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Old 06-06-2019, 12:23 PM   #21
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Larry:

Sorry I forgot to go over this point in detail when we were together. I think your surveyor was a bit obtuse. He asterisked the wot rpms which indicated it was a problem, but he never said so explicitly.

If an engine is over propped that means that the prop is demanding more hp than the engine can make at rated rpm so the engine slows down until the hp required by the prop equals the hp it can make. In your case the engine could only get to about 2,850 rpm. That also means at lower rpms it is lugging, just like driving a car up a hill in too high a gear. Downshift a gear or two and the engine speeds up and is much happier. The same is true of boat engines, raise the wot rpm to a bit over 3,000 (to allow for future bottom growth) and the engine will be happier even at lower rpms. A happier engine will last longer.


When we were delivering your boat it would have been better to slow it down even below the 2,450 rpm we ran to put less load on it. But the boat seemed to drop off plane a bit as the speed dropped significantly when I reduced the rpms to 2,300. I think 2,300 resulted in 12-13 kts. That implied that the lower speed was probably not on a clean plane and was pulling even more hp than it should.

So I chose 2,450 as a compromise between low enough and to let it stay on a clean, efficient plane.

Having said all of that, buy a phototach. Amazon and eBay sells them for about $20. You then put a strip of white tape on the flywheel, then rev the engine (use ear plugs) out of gear until it shows 3,000 on the helm tach and read the phototach. Then mentally (or maybe those Flowscan tachs have adjustments) adjust any reading you get on the helm tach by that same percentage. Do this for both tachs and engines.

Then take the boat out, warm it up and then advance the throttle to wot and see what rpm on each engine you get (adjusted by the phototach factor) and speed. Then go to a good prop shop in your area and give them that data, the diameter, pitch and number of blades on your props and see if they can rework the props to reduce the pitch. A rule of thumb is an inch of pitch reduces wot rpms by 100 and the most you can change is 2", so it just might work for you. Otherwise you have to buy new props.

Can you use the boat the rest of the season? Yes, but keep it down to 2,450, and don't run between 1,800 and 2,300 because that speed is the getting over the hump speed or not such a clean plane like I said above.

In doing so you are taking a bit of life out of your engine, but you decide. Boas are made for fun. If you run it below 1,800 you are so far below the wot speed that it won't matter how it is propped. And you will save lots of fuel.

When you do repitch the props you will notice that the engine has to rev to say 2,550 to reach 15 kts. That is fine. The engine will be happier turning a bit faster to make the same hp as before, just like downshifting a gear.

The other thing to do which will pay out tremendously if your engine ever overheats is to add an exhaust gas temp or exhaust gas flow alarms. Borel make the former and Aqualarm makes the latter. I like Borel's alarm because it has no moving parts to stick and is easy to install.


You wrap a narrow sensing strap around the exhaust hose just below the mixer and then wire it to the alarm on the helm. You do have to find a source of DC power at the helm, but there is lots of stuff powered up there and you may find a DC supply terminal strip underneath the helm. The alarm goes off when the hose hits 165 F and that gives plenty of time to react and back off the throttle.


Without that alarm you have to watch the engine temp gauge rigorously and in some cases you can do damage to an engine by losing raw water flow even though the coolant temp gauge barely moves, particularly at high speeds and engine loads.


You have a pair of one of the best engines made, but they can put out a lot of power, so protect them with that alarm.


David
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Old 06-06-2019, 01:27 PM   #22
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Thanks for the information. Funny how my surveyor did not mention any of this as I may have been able to negotiate this to be completed before closing.
I do not know what you hired the surveyor to do but unless he was able to seatrial the boat and do a WOT test he never would see this. Also he would have had to now exactly which engines ahead of time and be familiar with diesels as they are transplants I believe.

You are in a great area to boat - many places to go and see. Are you going to utilize Mt Sinai as your base for this season?
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Old 06-06-2019, 01:44 PM   #23
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Larry:

Sorry I forgot to go over this point in detail when we were together. I think your surveyor was a bit obtuse. He asterisked the wot rpms which indicated it was a problem, but he never said so explicitly.

