Hi,

My new-to-me boat (Crown 28) has a Volvo 10hp MD6A, and it's way
overpropped with a Campbell Sailor 11x6 3-blade (gets 1500 WOT instead
of 2400). The Campbell Sailor dealer says his calcs recommend a 12x8
(ie much MORE pitch). Now, my experience with CS props is you take the
calculated pitch and subtract 2", but I donno...

Engine IS very "tired" but it pushes the boat at 5+ knots with the
over-pitched prop, so it can't be TOO bad.

Those with MD6A's - what prop do you have on it?

druid
http://www.bcboatnet.org

On Thu, 18 Oct 2007 11:22:19 -0700, druid
wrote:

Hi,

My new-to-me boat (Crown 28) has a Volvo 10hp MD6A, and it's way
overpropped with a Campbell Sailor 11x6 3-blade (gets 1500 WOT instead
of 2400). The Campbell Sailor dealer says his calcs recommend a 12x8
(ie much MORE pitch). Now, my experience with CS props is you take the
calculated pitch and subtract 2", but I donno...

Engine IS very "tired" but it pushes the boat at 5+ knots with the
over-pitched prop, so it can't be TOO bad.

Those with MD6A's - what prop do you have on it?

druid
http://www.bcboatnet.org

You need to post the loaded displacement of the boat, the water line
length and the gear box ratio.

For example: if a LWL of, say 22 ft., a loaded displacement of 2,000
lbs. and a 1:1 gearbox then the prop should be about 9 X 6.

With a 1.5:1 box the prop is about 12 X 9

and with a 2:1 box about 14 X 13.

There various propeller calculation programs on the Net. Try a google
for "propeller calculations".

The calculator I used for the above is "PropCalc" from Marine Castle -
freeware - which seems compare favorably with other programs.


Bruce-in-Bangkok
(Note:displayed e-mail
address is a spam trap)

On Fri, 19 Oct 2007 08:40:57 +0700, Bruce in Bangkok wrote:

On Thu, 18 Oct 2007 11:22:19 -0700, druid
wrote:

Hi,

My new-to-me boat (Crown 28) has a Volvo 10hp MD6A, and it's way
overpropped with a Campbell Sailor 11x6 3-blade (gets 1500 WOT instead
of 2400). The Campbell Sailor dealer says his calcs recommend a 12x8
(ie much MORE pitch). Now, my experience with CS props is you take the
calculated pitch and subtract 2", but I donno...

Engine IS very "tired" but it pushes the boat at 5+ knots with the
over-pitched prop, so it can't be TOO bad.

Those with MD6A's - what prop do you have on it?

druid
http://www.bcboatnet.org

You need to post the loaded displacement of the boat, the water line
length and the gear box ratio.

For example: if a LWL of, say 22 ft., a loaded displacement of 2,000
lbs. and a 1:1 gearbox then the prop should be about 9 X 6.

With a 1.5:1 box the prop is about 12 X 9

and with a 2:1 box about 14 X 13.

There various propeller calculation programs on the Net. Try a google
for "propeller calculations".

The calculator I used for the above is "PropCalc" from Marine Castle -
freeware - which seems compare favorably with other programs.



Yeah....

I went through the prop-calculation thing on my last boat. Bottom line:
put a prop on that might be close and measure RPM. As I said, these prop
calcs don't work, especially for Campbell Sailor props. That's why I want
to hear from someone with the same engine, preferable on a similar boat,
and see what works.

One problem with most of the prop-calc programs I used is they don't take
into consideration the HORSEPOWER of the engine. Same prop for an 8hp as a
20hp? I don't think so!

druid
http://www.bcboatnet.org

On Thu, 18 Oct 2007 21:49:59 -0700, druid wrote:

On Fri, 19 Oct 2007 08:40:57 +0700, Bruce in Bangkok wrote:

On Thu, 18 Oct 2007 11:22:19 -0700, druid
wrote:

Hi,

My new-to-me boat (Crown 28) has a Volvo 10hp MD6A, and it's way
overpropped with a Campbell Sailor 11x6 3-blade (gets 1500 WOT instead
of 2400). The Campbell Sailor dealer says his calcs recommend a 12x8
(ie much MORE pitch). Now, my experience with CS props is you take the
calculated pitch and subtract 2", but I donno...

