boat speed and engine revs - 2009 Bav 34 cruiser

[Bobb]

I'm seeing conflicting posts on full throttle for a D1-30 and the expected boat speed.

I have a 2-blade Al prop and get about 5 kts at 2200 rpm. While i see others cruise at much higher revs, i was under the impression that most efficient and best torque came at just over 2k.

But theoretical boat speed for my LOA is over 7 kts, so I'm confused.

What do others get? what RPM do you cruse at?

thanks ..

Bob
2009 Bav 34 Cruiser
S/V Sympatheia

[Rampage]

We’ve got a 38, cruise at about 6 knots at 2000 rpm but that’s with a Brunton Autoprop.  When we had to fit the original 2 bladed Volvo prop, we needed to increase revs to 2500 to maintain the same speed.
We don’t worry too much about exact engine speeds, rather we adjust revs to maintain our speed.  So many factors affect speed that it’s difficult to advise about rpm v boat speed.  How clean is the prop and the bottom of the boat?  If either are fouled to any degree, it’ll have an effect. 

[Symphony]

Not far out, but speed a little on the low side. The test is whether you can achieve maximum rpm or just under - that is between 3-3200rpm and a speed of somewhere around the 7.4 knots mark in flat water. The optimum cruising revs is around 75% maximum, that is 2400rpm which should give you 5.5 knots. These numbers are not absolute and cruising 2200-2400 is comfortable.

I have the same engine in my 2015 B33. This came with a 16*13 2 blade but I fitted a FlexoFold 2 blade 16*11 and I get 5.5 knots at 2200rpm and a maximum of 7.6 knots at 3100rpm. I have checked these figures over the measured 1/2 mile in Poole Harbour which is far more accurate than either the log or GPS.

It might be worth checking the size of your prop. It will be 16" diameter, but not sure of what pitch it would be. 1" of pitch changes revs by approx 250, so if you can hit 3200 easily you could go up 1" in pitch which would reduce revs for any given speed, so you would get approx 5.5 knots at 2200rpm rather than 2400. To be honest, though, unless you need a new prop it is hardly worth it. It makes no difference to fuel consumption as you still use the same power for a given speed, but some engines have a sweet spot which differs from the optimum revs. Be aware that pitch can be different for different styles of prop, so if you are changing to a folder or a featherer, take the size recommended by the manufacturer.

Hope this helps.