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Old 11-15-2013, 09:41 AM   #21
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Why let something as trivial as gas vs diesel enter the conversation. Good grief guys!
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Old 11-15-2013, 09:59 AM   #22
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Why let something as trivial as gas vs diesel enter the conversation. Good grief guys!
Don't know if you're joshen or not! Guess you are.

Anyway, gas and diesel engine trivia makes for best understanding/comparison as to differences each engine type may provide... specially regarding altered prop configs to best accomidate altered speeds.
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Old 11-15-2013, 12:50 PM   #23
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Controllable pitch props make sense when the machine has a wide range of speed or load. A trawler that travels 3 days to the fish grounds then pulls nets for a few days before going home. An airplane that needs full power to take off at 60mph then cruises at 180mph. A ratio of 3 to 1. A semi displacement boat that gets "over the hump" at 12kts to cruise at 17kts is a much smaller ratio.

With gas engines it is important to maintain a ratio of RPM to manifold pressure. C. Lindbergh was used by the Army Air Corp to convince pilots that they could use power settings that were "over square" (IE more inches of manifold pressure than RPM in hundreds. 23" mp at 2300 RPM is considered "square". 25" mp at 2300 RPM is "over square".) Most supercharged or turbocharged airplane gas engines can live long lives operated modestly over square but high manifold pressures and very low RPM will shorten their life dramatically. Think trying to go up a steep hill at low speed with a manual transmission in top gear in a car. This would result in very high cylinder pressures.

Naturally aspirated Diesel engines are always at 29" mp (atmosphere) minus what ever loses are incurred in the intake tract from air filter ect. Power is controlled by fuel flow. Cylinder pressure is not the limiting factor. To avoid damage you need to manage exhaust temperature. Inline 6 cyl engines have good natural harmonic vibration patterns and can usually be operated at low rpm/high power without damage IF the safe EGT is not exceeded.

Super or turbo charged diesels do have to consider balance between mp an rpm but at the low power output that most small yachts use at displacement speeds it shouldn't be a factor. Consider the Flemming 55. Two 500 hp engines. At 8 kts it will burn about 5 GPH. If we assume about 15 hp per gallon per hour that would be about 55hp to push the boat. Or, 27.5hp per engine. You could put any prop you wanted on that and as long as you never went over 8 kts through the water you couldn't hurt those engines. (I know I didn't account for head seas, wind, dirty bottom ect but for our discussion I'll ignore that. These are BIG engines.). If you propped for 8 kts at dead idle you may save (a guess here, probably optimistic) 10-20% of fuel burn. The problem would be if you tried to go faster you would be trying to make power at too low an rpm and EGT would go way up. A friend who works at Case International in the "let's see what breaks" department says that when you put a turbo diesel on the dyno and keep adding fuel until it breaks the usual result is that the head of an exhaust valve comes off and does serious, expensive damage to the engine. As an interesting aside the engines he works with do not have water cooled exhaust and the exhaust system glows red all the way to the turbo starting about 200 degrees before "she blows".
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Old 11-15-2013, 02:30 PM   #24
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It's Settled

I spoke to 2 highly regarded prop guys in the Houston, Tx. area and they both came up with similar answers. I chose one of them.
I will be adding 1" pitch with a small cup. Everything here is a wild guess but they both said that it should increase my low speed (6-7 Kts) gas mileage by at least 15 to 20%. and put a little bit more load on my engine. Both of these were my goal.

This is not a radical change and my world won't end tomorrow if I decide to go into a plane..
The max RPS should not drop by more than 75-100.


Thanks to all for your ideas, both pro and con.

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Old 11-15-2013, 03:53 PM   #25
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I think that is a smart move. I would consider installing EGT gauge and check wide open temp before the change. Then I would not exceed that temp after increasing the pitch.
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Old 11-15-2013, 05:55 PM   #26
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" it should increase my low speed (6-7 Kts) gas mileage by at least 15 to 20%"

BS. I can't believe that a 1" pitch change and some cup will cause this effect. But hey, I advised talking to a prop shop. You did and that is what they said. But ask them to prove it with examples or theory.

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Old 11-15-2013, 07:13 PM   #27
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Why would I want to that? If someone came into my woodworking shop for a repair and I had to give them a hypothetical lifespan of the repair based on adhesive used, temperature, humidity, etc. I would tell them to take a flying F***.
Some things you just have to take on faith or reject it. It is only costing me another $75 to find out. This is just $75 more than the normal rebuild/repair of $650. No matter what the actual number turns out to be, It has got to be better than what I already have. How much better? I don't know and really don't care at this point. Better is still better. I It only makes sense. My total yard fees, everything included, will exceed $4,000. Another $75 gets lost in the wash.

I do however appreciate your concern

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Old 11-15-2013, 07:17 PM   #28
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Why would I want to that? If someone came into my woodworking shop for a repair and I had to give them a hypothetical lifespan of the repair based on adhesive used, temperature, humidity, etc. I would tell them to take a flying F***.
Some things you just have to take on faith or reject it. It is only costing me another $75 to find out. This is just $75 more than the normal rebuild/repair of $650. No matter what the actual number turns out to be, It has got to be better than what I already have. How much better? I don't know and really don't care at this point. Better is still better. I It only makes sense. My total yard fees, everything included, will exceed $4,000. Another $75 gets lost in the wash.

