Guys - these #'s can't be right? UPDATE

stevens · Oct 18, 2005

Hi guys, I need your help please, on an issue that I can't figure out myself. I sold a 22 pitch four-blade Turbo propeller this fall, but the buyer is not getting the results he expected. Here's the info he gave me: 21 foot daycruiser V-hull, kind of like mine in the picture Approx weight is 3600 lbs 1999 model 5,7L Mercruiser EFI 290 hp on the shaft Alpha drive Gen II, said to have a ratio of 1.5:1 With this prop, he is getting the following performance: WOT at 4800 rpm, 35,7 mph (31 knots) Cruise at 3200 rpm, 29.9 mph (26 knots) The speed is measured with GPS, I think. I figure this to be a slip rate of about 47% ??? Wouldn't you have expected this boat to be a lot faster, maybe more like 45-50+ mph? What do you think is wrong here? Is his tachometer off, does the drive maybe have a 2.4 ratio, is the engine not pulling properly, or is there something else wrong? On this setup, what approximate size propeller would you usually run? Please help, as I feel bad for this guy who just spent a lot of money on a new prop (from me...). I've recommended he check the drive ratio, tach and engine performance before he spends more money on another prop. He was thinking he'd buy & try a 24-25 pitch next...

TilliamWe · Oct 18, 2005

Re: Guys - these #'s can't be right? *UPDATE*

WOT of 4800 rpm is reasonable, but I agree with you, the speed should be more than 35mph. So either his tach, speedo, or drive ratio is off. Did you know that prop to be working properly when you sold it? But hey, if he wants to blindly try props, let him do it. If the prop you sold him is working correctly, he'll just lose RPM, and probably speed with the higher pitch props that he tries.

stevens · Oct 18, 2005

Re: Guys - these #'s can't be right? *UPDATE*

Hi Tilliam, and thanks for this. I know the hub is not spun and the prop is not otherwise defect, as I ran it myself not long ago and it worked like a charm on my own boat. With this same prop, I got approx. 34.5 knots/39.7 mph at WOT of 4400 rpm, 120 HP diesel, 2:1 Alpha drive, 14% slip. Here's a picture of it: Link to picture. I only sold it because I wanted to try out a 23P three-blade Vengeance (which is arriving soon). Now, he originally started out with a 19 pitch prop and described it as just whipping up a lot of bubbles back there... Here's how the Mercury Prop Selector came out: Mercury Prop Selector Result . If the drive ratio and other factors are as he believes them to be, the 22P would be close to perfect... He seems like quite a handy DIY kind of guy, but with little boating experience. Could someone please volunteer an appropriate procedure for diagnosing the problem on this one? Thanks again. Any input is appreciated.

walleyehed · Oct 18, 2005

Re: Guys - these #'s can't be right? *UPDATE*

First, we need to verify the gear ratio, then we can determine if the tach, or the speedo is off. Let's assume it's not GPS unless we have confirmation of that, and if it is GPS, the Tach is WAY off without knowing the gear ratio.

stevens · Oct 18, 2005

Re: Guys - these #'s can't be right? *UPDATE*

Thanks, Kenny. I also received an offline comment from Dhadley: "I wouldn't be surprised if the gear ratio is different than what he thinks. Only one way to find out and that's to count the turns manually. Not easy but I'd bet that's what throwing the calculations off." (Hope you don't mind the quote, Dhadley) Just for the sake of the curious, here's how to check the gear ratio: http://www.mercstuff.com/drivegearratio.htm How would one best verify the tach?

stevens · Oct 20, 2005

Re: Guys - these #'s can't be right? *UPDATE*

OK, just heard back from the guy. The speed was measured with GPS. He will be checking on the drive ratio this weekend, but was told it was a 1.5:1. It previously came off a boat with a V8 260 HP twin installation, if that is of any help. The connection between the engine and the drive (whatever that part is called in English) is new and installed this summer. Any common mistakes he may have made here? Hopefully, he'll get a chance to doublecheck on the tachometer as well. I'll keep you posted. Thanks for all your input so far.

TilliamWe · Oct 20, 2005

Re: Guys - these #'s can't be right? *UPDATE*

I believe you are referring to the "engine coupler"? If it was put in wrong, it wouldn't have bolted back together. I'll be real interested in that drive ratio. It should have been 1.5, but if that twin 260 V8 was in a big ole heavy cruier, it could be a 1.65. Try that ratio in your formula, what slippage do you get???

And I think you answered a question I had, do you speak/type in English or do you have some program translate? I am guessing you are typing in English, right?

stevens · Oct 20, 2005

Re: Guys - these #'s can't be right? *UPDATE*

If I plug in the 1.65 ratio, that still gives a slip rate of 41%. A 2.4 ratio would give 15%. That prop slip calculator is really neat: Go Fast I am afraid no computer program would be able to translate this fluently (yet). I am typing in English as I am originally an American but have spent most of my life abroad. Now, here's a translation service that is pretty good: Babelfish . Try it out - it's neat to be able to read, say, a Japanese page, or to see this iboats page in other languages. Thanks again - I'll keep you all posted on the prop problem.

