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Old 02-10-2018, 12:04 AM   #11
Lt. JG
 
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The stock outdrive for the v-6 is 1.81 to one if my info is correct. It is a alpha one gen 2 outdrive.

The stock outdrive for the v-8 that i am in the middle of sticking in it is 1.47 to one

The ratios are off but not a whole lot. I THINK that I can more than make up for it with prop changes.

As far as wide open I am thinking between 55 and 60 being able to cruise at 40 easily.

We will see when i get it on the water this year. I have a brand new v 8 outdrive but it is alpha one gen one and I think I would need to change the transom hardware to get it to match up with the gen 2 outdrive.

I appreciate you taking the time to talk with me about it.
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Old 02-10-2018, 01:26 AM   #12
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WOT is max RPM not speed. You would be surprised that prop changes are difficult to make up for improper gear ratio.

Just a fellow boater who likes to help where I can.

Gen 2 is a much better drive than the Gen 1, I would not use it.
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Old 02-10-2018, 05:32 AM   #13
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I very much appreciate the help mike I have a lot to learn.

yes i understand wot is rpm and that you prop the boat to the proper rpm

the outdrive i want to use is the gen 2 but that one is geared for v6

I am not sure but am putting alot of faith in what my mechanic is telling me but I have been told things wrong before.

In thinking about it myself for what is worth.

The v6 runs the higher gear ratio numerically. So at say 4500 rpm it should turn the prop at a certain speed. At the 1.81 gear ratio the prop shaft should be turning at about 2486 rpm.

If i mate the v8 up to a outdrive at the 1.81 ratio than at 4500 engine rpm the propshaft should be turning at about 2486 rpm.

the v8 mated up to its original outdrive with a 1.47 ration would be turning a propshaft speed of 3061. at the 4500 engine rpm.

if we compare the two v8 set ups we see that with the v6 outdrive the v8 is turning less rpm propshaft speed than with the v8 outdrive. at the 4500 rpm.

this should mean if i am thinking about it correctly that i will be able to add a prop with more pitch to get back to the final gear ration but it will be accomplished at a slower propspeed which some feel is a more efficient relationship between the prop and the water. The thought being that the faster the prop turns in the water the more losses in efficiency it has. So if i can get the same amount of push out of the prop at a slower speed i will have a more efficient connection between the prop and the water.

I am not sure that i am thinking about it correctly, it is a bit confusing.
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Old 02-10-2018, 12:48 PM   #14
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Your thinking is good except to increase rpm you need to decrease pitch. One change in pitch give about 250 rpm so you need to decrease a lot of pitch. I.e. if the initial set up used a 19 inch pitch your thinking has you at a 15 or 16 inch pitch which typically is not a good prop for a big boat.

The one thing you missed is the load on the boat due to its mass an power needed to overcome drag from the water all of which is load on the prop.

I have read on other forums of folks trying to do the same but not being able to achieve satisfactory results with the improper gear ratio. All I can say is try it but be prepared to get the correct drive.
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Old 02-12-2018, 06:12 AM   #15
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I was thinking the same thing that I would have to decrease pitch and i definetely dont want to do that.
On thinking about it more I keep coming up with that it should be the opposite.
The v6 outdrive turns the prop less turns per rev of the motor than the v8 outdrive does at the same rpm. So I think if using the v6 outdrive I would have to increase prop size or pitch to have the same push forward in the water(overall gearing prop included)
Im not sure it is the proper way to think about it but i am thinking about it with the prop included in the gear ratio. The overall gearing and if I can make up for the gearing difference by going larger in prop size or pitch I think it will be good. If it does turn out that my thinking is backwards and i need to go to a smaller pitch prop. than I am not so optimistic.
Like you say I will try it with the thought in my head that it might not work.
I will let you know how it turns out.
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