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Old 11-05-2015, 01:56 PM   #181
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At 1600 RPM we'll do 7kts (about 4 gal/hr)
At 1800 RPM we'll do 8.2kts (about 6 gal/hr)
Full power we can move at 11.5kts but we never do.

Add or subtract for current, wind, and added friction in higher seas and real world ground speed is going to be from 5 or 6kts on the low side to 10kts on the higher side.

We just finished our run South from Sandy Hook NJ to Charleston SC for the winter. We ran the ICW from Norfolk to Beaufort NC and outside before and after it. We averaged 4.8 gal/hr putting 106 hours on the main.
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Old 11-05-2015, 02:35 PM   #182
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Originally Posted by Ski in NC View Post
Yea, calling BS on TDunn's Volvo burn numbers. Beautiful old boat, kudos there, but a 2liter engine running 2000rpm pushing a 33footer is going to burn more than 0.55gph.

My 2liter tdi vw cruises down the road at about 2000rpm and 60mph and gets 50mpg. That's a bit above 1gph. Much easier to push than a 33foot boat at 7kts.
My boat is a pure displacement hull with a 32' waterline which gives a nominal waterline speed of 7.58 knots. The boat also has only an 8' waterline beam and displaces 9500 pounds. The small displacement, high length to beam ratio and hull form result in a very easily pushed boat up to around 7 knots. 7 knots is the transition point where the boat starts needing a lot more power to move it faster. At 7 knots I have almost no wake. All the calculations show that I should need just over 10 hp to make 7 knots. Note that 9 knots requires a fuel burn of about 2.4 gph. Of course it is your choice to not believe it.
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Old 11-05-2015, 02:38 PM   #183
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PI'm going to say good go'in Ski for call'in BS on fuel burn numbers posted! Way too many numbers are questionable IMO.

I've never had the guts to come right out and say it many times in the past.

As to TDunn's numbers it's not far off though if you fill in the blanks and assume he's been calculating his fuel burn from strictly hours and fuel that goes into the tank at fuel up.
With a diesel it (fuel burn) all has to do w the governor, load, rpm and engine displacement. One would think if the load was low enough and the rpm low enough it could be done .. but not likely. It makes me wonder how lean the fuel mixture can get on a diesel and still have smooth running.

Just read post #182 and need to ask .. Is part of your transom submerged at rest TDunn? If so I would'nt call your boat "pure" FD. Your displacement of less than 10,000lbs will go a long way to support your numbers. Also what is your idle rpm?
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Old 11-05-2015, 03:26 PM   #184
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Quote:
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My boat is a pure displacement hull with a 32' waterline which gives a nominal waterline speed of 7.58 knots. The boat also has only an 8' waterline beam and displaces 9500 pounds. The small displacement, high length to beam ratio and hull form result in a very easily pushed boat up to around 7 knots. 7 knots is the transition point where the boat starts needing a lot more power to move it faster. At 7 knots I have almost no wake. All the calculations show that I should need just over 10 hp to make 7 knots. Note that 9 knots requires a fuel burn of about 2.4 gph. Of course it is your choice to not believe it.
TDunn- Calling BS was a little harsh, I appologize for that. But your numbers do not pass the smell test. A 2liter diesel at 2000rpm will burn around 0.2- 0.3gph at no load, i.e., neutral. Then at 0.55gph that leaves little to make propulsive power. Your boat may only need 10hp to push, but at 2000rpm it's going to take more than 0.55gph to do it.

It is possible you have both a super efficient engine and a super easy to push boat, but to believe it I'd have to see good hard numbers.

How have you arrived at your 0.55gph number?

And again, kudos on the old cruiser..
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Old 11-05-2015, 03:37 PM   #185
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We cruise at one of two speeds for the most part either....