Engine IS very "tired" but it pushes the boat at 5+ knots with the
over-pitched prop, so it can't be TOO bad.

Those with MD6A's - what prop do you have on it?

druid
http://www.bcboatnet.org

You need to post the loaded displacement of the boat, the water line
length and the gear box ratio.

For example: if a LWL of, say 22 ft., a loaded displacement of 2,000
lbs. and a 1:1 gearbox then the prop should be about 9 X 6.

With a 1.5:1 box the prop is about 12 X 9

and with a 2:1 box about 14 X 13.

There various propeller calculation programs on the Net. Try a google
for "propeller calculations".

The calculator I used for the above is "PropCalc" from Marine Castle -
freeware - which seems compare favorably with other programs.



Yeah....

I went through the prop-calculation thing on my last boat. Bottom line:
put a prop on that might be close and measure RPM. As I said, these prop
calcs don't work, especially for Campbell Sailor props. That's why I want
to hear from someone with the same engine, preferable on a similar boat,
and see what works.

One problem with most of the prop-calc programs I used is they don't take
into consideration the HORSEPOWER of the engine. Same prop for an 8hp as a
20hp? I don't think so!

druid
http://www.bcboatnet.org

I don't know what prop calcs you used that didn't take into
consideration the Horsepower of the engine as that is essentially the
basis for the whole exercise since the H.P. and the gearbox reduction
give the diameter of the propeller while the pitch is determined by
the expected speed; in the simpler calculations, the hull speed. In
addition the more sophisticated formula include factors for number of
blades and blade width.

I agree that these calcs don't always give a perfect match but they
certainly should get you in the ball park but no formula is going to
give you the perfect match of propeller and boat.


Bruce-in-Bangkok
(Note:displayed e-mail
address is a spam trap)

On Fri, 19 Oct 2007 17:37:17 +0700, Bruce in Bangkok wrote:

On Thu, 18 Oct 2007 21:49:59 -0700, druid wrote:

On Fri, 19 Oct 2007 08:40:57 +0700, Bruce in Bangkok wrote:


There various propeller calculation programs on the Net. Try a google
for "propeller calculations".

The calculator I used for the above is "PropCalc" from Marine Castle -
freeware - which seems compare favorably with other programs.



Yeah....

I went through the prop-calculation thing on my last boat. Bottom line:
put a prop on that might be close and measure RPM. As I said, these prop
calcs don't work, especially for Campbell Sailor props. That's why I want
to hear from someone with the same engine, preferable on a similar boat,
and see what works.

One problem with most of the prop-calc programs I used is they don't take
into consideration the HORSEPOWER of the engine. Same prop for an 8hp as a
20hp? I don't think so!

druid
http://www.bcboatnet.org

I don't know what prop calcs you used that didn't take into
consideration the Horsepower of the engine as that is essentially the
basis for the whole exercise since the H.P. and the gearbox reduction
give the diameter of the propeller while the pitch is determined by
the expected speed; in the simpler calculations, the hull speed. In
addition the more sophisticated formula include factors for number of
blades and blade width.


Every calc I got when I googled "propeller calculations online" did not
include HP. They all were based on the definition of "pitch" (ie with no
slip, a 6" pitch prop will go 6" through the water in one revolution).

Two problems with that: first, slip is CRITICAL, and yet you're expected
to guess it and type it in. Second, CS props do not do this: a 6" pitch
CS prop will go about 8" in one rev without slip. CS calls this an
advantage, saying "our props have more thrust than any other!". They do
this apparently because the ANGLE of their blade corresponds to a 6" pitch
(they curve the blade to get the extra thrust).

So, back to my original stand: prop calcs don't work for CS props. So:
anybody have a MD6A and what prop do you have?

druid
http://www.bcboatnet.org

Druid,
Try Googling "propcalc" - it will give you a different set of programs
and some of these, including the freeware excel program named
*PropCalc*, will require input of engine horsepower. In fact, PropClac
requires it in the first entry.
The following is a copy of the first calc page. Some of the entries are
user inputs, some are the calculations. For ex. you put in engine
horsepower and max rpm, and out comes torque, etc.

