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My 1996 custom Striper with a 1999 two stroke 70hp Yamaha mounted four holes up spinning a 13"x17 Turbo propeller. At wide open throttle, motor trimmed down turning about 5800 rpm, I'm doing 34 mph. Motor trimmed up enough to start to get loose, motor turning 6000 rpm, I run about 40-42 depending on the wind and current if any. I'm not sure if I need to do some link and sync timing adjustments because I'm not seeing the performance that others claim to have out of this same set up. I hear others claim they average 45 mph.

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Huckleberry:

How much beer you carrying? That might be the problem.

My 15 GLS with the 50hp Yammy 4 stroke would do in the low to mid 30's. A 15 with a 70 should be a speedboat.

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1985. super sport with yamaha 60 4 stroke (996ccm) with 2 persons at 5700, full throttle 36-37kt


(changed abbreviated year to full 4 digit year for clarification)

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1985 sport with 60HP yamaha (2 stroke)...w/ 2 adult males, it runs right at 40mph...3 holes up...trimmed all way up


(changed abbreviated year to full 4 digit year for clarification)

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I guess not enough beer. Compression is fine, everything checks out except I haven't checked the timing at wot. Something ain't adding up. My 11' does 32 mph with a 25 horse and my 15' will only do eight mph more with a 70 horse. I guess I'll have to start doing some research.

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Did you say the outboard has a 17 pitch prop?

If so, I would confirm the engine is mounted has high as possible and start testing some props. I am thinking a 13 or 14 might be a better fit considering the weight you have with trolling motor and extra batteries.

Joey

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Motor is all the way up. I know because I put it all the way up the night before I took it out. I'm dropping it down one hole though because it doesn't handle as well, corner as well all the way up nor does the speed increase with it four holes up.
I can't go down in pitch, my engine will be out of the rpm range, too high. What I need to do to go faster is go higher in pitch, like a 19 but keep the rpm at 5900, which would happen if I either found some more horsepower or lost some weight from the hull.

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When I had a Yamaha F70 mounted on my 1985 15' CC, I was running a 15" pitch Turbo. I measured 42-43 mph with a hand held gps the few times I went that fast, but it was not a rigorous testing. No idea if I was going with or against current/wind etc.

I do agree that a 17P might be too large, given my experiences though.

Dennis

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Sorry, I was posting at the same time as your reply, and did not see you were already at max rpm.

Dennis

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It depends upon the gear ratio of your lower unit. See this link: http://www.mercuryracing.com/prop-sli...alculator/. Assume a slip of 5% as a start to get an idea of whether your prop is in the ballpark of what you need.

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With the speed/rpm values previously given with a 2.33:1 gearcase, motor trimmed down , 15% slip, trimmed up 4%.

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4% slip and 42mph with RPM at max for operating range is pretty good. If you want more have you considered an atlas microjacker.

Orher things to consider doing: 1. Confirm compression within spec, 2. Confirn linkage is opening carb butterflys fully and they are synchronized, 3. Clean rebuild carbs. Agree with confirming toming is within spec.

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Two different compression gauges verify compression is well within specifications, carb butterflies open as far as mechanically possible. There is about 2° left that the butterflies can open but the cam roller isn't big enough to open it the rest of the way. Also, I wonder if the carbs might be getting too much fuel. I'll check top speed wot timing when I get both a nice day and a friend to drive the boat for me.

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Yamaha 70's aren't the strongest rated 70's, so 45mph might be a little optimistic. I would think it would be in the 43-44mph range with a light load.

It seems like you the right set up engine height & prop. In your picture it seems like you have a fin on the cavitation plate. Have you tried removing that? (or is even still on) Are you running with 1 person or more? Are you in fresh water or saltwater?

The trolling motor in the front is probably why isn't handling great with the motor 4 holes up. When I have 2 people in my boat it makes a difference in speed and how much I have to trim the motor with the person sitting on the front seat vs sitting on the rear seat with me. It could also trimming a slight bit of mph off the top end.

My boat definitely is faster than 34mph before trimming. It did 33mph with a 60hp motor and mounted in the bottom holes (motor all the way day) If you have the fin on the motor, that may explain the problem.

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Thanks Dave and w.largarde for the info and comparisons. Unfortunately, the hydrafoil will be a permanent installation on this boat. With or without the trolling motor, battery for it, one or two people, this boat will porpoise violently with any kind of prop that will give the hull a bow high attitude.

One thing I have to remember is that this is not a Sport model with the light wood console and thwart seats, my Striper has a heavy fiberglass console mounted starboard with a large 12 gallon fuel cell and a igloo cooler helm seat with chocks all on the right side of the boat, not to mention the added trolling motor and my 180lbs. self. I guess I'm just wanting to make sure that I'm not utilizing power that should be in the motor but isn't because of something not properly adjusted or timed.

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I think your in the range. The factors you posted above are what is holding your top speed down a a little. I rarely drive at top speed anyways and couple mph difference doesn't feel that much faster.

A little more on the 15' speeds from my observation. There is a speed (which is probably prop dependent on lift) where the boat lifts out of the water. Thinking back to when my boat had a 60Hp Evinrude and Jack Plate {jackplate is worth 2-3mph} The best top speed was 45mph in the right conditions. If the hull would lift I could hit 44/45 mph. If the conditions were not right, I ran 42/43mph. The downside to the hull lifting out the water, is chine walking shortly follows.

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I saw one in Georgia, had an old Johnson 35 and was doing about 75 mph.
On I-75 north.

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Hahaha

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I will say this about the boat, it will plane off with no problem and handles well with five adults and everything for a day on the water. It has the low end power that I want so overall I'm happy with it. I do remember when I got the 70 a couple of years ago and was in the market for a propeller that is what I said I wanted, more handling than speed.

Now if the rain would only let up a little.