what should i do???

[ChrisCraftfisher]

i have a 1989 22ft chris ccraft sea hawk that i repowered with a 190hp v6 4.3l I/O in 2006. at top speed the rpms are about 600 more then they should be. i wanted to put a prop on with a smaller pitch to bring the rpms down but the boat was hard to get up on plan and would only stay on plan at high speed. i fixed the planing problem with a hydrofoil but i was wondering if it would still benefit to change out my prop. i have a 3 blade 14x19 stainless right now.
can anyone help??

 

[steelespike]

Re: what should i do???

First of all you would be better off with something like smart tabs.Foils can solve problems and create problems.In the extreme they can damage your antiventilation plate.What are the actual max rpm and speed?Difficult to tell whats up with only rpm.Maybe you could borrow a tach to check if your tach is accurate.I'm not an expert but it doesn't seem to me a 14 X 19 would be that far out on a 22 ft boat unless its a lightweight.Did you change it to an I/O or was it always an I/O?What make I/O?Year?

 

[oops!]

Re: what should i do???

hi chris....

WELCOME TO I BOATS

i am not a prop pro...but....

forget about top speed right now......

take your boat to a place that will let you try a bunch of props.....

you are currently ..UNDERPROPPED.....that means the prop pitch is too small and the engine is screeming.....

you want to up size the pitch till you are at the right rpms....then play with cup and diamater for best hole shot and top end while the motor is at wot...(Wide Open Throttle...= wot)

but it sounds to me like someone has been playing with your prop size......and got you all outta whack..... the guys here will be able to give you a baseline pitch.....

but that motor should be able to put you on plane easy...especially underproped.....you might also have weight issues..or lord forbid...compression issues

 

[ChrisCraftfisher]

Re: what should i do???

hey guys thanks for the replies! i bought the boat from my dad's friend and the old motor was seized. my dad's friend replaced it with the same size motor (4.3l V6 mercruiser) but the new motor had a 15hp increase from175hp to 190hp. the max rpm range that i read in the manual is 4400-4800rpm. i can't remember the exact diameter of the original prop i think it was 14 1/4 or 14 1/5 by 21 pitch, with that prop the rpm range was about 5800, i replaced that prop with a 14x19pitch prop and that brought me to 5400rpm. i wanted to put a bigger pitched prop on but i was told i shouldn't do that because my boat porpoise's. if i was by myself the boat would run ok, but if i had any other weight or people it would only stay on plan at 4000rpm or higher, other then that the bow would go straight up and the stern would sink down. i couldn't get out of my own way. i put a stingray hydrofoil on last year and it improved my performance greatly. i have better hole shot, boat rides nicer, and i can stay on plan with 4 guys and a boat lot of fishing gear at 3300rpm's. my rpm range is still out of whack though. at about 3700rpm's i can cruise at 16-17mph, that also seemed a little slow to me, but i don't really know. also my boat is about 3600lbs, so i wonder if it is just a heavy boat too. any ideas???

 

[steelespike]

Re: what should i do???

!4 X 21 pitch and a 3,600 lb boat 5,800 rpm is way out of whack something is wrong my guess is the tach is off. Also dropping in pitch should result in an INCREASE in rpm by about 400 if you went to stainless it may have been slightly less increase.Its possible the original prop is a different pitch or was reworked/damaged or something but 5,800 is way out.When you want to decrease rpm you increase pitch (higher number)Diameter is a result of design and you wouldn,t normally seek a particular diameter.
If you wanted to drop rpm by 1,000 you would need to go to a 5 inch increase in pitch that would result in a 26 inch pitch that definately is not for that combination.
.

 

[ChrisCraftfisher]

Re: what should i do???

how can i find out what the right prop is for my boat?

 

[Silvertip]

Re: what should i do???

You first need to get this pitch theory straight in your own mind and then do your testing with what you consider and average load and you must do that for each test. Increasing pitch (e.g., a higher number ) lowers rpm. Decreasing pitch (lower number) increases rpm. So by going from a 21 that you said resulted in 5800 rpm to a 19 that gave you 5400 does not make sense unless the prop diameters were drastically different (which they weren't). You can measure it's diameter. Measure from blade tip to the center of the prop shaft and multiply by 2. I think you have a bad tach and/or were trimmed out too far to get 5800 rpm with a 21P prop on that boat.

 

[ChrisCraftfisher]

Re: what should i do???

yeah i might be wrong i have to double check my old prop.

 

[oops!]

Re: what should i do???

as far as a heavy boat goes.....

check the hull vin plate......

it should tell you....

then weigh the boat at the local free scales....subtract the trailer weight....(its a little more complicated than that but it will give you a ballpark......
then you will know if your grossly overweight)

im guessing you have a few issues...

 

[oops!]

Re: what should i do???

i pulled a search for you.....

i hope silvertip doesnt mind.......but read this........

this was a post silvertip did a while back.......

quote
"Since many of the questions asked on this forum are from folks that don't understand why we need the information we do in an attempt to help, I suggest they visit the link below. There are many sites that describe propeller basics but one that does a good job of explaining this in as few a words as possible and which includes actual pictures is provided below. Read the info to help you understand the terminology (diameter, pitch, rake, and slip) and what the affects on each are on performance."

then read this......forget about the suzuki part......pay particular attention to the pitch part.....

http://www.suzukimarine.com/_m/accessories/propellers.pdf

thanks to silvertip for putting togeather the origonal post.

i hope that will help you understand some more about props....and how to proceed

cheers
oops