Newbi ? About wot rpm and prop sizing

[galvprobigd]

I've done a lot of lookin and haven't really found a awnser to my question. I've got a 90 elpto on a. 180 2000 fisher marsh hawk. At 5500 rpm I'm doin 45-47 mph but can run to almost 7000 rpm. My question is should the prop be sized to keep the motor from going above its 5500rpm at wot? I have no idea the pitch or dia of the current prop there is no numbers any where on it is is stainless. This might be a stupid question but was wondering if I need to reprop. Also at 45 the front of the boat will start to lift and come back down and smack the water. Trimming helps with that to a point but does get hairy. This happens on glass water or rough. Not sure what the speed of the boat is a 7000rpm bouncing so much I was just holding on to the wheel and backing her down. Any help would be much apriciated

 

[emoney]

Re: Newbi ? About wot rpm and prop sizing

You need a factory service manual for your motor to find out what max RPM is. 7k sounds like a LOT! Next, you need to confirm that your tachometer is working correctly. If it is, then it won't be long before that motor decides to punch out and go home, so-to-speak. As for the prop/pitch numbers on the current prop, have you pulled it off? A lot of times they're stamped on the inside. The reason why the hull is so squirlly at those speeds is it wasn't designed to run that fast, I'd imagine. That bouncing you're experiencing is called porpoising and you want to avoid that as it could result in the boat flipping (watch a few of those race boat videos on Youtube and you'll see where the concern lies). I see that model number but what's the make? Evinrude? Whatever it is, usually a Google search of "Evinrude 90hp max rpm" will get you the #'s if you don't have a service manual.

How are you measuring speed, btw? Is this from an on board speedometer or a GPS? GPS is the only way to know for certain, not that it's a big concern, but helpful when it comes to tuning the motor and for recognizing changes that a prop change may effect. First things first, however, and that's to confirm about your WOT rpm. If your gauge is accurate, I'd be willing to wager you'll find that you're way over. Pull the prop and see if the #'s are back there and then post what you find.

 

[steelespike]

Re: Newbi ? About wot rpm and prop sizing

Did you look under the prop nut for number? Without your prop size and speed by a gps we can't be sure if your tach is off or your prop is wrong.I have a feeling its the tach.

 

[cyclops2]

Re: Newbi ? About wot rpm and prop sizing

Find a KNOWN GOOD tachometer. Dashmounts can be inaccurate for many reasons. Do you know how to safely be ejected from your boat ?
If you continue to play around with that bobbing around bow...........A gust of wind will teach you a lesson. Especially if you do a long high speed 180 degree turn. .............Get more useless gear into the very bow compartment. Weigh it down solidly

I can flip my 16' Aluminum open fish boat with a 9.9 ................IF I want to on a windy day.

Stop playing macho racer. You will eventually wind up in the water injured.

 

[jestor68]

Re: Newbi ? About wot rpm and prop sizing

One source I checked says that boat is rated to 125 hp. Another says 90 hp.

You can usually control the porpoising with weight distribution. Bass boats all have too much weight in the rear portion of the boat. You need at least 100 lbs forward to hold the bow down.

Other like boats are running 45-48 mph at 5500-5600 rpm with Merc 90"s and 21 pitch props.

You say you get that speed at 5500 rpm. What happens when you push the throttle farther? Does the speed increase, or just the rpm? The motor could mounted high enough that the prop is ventilating, causing the motor to over rev with no increase in speed.

 

[82rude]

Re: Newbi ? About wot rpm and prop sizing

Did you look under the prop nut for number? Without your prop size and speed by a gps we can't be sure if your tach is off or your prop is wrong.I have a feeling its the tach.

give my tac a good rap when it says 7000k and she drops to 6200rpm.since it hits the rev limiter at 6250 and it never has i know that the tac is faulty.yours might be also.can you grab any of the wires on the tac (back of)and give a giggle maybe somethings loose?

 

[cyclops2]

Re: Newbi ? About wot rpm and prop sizing

I saw a used 1999 170 marsh hawk.

It is not a bass boat. It is a light duty Aluminum Jon boat with a slight V in the bow......That boat is really a Jon boat in handling horsepower waves & wind. People put whatever they want to in a boat. I am going to be nice.
The way over powered boat has the 90 hp because the guys in it were BIG.

Keep the 90 ??.............Learn to drive correctly when alone or with light loads.

 

[galvprobigd]

Re: Newbi ? About wot rpm and prop sizing

Thanks 82rude ill try that next time I'm on the water. And I'll check the wires when I get home.

