What pitch are you using 5.7 GSI -1.43 SX

Shaper79

Petty Officer 1st Class
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Mar 25, 2016
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Just wanted to see what pitch / size most folks are running on their 21 footer chaparral GSI engines and 1.43 gears.
 

Texasmark

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Dec 20, 2005
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I don't have one but a general reply would be to tune your prop for your engine operating in the mfg's. WOT operating range. If you do a lot of water toys and WOT cruising I'd go for the middle of it which would allow you to "lug a bit" with water toys and stay in and "spin up a bit" at WOT cruising. If you concentrate on one type or the other, then apply the general rule listed here for that end of the band.
 

Shaper79

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Mar 25, 2016
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Just asking. Because I could go Only get about 4200 rpms. The boat was just serviced. Everything checked out good. But it was a bit windy and choppy. I had the canvas top up. With 2 adults and a full cooler. I showed about 40 mph on the gage. No gps. But hell the Tac could be off. And the speed o. It's a 19 by 14.5 prop. I guess I'll try a 17 pitch and see what happens. To closer to 4600-5000 rpm range. It seems like it does plane fairly fast though.
 

Texasmark

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Just asking. Because I could go Only get about 4200 rpms. The boat was just serviced. Everything checked out good. But it was a bit windy and choppy. I had the canvas top up. With 2 adults and a full cooler. I showed about 40 mph on the gage. No gps. But hell the Tac could be off. And the speed o. It's a 19 by 14.5 prop. I guess I'll try a 17 pitch and see what happens. To closer to 4600-5000 rpm range. It seems like it does plane fairly fast though.

Obviously weight and environmental effects take their toll. For apples and apples you need similar test conditions. If you can give us your outdrive gear ratio (1.64, 1.83, 2.0, 2.33 sort of thing) it would help a SWAG from the calculator. You didn't say what outdrive you are running but Alpha comes with a lot of variables....if running Mercruiser.

Too many postings. Sorry. I just realized you gave us your ratio at 1.43. Let's run a set of numbers and see what comes of it.

4200, 1.43, 40 mph, 19P = 25% slip. That's a lot of slip. If your boat is a heavy deep V, a 5.7 in the stern does add weight you might find that realistic. I don't have a rule of thumb slip for that type of boat nor do I know if your current prop is a stern lifting type or not which would help the boat level out and not stern drag. What kind of wake do you make at 40? Nice and flat or good sized peaks and troughs, including where your wake converges directly behind the transom?
 
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Shaper79

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Mar 25, 2016
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It's a Volvo penta SX out drive 1.43 gears ratio. That's all I know about the outdrive. But. It was a bit windy with a little chop. And I did have the Bimini top up. So If this is going to be normally how we boat and usual conditions. I should prolly go with a 17 pitch 14.5 diameter... That should at least put me at 4600. In windy / chop condition and have the top up. With 2 adults. And full cooler. Don't ya thing.? It's alway possible my Tac is off. But it's a very good condition boat. It's only ever been in Arizona. So no corrosion. And o checked. The back of the Tac gage for loose connections. Ho yea. And it had a full tank of gas. I think it's about 60
Gallons.
 
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Shaper79

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Ummm. I guess I never really tried to study how the wake looks. I will next time out. It's an extended V plane chap. Sterndrive. And the prop is a 3 blade Volvo penta. But it does ride very smooth and strait. When I trim out about a quarter or or so.
 

Texasmark

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I think you are on the right track. You will find the boat responds a lot better with a 17. Your hole shot....punching out from a dead stop, will be improved....faster response and more solid. If you like to cruise just above planing speed, you will be able to maintain your set mph better and reduce the tendency to fall back off plane requiring a throttle advance, which gets the boat back on plane which then runs away with you and you have to cut back to where you wanted to run and etc. Last it will get your rpms up where they belong which will help to preserve your equipment and higher rpm throttle advances will be more responsive. Depending on what changes as a result of reducing the pitch, you may not loose any top end or if you do, not much.

To keep things where you expect them to fall, get the part number off your prop and somewhere in the number will probably be the numbers 19. That is probably the pitch and you will be wanting a prop with the exact numbers except change the 9 to 7. Otherwise look up the number which will put you in a type/style/function....etc of a series and just move up to the previous prop which normally would be 2" less pitch.

Stay in touch.

Mark
 

Shaper79

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Mar 25, 2016
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Thanks mark for embracing what I think needs to happen.it will be a month or so. Before I get back to the lake. But yea that 17 is defently the way I think I need to go. I'll check back in with my results.
 

Shaper79

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Mar 25, 2016
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Well I found a Volvo penta brand here in Iboats. It's a 17 pitch. 15 diameter. It their much difference in a 14.5 diameter. Over a 15 diameter performance wise. I have 19 by 14.5 now. And I need to come up at least 400 rpms.
 

Texasmark

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The diameter is usually defined by the prop characteristics, once the general diameter is defined......defined by the spacing between the prop shaft and the anti-vent plate. Difference between the area of a 15" circle and 14 ? isn't all that much. I'd go for it. Also, in what I said, as pitch decreases, diameter increases and vice versa again due to prop characteristics.
 
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