If an engine is over propped that means that the prop is demanding more hp than the engine can make at rated rpm so the engine slows down until the hp required by the prop equals the hp it can make. In your case the engine could only get to about 2,850 rpm. That also means at lower rpms it is lugging, just like driving a car up a hill in too high a gear. Downshift a gear or two and the engine speeds up and is much happier. The same is true of boat engines, raise the wot rpm to a bit over 3,000 (to allow for future bottom growth) and the engine will be happier even at lower rpms. A happier engine will last longer.


When we were delivering your boat it would have been better to slow it down even below the 2,450 rpm we ran to put less load on it. But the boat seemed to drop off plane a bit as the speed dropped significantly when I reduced the rpms to 2,300. I think 2,300 resulted in 12-13 kts. That implied that the lower speed was probably not on a clean plane and was pulling even more hp than it should.

So I chose 2,450 as a compromise between low enough and to let it stay on a clean, efficient plane.

Having said all of that, buy a phototach. Amazon and eBay sells them for about $20. You then put a strip of white tape on the flywheel, then rev the engine (use ear plugs) out of gear until it shows 3,000 on the helm tach and read the phototach. Then mentally (or maybe those Flowscan tachs have adjustments) adjust any reading you get on the helm tach by that same percentage. Do this for both tachs and engines.

Then take the boat out, warm it up and then advance the throttle to wot and see what rpm on each engine you get (adjusted by the phototach factor) and speed. Then go to a good prop shop in your area and give them that data, the diameter, pitch and number of blades on your props and see if they can rework the props to reduce the pitch. A rule of thumb is an inch of pitch reduces wot rpms by 100 and the most you can change is 2", so it just might work for you. Otherwise you have to buy new props.

Can you use the boat the rest of the season? Yes, but keep it down to 2,450, and don't run between 1,800 and 2,300 because that speed is the getting over the hump speed or not such a clean plane like I said above.

In doing so you are taking a bit of life out of your engine, but you decide. Boas are made for fun. If you run it below 1,800 you are so far below the wot speed that it won't matter how it is propped. And you will save lots of fuel.

When you do repitch the props you will notice that the engine has to rev to say 2,550 to reach 15 kts. That is fine. The engine will be happier turning a bit faster to make the same hp as before, just like downshifting a gear.

The other thing to do which will pay out tremendously if your engine ever overheats is to add an exhaust gas temp or exhaust gas flow alarms. Borel make the former and Aqualarm makes the latter. I like Borel's alarm because it has no moving parts to stick and is easy to install.


You wrap a narrow sensing strap around the exhaust hose just below the mixer and then wire it to the alarm on the helm. You do have to find a source of DC power at the helm, but there is lots of stuff powered up there and you may find a DC supply terminal strip underneath the helm. The alarm goes off when the hose hits 165 F and that gives plenty of time to react and back off the throttle.


Without that alarm you have to watch the engine temp gauge rigorously and in some cases you can do damage to an engine by losing raw water flow even though the coolant temp gauge barely moves, particularly at high speeds and engine loads.


You have a pair of one of the best engines made, but they can put out a lot of power, so protect them with that alarm.


David

Hello David - I agree with everything you say maybe expect for some of these:
"I think 2,300 resulted in 12-13 kts."
I agree - those shorter narrower hulls with the diesels tend to drop the stern a lot at lower speeds and cause that issue.

"You then put a strip of white tape on the flywheel, then rev the engine (use ear plugs) out of gear until it shows 3,000 on the helm tach and read the phototach."
Agree - also rev up and down (ramp) a bit slower than you would normally think. Record the top rpm on each engine - this should be about 300 in excess of rated WOT rpm. Your rating is 3,000 so high idle would be 3,300.

"Can you use the boat the rest of the season? Yes, but keep it down to 2,450,"
Do not agree - by the end of the season that 2,450 rpm might be a firewalled number. I guarantee the rpm's will drop due to increasing loads adding heat and stress over the next weeks/months.

