Man oh man, what a ride.

[Bushwacker]

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.

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

[riptide]

Hey John,
Boat looks sweet.Great pics.
I too am impressed with the ride on my Sceptre coming from a Mako 228.You are absolutely right about increasing speed in a chop to smooth out the ride.She also doesn't roll as much at drift like my Mako did.These 23 Seacrafts have that "big boat feel" to them.
Jerry

[1bayouboy]

That is such a great looking boat...I like the enclosure with arch...seems the perfect blend or protection, weight and function....

[Fr. Frank]

Your gear ratio is fine. Your prop needs changing.

Allow me to use a pickup truck as an analogy. The differential gearing is analogous to the lower unit gearing, and the tire diameter is analogous to the propeller pitch.

If I want to apply torque to the pavement more quickly I can change out the gears in my differential from the current 3.73:1 to 4.10:1 gears. If I do this, I will have better acceleration under load, higher rpms at cruise, lower axle rotation for a given rpm, and better responsiveness to power inputs.

OR, I can change my tires on the rear to a smaller diameter tire, which changes my applied drive ratio just like changing the diff gearing. Dropping from my current 265x75-16 tires to 245x65-16 tires gives me almost the exact same effective results as changing to 4.10 gears.

On the other hand, if I want to have lower rpms at cruise and a higher top speed, I can change the diff gearing to the optional 3.45:1 gears. This will reduce my acceleration, but give me lower rpms at a given speed, and probably increase my top speed.

OR, I can change my tires on the rear to a larger diameter tire, which changes my applied drive ratio just like changing the diff gearing. Increasing from my current 265x75-16 tires to 285x70-16 tires gives me almost the exact same effective results as changing to 3.45 gears.

In both cases, changing the tire size when it comes time to replace the tires costs a lot less, and produces the same results.

If your current gearset is undamaged, you can more economically change your effective power to thrust ratio by just changing the prop size.

[Islandtrader]

OR, I can change my tires on the rear to a smaller diameter tire,

This is the best analogy I have seen on the prop discussion...I now understand.

Thanks,