Prop Experiment SEEMS to go VERY WELL

"In the few years I have faith

"In the few years I have faithfully learned so much from this web site, I always try to explain my results in case someone else may find a benefit as I have... Here goes. THIS IS A TEST as inspired by FastJeff....

The short test data is : WOW ! WOW ! GEE WHIZ !....for a more detailed observation, read on:

Several posts are mentioned by this highly experienced floating wizard with regard to overproping the boat. To boil down his comments it is my INTERPRETATION that with 454's spinning larger wheels, it would be difficult to bog the engines down with bigger props but it is do-able. After 2 years of looking I'd FINALLY found a set of 21 x 23 4 blade props to replace my original 18 x 20 3-blade ones.

I knew in advance that these were going to be a BIG increase in prop size and can likely drop my 5000 rpm down 1000 or so...HOWEVER, the engines had NO trouble in getting the rpm's up and going... AT ALL, and the short story is, I might take them to a shop NEXT WINTER to take out a little pitch, MAYBE . Right now they seam really really good...I have no complaints in the quick 4 mile run we took to get gas at a local marina.....possibly 21 x 21.5 or 22 is a better way to go. I can easily reach 3600- 3800 rpm... and top speed was reached VERY quickly at about 28.5 knots but I did lose about 1200 rpm...IF I CAN GAIN 500 or so rpm, maybe it can be back at 32 knots. FUEL burn rate at 28.5 was about 43.5 gph, cruise at 3000 was a nice 23.5 knots.


Fuel economy indicated on the Garmin integrated GFS 10 was about .8 nautical miles per gallon at speed which was usually around .6 or even .5 ....HERE IS THE REAL THRILL....Once you get out of the no wake zone and hit the throttles, you get pushed into the seat a bit from the g force of acceleration, reminiscent of the 1969 Firebird or Z-28....The old props almost felt like they were trying to push the water out of the way so they can get moving. We hit plane within 15 seconds. My brother and I were both very highly amazed and impressed. No out of balance vibrations either so the prop shop can wait till next winter....At slow maneuvering speeds, there is a NOTICEABLE bite from the forward and reverse action on the shifters and the immediate reaction of the boat makes PRECISE handling for docking type slow speed stuff quite impressive also...

Here are my new questions....

I believe we are reaching full Delco EST distributor advance at this rpm so I am not affecting that parameter or overheating....True?

My timing setting currently are exact 11 degrees so that should be fine, I get no engine detonation the engines respond IMMEDIATELY....true?

Will extended Full throttle running at 3600 or so be problem since this is now the new WOT, I think not since the engine design suggests 4400?

Is it advisable to increase IDLE speed or MIXTURE to account for the BIG BITE these props seem to take from a maneuvering stop and go event which causes my port engine to SOMETIMES stall? This was not a problem with the smaller wheels as my idle speed is textbook at 675 rpm +/-....Thanks Again all....

AND, as for that steam thing...it was very humid today but at 3000 or above, I get this with 140 on the gauges

Jeff......Fantastic !

steam trail

"Al very intersting and thanks

"Al very intersting and thanks for the post. What kind, size, weight boat and what year motors and hours are you running? I have been looking at this on my boat that is 31' but common response is not worth it on this size boat. I know on previous single screw the switch made a big difference but that prop also cost $150 not $2000.

Next question - you were previously running 5000rpm on 454s?? Seems high, I know my manual states 4200-4000. I think you need to get the pitch reduced to 21 or so to get back to this range or you will be overloading the engines with a top of only 3600rpm.

Curius to see if changed pitch how the gph is affected. You indicated fuel economy numbers but at those numbers what were cruising at before RPM and speed and what now?