I do however appreciate your concern

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I am not surprised..my prop guy said should add some cup too...I explained how slow I run and we said maybe next time....real slow and it does matter less.
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Old 11-15-2013, 08:45 PM   #29
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If someone other than you runs the boat and isn't aware they can over-rev the engine easily they will blow it up! I hate to be the "you will blow it up" guy, but them is the facts. Most boats are set up so maximum rpm's are being limited by the propeller so as to not over-rev. YOU can do it, but someone not used to your set up will assume it is not possible to over-rev the engines as the props will prevent that. "Blowing out the carbon" will involve you watching your rpm's carefully when you run it up so you don't over-rev and damage something along the way :-)
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Old 11-15-2013, 09:13 PM   #30
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Good call Tony and great logic too. I have the same gas engine you do and would love to hear your thoughts of how the changes have benefited you. If and when I somehow manage to grow up and stop running flat out all the time may make a similar change myself.
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Old 11-15-2013, 09:40 PM   #31
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AkDoug,
He is adding pitch, so full throttle will be less rpm, not more. Experienced prop guys I've talked to say that with a 2 to1 gear ratio 1 inch should make about 200 rpm difference. So what Tony is doing with the added cup sounds reasonable to me. I'm looking forward to hearing results.
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Old 11-15-2013, 09:55 PM   #32
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The actual data may be a while coming. I don't have a flow scan meter for actual accurate data accumulation. We are already 9 months late in our 'running away from home" plans. Right now we are the final stage before our disappearing act - the dreaded yard routine. Hopefully we will be out of here by Wednesday and heading back to our marina. Then I want 2 weeks for shakedown to make sure all of the work done actually works.
Then as weather permits, we will be heading from Galveston Bay, Tx to Biloxi, Ms. for the rest of the winter. I suspect that my overall averages will be enough to tell whether it made a noticeable difference in fuel consumption or not. As far as performance is concerned, I might actually notice the difference in power when pulling in and out of my slip in idle.

Would it be acceptable to create a Tony B Foundation with a collection plate here on the forum? In no time at all, I will have enough money to get a flow scan meter and I can give immediate data. Just sayin'.
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Old 11-15-2013, 10:11 PM   #33
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When I re-powered from a pair of 145 hp engines to a pair of 200 hp engines, my mechanic suggested I re-pitch my props to lower the operating rpm of my engines and improve efficiency.
I had 23x13 props. I was getting 5 gph at 7 knots on the old engines. I re-powered and before re-pitching my fuel efficiency improved by a noticeable amount. I guess 10%. A couple of years later I re-pitched to 23 x 17, lowered the rpm from 2700 to 2000 rpm at 8 knots, now I get 4 gpm at 8 knots. I estimate the re-pitching saved another 5%. My mechanic told me the engine life will improve due to running slower, so its all on the win side.
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Old 11-15-2013, 10:25 PM   #34
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Tony, I've no flow scans either and no interest in acquiring them. Fuel mileage averages and "how it feels" in the seat of the pants is good enough for me. I'm pretty simple and low maintenance that way, kinda the California way.

You know what's funny? Before we bought a boat the wife didn't like the idea of a slow boat and I could go either way on the issue. Now that we have a planing boat she likes slow cruising over fast because the exhaust noise disturbs her reading
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Old 11-15-2013, 10:34 PM   #35
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I generally find that most women like a slow motion. I think the speed and noise is a testosterone thing. Gets the blood pumping.
Also, women seem to be more noise sensitive than men. My wife could hear a mouse fart. It's very annoying to me when she complains about noise because I don't hear all that well.
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Old 11-15-2013, 11:13 PM   #36
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Thanks for the wisdom, I'll give it a try in a couple hours
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Old 11-16-2013, 06:31 AM   #37
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There big if efficient,, but water lift mufflers might quiet the beast.

On most gas cars the exhaust can not be heard , so it cant be that hard on a boat.
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Old 11-16-2013, 06:46 AM   #38
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I don't know jack about motors which is obvious by now.
Anyway, when I bought the boat I joined a Mainship forum of sorts on Yahoo Groups. One of the older moderators had done the Great Loop and had kept meticulous records of his trip of almost 6,000 miles. He had flow scan meters which I was told were very accurate. Between the Flow Scan gal/hr data and his GPS he came up with these charts which there is no reason for me to believe that they are anything but accurate. On my little day trips and fuel sounding stick data I pretty much track with his charts.
Just by sheer coinkydink, he had the same make, model and year as my boat. A true twin.
Anyway, attached is some of what he sent me.
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Old 11-16-2013, 07:26 AM   #39
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If someone other than you runs the boat and isn't aware they can over-rev the engine easily they will blow it up! I hate to be the "you will blow it up" guy, but them is the facts. Most boats are set up so maximum rpm's are being limited by the propeller so as to not over-rev.
Diesel engine RPM is limited by the governor. The limiting rpm is called "high idle" and is what you check when you put the gear in neutral and advance the "throttle" to maximum. It is a normal condition and won't hurt anything.

If the prop were to limit rpm at some point below the high idle rpm then you might have problems. The condition you describe is a prop capable of absorbing more power than the engine can produce. In that case the governor fuel stop is supposed to limit the amount of fuel it is possible to deliver.


Running the engine at its designed "high idle" speed is not going to create an instant disaster or even one in the near future. It just sounds scary to some people because they have never done it before.

The rpm at which normal "full throttle" operation occurs is set by the high speed screw, it limits speed lever movement and nothing else, it only sets the point at which the governor is happy with the engine rpm and delivers just the right amount of fuel to keep it there.

In any event, when a prop limits engine rpm, you have an overloaded engine and a lot of smoke. The prop is NOT supposed to be a governor.
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Old 11-16-2013, 09:30 AM   #40
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I'm of the opinion that an overloaded engine is overloaded at all rpm's. If you are unable to get rated WOT rpm due to prop pitch then the engine is overloaded period, at all rpm's.

If Tony believes this then he must decide if the slight increase in pitch is worth the potential damage to the engine vs the increase in fuel efficiency. It could very well be, but I don't think there is any chart or formula anywhere that can tell you this. Entirely a judgement call.
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