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Old 04-20-2022, 07:24 AM   #1
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Choosing the more efficient boat

While I realize there can't be a true answer until the boat is actually tested in the water, I wanted to ask for some help on fuel efficiency and guidance making the better decision.



I am currently looking at two boats:



First - Twin Detroit Diesels 8v71 reported 318hp, 1150gal fuel tank, prop unknown, direct drive, 70-feet lwl, draft 5 feet, wood, seller says 30,000lbs but I feel that's about 20ton low. Seller claims 500 mile range. I really like this boat.



Second - Single Diesel 420hp (make unknown), 1400gal fuel tank, prop unknown, 55-feet lwl, draft 6.5 feet, steel, seller says 82,000lbs, unknown range.



Is the best I'm going to get out of the DD's 1 nmpg? I have searched here and seen others with various Detroit flavors getting almost 2 nmpg! I don't have a good understanding about this figure, if it is calculated with both engines running at 0.5 gph each?



The other diesel is yellow, I haven't seen a clear picture. I assume its a cat. I do know it has a 6:1 reduction gear, which probably means it has a big prop. However, if this single engine uses 1gph and can push the boat to 6 or 7 kts, I feel that extra 200 miles is way better than the other, larger boat.



Thoughts?
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Old 04-20-2022, 08:20 AM   #2
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There simply is not enough information for anyone to hazard a guess with this level of detail.

I would suggest that you are evaluating the total cost of ownership as maintenance, storage, major repairs, insurance and moorage are greater factors to the cost of ownership for the vast majority of recreational boaters.
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Old 04-20-2022, 09:16 AM   #3
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Here are some parameters to investigate when making your decision:

Weight has the most direct influence on efficiency. With the same hull shape and lenght, if one boat weighs 25% more than another, it will take 25% more force to move it at the same speed. Notice I didn't say fuel consumption as that depends on the drive train, see below.

Speed is obviously another one and the force required to move a boat goes up exponentially the faster you go.

Hull type affects the force to move a boat. If a full displacement hull is 1 then a semidisplacement hull can be 1.5 times the force.

And finally engine type. Older Lehmans, DDs, etc typically make 16-17 hp per gallon of fuel at cruising speeds. Newer small displacement turbocharged, common rail injected engines can make 18-19 hp per gallon of fuel at cruising speeds. As you can see, it doesn't affect overall fuel consumption much, maybe 10%.

So the most efficient boat will be the lightest, slowest, displacement hull with a small 100 hp Yanmar or John Deere common rail engine.

Is that what you want?

David
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Old 04-20-2022, 09:18 AM   #4
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It also depends entirely on the shape of the hull and what speed you wish to achieve and how far you wish to travel. Crossing oceans or a quick weekend blast to impress the girlfriend
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Old 04-20-2022, 09:26 AM   #5
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Originally Posted by DavidM View Post
Here are some parameters to investigate when making your decision:

Weight has the most direct influence on efficiency. With the same hull shape and lenght, if one boat weighs 25% more than another, it will take 25% more force to move it at the same speed. Notice I didn't say fuel consumption as that depends on the drive train, see below.

Speed is obviously another one and the force required to move a boat goes up exponentially the faster you go.

Hull type affects the force to move a boat. If a full displacement hull is 1 then a semidisplacement hull can be 1.5 times the force.

And finally engine type. Older Lehmans, DDs, etc typically make 16-17 hp per gallon of fuel at cruising speeds. Newer small displacement turbocharged, common rail injected engines can make 18-19 hp per gallon of fuel at cruising speeds. As you can see, it doesn't affect overall fuel consumption much, maybe 10%.

So the most efficient boat will be the lightest, slowest, displacement hull with a small 100 hp Yanmar or John Deere common rail engine.

Is that what you want?

David

Don't forget, that while an SD or planing hull may take significantly more fuel to push at the same weight (and same speed), in reality the difference is typically smaller. The SD and planing hulls are often lighter, which offsets some of the efficiency loss from the hull shape.
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Old 04-20-2022, 12:23 PM   #6
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Thanks for the replies. I have a better understanding.

To answer the questions, range and comfort at sea would be the most important options. Sure, being able to get out of weather would be ok, but being hove to is a fine option as well, so the need to get fast is not a necessity.


Slow is great, I dont mind long passages. It's just the rising price of fuel that has me concerned.

Best I understand, both of those options are full displacement.
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Old 04-20-2022, 12:24 PM   #7
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On a 70' wood boat, fuel costs will be lost in the roundoff error when you account for your overall expenses.
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Old 04-20-2022, 01:32 PM   #8
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My first private boat, 59 years ago, was similar to your 70'. It was SD, 65'x16' and about a 5' draft w/twin 671 natural DDs. Diesel was about 18˘/gallon, a much better burning fuel, and stable for years. I ran 1800 to 2100 rpm and made 10kts and above burning 10 to 14 gph. I never ran slow. Max continuous is 1800 but they are DDs.

My current boat is similar, too. SD, 83'x17' and near 5' draft. Twin 671 natural DDs. Diesel is $5+/gallon, burns poorly, and is about as stable as a crackhead. I always use an additive that improves combustion, etc. I always run at 1800 but POs ran at 2100+. I burn 8.5 gph @10kts. At 6.5kts about double the mileage. The better mileage over my 1st boat is longer hull, fuel additive, and I tune DDs better now.

A wood boat requires a lot of maintenance. It's fine for me, I'm a shipwright, I've had lots of wood boats, and my requirements for the current boat is it has DDs, and live longer than I do. Except for clean oil and fuel, I haven't touched my engines in 10 years. There has been no measurable valve wear, no injector changes, no pump failures, etc.
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Old 04-20-2022, 01:59 PM   #9
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The difference in dockage between a 70 and a 55 foot boat will likely be more than the fuel difference

The comfort difference in a seaway between a 70 and 55 footer will be hugely different ( assuming similar hulls ).

