gas compared to diesel in cruisers 3470

[Jerry l Jones]

I don't understand, powerboat reports did a test on a cruisers 3470 with 2x yanmar 4lha-stp (240 hp) diesels and it got poor milage, I found a test by boattest.com with the same boat but with 2x310-hp MerCruiser 7.4L MPI v drives and it did much better. I thought diesels should do better,

diesel ---------------gas
3000 rpm------------3500 rpm
22.2 kt ---------------27 kt
.81 nmpg------------.94 nmpg

top speed ----------------top speed
3400 rpm------------------4300 rpm
27.8 kt----------------------33.2 kt
.67 nmpg-------------------.68 nmpg

[yipster]

hello,

consider the diesel engines weight and also dieselfuel is heavyer than gas, still i would have expected better diesel performance also...

yip

[Nomad]

It's because thae engines are Yanmars. They are higher rev than most others.

[Willallison]

There's a whole host of reasons why this could be the case. Test conditions - ie the weather, different fuel / water / people / gear loads, fresh or salt water etc etc all need to be the same for accurate comparisons. The only way to do this properly is to run the boats one after the other, using the same test equipment at the same time in the same place, by the same people. Incorrect props, incorrect data.... the list goes on.
Nomad's correct about Yanmars tending to rev a little higher than other diesels - though I've never heard of them having a reputation for being thirsty.....
Perhaps the boat is simply underpowered with the 2 x240 hp engines. My 27ft Searay (1 x260hp petrol) returns very similar consumprion figures to the same boat powered by the larger 330hp, 7.4L motor. Both boats get approx 2.3 to 2.5 nmpg - yet mine is cruising at around 20 knots, whereas the bigger engine cruises at around 25.

[Nomad]

Wen't I said they were thirsty I was suggestion an under power vessel. They are also hight rev. All of Will's other suggestions are good ones.