Most economical cruising speed

[MyPleasure]

Hey there Carver owners, this is our first big block 454 engined Carver, and I was wondering if anyone had any idea of the best cruising speed for the 36 aft cabin? Right now we’re cruising at about 2000 RPM and getting about 8-9 knots depending on the tides, burning about 12 gph, Has anyone found a faster cruising speed, such as on plane at 3-3200 RPM any more economical than what we’re doing now? I know that our 42 aft cabin with the 375hp Cats was got about 9knots at 1800RPM burning about 6gph, but I know that you can’t compared gas to diesel. Our 36 Uniflite aft cabin with small blocks got about 6 gph at 9 knots, but this Carver 36 aft cabin with the big blocks burns a whole lot more than the Uniflite with the small blocks, but then the Carver is a lot more boat than the Uniflite was.
I know that any big block powered boat is not going to get good fuel mileage, but anything better than burning $125 of fuel and hour would be nice. By the way, I’m going by the Navman floscans that are on the boat.
Thanks for any suggestions

[kgarguilo]

Funny you should ask, I just did a test recently on my 1999 Carver Mercruiser 454's below are the readings while under way PER engines.
S0 2X for total gph.
@1050 RPM 1.57 GPH
@2000 RPM 4.54 GPH
@2776 RPM 9.46 GPH
@3550 RPM 14.95 GPH
@3950 RPM 21.14 GPH

Measurements taken from the MEFI 3 ECM via Diaacom software

[buster53]

Source of the post Funny you should ask, I just did a test recently on my 1999 Carver Mercruiser 454's below are the readings while under way PER engines.
S0 2X for total gph.
@1050 RPM 1.57 GPH
@2000 RPM 4.54 GPH
@2776 RPM 9.46 GPH
@3550 RPM 14.95 GPH
@3950 RPM 21.14 GPH

Measurements taken from the MEFI 3 ECM via Diaacom software

Those are nice numbers to have, but not knowing what speed goes with them to determine MPG, no way to tell the most economical cruise speed.
To the OP, most economical speed is hull speed or under, 7-7.5 knots or less. If you want a good cruising speed, get up on a decent plane. Anything between hull speed and decent plane, you are wasting fuel.

[pepmyster]

True. Hull speed is very economical for our 360 with 8.1VP. Above that gets wasteful till ,I believe, you get on plane. Then she purrs. We are not in a rush while using her.

[Midnightsun]

All engines are relatively the same regarding consumption at planing speed vs hull speed being the difference is substantial. Your most economical form of travel will always be at hull speed regardless of engine size or fuel used.

In my case (diesels) I burn around 0.75L/km at 1250rpm which is hull speed. Up on plane at best consumption is 2.7L/km. Difference is 3 1/2 times more fuel burned up on plane for the same distance covered compared with hull speed.

[bud37]

Source of the post Our 36 Uniflite aft cabin with small blocks got about 6 gph at 9 knots, but this Carver 36 aft cabin with the big blocks burns a whole lot more than the Uniflite with the small blocks, but then the Carver is a lot more boat than the Uniflite was.

It is difficult to compare different hulls even by the same maker.......LWL is will make quite a difference big block or small.....longer hulls will use less fuel to go the same speed for the most part.
9 knots is just pushing water and digging a hole, very inefficient speed for a 36' boat. Like has been pointed out hull speed is with the boat on the water , you can see it as you speed up......most boats around 1500-1650 RPM, depending on propping. This is why some boats become overpropped by owners looking for more hull speed at a lower rpm, therefore using less fuel....the trade off being planing becomes a problem sometimes depending on how far the over propping has been taken.

Nice you have the floscans, they will allow you tune your cruise accordingly.....have fun with your boating.

[bud37]

If the time to get there is important then that changes things greatly.....

[km1125]

I had a 36 aft cabin with the 350HP Crusaders, and my best speed/consumption travel method was to get up on plane quickly, then back off and stay on plane. Push the throttles at least 3/4 until the boat pops up on plane, then back off until the boat settles around 14MPH. Sometimes you need a little tab to keep it up on plane or to smooth the ride (depending on seas).

It's not the most economical in terms of nMPG (which is always going to be best at hull speed), but the best compromise of ride quality, fuel economy, passenger comfort and getting somewhere without putting the whole crew to sleep via boredom.

[Midnightsun]

This is why some boats become overpropped by owners looking for more hull speed at a lower rpm, therefore using less fuel....the trade off being planing becomes a problem sometimes depending on how far the over propping has been taken.

Over propping does not help fuel consumption at low speeds or any speed for that matter. Putting on a bigger prop while maintaining the same rpm will increase speed but does require more HP to do so which ends up being more throttle (higher fuel consumption) to achieve the same RPM. Bottom line is you have not saved anything by over propping however psychologically since one is travelling faster at the same RPM one tends to wrongfully THINK consumption is better.

[bud37]

More speed at lower rpm.

I agree it may not work when the boat is being pushed but...... in effect to make approximately 6 knots ( hull speed approximation ) for a 36' boat ( waterline length maybe 32' or less ) your rpm may be around 1500-1600.....granted it works if you consider going down towards 1200 rpm. Imo it works within reason because the hp output at those rpms is no great difference with the added benefit of big block torque. It does require the boat to be on top of the water and not pushing a large wake though, therefore requiring less force to be applied. One other benefit may be more control at the dock like you diesel guys......

Remember all hull shapes react differently and the longer the water line length the more efficient the hull may be. Also we are of course talking about planing style hulls here and not displacement.

I am not a fan of this but I can see why folks attempt this and lots do.....everything is a trade off compromise of some sort.

I had a boating friend who had a way over propped diesel boat........he made 9 knots with a 30' boat at 1100 rpm......couldn't get it on plane to save his life though......yup there are lots of stories.....I think he may still be on the same tank....