Just wanted to get some suggestions on propping. Right now my 28 cruises right around 22-23kts. at 3300-3400rpm. WOT is about 27 kts. Engines are rated at 4300rpm. Port engine is hitting 4300 while the the starboard is at 4050. The props are 17x15. I have tried 6 different sets of props on my work boat and none gave me the results I expected based on my knowledge in props. Obviously I would like to change props on this boat once, and only once.
I know that smaller wheel size would decrease the efficiancy. When pitch is added would it help. During the re power the Steyr guys were expecting a mid 20's cruise with a 30kt top end. I am close to that but need just a little more and am assuming some brands of props are better than others.
My engine and gear box wiegh about 733 per side. Total wieght savings of about 300-400 lbs over the small blocks. The horse power is 212 and I think the torque is around 320. Still need to get some more hours on the engines but will need prop work here pretty soon Kevin
props and speed
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mike ohlstein
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Considering tachs are notorius for being wrong, I guess I am not positive. I need to get a laptop computer to hook up to the engines. I have the software and things like idle adjustment can be made. It also will tell me throttle position sensor voltage. We checked on the sea trial and things were good, but it was nasty out and could not stay at WOT for long.
If both engines were at 4300, would I benefit from less diameter and more pitch? I recall hearing that cup is useless unless the boat can achieve 30 kts. To me it is a very interesting subject. Like I mentioned before, I thought I understood diameter, pitch, cup and the associated results with changes made to the prop. That all went out the window on my work boat. 500 hp on a light 29 ft boat got me 61.5mph. Despite being at the high side of the max rpm, there was a point when added pitch brought rpm's down but the speed remained constant. Kevin
If both engines were at 4300, would I benefit from less diameter and more pitch? I recall hearing that cup is useless unless the boat can achieve 30 kts. To me it is a very interesting subject. Like I mentioned before, I thought I understood diameter, pitch, cup and the associated results with changes made to the prop. That all went out the window on my work boat. 500 hp on a light 29 ft boat got me 61.5mph. Despite being at the high side of the max rpm, there was a point when added pitch brought rpm's down but the speed remained constant. Kevin
Kevin,
Make sure of the rpm numbers cause thats quite a difference between the two sides even accounting for the slight difference in ratio of the ZF's when you reverse direction.
Check for full open throttle and make sure that the props are indeed sized as marked.
I told Sid I have 4 blade props off a 28 with 1-1/8 shaft holes that you can try.
Will be going to Homestead next Friday for the truck race then head to the keys for Fri night and Sat. Leave early Sun back for the cup race. Can drop off if they are right.
Staying at the Crystal Bay Resort and Marina in Marathon.
Check all your numbers and get back to me.
Make sure of the rpm numbers cause thats quite a difference between the two sides even accounting for the slight difference in ratio of the ZF's when you reverse direction.
Check for full open throttle and make sure that the props are indeed sized as marked.
I told Sid I have 4 blade props off a 28 with 1-1/8 shaft holes that you can try.
Will be going to Homestead next Friday for the truck race then head to the keys for Fri night and Sat. Leave early Sun back for the cup race. Can drop off if they are right.
Staying at the Crystal Bay Resort and Marina in Marathon.
Check all your numbers and get back to me.
Last edited by Bruce on Nov 7th, '06, 17:10, edited 1 time in total.
You may want to check for slipage, plug in your info with reduction and see where your slippage falls. If excessive you may want to add some cup while reducing the pitch.
But first I would find out why RPM's are off so much. Fuel restriction, clogged filter, bad wheel, stuffing box too tight.. its gottat be something cause that is alot of rpm to be off for diesel especially.
Carl
But first I would find out why RPM's are off so much. Fuel restriction, clogged filter, bad wheel, stuffing box too tight.. its gottat be something cause that is alot of rpm to be off for diesel especially.
Carl
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In Memory Walter K
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I think your theory on cupping is incorrect. I have 210 Cummins 6BT's that I wanted to keep the slow side slow for trolling and docking so I kept the pitch down and added cupping. 6BT's WOT is 2650 and cruise at 2400. The cupping added 1.5-2 kts at cruise so I know: 1 it works on the high end, 2. The high end isn't 30 kts. I would think that if yours are high rpm engines, cupping would work even better for you. Walter
Thought I read it here on this sight about cup, not sure though. Even though the tank was just cleaned, should I check or replace the filters?. 22hrs since repower/tank polish. The props that I am using were my spares which were reconditioned last year. They maybe had 15 hours on them, but the boat was fork lifted a few times and even trailered. No vibrations of any kind. I have dripless seals, so no resistance there. I need to snag a laptop and check for 5 volts on WOT and get accurate rpm reading before I start messing with the props as a culprit. I still want to get more speed out of it though. From some of the other posts subjects on here it sounds like computer balanced props make a big difference. Bruce, are you going fishing, drinking or both while you are here in Marathon. Call me and we can get together if you have the time. Heck, if you need a boat, you can take mine! Kevin
Kevin,
Don't jump into the computer balance if you have no vibration.
