Re: Here We Go: My 383 Stroker Winter Project
I'd be happy with 65. Ideally I'd like to see 70 but I'm also realistic.
Tim, I remember somewhere in other threads that you said have better luck running a higher ratio and bigger prop. Do you think I'll see a speed increase going from 1.5 to 1.65 or 1.8 drive with a bigger prop? I'll probably end up buying a new drive either way when this one goes..
I've run a standard alpha drive behind the same engine with 1.5, 1.61 and 1.84 gear ratios. It was several miles per hour faster for each step up in gear ratio (ie 1.84 was a good bit faster than 1.5), and the short drive with a 1.32 ratio was the fastest of the lot, but only by a few mph over the 1.84. That was after each was propped right (there's a line of props in my garage even after I've sold some that I bought while playing). This is consistent with what folks have told me and what I've read about setting up I/O boats for speed.
The issue of a bigger prop creating more drag is with respect to diameter and surface area. Also, most all decent props are progressively pitched anyhow (the pitch is different for different parts of the blade).
The engine / offshore builder that put my bottom end together told me to keep the 1.84 ratio I had when I put it together the first time (I started with a 4.3). I was assuming I needed a 1.5 ratio and ready to buy a new upper drive. he winked at me and told me it might be a little bit faster... spot on.
The biggest reason is that you can get into props that are best for high performance - where pitch is 1.8-2.0 times diameter for anything going over 50 or 60 mph... and the blade has just enough surface area and diameter to hold (too small a prop, it'll blow out in corners, cavitate more easily, etc..., too big and you won't get any slip and will be paying a high cost in drag as well as straining the heck out of the drive).
Turns out that the sweet spot for most boats in our size range is 13 1/2" - 15" in diameter so the best props for speed / performance are going to be in the 25-30" pitch range. run some numbers using a prop slip calculator or doing a little simple math with the assumption that a boat's max speed is going to be around 70, assume around 8-10% slip, redline the engine at 5500 rpms and you're running something like a 28" prop with a 1.84 gear ratio...
Turning a higher pitch prop slower has a couple of advantages. It gives you less drag (a lower pitch prop turns faster and so the blade surface area contacts quite a bit more water, the other is that for modern prop designs (3 and 4 blade, mostly submerged, changes when it is surfaced), somewhere in that pitch range is the most efficient conversion of torque (rotational force) to forward thrust.
Now, before you go jumping onto getting a different gear ratio.
1.6 is a no no - smallest teeth (24 - 24), weakest gears hardest to get tolerances set up right. The ratio would be perfect for chasing 90's in my boat, but I learned the hard way that the 24-24 gear set sucks and is really easy to break.
1.81 / 1.84 works well and is a strong gear set, but you'll generally pay a premium for props that have higher pitch (ie 26-30"), are designed to be submerged and have small enough blade diameter and area so you get some slip. Also, you'll have the added potential issue that you may have problems with engine cooling - as the gear ratio goes up, the vertical shaft turns slower... and so does your impeller / water pump... A lot of the higher pitch props (around here anyhow), are designed for big horsepower boats (ie bigblock, 500+++ hp, etc...) and are too much wheel for a little alpha drive. you need some slip, especially out of the hole to keep from putting too much strain on the drive.
The bottom line, why buy a new drive? run what you have and prop it for your peak horsepower to be right at wot. you might give up a 4-5 mph top end over a 1.8 gear drive, BUT you'll sacrifice cooling efficiency (a weak impeller may not keep you cool enough to run as long...), and you'll not be able to use the most widely available (and cheapest...) props.
of course... I didn't pay attention to this, didn't have any real cooling problems and have a half a dozen props that work great in that range at this point, but I tend to do things the long way around...
Hope this is helpful... Just my current understanding which might be worth what you paid for it... ;-)
