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Author Topic:   I'm doing 34mph WOT with a 100 is my anchor out?
Denny posted 07-09-2003 04:52 PM ET (US)   Profile for Denny   Send Email to Denny  

Iíve done a search on the Forum on this but canít find an answer I could use.

I have a 89 Montauk with a 89 Merc 100 hp on it. I can get it to 34 mph tops. I trim it out as much as I can until I start to chine walk.

I have a 28 gal Pate tank and a 4.5 kicker on the port side. What should I do to get the boat to move faster? (I know buy a 4 stroke 90 Yamaha).

Any ideas ,recommendations?


UnderDog posted 07-09-2003 05:02 PM ET (US)     Profile for UnderDog  Send Email to UnderDog     
Denny, What is your engine height at(cav plate too bottom of the boat) and what are your rpms and what should your wot rpms be and what prop are you using. this is all useful information that we need to help you.

the engine height should be from min 3/4 inch higher then bottom to 1 1/2 inches higher.

prop size may be wrong and you could be at the wrong wot rmps.

we need more info to help you.
good luck


ps if Sal says different than I.... Sal is the Man, Guru, Superstar ECT. ECT. ECT.

andygere posted 07-09-2003 07:36 PM ET (US)     Profile for andygere  Send Email to andygere     
Answer these questions and the motor and prop gurus will be able to help you out.

1. What is your wide open throttle rpms?

2. What is the diameter and pitch of your prop?

3. How are you measuring your speed (GPS or pitot tube speedo)?

4. Is the bottom of your boat clean, or does it look like a shag carpet?

All else being equal, I would expect a 2-stroke 100 to be faster than a 4-stroke 90 (the 4-stroke has more weight, less power).

Sal DiMercurio posted 07-09-2003 08:07 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
These guys hit the nail on the head, come back with those answers & you will have more help then you need.
Chine walking at 34 mph ? nope, poroising maybe but not chine walking.
When you chine walk, your boat rocks from side to side, first on 1 chine, then the other kind of like poroising sideways.
You usually don't start to chine walk till about 50 + mph, not 34.
How are you checking your speed, ???? gps, speedo, radar, sonar or guessing?
Is your engine coming up to rpms it should be running?
Denny posted 07-22-2003 01:05 AM ET (US)     Profile for Denny  Send Email to Denny     
Hi guys,

Sorry for the delay in the reply to your questions but Iíve been out of town and just now have a chance to come up with the answers.

The motor measures 1 ľ higher than the bottom
The RPMís are 5200
The speed is GPS
The number on the Aluminum prop is 48-77346 A40 19P
There is a 14 above the cav plate so I assume that that is the diameter of the prop
The bottom of the boat is clean as can be

Sal, my confusion between chine walking and porpoising was not on ďporpoiseĒ. ;-)

Thanks for the replies on this folks, I appreciate it.


Sal DiMercurio posted 07-22-2003 11:07 AM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
A 14 above the cavitation plate ???????
Whoa just a second.
The prop is totally seperate from the engine & the prop will have it's size numbers on it, not the engine [ Antivent plate ].
The plate your refering to is called "antivent plate" & it's located right above where the prop goes.
There is no connection what so ever in numbers on the prop and the plate you refer to.
The numbers on the prop should read...14 x 19......the other numbers are just the serial number
If you drop down 2' in pitch to a 17", your engine will run at 5,600 rpms & give you a little more speed.
If in fact your prop is a 14 x 19, my suggestion to you is, try a 13 x 19 or 13 x 17 with a dash of cup put in , as you really do "NOT" need 14 inches of diameter to get your Montauk on plane.
Bigshot posted 07-22-2003 11:36 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Sal the 14 is a decal that merc puts on the foot for some reason, has nothing to do with prop. That engine redlines at 5250 I believe.....double check.
lhg posted 07-22-2003 02:24 PM ET (US)     Profile for lhg    
The 4 cylinder Mercury 100 is really not any more powerful, or faster, than the 3 cylinder 90. Don't ask me why.

The Montauk should run about 46-48 with that engine. So either your GPS is bad, a prop hub is "spun" (sheared and slipping) or the engine is needs repair. First thing I would do is inspect the back side of the prop hub. If the rubber is all torn up, it need's to be re-hubbed or replaced. My bet is on the hub, onless the GPS is broken.

lhg posted 07-22-2003 02:29 PM ET (US)     Profile for lhg    
Further, checking Mercury's prop charts, the 19" aluminum will give you 43 MPH, so your prop hub is slipping. If you want the boat to get up and run, switch to a 20" Laser II, for 47 MPH, engine in middle holes (3rd). Aluminum props cannot be re-hubbed very successfully.
Denny posted 07-22-2003 02:37 PM ET (US)     Profile for Denny  Send Email to Denny     
Thanks for the info. I'll let you know what I find out and do with this. It's great to have people who can help with kind of stuff.