RM"

"RM,

When the boat , a 34C

"RM,

When the boat , a 34C Silverton was repowered from the 350cuin blocks to the current 1992 454's in 2002, the owner re-used the same props. As a result they were too small. The 5K was routine and too much. We have about 650 hours on the engines and I am not too sure about overloading. This was a discussion on these boards about 1 year ago and I decided to go bigger to see the end results and re-pitch IF necessary. Of course a minimal 10 mile trip is nothing compared to a season of cipherin' and figurin' it all out. It does appear that the GPH is about 10% lower at top end but then TOP end is about 20% lower than before. Speed is minimally affected and those numbers are as of today. There is an overall 10% increase in fuel economy today...It will surely take a few tanks to get it under control but one thing for sure...it does not seem like the engines are overloaded. The new props squat the stern for about 3 seconds and then within about 10-12 more seconds she is on plane at about 18 knots ..Immediately running full throttle from there brought the boat immediately to 23-25kts without delay...that feeling like you are trying to push the water out of your way...it just jumps into plane and moves like running big gears in your car or truck...it may only do 75, but it gets there really quickly. I am anxious to read some comments..."

"Al; what is the clearance be

"Al; what is the clearance between the prop tip and the hull? Should be 3 to 4 inches to prevent hull pounding or paint erosion there. I think the move to 4 blades was a good one. It should run smoother as well. Yeah, you should prolly take out an inch of pitch, and get even faster accel and slightly more top end speed."

"Maybe yopu got a bit too much

"Maybe yopu got a bit too much prop on there, but she's happy with it, so.... I suggest adjusting the idle speed to around 800 IN GEAR fully warmed up.

Nice to know you're running in high gear instead of low, eh!

Jeff"

"Sounds like you're going

"Sounds like you're going in the right direction. I'd be inclined to take two inches of pitch but will defer to the propscan shop you use. I'd prefer to see 4000 wot with a full load of fuel and gear and people.

If you increase the idle speed, I'd make sure it isn't over 1000 rpm in neutral. Much over that and you'll be flying Dave up after he overhauls his gear to do yours."

"Dave, I have about 2.5" o

"Dave, I have about 2.5" of clearance to the tangent of a 90 degree molded line in the hull but directly above the tips it is about 2.5 to 3".

Jeff, That is a good point...you can hear the reduction in the idle when you throw from neutral. Surely she must be taking a a good chunk of water. In fact, my brother and I were looking at the stern water flow during docking and you could see the MUCH bigger swirls of churning water reaching the surface. Never had that before. Looked like toilet bowl flushes.

Mark, I HEAR YOU...! I do not need to be pulling the trans out next. I try to keep a nice smooth engagement..so far it appears to be good but I do get it...

HERE IS THE BIG QUESTION: What I have been told is that you will overload the engines and put premature wear on them if you run the engines at 3600 rpm which is the new WOT. From what is being said, the critical number is not a specific quantification, but rather a ratio (75-80%) of the total WOT. If my new WOT is 3600 rpm, then my cruise speed should be 2700-2880 rpm.....

ANY Comments on this one?"

"Spot on, Al. Only a moron, o

"Spot on, Al. Only a moron, or an oil sheik, runs his engines wide open! With the throttles backed way off, as one does in cruise mode, the motors are no longer straining. Note that this is only true for boats that are NOT underpowered! Heavy boats--like my favorite whipping boy (old 28 ' Carver Mariners)--need lots of revs and throttle just to maintain a plane.

Try this little experiment (and you'll love the answer): While going along at cruising speeds--at the "sweet spot" for your boat--mark exactly where the throttles are. This requires a bit of ingenuity, some masking tape, and a magic marker.

Later, with the engines OFF, pull the arrestors, set the throttles at the mark, and see how much the butterflies are open. On mine, you couldn't slip a pencil into the opening!

Jeff"

"Will do Jeff...I am sure I al

"Will do Jeff...I am sure I already have that answer

However, What about this potential pre-mature wear IF these rpms run 3600 for a while?

One other comment...IT APPEARS that the boat is now riding higher in the water at plane and instead of squatting below the water line, she rides above it with lower and much less wake trail"

"Al:

The boat being high an

"Al:

The boat being high and the wake being smaller is a good sign of efficient planing.