"You have a pair of one of the best engines made,"
The 6B is also sensitive to internal heat and so is a delicate engine when it is rated at this HP and when utilized in an overloaded condition.
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Old 06-06-2019, 02:16 PM   #24
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Smitty said:

""Can you use the boat the rest of the season? Yes, but keep it down to 2,450,"
Do not agree - by the end of the season that 2,450 rpm might be a firewalled number. I guarantee the rpm's will drop due to increasing loads adding heat and stress over the next weeks/months."

Smitty has a good point. I would use those engines at 2,450 only if the wot rpms stayed at 2,850. Presumably that high speed would keep some of the bottom slime off, but who knows what the bottom paint is like.

I think Larry will probably mostly use the boat at displacement speeds. 24-27 gph and $500+ fill ups was an eye opener. That is why we like slow trawlers, right!!!!

David
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Old 06-06-2019, 07:40 PM   #25
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I am researching for a prop shop. I have one local but I am not sure if there work is good. Not sure what the turn around time would be? Also need to find out where I can get a haul out. I don’t plan on going faster than 10 knots or haul speed I will be well under the 2450 rpms so if I get some crazy prices it will wait till end of season when I haul out for the winter.
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Old 06-06-2019, 08:37 PM   #26
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Not exactly near you but if you don't get other recommendations...
I can recommend Precision Propellers in Poughkeepsie. I have dealt w Gabe on a few occasions and I have been very pleased. He worked miracles on a set of IPS props and that made me a believer.
https://www.precision-props.com/
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Old 06-06-2019, 10:08 PM   #27
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Well if I haven't missed something, the PO ran the boat that way for 10 years since repower. Hard to imagine how the 11th summer is gonna be the deal killer.
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Old 06-07-2019, 05:06 AM   #28
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I do not know what you hired the surveyor to do but unless he was able to seatrial the boat and do a WOT test he never would see this. Also he would have had to now exactly which engines ahead of time and be familiar with diesels as they are transplants I believe.

You are in a great area to boat - many places to go and see. Are you going to utilize Mt Sinai as your base for this season?
I hired both a boat surveyor and engine surveyor. The boat was sea trialed at wot.
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Old 06-07-2019, 05:29 AM   #29
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I am researching for a prop shop. I have one local but I am not sure if there work is good. Not sure what the turn around time would be? Also need to find out where I can get a haul out. I don’t plan on going faster than 10 knots or haul speed I will be well under the 2450 rpms so if I get some crazy prices it will wait till end of season when I haul out for the winter.
At 10 knots you should be fine - check the pyros at 10 knots they may not even be off the post.
FWIW - at 10 knots your fuel efficiency may be very near that of 16 knots.
At 8 knots you will likely get almost double the nmpg.
And at 6 knots you will likely get almost triple the mileage from 10 knots.
It would be great of you could plan some of your travel around the Long Island tides/currents as well - sometimes its easy to do that.
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Old 06-07-2019, 05:31 AM   #30
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I hired both a boat surveyor and engine surveyor. The boat was sea trialed at wot.
Then the survey should contain the strobed and corrected rpm at WOT.
It would typically state the rated rpm and all gage readings as well as any comments on the deviations seen.
It should also have a baseline for all the gages at a couple or more rpms.
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Old 06-07-2019, 05:38 AM   #31
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Well if I haven't missed something, the PO ran the boat that way for 10 years since repower. Hard to imagine how the 11th summer is gonna be the deal killer.
That would depend a whole lot on how those 650 hours were previously run, whether or not the same props were in use the previous 650 hours, and what the current blow by test and other engine baselines set the current condition in.
This boat with healthy 370 engines and correct propping should be near 24 knts at WOT.
This post does not contain enough information to determine any reasonable conclusions on current condition of engines or anything about the props themselves. We cannot even confirm that the gage readings are correct at this point in time - so much of this is a best guess at this time.
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Old 06-07-2019, 07:01 AM   #32
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That would depend a whole lot on how those 650 hours were previously run, whether or not the same props were in use the previous 650 hours, and what the current blow by test and other engine baselines set the current condition in.
This boat with healthy 370 engines and correct propping should be near 24 knts at WOT.
This post does not contain enough information to determine any reasonable conclusions on current condition of engines or anything about the props themselves. We cannot even confirm that the gage readings are correct at this point in time - so much of this is a best guess at this time.
Apologies....
I have gone back and reread this post as well as opening the listing and reading that.
It may or may not be a prop issue - a very poor move on my part to just assume that the boat and engines were in good to best shape as a result of my original reading.

