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Old 05-15-2017, 12:28 PM   #1
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For smart guys and gals and problem solvers

The facts:

Cummins QSB 6.7 480 hp

When new with fuel, water and minimal supplies. 3350 rpm @ 23.2 knts. WOT

Now 51000 lbs fuel, water normal supplies . 3050 rpm 19.5 knts. WOT

Boat has Zeus pods so new props (2 / engine) is 4 props. $9100.00 to replace.



I want to get the boat to top rpm of 3350 again. The boat has always felt under powered to me. A friend has an East Bay (12 inches longer and 10000 lbs less w/o a bridge and the same maker put the same engines in his boat but they were rated at 550 hp. (140 hp more than my boat) and his performs much better.

I can get Cummins to rerate these engines to 550 hp for $10000.00 ( includes new water pumps at $1000.00 each).



To me it's a no brainer to rerate the engines ( I know it will use more fuel and that's not a factor). The boat now cruises at 2800 and 17.8 knts but I want it slightly higher at 3000 rpm and 3350 WOT.



What am I missing? What say you?

TIA
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Old 05-15-2017, 12:40 PM   #2
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"What am I missing? What say you?"


Re- pitch or re-prop the boat so that it can easily hit rated WOT rpm on a fully loaded boat on a hot and humid day.
After that run the boat at cruise rpm that you require to meet your speeds.
Adding Hp that you will never use is not a best solution - and if you require that extra 12.5% of hp to cruise I suggest you will have other issues before long.
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Old 05-15-2017, 12:53 PM   #3
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1. "When new with fuel, water and minimal supplies. 3350 rpm @ 23.2 knts. WOT

2. Now 51000 lbs fuel, water normal supplies . 3050 rpm 19.5 knts. WOT"


Do you happen to have EGT and boost readings for these two data points?
Do you happen to have fuel flow readings for these two data points?
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Old 05-15-2017, 01:10 PM   #4
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I do have those but would have to check to make sure, I do know at 100% throttle I am at 98% boost at 3950 WOT.

At 3000 rpm engines with gen the totalburn 42 gallons per hour.
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Old 05-15-2017, 02:09 PM   #5
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Originally Posted by Bigsfish View Post
I do have those but would have to check to make sure, I do know at 100% throttle I am at 98% boost at 3950 WOT."

Is that a typo - 3950- or is that at a no load / high idle) reading?
What is the rated WOT rpm on your engines?
The more data that you have on the engines the better to work with.


"At 3000 rpm engines with gen the totalburn 42 gallons per hour.
"
Would that be before as in number 1 above or later as in number 2 above?
Do you know what your genset draws in fuel use?
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Old 05-15-2017, 02:27 PM   #6
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How much weight have you added? 300rpm and 3kt drop is a lot of change.

Some semi-planing boats are super sensitive to weight. 51000lb is heavy for a 43.
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Old 05-15-2017, 02:54 PM   #7
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Smitty

It's now but would be the same I think because of fuel burn is a function of RPM. Gen burns 1-2 gph.
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Old 05-15-2017, 03:02 PM   #8
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Fuel burn is a function of load that is why it is not the same when out of gear as in gear at the same rpm. Fuel burn is not a function of rpm on diesels as they have a very large range in air fuel ratio based upon load.


What about the boost at 98% at 3,950 - is that the real rpm?
Is that in neutral" what is the rated rpm of the engines?
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Old 05-15-2017, 03:02 PM   #9
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Ski. Grand Banks listed the boat at 42,061 lbs but that was without the 800 pound dinghy and the had to add 1600 pounds of lead to the bilge to balance it out plus fuel, water, parts, tools and other supplies and it now weighs 50,743 on the travelift.

I think it was under powered from the beginning.
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Old 05-15-2017, 03:23 PM   #10
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Big fish when was the last time you changed the secondary fuel filters. When they get fouled you won't get full rpm.
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Old 05-15-2017, 05:41 PM   #11
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Quote:
Originally Posted by Bigsfish View Post
Ski. Grand Banks listed the boat at 42,061 lbs but that was without the 800 pound dinghy and the had to add 1600 pounds of lead to the bilge to balance it out plus fuel, water, parts, tools and other supplies and it now weighs 50,743 on the travelift.

I think it was under powered from the beginning.

It probably was a bit underpowered even at the start. But you have added a LOT of weight to an already heavy boat. Performance numbers don't surprise me.

