how to choose a prop

[thrasher]

Okay guys,
This is all new to me, so how do you choose a prop for a boat? I have a 19 foot Glastron boat with a 4.3L V6 Mercruiser engine and Alpha 1 gen 2 outboard. This is my first boat and the prop on it is a bit battered!! I would like to get a new aluminium prop (I know I will damage the prop learning so prefer to buy a cheap one for the minute). The prop on it seems to be 13" diameter with a 19" pitch, but the engine seems capable of turning something with more pitch.

How do I go about finding what diameter and pitch prop would be best for my boat? I don't want to buy 10 different props and try each one to find the best (costs too much and takes too long!). Is there a WEB site or a table that I can refer to?
Or any recomendations for a prop?

Thanks Gary

 

[Bondo]

Re: how to choose a prop

What's your rpms at WOT,..??..??.....

How do I go about finding what diameter and pitch prop would be best for my boat?

Forget about Diameter,+ pick a Pitch that'll give you the require rpms at WOT.........

 

[thrasher]

Re: how to choose a prop

When you say WOT, do you mean if I just gas it from standstill (and load it for 10 to 20 seconds) what is the maximum the motor will rev to? or do you mean if I give it a long straight with WOT, what RPM (and top speed) does the engine get to?
This is my first ever boat and I have only had it on the water twice, I have not yet given it WOT (I wanted too, but the boss was not in agreement! She's gone away for a few days so I can now try WOT..) Just want to make sure I get the measurement right.

 

[Esox]

Re: how to choose a prop

or do you mean if I give it a long straight with WOT, what RPM (and top speed) does the engine get to?

You need to run it "wide open" and note the max RPM and speed achieved. Preferably with a normal load.

 

[Silvertip]

Re: how to choose a prop

Prop selection requires some thought. For all we know the prop you have (before it was screwed up) may be the best for your type of operation. The best prop allows the engine to operate at its maximum recommended wide open throttle rpm with a normal load. If the engine runs faster than that rpm you need to slow it down by increasing pitch. If runs slower than that rpm you need to decrease pitch. Remember, a boat has no transmission like a car so whatever prop you end up with will be compromise between hole shot and top end speed. Without any base line numbers to go by, we can't help you. But you can help yourself by typing "Propeller Selection" in the search box for Yahoo or Google. Do the research. Then with your base line numbers, you can determine how you normally run. If you tow water toys a lot, you may want two props, one for power, and another for cruising. If you can only afford one or don't want to bother changing them, then you have the compromise situation. You then select from aluminum or stainless steel, and three or four blade depending on your requirements.

 

[thrasher]

Re: how to choose a prop

Many thanks Silvertip.
I had understood pretty much everything you said. My problem is that the boat is 15 years old and I have no idea if the prop on it was good or bad (in terms of being a good pitch for my engine and load). As the prop is damaged I am not sure that any results I get with it will really reflect what the same prop in good condition should give. I was hoping someone could give me a good starting point. The boat didn't come with any manuals so I am completely in the dark for a starting pitch!!
I will mainly use this boat to pull my two daughters in a bouy (they are 7 & 9 years old and weigh almost nothing!!), with myself and my wife in the boat. I am not looking to set a new water speed record, but just wanted to check the prop was reasonable for the engine and boat. I will try running WOT on the local lake and see what speed and RPM I get. I have a GPS, so it will be interesting to see how accurate the boat speedo is compared to my GPS reading. It may take a week or two to get some answers as I am all on my own for a few weeks, and it's going to be tricky to load the boat up by myself! It's in my garage which is on a slope and so far has needed 3 people to man handle the boat out and onto my car..

Thanks for all the replies.

Gary

 

[Silvertip]

Re: how to choose a prop

13-inch diameter is too small for that boat. In fact a 13 - 13-1/2 inch props are used on mid-horsepower outboards. Go to the link below (right here on iBoats) to find your motor and outdrive and the various available props. I would also suggest you stop at a Merc dealer or use Mercury's Prop Selector (on their web site) to establish a starting point. My guess is you should have at least a 21P or 23P prop. I run a 19P on my 75HP outboard.

http://www.iboats.com/mall/propfind...rcruiser&year=1992&rotation=Standard&x=12&y=9

 

[thrasher]

Re: how to choose a prop

Thanks Silvertip,

The link seems to suggest that if I go for a 21" or 23" prop that it should be 14" diameter. You say a 13" is too small, but does 1" really make that much difference? I tried using the mercury prop selector, but I need to gather additional data (like the stern drive ratio!) and the boat weight.. I will try the site again once I get home and have the weights.. Do you know how to find the stern drive ratio? Its a 1991 Alpha 1 gen 2 stern drive? I have the serial number at home.

Regards Gary