Comparisons Between 2 Cruisers

[richrob23]

My main question is which boat would burn the most fuel at 10 knots?

Ocean Alexander with a displacement of 18,200 kg and twin Cummins 385hp
13.96 meter length

verses

President 47 with a displacement of 23,000kg and twin Caterpillar 3208 375hp
14.43 meter length

both boats are semi-displacement GRP with a beam of 4.80 meters.

Which is the most fuel-efficient? The President is heavier with smaller engines. In theory, this would mean that it would burn more fuel than the Ocean Alexander. But is this true in practice?

[tkeithlu]

The 47 has a greater displacement, so yes, all things otherwise equal it will use more fuel. But other things aren't equal. The larger engines eat up more HP just turning themselves, for starters.

If you are keeping to hull speed the difference might be trivial. There are other factors that may influence fuel burn in any case, such as the condition of the respective engines, the air supply, the state of the bottom paint, the hull shape, and on and on.

[Cheechako]

I would guess the longest waterline will be the most efficient. Very little HP will get you most of the available WL speed. It's gaining the last 15-20% (?) of speed that eats up the fuel.

[goboatingnow]

By no means scientific but I once had a 55hp nanni mobo ( 36 feet ) and then many years later a 612hp TSDY( 44feet )

A displacement hull speed 6 knots of the nanni , and similarity 8 knots for the TSDY , the two did not consume that much difference in diesel ( about 1.3x for the bigger engines ) the nanni would be at 2400 rpm and the TSDY would be at 1100 rpm.

Of course once you opened the throttles the bigger boat began to plane and the fuel gauge seemed to be mechanically linked to the Rev counters !

[denverd0n]

Both boats are going to have to go significantly above hull speed to hit 10 knots. As such, the lighter boat will probably burn less fuel at that speed.

Though, the truth is that if you are concerned about fuel burn, you should keep the speed below hull speed. As Cheechako alluded to, once you get past hull speed in this kind of boat, the fuel burn goes up dramatically.

[richrob23]

Thanks for your comments. Yes, I agree, accurately determining the max hull speed is critical. One study I saw carried out demonstrated this well. The difference in fuel burn between 8 knots and 10 knots was double!!

I see there are many online calculators for max hull speed. I just enter the waterline length and press 'go'. As that is the only data point used I am not sure how accurate the result would be. I would have thought that draught, hull shape, keel type would all affect the outcome. Comments?

[valhalla360]

I didn't find the waterline length but assuming 40ft, that's going to give you a hull speed of around 8.5kt...so it's going to take a good bit of fuel to push to 10kts.

Engine sizes are pretty similar so any internal loss difference will be negligible.

Worlds difference will be slowing down to maybe 7kts. That will probably give you an MPG 2-4 times higher than 10kts.

[skipmac]

Originally Posted by richrob23

Thanks for your comments. Yes, I agree, accurately determining the max hull speed is critical. One study I saw carried out demonstrated this well. The difference in fuel burn between 8 knots and 10 knots was double!!

I see there are many online calculators for max hull speed. I just enter the waterline length and press 'go'. As that is the only data point used I am not sure how accurate the result would be. I would have thought that draught, hull shape, keel type would all affect the outcome. Comments?

The standard formula for hull speed (some use slightly different constant) is 1.35 X square root of the waterline. Yes this is more or less a rule of thumb and many other factors will effect the exact speed. However except in extreme cases the difference between theoretical and actual hull speeds will be fairly small.

For a specific boat a simple test would give a pretty good idea of the most efficient top speed. Run up to a speed maybe a couple of knots less than the theoretical hull speed and note the RPMs. Increase in very small increments and watch the increase in speed. You can plot all these on a graph and at some point you will see the line quickly flatten when you reach the point of diminishing returns.

[rondelais]

As others have mentioned, “hull speed” as obtained from the formula is not a perfect measure. At low speed, the resistance to going through the water increases as the square of speed. However, when speed becomes high enough to develop a significant bow wave, resistance starts to increase very rapidly. Thus, a graph of resistance versus speed shows a fairly abrupt hook upward. The exact shape of the curve does depend on the hull geometry, but the point of inflection of this curve should be somewhere close to the calculated “hull speed”. The reason that this calculation depends on waterline length is that what is actually being calculated is the speed at which the length of the bow wave is about the same as the waterline length. A simple way to think about the effect is that for a speed just below “hull speed”, the boat is supported at both ends, and at a speed above, the boat has to go uphill, taking a lot more power. Of course, it’s more complicated than this, especially when planing occurs.

[mickand]

Assuming a waterline of 43ft, your hull speed would be around 8kts. Cruising at 10kts and pushing water may increase fuel consumption dramatically.

[nortonscove]

Add propeller size and pitch, gearbox ratios, and a whole host of other factors to your consideration and you'll begin to understand the complexity of the question. I would think nothing short of a lengthy cruise in both would give you much of an idea given the propensity of most sellers to state best case scenarios when filling in this information.

[mvweebles]

Past delivery skipper here, primarily motor yachts. Both boats will get approximately 1 nmpg at 10 kts. Not an appreciable difference.

Peter.