The initial boat performance data is in on the Spitfire X7 propeller on my 160 DAUNTLESS. Note that I replaced the Trophy Plus, which was 13.75 x 17, with a Spitfire X7 in 13.5 x15. I delayed and delayed worrying about whether to get a 17-pitch or a 15-pitch pitch. I was okay giving up a little top speed for better acceleration, but I didn't want engine speed to go up too much in my 22 to 25-MPH sweet spot for cruising. Eventually I came across a performance report on the Boston Whaler website for the 160 Super Sport, which used a 15-pitch Spitfire X7 on a 90-HP FOURSTROKE with the same 2.07:1 gear ratio as my 115-HP engine. My 160 Dauntless is significantly heavier than the 160 SUPER SPORT, but my boat has 25-HP additional, and that gave me confidence to try out the 15-pitch propeller
As an aside, it's not easy to find a shop that will "work with you" on propellers. Everybody I spoke with locally said "all prop sales are final." They also said "I don't have any stainless props in stock and don't know when I'll get any." So taking something for a spin just wasn't going to work out.
Qualitatively, the Spitfire X7 propeller is just fine. The boat speed acceleration feels prompt, and engine speeds in my cruising range are not dramatically different. The boat rides and handles much like it did with the Trophy Plus (both of which are WAY better than my Black Max backup prop, which just feels mushy and requires a lot more bow-down trim on the trim tabs to get a decent ride).
The Spitfire X7 holds the water in tight turns even better than the Trophy Plus, and it takes an extremely tight turn to make it ventilate; the engine mounting height is the same and the engine trim is approximately the same. Shifting is now much quieter, though whether that's due to the propeller or to the new Flo-Torq SSR that replaced my old Flo-Torq III, I do not know. Qualitative summary: the Spitfire X7 propeller may be a little faster to accelerate, may hold the water a little better, but it is really not that different than the Trophy Plus.
Quantitatively, the story is similar. Below is a plot of boat speeds at different engine speed (RPM) for all four props that I've tried on the boat over the years. The Spitfire X7 15-pitch has a bit slower boat speed at every RPM setting compared to the Trophy Plus 17-pitch. That's no surprise, but it is perhaps a surprise that it's only a
little bit slower. Top boat speed dropped from about 42.5-MPH with the Trophy Plus to about 40.5-MPH with the Spitfire X7.
- MPHvsRPM.png (27.7 KiB) Viewed 5553 times
Why is it only a little slower? Because at all boat speeds, it has less slip than the Trophy Plus. That's really just another way of saying the effective pitch of the Spitfire X7 is not really a full two-inches-less than the Trophy Plus. To some extent, especially at low speeds, that may be because I had medium vent plugs in the Trophy Plus and solid vent plugs in the Spitfire X7.
- SLIPvsMPH.png (30 KiB) Viewed 5553 times
As I mentioned, I was willing to give up a little top speed for acceleration. And to a limited extent, that was successful. The Spitfire X7 is the fastest-accelerating propeller of all that I've had on the boat, but only by a little. Keep in mind that the fidelity here isn't that great; boat speed and engine RPM are taken from a video so they are synchronized, but my GPS appears to provide approximately a one-second (or maybe 0.5 second) refresh. It's also a different GPS than what I used for the older propellers, and the boat now has trim tabs instead of a lower-unit hydrofoil. In other words, this experiment is not particularly well-controlled.
- MPHvsSEC.png (40.92 KiB) Viewed 5553 times
I'd say that the quantitative analysis pretty much matches the qualitative one -- the Spitfire X7 is maybe a little better on the acceleration, gives a little slower boat speed at all speeds on plane, and holds the water a little better in turns.
For replacing a damaged prop, I'm happy with the results, and I think this prop will work very well for me. But frankly, I'd be pretty disappointed if I had spent $600 looking for some big advantage over the Trophy Plus. Perhaps the biggest improvement came from the $90 Flo-Torq SSR hub kit.
-Bob