Thanks for putting up some good stuff Kev.
Bob
--------------------------------------------
On Thu, 11/10/16, Kev captainyoung@gmail.com [electricboats] <electricboats@yahoogroups.com> wrote:
Subject: Re: [Electric Boats] General direction in propeller efficiencies
To: electricboats@yahoogroups.com
Date: Thursday, November 10, 2016, 5:21 PM
I went to the Landing
School for yacht design. This was given to us as a
rule of thumb, mostly for small yachts. And
this is what I remembered off
the top of my
head.
I think this rule is for designing new
boats. If you have the prop to close
to the
hull, you can get a slapping noise on the hull, and it
could
possibly cause a turbulence around the
prop, which would effect the
performance of
the prop.
You wouldn't want to build boat mold($$$$)
for a production boat, to find
out you can
not put the size prop you designed it for. So the rule
might
error on the side of caution.
Although, you are right, when the prop gets
real big, 2 inches might not be enough
clearance.
Alright, I looked it up in
Gerr's book, so this is the professional way to
calculate prop clearance for displacement
hulls.
"
Genereally,
there should be a tip clearance of 15%..
Ideally, it should be 20%...
But, here is
Gerr's chart for slower shaft RPM
RPM 200-500 SL Ratio under
1.2 Min Tip Clearance 8%
RPM 300-1800 SL
Ratio 1.2-2.5 Min Tip Clearance 10%
RPM
1000 and above SL Ratio over 2.5 Min Tip Clearance 15%
"
Note in the above table SL Ratio is speed to
length ratio.
SL Ratio = Kts / square root
of Waterline length in feet
So for a boat to do 6 knots,
and it's waterline length is 30 feet, you
would get a SL Ratio of 1.095
What king of
neworleand said is also true, there are other factors in
specifying a propeller besides prop diameter,
pitch and blade count.
Here is a list of things you need to specify a
prop: (from "Propeller
Handbook"
by Dave Gerr)
"
1.
Diameter
2. Pitch
3. Number
of Blades
4. Hand (Left or Right turning)
5. Propeller shaft diameter and keyway
6. Blade Area
7. Cupped or
uncupped blades
8. Supercavitating or
standard noncacitating blades
9. Blade
section shape (airfoil, ogival or compbined)
10. Skew
11. Rake
12. Blade thickness
13. Hub
Diameter
1-6 must be specified for every propeller and
every installation. 7-13 are
of greater importance for different types of
craft and in solving specific
problems."
On Tue, Nov
8, 2016 at 2:59 PM, sabre281@yahoo.com
[electricboats] <
electricboats@yahoogroups.com>
wrote:
>
>
>
Kev, where did you come up with 2" of clearance from
the hull, just
> wondering.
> If you size a wheel to that becomes a
pretty small wheel.
> The rule of thumb
has been 1" if memory serves me well.
> My Sabre 28 turned a 14" wheel which
gave me 1" clearance. Some of the
>
folding prop vendors claim it can be within 1/2"
clearance.
>
>
Personally I'd want to run the largest wheel I could get
away with going
> the EP route.
>
> Bob
>
>
>
>
Posted by: bob jennings <sabre281@yahoo.com>
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