After rereading the post and listing here are some new questions:
- Did the boat do 20 knots at cruise and 26 knots top speed at Seatrial?
- Did the boat ever do 20 knots cruise an d 26 knots top?
- Were the above speeds achieved with the same props as on the boat now?
- Are the trim tabs operational?
- Are the tachs know accurate? (mech survey)
- What were the gage readings at Seatrial?
- Did the mechanical survey include a blow by test? A high idle rpm check? A boost test? etc?
- Why were the aftercoolers etc just recently serviced?
- What other parts were recently replaced?
- On the trip home what did the hull/rpm do with extended tabs?
- What other mechanical maintenance records does the boat have?

These would allow you to see if the prop slips are/were in the ballpark for the boat...
- What are the prop specs? (D X P X # blades by material)
- What are the transmission ratio's
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Old 06-07-2019, 07:17 AM   #33
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Smitty said:

""Can you use the boat the rest of the season? Yes, but keep it down to 2,450,"
Do not agree - by the end of the season that 2,450 rpm might be a firewalled number. I guarantee the rpm's will drop due to increasing loads adding heat and stress over the next weeks/months."

Smitty has a good point. I would use those engines at 2,450 only if the wot rpms stayed at 2,850. Presumably that high speed would keep some of the bottom slime off, but who knows what the bottom paint is like.

I think Larry will probably mostly use the boat at displacement speeds. 24-27 gph and $500+ fill ups was an eye opener. That is why we like slow trawlers, right!!!!

David
"I think Larry will probably mostly use the boat at displacement speeds. 24-27 gph and $500+ fill ups was an eye opener. That is why we like slow trawlers, right!!!!"

FWIW - our larger boats always used much less than those figures at the same cruising speeds in LI sound over maybe 25 seasons. If the boat is reasonably clean with tuned engines and well sized props I find the fuel use to be one of the lower costs of boat ownership. I do not know that hull form so comparisons will be somewhat inaccurate but these should have some value. We cruised 2 - 45' boats and one 47' boat (approx. 35,000#) mostly in Long Island sound
and the Hudson traveling at 15-17 knots for about 800-900 of our typical 1,000+ nm total season. These boats all had similar mechanical diesels - 6 cyl, 5.9L, turbo, aftercooled, and 3,000 rated rpm.
With a little rounding and a bit of nudging for ease of memory these are the typical fuel burns we would see over many seasons use:

6 kts - 3+ nmpg
8 kts - 2 nmpg
10 knts - 1 nmpg
16 knts - 0.92 - 0.95 nmpg
Towing a larger RIB at 16 knts - 0.90 nmpg

IMHO - We would hesitate to cruise the boat or those 5.9 engines at a 12+ gph rating. I would specifically avoid cruising the 5.9 engines at 12+ gph at 2,450 rpm.
Our expected fuel use at 16 knots and about 2,500 rpm was right about 17-18 gph between two engines or between 8-9 gph per engine.
If we were not going to travel at 6 knots then we would just about always move it up to 15-17 knts.
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Old 06-07-2019, 08:55 AM   #34
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The following is a little geeky, maybe a lot geeky, but I am a geeky kind of guy:

Larry's boat used 12 gph per engine to go 15 kts. This was the result of a fill to fill measurement.

Looking at the Cummins data sheet for this engine, 12 gph is 225 hp. If it were propped correctly I believe 225 hp would be fine for long term cruising use. Tony Athens advised me personally to that effect when I was considering buying a boat with a Cummins 6BTA. That is 38 hp per liter and is just under Tony's magic 40 hp per liter max for continuous duty.

In its over propped state 225 hp is certainly high, because it is making that 225 hp at a lower rpm than Cummins specifies in its data sheet. Making the same hp at a lower rpm means the torque is higher so the engine stress is higher.

But recreational boat engines are made for having fun and if going 15 kts for a few dozen hours over the summer makes it more fun, well ok. It will result in a little bit less engine life, but as pointed out by a PP it is probably no worse than the previous 10 years since its repower.

