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Props - RPM's and Cruising Speed
I have a 30' Sea Ray Weekender with straight, 260HP, 5.7L inboards.
Currently, the boat cruises at about 20kts at 3,100 RPM's. At this speed, I get reasonable fuel economy, because it's not until about 3,200 RPM's that the secondaries join the party and start tapping the keg, if you will. Max RPM on the boat is 4,100, at a speed of around 30kts. I very, very rarely operate at this speed, and when I do, it's usually for less than a minute. I'm wet behind the ears when it comes to propping, so a question for those in the know: At 20kts, the boat requires slight tabbing, so it's not quite 100% on plane. If I bump the RPM's up to 3,200-3,300, the boat cruises at about 24kts, and will plane without tabs. The first comment will likely be that operating on a full plane is more fuel efficient, and I would normally concur, but it's not the case with this boat. I've done the calculations, and it's more efficient for me to keep out of the 4 barrels. My current props are 3-blade. Would it be possible to prop the boat so that the cruise speed at 3,000 RPM's was bumped up a few knots? My thinking is that if I can get on full plane while keeping out of the secondaries, I'll enjoy the best of both worlds, higher cruising speed while maintaining fuel economy (if that's really such a thing on a boat). 8) I'm not concerned with losing a couple of hundred RPM's on the top, as I simply don't operate the boat in that range. Another benefit is noise. Above 3,100 RPM's, the engines generate significantly more noise. Is a 4 blade prop an option? More cupping on my current prop? More diameter? Other? Thanks! |
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Props - RPM's and Cruising Speed
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Props - RPM's and Cruising Speed
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#4
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Props - RPM's and Cruising Speed
On Fri, 28 May 2004 16:47:39 GMT, WaIIy
wrote: I disagree, the boat weighs about 9500 lbs dry and that is plenty of power. ========================================= Depends on your point of view. My 24 ft I/O cuddy with the same enfine weighs about 5,000 lbs with fuel, gear, etc. At 3,400 RPM it cruises at 24 to 25 kts which I regard as adequate, but just barely. I'd really like to be running at 30 kts in flat water. The fact that the boat in question can plane off and run and 20 kts at a reasonable RPM means that he has enough power, but barely enough in my opinion. Most 10,000 lb boats will either be running a single 454 or a pair of 350s, giving them a cruising speed of 25 to 27 kts. |
#5
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Props - RPM's and Cruising Speed
On Fri, 28 May 2004 14:40:39 -0400, Wayne.B
wrote: On Fri, 28 May 2004 16:47:39 GMT, WaIIy wrote: I disagree, the boat weighs about 9500 lbs dry and that is plenty of power. ========================================= Depends on your point of view. My 24 ft I/O cuddy with the same enfine weighs about 5,000 lbs with fuel, gear, etc. At 3,400 RPM it cruises at 24 to 25 kts which I regard as adequate, but just barely. I'd really like to be running at 30 kts in flat water. The fact that the boat in question can plane off and run and 20 kts at a reasonable RPM means that he has enough power, but barely enough in my opinion. Most 10,000 lb boats will either be running a single 454 or a pair of 350s, giving them a cruising speed of 25 to 27 kts. Just to clarify, the boat does have twin engines..........Also, the recommended RPM range at WOT is 4K-4.4K.......... I thought too that I should have a higher cruising speed at 3K RPM's, but 19kt-20kt is it. |
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Props - RPM's and Cruising Speed
On Fri, 28 May 2004 21:04:08 GMT, WaIIy
wrote: There might be a mixup here, Wayne. I would agree that one 5.7 is underpowered. He is runnning twin 5.7 straight inboards. =========================================== One of my neighbors has the same boat with twin 350s and I/Os. He cruises at 26 to 27 kts but he's pushing his engines to 3600 RPM most of the time. Conventional wisdom around here is that the I/Os run a bit faster than straight inboards. |
#7
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Props - RPM's and Cruising Speed
I have a 30' Sea Ray Weekender with straight, 260HP, 5.7L inboards.
