Max RPM tuning questions.

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wtrojan

Seaman
Joined
Feb 16, 2003
Messages
50
Hello,<br />I have been to some max RPM tuning recently (motor tuning, motor transom height, new prop) and generally I’m OK with the results. To start with, I was getting 4600rpm with 21’’ pitch aluminum prop (25’ panga style boat, 140 HP). After adjusting the WOT (was not getting a 100% aperture), fixing some minor flows, raising the motor a notch on the transom and putting 19’’ SS she makes some 5300rpm at WOT and almost 34mph with medium load (4 persons, regular fishing equipment). I’m glad with the acceleration and planning, so assuming the motor is in proper position on the transom (several opinions agree here), my questions are:<br /><br />1. Is it worth putting 17’’ prop to meet the 5500rpm? From the engine degradation point off view. I’d gladly stick to 19’’, I’m more interested in speed@rpm than in the hole shot, I’m also getting the 19’’ SS for a nice price… How about the over revving risk?<br />2. I am considering dropping the hydrofoil to limit the drag, as my experienced friends and I see no big improvement in planning performance (long boat, no deep V, usually running fast). How much rpm change (if any) could I expect in the planning range without the stingray?<br />3. In your particular cases, how much rpm/speed difference have you obtained raising the motor on the transom, by say ½ or 1’’?<br /><br />Any opinion will be appreciated!
 
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wtrojan

Seaman
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Feb 16, 2003
Messages
50
Re: Max RPM tuning questions.

Could anyone share his experience, please?
 
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rodbolt

Supreme Mariner
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Sep 1, 2003
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Re: Max RPM tuning questions.

Hello<br /> I am not sure of your enines WOT requirements. we need model numbers to try to tell. first thing I would do is run it on the old dyno to insure the motor is capeable of making the required power. if it makes power ok then play with props. first make sure the engine does what it was designed to do. Good luck and keep posting
 
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walleyehed

Admiral
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Jun 29, 2003
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6,767
Re: Max RPM tuning questions.

First of all, I would recommend raising the engine, with the 19" prop, until you begin to notice either blow-out on low end, or extreme ventilation at top-end. It may be that it will ventilate in corners, as well, if trimmed up-this is not bad as long as you trim down slightly when/before turning.<br /> You may find that your 19" will turn more RPM, with the eng. mounted higher.<br /> I've found on my boat, I got about 125 rpm per hole up. I'm now in the 3rd hole and am building a setback plate to attempt to raise a little more.<br /> The foil will affect different boats in different ways...in my case, it helped considerably with planning attitude-RPM is a result of what the boat is doing at that point, not a loss or gain-it planes at the same RPM, just quicker-timewise.... :)
 
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wtrojan

Seaman
Joined
Feb 16, 2003
Messages
50
Re: Max RPM tuning questions.

Thanks for your input rodbolt, walleyehed!<br /> <br />About the engine, this is Yamaha 140HP two stroke (not sold in the US). Same V4 design as known 115HP and 130HP models. Recommended rpm range is 4500-5500, so I’m close with my 5300.<br /><br />Walleyehed, I already tried one more hole up on the transom before (two holes altogether from the original position), it was ventilating quite a lot and leaving very narrow trimming range. I just didn’t want the first post to be too long, so I skipped this part … :) That’s why we lowered the engine one hole (leaving one up from original), it works well now, thus I consider the X dimension being optimum. We also got some 125-150rpm per hole.<br />Before mounting the foil, we had nice planning capability, and after the mounting we saw no particular difference, so that’s why I’ve been considering dropping the foil to limit the drag (and possibly raising rpm a little). I read a lot about foils and other trimming devices and understand that sometimes the change is dramatic, but in my case it was minimal.<br /><br />So, is it worth trying 17’’ prop (5700rpm?) , or leave the 19’’ as I’m close to the max rpm limit and glad with the performance? And how about the foil, leaving it on does not hurt me, is this worth dropping than? Thanks!
 
LubeDude

LubeDude

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Oct 8, 2003
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Re: Max RPM tuning questions.

I think you are there! Dropping to 17 will help your hole shot but you may lose a little top end! You never know though untill you try it!<br /><br />LubeDude
 
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kenneths

Petty Officer 2nd Class
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Oct 3, 2003
Messages
154
Re: Max RPM tuning questions.

Spearo, I think the 17" would be a good choice because 5700 r's won't hurt anything, and you'll get a better hole-shot. With a load of people, gear and what-not, you won't be lugging as much. When you're alone, pull back to 5500.<br /> If I may add 1 more note to this, you might consider a good quality SS prop for your application-they generally have a bit more cup, pullling just a little harder as a result, but allow higher trim angles-you may only gain 200-300 r's with this type of prop in 17". :)
 
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wtrojan

Seaman
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Feb 16, 2003
Messages
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Re: Max RPM tuning questions.

Thanks everyone.<br /><br />I made another day fishing during weekend, still with the 19’’ and again, it was nice – enough hole shot for me, good cruising and top speed. To make it clear, the 19’’ is SS, and the 17’’ I’d try in near future is SS also. I guess I’ll try the 17 to be able to compare them, though the 19 makes me happy already. I don’t think I’d have a full load situation ever, the boat easily accommodates 6-7 adults with stuff and in 95% cases we are 4 persons maximum. This is why I’m not quite sure to prop her suiting the 5% of the cases, I admit... Well, I’ll try the 17’’ and will see.<br /><br />However, any opinions about the hydrofoil I mentioned in previous posts?<br /><br />BTW, assuming that with the 19’’ prop, the max mpg is at 4000rpm, would the 4000rpm be optimum for a 17’’ prop also??
 
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walleyehed

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Re: Max RPM tuning questions.

As long as you're getting the 5300RPM with the 4-5 adults, I would say OK.... If you're getting that by yourself, you're over-proped.
 
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walleyehed

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Jun 29, 2003
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Re: Max RPM tuning questions.

As the foil goes, some boats it works, others it don't (I'm told). I personally am a fan of hydrofoils, and have not seen one single boat I've put them on that it didn't help. I did have one guy tell me it was a safety hazard, as far as he was concerned, and wanted to remove it. I asked him to show me what it was doing that he didn't like........I quickly found he had no understanding of how the trim system worked. After giving him alittle dual instruction, he found his RPM increased, as well as top-speed by 4 mph. He was pushing the nose down, with full down trim at wot.........No, it Won't work that way........lessons learned, now he's happy.<br /> Again, some work better than others and can only be judged by what you want from YOUR boat........... :)
 
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mudcat1

Seaman Apprentice
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Jun 16, 2003
Messages
34
Re: Max RPM tuning questions.

Same question almost.I trust My dealer,but I have a 16' Skiff with 50 Ya 4 and alone I get only 4700 rpm's at wot.Is there something wrong here.I have called the dealer and of course he say's bring it back in for a check,but I live in a rural area and thats not that easy.Timing tells me I'm getting 30plus mph which is not bad but book says 5000-6000rpm.
 
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walleyehed

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Jun 29, 2003
Messages
6,767
Re: Max RPM tuning questions.

mudcat, is this a new rig, or one you've had for a time???? If it's new, it's hard to say what might be going on, not knowing what prop you have, and how it's set up.<br /> If the book says 5000-6000, I'd be looking for 5800 minimun. The question is...has it been this way??????? :)
 
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