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  #1  
Old 05-12-2014, 12:58 PM
rwl rwl is offline
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Default Answer this about boat speeds

So I have been wondering. I have a boat with a 175 V-Rod, when I bought the boat it was max HP. Now they rate it for 200hp. I see reports of the boat doing 5 MPH more with the 200.

The questions is how, if I run max RPM already, the motors have the same gear ratio and max rpm, and the prop pitch is the same, can the boat be any faster?

I can see a better holeshot, by being able to spin the prop up quicker with more HP, but top speed I just don't get it, all things being equal?
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  #2  
Old 05-12-2014, 01:18 PM
REW REW is offline
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Rwl,
If you add 25 hp to a boat, you should be able to spin a prop with a heavier pitch at the same speed.

But, I believe that you are right.
If you spin the identical prop at the same rpm, the boat will go the identical speed, no matter which motor is twisting the prop.

Be safe
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Old 05-12-2014, 01:23 PM
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Gregking Gregking is offline
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So what you are saying is you hit the rev limiter on both motors with the same prop it will go the same speed ?

Sure, because you limited the extra 25 hp on the top end. Not sure why anyone would.

However if your 175 had not hit the limiter the 200 probably will with the same prop. The difference in rpm from the 175 to 200 would be a difference in speed.
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Old 05-12-2014, 02:06 PM
rwl rwl is offline
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Maybe the tachometers or the speedometer aren't accurate enough, or the guy reading them? Anyway I see the same pitch props supposedly running faster, but I am hitting real close to max rpm, so I don't know how they can get 5 mph more (supposedly gps), unless maybe I'm venting cause my motor is too high? I once raised my motor two holes, didn't like it and lowered one, maybe go back to original, it did not gain me anything to raise the motor.
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Old 05-12-2014, 03:05 PM
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Gregking Gregking is offline
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same boat model with the same hull design?
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Old 05-12-2014, 04:58 PM
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ptm ptm is offline
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Quote:
Originally Posted by rwl View Post
So I have been wondering. I have a boat with a 175 V-Rod, when I bought the boat it was max HP. Now they rate it for 200hp. I see reports of the boat doing 5 MPH more with the 200.

The questions is how, if I run max RPM already, the motors have the same gear ratio and max rpm, and the prop pitch is the same, can the boat be any faster?

I can see a better holeshot, by being able to spin the prop up quicker with more HP, but top speed I just don't get it, all things being equal?
When I bought my Verado that was a gripe people were addressing, I think the 175 was going as fast as the 200. I can run 51 pretty well loaded and the 200 was pushing the same boat around 52, 53 tops. I think the new verado pro or the optimax 200 are going to push the extra 4 or 5 mph.
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Old 05-12-2014, 05:12 PM
REW REW is offline
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Simply put, if your prop shaft HP is greater, you will be able to spin a larger pitch prop at the same rpm that a less powerful motor will turn the same prop.

Hence, a larger hp motor will be faster - all other conditions being equal.

Remember, just because there is a different label on the motor does not actually guarantee that there is a large difference in shaft HP.

Be safe
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Old 05-13-2014, 11:29 AM
JohnN JohnN is offline
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I'm not an expert by any stretch, but if one setup has more prop slip than the other it will be slower. Plus, it rarely seems like you can get two boats that are setup "exactly the same" to go the same speed.
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Old 05-13-2014, 11:56 AM
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the only other thing not addressed is the 10% rule. Maybe the 175 is running 192 and the 200 is running 180. This is the extreme of course.
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Old 05-13-2014, 02:46 PM
jimmythegreek jimmythegreek is offline
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also the lower unit gearcase ration is the main key. Different HP ratings often have diff gearcase rations, so the same prop and rpms will equal different speeds due to the multiplier in gearing
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