walleyehed · Oct 21, 2005

Re: Guys - these #'s can't be right? *UPDATE*

stevens, Obviously his tach if off. the only way to make any prop slip that much is tie it to the dock and floor it.... What really bothers me is the fact we have a 21ft wellcraft cuddy with a 5.0L that runs 48-50MPH. A 21P SS is the best all-around for this boat. Something is astray with your situation there...

stevens · Oct 30, 2005

Re: Guys - these #'s can't be right? *UPDATE*

Just heard back from the guy again. He checked it manually, and his outdrive really does have a ratio of 1.5:1 BTW, it is an Alpha drive with the serial number OF 505304 . The engine (1999 model 5,7L Mercruiser EFI 290 hp) seems to run fine and only has about 50 hours on it. The tachometer is a Teleflex for 8 cylinder, bought new and installed this year. Do you still think the problem is the tachometer? I seem to recall some grumbling here about Teleflex tachometers? Now, how should he check it, and what should he replace it with? Thanks, guys.

bdirect · Nov 1, 2005

Re: Guys - these #'s can't be right? *UPDATE*

I checked my numbers and His tach has got to be wrong. I have a 24ft v cuddy (4,000lbs )350 mercruiser Bravo 250 hp Mirage Plus 19 prop fully loaded I get 34knts (GPS & Speedo) turning 3100 RPM

walleyehed · Nov 1, 2005

Re: Guys - these #'s can't be right? *UPDATE*

The tach needs to be tested against a shop tach...if that pans out, we may have gear ratio unknowns....????

phatmanmike · Nov 2, 2005

Re: Guys - these #'s can't be right? *UPDATE*

if you move to arkasas , itll all work out in the end!

stevens · Nov 3, 2005

Re: Guys - these #'s can't be right? *UPDATE*

The boat in question is now on land for the winter. He'll check on the tachometer and the gross weight and report back to us. The anti-ventilation sits about 1/2 inch above the bottom of the hull, so that should be about OK. Powertrim works, but he hasn't used it much, and kept the trim in a mid-position all along. Two monster batteries aft, so it's a bit stern heavy. Here's a picture of the boat: (picture) Thanks for all your help so far. I really appreciate it. Arkansas - wouldn't that be something! And, I think I'd want Walleyehed's job too... Or maybe Denver... Link

stevens · Nov 14, 2005

Re: Guys - these #'s can't be right? *UPDATE*

OK, this is getting really interesting! This is what he just wrote me: "I've studied the manual and have found something about the shift interrupt switch (the switch that is placed on the gear shift plate). It says that if faulty, the wrong type, or the wrong setting of this type of switch you will get a decreased performance of the engine, and problems shifting gear - which I have. Have tried adjusting the gear according to the manual but can't get it right, it remains hard to shift. According to the local dealer, this switch is not something I needed, but the manual says otherwise. I enclose a copy of the manual ( link ) plus the wiring diagram of the engine ( link ). Maybe this is something the Americans (at iboats) could have a go at?" I am way (!) out of my league here. Any thoughts on this one? Please?

imported_bjs · Nov 14, 2005

Re: Guys - these #'s can't be right? *UPDATE*

i dont see how the tach could be accurate after having racing exp.theres no tach thats accurate only if its cable drive.i put 4 motors on a dyno my tach was 200 rpms off checked another tach it was 150 rmp off.i dont care what that boat shop or teleflex is telling him there full of crap.checked a cable drive tach and it was right on the money a ele tach is only to get you close to rpm's.i wish i could run my motor below max rpm's.but that depends on how much i trim the motor up.

walleyehed · Nov 14, 2005

Re: Guys - these #'s can't be right? *UPDATE*

That place in Denver is only 165 miles from me stevens...want me to go check her.....er, I mean "it" out???

stevens · Nov 15, 2005

Re: Guys - these #'s can't be right? *UPDATE*

Walleyehed - thanks for the kind offer of assistance. Seems a lot of iboaters wanted to chip in ( thread )... Moderators - perhaps we should move this thread to the Mercruiser I/O forum?

stevens · Jun 8, 2006

Re: Guys - these #'s can't be right? *UPDATE*

*UPDATE* Remember this one? I sold a 22P four-blade Turbo and the guy wasn't getting anywhere near the speed he expected? ( It's a 21 foot boat with 5,7L EFI 290 HP with an Alpha drive and a gear ratio of 1.5:1 ) The riddle has been solved, and it was all because his brand new Teleflex tachometer was completely out of whack and he was actually not revving the engine up to where he should. He has now had the tachometer calibrated and verified, and the numbers he gets are great! He hasn't dared yet to go faster than 40 knots (46 MPH), which he attained at 3600 RPM. This equals an 8% slip, which is great! Also, these figures would indicate a probable top speed at 4900 RPM of about 64 MPH (55 knots). Better still, he can now cruise gently along at 20 knots and only 1800 RPM, which is great for both fuel consumption and noise levels. So much hassle over something so stupid as a faulty tachometer... Thanks for all your help, guys.

Dhadley · Jun 8, 2006

Re: Guys - these #'s can't be right? *UPDATE*

So much hassle over something so stupid as a faulty tachometer...

Way more common than you might think. Way more common than it should be, come to think of it. Glad you got it figured out! Thanks for the update. That's how we all learn.

Guys - these #'s can't be right? *UPDATE*

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Guys - these #'s can't be right? UPDATE