I am considering a new 3 blade prop for my 30' Hunter with 2GM20 engine. Everybody from A to Z says to select a prop that will let the engine reach 3400 RPM under calm conditions. I cruise my boat at 2400 RPM and on rare instances 2600 RPM. Because of the difference in the engine power curve and the propellor power curve I am extracting approximately 6 hp of the avaliable 12 at 2400 RPM. As long as I realize my limitations, it would seem to be to my(and the engines') advantage to overprop a bit to more closely load the engine at these speeds. Would somebody kindly explain the error in my thinking.
- Status
- Not open for further replies.
J
Justin Wolfe/PYI
Kevins travelling...
So I'm all you get today. Okay, so answer me this. Why are you considering a 3 blade prop? Replacing an old 3 or an old 2? I'd be assuming too much if I went further without knowing what prop you have now. Here's a simple answer to your question though. For this example lets assume you have a 2GM20 rated for 20hp max @ 3400 rpm (I think its a 3600 rpm engine by the way). Lets assume you have the perfect setup with a 14x11 prop. Lets assume that at normal cruising rpm of 2800 rpms you are generating 17hp (I'm guessing, but lets assume that) and that you are achieving 6.0 knots of boat speed. In this situation everything is good. The engine isn't overloaded, fuel efficiency is reasonable and boat speed is good. Now lets slow the rpms to 2400 rpm and increase the pitch by 3" to a 14x14. I don't have an 2GM hp curve but lets assume you are generating 10 hp at 2400 rpm and with the new prop can still get 6.0 knots. Sounds good? Here's the rub. It takes more than 10 hp to achieve 6.0 knots on a 10,000 lb boat with a 27' waterline, so you are overloading the engine and most likely will not get to 6.0 knots. The fact that the rpms are 2400 does not imply that the engine is working less. Add to this the fact that a 14"x14" prop is not as efficient as a 14"x11" and you've decreased the boat speed even further, but still worked the engine too hard. 14x14" is roughly 28 degrees of pitch which on our scale of 10-30 degrees for Max-Props is right at the top end. In the "not so efficient" range. A lot of assumptions for the example, but I hope it makes the point. The bottom line is that by running your engine at 2800 rpms for cruising you get more boat speed while working the engine less. A win-win situation. I'm sure this generated a question or two... Ask away.
So I'm all you get today. Okay, so answer me this. Why are you considering a 3 blade prop? Replacing an old 3 or an old 2? I'd be assuming too much if I went further without knowing what prop you have now. Here's a simple answer to your question though. For this example lets assume you have a 2GM20 rated for 20hp max @ 3400 rpm (I think its a 3600 rpm engine by the way). Lets assume you have the perfect setup with a 14x11 prop. Lets assume that at normal cruising rpm of 2800 rpms you are generating 17hp (I'm guessing, but lets assume that) and that you are achieving 6.0 knots of boat speed. In this situation everything is good. The engine isn't overloaded, fuel efficiency is reasonable and boat speed is good. Now lets slow the rpms to 2400 rpm and increase the pitch by 3" to a 14x14. I don't have an 2GM hp curve but lets assume you are generating 10 hp at 2400 rpm and with the new prop can still get 6.0 knots. Sounds good? Here's the rub. It takes more than 10 hp to achieve 6.0 knots on a 10,000 lb boat with a 27' waterline, so you are overloading the engine and most likely will not get to 6.0 knots. The fact that the rpms are 2400 does not imply that the engine is working less. Add to this the fact that a 14"x14" prop is not as efficient as a 14"x11" and you've decreased the boat speed even further, but still worked the engine too hard. 14x14" is roughly 28 degrees of pitch which on our scale of 10-30 degrees for Max-Props is right at the top end. In the "not so efficient" range. A lot of assumptions for the example, but I hope it makes the point. The bottom line is that by running your engine at 2800 rpms for cruising you get more boat speed while working the engine less. A win-win situation. I'm sure this generated a question or two... Ask away.
