19p to 17p and 1000 rpm gain??

[AEROCOOK]

I have a 1988 Johnson 110 on a 16 foot aluminum bowrider. My tach was broken for most of the summer and I just got it fixed, only to discover that at WOT I was only getting 4700 rpm with a 19 pitch 13" dia. aluminum prop.
This really surprised me since the Motor (with said prop) was just removed from a larger heavier boat. Also, my boat is only rated for a 90 hp, so I kind of thought I could increase the pitch since I had "extra" HP on tap. Anyway, a friend had a 17p x 13 3/4 aluminum prop, so I borrowed it as a test and was surprised to see a 1000 rpm gain at WOT!
I know that there is a lot more to "Propeller science" than HP and size of boat, but from what I have read, 200 rpms per pitch is a general rule of thumb, so why 1000?
I should also mention that the motor is currently 1.5 inches too low on the transom, (the cavitation plate is 1.5 inches below the bottom of the keel, I will be correcting this on the weekend) Motor runs strong smooth and flawlessly.

 

[Silvertip]

Re: 19p to 17p and 1000 rpm gain??

Since you had a tach problem -- you may still have a tach problem even though you think you fixed it. Verify it with a shop tach.

 

[walleyehed]

Re: 19p to 17p and 1000 rpm gain??

I would suspect the same as silvertip....
I will also add that the 4700 with a 19P on that boat throws the red flag for me...unless this eng is very sick or the boat is well over 2000-2200lbs or so, the 19P shouldn't be far off of the 5700 mark.

 

[AEROCOOK]

Re: 19p to 17p and 1000 rpm gain??

Let me first clarify that it wasnt the tach that was broken, it was the rectifier and I bought a new one from BRP. It's possible that the there could also be a problem with the tach it's self, so I will check that first. (btw, tach is set to #6 setting)
I have an automotive diagnostic tool that has an inductive pick up tach function, can I use that?
I believe it would show twice the actual rpms on my motor since its designed for use on 4 cycle engines, right??
Also, what kind of effect (on RPM) could the incorrect engine height have?

 

[Tacklewasher]

Re: 19p to 17p and 1000 rpm gain??

I can get about the same difference between two of my props. 19p SS OMC vs 17p aluminum.

When I finally looked at them side by side, my 17p aluminum had been repaired so many times that width of the blades had been reduced by about 1/3.

What speed are you getting out of both props? Give us the speed and rpm and we can try to compare the slip percentages.

Still think your tach is at issue, but maybe post a picture of your props.

 

[AEROCOOK]

Re: 19p to 17p and 1000 rpm gain??

Thanks for the replies. This weeked I will be checking / verifying the tach operation, raising the motor to the correct position and verfiying ignition timing at WOT. (I have performed all steps in the link n sync procedure except the timing) I will post again on Monday with an update.

 

[walleyehed]

Re: 19p to 17p and 1000 rpm gain??

Aerocook, you may already know this, but if you are below 1000ft elevation, make sure the static timing is 1-2 degs retarded from Max advance.

 

[AEROCOOK]

Re: 19p to 17p and 1000 rpm gain??

Just got back from the lake and I thought I would post with an update.
On Saturday I raised motor to correct transom height, checked WOT ignition timing using Joe Reeves non running method. I discovered that it was retarded 5 degrees too far and all adjustments were at their max, so I removed the timing advance rod and bent it till timing was in spec. I tried using a Snap-On brand automotive inductive pick-up tach to compare and verify the accuracy of the tach in the boat but it didnt seem to work, so unfortunately I was unable to confirm accurate RPMs.
Anyhow, the adjustments I made had a positive effect as I was able to get 5000 RPM with the 19p (I know, still too low) and very close to 6000 with the 17p. I hope to borrow a GPS next weekend to compare speeds, but for now Im only using the 17p just to be safe.
Maybe Im mistaken, but in the back of my mind, I cant help but think that the motor has more to offer than 5000 RPM with the 19p so I will keep tinkering at it till Im satisfied.
Thanks for the input.

 

[walleyehed]

Re: 19p to 17p and 1000 rpm gain??

Can you get us a make/model of both props????
I'm concerned about the diameter of the 17P...Now don't go thinking I'm going to back out and say the diameter IS a Big issue, but it's throwing up another flag for me as to it's intended application. Most std alum OB props are 13.25 in 17P which has the correct blade design for your type of boat.
And...if it's what I think it is, it will have less cup and in a different location than the std 13.25X17 would have.
OMC/BRP also had a 14X17 which runs about the same as the older 13.25X17, but has a tad better hole-shot and runs a bit higher.

 

[steelespike]

Re: 19p to 17p and 1000 rpm gain??

Having the antivent plate even with the bottom is considered a starting point.
You should raise the motor to a point just before venting becomes a problem.A stainless will usually allow even more height.A 1" increase in height can net a couple of mph under ideal conditions. You can tell much more about a prop and the tach with the prop size,rpm and gps speed.

 

[AEROCOOK]

Re: 19p to 17p and 1000 rpm gain??

Can you get us a make/model of both props????
I'm concerned about the diameter of the 17P...Now don't go thinking I'm going to back out and say the diameter IS a Big issue, but it's throwing up another flag for me as to it's intended application. Most std alum OB props are 13.25 in 17P which has the correct blade design for your type of boat.
And...if it's what I think it is, it will have less cup and in a different location than the std 13.25X17 would have.
OMC/BRP also had a 14X17 which runs about the same as the older 13.25X17, but has a tad better hole-shot and runs a bit higher.

Sorry walleyhead, I just realized that I provided incorrect info on my original post. The 17 pitch prop is 13.25, not 13.75 as stated.
Sorry to send you down the wrong road in assessing the situation.
I knew what the size was in my mind, but my fingers typed somthing different... My bad.
I will take some pictures when Im back at the lake this weekend but Im pretty sure that both are standard OMC aluminum props and both are in very good condition with no nicks or bends etc.
I plan on having the boat weighed as well because if it weighs more that expected I could be wasting everybodys time... Heavy boat requires less pitch right?