Kamd43 optimum fuel economy

[Momac]

To help resolve a friendly debate....
What is the engine speed for optimum fuel economy in a kad43 engine in a planning motorboat (not including going at very slow , around 5 knots) assuming a calm sea.

 

[Mr Googler]

0 rpm......whilst being towed at 25 knots :encouragement:

 

[BruceK]

Too many variables. It will be the speed he is just comfortably on the plane at circa 18-20 knts and somewhere between 2900 and 3100 rpm or 65% throttle. At least that is mine on KAD42's (omitting displacement speeds)

 

[Momac]

Too many variables. It will be the speed he is just comfortably on the plane at circa 18-20 knts and somewhere between 2900 and 3100 rpm or 65% throttle. At least that is mine on KAD42's (omitting displacement speeds)

I didn't expect precision .That's near enough.
I had thought more like 3300rpm since a tiny bit of throttle takes my boat from 3000 to 3300 or thereabouts and takes me from 20 to 24 knots on smooth water..... but that's with kad32,s. It feels like a lot of gain for a little throttle.
A shame a trip computer seems difficult to fit.

 

[BruceK]

when I was saying comfortably on the plane I was referring more to what you have experienced. There is a point I find when the boat doesn't feel like it's being sucked back any more. When it seems to suddenly step up and become quite light. For you it's still around the 20 - 21 knt mark. That little bit of performance gain is due to your hull coming on song but you are still buying it with fuel.

 

[Restoration man]

I find if I’m just on the plane + say another 200 revs max that’s about the optimal speed /revs ,I can plane at 14 kts but as others have said it feels just bit lighter with the extra 200 revs and I pick up about another 2 knots so prefer that speed ,16kts is not to bad imho you get places at an ok pace , but if I’m going at 20kts you really don’t get there much faster and it’s not worth the extra fuel , anything more than just over planning speed and it drinking it faster and faster but you don’t go significantly faster to out way the extra fuel , ours are kad 42

 

[PowerYachtBlog]

In my experience and on my boat it is around 3 to 3100. I achieve usually about 2.6 and or 2.7 liter per nautical miles.

But each boat has a variation on this. A Cranchi Zaffiro achieves about 2.8 and Fairline Targa 37 should be about 2.9 at best while a Targa 34 or Sessa 35 Oyster achieves 2.6/.5.

If your Sealine has Kad43 with shafts I think you are looking at around 3100/3200, the boost of the turbos increases quite a bit over 3300 so does fuel consumption, and I think you should be around 3.3/.5 liters per nautical mile.

 

[gordmac]

My friend's BT uses about the same fuel at 20kn as 28kn with slightly less at around 25kn.

 

[Momac]

My friend's BT uses about the same fuel at 20kn as 28kn with slightly less at around 25kn.

Not much in it then.
So may as well go at whatever speed is comfortable.