K
Kevin L. Woody
Additional stuff
Dear Frank: Thanks for your letter and thanks to Justin in sitting in my chair while I was away. I would like to add some more info. When you add pitch to a propeller you create additional load to the engine. It is true that if you add pitch you will increase the vessels speed at a lower rpm. This action however causes the system to over-load. Because of the additional loading it requires additional fuel to obtain the same amount of rpm’s. This additional fuel causes carboning, which will create heat and possible scoring to the cylinders. We assume that because the engine works at a lower rpm, we think the engine is working easier. In fact when we overload a engine we cause more problems not allowing it to obtain the upper rpm’s. Hope this helps. Sincerely, Kevin L. Woody
Dear Frank: Thanks for your letter and thanks to Justin in sitting in my chair while I was away. I would like to add some more info. When you add pitch to a propeller you create additional load to the engine. It is true that if you add pitch you will increase the vessels speed at a lower rpm. This action however causes the system to over-load. Because of the additional loading it requires additional fuel to obtain the same amount of rpm’s. This additional fuel causes carboning, which will create heat and possible scoring to the cylinders. We assume that because the engine works at a lower rpm, we think the engine is working easier. In fact when we overload a engine we cause more problems not allowing it to obtain the upper rpm’s. Hope this helps. Sincerely, Kevin L. Woody
F
Frank Walkker
More Info
Justin thanks for responding to my question. The reason that I cruise at 2400 RPM is that this produces the least vibration, reasonable noise level and 5.25 calm nots (hull speed approx 6.75). I am now swinging a 16x11 2 blade(stock prop is 15x12). We do a lot of ICW motoring here and I am looking to up my cruise speed a little and smooth out vibrations. I appreciate not getting an inefficient diameter/ pitch combination but am just trying to err on the overprop side (say 2" pitch max) to maximize my speed at my normal cruise. Tx Frank
Justin thanks for responding to my question. The reason that I cruise at 2400 RPM is that this produces the least vibration, reasonable noise level and 5.25 calm nots (hull speed approx 6.75). I am now swinging a 16x11 2 blade(stock prop is 15x12). We do a lot of ICW motoring here and I am looking to up my cruise speed a little and smooth out vibrations. I appreciate not getting an inefficient diameter/ pitch combination but am just trying to err on the overprop side (say 2" pitch max) to maximize my speed at my normal cruise. Tx Frank
F
Frank Walkker
Look at the Curves
Kevin, thanks for the info. I understand about overloading the engine, but when I am extracting at 2400 RPM 10 HP of an avaliable 12 HP I fail to see that this is overloading the engine. I fullly understand that if I put the hammer down with an oversize prop then I risk damage to the 2GM20. Tx Frank
Kevin, thanks for the info. I understand about overloading the engine, but when I am extracting at 2400 RPM 10 HP of an avaliable 12 HP I fail to see that this is overloading the engine. I fullly understand that if I put the hammer down with an oversize prop then I risk damage to the 2GM20. Tx Frank
J
Justin Wolfe/PYI
Hi Frank
You've confused me a bit here. I don't have the curve of your engine but something isn't adding up. If you've got a 2GM20, then you've got an 18hp engine rated for 3600 rpm max. You're saying you only get 12hp max. Is it really a YSB-12 Yanmar? In which case it's only rated for 3000 rpm max & 2400 is the proper cruising rpm... Tell me again what engine you have. I show a 1 cylinder Yanmar for the older Hunter 30's in my book.
You've confused me a bit here. I don't have the curve of your engine but something isn't adding up. If you've got a 2GM20, then you've got an 18hp engine rated for 3600 rpm max. You're saying you only get 12hp max. Is it really a YSB-12 Yanmar? In which case it's only rated for 3000 rpm max & 2400 is the proper cruising rpm... Tell me again what engine you have. I show a 1 cylinder Yanmar for the older Hunter 30's in my book.
