Cruising Speeds

[Maxumise]

I did some math and used a mercruiser fuel calculator website to find out the following:

If I need to be somewhere and want the most fuel efficiency, I should cruise at 3.000 RPM burning the least fuel per mile. But if I am just cruising around and want to boat for as long as possible with the least fuel use in total, then I cruise at +/- 1300 rpm. That way I use the least amount of fuel per hour.

So it al depends on what you try to archive. More time on the water or more miles to cover

[PJHoffnet]

Originally Posted by CapPap

Thanks PJ. That’s what I am curious about. However, I wonder if the larger yachts such as yours with diesels are more likely to travel at the slower speeds. My 35 footer has gas engines. I may try a journey at slower speed this summer to test it out. Time is less of an issue for me these days.

Concur, gas guzzlers are a completely game then my slow rolling diesel. 'fraid I can't speak to that, but there are plenty here who can.

[Gargiulotime]

Steve I am still chasing the expected knots at 22-2400 rpm. I had my props pitched as you suggested yours were. I’m wondering if you have any other information about your props meaning are they three blade four blade and somebody was asking me about any blueprinting you might have or not. He asked if they “scanned your props” I’m sorry to continue to bother you with this subject. And
Appreciate your responses

[shrew]

Discussion of RPM as a variable is almost moot here. RPM will dictate fuel burn over time, however that is out of context unless factor in distance.

Gear ratio of the gear set/outdrive will impact the speed by RPM dramatically. V=S/T

V = Velocity
S = Distance
T = Time

I have a single 6 cylinder diesel running at 2,400 rpms making +/- 8 knots (depending on tide) burning roughly 3 Gal./hr.

The real calculation is Efficiency = Fuel Burn/Distance

[SteveP]

Hi G,

I have a pair of the OEM props. They are CJR NIBRAL 4-blade 2" shaft and were originally 24 x 22. I had them both de-tuned a bit to get higher WOT. Currently 24 x 20.5

[Captain_Ken]

Diesels are a lot better at low speed efficiency, so I'm told. And I have lots of experience to back that up with a 33 Bertram, 42 Hi Star and 27 Blackfin (which gets about 7 kmpg at 7 knots).

If you are willing to pretend you are a sailboat or trawler, you can save a lot of fuel. Just try to find that sweet spot where you do not exceed the boats hull speed. More than this, and the hull sucks itself down excessively due to the airplane wing effect, pushing more water and thus more drag instead of more speed, beyond that hull speed.

To calculate the theoretical hull speed of a displacement yacht, or a planing hull running slow, use the formula: max hull speed (in knots) = 1.34 x the square root of the waterline length (in feet).

This formula provides a general estimate of the maximum speed a displacement hull can reach efficiently. If you try to push it faster, but not on plane, you just burn fuel without much increase in speed. But if you run slower than this trawler-like maximum, you will burn a LOT less fuel. Perhaps as much as 1/4 on a per-mile basis when compared to normal planing.

The waterline length of my 4100 SCB is about 35 feet. This results in about 8 knots per the formula. I can push to this speed with 1150 rpm. I just got the boat and am exploring the actual burn on a trip from FLL to BWI.

It is easy to go slow when the weather is good. Current and wind from behind are deeply appreciated in this trawler mode for sure. The real test is to keep it up when the seas are pitching or weather is closing in on you.

[Captain_Ken]

Originally Posted by SteveP

Hi G,

I have a pair of the OEM props. They are CJR NIBRAL 4-blade 2" shaft and were originally 24 x 22. I had them both de-tuned a bit to get higher WOT. Currently 24 x 20.5

Thank you. That is very helpful. I have my 4100 SCB props in the shop now for a make over. I have Cummins 370 HP 6BTAs, rated 3000. Is that similar to your boat? I can only pull 2700 on port and 2900 on Stb. Engines have been looked at pretty closely and just serviced. Don't know the specs on the props, and that will be the first thing to get with the as-found scan. They are not marked.

[Glenn111]

Originally Posted by Captain_Ken

Thank you. That is very helpful. I have my 4100 SCB props in the shop now for a make over. I have Cummins 370 HP 6BTAs, rated 3000. Is that similar to your boat? I can only pull 2700 on port and 2900 on Stb. Engines have been looked at pretty closely and just serviced. Don't know the specs on the props, and that will be the first thing to get with the as-found scan. They are not marked.

I suggest you prop the boat to at least hit recommended WOT rpm's. So you want to prop WOT it to hit 3,000 rpms at least on the Cummins 370 hp, BTA's. Clean bottom and running gear, of course. Let the engines breath.....they will last longer. Speed should be a very little concern....max rpm's is a BIG deal.