Prop to Crank HP rated engines?

[Dhadley]

Re: Prop to Crank HP rated engines?

Did he say what was 3" below the pad?

 

[imported_Jimbo]

Re: Prop to Crank HP rated engines?

Hey Jedi, since we seem to share the same problem, please get back with your results maybee I should do the same thing. First of all I will raise the engine a few notches and see what happens. BTW this is what my set-up looks like:

BR Jimbo

 

[jedimaster]

Re: Prop to Crank HP rated engines?

Center of the prop shaft 3 inches below the pad. Currently I am at 5.5 inches with a half inch more travel available on the bracket. But tomorow I think I will go pick up a jackplate to get up an extra 2 inches.

 

[Dhadley]

Re: Prop to Crank HP rated engines?

OK, that's good. A lot of times folks miss the idea that they mean the propshaft.

 

[jedimaster]

Re: Prop to Crank HP rated engines?

Any guesses on what my max rpms will go up to based on moving from a 3 blade to a 4 balde in the same pitch and moving the motor up 2.5 inches?

 

[Dhadley]

Re: Prop to Crank HP rated engines?

Could be anything. If you're using the same style 4 blade as the 3 blade from the same manufacturer it'll probably be roughly the same. If you're going from a bow lifting 3 blade to a stern lifting 4 blade, and your boat needs bow lift, the rpms could be less. And so on....

 

[jedimaster]

Re: Prop to Crank HP rated engines?

The prop I bought is an amita 4 blade.
Its listed as a stern lifting prop.

However I just read that it you usually need to decrease the pitch by 1 or 2 pitch sizes to keep the same rpms. So that would mean by keeping the same pitch my rpms will now go down? I have also read in other articles that depending on the hull design, top end RPM's can actually go up when going to a 4 blade from a 3 blade.

I was oringially going to get an HR Titan 4 blade 17 pitch, would I be better off with the Stainless 4 blade prop or the alluminum 4 blade? or should I have dropped a pitch to a 4 blade 15 pitch SS or AL, or a 3 Blade 15 pitch SS or AL?
Basically I think I have a few options.

17 SS 3 blade
17 SS 4 blade
17 AL 4 blade
15 SS 3 Blade
15 SS 4 Blade
15 AL 3 Blade
15 AL 4 Blade

I was thinking the best three options were a 17 pitch 4 blade AL, 15 pitch SS 3 blade, or a 17 pitch SS 4 blade, all three options were in my price range, but the tech on the phone said the 4 blade AL would provide the best results.

 

[Dhadley]

Re: Prop to Crank HP rated engines?

If your boat needs help to carry the bow you'll be better off with a bow lifting prop. Some boats like Allison, Stroker, Bullet have a lot of natural bow lift and benifit more from a stern lifting prop.

It all depends on your hull design. If you use a lot of positive trim then you'd be better off with a bow lifting prop. If the nose floats pretty easy on it's own then a stern lifting prop will probably get more hull out and create less drag.

 

[jedimaster]

Re: Prop to Crank HP rated engines?

How would one determine weather or not a particular hull has natural bow lift?
as my calculations went before, I was turning 5400 rpm's with a 17 pitch prop, at 33.6 mph, averged out in two directions.

The prop slip is 6.46 percent. To my understanding 8-10 is an ideal setup for a stepped hull bass boat, so I would think I am running very efficiently up top, but still horrible down low, not too sure how if at all that would relate to bow lift. I don't see me being able to add much efficiency up top, but possibly an increase in motor height and I may get some more rpm's, with less of the lower unit in the water to drag.

 

[Dhadley]

Re: Prop to Crank HP rated engines?

To get your max speed, is the motor at about 50% of the trim range or at 75%? Or 90%?

 

[jedimaster]

Re: Prop to Crank HP rated engines?

Not too sure, I have to kinda go by feel the trim guage is broken. I don't notice much a a difference in speed until it starts to plow. Not like my bowrider, it was really sensitive to trim angle. Just a tick on the switch and you would drop a few mph. I was ticking the trim up and down and couldn't see much of a difference in rpm's or speed until it noticably started to plow water with the bow.

 

[jedimaster]

Re: Prop to Crank HP rated engines?

So I installed the jack plate. The prop shaft is now sitting at 3 inches below the pad.

Before Jack Plate
5400rpm's
33.6 mph

After Jack plate
5700 rpm's
36.5 mph

However if I trimmed up while on plane beyond a certain point I would ventilate, also any corenering maneuouver would ventilate.
Water presure is nice and high so no worries about that.

Hole shot still sucks, and I can't get out of the hole without moving the trolling motor batteries the console.

It now seems like the bow isn't lifting enough on plane. I think maybe a bow liftingprop would better. also with the RPM's up now possibly an 18 pitch raker would be a good move? Supposedly they give good bow lift and can't be run very near the surface.

I'll have to wait for the 4 blade alluminum to see what sort of affect that has. To have some sort of comparision.
Any further advise on the setup?