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Prop for MD6A?
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 |
Prop for MD6A?
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) |
Prop for MD6A?
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/ |
Prop for MD6A?
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 |
Prop for MD6A?
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) |
Prop for MD6A?
On Fri, 19 Oct 2007 04:46:24 GMT, Jere Lull wrote:
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. Further to my previous post: I'm sorry to say that I did not take into account that the Campbell Sailor propeller is a narrow blade, low drag, propeller and the normal propeller calculation formula will not be accurate with this type of propeller as nearly all formulas are based on 3 bladed propeller having a blade area of 55%.DAR. (D.A.R. stands for Disc Area Ratio and is the developed area of all the blades as a percentage of the total area of the circle described by the propeller) The more sophisticated formula include correction factors for this difference in blade width but the normal run of the mill formulas do not as they appear to be based on power boat requirements. Bruce-in-Bangkok (Note:displayed e-mail address is a spam trap) |
Prop for MD6A?
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 |
Prop for MD6A?
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 |
Prop for MD6A?
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? What other people do with their boats may or may not impart something useful. You have your very own performance figures, and they are telling you less pitch. Maybe less diameter. Perhaps both, but whatever, you want one that is easier to spin. Did the engine turn up more RPM when it was new? I mean you can run a worn out car in a lower gear because it lacks the suds for a higher one, but it isn't really the answer. Casady |
Prop for MD6A?
On Fri, 19 Oct 2007 16:53:13 +0000, Richard Casady 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? What other people do with their boats may or may not impart something useful. You have your very own performance figures, and they are telling you less pitch. Maybe less diameter. Perhaps both, but whatever, you want one that is easier to spin. Did the engine turn up more RPM when it was new? I mean you can run a worn out car in a lower gear because it lacks the suds for a higher one, but it isn't really the answer. Casady OK, here's the Deal: I've already decided to take 2" pitch (or at least 1 1/2" ) out of my prop. What I WANT to know is what other MD6A folks are spinning so I can get a feel for how tired my engine is. Yes, I will take into account things like the "boosted" CS thrust, differences in boat size/weight/LWL, possible differences in gear ratios, phase of the moon... I just wanna know, ok? druid http://www.bcboatnet.org |
Prop for MD6A?
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. |
Prop for MD6A?
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. |
Prop for MD6A?
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. |
Prop for MD6A?
On Fri, 19 Oct 2007 11:01:33 -0400, Red wrote:
.... 118 Engine Torque ft/lb .... 172 Propeller Torque ft/lb Red This is a nit-pick, but everyone has his hobby-horse. A measure of work is raising some weight through a particular height. If you time how long this takes, you have an early way of specifying power - James Watts's way, in fact. That's feet times pounds all divided by time . For example ft.lbs /min A measure of torque is how much force a crank can apply at a given distance from the shaft's axis of rotation. This does not signify power, or work but it's sometimes good to know. The units for torque can be force in pounds TIMES distance from the center in feet. This time, the force is at right angles to the distance from the rotation axis. So it can be given as ft.lbs also, but this makes it look like work, just one step away from power. But it isn't. So some people who like to keep the difference between work and torque in mind, call the one unit ft.lbs and the other unit lb. ft. To summarize: there is no such unit as ft/lbs. / means divide by.... There, I said it! :-) Brian W |
Prop for MD6A?
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 [based on 2 minutes back of the envelope and positively NO experience with a MD6A].... Borrow a 10 X 5 and look for 6+ kts at 2200 revs out of your ol' tired engine. :-) Brian W |
Prop for MD6A?
On Oct 19, 8:35 pm, Brian Whatcott 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 [based on 2 minutes back of the envelope and positively NO experience with a MD6A].... Borrow a 10 X 5 and look for 6+ kts at 2200 revs out of your ol' tired engine. :-) Brian W Interesting discussion - here's an "update": First, I realized I have a spare prop: a 14x8(? have to look...) 2- blade non CS (BTW, DON'T tell CS that their patented curve is simply "cupping" ). I can maybe repitch it or just leave it as is - in any case, I can put it on with a quick-lift rather than lifting the boat out, leaving it on the hard while I remove the CS, get it repitched and put it back on. Second, I've decided to pull the head and injectors and try to squeeze a bit more life out of the engine. Already got them off, getting them to the rebuild shops on Mon, but 'er back together next weekend, and see if I can get a few more RPMs. If not, I can still slap on the 2- blade... druid http://www.bcboatnet.org |
Prop for MD6A?
On Sun, 18 Nov 2007 19:46:02 -0800 (PST), druid
wrote: Interesting discussion - here's an "update": First, I realized I have a spare prop: a 14x8(? have to look...) 2- blade non CS (BTW, DON'T tell CS that their patented curve is simply "cupping" ). I can maybe repitch it or just leave it as is - in any case, I can put it on with a quick-lift rather than lifting the boat out, leaving it on the hard while I remove the CS, get it repitched and put it back on. Second, I've decided to pull the head and injectors and try to squeeze a bit more life out of the engine. Already got them off, getting them to the rebuild shops on Mon, but 'er back together next weekend, and see if I can get a few more RPMs. If not, I can still slap on the 2- blade... druid http://www.bcboatnet.org ======================= Let us know how it works out. The real world has a way of confounding the theoretical at times. |
Prop for MD6A?
On Nov 22, 7:36 am, Wayne.B wrote:
On Sun, 18 Nov 2007 19:46:02 -0800 (PST), druid wrote: Interesting discussion - here's an "update": First, I realized I have a spare prop: a 14x8(? have to look...) 2- blade non CS (BTW, DON'T tell CS that their patented curve is simply "cupping" ). I can maybe repitch it or just leave it as is - in any case, I can put it on with a quick-lift rather than lifting the boat out, leaving it on the hard while I remove the CS, get it repitched and put it back on. Second, I've decided to pull the head and injectors and try to squeeze a bit more life out of the engine. Already got them off, getting them to the rebuild shops on Mon, but 'er back together next weekend, and see if I can get a few more RPMs. If not, I can still slap on the 2- blade... druid http://www.bcboatnet.org ======================= Let us know how it works out. The real world has a way of confounding the theoretical at times. Well, I FINALLY got the engine back together and running! Rebuilding the head and injectors only cost $650, but lots of grunting and general blue-air. Took her out for a bit, but there was quite a bit of tide and gusty winds, so it was a bit hard to see any changes. I think I got maybe an extra 1/2 knot (didn't have GPS and was too busy to check RPM on the hand tach). One thing that DID happen is now I get the "classic" grey/black smoke when I push up the throttle past where the RPMs get any higher, telling me too much fuel for the engine speed. DEFINITELY a sign of over-pitched prop! So: gotta pull her in the spring to check/replace a thru-hull, so I'll reduce the pitch an inch, maybe 1.5 inches.... druid http://www.bcboatnet.org |
Prop for MD6A?
On 2007-12-17 17:06:03 -0500, druid said:
One thing that DID happen is now I get the "classic" grey/black smoke when I push up the throttle past where the RPMs get any higher, telling me too much fuel for the engine speed. DEFINITELY a sign of over-pitched prop! So: gotta pull her in the spring to check/replace a thru-hull, so I'll reduce the pitch an inch, maybe 1.5 inches.... Not necessarily. We are properly pitched, but have a little extra throttle to throw in which blows pretty black. Even when we were under-pitched our first season, that happened once we achieved the engine's max RPMs and the governor cut in. Measure the revs before you change the prop. -- 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/ |
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