Prop Selection

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WaxedElvis

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I posted in ATC yesterday - got nothing so I thought I'd try here - Thanks

If someone would be so kind as to educate me regarding props –

Twin 200 o/b's, I want to maximize range and have been told the right props can make a difference. This leads to several questions –
1) Is that true, if so what props do that?
2) How do I know what props I have now?
3) How much $ am I looking at to change them?
4) What do you do with used props?
5) Does it really make enough of a difference in effeciecy to justify the cost? (I'm assuming the cost is high because it's boat related.)
6) The answer to any other question I don't know enough to ask….

Thanks in advance.
 
Morning Mike,

Saw your post yesterday, and couldn't think of a simple answer to complex questions. Your engines are what they are, with a recommended prop size, and WOT rpm setting...don't know what that is for your boat. To "maximize range" is a go farther on a gallon of gas question (not adding extra fuel storage), I think. The pitch of the prop will affect allot of things, but not necessarily MPG. You want to shoot for proper (for your engines) WOT. Your prop sizes will be stamped on the hubs of the props and you should see if this is the manufactures recommended size. A fuel flow gauge will help you find the "sweet spot" for best MPG results...hope this helps…..Stan
 
(1) You might get a small range increase by going with a smaller diameter higher pitch prop. This might allow you to stay on plane at a lower RPM. You always want to insure with any prop change that you can still reach to engines rated RPM.

(2) They are stamped on the prop hub. Example 15x19. This means the prop is 15" in diameter and has a pitch of 19". The pitch refers to how far the prop will travel forward in the water with one revolution assuming n100% effeciency.

(3)Depends on the size and materials used in construction. An aluminum prop is cheaper then a stainless steel prop. Expect to pay between 200 and 800 dollars.

(4) Keep them as spares on the boat

(5) No. You will do better keeping the bottom clean, the weight of the boat down and engines well tuned.

Dave

Faster now but still steady
 
Suggest you post your query on THT where a lot of outboard boaters congregate. You'll surely get a lot more replies and some genuine propeller experts will come to your aid. Be sure to include information about your boat such as make, model, type, size and include the make and model and size of your current props.
 
what is THT is it on this site or another site because I need to find out what prop to start out with on my 1987 17ft cuddy cabin bayliner. See I brought it and it needed a new lower unit because the guy backed into a rock so I got the boat cheap $350 and brought the lower unit off ebay for $400. It has a 1987 75 hp mariner motor and I put a 1981 80 hp mercury lower unit on it now I just need to find out what prop to start out with before I put it in the water the prop that was on it was a 13x21 but my buddie thinks thats to big of a pitch for the size of the boat and the size of me and my family so if anyone could help me out here that would be great. I am new to boating This is my first boat so please someone help me!
 
The Hull Truth, another website. Nobody can tell you exactly what pitch is going to work for your boat, too many variables. That being said, 13X21 sounds too steep. A wild guess is a 17 or maybe even a 15 pitch. Having a tachometer in the boat is very important in determining prop pitch. What you are looking for is something close to max recommended rpm at wide open throttle. If you try out the 21 pitch, report back what your rpm is and the speed you get, preferably according to GPS reading.
 
THT is on www dot thehulltruth dot com. Participants such as Tom W Clark can put you very, very close to the exact best propeller for your application. You will be asked to provide exact boat, motor to include gear ratio, and propeller information. You'll also be asked what your current performance numbers are and what your goal is.
 
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