The results are in on new props

[bobk]

We just completed the annual trip from Baltimore to Port St. Lucie, FL on the 1981 48MY with 6V92's (425hp). Encouraged by Brian's results, I had the spare props repitched from 26X22 (with a cup) to 26X23.5 without a cup. Hatteras literature for the boat suggested the original wheels were 26X23 so I was not aggressive but many 48 MY owners are using 26X22.

The boat was fully loaded as a liveaboard, lots of tools and three adults. Prior to changing the props I cruised at 7.8 knots at 1000 rpm and had a hard time getting up on plane. With the new props, we achiece 2300 rpm, max speed of 18.5 knots and can hold 14.5-15 at 1950 rpm. It falls off fast below that.

The real data that I suspect others will be interested in is the fuel burn. At the same 1000 rpm I'm running 8.3 knots, the engine temps are up a little bit and I calculate the fuel burn is 1.5 smpg vs 1.2 on the old props. The entire trip covered 1092 nautical miles (gps odometer) in 141 hours on 878 gal of fuel; 6.23 gph. However we anchored 13 days so I estimate 30 gal or so for generator burn. Allowing for the generator, it is 6.0 gph for the engines.

I should add that the transom is the cleanest it has even been for this length of trip. Just barely a dusting of grey. Not sure if this is related to the props or the use of MMO plus Howes MPK.

Regards,
Bob
Chateau de Mer
1981 48MY

 

[Walter P]

Hello Bob:

Thanks for the information. I believe you failed to add into the mix however. Do your figures factor in those periods that you run the engines up to clear them out?

I wish you had FloScans. I have them, but I haven't made the trip yet with this boat (sistership - '82, Series 1 48 MY).

With your new props, what speed do you make at 1150-1200 rpms?

Talk to you soon.....

Walt

 

[Brian Degulis]

It would be logical to see less soot on the transom because the engines are loaded a little more at low RPM resulting in a cleaner burn.

Brian

 

[bobk]

Hello Bob:

Thanks for the information. I believe you failed to add into the mix however. Do your figures factor in those periods that you run the engines up to clear them out?

I wish you had FloScans. I have them, but I haven't made the trip yet with this boat (sistership - '82, Series 1 48 MY).

With your new props, what speed do you make at 1150-1200 rpms?

Talk to you soon.....

Walt

Walt, I push them up once a day if I think of it, otherwise it might go two days (typically 10 hour days). I usually see the smoke clear in a couple of minutes and then give it an additional couple of minutes and pull them back down. So yes, the overall fuel burn includes the whole deal. I did see one leg where the burn was up.... we ran at 1950 for thirty minutes to make a bridge and then followed a shrimper at 9 knots for several hours, 1150-1200 rpm, in order to get through the bridges in NC. MPG for that stretch dropped to an estimated just over 1mpg. 1200 gets to about 9 knots, but probably 9 gph. Looked at as I do, the extra 0.7 mph costs well over two gph. I'm too cheap to do it long term.

Overall, I believe I got a win win situation. It gets tempting to add another half inch of pitch....

Regards from warm sunny Florida,
Bob

 

[Brian Degulis]

If your like me and run below 10kts just about all the time then you should consider adding more like 2". You might even consider larger dia? You could probably gain a free Kt or 2 and eliminate the need to run them up and clear them out.

Brian

 

[Walter P]

Bob, You keep this stuff up and I'll have to consider following your lead. I hate to be un patriotic and not spend more $ however. Seriously, Brian may have a good point.... add another 2" of pitch. Might be worth a try on your trip north in the spring. (that way I'll have the summer to get ready for our joint trip south next year)

Since running at higher speeds (planing) is not important, I wonder if removing the turbos and installing smaller injectors would be a good idea. While we're at it, remove the aftercoolers as well. These changes will also mean less maintainance.

Walt

PS Diesel today at Atlantic Highlands Municipal Marina was $3.05 gal, less 5 cents gal if you take over 200 gals.

 

[Bill Allen]

Well thanks to the politicains I'm stuck here for this year now next years another story, If there's any country left. I'm puttin my 24 x 26 wheels back on the hatt. She'll still plane with them I actually thought about another inch of pitch since who the hell can aford to go on plane when travelling to Fl. from N.J. Bill

 

[krush]

Since running at higher speeds (planing) is not important, I wonder if removing the turbos and installing smaller injectors would be a good idea. While we're at it, remove the aftercoolers as well. These changes will also mean less maintainance.

That's a horrible idea. The engine was designed for the turbo. Turbo engines usually have lower compression than naturals.

Trying to outsmart the original manufacturer usually doesn't work for most people LOL

 

[bobk]

If your like me and run below 10kts just about all the time then you should consider adding more like 2". You might even consider larger dia? You could probably gain a free Kt or 2 and eliminate the need to run them up and clear them out.

