question about de-rating dd?

[nor'easter]

I was thinking about this as I rolled out of bed this am.
This is a hypothetical situation on a 50-54 Hatt/Bert C with DDs. I have been casually looking for a while, and all boats I have looked at are DD powered. Never being able to meet and know the owners running habits always makes me weary of the "super clean, turnkey, fish today" rig that may go boom mechanically in 200 hrs.
If one were to de-rate by underpropping (not excessively) to unload a set of DDs that ran well, but had a high hp/cid rating....what would happen to fuel burn? I assume it would go down at a given rpm, and so would speed.
I have 692s at 565 and run them 1600-1850 and they treat me well. I have seen and heard discussions about derating a 92 by underpropping it to get better longevity...all else being equal.
When I run my boat light, or not in a big sea, I see lower load #s (DDEC) and small but noticeable drop in fuel useage. I assume it would be the same with underpropping, but you would also see a drop in speed.
Just the random thought of a boat lover/shopper.
Thanks

 

[ThirdHatt]

There was a thread on this subject within the last couple of months that ran several pages and had some excellent information.

Search for a thread named "Larger Props For Efiency And Engine Life When Running Big Engines Slow" by Brian Degulis, a very knowledgeable member with a 61MY that did what you are thinking about to his boat.

 

[Genesis]

Just take 1-2" out of the props.

That is, quite seriously, all you need to do.

 

[nor'easter]

Thanks,
Thirdhatt, I remember the thread and looked back through it. I am pretty sure, unless as I am confused, that the situation would be the opposite. If I recall Brian wanted to put big wheels on to load the engines at a lower rpm and get more efficiency at displacement speeds rather than run raster.
I am curious about using less pitch to unload them and still run on plane at cruise speed. The primary goal would be to get more life out of a "pumped up" DD with a service life I didn't know before purchase....assuming surveyed out well.
I know that speed will drop, I was just curious about a drop in fuel consumption.
Genesis, that is what I thought, but I didn't know if there was any guidance about how much pitch you need to take out to derate them sufficeintly to take it easier on them.

 

[Brian Degulis]

Before you do that take a look at an engine/prop power curve for your engines. You will see very quickly that the only time your using the available HP is at WOT. So as soon as you back of the throttle you are in fact under loaded. If you remove pitch you will be even more underloaded and you will need to turn higher RPM to obtain a given speed. The down side of a fixed pitch proppler is that it is underloaded at anything below WOT. With de pitching you make that problem worse you will lose fuel efiecency and shorten engine life. As long as your making full rated RPM at WOT then the best way to take the strain off is to simply throttle back. There is no point to Raising the RPM and lightening the load when it's already under loaded. This is more about the prop and the way it absorbs power than it is about the engine.

Brian

 

[Maynard Rupp]

Don't try that with gas engines. You need as low a pitch you can stand. If you have too much pitch they will detonate and the boat will slow down. I guess that happens because gas engines have less torque especially at lower RPMs.

 

[REBrueckner]

I agree precisly with Brian's description. I see no advantage to decreasing prop size...it's likely less efficient.

Instead of running at a given RPM with smaller props, as you are thinking, just throttle back a hundred or two hundred RPM. As you already imply in your post, you know in your heart that running a DD at say,1800 RPM is a LOT easier on the engine that trying to squeeze out a few more knots at 2050 RPM....

 

[nor'easter]

Thanks all,
I guess I arrived at this answer with my current boat already. I am happy with my cruise at 18-20 kts and that is 1650-1800 for my 39 w/692s. My engines run great and are well taken care of. A few months after I surveyed and bought her, I met the captain of a 54 Bert that new my boat and her prior owner. He said he was a speed demon. I am not sure if he saw my heart sink or not, but I don't think he was trying to beak my chops.
So, thinking about the next boat had me thinking about this topic. Power will likely be bigger, and problems more expensive. Assuming they survey well, and turn up to/past rated rpm maybe the answer is the same one I came to on my own.
Thanks for the thoughts/experience.

 

[REBrueckner]

When buying any boat, during the general discussion with the present owner, always ask what speed they run...maybe how many hours at that speed each year...then come back after some other discussion and inquire about what RPM the owner normally runs....you'll get some good insights on what really happened or a real contradiction. If the answer don't mesh, you can always compare the owners annual reported hours with total fuel utilized to gauge his REAL operation. ...and whether the owner can be trusted or even knows what's going on....most don't really know...

 

[Brian Degulis]

If for some reason (like maybe a hired captain) you wanted to prevent the engines from being run excessively hard you could simply re shim the governer from 2300 to 2100. That would make it impossible for anyone to operate the boat at 100% out put. You would need to be carefull as some boats need full rated power to come up on plane. You also need to know exactly where you are at now. You would need to confirm that when you are at WOT you are turning rated RPM. You also need to confirm that the governer not the prop load is holding the engine at rated RPM by not more than around 50 RPM.

So you would need to run them up to WOT confirm your reaching rated RPM then manualy try to push the rack up (this can be done by removing the buffer screw and pushing in on the fuel lever). If you cant' get any more RPM then the props are holding the engine at full rated RPM and you need to reduce pitch a little or look for a problem in the engine. If you can get more RPM by pushing the rack it means the governer is holding the RPM. If you can raise the RPM by more than 50 it means the prop load is to light to utilize full rated power.

Brian

 

[krush]

Brian, shimming the gov (lets say 2400 to 2000) just prevents the engine from reaching 2400 rpm. It doesn't prevent the rack from fully opening the injectors when getting up on plane, does it?

 

[nor'easter]

Rob,
I never met the owner of my boat. He was a long distance owner and hadn't been on the boat in a LONG time.
Might be a different situation next time around....I hope so.
I will be the one running it, so i am not worried that I need to protect it from a heavy left handed captain.
As an aside, the 53 vs 55 thread brought another question up for me. The 55 was equipped with 1271s 650 and 840 hp I think, as well as 1292s later on. The same boat with different power will need the same hp(roughly) to run at the same speed. If a 1271 boat runs at 17kts at 1800, the the pumped up 1271 boat and the 1292 boat should run at the same speed with a lower rpm and similar fuel burn. Same hull at same speed uses same hp....and same fuel!? Do I have this right? Taking it easier on bigger power can work out, all other things being equal.?
Paul

 

[Brian Degulis]

Brian, shimming the gov (lets say 2400 to 2000) just prevents the engine from reaching 2400 rpm. It doesn't prevent the rack from fully opening the injectors when getting up on plane, does it?

No it won't prevent it from happening but without putting the RPM to the prop it probably won't require enough HP to cause the rack to open. If the prop requires full rated power at 2400 RPM which it typicaly does. Then it requires a lot less HP at 2000 and it's not like a truck that's got a solid conection to the road. Even though the boats struggling to get up on plane it's really anyones guess weather or not that's going to cause the prop to require enough HP.

Here's a good example the 50' towboat we build used to be powered with 871s. To get the right prop size we would put that boat up against a seawall and manually push the rack to full fuel. It's a towboat so when up against a wall it still has to turn rated RPM 1800 continous. After doing that and getting the right prop size if you took the boat out and just let it run free and pushed the rack up you got only around 100 RPM more. So standing still and just moving water under the boat or running free at 10 kts only a 100 RPM diffrence.

Brian