Under proped? thoughts?

[eseyoung]

I know that this has been talked about many times...and i have read a great deal on the topic but i would like specific comments about my situation.

Boat is a 27FC with the 78 Nissan.

for a cruising speed i am running 1800 at 1/3 throttle which gets me, in no current, about 6knts. I am able to hit the redline at slightly more than half throttle and that gives me 7-7.5knts. I am told by boat owners that the redline is 3K. The Nissan car people (ld28 is early/mid 80's Maxima) are saying 3800. Id rather error on the side of caution and call it 3.

I am not sure what pitch the prop is currently.

As for what i would like to accomplish, I am not necessary trying to gain cruising speed but i would like to have ability to run a little harder to fight currents when necessary. 6knots is OK but when i am fighting a 3knt current it makes for a slow ride and and somewhat limited maneuverability.

Not somethign that NEEDS done but just a thought.

[RobS]

You have to know what rpm she's rated at, your 800rpm swing makes this all guess work..

[eseyoung]

It is a bit of a matter of debate...I had been sticking with what the industrial application guys said. Since they are the only OEM parts source, i give their advice more credibility because they are an 'official' source and their rating is more conservative. the car guys (a bit of a cult following) can't agree 100%, some say 3500 and others say 3800.

that said, I have no doubt that i could get to 3800 with less than 3/4 throttle.

The LD28 in its marine form is a true redheaded stepchild. there are virtually no resources for marine application owners.

[Jay Knoll]

Eric

You might want to dig deeper in here to find out more specific information

http://nissandiesel.dyndns.org/viewtopic.php?t=197

[eseyoung]

Jay,

Thanks for the info! looks like that old gal is turning closer to 3800.

WOOOO i thought she was screaming at 3000. I will run her up this weekend and see what happens and note throttle position.

i have looked at this site before but not that specific post.

[eseyoung]

Well i did some digging in places i hadn't yet dug.

I am going to stick whit my original 3000 max rpm. unless someone can help me interpret this differently

[jleonard]

What does it turn for "high idle" or WOT in neutral? You should do this with the cable linkage disconnected so you know you are at full fuel.
That is what the pump is set for.
You probably want to run 300 or so rpm below that when under load. That's pretty typical.
The Bosch VE pump is easy to tune if you need to. I am very familiar with this pump as I had one on my Dodge/Cummins truck. Also had one on an old VW diesel. I made several adjustments on both, actually lots of them on the Cummins.
If running 300 below high idle it can be adjusted to make sure there is no smoke, that will make sure EGT stays low (and internal stress).
It's really not the rpm that hurts, it's the heat/stress that does.

[RobS]

Well i did some digging in places i hadn't yet dug.

I am going to stick whit my original 3000 max rpm. unless someone can help me interpret this differently

The automotive rating is not applicable here. Well the industrial rating is at max 2,600 so if you're propped to hit 3,000 and run her at between 1,800 and 2,200 I think she will serve you well. That is my "theoretical" input. Jay has a wealth of hands on experience so I would follow his recommendation on checks and balances.

[don123]

As to pitching the prop - If you never intend to run her at more than 3,000, you might need a little more prop for best performance. Since you can easily attain 3,000 with only slightly more than half throttle, you may have lots of wasted 'headroom' above 3,000. I think you should see what the boat/engine is capable of and then make a decision as to where to go from there. It certainly won't hurt anything to briefly run it up and see where your current max point is - The point where adding more fuel doesn't get you any more RPM. This will tell you what you need to know about your current prop. If it will run up to 3,500 or 3,750 and you desire to use 3,000 as your max, then re-propping to where you get a maximum of 3200 to 3300 will give you the best performance at 3,000 and below - You might even improve your overall economy cruising at 1,800 to 2,200 RPM

Ideally, you want the maximum attainable, power limited RPM to be a couple hundred above your desired max cruise, to ensure you're not bogging the engine down with too much prop. As an example, my Yanmar is rated at 4,000 intermittent max and 3,800 continuous. If I desire 3,500 to be my max, the ideal prop would be one that the engine/boat combination will allow 3,800 before adding throttle gets me no more RPM - That way, at 3,500 I still have some headroom left, but I'm propped to make the most of the engine's power

If you decide to re-prop, adding 1 inch of diameter is the equivalent of adding one more inch of pitch, according to the experts at Michigan Wheel. Adding diameter is more efficient than adding pitch, because you're getting a bigger 'bite' on the water. When I repowered, I went from a 15 to a 17 diameter and there is adequate clearance there for probably an 18 inch maximum diameter. I don't know what prop came on the LD28 equipped boats, but the 4D61's came with a Michigan Dyna-Jet 15 X 11 RH

Don

[eseyoung]

Interesting thing is that in its marine form it is rated at 78HP. Which is less than the automotive rating and more than the industrial rating. I have not been able to find anything specific to the marine version.


Either way, I opened it up this weekend and cruised at 2400 which, according to the above referenced document is in the power curve of the engine. Which was more comfortable as far as speed and my ability to run against the tide and in the slack tide increased my speed to 8 +/- knts. the engine sounded happy and had no issues with heat mitigation, smoke or other issues.

I am able to hit 3800 but that seems to be where the prop is limiting...as i come above the torque curve of the engine and go faster over hull speed am to the point of diminished returns to push much harder than 3000 for any reason.


Thanks for the information everyone.

[don123]

The automotive version was rated at 90 HP, but that was at 4600 RPM. I'm sure they wouldn't recommend anyone run it there consistently. I can't find any specifications for the power or torque numbers for marine engine usage either, but it is the same engine as the automotive version so your 78 HP was likely taken at 4,000 or possibly 3800 RPM, which is probably the design maximum consistent RPM for marine use, where it would be expected to run all day - It's certainly not making 78 HP at only 3,000 RPM

You can tell about how much power you're making by monitoring your fuel consumption - Any properly tuned diesel engine which burns 1 gallon per hour of fuel is producing approximately 18 HP when doing so - 36 HP for 2 gallons per hour and 72 HP for 4 gallons per hour. The 18 HP number at 1 gallon is probably about what you need to achieve hull speed. To go any faster, you'll need to move lots more water and doubling your fuel use will likely only get you 10 to 15% more speed, but it certainly won't hurt your engine any. It would probably be a good practice to run it up to 3000 or 3500 for 10 or 15 minutes on your way back to the dock after a day of 1800 to 2000 cruising

There was an LD-28T turbo charged version manufactured for stationary genset and also marine use and likely it had a higher power output and at a higher RPM

I assume you meant to say 3400 RPM and not 2400 when you say you 'ran it up' and if that's the case, I would say your current prop is very well matched to your engine power in your boat hull - Maxxed out at 3800 will give you a constant safe cruise at about 3500, which as you say isn't something you'll be doing often because you're pressing your boat well above it's hull speed and you're paying quite a price in fuel economy to do it

Don