45 Engine Horsepower
2000 Engine R.P.M. (max)
118 Engine Torque ft/lb

44 SHP - Shaft Horsepower at gearbox output.
3 Enter number of bearings between gearbox output and propeller.
1.50 Enter Gearbox reduction ratio. Eg. 1.5

4.50% Percentage power loss due to shaft bearings.

44 Shaft Horsepower at propeller.
1333 Propeller RPM
172 Propeller Torque ft/lb


Red

On Fri, 19 Oct 2007 07:18:08 -0700, druid wrote:

They do
this apparently because the ANGLE of their blade corresponds to a 6" pitch
(they curve the blade to get the extra thrust).

That is called "cupping" in prop language. Cupping will increase the
effective pitch which is fine if your engine has enough torque to
maintain RPMs. If not you haven't accomplished anything. There is no
free lunch with props.

On Fri, 19 Oct 2007 17:37:17 +0700, Bruce in Bangkok
wrote:

the H.P. and the gearbox reduction
give the diameter of the propeller

Not really. Diameter of the prop is limited by tip clearance of the
blades. Generally speaking you should use the biggest diameter that
will still give 10 to 15% clearance. Too little clearance will cause
vibration as the prop thrust interacts with the hull and/or aperature.

There is a way of backing into the prop numbers based on the estimated
hull speed and slippage factor assuming that the engine will deliver
the required power to maintain RPMs.

Required horsepower is a function of displacement and drag. If the
prop pitch is too great, or the available torque is too low, the
engine will not rev high enough to deliver its rated power.

On Thu, 18 Oct 2007 21:49:59 -0700, druid wrote:

One problem with most of the prop-calc programs I used is they don't take
into consideration the HORSEPOWER of the engine. Same prop for an 8hp as a
20hp? I don't think so!

That is correct. More horsepower generally means more torque, and
more torque gives you the ability spin a prop with more pitch, all
other things being equal.

On 2007-10-18 21:40:57 -0400, Bruce in Bangkok said:

On Thu, 18 Oct 2007 11:22:19 -0700, druid
wrote:

My new-to-me boat (Crown 28) has a Volvo 10hp MD6A, and it's way
overpropped with a Campbell Sailor 11x6 3-blade (gets 1500 WOT instead
of 2400). The Campbell Sailor dealer says his calcs recommend a 12x8
(ie much MORE pitch). Now, my experience with CS props is you take the
calculated pitch and subtract 2", but I donno...

Engine IS very "tired" but it pushes the boat at 5+ knots with the
over-pitched prop, so it can't be TOO bad.

Those with MD6A's - what prop do you have on it?

You need to post the loaded displacement of the boat, the water line
length and the gear box ratio.

For example: if a LWL of, say 22 ft., a loaded displacement of 2,000
lbs. and a 1:1 gearbox then the prop should be about 9 X 6.

With a 1.5:1 box the prop is about 12 X 9 and with a 2:1 box about 14 X 13.

There various propeller calculation programs on the Net. Try a google
for "propeller calculations".

The calculator I used for the above is "PropCalc" from Marine Castle -
freeware - which seems compare favorably with other programs.

I included Bruce's comments solely because he's right, too, in an
absolute sense.

But 12x8 was about the usual prop in that era, and it did reasonably well.

If I were in your situation, I'd look to CDI for their recommended
plastic prop. In fact, when other Tanzers ask who have similar old
Volvos, I give the same answer. The bronze props I see today are
identical to what we would have seen 30+ years ago. CDI's props deliver
the equivalent of a new motor without the $10k cost.

Check out http://www.sailcdi.com/ppmain.htm

NOT affiliated with them other than being a satisfied customer of their
furlers.

--
Jere Lull
Tanzer 28 #4 out of Tolchester, MD
Xan's pages: http://web.mac.com/jerelull/iWeb/Xan/
Our BVI trips & tips: http://homepage.mac.com/jerelull/BVI/