If we accept that the location of the "dynamic" center of gravity is constantly changing depending on the speed of the boat then it becomes obvious that we cannot achieve the ideal balance in the whole range of speeds.
It is therefore wiser to choose in advance the range of speeds we are most interested in and try to keep the "dynamic" center of gravity location as close as possible with the "static" one at the specific speed we usually run.
Based on the above, we consider that the placement of the 10-inch jack plate on the Focchi’s 7.30 transom on which the V8 Pro XS 300hp engine of Mercury was hung is correct.
With a reduced weight of 43 kgs of the V8 300hp Verado as well as 200 more rpm at WOT - up to 6200 -, the V8 Pro XS 300hp engine promises more overall power and higher final speed provided however that the appropriate boat-propeller-load combination is applied so as to be capable of reaching its maximum efficiency.
For more information about Focchi 7.30 you can read our earlier article: Focchi 7.30 – Six Cylinder Verado 275+hp
Our sea trial took place under ideal weather conditions without any wind, with 80 liters of fuel and two people on board. The Rib had full equipment and an auxiliary engine while it had more weight than usual due to the larger custom console and an extra battery.
In order to be able to calculate with relative accuracy the appropriate propeller for the specific boat-engine combination, we have placed on purpose the longest Revolution propeller on the propeller shaft of the Pro XS so that we do not exceed in any case the maximum rpm limit recommended by Mercury.
Focchi 7.30 was delivering quick acceleration in the whole rpm range with a very good throttle response, due to both the amlpe torque generated from the 4.6 liter engine and the design features of the Revolution propeller.
It is worth noting that with the moving of the cg backwards due to the placement of the jack plate, the bow tends to ride higher and clearly out of the water at low speeds while when exceeding 30 knots the boat insisted on maintaining its absolutely level cruising. With the appropriate adjustment of the jack plate depending on our speed we were also able to control the lift of the propeller and therefore the hull-water surface relationship we desired in each condition.
Focchi jumped with ease on plane with little bowrise while it was characteristic that at just 2500 rpm we were traveling at 22.5 knots where we recorded the most economical cruising speed burning 0.84 liters for each nautical mile. This means that at this speed we have the greatest possible autonomy that is 95 nautical miles with 80 liters of fuel in our tanks.
At 3000 rpm, our speed was 29 knots with a fuel consumption of 30 liters per hour, while at 3500 rpm we ran at 35.5 knots burning 40 liters per hour. These are really high cruising speeds that allow us to reach our destinations very quickly, even having our engine running completely effortless.
It is worth noting the low fuel consumption at the above speeds, which range around 1 liter per nautical mile.
At 4000 rpm we were running with 41 knots consuming 1.2 liters per nautical mile, while at 4500 rpm we reached 48 knots burning 1.5 liters for each mile.
At the wide open throttle we flirted with 60 knots while the engine reached 5500 rpm. It is characteristic that the Rib was comfortably reaching 57 knots maintaining very good stability with absolute control in its handling.
You can see our detailed measurements in the following table:
With a better look at our measurements we will see the very low slip values that Revolution gives us in the whole rpm range, a fact which also leads to the very low consumptions we recorded.
The 8% slippage we calculated at the wide open throttle shows also that there is still a lot of room for improvement.
It is obvious that Focchi 7.30 can easily exceed 60 knots if we experiment with the position of the jack plate and the trim trying also different propellers.
A very good choice for this hull seems to be the Bravo I LT propeller which is suggested to be one inch less in pitch than the Revolution one, so as to allow the engine to reach very close to its maximum rpm limit.
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