Wide Open Throttle

KCLOST

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Jun 22, 2002
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Re: Wide Open Throttle

walleyehed,<br /><br />I'm running a Laser II 25P 3-blade, not the Trophy anymore...
 

KCLOST

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Jun 22, 2002
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Re: Wide Open Throttle

Dh, I assume I have the proper stock lower unit for a Merc. 175 and 200hp... Which I think is a 2:1 gear ratio for that time period????? <br />Compared to a 1.86:1 for the 135 and 150hp models????<br /><br />Is there an easy way to confirm this? I actually have the lower unit off the motor at this time.
 

Dhadley

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Feb 4, 2001
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Re: Wide Open Throttle

Ron did mention that his stock speed-o was showing 65 but he had not checked it with a GPS. He has not mentioned, that I saw, what the actual rpm's are either. And the boat is in storage so the actual pitch wasnt posted. So, we have to make some assumptions here.<br /><br />First we'll assume the boat speed-o is correct. Who knows? Maybe it a liquid filled Gaffrigg and is real close if not dead on....<br /><br />Assuming the motor will hit 6000 rpm with a 24 Raker, that means that at 65 mph he has about 12.5% slip. Pretty good but he said 65 wasnt as fast as it would go. So a further assumption; At WOT it will hit 70. Thats about 6-7% slip. Pretty amazing but not likely. 10% slip would be right at 67 mph. Hummm...<br /><br />Lets now assume he has a 26 Raker and can still achieve 6000 rpm (see where we're going with all the assumptions?). At wot of 70 mph that'll be about 13% slip and a bit more believable.<br /><br />Lets look at all the assumptions we just made. Accurate speed, pitch and rpm's. Maybe everything is a good guess except for the rpm's. Maybe it really turns 5500 or 6500. Heck, the only thing we do know is the gear ratio!<br /><br />This is why we do all testing with known accurate instruments. We can still use a dash speed-o. If we make a change and the speed goes down, we most likely went in the wrong direction. We may not be able to calculate accurate slip but we know we most likely went the wrong direction.<br /><br />Aint this fun?
 

Dhadley

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Re: Wide Open Throttle

It real easy. If the water pump is off it'll help but no big deal. <br /><br />Mark a spot on the d/shaft and propshaft. Turn the driveshaft clockwise with the unit in forward. Count the times you turn it as someone counts the propshaft turns. Stop when the propshaft has turned 10 times. <br /><br />If youve turned the d/shaft 20 times, its a 2:1 ratio. If youve turned the d/shaft a little over 18 and 1/2 times (18.6 to be exact) its a 1.86:1 ratio. If youve turned the d/shaft about 17 and 1/2 times (17.6 times) itll be a 1.76:1 ratio.
 

KCLOST

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Re: Wide Open Throttle

I'll check it,,,<br /><br />Any idea as to what ratio I should have for a 1986 merc. 175?
 

Paul Moir

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Nov 5, 2002
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6,847
Re: Wide Open Throttle

My sources show it ought to be 15:28, or 1:8666... for an '86 175hp.
 

rdgallo

Petty Officer 2nd Class
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Sep 19, 2004
Messages
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Re: Wide Open Throttle

Guys, I don't know what my RPM was at 65. I don't remember, but I do know I would not exceed the 5500 max. and I did not run very long at that speed - just way too fast for my comfort level. I guess I just don't have the guts. I didn't mention that the dealer did rejet the carbs at one point as I was having a problem with it wanting to die when I accelerated. Hope this helps some. Sorry I don't remember the rpm at 65.<br />Ron
 

Dhadley

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Re: Wide Open Throttle

Humm...you do understand its perfectly OK to exceed 5500, right? In fact you want that motor set up at or near 6000. More wont hurt. The rev limiter is either at 6200 or 6700 depending on which ignition components you have. You can run it at any rpm you want as long as its capable of 6000. <br /><br />I get the impression someone has told you not to exceed 5500.
 

rdgallo

Petty Officer 2nd Class
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Sep 19, 2004
Messages
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Re: Wide Open Throttle

No, I did not know it was ok to exceed 5500 rpm. I thought I read it in the manual not to exceed that. I will have to go back and reread. I didn't even know it had a rev limiter. Maybe someday when the water is like glass I will let it rip and see what it will do. Thanks for the info though.
 

KCLOST

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Jun 22, 2002
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Re: Wide Open Throttle

Dh and Paul,<br /><br />I checked the gearing last night and it's 1.86 to 1... Looks like I have the right set-up at least...
 

KCLOST

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Jun 22, 2002
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Re: Wide Open Throttle

With a 25P Lazer, I can reach 5700-5800 rpms with the motor Tilted out to nearly max.<br /><br />With a 24P trophy around 6000....
 

Dhadley

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Re: Wide Open Throttle

With the 25 youre at about 17-18% slip. That's OK but it could be better. If you can use all the trim we might look at raising the motor.
 

KCLOST

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Re: Wide Open Throttle

Dh,<br /><br />I've been down that path... I've raised the motor already as high as I can go.... Meaning that when using the trim to what I consider "close to max" I do lose signifigant water pressure...<br /><br />Do you think a different prop would make a big difference, hence the Raker for example? Or going with more set-back, 8-10" jack plate?
 

Dhadley

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Re: Wide Open Throttle

A general rule of thumb is that if you can use all of your trim range and your particuliar prop does not lose bite, you can go higher on the X dimension. If your already as high as you can go on the transom the a jackplate is the next logical step. <br /><br />With todays props it is possible to start to lose water pressure while the prop is still hooked up. At that point we start to look at alternative ways to pick up water. Scoops, nose cones with low water inlets, sportsman style pick ups (inlets on top of the nose cone), transom mounted pick ups etc.<br /><br />If your going with a manual plate I really like the 5" convertible from Bobs Machine. OK - so I like all Bob's stuff. In the standard configuration it'll have a 5" set back. You can reverse the outer plates and get a 3" setback. With a set of 2" spacers you can go as much as 7". <br /><br />I see some testing in your future.....
 

KCLOST

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Jun 22, 2002
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Re: Wide Open Throttle

I already have a 6" jack plate, sorry I didn't mention that..... So will a 8-10" plate help signifigantly?
 

Dhadley

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Re: Wide Open Throttle

Can you go any higher with the plate or raise the motor on the plate?
 

Dhadley

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Re: Wide Open Throttle

Since youre pretty close to optimum now, you might try going up in small increments -- say 1/2" or so at a time. With the 25 at 5800 youre very close to that props maximum efficiency for the set up. <br /><br />To answer your question about more set back we look at a couple of things. First, more set back helps raise the bow. Basically more set back will change the center of balance or lift point. It is helpful to get the bow up without using too much positive trim. In a perfect world we want the thrust (centerline of the propshaft) going parallel to the waters surface with the least ammount of the hull in the water. <br /><br />So, if you need excessive positive trim to raise the bow, yes more set back will probably help. A very easy way to find out is with the 2" spacers. That will put you at 8" back. If it helps, then youre going in the right direction. All you need is the spacers and a bolt kit. <br /><br />Secondly, we look at holeshot. Obviously the more set back we have the more the bow tends to rise on holeshot. How much it rises depends on the load distribution in your particular boat and hull design. Just something to keep in mind.
 
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