Basic question about testing for a new prop

keninaz

Chief Petty Officer
Joined
Dec 15, 2010
Messages
448
Tomorrow I will test my boat/motor with an induction tach for RPM at elevation to select the proper pitch for operation at elevation vs. the standard prop that came with my small motor.
I have been told to do this with minimum load in the boat. Then I got to thinking about it and why would I not test the boat with a typical load the way I will normally use the boat.
I am a bit confused.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
By doing a lightly loaded run we can get a better idea if your setup is working.
We will need the gps speed, details on the motor,prop size)near the base of the blades
or under the prop nut.Need the gear ratio if you have it.
 

keninaz

Chief Petty Officer
Joined
Dec 15, 2010
Messages
448
Ok, just got back from my local lake. No wind today and the lake was like glass so a good day to test the motor out.
The boat, a Lowe 1467WT (14' deep V) weighs I around 262# according to the manufacturer. I hit the scales at 232# and the motor weighs in at 87#. The gas can hits around 20# (2 gallons in tank) and other than that there might be 30# more in the boat in misc is all. So the gross weight of the boat is around 631# or so.
The prop from the factory is a 9x8.75.
I tried all tiller angles to see if it made any difference in the boat but I did not see much difference. With me and most of the weight on the rear end the boat rode high in the front. Using the Tiny Tach for accurate RPM display I did not see much in the way of change in RPM either at the various angles of the motor. I suspect as it's not planning at all (or so I think).
I was shocked when regardless of angle of the motor as about as light as I can get it the motor RPM only varies from 3780 to just over 3900 RPM and the GPS measured speed was about the same regardless with WOT making 6.4-6.7 MPH.
The factory says this motor should run best at 5700-5800 RPM with spark reduction occurring at 5850 for motor safety.
At this point I don't know if the loss in RPM is all due to the sea level factory prop or loss of engine RPM/power at 5100'.
The motor is a new 2014 Mercury 9.9 HP Non Bigfoot with a gear ratio of 2.06:1

Anyway, that's about the best testing I can do at this point. I am thinking a 9x6.5 prop might help it a bit but I don't know how much.
 
Last edited:

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Are we sure the brand new motor is running on 2 cylinders?
Have we grounded one plug wire at a time . Is the throttle opening all the way?
6 mph you could row that fast!
Not saying it should plane at 5000 ft but it should be really busy.8 or 10 mph!
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
The loss of achieved rpm is mostly due to engine not being opt suitable for that boat, more hull drag to overcome fast + high altitude issues.

At just 3900 wot rpm you're extremely lugging the engine. Now imagine the extra poorer performance you would achieve under more load or + passengers. At least you count with an induction tach to know how well or not that engine is revving at altitude. Other boaters using same without a tach will be killing the engine extremely lugging it without knowing this condition is actually happening.

Go for a second wot spin as before, trim engine to be perpendicular to water level and move all the weight forward, see if by moving weight can make the combo to plane, if so rpm will raise accordingly.

A 15 or larger HP OB with factory delivered prop will perform much better than a 9.9, although not top as at sea level use, if you can't plane that combo and only getting fast displacements speeds, you are increasing hull drag difficult to overcome fast with an engine that's under rated and has already jets issues.

You can expect a 500-600 rpm increase per pitch decrease on a light combo at sea level, at altitude probably 50% increase in which case will need to decrese several pitch numbers to pull revs to 5850 with just pitch play. Anyway should achieve faster hole shot but with terrible top end speed decrease.

Happy Boating
 
Last edited:

keninaz

Chief Petty Officer
Joined
Dec 15, 2010
Messages
448
It starts and runs smooth. I have not grounded one of the spark wires.
I looked at the throttle linkage and it appears not to be working correctly to me but I will have to review the service manual in hopes of telling for sure.
Of note, a couple weeks ago I took the boat to a lower elevation, 1600' where the motor and prop should perform well but I did have an additional load at that time and my results MPH wise were the same as elevation here.
I had the feeling all along that the
RPM was not coming up to par lower or up here so maybe the throttle linkage is the problem. Again I will have to reference the service manual online. It seems that I can push it further by hand than the way it's set now.
 

keninaz

Chief Petty Officer
Joined
Dec 15, 2010
Messages
448
Well I looked at the service manual but noted that what I had was a 1998 printing but the motor is a 2014 and the linkages are not the same.
I will have to try and find a newer service manual or haul this 100 miles to the dealer I bought it from for check out I guess.
 

keninaz

Chief Petty Officer
Joined
Dec 15, 2010
Messages
448
I just got off the phone with the dealer. His service guy pulled the hood off a 9.9 and told me what to look for. As it turns out, somehow the throttle linkage was not hitting the WOT position stop and in fact was about 1/2" from that stop point. I readjusted the linkage and it's much much better now.
Now I have to take the boat back to the water and run another test session with it.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
At least you found the culprit, now will work much better. Go for that wot test again and report back your findings.