Actually I think installing exhaust gas temp or low r/w flow alarms is more urgent than repropping. The current prop situation isn't by itself going to blow the engine but an undetected overheating situation could.

How about fuel economy. Well 24 gph total at 15 kts is .625 NM/gallon, pretty high.

Running at 10 kts and below should be even better for this boat. Fuel economy will be better at 10, much better at 8 and the engine will be happy at either speed. My SWAG is about 1 NM/gal at 10 kts and maybe 1.5 at 8 kts. That is where Larry reported he plans to run the boat until he corrects the prop pitch.

David
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Old 06-07-2019, 09:23 AM   #35
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The following is a little geeky, maybe a lot geeky, but I am a geeky kind of guy:

Larry's boat used 12 gph per engine to go 15 kts. This was the result of a fill to fill measurement.

Looking at the Cummins data sheet for this engine, 12 gph is 225 hp. If it were propped correctly I believe 225 hp would be fine for long term cruising use. Tony Athens advised me personally to that effect when I was considering buying a boat with a Cummins 6BTA. That is 38 hp per liter and is just under Tony's magic 40 hp per liter max for continuous duty.

In its over propped state 225 hp is certainly high, because it is making that 225 hp at a lower rpm than Cummins specifies in its data sheet. Making the same hp at a lower rpm means the torque is higher so the engine stress is higher.


But recreational boat engines are made for fun and if going 15 kts for a few dozen hours over the summer makes it more fun, well ok. It will result in a little bit less engine life, but as pointed out by a PP it is probably no worse than the previous 10 years since its repower.

How about fuel economy. Well 24 gph total at 15 kts is .625 NM/gallon, pretty high.

Running at 10 kts and below should be even better for this boat. Fuel economy will be better at 10, much better at 8 and the engine will be happy at either speed. My SWAG is about 1 NM/gal at 10 kts and maybe 1.5 at 8 kts. That is where Larry reported he plans to run the boat until he corrects the prop pitch.

David

"In its over propped state 225 hp is certainly high, because it is making that 225 hp at a lower rpm than Cummins specifies in its data sheet. Making the same hp at a lower rpm means the torque is higher so the engine stress is higher."
And the EGT is over the limit with unknown pyro accuracy, boost is high - lower rpm HP extraction is always a tight edge to walk.

"But recreational boat engines are made for fun and if going 15 kts for a few dozen hours over the summer makes it more fun, well ok."
My experience with the 6B has led me to conclude that utilizing these engines over the edge will lead to failures in the hundreds of hours and not thousands. There are at least 3 persons on this site that have replaced there 6B's due to these reasons and many more I have been in touch with over the years at other sites - they have sent me similar rpm, speed , boost and EGT data with some just above these readings having dire consequences.

"It will result in a little bit less engine life, but as pointed out by a PP it is probably no worse than the previous 10 years since its repower."
Please advise these details about the previous 10 yrs / 650 hours:
- were they run with the same props?
- are the tachs know accurate?
- did they achieve the posted 26 knts WOT?
- did the engines cruise at 20 kts?
- was the boat ever run on plane?
- what # hrs was the boat run on plane?

I know almost nothing about this boat and these engines at this time so it is very hard to agree or disagree with anything.
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Old 06-07-2019, 09:32 AM   #36
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The following is a little geeky, maybe a lot geeky, but I am a geeky kind of guy:

Larry's boat used 12 gph per engine to go 15 kts. This was the result of a fill to fill measurement.

Looking at the Cummins data sheet for this engine, 12 gph is 225 hp. If it were propped correctly I believe 225 hp would be fine for long term cruising use. Tony Athens advised me personally to that effect when I was considering buying a boat with a Cummins 6BTA. That is 38 hp per liter and is just under Tony's magic 40 hp per liter max for continuous duty.

In its over propped state 225 hp is certainly high, because it is making that 225 hp at a lower rpm than Cummins specifies in its data sheet. Making the same hp at a lower rpm means the torque is higher so the engine stress is higher.

