Which Prop Should I Choose? Also Help With Identification?

[JarJarBlinks]

Hey all,

Background information:

1972 Glastron GT150

Mercury 115 HP Outboard Engine
5000-5500RPMS Max Recommended
Great Condition Engine and Hull
I want highest possible top end and best cruising fuel economy from the prop


With Propeller #2 I was making 5350RPMs and doing 45 MPH according to the notoriously inaccurate pitot static tube-driven gauge.


Here is my dilemma. I have 4 propellers that came with the boat to choose from. All are aluminum with rubber hubs. They are all in fair shape and one is not in a drastically better condition than the rest.

None of the propellers are labeled with measurements. I am assuming that they are all progressive propellers as the blades are "cupped". I just measured the blades myself using the methods posted in another thread; it may not be perfect but the measurements will show the differences.

Ill list the markings on the propellers, perhaps someone may know who manufactured them or how to cross reference manufacture measurements (No big deal if we can?t).

Prop 1:
"5768"
Diameter = 13.2"
Pitch = 15*

Prop 2:
"7680
6994"
Diameter = 13"
Pitch = 16"

Prop 3:
"12569
5629
48 32746A32 1P"
Diameter = 12"
Pitch = 15.5*

Prop 4:
"6110"
Diameter = 13"
Pitch = 17.3*

Which prop should I install on this boat?

 

[hwsiii]

Re: Which Prop Should I Choose? Also Help With Identification?

Try propeller No 1, it has less pitch and should raise the RPM right about maximum and maybe a little over, depends on the blade surface area.


H

 

[Varago]

Re: Which Prop Should I Choose? Also Help With Identification?

JarJar the 48 32746A32 1P is a merc 21" by 13" prop.

What I dont understand is why you dont test drive each prop, there is no way that numbers alone will tell you whcih is best for you. There is SO much more to a prop then dia and pitch.

Dave

 

[JarJarBlinks]

Re: Which Prop Should I Choose? Also Help With Identification?

JarJar the 48 32746A32 1P is a merc 21" by 13" prop.

What I dont understand is why you dont test drive each prop, there is no way that numbers alone will tell you whcih is best for you. There is SO much more to a prop then dia and pitch.

Dave

This is only a pleasure boat and I simply don't have the time to test drive the boat with all 4 props. A combination of irreplicable conditions, relatively small differences, and an innacurate pitot tube that rarely works lends me to rather ask the experts on the forum rather than to try and figure out the differences by trying them and comparing.

Am I looking for the prop that gives me max RPM? What will that do for me? Better top speed, fuel economy, or slower overall but just better for the engine?

 

[hwsiii]

Re: Which Prop Should I Choose? Also Help With Identification?

Usually it is best to run at the Top of your maximum RPM range as this is normally the best for the motor and will deliver top performance, but that is not always the case.


H

 

[Frank Acampora]

Re: Which Prop Should I Choose? Also Help With Identification?

Why does that Mercury 115 look suspiciously like a white OMC engine?

At any rate, that boat with a 115 should do closer to 55 MPH , not 45. If I am not mistaken, the GT 150 was rated for 90 HP so she should go like a scalded cat with the tower of power.

Something does not add up. With that power and that hull, you should be able to swing a 19 or even a 21 pitch prop at close to maximum RPM.

You need to check the engine height on the transom, the trim position, and for good measure, the compression of all cylinders.

Go to the classic Glastron site. A Glastron V153 15 foot runabout with a Merc 115 there did 55 with two people and two 6 gallon tanks of fuel. ( same basic hull as the Gt 150 with a different deck and seating.) My V153 is running a three cylinder 90 Chrysler with a 21 pitch stainless prop at 5000 RPM and 43 MPH, GPS, 19 pitch stainless prop at 5500 RPM and still 43 GPS. And a 90 Chrysler was not known to be a firebreathing engine.

 

[JarJarBlinks]

Re: Which Prop Should I Choose? Also Help With Identification?

2 questions:

1. I have included pictures of the leading side of the propeller hubs. Why does 1 hub look alot different than the other 3 (It has like a pyramid indentation to it.

