Pitch problem?

[Ranger330]

Just re powered with a 98 johnson 150 and 6" jackplate. Have the prop shaft 4" below the pad. Have a 14.5 x 21 prop on now. I'm running 43 mph at 5200 rpm. What should I do? I'm thinking 26 pitch prop. What about a 4 blade?

 

[roscoe]

What should you do?

What are you trying to accomplish ?

Going to a 26 pitch prop will drop your wot rpm to 4500 or lower, seriously low. Will be a dog out of the hole and pretty much ruin everything you have now.

Dropping down to a 19p would make more sense. Will improve hole shot and get rpm up closer to where they should be. Should maintain top speed as well.

Now, switching make model and design of the prop will present slightly different results.
What were you hoping to get from a 4 blade?

 

[emdsapmgr]

Likely that engine's normal WOT operating range is 4500-5500. Current thinking is that you need to target your prop selection so that the engine will run right at the top of that "normal" range, or 5500 rpm's at WOT. At 5200 rpm's you are about 1 prop size too large (too much bite.) You need to go down to a 19 pitch to hit your rpm target. 4 blade props are good, esp on heavier boats, like a bass boat. They help lift the hull out of the water, for improved performance. Since you've got a jack plate, make sure you optimize your engine setup before you get into switching/replacing expensive stainless props. The stated mounting height you've given is not as important as where the water flows/hits the lower unit when at WOT in the lake. The lower unit's anti-ventilation plate needs to be above that water flow for max performance. Stainless props permit high transom mounting, as they still bite with one blade tip of the prop actually piercing the water surface. Be sure you watch your engine water pressure when changing engine heights. (are you running a water pressure gauge?) Once you get the setup dialed in, you'll know your rpm's and then go for prop pitch selection.

 

[steelespike]

Do you feel the boat is slow?
43 @ 5200 rpm indicates 22% slip. Is your speed by gps?The tach is usually suspect when slip is high.
Slip is usually around the low double digits.
gps is the only sure way to check speed. Tell us about your boat,tell us about your prop.

 

[Ranger330]

My rpms are 5200 but I still have 1/2 throttle to go. I could run it up to 7000 rpms if the engine could take it. I figure I need to shave 1000 rpms off by going up to a 26

 

[SkiDad]

i would think you should be closer to 50 mph with that prop. I wonder if you are too high ?

 

[Scott Danforth]

confirm your RPM's with a service tach. the inexpensive dash tachs are notoriously inacurate
where is the Anti-cav plate with respect to the pad, and how much set-back does the jack plate add.

two questions:
what is wrong with running 5200 RPM your max WOT is 5500.
What problem do you want to fix.

Assuming your running a bit low on RPM's and you have gone thru the stickies at the top of the forum
my first thought is raise the motor a bit to see if you start to ventilate.
Second thought is you may want to drop down to a 17P 4-blade which will raise your RPM up to about 5400 from 5200

 

[Ranger330]

I'm at 5200 rpms at 1/2 throttle. I've got enough throttle left to turn 7000 rpms. I don't think I need a lower pitch. I want to lower my rpms not raise them. Don't I need to be around 5500 WOT? Well I'm at probably 7000. Of course I don't really know because I haven't pushed it over 5500-5800

 

[GA_Boater]

Ranger - Both of your threads have been merged.

 

[steelespike]

As suggested you need to confirm the tach is accurate.
I'll stop being subtle What brand,model size and weight boat With the reference to a pad I suspect intended fort speed.
What brand,model,material prop.I suspect your rpm could be at 4500. I'll run some numbers.

 

[steelespike]

If we run your numbers and use 12% slip to solve for rpm we get 4570.

 

[roscoe]

My rpms are 5200 but I still have 1/2 throttle to go. I could run it up to 7000 rpms if the engine could take it. I figure I need to shave 1000 rpms off by going up to a 26

That is pretty much vital info, that should have been in the first post, instead of misleading us with irrelevant info.

 

[emdsapmgr]

I agree with Roscoe's comment. It's a different solution if it will run 7000 rpm's at WOT. Yes, you should start trying some taller props. Trying a 26 would be as good as any prop to start with. Run it and post the max RPM's. All solutions depend on a maxmized setup, accurate tach and the WOT max rpm's. It's hard to calculate just what the end result with a 26 will be. When your max rpm's are so far off, the old rule of "changing one prop size yielding a 300 rpm difference"-doesn't get you anywhere to helping determine a good final prop pitch at this point. That rule applies if your rpm's are already close to the target rpm's. You are looking at multiple props/size changes. Hopefully you've got some buddies that you can swap props with.

 

[Ranger330]

Thanks. I've got a 14.5 x 26 Raker to try. Maybe that will get me close. I do appreciate the help.

 

[steelespike]

How is the hole shot with the 21" prop?

 

[Ranger330]

Hole shot is awesome. It practically jumps out of the water.

 

[steelespike]

That is an indication it may be under propped. The boat must be light and fast.? My limited experience a 150 on a suitable boat would be propped with props under 20".Including Anglers and Rangers bass types..

 

[Scott Danforth]

First read the sticky that says "Read First" http://forums.iboats.com/forum/gene...p-questions-and-topics/295058-read-this-first

second, verify your RPM's
third, throttle position and engine RPM are not linear. while you may be at half throttle and 5200 RPM, it may only go to 5250 with the remainder of throttle, it may go to 7000 RPM if the rev limiter is removed. this has to do with prop curves, fuel curves, hull curves and engine loading. if you truly are getting 7000 verified RPM at WOT, then we need to load the motor up a bit (prop pitch, diameter, etc.)
fourth. as requested, what is the ACTUAL WOT RPM f your current setup with the throttle mashed all the way against the throttle stops, the motor trimmed properly., and you are carrying all the gear you normally bring with you.
firth, as requested, what is the ACTUAL WOT speed as indicated by GPS
6th, as requested, what is make, model, dimensions, weight, etc. of the boat
7th, as requested, what is make, model, dimensions, material, etc. of the prop.

 

[gm280]

You can go to a harder pitch to bring down the WOT RPMs IF your tack if displaying properly. If that is found to be true, yes then go up to a more aggressive pitch. But understand you will be shifting the performance curve towards the upper end and hole shot will be longer the high pitch you go... If your engine is capable of turning the prop you can calculate what any prop will give you at WOT within limits. But it all depends on IF the engine can turn it to the RPM range...it is mathematics after all!

 

[emdsapmgr]

The raker 26 is a great top end prop in any application. I've also had good results with the Renegade 4 blade ( 25 and 27 pitch) prop. (takes a different forward thrust hub) They work well on heavier bass boats, yet their design still gives good top end performance. They give better hole shot performance than the raker, but may loose a little to them on top end.