Prop question?

[oldguy60]

Hi all.
I recently bought a 16' runabout with a 1976 Mercury 115HP outboard and it came with an aluminum 13 x 17P prop and when I tested it on the water with 3 people, 15 gal. gas, 6 1/2 hp kicker (basically a full load) it was revving above 6000RPM ( my tack only goes to 6000) and only giving 31 knots (GPS reading) at WOT.
My question is: I have access to a 13-1/2 X 17P Ballistic SS prop and I want to know what changes in RPM to expect "going from Aluminum 13 x 17P standard rake style prop to the Ballistic 13 1/2 x 17P stainless steel which has more rake and different blade design and 1/2" larger diameter"?
My operating range is 4750 - 5250 rpm so I need to drop about about 800 - 1000 rpm.

 

[hwsiii]

Re: Prop question?

Guy, i show your motor to have a manufaturer reommended WOT RPM should be 5,300 RPM and your gear ratio to be 1.7:1, please verify this. If this is orret then you have a prop slip of 39% and I just find that hard to believe. If you will fill out the following form I will do some further analysis.

Iboats Boat and Motor Info
1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
2a.What is the recommended HP for your boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make and model of motor
7. HP and gear ratio of motor IMPORTANT
If you don?t know the ratio, you need to pull the plugs out and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing and turn the motor until the tape lines up with each other, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
8. Manufacturer?s recommended Wide Open Throttle (WOT) range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches
10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil, dolefin or trim tabs
12. Make, model, diameter, pitch and whether SS or aluminum prop
13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. If you do not have a tach you can buy a Tiny Tach for $ 50
14. Are you at sea level or a higher elevation, give us the elevation in feet
15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way, checked compression, and looked at the plugs and checked spark, all of the foregoing could be the reason your prop is not attaining full RPM
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
17. Does the prop show any damage that you can see
18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations
19. If you are trying to attain a better cruising speed and fuel savings or trying to attain a faster speed I will want you to take your boat and run it with 1 or 2 people and give me the RPM and speed readings starting at 3,000 RPM in 500 RPM increments all the way to WOT.

REMEMBER, The numbers I give you will be NO better than the information you give me

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.


H

 

[oldguy60]

Re: Prop question?

1.- 1966 16ft. Glasspar (I believe the model to be an Avalon)
2.- length=16ft., width= 6ft. 5in.. don't know the weight (but it is fairly light as I can move it around on a level surface by myself and I am not a big guy)
2a. - don't know the recommended horse power as this is the second motor on this boat and there is no decal with that info.
3.- normally 2 people (but sometimes 4) on the boat and 15 gal. of gas
4.- normally just used as a runabout and sometimes fishing (no skiing or tubing etc.)
5. - It is a deep-V but not a really steep deep-V like you see in ocean type deep_v's (ones with high gunwhales like a double eagle hull). I believe it to be more of a lake boat.
6.- motor is a 1976 Mercury 1150 outboard
7. - it is 115hp and has a gear ratio of 1.7:1
8. - manufacturer's WOT range is: 4800 - 5300 rpm
9. - anti-ventilation plate is level with keel ( with in one inch)
10. - it is not a bass boat and I don't know what a pad bottom is but I will try to attach a picture of the boat
11. - it does not have a have a hydrafoil, dolefin or trim tabs
12. + 13. - the above test figures given (WOT gave more than 6000rpm and 31 knots by GPS with 13 x 17P aluminum stock Merc prop and 3 people in boat and 6 1/2 hp kicker and 15 gal gas)was with a stock Mercury 13 x 17P aluminum prop that was in decent shape but I now have a Ballistic 13 1/2 x 17P Stainless steel prop on it (that I haven't tested yet)
14. - I am not right at sea level but damn close (maybe 100 - 200 ft above)
15. - The engine was just tuned up and the old plugs looked good, and the spark was excellent ( according to the mechanic)and the compressions were 150,155,155,155,155,140,. I don't know about the butterfly but I am sure the mechanic would have checked that when he checked the timing and spark.
16. - The prop that was on the boat when I bought it was a Mercury aluminum 13 x 21P and it had some damage so I took it off (with much trouble as it was seized on the spline) and replaced it with the extra stock prop that came with the boat (a 13 x 17P Merc. aluminum). And when i asked the local Mercury dealer what I should use to give me the best combination of economy and top end and still put me in the right RPM range he suggested A 13 x 19P maximum and he thought that the 13 x 21P was too much for this engine and to try the 19 pitch. The Ballistic 13 1/2 x 17 SS I have on it now is a loaner from a friend and as I said earlier I haven't tried that prop yet.
17. - The 13 x 17P Stock Merc. aluminum prop that gave me the RPM that was too high at WOT (above 6000 rpm) and a top speed of 31 knots had some slight damage (a few very small nicks to a couple of the blades).
18 + 19 - I am trying to attain the best combination of cruising speed and economy and hopefully a better plane to combat the rough water that can come up on the lake we frequent (Powell Lake, B.C., Canada). I see some other small runabouts that seem to have only a small portion of the stern in the water when they hit the rough chop and they just seem to skim over it with no bounce at all while our boat as it is now is terrible in the waves as it bounces and shakes your eyeballs out. I hope to go out on Monday or Tuesday coming and try the stainless steel Ballistic 13 1/2 x 17P prop out and I will try to get some readings of RPM and speed every 500RPM starting at 3000 as you stated.