But recreational boat engines are made for having fun and if going 15 kts for a few dozen hours over the summer makes it more fun, well ok. It will result in a little bit less engine life, but as pointed out by a PP it is probably no worse than the previous 10 years since its repower.

Actually I think installing exhaust gas temp or low r/w flow alarms is more urgent than repropping. The current prop situation isn't by itself going to blow the engine but an undetected overheating situation could.

How about fuel economy. Well 24 gph total at 15 kts is .625 NM/gallon, pretty high.

Running at 10 kts and below should be even better for this boat. Fuel economy will be better at 10, much better at 8 and the engine will be happy at either speed. My SWAG is about 1 NM/gal at 10 kts and maybe 1.5 at 8 kts. That is where Larry reported he plans to run the boat until he corrects the prop pitch.

David
"Actually I think installing exhaust gas temp or low r/w flow alarms is more urgent than repropping. The current prop situation isn't by itself going to blow the engine but an undetected overheating situation could."

If we knew the pyros were installed with the tips correctly, if we knew they were reading immediately after the turbine , if we knew they were accurate we would know he is now on the edge at 2,450 (actually a bit over).
Since we all know that there are many ways this will only get worse over the next weeks and months there is no real margin IF anyone should advance the throttles.
Temp differences between the post turbine location where I am guessing these senders are located and the actual internal engine temps will be in excess of 350 degrees. I have seen cummins 6b's that have these post turbine temps rise near 1,000 have issues quickly - so knowing whether the gage is accurate or not when currently already at or over 900 is a focus.
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Old 06-07-2019, 09:39 AM   #37
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Originally Posted by djmarchand View Post
The following is a little geeky, maybe a lot geeky, but I am a geeky kind of guy:

Larry's boat used 12 gph per engine to go 15 kts. This was the result of a fill to fill measurement.

Looking at the Cummins data sheet for this engine, 12 gph is 225 hp. If it were propped correctly I believe 225 hp would be fine for long term cruising use. Tony Athens advised me personally to that effect when I was considering buying a boat with a Cummins 6BTA. That is 38 hp per liter and is just under Tony's magic 40 hp per liter max for continuous duty.

In its over propped state 225 hp is certainly high, because it is making that 225 hp at a lower rpm than Cummins specifies in its data sheet. Making the same hp at a lower rpm means the torque is higher so the engine stress is higher.

But recreational boat engines are made for having fun and if going 15 kts for a few dozen hours over the summer makes it more fun, well ok. It will result in a little bit less engine life, but as pointed out by a PP it is probably no worse than the previous 10 years since its repower.

Actually I think installing exhaust gas temp or low r/w flow alarms is more urgent than repropping. The current prop situation isn't by itself going to blow the engine but an undetected overheating situation could.

How about fuel economy. Well 24 gph total at 15 kts is .625 NM/gallon, pretty high.

Running at 10 kts and below should be even better for this boat. Fuel economy will be better at 10, much better at 8 and the engine will be happy at either speed. My SWAG is about 1 NM/gal at 10 kts and maybe 1.5 at 8 kts. That is where Larry reported he plans to run the boat until he corrects the prop pitch.

David

"Tony Athens advised me personally to that effect when I was considering buying a boat with a Cummins 6BTA. That is 38 hp per liter and is just under Tony's magic 40 hp per liter max for continuous duty."

It has been a while but I had spoken to Tony often in the past and agree with everything he has done with and about the Cummins 6b.
If you take the above quote and add this to it:
"as long as the engines are propped so as to always achieve rated WOT rpm + 3-5%"
Then I agree.
Leaving out that part of his 'rule' is not a good idea as it skirts the key method to avoid the problems with running near those limits.
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Old 06-07-2019, 09:44 AM   #38
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Interesting conversation and I think we have scared Larry off of ever running those engines again in their current over propped state at 2,450 rpm.


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Old 06-07-2019, 09:49 AM   #39
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Quote:
Originally Posted by djmarchand View Post
Interesting conversation and I think we have scared Larry off of ever running those engines again in their current over propped state at 2,450 rpm.


David
I no longer can say the boat is over propped , I do not know enough…..
Knowledge can reduce and/or correct problems and fears.
Do we know any of the important specs about that boat and the engines that are posted above?
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