When it comes to choosing propellers for any boat, a fair amount of trial and error is required to get the performance you want.
When I sourced the replacement motors for Divecat I had no idea what propellers would be needed. Of course, these second-hand Yamahas came complete with a set of 15½ by 17-inch propellers (15½-inch diameter and 17-inch pitch) but those were suitable for the boat that they were originally fitted to. As anyone who has been down this route knows, selecting the right prop for your specific boat and desired performance is part science, part mathematics and, perhaps, part dark arts and personal preference.
The hull design, weight and waterline length are of course all factors in the ultimate performance of the motor and hull. If you have a production boat, then the manufacturer will generally have a good idea on what are likely to be a good motor and propeller configuration. But even if that is all known, there are different requirements for racing, fishing, water-skiing or towing (very relevant for Coastguard, who once owned my motors). Fuel consumption, speed, loaded weight and acceleration at different RPM ranges are all considerations. There is no such thing as “the best propeller for my motor” – the same motor on two different boats will likely need different propellers, and even two identical boats but with different roles/uses might benefit from different props.
The original Suzukis that were fitted to DiveCat had a pair of 16-inch diameter by 15-inch pitch props, but those motors had a different gearbox ratio and a different torque curve. I suspected the 17-inch pitch on the Yamaha’s would be too high, but I could only validate this once I had the boat back in the water and ran a set of speeds tests.
And indeed, the tests showed the 17-inch props weren’t right, even though the boat reached the same 32-knot top speed as before her long swim. It is very important for a motor to be able to get close to its rated maximum RPM, which for the Yamaha F250 is 6,000 to 6,300. With those 17-inch props the motor would only get to 5,200rpm at wide open throttle. At that point they were burning the rated maximum fuel consumption despite being some 15% short of maximum revs. Thus, they were working excessively hard to get to that point, which over time would translate to shortened engine life and potentially serious damage. New props were needed!
But where to from here? Stainless props are considerably more expensive, but also more robust, than aluminium. Thanks to the thinner, stiffer blades, stainless propellers are also slightly more efficient – you can expect up to one knot more speed over an identically-sized aluminium propeller.
The second consideration is the number and type of blades. Extra blades generate more thrust but also add drag which negatively impacts fuel consumption. Racing boats often have four- or even five-bladed props, as do boats used for towing skiers or wakeboards. But for most users the extra fuel usage makes them unattractive.
Blade shape is also important, with stainless steel able to support cupping on the blades. This imparts a slight curve to the edge of the blade and helps in reducing slippage.
The prop diameter is generally limited by your motor, specifically the clearance under the gearbox. You can mount a smaller diameter prop, but there is an upper limit to what will fit. So there may be an inch or so to play with but not much more. In the case of my Yamahas, the available options ranged from 14½- to 15¾-inch diameters.
And the last parameter is pitch. This is the theoretical distance the prop will move forward in one complete revolution. So a 20-inch pitch prop will move the boat twice as far forward as a 10-incher, assuming the there is no slippage and the boat moves perfectly. The pitch is the most common parameter that we get to play with and the easiest to quantify mathematically.
Since I had a baseline value to start with, I was able to make a calculated guess what would be the best option. At this point there is nothing quite like getting an expert opinion, and Ben Taylor from Promt Parts in Manukau was my man. Solas are the biggest-selling brand of third-party propellers on the market, and their Rubex hub system allows their props to fit a variety of outboard motor makes and models. Promt Parts has been supplying Solas and other brands of propellers to dealers in New Zealand for nearly 20 years.
I started off by trialling a pair of 15 by 15-inch aluminium ‘Rubex 3’ props. These were the most economical option, coming in just over $400 each including GST with the hub system to fit Yamaha. Because I needed a counter-rotating pair, I was slightly limited for options by what was in stock, but Ben assured me that I could swap these out if I returned them in pristine condition.