I tend to think of being "hove to" as a sailboat concept. Not all powerboats are able to do that. ( engine vents, window size, stability limitations )
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Old 04-20-2022, 03:34 PM   #10
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Quote:
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I tend to think of being "hove to" as a sailboat concept. Not all powerboats are able to do that. ( engine vents, window size, stability limitations )
Hove to is a purely sailboat or motor sailor concept. It means backing the sails so the boat is stable and moving forward slightly or none at all. It just doesn't apply to a boat without sails.

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Old 04-20-2022, 03:45 PM   #11
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I would go with the steel single engine in a heartbeat. Single engine will have the prop and rudder protected by the keel. Better fuel consumption. Cat over Detroit. Then there is the maintenance of a wooden boat. Had both. No comparison! A final consideration will be costs and availability of dockage.
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Old 04-20-2022, 08:56 PM   #12
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I get your concern with fuel but my experience has been the 15 to 20k annually, not including any separate upgrades ,is where the cost is.
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Old 04-20-2022, 09:17 PM   #13
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Quote:
Originally Posted by JohnRichard View Post
While I realize there can't be a true answer until the boat is actually tested in the water, I wanted to ask for some help on fuel efficiency and guidance making the better decision.



I am currently looking at two boats:



First - Twin Detroit Diesels 8v71 reported 318hp, 1150gal fuel tank, prop unknown, direct drive, 70-feet lwl, draft 5 feet, wood, seller says 30,000lbs but I feel that's about 20ton low. Seller claims 500 mile range. I really like this boat.



Second - Single Diesel 420hp (make unknown), 1400gal fuel tank, prop unknown, 55-feet lwl, draft 6.5 feet, steel, seller says 82,000lbs, unknown range.



Is the best I'm going to get out of the DD's 1 nmpg? I have searched here and seen others with various Detroit flavors getting almost 2 nmpg! I don't have a good understanding about this figure, if it is calculated with both engines running at 0.5 gph each?



The other diesel is yellow, I haven't seen a clear picture. I assume its a cat. I do know it has a 6:1 reduction gear, which probably means it has a big prop. However, if this single engine uses 1gph and can push the boat to 6 or 7 kts, I feel that extra 200 miles is way better than the other, larger boat.



Thoughts?
Gut feel, but an educated gut from an ex delivery skipper who "ate my own dogfood" with guesses like these.

#1 with larger twins will get right at 1 nmpg over long runs. #2 with a single will get closer to 1-1/2 nmpg. Although #1 has longer lwl, at 20t, designed for higher speed based on PO statement of 500 nm range with 1150g diesel. #2 being slower will have more generator time and more hours and oil changes.

In the end, difference will be negligible. If you're doing some sort of T-Chart, could go either way. Whenever I've done a T-Chart to arrive at an objective decision, I've always managed to skew the data a bit to reinforce my lean anyway.

I have to say, I'm stunned I've written this much given the paucity of information the OP has offered. No makes, no links, nothing.

Peter
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Old 04-22-2022, 07:33 AM   #14
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I would go with the steel single engine in a heartbeat. Single engine will have the prop and rudder protected by the keel. Better fuel consumption. Cat over Detroit. Then there is the maintenance of a wooden boat. Had both. No comparison! A final consideration will be costs and availability of dockage.
What he said, except I would add that since this sounds like it may be a project boat or complete refit, if you’re planning on keeping it long term and doing long distance cruising, then I would repower.

You don’t need half that HP, and old Cats are expensive to maintain and lousy with fuel efficiency.
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Old 04-23-2022, 06:07 AM   #15
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Think I'd pass on both boats. Don't have enough resource to deal with wood. Don't know enough about steel to have a clue... and for us, shallower draft is better, so 6˝' isn't attractive.

Think I'd find a boat with creature features we need/want/etc. (none mentioned in OP) and then re-evaluate for whatever engine(s) that boat came with. Double, single, either good. Fuel, just a cost of doing business, not the biggest blip on the wallet.

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Old 04-25-2022, 04:12 PM   #16
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Go twin screws. Go with the boat that is in the bast condition.
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Old 04-25-2022, 04:35 PM   #17
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. Diesel was about 18˘/gallon
And dinosaurs still roamed the earth

Clearly not all of them, some had to be made into diesel
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Old 04-25-2022, 04:44 PM   #18
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You are comparing apples to watermelons.
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Old 04-25-2022, 05:20 PM   #19
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You are comparing apples to watermelons.
That's what I was thinking, but I'd go with the "twin" option.
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Old 04-26-2022, 12:18 AM   #20
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Yup this article is definitely clickbait. Wood v steel, single v twin, Cat or DD's
OK, I may as well join in and create more confusion so here goes.
Wood v steel, a wooden boat is like a bad marriage, its gotta go, the only difference is time, as you get older the constant maintenance wears you down.
Single v twin. K.I.S.S diesels now are way more reliable so take yourself off to a diesel maintenance class and learn about diesel tlc.
Cat or DD, Cat are in my opinion a better engine because from the design concept they are built to provide torque, have worldwide service backup and ideal for swinging a big prop, DD are dated and older technology.
Whichever boat you choose just easing up on the throttle just a couple of hundred RPM will make little or no difference to your ETA but will certainly make a difference to your fuel and way the cost is going up you will thank me in years to come.
If cruising on the sea, take time to get in the rhythm of mother nature and use the tides, use the wind direction, less waves head on, mean less resistance = less fuel.
And above all that think of you and your partners personal comfort onboard because a happy crew means you get to keep the boat.
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