I can't figure out this math, please help.
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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???
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???
Last edited by Anarchy Powerboats; 08-23-2008 at 03:17 PM.
#2
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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.
Just guessing on all this though.
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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.
Just guessing on all this though.
BTW 65mph is my target for this boat.
Last edited by Anarchy Powerboats; 08-23-2008 at 03:19 PM.
#4
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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
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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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
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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I'll post the Dynos and curves this Monday, hopefully it will shed more light on it.
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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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