6.5-7 knots when we are happy slow
16-17 knots when we want to cover distance quicker


The speeds in between and above just do not make any sense.
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Old 11-05-2015, 03:49 PM   #186
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Rainha Jannota does 7.3 knts @ 2000 RPM burning 3.3 GPH. @ 1800RPM, our favorite, she does 6.6 knts burning 2.35 GPH.
Not bad for a floating 2 bedroom house with 360 degrees ocean view.
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Old 11-05-2015, 04:02 PM   #187
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I have the D2-75. I find the Volvo curves fairly accurate. Here are the curves for TDunn's D2-40. If he nailed the prop sizing, he might have hit the sweet spot.
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Old 11-05-2015, 06:17 PM   #188
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We can do 9-10kts @ 5gph.....or 20kts for not that. I think our regular cruising will be around 9kt with the ability to get out of weather. (Loving this boat so far!!)
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Old 11-05-2015, 06:45 PM   #189
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My prop is sized so that I am very slightly under propped. My fuel burn is measured by checking how much fuel is in the tanks before and after a run. I have plastic tanks that are fully visible on one side. When I initially filled them I marked them at one gallon intervals. So, with a ruler I can determine my fuel burn within 0.1 gallons. My fuel burn numbers are based on 4-5 hour runs made at given rpms. Those runs were made specifically to determine fuel burn.

As far as my transom goes, the waterline is painted about 4 inches above the bottom of the transom. Here is a picture of the transom showing that there is about 2" of bottom paint showing so I have maybe 2" of immersion at the transom.



My idle rpms are about 860. The D2-40 is a 1.51 liter engine. Published fuel consumption curves are at maximum power for each rpm value. Engines generally run at much lower power except at their top end where the propeller curve intersects the maximum power curve. Consequently, the published fuel consumption curves are the maximum fuel consumption at each engine rpm value. In operation fuel consumption will be less than those values by as much as 70-80% at the lower end of the rpm range. Fuel consumption for a diesel is based on the amount of power produced. Produce less power and you use less fuel.
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Old 11-05-2015, 08:05 PM   #190
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We can do 9-10kts @ 5gph.....or 20kts for not that. I think our regular cruising will be around 9kt with the ability to get out of weather. (Loving this boat so far!!)
Having that extra speed at your command is great isn't it?
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Old 11-05-2015, 09:55 PM   #191
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OOps, I misspoke. Published fuel burn curves like the one posted above for the D2-40 are along the hypothetical prop curve on the diagram. The real prop curve can be quite different. My propeller curve, confirmed by speed measurements as a function of engine rpms is shown below. The rms difference between predicted and observed boat speed versus rpms was 0.3 knots. As you can see my boat needs between 6 and 11 hp (depending on seastate) at 2,000 rpms which is very close to 7 knots.



The curve above was calculated for a 13" pitch prop. When I had my prop reworked it actually came out to 13.35" pitch. The result is that I run at 6.9-7 knots at 2,000 rpms using just under 8 hp on flat water. My predicted fuel burn is 0.53 gph, so my actual fuel burn is a bit higher. At 1,500 rpms the boat is predicted to go 5.6 knots on 0.22 gph using 2.9 hp, which is again just a bit under observed results.
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Old 11-05-2015, 10:44 PM   #192
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OK I think TD is probably correct.

Just as an unequal but meaningful comparison the Albin 25 w a 27hp Yanmar can consistantly run 6 knots on 1/2 gph. Many in the Albineer's of BC report that and I went much faster (8.5 knots) burning .85gph w my Albin. Tortuga is a longer and generally much slicker hull but weighs double what Tortuga does. Another meaningful comparison is my own Willard w a hull that's like a barge compared to Tortuga and weighing 3.5 tons more cruises at only double the Tortuga's burn numbers. Still to me I think TD's numbers are in the ballpark.

For most people here w much much bigger boats and engines 1/2 GPH probably sounds like a guy in a bar saying he's getting 100mpg w his Prius. And there are few of us that run on 20hp or less and can relate to these low numbers.
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Old 11-05-2015, 11:48 PM   #193
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I have two boats.. On the boat in my avatar (1936 Nunes Brothers 33 with a 2011 Volvo-penta D2-40) I like to run at about 2000-2050 rpms which gives me about 7 knots at 0.55 gallons per hour (12.7 mpg). However, my wife prefers that I run at about 1,600 rpms for about 5.5 knots at about 0.25 gallons per hour (22 mpg).This summer I averaged 0.4 gph. I can do just over 9 knots wide open throttle.
I have a similar style boat (full displacement) with the same size engine (40 HP) and will say that these fuel burn numbers are exactly what I see. My water line is shorter so the the speeds are about a knot less.