After very carefully removing the existing props and fitting the new ones without scratching them, I took DiveCat for a run. Performance initially seemed just about perfect. The motors reached 6,000rpm, and top speed lifted to 33 knots. Fuel consumption, on the other hand, was worse at the lower end, although at top speed it was better. This is consistent with the theory that bigger props spun slowly are more fuel-efficient than smaller props spun fast.
I then had a discussion with Ben about the available options in stainless steel. It seemed like the 15-inch pitch was pretty much spot on at the top end, but I wanted better performance in that low to mid-range. His recommendation was the Lexor model three-blade stainless prop in 15¾-inch by 15-inch size, coming in just over $1,200 per prop, including GST and the required hub. The increased diameter would give me better mid-speed fuel consumption, while the 15-inch pitch should allow the motors to rev close to the required maximum without overloading them.
Purely for the purposes of research I also twisted his arm to let me try out two other options: a three-blade in 15 by 14inch, in case smaller pitch was better, and a set of four-bladers in 15½ by 15 inches. I then ran a series of identical trial runs with all three options up the same stretch of flat water, and compared the results with those of both the original props and the aluminium Rubex. I graphed speed against RPM, fuel consumption against RPM, and fuel consumption against speed. The results were interesting. See attached graphs.
The blue line on all the graphs is for the Yamahas’ original 15.5 by 17-inch props. At almost all revs the speed was highest, and at the bottom end of speed range, the fuel consumption is the lowest of all the props tested. The inability for the motors to reach maximum revolutions was the biggest issue – the engines clearly have to work extra-hard to turn those props, as borne out by the fuel consumption climbing sharply at the top end of the rev range. Not a good option.
The green line is the aluminium 15 by 15-inch. The rev range was perfect, and although the fuel consumption was higher at speeds below 25 knots, the smaller props were more economical at the top end. However I wanted stainless steel props, so this was a test rather than the solution.
The grey line is the 151/2 by 15-incoh four-blade. Interestingly, this was almost linear in terms of speed versus RPM, and the motors reached 6,000rpm. Speeds were higher throughout the range and fuel consumption was better than with the aluminium props, so if I was going to be pushing DiveCat at 90% or more of maximum revs, this might be the prop to choose. But overall fuel consumption was even higher than with the original props, so I was sure there was still a better option.
The orange line plots Ben’s recommendation of the 15¾ by 15-inch Lexor. The speed curve was a close match to the original propellers once the boat got to planing speed, and fuel consumption is better from 4,000rpm upwards. This is where my cruise range lies – 23 knots seems to be the sweet spot, with consumption just under four litres per nautical mile. The maximum revs – 5,600-5,700rpm – is still slightly low, but within the acceptable range.
And lastly the yellow line is the 15 x 14-inch, which we tested just to make sure we were not missing something. As expected, speed over entire rev range was lower, but we did reach 6,100rpm with the same top speed (32 knots) and same maximum fuel burn. The hump in the middle is getting up on to the plane – the motor is revving very much higher to get to that point, which means that at my preferred cruise speed of 20-25 knots I would be pushing 5,000-plus rpm. Because of the higher revs the fuel burn is higher over most of the range, although the curve is fairly flat once she is planing. Again, more economical at the very top end, but definitely not good in the mid-range.
So, in conclusion: The smaller props (both the 15 x 14-inch and the four-blade) reached the higher revs, and at speeds above 25 knots their fuel consumption was lower. But at the target range of between 20 and 25 knots, the bigger props spin 500 to 1,000rpm slower, using 0.5 to 0.75 litres per nautical mile less.
For my usage pattern the 15¾ by 15-inch Lexor SS is my ‘best’ prop, shown in orange on all the graphs. I can still push hard to over 30 knots if required, but would be foolish to run at that speed for extended periods.
For reference, DiveCat weighs around 6.5 tonnes and has a waterline length of 9.5m. At 23 knots the engines are using a combined 90 litres per hour and burning 3.91 litres per nautical mile. This gives me a working range of 150 nautical miles from her 700 litres of fuel on board, allowing a 15% reserve. BNZ
