Is this right?

[mike]

See attached DD fuel curve file. If you are prop'd correctly these curves are spot on.

 

[MikeP]

Thanks - I have copies of those tables. Frankly, I have never understood them. In reading the table, I'm looking at the upper (straight) line of the fuel consumption portion of the graph, not the lower (curved) line which refers to propellor load. If I am supposed to be looking at the lower (curved) portion to determine flow, it does'nt make any sense to me. The fuel flow WITH propellor load can't be less than it is without any propellor load at the same RPM.

'Splain please!

 

[Passages]

Mike,

That graph is great!
Do you, or anyone else, have a similar table for 6-71's?

 

[Nonchalant1]

If we assume that the curves are accurate, and then assume we really understand them, wouldn't we then be back to the high degree of variability question of how many RPM's does it take to get a 53MY to do hull speed at 9.1 knots before we could calculate the NM/gal? I'm sure that flat water takes less power to achieve the same speed as when there are waves. I'm sure that more weight = more fuel for the same speed (i.e. water, fuel, holding tanks, gear, dinghy, and enough beer for 2 weeks all add weight). etc., etc. Seems like empirical evidence is the best way to tell what we're really getting. Either I have to believe the FloScans or I have to find a long river with no turns and no "no wake" or "low wake" zones so I can measure.

Doug

 

[Genesis]

RPMs aren't the big deal. Horsepower is.

An engine is most efficient at one given RPM and power output, but the variability there isn't huge.

For all intents and purposes, its a matter of power demanded.

If you're propped "correctly" (meaning right to the rail) then the curves are accurate. Otherwise they are not.

Your best way to KNOW is to measure fuel flow at a given desired boat speed. Then you KNOW what you're burning.

 

[Nonchalant1]

Thanks - I have copies of those tables. Frankly, I have never understood them. In reading the table, I'm looking at the upper (straight) line of the fuel consumption portion of the graph, not the lower (curved) line which refers to propellor load. If I am supposed to be looking at the lower (curved) portion to determine flow, it does'nt make any sense to me. The fuel flow WITH propellor load can't be less than it is without any propellor load at the same RPM.

'Splain please!

Mike,

I think that both curves just relate horsepower to fuel consumption at a given RPM. As you know, your throttles are really not throttles, they're RPM controls. The rack will adjust to supply enough fuel to maintain RPMs so as the boat is more loaded, fouled bottom, more beer onboard, etc. more fuel and more HP are needed to maintain those RPMS. Therefore you need a "range" of HP and fuel for any given RPM. For example, with the N90 injector curve, at 1400 RPM the HP can be between 100HP and 300HP with corresponding fuel use between 6GPH and 18GPH. Yes, the engine would be way overloaded at 300HP and 1400RPMs, but that's what it's capable of doing at maximum. If your boat goes 12 knots at that 1400 RPM, the chart says you'd get between 1.0 and 2.9 gal/NM (2 engines), depending on how much HP is required to maintain that 1400 RPMs (engine loading).

This JPEG is a little fuzzy, but it looks like the solid curves are all SHP at the shaft. The one dotted line above is the max fuel use BHP which is higher because it's gross horsepower.

The problem is that the range is really big. If we know we can do 14 knots at 1850 RPM all day, the chart says it'll be between 15 and 22 GPH per engine so 30 to 44 GPH for the boat. Therefore it'll be between 2.1 and 3.1 gal/NM. That's one RPM/speed/fuel I've actually measured on an all day cruise and I actually do that at about 2.1 to 2.2 gal/NM, so it's within the right range, but at the low end. Again, it comes down to how many RPM does it take for a given speed and where in the range you are for required HP.

Doug Shuman

 

[MikeP]

Today I finally got around to checking the props on the boat - they are 28x29 and were installed in 1997. According to the reciepts, they were apparently modified from a pair of oem 28x31 props. 28x31 were oem per Hatt specs in the owners book for this boat and there is a never-used spare pair of them on board still in the oem factory wrapping.

I would ASSUME, based on my experience with automotive differential ratios, that the oem props would yield a better cruise efficiency than the de-propped versions. However, I have NO experience with propping boats, so this assumption may be faulty.

I' m sure that whatever the floscans are showing (assuming they have been calibrated) is correct. So there is no need to look for a long, flat river with no current for test purposes!

 

[SKYCHENEY]

SKYCHENY,

Wow! 28 X 28 on a 465 hp boat is over propped? I thought that most of the 53MY's with 435 hp 8V71TI's ran 28 X 31's? Is your displacement weight 55,000 lbs. like the MY? (I think that's dry weight).

