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Kennneee

Fuel Consumption/Propellers

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Hi Guys- I would love to know what kind of fuel consumption you are seeing with your Bluejackets. Have you played with different propellers and what kind of changes have you seen in your fuel burn and performance? Three blade vs. four blade? 

Ken

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Is your engine hooked up to a chart plotter, that will give some info about the fuel consumption.  Mine does tell the "Miles per Gallon"  and it doesn't seem to make much difference throughout the speed range.  I would have thought going slow would give a better mileage.  About 3.5 mpg.

I know little about propellers, just that they need to be sized to get the rated engine rpms at full throttle.  Larger diameter gives more pushing power, greater pitch more speed.  I had to go back in pitch a bit to get the rated rpms.  Just kept the original propeller as a spare.

I would like to know what the difference is between 3 or 4 blades.

I am building a push boat for a Chesapeake Skipjack at the Reedville Fishermen's Museum .  Hopefully through that I can find out what a 4 blade vs. a 3 blade does.  But that will be a while.

Your boat just looks astonishing and you should be proud of it.

 

Egbert

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14 hours ago, ejds said:

Egbert- I am waiting to receive the NEMA 2000 cabling I need to hook the motor up to my chart plotter. It will probably be a couple of weeks since it is coming from the US and things can be slow getting here. I did fill my fuel tank and checked the consumption the old fashioned way by doing the math. So far she is getting just under 5mpg. That said, like many young things, she will never be lighter. No water and other junk aboard yet. On the other side of the coin, I am still playing with getting her properly propped. My first prop was a 13.5x16- 3 blade and I couldn't get the engine to rev higher than 5200RPM . Redline is 6000rpm. I have have a 13.5x14 on her now and still can't get the RPM above 5600RPM which might be ok. I don't expect to be cruising at the 26 -27 knot speed I am at WOT. I have been advised to try a Solas Amita 4 blade which I will probably do. Still much to learn.

She seems to get up on plane effortlessly and her nose points up around 3.5 degrees (digital tiltbox). I find very little difference in speed and bow angle when changing engine tilt. I run the engine is almost all the way in most of the time. I wonder if that will change as she gains weight and sits lower in the water. I will let you know how the 4 blade prop works when I get one.

Thanks for the compliment. I just followed the directions on the back of the box:).

Cheers,

Ken

 

 

Is your engine hooked up to a chart plotter, that will give some info about the fuel consumption.  Mine does tell the "Miles per Gallon"  and it doesn't seem to make much difference throughout the speed range.  I would have thought going slow would give a better mileage.  About 3.5 mpg.

I know little about propellers, just that they need to be sized to get the rated engine rpms at full throttle.  Larger diameter gives more pushing power, greater pitch more speed.  I had to go back in pitch a bit to get the rated rpms.  Just kept the original propeller as a spare.

I would like to know what the difference is between 3 or 4 blades.

I am building a push boat for a Chesapeake Skipjack at the Reedville Fishermen's Museum .  Hopefully through that I can find out what a 4 blade vs. a 3 blade does.  But that will be a while.

Your boat just looks astonishing and you should be proud of it.

 

Egbert

 

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Egbert- I hooked up a NEMA 2000 network to my engine and now get mileage readouts. I am currently getting around 4.25-4.5 NMG in the mid to high teens(knots). I can get it up to around 6 NMPG+ at 7-8knots. I will plot it more accurately soon and compare it to my 4 blade when it arrives.

i often see mileage posted for different boats and am not sure if the numbers are in nautical miles or statute miles. What are yours? My 4 blade prop should be here on Monday so I will have some info soon after.

Ken

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I use statute miles for everything.  I went on a trip last weekend onto the Chesapeake Bay and was getting at the most 3 MPG.  This was with a propeller that is a little too big for my engine and boat.  13 pitch instead of 11.  I was also towing my little Nutshell Pram.  It was hard to find the optimal speed towing a dinghy.  With the waves on the Bay and my wake the pram was also filling up with water.  

Looking forward to your 4 blade findings.

