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Old 26 March 2008, 10:08   #11
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I managed a GPS 26.1 knots at that rpm. Engine was screaming so didn't hang around there for long. At 5500 it was moving at around the 20-21 Knt mark. I've run the numbers through a couple of "prop guestimators" that have come out at around the 15" mark too.

Other thing to bear in mind is this boat is "only" about 550 KG fully fuelled.

As for the gear ratio, I'll need to take the bottom end to bits to figure that out unless someone has the specs to hand......
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Old 26 March 2008, 11:33   #12
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At 5500 it was moving at around the 20-21 Knt mark.
Wasn't a bad guess then, was it?

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As for the gear ratio, I'll need to take the bottom end to bits to figure that out unless someone has the specs to hand......
Make sure the kill switch is OFF, put engine into gear, take note/mark the flywheel position and turn the engine until the prop had done one complete revolution. Count the turns of the engine to get your gear ratio. I was guessing around 2:1.
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Old 26 March 2008, 12:39   #13
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Yep. But the fact it could then carry on up to 6.5K RPM kind of hints the prop is a touch on the low pitch side......

Good call! My mind is elsewhere this afternoon..... I'd have the plugs out anyway, its got a huge stroke so compression is a bit much to turn the flywheel by hand against! From memory the Mariner spec I saw said something like 1.7 or 1.8:1, for what that's worth.

Will let you know either a lot later tonight or tomorrow......
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Old 08 April 2008, 09:55   #14
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Suddenly all becomes clear......

It's amazing how much fog clears when you find a delaer that actually knows the products they sell.....

Turns out I have a "High rake" prop. (I think that's what he called it) Anyway, the bottom line is that Yam appaerently did a series of smaller diameter props to allow faster boating - I guess the theory is that by reducing the drag due to diameter you can up the pitch a bit and take out the reduced drag with higher pitch and so go forwards faster. Probably also explains why it let go with the slightest provocation!

Turns out the "standard" (I now know it's a G-series) prop is slightly over an inch greater diameter than what's on the bottom of my leg. Kind of explains why I thought it looked a touch on the small side!

So, now the experimentation really begins - I'm either looking for a 15" "High Rake" or a 13" "standard.
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Old 15 April 2008, 12:20   #15
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I get 34 knots out of my boat. I didn't have a rev counter but the 15" prop sounded too short. I went up to a 17" one and gained a little on top end but alot at cruising speed. The engine sounded right at 28 knots where before it was around 24 knots.
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Old 16 April 2008, 05:31   #16
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Pablo,

Sounds like we have similar setups (because they both go through a garage door? )

Is yours a 2 or a 3 cyl engine? Also any idea what diameter prop you are running?
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Old 16 April 2008, 11:23   #17
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3 cylinders, i'll ask the prop its dimensions tonight (hopefully I won't get a slap in the face )
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Old 19 April 2008, 10:47   #18
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Pablo,

Sounds like we have similar setups (because they both go through a garage door? )

Is yours a 2 or a 3 cyl engine? Also any idea what diameter prop you are running?
It's a Solas Myth 13.25 x 17
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Old 22 April 2008, 13:30   #19
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Isn't that precariously close to the underside of your cav. plate?? Mind you if you have the 3- cyl there's maybe a bit more room in there.

What do you reckon your overall weight is?

Or maybe I just need to polish my hull?

Thanks, that's useful info.
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Old 09 June 2008, 05:15   #20
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The fog clears.........

Well, finally managed to borrow a G- series 14" pitch prop yesterday, and went for a spin.

In summary - 4 up + 30L fuel - 4500rpm, one up + 15L = 5500rpm.

So, assuming the pitch increase of 2" takes out about 400rpm, looks like the extra inch in diameter accounted for 600 rpm (using the 1-up rpm compared to the original test).

Unfortunatley no GPS this time as it was all a bit last minute and it was left behind (d'oh!)

So, if anyone has a 14" G- series that needs a good home......... (or a 13" for those days when I have passengers!)
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