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Old 08-29-2016, 06:06 PM   #1
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Grand Banks 1985 32' 135 Ford Lehman Fuel Consumption Data and Plots

Here is a link to a Google sheet that I am in the process of putting together for my own use.

https://docs.google.com/spreadsheets...it?usp=sharing

Some of the data comes from the Factory specs, some the Sea Trial, and some from my very limited experience with the boat (purchased it August 18, 2016). I hope to update the Factory hull speed in the next few days. Then I will see if the over propping might have given better fuel economy. If it doesn't I may switch back to the original pitch to lower my idle speed and let the engine run a little easier (less fuel per cycle for a given hp).

I used the prop loading curve for factory fuel curve. Right now my fuel consumption is my best guess. But it matches the generic formula for fuel consumption at hull speed pretty well.

Hull Speed knots (sqrt LWL * 1.15)
For my boat this is 6.1 knots

(Disp in metric tons * 2.5) Boat weight loaded / 2240 * 2.5 = hp required @ HS
I used 20500 lbs for my boat (full fuel and water tanks) Boat empty weights 17500 lbs I think.

For a single Diesel HP per Gal = 18.
I've seen numbers from 15 to 18 for this calculation. The 18 HP per Gal seems to fit my boat pretty well.

I would welcome any comments, questions, or corrections.

Hope someone finds this a little useful.

Also note that my prop has 4" more pitch than the original prop hence the lower max RPM from the factory specs.
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Old 08-29-2016, 07:53 PM   #2
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Don:

You definitely are a numbers guy and maybe an engineer as well, so let me offer some rules of thumb, mathematics and data from Ford's data sheet for that engine:

Hull speed is a qualitative thing as well as a mathematical/geometrical thing. Qualitatively it is when the speed of the boat matches the bow wave speed, ie it starts to climb the bow wave and the horsepower required goes up dramatically. Mathematically/geometrically it is usually expressed in knots as 1.34*sqrt(LWL).

Empirically there is a rule of thumb for true displacement speed hulls (which the GB32 is not) that says that it takes 1.5 * displacement in thousands of pounds to push the boat to hull speed as defined above. I have verified this rule of thumb pretty closely with the data on 6-8 boats that Lieshman included in his revision of Bebe's Voyaging Under Power.

Semi-displacement hulls require more power than the factor of 1.5. Opinions vary from 2.0 to 2.5, but my downeast style Mainship Pilot 34 is more like 3.0.

So your GB 32 probably has a LWL of 29 feet so its hull speed is 1.34*sqrt(29) = 7.2 kts. At your 20,500 displacement, it will take 41 to 51 hp to push it to 7.2 kts.

The Ford data sheet (for a 120 hp version, I can't find it for the 135 hp version) shows it using .39 lb (of fuel)/hp hour at its most efficient point, about 1,600 rpm. Diesel weighs about 7 lbs per gallon, so that number converts to 17.9 hp/gph, pretty close to your number above. It indicates that you will burn 2.3 to 2.8 gph at 7.2 kts

Finally, a crude way to estimate power required to turn a prop is to use the formula power= fudge factor*rpm^2.5 (or a 2.5 to 3.0 exponent, 2.7 being used most often). This formula is a bit empirical with a bit of theory thrown in, but don't ask me to explain the theory. The fudge factor can be derived I suppose from principles but an empirical way is to take the rated maximum rpm of an engine, plug the formula into a calculator (or Excel spread sheet) and play with the fudge factor until you get the rated wot power. Then you can use the fudge factor at lower rpms to calculate the power required.

All of the above can be used to construct power required vs rpm vs fuel burned and relate that to boat's displacement speed. If you develop empirical data for your boat's speed vs rpm then you can use the 2.7 formula to calculate the power required at any speed. It will be reasonably accurate up to about 1.3*sqrt(LWL) then it will calculate low up to about 2*sqrt(LWL) as the boat climbs over the hump, then it will come back to reasonably accurate after that.

But an easier way is to use boatdiesel's advanced prop calculator. It uses actual wot speed, rpm and hp for your specific boat to calculate a hull form factor. It then constructs the above curves for your specific boat and does use a more sophisticated power formula than the simple exponent one. It requires you to join boatdiesel for $25/yr. dues, but given your interest it will be the best $25 you spend on boating as it gives you access to some real pros in marine engines and troubleshooting them.

So I applaud your interest. I will be happy to explain any of the above, but don't ask me to defend it ;-).

David
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Old 08-29-2016, 09:03 PM   #3
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GB 32 fuel consumption

djmarchand Thanks for your input!

FYI the GB32 has a LWL of 30' 11" So that would boost the hull speed a little bit from your calculation.

I have also seen most places use 1.34*sqrt(LWL). I choose to use the 1.15 as some others do because it is more conservative, and for my boat, the efficiency starts to drop just before the calculated hull speed from using 1.15. Not saying that is correct just explaining why I choose 1.15 vs 1.34.

Climbing over the hump with my boat is not going to happen. 135 hp vs 10 tonnes kind of limits me to a 8 knot cruise speed (max with my prop is 9.2). But I don't mind, I'm retired an my philosophy is it's about the journey not the destination. Not for everyone but that's how I am. Plus I'm a cheap SOG and will do most of my running at 1000 to 1200 rpm.

Why do you say the GB is not a displacement hull? Is it because of the squared off stern? I know that creates a lot of drag compared to a rounded stern.

I ran the boat from Charlevoix, MI to Port Huron, MI (285 km) so I have a fair idea of my fuel consumption at 1200 rpm and it matches my data very well.

