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powbmps

Max. RPM

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powbmps

I'm totally geeking out over this boat, but since everyone is so nice around here, I'll keep asking questions.

1995 Response with the 350 MTS. Calibrated the speedos last night with GPS. Stock Dymex 13 x 13 prop with a few small dings in the edges.1/4 tank gas and 280 pounds of people with calm water. Max. RPM 4,150 and 43.5 mph. According to the "service bulletin" I should be in the range of 4,400 to 4,800.

Seems low compared to all of the boats on the prop list, but those all seem to be newer boats with upgraded props. What gives?

Thanks!

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GreenMan

There's lot's of options there...!

Did you do the speedo cal in still water? (Versus in a river with a current that needs to be allowed for).

'A few small dings' could also suggest that the prop might be deformed and losing efficiency. Does it vibrate?

Are you sure you are getting full opening on the throttle butterfly?

If the tacho is accurate, seems like high mph for those rpms?

Do you know if the engine is in good shape? Properly tuned, good compression etc.

At high altitude (reduced engine performance)?

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Mackie 12

I agree with Green man, That's fast for a low RPM. I don't think a dinged prop will limit RPM's though?

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MadMan

I agree with Greeman also, but in addition, my experience has shown, I can go something like 42 mph at 4200 rpm, but to go 44 mph I need to be at 5000 rpm. A lot more rpm just give a little more mph.

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asp13

I agree with Greeman also, but in addition, my experience has shown, I can go something like 42 mph at 4200 rpm, but to go 44 mph I need to be at 5000 rpm. A lot more rpm just give a little more mph.

Very true, I heard that with the Bugatti Veyron as an example, it has 1,000 hp. It only needs 275hp of that to hit 150 mph. to top out over 200mph it needs the remaining 725hp.

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GreenMan

Very true, I heard that with the Bugatti Veyron as an example, it has 1,000 hp. It only needs 275hp of that to hit 150 mph. to top out over 200mph it needs the remaining 725hp.

I think the rule-of-thumb is square the power to double the speed...?

UPDATE: Hang on, that cant be right. Maybe drag increases with the square of speed? I should have Googled it. Oh well, it's something like that which means you need a massive increase in power to achieve a modest increase in speed. I shouldn't have started this post...

Edited by GreenMan

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powbmps

You guys are right. Too many variables. I'm going to check it out again tomorrow. Motor seems to run well. I don't notice any vibration. It's on a small lake, so no current. Motor was just tuned up with new plugs, etc.

I'll just stop worrying about it for now. Need to enjoy the warm weather we have left.

Thanks again!

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GreenMan

Yep! If the the engine and prop are in good shape, then that is all you are going to get from that combo.

ACME or OJ may happily advise on prop choice for your application.

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DOOFUSisaCAT

the small nicks that you are talking about can easily take 3,4,5 mph off the top end.....suggest you find a good prop shop, or Acme will repair it, have either add some cup...after you talk to the reapir shop about what you are trying to accomplish...then see how much better and more speed you get.....or order a new CNC prop

if the prop is original, it is most likely not a CNC prop.....CNC props are just better, and more importantly, more efficient at getting your from point a to point b. or getting you outta the water.

That small nick is the same as leaving the parking brake on as you try to get on down the road....that nick is cutting thru the water at say 3000 rpm. So that nick comes into contact with a lot of water.

velocity is always squared

Edited by DOOFUSisaCAT

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powbmps

Awwwww screw it. Just ordered an Acme 515 from Wakemakers. Will update when I run it with that.

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powbmps

The boat was "tuned up" just before I bought it, and NGKR V-Power UR5 plugs were installed. Pulled them out to check and there was unburned fuel on the tips of the plugs. Replaced them with the recommended AC-MR43T plugs. Not sure if it is running differently, but the fuel is gone.

Still trying to figure out how an NGK 5 heat range compares to an AC 3 heat range. Looks like NGK numbers theirs higher = colder and AC is opposite.....

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powbmps

I'll just keep adding random stuff to this thread. One of the exhaust hoses has a pretty good kink in it, partially blocking the flow right out of the manifold. Replacing it today.

20150923_090645.jpg20150923_090706.jpg

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powbmps

Plug comparison. Opposite ends of the spectrum. Those NGKs look like they would be a bit closer to the pistons.

20150923_184043.jpg

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powbmps

Compression check. #1 is a little low, but still within 10% of the rest.

165

180

170

180

175

180

170

175

Prop is a little more beat up than I thought. New one arrives tomorrow, so we'll see what that does.

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powbmps

Tried out the new prop today. 480 pounds in the boat, 44 mph at 4,400 RPM's. Nice improvement on the hole shot for sure. Gets up to speed fast. Wake seems a bit smoother as well (32 mph). $380 well spent!

20150926_112101.jpgIMG_0317.jpg

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GreenMan

Nice outcome!

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