WOT RPM Range

[pjbass]

I have a rebuilt 1997 Johnson Faststrike 175 hp and i'm trying to find the WOT RPM range? Second, I have a stainless steel prop and it doesn't pitch or size stamped on it. How do I figure that out? Is there a way to measure it?

Any help or reference will be great.

 

[bob johnson]

Re: WOT RPM Range

book says 4500-5500, but I have heard many places that the 60*V motors love higher rpms up to 6000!!

pitch is the theoretical forward distance the boat would move in one turn of the prop....

you could try and interpolate that with some testing on a rod in your shop.

the other is the dia, measure from the HUB OD to the edge of one blade, then add half the dia of the hub. and double your result...thats the DIAMETER of the prop

ex: 17 X 21

dia X pitch

dia is always first

i bet there is more help on the way for you though!!!!

The prop and rigging forum might be a good place to ask this question


bob

 

[pjbass]

Re: WOT RPM Range

I bought the motor last year and still testing the WOT and watching max RPM. I've been ranging between 4300 to 4600 rpms just in/out of 50 mph(GPS). I have the motor mounted down to the second last hole and I was wondering if I should raise it up one notch or try to find out if it is the prop. I"ll post in the Prop section for that answer. But at least you answered my operating rpm range question. thank you.

 

[Silvertip]

Re: WOT RPM Range

Where is the AV plate in relation to the keel? Should be about 1-inch above as a starting point. Go up from there but not so high you lose water pressure. Are you playing with trim at WOT? Is the engine properly tuned and making full power. Check the hub of the prop behind the nut for a part number or diameter and pitch info. Trying to determine pitch is best done by a prop shop using a pitch block. You can measure the diameter.

 

[bob johnson]

Re: WOT RPM Range

what is the relationship of the cavitation plate on the lower unit to the bottom of the hull right in front of the motor???

thats the place to start...then id go with a lower pitch prop....you can hold a new prop next to the one on the boat and look for a STEEPER angle....

most places will sell you a prop, then let you try a few to find the one you can keep... you will have to go down a lot in pitch i think..at least 4 "...what ever prop you have get your best speed at the highest rpm you can get...then trim out very slowly and watch the speed and rpms..... you should be able to pick up some rpms, maybe 200 or more by trimming out once up on plane!!!!! you can trim until your speed peaks and starts to fall again... or you get into cavitation...


seldom if ever is it pick a prop and you are done...trial and error is the name of the game


good luck

bob

 

[wilde1j]

Re: WOT RPM Range

Look at the AV plate @ WOT. If it's buried in water you can come up a hole. Continue raising until the top of the AV plate is just uncovered. This is totally an empirical process and there's no magic relationship that you can just measure, although AV plate even with the hull bottom is a good place to start. At each raise increment, try some hard turns. If you get excessive cavitation, it may be time to back off a hole.

BTW, any decent prop shop will have pitch blocks and can tell you the prop pitch in seconds.

You want to aim for 5800 RPM @ WOT. Hold off on any pitch change until you get the "X" dimension right. You will pick up some RPM and top speed as you raise the motor.

 

[pjbass]

Re: WOT RPM Range

The boat I have is 1996 Stratos 280 Fish-n-Ski that has stepped or notched transom. I am still puzzled on the right engine height if I should set the height based AV or the centerline of prop shaft. The boat has a keel and not a pad. This is where the confusion begins because I posted here before and got too many opinions.

Attached is a photo of my transom with my "old" motor. I don't have a photo with the new motor but it is shorter shaft than this one.

 

[bob johnson]

Re: WOT RPM Range

stick a straight edge up against the bottom of the hull right in the center of the boat, if you got 3 feet of the straight edge contacting the bottom of the hull, thats agood...then see where the top edge of that straight edge comes across the lower unitm , WHEN the lower unit is all the way down!!


it should be about an inch with in the cavitation /ventilation plate...

that older motor looked like someone tried to dress it up with a can of white spray paint..

thats always a big warning sign to me!!


good luck

bob

 

[pjbass]

Re: WOT RPM Range

Yeah that was the old motor. I got rid of it quickly and it was the wrong shaft size for the boat.
Attached photo shows the new motor on the boat.

Right now, the motor is set with AV plate an inch above the keel and I was wonder if I should raise it up one more bolt height.

 

[bob johnson]

Re: WOT RPM Range

can you take a picture of the back of the boat showing the motor down and the view looking level with the bottom of the hull?

you got to get that set up close before you start this process....IE getting your motor in the right rpms range and getting all the performance you can

PS the previous photo with the old motor was too close!! you need to back off a little trim the motor all the way down( that wants close in the old photo) and get the cav plate and hull in the same line of sight

bob