What determines diesle boat economy?
- Thread starter pdecat
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Ligghter is better. It's a combination thing. Hull shape, length, size of engine etc. determine economy. Some criteria affect it more than others but inthe end it's the package.
Agree, it is a combination of things. IMO the two that factor the most are hull shape & waterline length. The more slippery the hull shape the better. The longer the waterline length the faster the hull speed. If a single will comfortably push the boat at its cruising speed there is no reason to have two engines with all of the added costs of having a second beast in the bilge. Weight does not matter much. Once weight is moving it does not take much to keep it moving. A 300hp motor running only a little above idle to produce 50hp to move at a desired speed will not burn much more than a 50hp motor running WOT to produce that 50hp & same speed.
1200, maybe 1400rpm gave me 6kts in my 100hp 27' Albin. Only went to the fuel dock a few times a season & the boat was out every weekend. While friends in their go fast boats were in line at the fuel dock I would cruise sedately by, beverage in hand cause I did not have to worry about spilling it at my speed, & they might catch up to me a little before our agreed anchorage. Going slow has its benefits.
1200, maybe 1400rpm gave me 6kts in my 100hp 27' Albin. Only went to the fuel dock a few times a season & the boat was out every weekend. While friends in their go fast boats were in line at the fuel dock I would cruise sedately by, beverage in hand cause I did not have to worry about spilling it at my speed, & they might catch up to me a little before our agreed anchorage. Going slow has its benefits.
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Torque power..
Since I screwed up the original question when trying to get it back as active I'll answer my own question
Displacement is the primary factor determining fuel use. Hull shape has some bearing but is relatively minor, within the usual boat types, compared to displacement.
How much water the boat disturbs and the friction moving that water creates determines how mush power it requires to move at 6 knots.
A heavier boat of the same length sinks lower or displaces more water that must be moved by the engine power. That is the reason racing boats always try to be lighter.
Consider an empty 40 ' rectangular floating box vs a 40' boat of any shape. The boat would be significantly heavier than the box and need more power ( fuel) to move it.. We have all experienced that lighter boats seem blown about by wind much more easily that heavier boats.
Now of course if we load that floating box to the same weight as the boat we would then see more power required to move it six knots and because it would be less slippery than any boat shape the shape factor would come into play.
So it is displacement that is the primary factor in fuel use, shape as in normal boat designs is a smaller factor.
Displacement is the primary factor determining fuel use. Hull shape has some bearing but is relatively minor, within the usual boat types, compared to displacement.
How much water the boat disturbs and the friction moving that water creates determines how mush power it requires to move at 6 knots.
A heavier boat of the same length sinks lower or displaces more water that must be moved by the engine power. That is the reason racing boats always try to be lighter.
Consider an empty 40 ' rectangular floating box vs a 40' boat of any shape. The boat would be significantly heavier than the box and need more power ( fuel) to move it.. We have all experienced that lighter boats seem blown about by wind much more easily that heavier boats.
Now of course if we load that floating box to the same weight as the boat we would then see more power required to move it six knots and because it would be less slippery than any boat shape the shape factor would come into play.
So it is displacement that is the primary factor in fuel use, shape as in normal boat designs is a smaller factor.
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I have no opinion on that because any one application could have dependencies that drive the economy.
If this is merely about fuel economy I thought it was a water length vs. beam kind of thing. Otherwise tankers would be short and wide.
If this is merely about fuel economy I thought it was a water length vs. beam kind of thing. Otherwise tankers would be short and wide.
Rated engine HP is often equated with fuel use by people looking at boats. They somehow imagine 400 hp engines will use much more fuel than 100 hp engines at the same speed
It takes a certain amount of power to move any boat of any design at six knots. That same amount of power can be provided by high or low max power engines but since the power produced by a diesel engine is mostly similar in 100 hp and 400 hp diesels at about 18-20 HP per gallon of fuel per hour if you are not using the power you are not using the fuel.
So our six knot diesel boats fuel use will not primarily be affected by engine max hp it would be mostly similar.
It takes a certain amount of power to move any boat of any design at six knots. That same amount of power can be provided by high or low max power engines but since the power produced by a diesel engine is mostly similar in 100 hp and 400 hp diesels at about 18-20 HP per gallon of fuel per hour if you are not using the power you are not using the fuel.
So our six knot diesel boats fuel use will not primarily be affected by engine max hp it would be mostly similar.
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Obviously displacement will have the most impact but for practical purpose when this question comes up it's going to be to compare boats of fairly similar size and displacement, ex a prospective buyer looking at two or three 50 footers with different hull shakes and beams. In that case hull shape and beams will have the biggest impact in fuel economy, not displacemnt which is likely to be pretty similar,
Beam has a big effect and was not included in your poll. I ve run different 50 something hatteras at hull speed and there is roughly a 20% difference fuel burn between the earlier narrow 16' beam hull and the later wider 18' beam boats
Beam has a big effect and was not included in your poll. I ve run different 50 something hatteras at hull speed and there is roughly a 20% difference fuel burn between the earlier narrow 16' beam hull and the later wider 18' beam boats
Pascal,
"I ve run different 50 something hatteras at hull speed and there is roughly a 20% difference fuel burn between the earlier narrow 16' beam hull and the later wider 18' beam boats"
Was just thinking the "earlier" boats were more weight and the motors were not modern (efficient) I think that would play more into it than beam? Your thoughts? I do get that a wider beam (most cases) makes the boat have less hull in the water , hence lest resistance, but not sure that alone is the difference.
"I ve run different 50 something hatteras at hull speed and there is roughly a 20% difference fuel burn between the earlier narrow 16' beam hull and the later wider 18' beam boats"
Was just thinking the "earlier" boats were more weight and the motors were not modern (efficient) I think that would play more into it than beam? Your thoughts? I do get that a wider beam (most cases) makes the boat have less hull in the water , hence lest resistance, but not sure that alone is the difference.
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Isn't Pascal saying that the 18' beam boat presents more drag and burns 20% MORE fuel than the 16' beam boat, i.e., it's cheaper run the 16' beam, all other things being equal?
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