Doug

Okay, heres the deal.....well, sort of. When the boat was pulled today, I checked the props. They have no markings as to size. Just the date, NiBral, and some code numbers are all that is stamped on them. So, I decided to check the spares. The spares are stamped 28 x 31, but where the 1 is stamped is also a 2 stamped over the top of it or the other way around . I think that someone changed the 28 x 31's to 28x32's, but I'm still not positive. All I know is that I'm overpropped now, so I will pull the current set and send them in for scan. If the current is 32"pitch and I can only reach 2300rpms at WOT, should I go back to 31" or do I need to take out more? My no load R's are at 2450, so no problem there.

 

[Genesis]

2300 is a MINIMUM for V92 engines at FULL load (full water, fuel, people, etc)

I like 2350 better. A LOT better. Your engines will like it too, and reward you with a longer life between overhauls. BTW, that number has to be on a phototach or known-accurate digital meter - analog tachs are not accurate enough.

No-load should be between 2450 and 2500. If its not in those specs, you must set it first before evaluating full-load WOT.

 

[Nonchalant1]

Sky,

The RPM-Speed-GPH measures I said I'd do above are posted in a new thread today. This thread is what got me going on writing it down.

Doug Shuman

 

[SKYCHENEY]

Thanks guys,

DD checked my no-load RPM a few months ago, so I know that is okay. And, I have digital tachs that seem to be accurate to within 50rpm. I think I will make sure I have the original spec 28x31's on there and see what that does. Thanks for the help.

 

[Nonchalant1]

SKY,

Maybe pull your spares out and take them to a PropScan place for free? If they're reasonably near 28X31, try them first before you have work done on them.

Doug

 

[Nick]

I found a copy of the similar curves for the 671-TIs 450HP and 430HP. If Passages or anyone else is interested send me your fax or email address. If I can I'll scan it and post it.

Nick

 

[SKYCHENEY]

Okay, back to the prop issue. I pulled my props and they were stamped 28x32, but Propscan said that they were way off. Some blades were as much as an inch of pitch off between them. Anyway, they recommend retuning these Federals to 28x30 with is the same as DLCameron's 53EDMY. Next I talked to Michigan Wheel and they suggested going down to 28x29. I ended up going with what Propscan said, so we'll see how it does this spring.

Thanks everyone for the help. My 6v92's should breath easier now.

 

[spartonboat1]

Re: Is this right?/ Reinstall

Props have been pulled, which means re-installing same. Since I now have religion on reinstalling props, based on threads here and some very good articles in Passagmaker, thought I'd mention about lapping in the prop hubs w/compound, checked with bluing in addition to correct installation of the nuts (first big on, then off, then small before big, etc.) and correct size cotter pin that actually fits the pin hole.

FWIW...

 

[SKYCHENEY]

Thanks, I will do it "by the book". They were installed wrong before, ie big nut first, but after reading the Passagemaker articles, I think I can get it right. Although, I can't seem to find the issue with part 2 of the prop series in it. I was going to re-read it, but I can only find part 1 and the shaft alignment issues. I seem to me missing the one I need.

 

[Nonchalant1]

Sky,

Did'ja talk to Hatteras or Steve McPhereson? Find out what pitch they were supposed to be when the boat was new?

Doug

 

[ThirdHatt]

Gentlemen,

When I purchased my 1973 58YF this summer in Palm Beach, FL the lift machine that lowered me into the water had two guages that are calibrated monthly to measure weight. I asked the lift operator to tell me what my boat weighed before he lowered her into the water. Once he got on level pavement right near the slip, he gave me two weights (don't remember the breakdown) but they totalled 79,200 lbs! This was with 2/3 fuel (forward and mid tanks pressed, cockpit tank empty) and full water. I was surprised she was so heavy as the dry weight as Hatteras states it was 62,500.

My boat has the 8V71TI's with fresh rebuilds (177hrs) and will step away from the wake and get on plane at around 13.5 knots and achieve 17 knots (according to several GPS) at 2300rpm (WOT). I had the props tuned and they are 28x29. My spare set is 28x30. I cruise mostly at 1500-1600rpm at 10-11kts, with occasional 1950rpm stints at around 14kts.

Best Regards,
Byron

 

[SKYCHENEY]

Doug,
Yes, the original props were 28x31, but it appears that someone changed them to 28x32. I think the spares were at one time the originals as they were stamped 28x31 with the 1 x-ed out and a 2 stamped next to it. I knew that I was overpropped, I just didn't know how much pitch to take out. I'm pretty confident that the 28x30 should be close to what I need.