2. Should I be using a thrust washer? When I removed the prop after I bought the boat, I found that it didn't have a washer installed. But there was one in the glove box! I don?t know much about thrust washers so I have included pics of the washer, the hubs, and the "spline" area where the prop meets the drive. The thrust washer I found on the boat is imprinted with the numbers "13282".


Replies:

Picture is a random picture I found online to show what the boat looks like.


Dave, not to say that you are wrong but I can easily measure the diameter of that prop and it there is no way that it is even close to 21".


Frank, I wondered about why this boat seems so "slow". I first noticed that the James Bond boat did 65 MPH to do the famous jump with a 90HP engine. At about 50, this boat will start to porpoise because it does not have a pad designed into the hull. And I know that the boat could go about 52MPH indicated because the PO used to have a stainless steel prop (huge diameter so the anode wouldn?t fit, exhaust over the blades to allow for ventilation for slow speeds, and a solid hub[gasp!]) that he would run with. I have included what you have asked for: pictures of the engine height w/ string representing bottom of hull and pictures of the engine trimmed all the way down.

Of course I trim up when going fast, up far enough so that it won't porpoise. I?ll hit 50 MPH indicated on a good day w/ light chop and a whole lot of up trim and slight porpoising. But I still think I should be faster, yes?

Also, Frank, I didn't check the compression of the engine. Ill do it when I am performing my winterization steps later in the year. But let me pose this to you: if it was engine related then shouldn't the engine not be making max RPMs?

 

[JarJarBlinks]

Re: Which Prop Should I Choose? Also Help With Identification?

 

[Varago]

Re: Which Prop Should I Choose? Also Help With Identification?

Sorry JarJar I meant 21" pitch and 13" dia, this comes from merc on the 48-32746A3 21P (top one in your pictures)

Dave

 

[JarJarBlinks]

Re: Which Prop Should I Choose? Also Help With Identification?

Any ideas on my previous questions about why the hub of one propeller is different than the others and if I should be using a thrust washer?

 

[Dhadley]

Re: Which Prop Should I Choose? Also Help With Identification?

Wow, what memories! I had one of those "back in the day". That was the first boat I straightened the bottom on. What a difference! I had an old 115 E-rude, no trim, an old 23 solid hub clever and it ran about 63. What a blast!

 

[JarJarBlinks]

Re: Which Prop Should I Choose? Also Help With Identification?

I awnsered my own question. The lower unit had a thrust washer on it, it was just stick becasue of caked on grease. I was able to pop it off by using a screwdriver as a lever. The thrust washer that was on the boat fits perfectly into the "odd looking" propeller, whereas I would guess that the other thrust washer is for the rubber-hubbed propellers.

Any ideas why I am not getting 60-65 MPH? How does the height of the engine look?

 

[Dhadley]

Re: Which Prop Should I Choose? Also Help With Identification?

I put a ton of work into mine to get it to run like that. It ran in the low to mid 50s before the bottom was done (if I remember correctly). I'd probably start with a good 19" pitch and work the set up. Mine seemed to like a lot of motor height.

 

[Frank Acampora]

Re: Which Prop Should I Choose? Also Help With Identification?

You won't get 60-65 with your 115! 55 is more like it. The James Bond boat had an OMC 135 for the jump!

Go to classic Glastron and read about it.

Height of the engine APPEARS OK, but you really need to experiment. You need to raise it in 1/4 inch increments (if possible) until it ventilatescavitates on the straightaway, then lower it 1/4 inch. My 90 is set exactly 1 inch above the apex of the VEE and it just barely gurgles on the straights with an uncupped prop. BUT: Every engine is different.

 

[JarJarBlinks]

Re: Which Prop Should I Choose? Also Help With Identification?

With Prop #4, I was getting 5100 RPMs. Same load as before (2 people, half tank of gas). I couldn't get a good speed reading.

So I take it that this is not an optimal prop for this engine/boat?

Also, how do people get good speed readings? I hear about GPS, but the only boat GPS's I see are $750-$1000!!!!

 

[hwsiii]

Re: Which Prop Should I Choose? Also Help With Identification?

You an buy a handheld one for about $ 200, even some phones have a GPS so you can measure speed.


H