 

Egbert

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Here are some readings I took earlier.

rpm      mph       mpg        gph

2000    6.5          7.3           .9

2500    7.5           3.3          2.3

3000     8.0          2.5           3.2

3500     13            3.1         4.3

4000     16           2.82        5.8

4500     18.5        2.94       6.3

4900      21.7       2.97       7.3

This was on our nice flat creek with just me and 45 gallons of gas on board.  It doesn't help that my boat is over 4000 LBS 

Same here as you mentioned in an earlier post playing with the engine trim seem to make very little difference.  The bow raised to about 3 degrees max 

Egbert

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Egbert- Thanks , that is useful information. Do you know if Tom’s figures for Liz are in statute or nautical mpg? In either case the mileage is impressive. I would like to know since Liz seems to be a benchmark for exceptional efficiency.

You mentioned your dinghy. I seem to remember a photo of a skin on frame dingy in your building album. How did that work out? I have built a few SOF kayaks over the years and they can be quite lightweight. I am starting to think about a dinghy for Rosie and a SOF crossed my mind. Graham has some lovely dinghy designs that are also in my sights. Weight is an issue unless I come up with a good lifting boom design.

Ken

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I can't imagine anyone using the statute mile for anything to do with boats.  It is the one odd ball distance that is still universal and should always be used. 1 nautical mile =  1 minute of latitude.

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Dave- Learning how to navigate over 40 years ago there was no other option(as far as I know) to using knots. That was the era of compass, sextant,  dead reckoning etc. Paper charts were the only thing available. 

Around 25 years ago I started seriously racing kayaks and surfskis. All of the racers talked about speed in terms of statute MPH. I did my best to stick with knots but finally relented and went with the rest of the crowd. I was convinced that they used statute mph because it sounded faster. Easier to go 5,280' that 6,000....

Fifteen years ago I moved to Canada. Oh no, KILOMETERS PER HOUR! Well, it sounds really fast. Maybe because I got old and crunchy, I stuck with MPH. At least my US paddling friends would know how fast I was going when we told after race lies.

It seems like most of the boaters I talk to now never use paper charts. I get it. I recently bought a chart plotter and love it. My sextant is on consignment to be sold. That said, I still like to look at a paper chart and get to overview, etc. I suppose if you are relying totally on digital charts and electronics the way speed and distance are measured becomes less important. 

Yesterday I spoke to a young dock mate and I asked him how his trip was. He told me his only trip wound up being to a boatyard. He had hit a rock. "I wasn't paying attention and didn't look at the MAP". Ok, I am from another world. I think there was a time you could get tared and feathered  or keel hauled for calling a chart a map.

So, I guess the old rules are squishy and whatever works, eh. I will stick to knots but will still drink a beer with someone that uses statute MPH. If he says MAP, maybe knot.

Ken

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There is absolutely nothing wrong in using mph. Running inland and thru winding rivers, streams and across small bays to creeks, where distance is measured in statured miles, and you are not transiting in a direct route., you do measure your distances over ground.  So this is where numerous old pharts accept the terminology. Personally I measure my speed on all inland waters in mph. Of course  I still shave with a single edge razor on most occasions.  Operating in open water calls for the knot methodology.  I still on occasions use dividers when measuring running distances on my charts. And yes I use "charts" in both instances. By the way the ICW charts are in STM.

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OK, I am able to follow the logic of whatever system anyone chooses to use.  In the USN we used knots, etc and that was fine as charts and literature followed that system.   When I started to use small fishing boats in the 1950's, on inland lakes where no one had ever even seen a nautical chart, only statute made any sense if you wanted to be understood correctly as everyone clearly assumed that was the system used. 

 

Our first sailboat in 1970 had us using knots and nautical miles but all the waters we cruised and the waterway charts were in statute miles. Navigation was with a lead line, compass and chart.  This meant that all such measures had to be translated clearly to avoid confusion.  There does not seem to be any system that all can agree on but I find it silly to use any system that needs constant explanation to be interpreted correctly.

 

I use statute miles and MPH without apology to any who think it less nautical because I think it causes less confusion..  Frankly, I wish we had opted for the metric system for everything long ago.

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I'll just stick with what I finally got used to, Miles, Inches, Feet etc.  My boat goes faster and can cover more distance in statute miles.   I grew up with the metric system which is even faster.  

 

I haven't tried the SOF dinghy in the water yet.  Have been to busy with other things.  I did learn that towing a dinghy is a major pain.  The only way to carry it would be a couple of davits on the transom because I have no roof space left.

The dinghy has nesting capability but that is more to carry it on the truck.

Egbert

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