On the engine load factor, the OEM gives a WOT range to ensure the load on the engine isn't too much. As you've found, the 'load' that the engine 'sees' is controlled by many factors, reduction ratio and prop sizing being two of the big ones. You know from a 'spec' view that the load is a bit large. Running WOT for a several minutes here and there shouldn't have a big impact but doing it for a two hour run offshore may start the process. I'd bet there is "margin" in the OEMs numbers, if for no other reason than manufacturing tolerances. You're within 10% so as long as the engines don't "complain" and maintain RPM you should be fine."

"Al:

Just to reiterate what

"Al:

Just to reiterate what Diver and Mako have pointed out, from your data you are overpropped by about 2" of pitch. This limits the RPM's the engine can achieve because they see a larger load, hence 3600 RPM's at WOT. You cannot run at WOT for more than a few minutes without risking engine damage, as Jeff implied. It would be best to take out a couple of inches of pitch to achieve 4000 at WOT and get your cruise RPMS's back in the 3000-3400 range, both for longevity and for fuel economy. If you plan to run offshore alot (and I know you do), it would appear re-pitching is in your future. Congratulations on a major performance improvement; you just need to dial it in a little more.

Best wishes,"

"If a 470 c.i. Lycoming gas ai

"If a 470 c.i. Lycoming gas airplane engine can be rated at WOT at 2500rpm, why can't the BBC run WOT at lower than 4000? Not sure, myself. I do have some random thoughts on it:
At high throttle settings, but lower rpm, the cooling system MIGHT have an issue. Both the oil pump and coolant pump are turning slower, but still the demand for high flow cooling is there. The other item is ignition timing. The first sign of pinging is usually high load, low rpm operation. I suspect continuous high power demands at "lowish" rpms may stress both these factors.
Also, to maybe a lesser extent, is that you are running at a higher torque setting, than the same HP at a higher rpm. This may stress the transmission a bit more; but that would not be my top concern. (unless you own a old, already stressed tranny like me )"

"RPM isn't as much of a fa

"RPM isn't as much of a factor as heat, power and throttle position, although I agree that cooling might be the limiting factor. These engines (and nearly all gas engines for that matter) are not rated for continuous full power output. You must differentiate between engines designed to operate at full power (continuous duty) and intermittant duty engines that are designed for continuous operation at reduced power and only tolerate full power operation for short periods. The SAE continuous duty rating for a 454 big block is probably a lot closer to 250 HP than 350. Full throttle operation probably puts you nearer to the 350 HP, a power level the engine was not designed to sustain."

"Correct-a-mundo! So run it at

"Correct-a-mundo! So run it at 2,500 rpms cruise speed and all will be well. The hp produced at this speed--and the oiling/ cooling/ etc. required to do it--is substantially reduced at the same time from that required at WOT.

Jeff

PS: Contrary to what Thomas said about reducing pitch, fuel mileage would NOT be improved."

"Reducing pitch per se won&#39

"Reducing pitch per se won't improve economy but pulling back from full throttle will, and that's more or less what I was speaking to. Its a balancing act and I don't think he's quite there yet..."

"Here's a link to some oth

"Here's a link to some other thoughts on the HP rating topic. the gent with the extensive diesel background summarizes reality best - the higher the RPMs an engine runs at the faster it wears out.

http://www.boatdesign.net/forums/diy-marinizing/auto-engine-marinization-650.htm l"

"Thanks to ALL of you for the

"Thanks to ALL of you for the above HIGHLY interesting and needed comments...now I am getting into the meat of this topic and can get a better grip on the how's and why's. I am finally at rest with the theory that, as Mark states " the higher the RPMS an engine runs at, the faster it wears out".

I have some balancing act to create between the two limits of rpm vs engine ,oiling and cooling. Unfortunately, the slight few miles yesterday was not enough for us to make any real determinations but by the next several weeks, I am SURE this will change and I will keep everyone updated.

I also recognize the 250 HP output as a more accurate representation of the engines typical performance. While my brain sees 350HP emblazoned on those valve covers times 2...yee ha! 700 screaming horsepower.

My real goal is the best possible combination of reliability and performance from these engines. I already know what it is like to lift the heads off...I'd hate to think of lifting the blocks out too!

Dave, I failed to find any information about your trans problem...did it need rebuilding?"