Thanks again,


Denny posted 07-22-2003 03:00 PM ET (US)     Profile for Denny  Send Email to Denny     
By the way as far as the speed on the GPS goes I cross checked the GPS speed off the Lowrance against the speed on my Bottom line and they were very very close. That points to the hub spinning theory. I favor that instead of the repair the motor again program.

Thanks guys,


jws posted 07-23-2003 07:23 PM ET (US)     Profile for jws  Send Email to jws     
Bigshot, Pretty sure the 14 sticker on the foot indicates the number of teeth in the gear.
simonmeridew posted 07-23-2003 07:51 PM ET (US)     Profile for simonmeridew  Send Email to simonmeridew     
If the prop is spun would the RPM's race above red line at WOT? Like a bad clutch in a big V8 car, the engine is not dragged or impeded and thus over revs.
I'm probably wrong but I think I'd do a compression test before spending a couple hundred on a new prop.
Bigshot posted 07-24-2003 11:01 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
People should do compression tests annually so they can monitor engine wear, etc. f course you need to keep a log. Whenever ANYTHING gets weird that is not fixed wth Techron or new a comp test.
ShrimpBurrito posted 07-25-2003 02:15 AM ET (US)     Profile for ShrimpBurrito  Send Email to ShrimpBurrito     
If you just look at the prop, you can tell if it's spun.

First, there likely will be a swirling look in the material that binds the hub to the prop. Second, you can pull the prop and look at the side that was facing the lower unit. If the part of the hub that is flush with the lower unit is shiny, clean, etc. (or at least not gummed up like the surrounding areas), then that means the hub was been spinning independent of the drive shaft.

GlennGlenn posted 07-25-2003 03:01 PM ET (US)     Profile for GlennGlenn  Send Email to GlennGlenn     
I have a 28 gal Pate tank and a 4.5 kicker on the port side. What should I do to get the boat to move faster? (I know buy a 4 stroke 90 Yamaha).

Any ideas ,recommendations?


Denny, four strokes are NOT the answer as far as performance, STILL to heavy and they are still not up to par per pound and price with a 2 stroker; Get a 90 HP Mercury Optimax and that'll solve your problems. Other than that, try to raise teh engine about 2 inches (as far as you can go and still draw water ) and reprop to a small diameter larger pitch.


Denny posted 08-07-2003 11:45 PM ET (US)     Profile for Denny  Send Email to Denny     
Hi Guys,

Just thought that I would give a final follow up on the question. I put a new prop on the Merc, Tec in the gas, and new plugs in the engine. I hit 44 mph. That was just about a 10mph improvment. The old prop was spun not doubt about it. Thanks a lot for your input.

The check is in the mail.


lhg posted 08-08-2003 02:48 PM ET (US)     Profile for lhg    
Glad you found the problem.

In response to Simon's post above, I have noticed that aluminum props act differently than the SS props when the rubber hub is sheared. The lightweight aluminum prop tends to retain some of the resistance, and gives the confusing results that Denny was dealing with. The heavier SS props seem to just let go completely, as Simon suggests.

Mercury's newer plastic "drop in" hub sleeve seems to be an improvement over the older rubber style. Easy to replace also. Michigan Wheel is now offering this system on their aluminum (only) props, probably purchased from Mercury.

simonmeridew posted 08-18-2003 08:44 PM ET (US)     Profile for simonmeridew  Send Email to simonmeridew     
Many thanks for your clarification on the spun hub aluminum vs stainless steel.
When we tow the boat to the Outer Banks in the spring 900 miles one way, I bring the old aluminum prop with me in the truck (not in the boat) figuring if something happens to the Rapture I won't lose a couple of vacation days looking for a new prop. Not a bad idea to keep neverseize on the splines either.
Gene in NC posted 08-19-2003 01:33 AM ET (US)     Profile for Gene in NC  Send Email to Gene in NC     
Whoa. Are we sure that the prop was the problem. Denny confounded the test results by combining the tune up (Techron for the carbs and new plugs) with the prop change. Would anyone besides Bigshot, simonmeridew and me like to see Denny retest with the old prop. We might all learn something from that. Bigshot might just be right.
Bigshot posted 08-19-2003 10:39 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Right about what?
Gene in NC posted 08-19-2003 11:52 AM ET (US)     Profile for Gene in NC  Send Email to Gene in NC     
Bigshot, right about doing a Techron and plug tuneup before spending on a new prop.
Denny posted 08-19-2003 01:14 PM ET (US)     Profile for Denny  Send Email to Denny     
Hi Guys,

I had a tune up on the engine about 15 hours before I changed the prop. I changed the plugs out of general principle and put in the Tec because it seemed like a good idea. The drop in power went on before and after the tune up.


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