop is too big. I have a 21 hp Westerbeke and spin a
> > 13 x 11  3-blade. It does not limit me - I can run full RPM of about
> > 3,500 if I want (which is trying to push me over hull speed). My only
> > problem is at idle it runs about 3 knots which makes for tricky docking.
> >
> >     Definitely if you can't get rated RPM you have too much prop. Isn't
> > a 14 pretty close to the aperture in the rudder? I think you are lugging
> > the engine & getting cavitation around the rudder. Not an expert though.
> >
> >                                         - Gordon White
> >
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