Before checking anything else, check for WOT. Thats the first thing.
I will get my prop guy to figure out some wheel sizes for you.
What's that 28 weight, 12K?
Remember when diagnosing any problem, always look at the simple things first and work your way forward.
When you have two engines feeding off the same pickup as is the 28, if the fuel were a problem you would more often see the same problem on both engines.
I'll just be relaxing with the wife.
Getting together and down a few clears is fine.
Don't jump into the computer balance if you have no vibration.
Before checking anything else, check for WOT. Thats the first thing.
I will get my prop guy to figure out some wheel sizes for you.
What's that 28 weight, 12K?
Remember when diagnosing any problem, always look at the simple things first and work your way forward.
When you have two engines feeding off the same pickup as is the 28, if the fuel were a problem you would more often see the same problem on both engines.
I'll just be relaxing with the wife.
Getting together and down a few clears is fine.
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Hyena Love
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Phot Tach
First, most prop shops will loan you a photo/laser tach. Shoot the engines in neutral in the slip. Do every 500 rpms. This will give you a quick and dirty answer to whether the tachs are wrong, how much wrong, and how the error changes with the rpms.
Speed looks about right for those rpms thru what - 1.5 to 1 trannies?
Close enought to what most small block boats do with the weight reduction.
Thus, assuming the engines are running right, the big question is if you can swing bigger wheels and still get to the desired WOT numbers.
Speed looks about right for those rpms thru what - 1.5 to 1 trannies?
Close enought to what most small block boats do with the weight reduction.
Thus, assuming the engines are running right, the big question is if you can swing bigger wheels and still get to the desired WOT numbers.
Kevin,
I normally only read here since I'm an aspiring Bert owner, not an actual owner. Props and speed have been a side interest of mine given my engineering background and diesel repower project I undertook on my boat a few years back. I agree with the others that your RPM issue should be confirmed and addressed before spending any real effort on propping.
Having said that, the Propellor Handbook by David Gerr, (a great read if your a techno geek like me), is the likely source of the no cup below 30 knots mindset. I was bitten by that comment as well and after talking to a lot of prop experts , including one that knows Dave personally, my impression is that there are more in favor of cupping than not, even at lower speeds. Your question about propellor size and pitch versus efficeincy is very appropriate. According to most, there are diameter to pitch ratio's that are more efficeint than others. Your props are almost identical to mine and the prop speed is also close. I have 17x14 Mich. DJ 3B med Cup ISO class I on my boat which has twin 200 hp 3800 RPM engines w/ 1.5 reduction. Our diameter to pitch ratio is not ideal. An over square ratio would be better but on my boat, I can't do it becuase of weight and blade loading (I weigh over 16000 lbs loaded). It seems you might be able to if your boat only weighs 12K#. The rule of thumb is 400 pounds per square inch of blade area... it might be worth checking your numbers.
I normally only read here since I'm an aspiring Bert owner, not an actual owner. Props and speed have been a side interest of mine given my engineering background and diesel repower project I undertook on my boat a few years back. I agree with the others that your RPM issue should be confirmed and addressed before spending any real effort on propping.
Having said that, the Propellor Handbook by David Gerr, (a great read if your a techno geek like me), is the likely source of the no cup below 30 knots mindset. I was bitten by that comment as well and after talking to a lot of prop experts , including one that knows Dave personally, my impression is that there are more in favor of cupping than not, even at lower speeds. Your question about propellor size and pitch versus efficeincy is very appropriate. According to most, there are diameter to pitch ratio's that are more efficeint than others. Your props are almost identical to mine and the prop speed is also close. I have 17x14 Mich. DJ 3B med Cup ISO class I on my boat which has twin 200 hp 3800 RPM engines w/ 1.5 reduction. Our diameter to pitch ratio is not ideal. An over square ratio would be better but on my boat, I can't do it becuase of weight and blade loading (I weigh over 16000 lbs loaded). It seems you might be able to if your boat only weighs 12K#. The rule of thumb is 400 pounds per square inch of blade area... it might be worth checking your numbers.
Yup, I agree with you guys Not going to spin my wheels chasing a ghost When I find a laptop I will check the WOT votage and RPM across the board Steyr guys tell me that the computer is actually a Ford computer, go figure, Europeans buying stuff from Ford Probably OBD II Still need to pput some hours on her anyways Sorry about the run ons,,,,,,,period button is broken
Kevin
Side note: 24 hours on the meter and the oil still so clean it is difficult to check the fill level Thought diesel always turned black pretty quick
Kevin
Side note: 24 hours on the meter and the oil still so clean it is difficult to check the fill level Thought diesel always turned black pretty quick
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