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Author Topic:   Wide-Open Throttle: Boat Speed Too Low
whalertex posted 12-23-2003 10:19 AM ET (US)   Profile for whalertex   Send Email to whalertex  
Would someone clue me in on this. I think I'm missing something. I've got an 85 montauk with a 90 hp Johnson VRO and I'm running a 13 3/8 x 17 stainless prop. My problem is I don't seem to have the top speed I should be getting at 5500 rpm; by myself but with gear (and I'm a big guy, 240lbs) she only goes about 28 mph at 5500. Hole shot is great and the engine seems to be running fine. What I've noticed too is that I have a lot of throttle left at 5500 rpm. I've always thought WOT is "wide open throttle" and your revs would be limited by the load on the engine in a correctly matched prop, motor, hull combo. I've pushed the control all the way forward for less than a minute and the revs climb to 6500 and the boat starts to sing across the water at about 38 mph and it will go higher, but I don't want to blow the engine. From what I've read in the archives, the motor seems to be at the correct height, but what else should I look for to get more top end at 5500 without risking this old engine? Boat has been trailered and garaged all its life, so increased hull weight because of water intrusion doesn't seem likely, but I haven't weighed it. Thanks in advance and happy holidays. This is a great site.
Peter posted 12-23-2003 11:27 AM ET (US)     Profile for Peter  Send Email to Peter     
Sounds like a bad tachometer.
Bigshot posted 12-23-2003 12:01 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Bad tach or tach setting or bad prop. I would try another 17" and see what gives before replacing the tach. The 17 should not even let you hit 5500, more like 5300 if stainless.
Sal DiMercurio posted 12-23-2003 09:19 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
I agree with the above posts, better check that tach.
Look on the back of the tach, theres a little dial & for an OMC engine it needs to point to #6.
That boat should run close to 42...45 mph with that 90.
From what your saying ,...if it's the prop, you would have to go to a 25 pitch prop & thats not in the books at all for that rig.
That 17p prop should allow the engine to run between 5,200 & 5,400 as Bigshot said.
How high is your engine mounted?
Are you checking your speed by gps, speedo ?
I'v seen several gps's not give the right speed for some reason or other.
In some cases you must run about 1/2 mile before the gps reads correctly, don't ask me why cause I really don't know.
Whats really throwing me is, even if that prop is spinning 6500 rpms, your speed should be in the mid 40s & it's not.
Check that tach.
WOT means your knuckles are bleeding from pushing the throttle handle as far as it go's & thats all she wrote.
rwill1 posted 12-23-2003 11:01 PM ET (US)     Profile for rwill1  Send Email to rwill1     
Slipping hub if not the tach?
Sal DiMercurio posted 12-23-2003 11:15 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Usually when the prop spins a hub, the prop will still hang on at idle but breaks loose as soon as you give her throttle, can't get on plane.
I'v never seen a hub partially slip & allow 38mph.
It's either being able to idle & move at about 3 mph or lots of rpms & going nowhere like not moving.
whalertex posted 03-03-2004 03:37 PM ET (US)     Profile for whalertex  Send Email to whalertex     
Just thought I'd let you all know it was a bad tach. Thanks for your help.
Moe posted 03-03-2004 04:05 PM ET (US)     Profile for Moe  Send Email to Moe     
Thanks for following up.

Bigshot posted 03-04-2004 10:56 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Yeah...thanks for the follow-up.....rarity. So how fast is she going and how long did you baby that engine? At 28mph you are only running MAYBE 4k. Bet there is some carbon buildup, run some Techron in your next tank of gas.
whalertex posted 03-04-2004 05:30 PM ET (US)     Profile for whalertex  Send Email to whalertex     
Haven't gotten out on the water yet, weather and personal time just haven't coincided. Bigshot, you're right on as usual. I've already bought some techron and I plan to put it in the tank when I gas up. When I replaced the tach, I replaced the plugs too. They were pretty black, but the engine seemed to run fine. Cheap insurance. I hope to take her out this weekend and I'll relay a final report.
jimh posted 03-04-2004 09:20 PM ET (US)     Profile for jimh  Send Email to jimh     
Hey Nick--

This thread reads like one of the Tom and Ray CAR TALK shows where they call the guy back to see if the advice they gave him was correct.

Good call on the bad tachometer!

JBCornwell posted 03-04-2004 11:15 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
You need a 19" prop with that setup, Whalertex.

You are reaching 5500rpm at part throttle. That is your first clue.

Red sky at night. . .

Bigshot posted 03-05-2004 11:03 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
His tach was bad JB....17" is what he needs and will run about 5300 WOT. ASSuming his 17" is still a 17"
whalertex posted 03-15-2004 11:10 PM ET (US)     Profile for whalertex  Send Email to whalertex     
Finally got the boat out but conditions weren't optimum (and I wanted to fish, not tweak the boat).
That said, against a slight incoming tide, into a slight chop with an 8kt headwind, heavy with 2 guys, gear and 27 gals of fuel, and the boat probably not trimmed out perfectly, still managed 36.4 mph per gps at 5300 rpm WOT. Plenty fast enough for me.
I probably won't push the old girl that hard again unless I have too. Happy whalerin'!

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