I already joined Boatdiesel an purchased the level 2 (I think that's what they called it) but haven't had the time to check out their calculator but will do that soon (next rainy day).

So my 2 cents says that at the end of the day experience will give you the best numbers for your boat. Hull condition, engine condition, and sea state play a big part in the fuel consumption. I know that 2' head seas slowed me down by about 0.2 knots or a little bit more. 6' to 8' following seas gave me about the same boost in hull speed. My engine has around 1800 hours on it and when I winterize it I'm going to have the oil tested. I couldn't have it tested before as the oil had been changed last fall when they winterized it and it hadn't been wet till we ran the sea trial.

Thanks again for the comments, formulas, and data.

Safe trips to you Don
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Old 08-29-2016, 11:37 PM   #4
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All said and done, wind and tide so effect the spread sheet formula that in the end in my opinion, experience/history will confirm fact and give one a range of both anticipated speed and fuel burn by one's seat of the pants numbers.
It is agreed, it is the voyage not the time involved when one has a slower boat (6-8 knots)
to wit: 1350-1400 RPM results in 6.3-6.9 SOG with the two elements mentioned creating any difference plus or minus. All this without any elaborate instruments or degrees. 1350-1400 RPM = 1.5 GPH fuel burn.

And yes, the boat is horribly 'over wheeled', (but quite 71 Dbs pilot house)

This is my history, i wont try to defend it given to debate.

Al-Ketchikan 27' Marben pocket CRUISER
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Old 08-30-2016, 05:36 AM   #5
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Thanks Al

Could you let me know what your prop specs are? Dia, pitch, and # of blades?
Mine has a 24X24 4 bladed prop. The original prop was a 24X20 4 blade. Is that the correct order for a 24" dia X 20" pitch or is it reversed?

No tides here but there is a 2 to 2.5 knot current in the river and it get to a little over 5 knots in the narrows under the Blue Water bridge.

Have a great summer or what's left of it. Don
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Old 08-30-2016, 06:07 AM   #6
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Don:


Read some of the epical threads on over propping an engine on this forum. IMO in some circumstances it is ok to do so up to about 4" of pitch, but you limit yourself to never running near wot as you will harm the engine.


But yours is a relatively small boat with a relatively large engine so here is what you gain and lose by over propping: If you needed to run at 1,600 rpm to push your boat to 7 kts with proper pitch (meaning you can reach rated rpm at wot) you will have to only run at 1,200 rpm with 4" more pitch. That provides slightly better fuel economy and lower noise.


But you will only be able to reach 2,000 rpm at wot and should never run it that hard, maybe up to 1,600 rpm to keep it from being lugged and over loaded.


And yes the Grand Banks 32 (as are all GBs) is a semi-displacement hull. It has a broad, flat run to the transom with little deadrise (the angle where the two sides meet at the stern). That hull form lets it produce lift at the stern at higher speeds but is not as efficient at slow speeds.


David


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Old 08-30-2016, 11:16 AM   #7
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Don- In general boats of similar yet different design perform differently.

Our hull is termed a SD, although given the performance, one would be inclined as we are, that it is more a displacement hull demonstrated by performance.

By calculation our boat shows a suggested prop somewhere around 19X13 3 blade. we are running a prop of 22X16. 3 blade.
Our specs used are 14000#, 25 LWL. 8.5 Beam at waterline, 2.5 draft ex keel depth (1foot) 85Hp at 3000 RPM.
Our current conditions as a result are a max of 2000 RPM WOT. Hull speed at WOT (Run at 5 min full WOT) was determined to be 8 knots. We run at a max of 1400 RPM resulting in an average of 6.5 knots. Our fuel burn as reported, averages 1.5 GPH.
As we maintain a reserve of 600 RPM or 1/4 of available RPM, the normal recommended for marine engine use as I understand the general understanding, we are content in the engine performance.

The Perkins 4-236 which is the model, has a wonderful reputation for being one tough platform for heavy duty application.
Given the manufacture gives three levels of RPM measurement, 2500 RPM for heavy application (My interpretation of the wording from memory) 2800 for intermediate duty, and 3000 for light duty.
My assumption is the 3000 with proper gear and prop is the more correct application intended for our boat.
To achieve that requires our changing to a 3:1 gear from current 2;1 or the recommended small replacement prop per the formula result.
The end result would increase our running RPM and increase the noise level with perhaps a anticipated fuel burn reduction to near 1 GPH,

So, in conclusion Don, we are way out of wack according to all the book calculations. yet,we have achieved near calculated hull speed (6.9-7 knots) at a reduced RPM,retaining a engine RPM margin,and with a satisfactory fuel burn with a quite confined pilot house noise level.

Hence the earlier comment of not going to defend what is working for this application.
other than to say, the end result was gained through 'seat of the pants' trial and error.

In reading djmarchand's response, there seems general agreement to much of our efforts and results.

Regards,
Al-Ketchikan, 27' Marben pocket CRUISER
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Old 08-30-2016, 04:24 PM   #8
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Interesting. We generally cruise our twin 135's at 1900 rpms which yields about 9.5kts. The factory chart (which I have) shows about 2.5 gph per engine at that speed, but we're seeing more like 3 per. Could be their chart is a bit optimistic, our tachs could be off a little, or maybe a bit overpropped. However your consumption numbers are also a fair bit over the factory numbers. (Granted you're definitely overpropped) so maybe it's just the nature of these engines. Like I said, interesting.

At some point I'll play with cruising on one engine to see what that does for fuel consumption. (Transmissions are ok with this and the boat handles fine in reasonable seas on one)

Ken
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