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I can't figure out this math, please help.

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Old 08-23-2008, 03:40 PM
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Default I can't figure out this math, please help.

7,500 lb boat ( fountain 35' ) single step
315 hp 3800 rpm diesel from yanmar with 705lbs torque
bravo xr 1.50 gears

What kind of speed do you estimate from this setup, the yanmar folk have no idea either? What prop is a good place to start with this???

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Old 08-23-2008, 03:57 PM
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Figuring backwards, if the boat is expected to run about 65mph, with 10% slip, 30pitch props looks about right. I expect the Yanmars are heavier than BBC's, so maybe a stern lifting prop like a Q4. The drives probably won't like 5 blades with all that torque.

Just guessing on all this though.
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Old 08-23-2008, 04:15 PM
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Originally Posted by Griff View Post
Figuring backwards, if the boat is expected to run about 65mph, with 10% slip, 30pitch props looks about right. I expect the Yanmars are heavier than BBC's, so maybe a stern lifting prop like a Q4. The drives probably won't like 5 blades with all that torque.

Just guessing on all this though.
The yanmar bobtail is 900lbs and the 496 ho was 1200 w/the xr so they are both pretty close weight wise.
BTW 65mph is my target for this boat.

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Old 08-23-2008, 10:08 PM
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No offense, I think if you went 50-55 mph you'd be doing good. Read all my posts, you have no prop speed with 1.5 reduction gears.

The boat will run abouy as fast as it did with gas engines at 3,800 rpm, not faster. Yes, you have more torque but that useless with a small drive like a Bravo (can't swing a BIG prop). You can add some dia and some pitch but you added weight. On top of all that your probably putting in less Hp than your taking out.

Find the 5 page paper I wrote and copied onto here somewhere, what was true in 1988 is still true today.

If you wanted to talk you can call my cell 732-718-2711 tomorrow.

Joe Gere
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Old 08-23-2008, 10:35 PM
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No offense, I think if you went 50-55 mph you'd be doing good. Read all my posts, you have no prop speed with 1.5 reduction gears.

The boat will run abouy as fast as it did with gas engines at 3,800 rpm, not faster. Yes, you have more torque but that useless with a small drive like a Bravo (can't swing a BIG prop). You can add some dia and some pitch but you added weight. On top of all that your probably putting in less Hp than your taking out.

Find the 5 page paper I wrote and copied onto here somewhere, what was true in 1988 is still true today.

If you wanted to talk you can call my cell 732-718-2711 tomorrow.

Joe Gere
Ill look for it, thanks Joe. They spec 1.63 or 1.65 with the sport package they ship. I was hoping to fix it at the prop?
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Old 08-23-2008, 10:49 PM
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The more reduction gear you have the slower the boat will go. You need to go the opposite direction like 1.33, 1.25 not 1.63.
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Old 08-23-2008, 11:04 PM
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Originally Posted by HabanaJoe View Post
The more reduction gear you have the slower the boat will go. You need to go the opposite direction like 1.33, 1.25 not 1.63.
I think this way too. I think the 1.50:1 is too tall for this set-up.
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Old 08-24-2008, 12:53 AM
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Originally Posted by HabanaJoe View Post
The more reduction gear you have the slower the boat will go. You need to go the opposite direction like 1.33, 1.25 not 1.63.
I think the guy at Boatswain doesn't know this particular package too well then. I really don't want to get new XR's because mine are almost new, I can send them to Max Machine Worx but I need to figure out exactly the right ratio.
I'll post the Dynos and curves this Monday, hopefully it will shed more light on it.
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Old 08-24-2008, 01:02 AM
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One question, at the 65 mph you want to go, how many rpm's are the engines turning? That's your starting point, to get those wheels to turn the same prop speed as you got with the gas engine to go the speed you want to run. Answer that and you can get gear ratios, whether you have the power to do it - that's next.
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Old 08-24-2008, 01:12 AM
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The torque curves and dyno is here http://www.yanmarmarine.com/uploads/...P_TechData.pdf
on page 2
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