Happy Boating
 

keninaz

Chief Petty Officer
Joined
Dec 15, 2010
Messages
448
I will test it again and see if the throttle linkage fix worked. But Mercury is recommending a 9x6.5 prop for me too. I will try to stock prop first and hope that the fix makes me happier.
 

keninaz

Chief Petty Officer
Joined
Dec 15, 2010
Messages
448
Well no luck. I may have moved the throttle linkage messing with it myself before I found I did not put it back where it belonged?
The bottom line is that today's test with the same load and distribution (I have no choice) again trying all trim angles the absolute best was just over 3900 RPM and 7.2 MPH observed. I am guessing that the elevation is killing the motor???
So I guess at this point I try a 9x6.5 prop that Mercury recommended and see what happens. Not quite sure what to expect but from what has been said here the RPM should go up, not sure about speed however.
Disappointing results for sure.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
I think I calculated the 6.5 at sea level and came up with 16 mph at 6000 rpm. and guessing 10% slip.
​ Of course that based on a boat thats a more reasonable weight.
 

keninaz

Chief Petty Officer
Joined
Dec 15, 2010
Messages
448
I can't bet the boat any lighter either, nor can I change the weight distribution.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
I can't bet the boat any lighter either, nor can I change the weight distribution.
I didn't intend you to make any changes only a reference number for a 6.5 prop in a more typical application.
Maybe you could try your 9.9 on a friends basic bare utility.
There must be other 9.9s near you how are they doing with the elevation.
Strike up a conversation at the launch ramp.
 

keninaz

Chief Petty Officer
Joined
Dec 15, 2010
Messages
448
Small community and only one local lake even allows the 9.9 HP motors. But I will look out for one. I thought of taking the motor back down to Phoenix for testing but they are at a much lower altitude and I ran the motor with a bit heavier and normal load at 1700' and did not see much difference. I did not have the tach at that time but I am beginning to think that the motor was not coming up to proper RPM with the supplied prop there either and it should have.
I am wondering if something is wrong with this motor? It seems like a loss of 1800 RPM from the factory WOT to only 3900 with the lightest boat load I can give it is strange.
Using rough math with an observed 7.2 MPH at best at elevation if I could get the RPMS up to proper level I should be doing some 10+ MPH. I am going to so other testing today looking for an answer.
The dealer is ordering a 6.5 pitch prop too for the motor and I will be anxious to test it.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
I am wondering if something is wrong with this motor? It seems like a loss of 1800 RPM from the factory WOT to only 3900 with the lightest boat load I can give it is strange.

Were you achieving at sea level around 6 K . 500-600 + rpm per one less pitch, does not enter the high altitude equation, probably + 300 or less rpm. A 6.5 prop should provide higher rpm, but don't expect a wow speed at altitude.

Happy Boating
 

fishrdan

Admiral
Joined
Jan 25, 2008
Messages
6,989
If you can't distribute what weight you have, then you may need to add some weight to the bow as far forward as possible, for testing purposes, try 50-100# and see how the boat reacts. (Bring along a blanket and pick up some big rocks by the launch ramp.)

I would also try the 5.5P now that you have the tach to see what happens, see if the outboard hits 5800 or close to it. I don't remember, but what happened with the 5.5P, still dogging on RPM or hitting the reving out and hitting the limiter?


Doelfin or a Stingray JR hydrofoil..............
 

keninaz

Chief Petty Officer
Joined
Dec 15, 2010
Messages
448
As of today I am almost certain that I have my problem solved.
Today our granddaughter is staying with us and wife wanted to take her for a ride in the boat has at 6 years old this is her 1st time ever in a boat.
So I added well over 300 pounds back to the boat with the extra passengers and gear and while boating I had the passengers sit in the very front of the boat.
So now the Lowe V1467 WT that weighs some 262# and is 13' 11" long is back where I would normally use it fishing and such.
Much to my surprise versus the results I had as suggested with the latest boat possible and only obtaining 3900 RPM the boat was able to reach nearly 4200 RPM and the boat actually planed and maintained that plane quite easily. I did not have the GPS with me to monitor speed but I would estimate that I obtained somewhere between 8-8 1/2 mph
The Dealer also called me and told me that my prop for correction at altitude is in stock and that is an 8.9 X6 .5 which when installed should raise the rpm's perhaps another 300 to 400 RPM.
I noticed yesterday that my plugs were pretty dirty and fouling due to the low RPM range I have been in so perhaps with better balance and load and the new prop I will get enough RPMs that they will burn clean. I could always go to a hotter burning plug to correct this problem however.
I have a few posts going so I will post this in all of them as it may help someone else along the line. I never figured that adding so much weight to this boat would increase the speed. Lessons learned.
 

keninaz

Chief Petty Officer
Joined
Dec 15, 2010
Messages
448
Today, with a full load in the boat, all but the trolling motor itself, and the new 8.9x6.5 prop that Mercury Marine said to use at 5000' I took the boat out and it hit 5400 RPM max and over 12 MPH max. Steady speeds against the wind and all was 10-11 MPH easily.
Problem finally solved.
The only other thing that I did was at idle it was loading up a bit so I drilled the plug they had installed over the fuel screw for idle and adjusted it. It now starts better and idles better and does not load up at extending idling.
 
Top