Jestor my plate says its rated for a 125hp. I got those speeds from two hand held GPS units. The boat started to bounce so I didn't see the speeds above 6000rpm but it was climbing. The bouncing starts around 5500rpm and 43-45 mph then I gotta start playin with the trim again to get it to slowdown.

Thanks emoney ill pull the prop to see if there is any numbers under there. My BIL has a 2001 18' triton with 175efi murk and a 71mph with 2 guy and gear at wot and the motor won't spin any faster. 74 is the fastest he's had it. Him and few guys I work with said I shouldn't be able to spin the motor faster than 5500 at wot with the right prop. I guess I'll move my trolling batteries up front there is a lot or dead wasted space under the front deck

 

[jestor68]

Re: Newbi ? About wot rpm and prop sizing

Jon boat.

 

[jestor68]

Re: Newbi ? About wot rpm and prop sizing

I saw a used 1999 170 marsh hawk.

It is not a bass boat.

Most folks would call that a bass boat.

 

[galvprobigd]

Re: Newbi ? About wot rpm and prop sizing

They the second one is what I'm talkin about.

 

[jestor68]

Re: Newbi ? About wot rpm and prop sizing

They the second one is what I'm talkin about.

Yes, I know.

There is nothing dangerous about your porpoising issue. It's just annoying.

That boat is capable of safely going over 50 mph.

As I stated earlier, according to a couple of bass fishing forums, guys with your boat and motor were getting 46-48 mph at 5500-5600 rpm with a 21 pitch stainless prop. I suspect your prop is something less or your tach is way off, or your motor height is causing ventilation at speed; which can cause your motor to over rev.

 

[galvprobigd]

Re: Newbi ? About wot rpm and prop sizing

Thanks jestor for the info, I was afraid the porpoising was from having the wrong prop installed. About motor height, the center line of my prop shaft level with the bottom of the boat is 10" and the po installed a "whale tail". Found the numbers on the prop, thanks steelspike. Turbo FC13 1/4x 22 is what was below the nut

 

[steelespike]

Re: Newbi ? About wot rpm and prop sizing

Assuming normal slip of about 10% at 7000 rpm you would be doing about 60 mph. A number of things may be going on
but I suspect the tach.

 

[galvprobigd]

Re: Newbi ? About wot rpm and prop sizing

Steelspike dose that sound like the right prop or should I change it. Or dose it even matter As long as I make sure my tach is working correctly? And dose the motor height sound right? The po said the "whale tail" gave it an awesome hole shot. It's nothing to write home about but haven't driven it without it either.

Thanks to everybody's help.

 

[Mi duckdown]

Re: Newbi ? About wot rpm and prop sizing

Motor height, the center line of my prop shaft level with the bottom of the boat is 10"
Don't understand.
I think a few pics of your motor height in relation to your transom are in order.

 

[82rude]

Re: Newbi ? About wot rpm and prop sizing

yes i agree.i just went out and looked at mine to be sure and the anti-ventalation plate or cav plate is 3/4 of a inch above the bottom of the boat.personally i think those whale tails are useless,want a better set up get the smart tabs.

 

[emoney]

Re: Newbi ? About wot rpm and prop sizing

You want you cavitation plate to be even with the bottom of your hole (that's where the PO installed those "fins"). If that's the case, when it's trimmed all the way down, then the motor height is fine. Your buddy's boat going 70mph, I'm sure sits flat and even, and that's all thanks to the hull design. Competition bass boats are designed for that speed since so many tournament fisherman are in a big hurry to go sit, lol. All boats are different, as you can find another boat the same size as his, with the same exact motor, but in say a "offshore-style" boat that will only got 45mph because the weight, and the hull. All things are relevant in boats.

Still need to pull that prop and find the numbers so we can see what you've got there. None of the things said have addressed that because we don't know what you're using. And yes, you can be "over" or "under" propped, as a lot of people that tell you they've "been boating for 20 years", have no idea how pitch/cupping/diameter/etc. affect performance.

 

[82rude]

Re: Newbi ? About wot rpm and prop sizing

agree 100percent emoney with your last post .if for some odd reason you cant find any numbers on your prop which i would find strange the simplest solution would be to take off the prop and go to a prop shop and they could tell you in about 10 seconds.still 7000k for rpms doesnt sound right atall id still be checking out that tac for sure.

 

[steelespike]

Re: Newbi ? About wot rpm and prop sizing

He found the numbers a few posts back 22" Turbo.
Your prop is about the right size.Perhaps slightly over but we need to be sure we have an accurate tach before making any changes.