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I'm spinning a 14-1/2 X 17 prop.I'm wondering if the out drive needs a gear change.
Currently,It's a 1.65 to 1 gear ratio,but my marine mechanic says that I would be much better off with a 1.50 to 1 gear ratio.
Anyone have any advice?would it be worth the cost,and would it improve the performance significantly.
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Great looking boat John! Glad to see you finally got her launched!
Regarding performance, I think you've already got the right gear ratio and that a change to a 1.5 would be going in the wrong direction. Unless you're running a very light high speed boat, a larger diameter/slower turning prop is generally more efficient. I think that's one advantage the 4-stroke Zuke's have over their competition - they get some extra gear reduction between the crank and drive shaft, so they can spin a larger prop. If that engine is new, it may still be a little tight also, so you may pick up some more rpm after getting some more hours on it. I'd focus on a prop change, maybe less pitch and a little more diameter. Stay with aluminum to keep the cost down until you get it dialed in. A good prop shop can also repitch an aluminum prop by a couple of inches for not too much money.
The reason I say the 1.65 is the better ratio is that a friend of mine that owned the SeaCraft 21 "Unohu" (see Proud Owners section of Carla's web site) did a major repower back in about 1975 from an I-6 Mercruiser to a 350 V-8, rated at 250 hp. His mechanic was Frank Brown at Brown & Hauptner Marine, who used to crew for Carl Moesly on some of his race boats, and was one of the best mechanics around at the time. The standard ratio on that setup was 1.5:1. Frank asked Bob if he would mind if he tried an experiment where he'd swap out the 1.5 gearbox for a 1.65 at no cost when he installed the new package. Bob said go for it, and was very pleased with the results. The most convincing part of this story is that the following year, that hp rating on that exact same package went up to 260 or 265 hp - the only change . . . a 1.65 gear instead of a 1.5!
With a heavy Bahamas-trip load the Unohu would cruise at 20 kts at an easy 9" of manifold vacuum and run over 40 kts @ WOT. (Frank said max continuous power was 5", so don't cruise any lower than that if you want a long life on the exhaust valves, which is generally the weak link on most 4-S engines!)
All the engineers at Pratt & Whitney that I worked with that ran inboards had installed vacuum gages and ran the engines to manifold vacuum. A plot of vacuum vs speed should help you find the "sweet spot" (most efficient trim & speed) on any inboard rig. I'm frankly amazed that no one on this site running inboards and discussing props seems to pay attention to manifold vacuum, because it provides a wealth of information. They're a very good indicator of engine load and health and can detect even minor changes in engine performance. I even use them on my vehicles, and if I was running a 4-stroke outboard I'd have one on it too! Generally the prop that allows the engine to wind up to it's rated rpm @ WOT will also give you the highest vacuum at cruise. I'd recommend installing a vacuum gage; with the existing prop you should start seeing higher vacuum at a given rpm as the engine breaks in. I would then record vacuum as well as speed vs rpm when you try a different prop. Should be a big help in getting her dialed in! Denny