3YM30AE fuel consumption

[geoffr]

I'm trying to figure out what the spec'd fuel consumption is for a 3YM30AE. I have two different curves from Yanmar which are very different.

(We are trying to figure out the sweet spot for our boat, but are questioning our correctness/ability to measure our fuel consumption ( vs RMP & speed)

thanks

[StuM]

I'd go with the figures here:


https://www.yanmar.com/media/global/...et-3YM30AE.pdf

[geoffr]

Stu,

That is one of the documents I have.. the other I just found again is this one: https://www.yanmarmarine.com/theme/y...et_3YM30AE.pdf

which would be right??

Originally Posted by StuM

I'd go with the figures here:


https://www.yanmar.com/media/global/...et-3YM30AE.pdf

[Sailbaum]

They are both "correct", but one shows fuel consumption with a prop load, based on theoretical efficiency of a three bladed prop. Note the legend at the top of one of the fuel curve tables. The other is based on a theoretical full load through rpm range.

[StuM]

Do you have a 120 or 125A alternator?
Saildrive? SD20 or 25 ?


More importantly, what sort of prop. That has an effect on fuel consumption at the same RPM.


One of those documents lists 1212 RPM @ 2.64 reduction (i.e. propped for 3200 WOT), the other lists 1364 RPM @2.64 reduction (3600 WOT).

Do you get 3200 or 3600 at WOT. That will dictate which curve is likely to be more accurate. You can also extrapolate if you are propped to some other RPM at WOT.

[geoffr]

Stu,

Thanks for the clarifications and pointing out some bits I missed.

We have the 2.62:1 transmission KM2P transmission which is driving a Sillette outdrive which has a 3 blade prop. We a slightly under propped and at WOT get to 3200rpm.

We have the 125A alternator, now connected to a Balmar MC-614 (which pushes it harder than the factory regulator). Which kicks over to float part way through each day of motoring. (can't sail as the mast is down right now)

Any thoughts on an extrapolation?.. I'm not sure how to go about it...

thanks!

[StuM]

If you are getting 3200 at WOT with your KM2P, the 1221 RPM @ 2.62:1 figure would be the appropriate one. That would match the data from
https://www.yanmar.com/media/global/...et-3YM30AE.pdf

[DougR]

If you are getting 3200 RPM at WOT you are propped just about correct, as the max. RPM for the 3YM is 3200.

For an approximation of your fuel consumption use the graph labeled " prop load exp. 3.0". The other product bulletin appears to be a misprint, and it's fuel curve appears to be based on a 3600 rpm engine prop curve. (Not a full load curve)

The hull resistance curve of a full displacement boat approximates the mathematical exponential curve of 3.0, and it is this resistance that effects the load on the prop and therefore the amount of fuel that will be consumed at the various RPM points. So Yanmar publishes this 3.0 prop load curve knowing that the engine will most likely be used for displacement hull sail boats. The graph has nothing to do with the number of blades on the propeller, and holds true for correctly propped engines with 2 or 3 or 4 blade props.

If you examine the fuel consumption curve of the Yanmar 4LV 230 engine you will see that it is labeled " prop load exp. 2.5 ". Does that mean it's for propellers with 2.5 blades? Of course not......The reason is that the hull resistance curves of most planning hulls approximates the shape of the mathematical exponent curve of 2.5, and this graph will give a good approximation of the fuel consumption of such a planning hull.

All of that said, the only way to get a really good handle of the consumption of your hull is to do a sea trial and measure it. Normally this is done with numerous
" up and back" runs at various rpms and load conditions with a fuel flow meter and graph it all out. If you don't have a fuel flow meter the same can be done with calibrated containers, it just takes a bit longer.

DougR