Currently, the boat cruises at about 20kts at 3,100 RPM's. At this speed, I get reasonable fuel economy, because it's not until about 3,200 RPM's that the secondaries join the party and start tapping the keg, if you will. Max RPM on the boat is 4,100, at a speed of around 30kts. I very, very rarely operate at this speed, and when I do, it's usually for less than a minute. There are several things to consider. As others point out, with a 4100 rpm max., there is a distinct risk that there will be detonation (knock) in the cylinders. High speed detonation is difficult to impossible to detect, especially when the engines are well insulated (as they would be in a SeaRay.) Detonation is like hitting the piston tops with a ball-pein hammer and can eventually punch holes in the pistons. Also, carburetors try to do the best they can at providing the right fuel-to-air ratio for combustion. That said, running the engine just shy of opening the second barrels tends to run the mixture lean, which will further aggrevate the detonation problem. I have read in boating mag advice colums NOT to run that way. You will get "save money" with better fuel economy, but lose in the long run. As to tabbing, I have a 1989 30ft. Sundancer with flowmeters with GPS input that shows instantaneous miles/gallon. I have observed several times that I get slightly better mpg (approx. .05 mpg) by pushing the bow down slightly with tabs when cruising at 20-25 mph. I would never have guessed that, 'cause the wetted area goes up (viscous drag), but it really does happen. |
#8
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Props - RPM's and Cruising Speed
Sounds like you need to drop prop pitch a bit to get the WOT RPM where it
should be. Reducing pitch reduces load on the engines at the same RPM and you might just be able to run a few 100 RPM more before the secondaries open. Another option may be to have a performance shop change the "rods" which control when the secondaries open. I presume you have the old Chevy standby Quadra-jet carbs. wrote in message ... I have a 30' Sea Ray Weekender with straight, 260HP, 5.7L inboards. Currently, the boat cruises at about 20kts at 3,100 RPM's. At this speed, I get reasonable fuel economy, because it's not until about 3,200 RPM's that the secondaries join the party and start tapping the keg, if you will. Max RPM on the boat is 4,100, at a speed of around 30kts. I very, very rarely operate at this speed, and when I do, it's usually for less than a minute. I'm wet behind the ears when it comes to propping, so a question for those in the know: At 20kts, the boat requires slight tabbing, so it's not quite 100% on plane. If I bump the RPM's up to 3,200-3,300, the boat cruises at about 24kts, and will plane without tabs. The first comment will likely be that operating on a full plane is more fuel efficient, and I would normally concur, but it's not the case with this boat. I've done the calculations, and it's more efficient for me to keep out of the 4 barrels. My current props are 3-blade. Would it be possible to prop the boat so that the cruise speed at 3,000 RPM's was bumped up a few knots? My thinking is that if I can get on full plane while keeping out of the secondaries, I'll enjoy the best of both worlds, higher cruising speed while maintaining fuel economy (if that's really such a thing on a boat). 8) I'm not concerned with losing a couple of hundred RPM's on the top, as I simply don't operate the boat in that range. Another benefit is noise. Above 3,100 RPM's, the engines generate significantly more noise. Is a 4 blade prop an option? More cupping on my current prop? More diameter? Other? Thanks! |
#9
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Props - RPM's and Cruising Speed
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#10
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Props - RPM's and Cruising Speed
On Fri, 28 May 2004 13:54:40 +0000, marbisiNsP wrote:
I have a 30' Sea Ray Weekender with straight, 260HP, 5.7L inboards. Currently, the boat cruises at about 20kts at 3,100 RPM's. At this speed, I get reasonable fuel economy, because it's not until about 3,200 RPM's that the secondaries join the party and start tapping the keg, if you will. Hi, Just thought of something as I was going over the Coquihalla Hy and slipping in and out of overdrive... First, I hear you about the noise. I like running my Campion at around 3K partly because of the noise. Sorry, can't help you there. But... The secondaries really open because of THROTTLE setting, not RPM, correct? So how about this: DECREASE your prop pitch. Then you're running at a higher rpm, where the engine puts out more power at lower throttle setting. I suspect you'll be able to get that few more knots to get fully on plane at a lower throttle setting than with the "steeper" prop. Worth a try... Lloyd Sumpter "Far Cove" Catalina 36 |
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