Brian

Brian, help me think through this please. The 'hull speed' on this boat is just shy of 9 knots. The performance data on a sister ship (see http://www.hatterasowners.com/Brochures/48CY48MY/48MYA-0781.htm ) shows it takes a lot more fuel at nine knots than at eight. I'm using the 8.3 knots (1000 rpm) because it is a good compromise for me and keeps me above the DD recommended minimum of 900 rpm. Is your expectation that if I increase the 26X23.5 to 26X25.5 that she will do nine knots at 1000 rpm and 6 gph? If so, I'll go for it, but my concern would be that the extra power needed to reach the nine knots will need more fuel. But 8.75 knots at 900 rpm and 6 gph would be great. What do you think? REtuning props last time cost over $800 so that has to be weighed in too.

Bob
1981 48MY

 

[Brian Degulis]

Brian, help me think through this please. The 'hull speed' on this boat is just shy of 9 knots. The performance data on a sister ship (see http://www.hatterasowners.com/Brochures/48CY48MY/48MYA-0781.htm ) shows it takes a lot more fuel at nine knots than at eight. I'm using the 8.3 knots (1000 rpm) because it is a good compromise for me and keeps me above the DD recommended minimum of 900 rpm. Is your expectation that if I increase the 26X23.5 to 26X25.5 that she will do nine knots at 1000 rpm and 6 gph? If so, I'll go for it, but my concern would be that the extra power needed to reach the nine knots will need more fuel. But 8.75 knots at 900 rpm and 6 gph would be great. What do you think? REtuning props last time cost over $800 so that has to be weighed in too.

Bob
1981 48MY

Your thoughts are correct it does take proportionately more HP as you get close to or sightly above hull speed. But there is another factor in play. A lightly loaded DD is horribly in efficient you can see as much as a 40% reduction in fuel economy. So I believe that would overcome the need to produce additional HP for the increased speed. Also the 900 RPM minimum is based on OEM prop size I believe running a loaded 800 RPM is better for the engine than a lightly loaded 900 - 1000 RPM. Because your engines were not designed to run at these low RPMs none of the power or fuel consumption data exists at anything lower than 1200 RPM. There is no way that I know of to figure it out on paper but I believe it would work very well for you. I base it on the data that does exist and the results I got on the boats I've had.

Brian

 

[bobk]

Brian, many thanks fore your detailed reply. I'm game to have a try. The props on the boat are 26X23.5. They reach 2300 rpm and 18.5 knots. How far should I tweek them? I have a spare set of 26X22 cupped props that I can work with.

WaltP you lucky dog, you get the benefit of my experiments. You better go south next year. I might even do nine knots with you.

Bob

 

[Brian Degulis]

I don't think the cup is doing you any good at displacement speed so I would have that taken out. Then I would see if the prop shop can add 4" of pitch? They always seem to say 2" is the max they can add and I never quite got that. There were times I had 2" "the max" acording to the prop shop then I would bring them in again and they would add another 2" so I guess it depends on what they are willing to do.

You might also think about locating some larger dia props if you have the clearance. Something like 28X23 adding diameter is the best way but of course much more expensive.

Brian

 

[bobk]

Brian, I agree on the cup issue. Millers Island Props told me that a cup is wasting power unless the prop can turn at least 1600 rpm.

I checked SAMS a while back for larger wheels but no luck. Can anyone recommend anyplace else?

Bob

 

[bobk]

Guys, I'm opening this thread again.

I hit something on the way north in 10' of water so assume it was a semi-submerged log. It seems fine at 1000 rpm but I got a high speed vibration so need to do a repair and was thinking of tweeking the spare set to 26X24 rather than going all the way to 26X26. BUT, 26X24 may not allow RPM for planing which won't bother me, but how about blowing it out? Will just running 1200 rpm for ten minutes do the job? It's just digging a deep hole at that point now and temperatures barely move up.... AND I recall a caution from someone about the extra load at that intermediate rpm.

Bob
Chateau de Mer
1981 48 MY
6V92's, 425 hp

 

[dakrhon]

Hey guys,

Great to hear some info on your boats, I own a 1981, 48 MY w/6v92's. When I bought the boat it had 26x22's, I had the props repitched to the specified 26x23's and hit 2300rpm plus or minus 10 rpm. I installed flowscans and am in the range you guys are noting. I'll keep closer tabs on your stories and add mine, been through refrigerator out the salon window, replacing window fleece, installed air seps, flow scans, fresh water pump, replaced vacuum flush pumps, touch up paint, repacked shafts, an on and on and on.

 

[bobk]

Hey guys,

Great to hear some info on your boats, I own a 1981, 48 MY w/6v92's. When I bought the boat it had 26x22's, I had the props repitched to the specified 26x23's and hit 2300rpm plus or minus 10 rpm. I installed flowscans and am in the range you guys are noting. I'll keep closer tabs on your stories and add mine, been through refrigerator out the salon window, replacing window fleece, installed air seps, flow scans, fresh water pump, replaced vacuum flush pumps, touch up paint, repacked shafts, an on and on and on.

How long have you owned it and what was she called before you got it. Just curious if I looked at her too before I bought mine.

Bob