Need the Prop Gurus!

[bsutravis]

I've never been happy with the hole shot or top speed at WOT with this old boat being pushed by an even older Tower of Power Mercury. Here's what I've got:

'77 Mercury 1150 - 115 HP
Boat is an '88 16' - Fabuglass Fish n Ski
Prop Numbers: 1411-133-17 and is Aluminum with a 13.25" diameter
Max RPM at WOT: 4600
Max Speed at WOT: 33mph (GPS verified)

With those numbers I think I'm overpropped and I'm not getting to the max RPM for a 115 Tower of Power.... correct? I think I should be on the 5200 RPM range.

So my question is... is it feasible to drop to a 15 pitch prop or should I stay with the 17 and drop the diameter? A 15 pitch just seems so small but I'm not sure.

Thoughts???

Thanks!

 

[racerone]

If your motor is not in good condition then going to 15" might get the revs back up.----Not the correct approach.-----Post the compression numbers.---Is the boat in good condition and not water logged ?

 

[Scott Danforth]

agreed, post your compression numbers. is hole #3 low?

4600 RPM is way to low of an RPM with the 17p prop

have you weighted the boat

 

[bsutravis]

The motor is in excellent condition, it was 100% rebuilt a few years ago and has less than 20 hrs on it since rebuild. There were 3 guys in the boat so we were weighted down I suppose, plus live well water.

 

[Sea Rider]

Has the motor been propped right after was rebuilt for that application ? If the motor runs healthy a 3 blade 15 pitch prop will pull the wot rpm up if happens that's not been propped right, the boat being waterlorged or a combo of both situations.

Happy Boating​

 

[bsutravis]

No, the prop is the same that was on it before rebuild... but the boat has never gotten decent speed or hole shot since I've owned it. With just myself in it I might get 36mph or so... unsure about the RPMs with just myself.
Would making the prop diameter give me more RPM's as well or should I just try a 15 pitch?

 

[Texasmark]

Don't know when Merc started prop rating their engines but they have been accused of running high for their posted ratings. So, you could be running a 100 hp by today's standards, but even at that, that's a big engine for a 16' boat with any kind of a planing hull.

What is the history on the boat? How about some pictures, lots, showing hull, engine mounting, stern shots showing the height of the lower unit anti ventilation plate to the hull, the hull at the transom.

Did the boat spend it's life sitting outside? A marina queen? Point here is saturated flotation foam increasing the weight of the hull big time.

How much did these 3 guys weigh? How big is your livewell or beer box or other heavy things.
------------
But, my opinion is that you have something dreadfully wrong. Your prop slip checks to be 11% which isn't all that bad meaning that your boat is responding well to the thrust the engine is providing. So it's not all the things mentioned above. That tells me that the problem isn't in the hull or the setup, it's in the engine itself.

When you said it has been rebuilt a few years ago, and I guess it ran like a scalded ape afterwards for that 20 hours, put on it when, immediately after the rebuild? Has it been run since or sitting up in a barn/garage/out in the yard...fuel turning to varnish in the carb bowls......I like Racerone's request for the compression numbers and to know whether or not all cylinders are firing. While taking pictures, how about a picture of the business end of all your spark plugs. With that solved do you know if you have unrestricted fuel flow to the engine besides the varnish comment above? When was the last time the fuel lines were changed; the fuel filter was changed...and inspected for debris? Can you disconnect the fuel line from the engine and using a suitable container, squeeze the bulb and get good squirts of fuel out the end of the hose? You seem to only be running on half your HP. IF your load weighed 200# apiece, and the setup was wrong, and the hull was dirty with barnacles and the trim angle was wrong and all that, your slip would be up around 25% and with that, a 15P might help some......, but at 150-200 RPM change per inch of pitch change, in either direction, not enough to get you up to your rated RPMs which I estimate to be at least 5500....I had an '88 115 and that was it's top end rating...but it liked to run 6k and 50 mph on a 171/2' Ranger with a 19P Laser SS. Merc doesn't have the cubes to get torque so they need RPMs to get HP...HP equalling torque x RPMs/5252.

My 2c

 

[Sea Rider]

If the prop was same as before the entire rebuilt means that motor was never before propped right to run towards it's max wot rpm range. Not all boaters props their combos right, some are content with the prop's water performance that's currently installed or factory deliver with if it were the case.

Propping right it's a fine tunning near science that many won't go for due to too much trouble, extensive water testing and money spent. Need to know the motor's min-max wot rpm range factory stated to prop it right as will be usually loaded, say 2-3 up to know how much prop pitch to drop...​

Happy Boating

 

[racerone]

Find a shop with a marine dynamometer.-----Have the motor output tested.-----Something is not right if you have to go to 15" pitch with that set-up !!!

