Correct propeller size HELP!

[specialk01]

I have a 1989 20.5ft Renken cuddy. It has a 5.0 liter OMC Cobra motor in it. I just bought it this year and don't know much about propeller size. Right now it has a 14 1/2 X 19 aluminum propeller on it which is in pretty poor shape. When I bought the boat the previous owner told me the boat should go approx. 50mph. I took the boat out and the top speed was only like 37mph at 4500 RPM's. I want to upgrade to a stainless steel propeller but i'm not exacly sure if I should keep the same prop. size that is on there or switch to a different size. If someone could help me answer this question I would greatly appreciate it. Even if someone might know some part numbers because I am kind of looking for a used propeller first. Thank you very much.

 

[MurdockJr]

Re: Correct propeller size HELP!

Marty at 4500rpm do you still have any throttle left or are you wide open?

 

[specialk01]

Re: Correct propeller size HELP!

At 4500 RPM thats pretty much all she's got. It doesn't seem to come out of the hole very well either. With only 4 people in the boat (2 in the back seats and 2 in front) it took me a very long time to get up on plain. I know installing trim tabs would help this situation but I would like to know if a prop. size change might help me get up on plain faster and still not loose all my top end speed. Thanks for your input.

 

[backdraft]

Re: Correct propeller size HELP!

Marty depending on the weight of your hull ,you might try a 14.5 by 15 inch pitch will a single cupped edge in a stainless prop. You can allways add more pitch with a double cupped edge. Acceleration will be greater and the cupping will hold the prop better in the water at the higher rpm. nevjb

 

[catfish1]

Re: Correct propeller size HELP!

marty, is the hull clean? <br />before you go experimenting with props, make sure the hull is clean. it sounds to me like a combination of a dirty hull & a wore out 19p prop. a hull cleaning can make a huge difference!<br />if you go down in pitch it should plane a little faster, but you will most likely sacrafice top end speed. your right about trim tabs helping the planeing. look at smart tabs, i just got a set for my bayliner and it made a huge difference! you wont be disappointed.<br />a clean hull, new 19pitch prop, and smart tabs, may be all you need.

 

[Capt. T]

Re: Correct propeller size HELP!

Marty K:<br /><br />4500 rpm out of a 5.0 L sounds reasonable, but on the low side for WOT, at least to me. Any others think the same? <br /><br />You've got some good advice about cleaning up the boat, I would extend this to doing a tuneup on the engine. An '89 could have a lot of hours (at least a lot of hours sitting). I would look at replacing plugs, wires etc.<br /><br />Next look at new props, you need two anyway (always keep a spare + parts on board). Find a prop shop that will work with you. They should let you try one out and switch if needed. Particularly if you buy two new props ( 1 Stainless, 1 alum=spare). <br /><br />I would try a 17" pitch stainless as a starting point. Try and prop the boat so you hit WOT rpm with your typical load of fuel & passengers. For the life of your engine (and maybe yours as well) always, ALWAYS respect the WOT rpm limit for the engine and prop accordingly.<br /><br />Catfish1: <br />What kind of differences did you see after the Smart Tabs? I'm thinking of putting a set on my boat (see my tag line) mostly for holeshot. I'm over propped right now at 19" pitch. Prop shop rebuild this winter down to 18" with cup.

 

[tylerin]

Re: Correct propeller size HELP!

Its hard to determine if you need a smaller prop if the one you've got, as you put it, is in "pretty poor shape" maybe you can just have that one fixed, depending on its condition. %That will at least get you in the door of a prop shop.

 

[catfish1]

Re: Correct propeller size HELP!

here ya go! http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=007133

 

[specialk01]

Re: Correct propeller size HELP!

Thanks for all the input. This site has been very helpful. As far as the condition of my hull goes, there is not even as much as a scratch on it let alone any other substances so I know thats not it. I'm just affraid if I go with to low of a pitch then I will exceed my WOT RPM's and burn up my motor. I don't wanna do that, but on the other had I don't want to go with too high of a pitch like say 21 inch and never be able to get out of the hole with more then 2 people in the boat let alone try and pull up a ski. Also as far as the numbers go for props, I know I have a 15 splined shaft and the prop. on it 14 1/2 X 19 can I also look at props that are like 14.75 or 14.25 X 19? Or does it have to be 14 1/2 X whatever? I liked what I heard from someone about the smart tabs. So if I prop up to like a 21 the I can get the tabs and not have a problem getting it to plane and I shouldn't have a problem getting a ski up then right? The only problem I have to deal with is coming up with like $250 for the SS prop and another $400 for the tabs. Well I guess thats something I will have to consider or if anyone knows a better cheaper way to get the boat to plane faster and get lots of speed out of it please let me know. Does anyone know how well those fins work for the lower unit? Will those help my situation at all. Thanks again.

 

[catfish1]

Re: Correct propeller size HELP!

hey marty, call this number 800-233-0194. it should be the number to nauticus, you'll find the tabs are nowhere near $400. good luck!

 

[Jdeagro]

Re: Correct propeller size HELP!

Marty K;<br /><br />First, regarding top end - 4500 RPMs is not far off the mark, could be a bit more, but that probably is not your trouble.<br /><br />Second, you say the existing prop is poor, and you want to go to a stainless steel prop. I think you are underestimating the effects of a "poor" condition prop. A new aluminium prop could do wonders for the performance - the turning point props are a great design ( cupped like a SS prop) and perform within 1 mph of their SS props. This will also be easier on your wallet.<br /><br />For everyday family use, I am not a fan of SS props simply because they are not very forgiving if you happen to hit something. <br /><br />Lastly, Smart Tabs for your boat retail between 179.00 and 199.00 depending on the company. Since your prop is dammaged that is the first thing I would handle. As for the Smart Tabs, you will pick up better top speed (2 to 4 MPH) along with better low end performance ( accelleration, plane easier, ride better etc.)

 

[crazy charlie]

Re: Correct propeller size HELP!

Marty,it has always been my preferred way of going about things on my boats and very rarely does it fail me.CHEAP WAY FIRST>Get the prop reconditioned for 40$ and see what the difference is before you make any $$$$$$Decisions.Charlie