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Old 11-06-2015, 12:48 AM   #194
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Having that extra speed at your command is great isn't it?
Yup!
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Old 11-06-2015, 01:08 AM   #195
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"5.5 knots at about 0.25 gallons per hour (22 mpg)." Really, four cups per hr to move many tons 5.5 nm through the water! That's unbelievably efficient!
After carefully reading all posts since I posted the quote above... I want to change the word "unbelievably" to incredibly.

TD - and others - thanks for backing up the very low, low burn numbers so
I can wrap my head around power boats that virtually don't use fuel.

At 1/2 gal per hr in 200 hrs use per year; averaging 5.5 nmph = 1,100 miles traveled on 100 gals X $3 per = $300 annual. Well less than a buck a day averaged out for full year... and... around 0.30 per mile! Not too shabby for pleasure boating!!
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Old 11-06-2015, 01:40 AM   #196
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The question that started this thread was asked back in 2011. And what I said in post #2 still holds today:

Quote:
Fuel is the cheapest thing in boating anyway--- if we were concerned about fuel burn and cost we wouldn't be boating at all.
I find this obsessing over fuel burn to be fairly bizzarre, particularly in the toy boat realm. If one is going to cross oceans or spend most of their lives on the move, sure, it makes sense to want to know what the fuel burn is so one can budget intelligently. If you're running a fleet of tankers or bulk carriers and trying to make money doing it, absolutely, fuel burn is a major consideration and it makes sense to calculate it down to the number of inches per pint.

But for the vast majority of recreational boaters, I don't see that it makes any difference whatsoever what their boat's fuel burn is. If a boater is like the BandB crowd and wants to go fast, it's not rocket science to figure out that it takes a lot of fuel to do that. One can either afford it or they can't.

If you are boating on a budget--- if there even is such a thing --- it's not rocket science to figure out that one needs a boat that doesn't burn much fuel aka small, slow, or both.

The 30,000 pound, twin-engine cabin cruiser we keep in the PNW burns about 5 gph at about 8 knots, give or take. The only time fuel consumption even enters our minds is when we want to make sure we don't run out of it on a given trip. So we do a basic calculation even our dog can do using distance, time, and a fudged fuel consumption with a margin of error in our favor thrown in to account for the unfavorable currents we'll have at least half the time and call it good. So far we've never been caught short.

As to cost, it costs what it costs. If it started costing enough that it became a concern we'd get out of this kind of boating and do something else. The difference between 5 gph and 5.5 gph and 6 gph isn't going to mean squat in terms of whether we can afford this kind of boating, nor is it going to mean squat to whether we can go on such-and-such a cruise. There are plenty of fuel stops around here should we find we need to get more during a trip.

The cruising boat we have in another part of the world burns more bloody fuel than I can get my head around. But the same thing applies--- if it was too much for us we wouldn't have gotten into this other type of boating.

Pissing about figuring out if the fuel burn is a tenth or a quarter of a gallon more or less an hour in the one boat and ten or fifteen gallons more or less an hour in the other one is a totally pointless exercise in my view because whatever the answer is, it's not going to change anything. We're going to use both boats until the day comes we decide we don't want to anymore.

This is one topic on which I totally agree with Art. I don't know when or where he said it, but I know in the past he's said (I'm paraphrasing) don't sweat the small stuff. Just go boating and enjoy the hell out of it.
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Old 11-06-2015, 06:34 AM   #197
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Marin, not sure why other TF members strive for efficiency bothers you so. People boat for all sorts of different reasons and work at making their boat the best it can be for them. Why don't you go spend your time on the "Making your cabin cruiser go faster" thread.

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Old 11-06-2015, 06:41 AM   #198
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Its not the fuel burn that creates a great trip,

its quiet sooth engine operation ,

that is only noticed when the engine is turned OFF!
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Old 11-06-2015, 07:11 AM   #199
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Having that extra speed at your command is great isn't it?
Yes!!! Lovin' it!! The extra space too!
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Old 11-06-2015, 07:24 AM   #200
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Marin, not sure why other TF members strive for efficiency bothers you so. People boat for all sorts of different reasons and work at making their boat the best it can be for them. Why don't you go spend your time on the "Making your cabin cruiser go faster" thread.

Ted
Some may have forgotten that back in 2011 many of us who burn a lot of fuel a year were worried about $5.00 a gallon or more fuel.

And for some of us that WOULD have been the biggest annual boating expense.

But I know it's hard for some to have the big boating overview...for some of us it's a major part of our lives...for others it's just something to do between other things.
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