reduce rev's and fuel with Jet

My sleek craft tunnel with 450hp and Berkeley 12JG make too easy high revs.
In the Netherlands the petrol cost almost 2 euro per liter so a few hours fun for a few hundred dollars spoils a little the fun.

The boat does over 80 mph and for me 40 is enough. I am looking for a way how to make the boat more fuel efficient at 40mph. I also do not need to be launched as a catapult.

It is an A impeller and I have read that AA will move more water at the same revs. On the other hand you move more water so you also need more power.

So is it likely to reduce significantly the fuel consumption by changing from A to AA?
If is it just a few liters a day,...

 

If it does over 80mph as-is, then 40mph should be well into the best fuel economy range rpm. I doubt you'll improve fuel consumption much fiddling with a coarser impeller.

 

Berkeley 12JG and "fuel efficient" are mutually exclusive concepts.
Berkeley made their jets in an era when petrol was almost for free as the cheapest way to propel a boat.

To make your Sleekcraft less thirsty you need a duoprop stern drive.

 

Replace the motor with 200HP.
The bigger the motor the worse the gas mileage.
Or put a smaller carb on the existing motor - may also require a different cam.

 

We have built many 21 foot, 3500 to 4000 lb, wet jet boats with Berkley pumps
In the old days the A impeller was the only impeller (excluding B for very low hp engines)
Our 21's would run in the 3600-3800 rpm at a cruise speed of 32 mph
Engines 330 hp 460 Ford or 454 Chevys
Fuel consumption at that speed about 12 gph for a 2.66 mpg fuel use
Then the AA came out, same hull, weight and speed, the fuel consumption went to 10gph for a 3.2 mpg rpm 3200-3400
When the AAA came out the the fuel burn was 8 gph giving us 4 mpg Rpm 2900-3100
The -AAA gave faster acceleration but with only 330 hp available wot the top speed decreased. I don't remember the figures as we were running rivers and 30-32 mph was a great speed for the hull. Responsive and light to the steering

So a AA will give you lower top speed, better acceleration and better mpg at say your 40 mph target

With 450 hp available I would go with the AAA for best efficiency. Efficiency for this reply is the best mpg at 40 mph comparing the 3 impellers
I am pretty sure that there are jet boat dedicated forums that. Will offer more accurate numbers for your hull type

 

I've never seen anything like those sort of mpg gains with propellor changes, maybe jets can, but it looks pretty optimistic to me. 50% is huge !

 

That was 1/3 reduction which is huge.

 

I fail to see how a boat that does 80 mph will benefit in any way from coarser pitch impellors at 40 mph. The answer may be, where fuel is so prohibitively expensive, to run over it with a bulldozer.

 

Nothing to do with optimistic
All measurements were taken with a Floscan meter. All engines had the non return fuel kit

But the trade off was top end/rpm
At 330hp prox the engines dropped maybe 200 to 250 rpm off the top with each impeller change

As an impeller builds pressure with a corresponding increase in nozzle velocity, the results do not mirror a change in prop pitch

The thrust from a mixed flow jet, Berkley, optimizing the mass flow rate change wrt to pressure/ produced good results for us
We were trying to get the lowest fuel burn at a target speed of 32 mph keeping a solid acceleration

We then were asked to install a 773 Hamilton, 3 stage, axial flow and it exceeded the Berkley performance,we stuck with these until the single stage Hamilton with the turbo out performed the 3 stage.With the new single/ turbo Hamilton discontinued the 773

Again a narrow target. Additionally the Hamilton could digest a bit of gravel whereas 3 or 4 stones stuck in a Berkley grate or a twig in the bowl, would cause extreme cavitation and the thrust could drop 50% until the debris/grate got cleaned