[alberg30] Re: Props >> AWLGRIP

[Jack Vanderloo]

From: Jack Vanderloo <jvdloo at cyberus.ca>

Ahoy John and Gordon:
John, the specificity of the totality of your elucidation is, like, wow, awesome!
Sure does it for me!  Am rushing out in minus 18 degrees Celcius this AM to check
on 10-15% clearance rule.
Now, for today's newest topic: any words of wisdom out there on the repair of
exteded-submergence-induced waterline blisters in AWLGRIPped topsides?  My three
month cruise on the Great Lakes last summer was embarked on and, unfortunately,
finished with the boat 4" down on her marks - due to excessive (anal-retentive?)
victualing of canned goods (enough to keep most food banks going for a year).
And, yes, Dorothy, they do have grocery stores along the North Channel!
Anyone out there who has tackled such blistering?  Preparation?  Fill & fair?
Colour matching?  Flattening the gloss of AWLGRIP to match the intentionally
flattened original paint job?  Brush vs. spay?  Techniques? Etc.
Thanks and ...
Cheers
Jack Vanderloo
ALDABI KC 641

sunstone wrote:

> 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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