 

[hwsiii]

Re: Prop question?

Guy, I will be waiting and please try to find a GPS to use for speed, either a hand held or some phones have them.


H

 

[oldguy60]

Re: Prop question?

OK - here is the results with the Ballistic 13 1/2 x 17P SS prop using a GPS and the altitude was 206 ft. above sea level.

3000 rpm = 17 knots
3500 rpm = 21 knots
4000 rpm = 25 knots
4500 rpm = 28 knots
5000 rpm = 32 knots
- at just over 5000 rpm the motor has a sudden drop in rpm and then after a slight pause climbed back up again so I guess there is another issue here going on besides the prop. I recently installed a fuel filter (the kind with a clear glass bowl and drain at the bottom) and the hose they sold me at the supplier to make the connections was not as thick as all the other stock fuel lines so I am replacing that first and see if that solves this problem.

Anyway the question of whether I should go to a 13 x 19P prop or a 13 x 21P prop to achieve the best combination of top speed and economy still begs for an answer??

 

[oldguy60]

Re: Prop question?

Oh I forgot to say that the WOT with the Ballistic prop (previously stated) was still up around the 6000 rpm range before the engine had the sudden drop in revs so I assume that the 17 pitch prop is not the correct one for this boat because it puts me above the correct rpm range?

 

[78ImperialT]

Re: Prop question?

ok...17p is 17p. so 17 inches per rotation. it doesnt matter what type of prop it is, its still just 17 inches.

i say the 21p is perfect! i have....had....a 16' trihull with a 75hp johnson and ran a TP hustler 21p...no probs. planed out great! top end was impressive! and fuel economy was super. i wouldnt plan on pulling an adult skier with that prop, so i kept the original 17p.

i say go for it!!!

 

[Randybeall]

Re: Prop question?

ok...17p is 17p. so 17 inches per rotation. it doesnt matter what type of prop it is, its still just 17 inches.

i say the 21p is perfect! i have....had....a 16' trihull with a 75hp johnson and ran a TP hustler 21p...no probs. planed out great! top end was impressive! and fuel economy was super. i wouldnt plan on pulling an adult skier with that prop, so i kept the original 17p.

i say go for it!!!

I have to agree with this. The 19 would give you in the 5600 range if 6000 was all the 17 would do but we don't know that. 21 makes since unless you need quick take off for skiing or tubing.

 

[hwsiii]

Re: Prop question?

Guy, I recommend a Solas Rubex 4 blade aluminum prop in a 19" pitch right here from Iboats. Right now you are showing a 21% prop slip with the Ballisitic prop at 5,000 RPM. I hear you say that the prop will turn 6,000 RPM but for some reason you don't show a speed for that, so I have to assume you are having a problem holding that RPM. I did use the 6,000 RPM as my basis for the recommendation of the new prop though. You will find the Rubex will turn about 5,300 RPM and your prop slip should go down to less than 10% which will give you an increase in speed and by running a 4 blade prop instead of 3 blades you should not lose any hole shot and you will have an increase in WOT speed as well as at all cruising speeds.

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H