 

[QBhoy]

Compression test time, I’d say. You’re already suspiciously low pitched if it’s a 17”, I’d say. Other than that...I haven’t ran any of your numbers through a prop slip calculator, but I’d be pretty sure they won’t be good or make sense. Something isn’t right.

 

[racerone]

Well----A 100% rebuild a few years ago means very little in my opinion.

 

[Sea Rider]

To the OP, to avoid tech speculation, perform a compression test with a fully warmed motor and report the individual cylinder numbers achieved.

I'm going to flip my desk's chair laughing out loud if that motor which has been totally rebuilt and has clocked less than 20 run hours dials high and even compression numbers between all cylinders. If all cylinders are firing spot on as they should, go for a15 pitch prop maintaining same 17 specs to bump up more wot revs than with current 17 pitch or send current 17 to a prop shop and have it repitched to a 15, little money to invest, much boating fun to gain if bumping rpm and speed up is what you want.

Happy Boating​

 

[QBhoy]

To the OP, to avoid tech speculation, perform a compression test with a fully warmed motor and report the individual cylinder numbers achieved.

I'm going to flip my desk's chair laughing out loud if that motor which has been totally rebuilt and has clocked less than 20 run hours dials high and even compression numbers between all cylinders. If all cylinders are firing spot on as they should, go for a15 pitch prop maintaining same 17 specs to bump up more wot revs than with current 17 pitch or send current 17 to a prop shop and have it repitched to a 15, little money to invest, much boating fun to gain if bumping rpm and speed up is what you want.

Happy Boating​

No way that should be having a 15” prop on it. Under normal circumstances and gear ratio. Not a chance.

 

[racerone]

I agree.----Why is this issue so confusing to some owners ?----Get the motor checked out !----Get the boat checked out.----Something is wrong.

 

[Sea Rider]

Issues that will contribute to make any motor not to run top :

-Check that the remote control and throttle cable allows the carbs to open fully at wot.
-Motor and transom are spot on height matched.
-Motor isn't excessively trimed up or down, running on plane at neutral trim is ideal.
-Hull is immaculate clean.
-Deck load is well balanced.

If we assume that the motor runs strong throughout the whole throttle range, compression is high and spark plugs fires strong, boat isn't waterlogged and copes to the letter with was mentioned earlier that motor running at just 4600 wot revs is 400 wot rpm under its max middle rpm range and 900 rpm from its max 5500 wot rpm range if assuming the wot rpm range for that motor is 5000-5500 ?

Happy Boating

​

 

[achris]

...assuming the wot rpm range for that motor is 5000-5500 ?​

That's the right range for those engines, yes. And overpropping them is the quickest way to an early grave! They tend to pop a hole in #3 when overloaded.

To the OP. You have no control over diameter. That's decided by the prop manufacturer, and based on the style of prop. You move up and down by PITCH, not diameter. SOME different diameters are available for a prop with the same pitch, but that's a style change. Like a Vengance 17" for my engine will have a 14" diameter, and a Mirage of the same pitch will have a 15" diameter. That's just the way props work. Forget the diameter, concentrate on pitch, that's what will ultimately control your engine revs.

Chris..........

 

[tbetty]

I'm running a 1971 -115hp tower of power on a 16ft Tahiti ski running a 17p X 13.25 aluminum 3 blade prop turning 4800 rpm (not verified) at 45mph GPS. Don't know if this helps or not but hole shot is right now. This is a 1970 boat so no flotation to be waterlogged but still a heavy boat with all gear and gas. You don't seem to be too far off on rpm, speed maybe. I question the hp and rpm rating from Merc.

 

[Faztbullet]

Sounds more like carbs are not open at WOT or ya got a hole down....

 

[Texasmark]

I'm running a 1971 -115hp tower of power on a 16ft Tahiti ski running a 17p X 13.25 aluminum 3 blade prop turning 4800 rpm (not verified) at 45mph GPS. Don't know if this helps or not but hole shot is right now. This is a 1970 boat so no flotation to be waterlogged but still a heavy boat with all gear and gas. You don't seem to be too far off on rpm, speed maybe. I question the hp and rpm rating from Merc.

Some body is telling you a story:

Go-Fast.com > Knowledge base > What is Prop Slip? > Prop slip calculator

Your slip is minus 15.....a screw turning in a block of wood is a slip of zero. Fast boats run in the positive single digits.

 

[Sea Rider]

Need to verify the max wot rpm achieved as loaded with a tach in good working order to get the max out of said motor and not screw it if suspicious that runs over propped...

Happy Boating​