Prop selection advice needed

[Prairie Bouy]

Greetings!

New to this web site and boating in general.

I recently aquired a 17' Smokercraft Lazer. It has a 115hp Johnson on it and is propped with a 13 1/2 X 20 Raker prop. I have done some research, including this web site, and it is clear that this boat is over propped. I confirmed this today by testing with WOT and using a GPS for speed and a Snap-On timing light / tachometer for RPM.

The result was 39.7 MPH at 4400 rpm. The boat was approximatly 1/2 to 2/3 loaded with two people and full of fuel. The purpose of the boat is for fishing and family fun. Towability is probably a higher requirement than Max speed. I'm from Brooks, Alberta, Canada and the altitude is 2490 ft.

I'm hoping from this that you can provide an opinion for a starting point for a prop that may suit our needs.

Also, should I be considering a 4 blade vs a three blade?

Thanks in advance! I appreciate any and all input.

Al

 

[Prairie Bouy]

Re: Prop selection advice needed

Didn't think your such a tight lipped bunch. eerrrmmm. I guess in this case it's tight fingered.

Just an update. From people I talked to they recommended a 17 pitch prop. So I found a Solas 12 3/4", 17 pitch, 4 blade and installed it. Now I'm waiting for weather to clear so I can test it. I'm thinking it will come out of the water like a rocket.

Cheers,

Al

 

[Ron G]

Re: Prop selection advice needed

welcome aboard were not that tight fingered you'll find out here soon dLets see were your setup is.how high is the cavation plate above the bottom of the hull?and is there a jackplate?and is the motor healthy?alright from were i can tell by the numbers your roughly 1200-1400 under were you need to be,alot of this may be prop but i bet alot is setup to.the 4 blade is going to need to be able to run high in the water to get the effectiveness out of it.rember the setup is probably the most important factor of anything.

 

[walleyehed]

Re: Prop selection advice needed

We also need to know the year model of this engine, or if you know if it's a 60 degree looper or is it a 90 degree crossflow...RPM will need to be a bit different between the 2.
The 17P 4-blade will kill the top-end, and will likely run about 4700RPM...still severly lugging the engine.
At 3000ft, you're gonna need a high quality 15P in SS or alum. When you give us engine model number, I can tell you what else you need to change to make it run where it should at 3000ft elevation.

 

[Prairie Bouy]

Re: Prop selection advice needed

Thanks fellas, I can imagine that these questions repeat themselves often. I appreciate your efforts.

For Rons questions, a picture is worth thousands of words. Here's my setup. Hopefully this can give you the answers you need. I don't know what a Jack plate is but I asume it will raise and lower the motor on the transom in fine increments.

(I tried the preview for these pics. Hopefully it works here. )

]

Ken, the motor is a 1991 Johnson VRO 115 hp.

I was able to try the boat out Saturday, it seems we are a bit shy of the 5500 RPM by about 5 or 600 RPM. I didn't have my buddies tach this time out, I did calibrate the boat tach to it last time and it did read close. I think the place I bought the prop from will let me exchange for a lower pitch. If thats what I need. A SS prop is not out of the question, but I think I should run AL until I get a little more boat handleing experience.

Hope this helps to help me.

Thanks again,

AL

 

[Plainsman]

Re: Prop selection advice needed

Al,
I can't help you with the prop question, I can with the pictures. Get a free account at http://www.photobucket.com
and post the link here so we can see the pictures.
And Welcome to Iboats!

 

[Crownie2]

Re: Prop selection advice needed

My advice would be to ask to try the 15" prop.

Bob

 

[Crownie2]

Re: Prop selection advice needed

OH - and yes - the setup is extremely important and you should be sure the engine is delivering full power.

Bob

 

[Prairie Bouy]

Re: Prop selection advice needed

Thanks Crownie!

I managed to get the pics posted from photobucket. I've used my web provider server on other forums......no workie here. Anyway, you can now see my motor setup.

Because of the time of season and the Alberta prairie is not the marine center of the world, I'm limited to prop choices. I was able to get a 13 3/4 X 15p Piranah 3 blade and try it. It's a comosite prop, but the name already told you that I think. Seems like the motor makes alot of noise, but we get no where fast. The hand held tach is for 4 stroke motors so the displayed RPM value must be divided by 2. It also runs out of digits at 9 999 RPM and gives an error after that. On test of the 15P at WOT I get high 9 k revs and flashes over to to err. This tells me I'm right around 5000 rpm. The boat tach says 6000. Are boat tachs usually out by this much? Why do they install them then? I'm only getting about 32 MPH as well. It's all very peculiar. Because there are many similar boats that are running 17P with 90hp motors around here. Lunds, Cresliners, Alumacrafts, etc.

Having said all that, I've pulled the spark plugs wich are all a nice tan color, except one being a bit darker. I also did a compression test and the results are, 107, 108, 112, 115 psi. Within 10 percent wich I think is acceptable.

BTW. The 17 P run 36 MPH at 4800 rpm.

If anybody has any suggestions I'd like to read about them.

Thanks in advance,

AL

 

[Crownie2]

Re: Prop selection advice needed

As I thought, the 15" would be better - the question is which one. Can you try to swap out the 17" four blade? Actually a three blade would be better. I don't know too much about the Pirhana and don't know why it performed so poorly.

Pics are worth a lot - yes you can raise the engine at least one hole on the transom. Get someone who knows how, to help or do it. I don't like the looks of that top mounting bolt - is that secure??

Good luck!!

Bob

 

[Ron G]

Re: Prop selection advice needed

IHows your water prassure now?if good raise it on up it should bring the rpms up a little.you may need to go to 15 to get the rpms up were they need to be.

 

[Prairie Bouy]

Re: Prop selection advice needed

The motor is secure at the top. There is a spacer there.

I wondered about raising the motor. I have nothing that tells me what the water pressure is. How and where do I connect?

Is there any chance that this motor may be geared funny?

It's a giving however, motor RPM must go up for the sake of the motor.....hence, my wallet.

Al

 

[Ron G]

Re: Prop selection advice needed

You need to get a water pressure gauge thats one of the most important things you can have.i would think by looking at the pics you could go up another hole.i dont know about the gearing being funny.but the rpms are going to need to come up your lugging that motor right now.

 

[Prairie Bouy]

Re: Prop selection advice needed

Ron,

Before I go through all this, how much do I chance to gain by moving the motor up one hole? A pressure gage is not a problem. Those are already in my shop. Where and how does it need to be connected? What is the max, min pressures I'm looking for?


As you can see I have plenty of questions. Thanks for the paitience to answer them.

AL

 

[Ron G]

Re: Prop selection advice needed

i dont know that much about were to conect the gauge.but theres alot here that will help with that end of it.
as for what your to gain not that much maybe around 100rpms if lucky.going down in pitch if lucky 200 rpms.which is still way below were you need to be.but raising the motor does help in the performance of the rig.but ya never know until you try it.

 

[walleyehed]

Re: Prop selection advice needed

Crossflow, we need 5800RPM. T in the pressure gauge line right into the tell-tail line at the rear of the block.
The line comes from the bottom of the block (90 degree fitting) loops up to the top and back down to the pi$$er port. T in anywhere on that line.
And, the engine is mounted way too low....you could run in the third set of holes from the top with no pressure issues....question is...is the prop good enough to bite at that height.