Why is 1600lb of lead needed to balance a 800lb dink? On a planing boat the last thing you want to do is add weight.

You need a bigger boat or a smaller dink!!!
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Old 05-15-2017, 05:46 PM   #12
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"think it was under powered from the beginning."


FWIW - from my perspective I do not think its under powered. I do believe it is overpropped and by getting back to correct loading you will gain both engine life, efficiency, and some speed.


Without more detailed data lets just take a "WAG" based upon this statement of yours....
"It's now but would be the same I think because of fuel burn is a function of RPM. Gen burns 1-2 gph"

So the Wag would say that you are left with 40 gallons per hour for the mains when running at about 18.5 knots.
That would mean your hull requires about 760 hp or likely less to run at 18.5 knots (40 X 19 = 760). That would be about 380 hp per engine or about 80% of the engines rating at 480 hp.
The main problem I see so far is that the engines are not able to reach near rated rpm which will cause them to run less efficiently and potentially ate very high EGT's (not good). Also if you can not get near rated rpm you will never realize the full rated hp at any of the rpm range as well as WOT.
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Old 05-15-2017, 06:22 PM   #13
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Smitty

Good info. The boat did get the proper RPMs and speed when new with little weight and little fuel.

Btw the 1600 pounds of lead were to balance the boat including the dingy (a Rib with 40 hp) that weighs 800 pounds. Not sure how the screw up happened on Grand Banks end but the dingy is not excessive in my mind.

I think what you are saying reproping is better than turning up the hp.and will increase the speed.

Thanks.
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Old 05-15-2017, 06:24 PM   #14
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I am planning to take my prop down a little during the next bottom job. Right now I am short about 75-100 rpms at WOT on my QSC 8.3 540 Cummins and getting light soot on the transom at other than displacement speeds.

My 1989 45' Californian MY with 6-71 TIB's ran like a SAA after I took out some pitch. My last NT 42' with the 5.9 Cummins got much better fuel economy after the pitch was reduced. I had an old Taiwanese Albin with a 120hp Ford Watermota that ran better after I had the prop corrected.

In my recollection I have never seen a new boat that was not over propped from the factory. Don't know why this is.
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Old 05-15-2017, 06:37 PM   #15
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So I have been looking into having the pitch (not diameter) corrected on the props. Is that the consensus of this group?

I have found since these are pods with two props per engine I'm having trouble finding a prop shop that really understands the relationship of the bigger prop to the smaller prop. This may become a mission.
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Old 05-15-2017, 06:43 PM   #16
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Smitty

Sorry, I didn't answer your question. Yes it was a typo, should read 3050. So the boat has to find 200 RPMs.

Thanks.
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Old 05-15-2017, 06:50 PM   #17
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Air and fuel. Clean filters all around? Max design flow? Backpressure on exhaust?
Are you carrying a huge oversize anchor and chain?
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Old 05-15-2017, 07:28 PM   #18
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If you reduce pitch you will loose speed for a given rpm at slow speed (hull speed). It will not effect your hull speed but your engines will turn faster to get there. However, re-pitching the props is pretty close to science with all the data you have so your result is pretty much guaranteed.

Adding horsepower may or may not get you what you need. You may spend the $10,000 to add the HP and not get the 200 rpms you are searching for. If Grand Banks has data that insures you will get the rpms with the upgraded HP I would consider that route. You need the added power when you are loaded and want to run faster. I bet most of the time you will not be tapping that resource.

Bottom line is re-pitching helps your engines but not your need for speed. Adding HP gives you another dimension to tap into but does not require you use it.
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Old 05-15-2017, 08:31 PM   #19
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A2010

Clean air filter and fuel filters.
I'll have to check the backpressure of the exhaust if it is listed on Vessel View.
I don't think the anchors are heavy, I have two 60 pound anchors on the bow platform. One with 300 feet of 5/16 ht chain and the other with 69 feet and a nylon ride. I wouldn't boat with less.

Thanks.
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Old 05-15-2017, 08:43 PM   #20
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Thanks. I'll check with Grand Banks to see if they have an answer on the props. You are correct that I won't run at WOT other than to clean the props for a few minutes but I would sure like to see 3000 RPMs.

I wish you were correct on reworking the props being a science, I've been getting answers that seem strange as most shops don't seem to understand I have two props per engine one smaller than the other. Interesting.
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