prop selection help please.

[Banditz]

Hello, i have a 50 hp 1990 force outboard on a 1979 14' astro glass fiberglass boat. Now i do not know the weight of the boat. I am sorry but I have no way of knowing. i would guesstamate at least 1400lbs with a full load and boat. But thats only a guess and heavy if I may say so.

Engine is of course a 50 hp force.

Speed now is GPS 27-28mph.

Blade has 11x12 stamped on it. So the pitch would be 12?

width of boat is right under 50" and motor is perfect under way. meaning the anti vent plate is right where the pics show it needs to be. Barely any water going over it...thanks for those pics..kinda made things a little easier.

Boat is 14' long...

Blade on her now is a three bladed aluminum prop. Kinda like to stay with a aluminum cause I go in shallow water often and dont wanna break a composite. Nor do i care enough for 1-3mph to justify the cost of a stainless prop.

I do not know the gear ratio. And have no way of finding out.

The max rpm is 5250...

Hope that gives someone enough info to help me out. Oh yeah, and which way does pitch go? meaning if I go to a 14 pitch will it go faster typical? Or would a 10 pitch go faster typical? thanks everyone.

 

[hwsiii]

Re: prop selection help please.

Banditz, You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057


In my opinion you should be getting about 3 to 4 MPH more than you are now.

Your RPM are perfect right now and your prop slip is right at 10% which is excellent, what you might try to gain a couple of MPH is raise the motor up a little higher on the transom.



H

 

[Banditz]

Re: prop selection help please.

How would i go about doing that? make new mounting holes? Wouldnt that make the transom weaker? Or do these things have a way to lower and raise without drilling new holes? Also I keep hearinf or reading that to raise speed a little to trim up the motor? How do you do that? I do have trim on it, just dont know exactly what I would be supposed to do in this case. THANKS!

 

[hwsiii]

Re: prop selection help please.

No, you should only have to move the motor up on the bolt holes. To get maximum speed from your motor you trim the motor UP until it starts ventilating and the just touch it down until it quits and you have maximum speed.



H

 

[Banditz]

Re: prop selection help please.

Sweet thank you!

 

[hwsiii]

Re: prop selection help please.

Banditz, you are very welcome, when you go out next we need some new WOT RPM and speed numbers with your NORMAL load.



H

 

[Banditz]

Re: prop selection help please.

Thanks man! I moved her up about 2 inches. had to drill two new holes, but the top holes worked fine. With my fluffy 250lb self, and ten gallons of gas, a UPS battery that weighs 102lbs up front for trolling, a 45lb deep cycle for starting..and of course, my fluffy brother. My rig now planes off smoothly,quickly, and above all she hits 32mph. I'm very grateful to you bro! Your advice was spot on. I am content as can be now. After trimming my half $@% broke trim pump up...she rides straight, smooth, and without a doubt fast! Gotta love a old boat that hauls butt! She hit 5200 rpm and she whines just a tad..kinda worried about that but I also figure its normal.since theres no oil to check bein a two stroke, and i know my mix is perfect, then I guess its ok. Water pump is blowing water out the back like a jet, and its barely warm. Outboard feels cool the whole time, and I dont even think its got a over heat alarm, never heard it anyway... Well thanks again man. I am very satisfied!

 

[hwsiii]

Re: prop selection help please.

Banditz, I am very happy to see you are happier. I told you you should be able to get another 3 to 4 MPH out of her.



H

 

[steelespike]

Re: prop selection help please.

As usual H hits it on the nose.
Your inprovement is why I always nag about motor height.H has the expertize to figure out if your prop selection is right and suggest another solution.
I usually suggest raise it to a point just before venting(cavitation) becomes a problem as there is no specific demension that is ideal on every setup..Yours is a classic example of what proper height can do.
Your motor will also be more efficient because it is producing the most speed for a given throttle setting.