F
Frank Walker
"Just the Facts Madam"
Justin, I have a 1988 30 ft Hunter with a 2GM20F (2.21 FWD Ratio) engine. The engine is rated at 16 Hp continuous at 3400 RPM and 18 Hp for 1 HR at 3600 RPM. I swing a 15x12 two blade propellor. Max calm RPM with this prop is 3400 RPM. For reasons previously explained I cruise at 2400 RPM and if the wind kicks up 2600 RPM. Because of the difference in shape of the power curves between the engine and prop they can only match up at one speed. If the prop/boat extracts 16 HP from the engine at 3400 RPM then the prop and engine are matched by conventional thinking. However, now consider 2400 RPM. The engine is capable of producing at that speed 11 HP continuous, but the same propellor is now only extracting 6 HP. It is at this point that I have 5 HP going to waste. I am suggesting that by overpropping slightly you capture some of this difference. I fully acknowledge that if I set one up this way then I can no longer spin the engine up to 3400 RPM because: one it will not turn up that fast, and two the engine might be damaged. This is not a charter boat, and I am the only guy that puts a hand on the throttle. It just strikes me that the engine would be better off than if I say ran it at 3000 RPM with a standard prop. The piston velocity would certainly be less. My original question is there something else that I am missing. Thanks Frank
Justin, I have a 1988 30 ft Hunter with a 2GM20F (2.21 FWD Ratio) engine. The engine is rated at 16 Hp continuous at 3400 RPM and 18 Hp for 1 HR at 3600 RPM. I swing a 15x12 two blade propellor. Max calm RPM with this prop is 3400 RPM. For reasons previously explained I cruise at 2400 RPM and if the wind kicks up 2600 RPM. Because of the difference in shape of the power curves between the engine and prop they can only match up at one speed. If the prop/boat extracts 16 HP from the engine at 3400 RPM then the prop and engine are matched by conventional thinking. However, now consider 2400 RPM. The engine is capable of producing at that speed 11 HP continuous, but the same propellor is now only extracting 6 HP. It is at this point that I have 5 HP going to waste. I am suggesting that by overpropping slightly you capture some of this difference. I fully acknowledge that if I set one up this way then I can no longer spin the engine up to 3400 RPM because: one it will not turn up that fast, and two the engine might be damaged. This is not a charter boat, and I am the only guy that puts a hand on the throttle. It just strikes me that the engine would be better off than if I say ran it at 3000 RPM with a standard prop. The piston velocity would certainly be less. My original question is there something else that I am missing. Thanks Frank
F
F. Yaskin
Diesel operation
You are thinking that the throttle directly controls RPM. It doesn't. It sets the governer to deliver a certain RPM based on the throttle position. If you are overpropped, you are running at 2400, but the governer, based on throttle position, is suppying fuel for ( as example) 2700 RPM, with the excess causing oil dilution, smoke and other bad operating conditions.. The HP vs RPM graphs can be deceptive in some cases. If you run the engine in neutral @2400 RPM, the engine is not creating the same HP as when under load @2400 RPM( again, an example). Its not easy to KNOW when your underpropped. If the correct prop is, say, a 14x11, a 14x9, 14x10, and 14x11 will all have the same max rpm. Its not until you go to 14x12, and note that you can't reach rated rpm, that you know ( as opposed to assume) that the correct prop is 1 step back.
You are thinking that the throttle directly controls RPM. It doesn't. It sets the governer to deliver a certain RPM based on the throttle position. If you are overpropped, you are running at 2400, but the governer, based on throttle position, is suppying fuel for ( as example) 2700 RPM, with the excess causing oil dilution, smoke and other bad operating conditions.. The HP vs RPM graphs can be deceptive in some cases. If you run the engine in neutral @2400 RPM, the engine is not creating the same HP as when under load @2400 RPM( again, an example). Its not easy to KNOW when your underpropped. If the correct prop is, say, a 14x11, a 14x9, 14x10, and 14x11 will all have the same max rpm. Its not until you go to 14x12, and note that you can't reach rated rpm, that you know ( as opposed to assume) that the correct prop is 1 step back.
- Status
- Not open for further replies.