How to determine prop size?

[Btuvi]

How would I determine the appropriate prop for a 175 Mercury opti on a 20'Hydra- sport dual console?

Just checked the Mercury site and narrowed down recommendations - sort of.

They gave me choices. Aluminum or stainless. 5 blade, 4 blade, or 3 blade.

This part confused me. The alum 3 blade is18.0 pitch and the ss is 17.0 pitch.

Why the difference?

Also, there must be a place where comparable props cost a bit less. I prefer not to refinance my car - unless absolutely necessary.

Thanks

 

[steelespike]

Re: How to determine prop size?

Before buying a prop lets discuss what your trying to do.
What is your present prop size,max rpm and speed.Max rated rpm for your motor.

 

[Btuvi]

Re: How to determine prop size?

I knew someone would ask me questions I couldn't answer. I suspected that it would be you, Steelspike.

I don't have the boat nor do I have the engine. I am contemplating purchasing the boat (used) from one source and the engine (used) from another. I may have an opportunity to trade out an aluminum prop for ss before purchasing engine so I'm trying to look ahead so I'll be a little more knowledgeable about my options when I bring this up. Dealer will probably know but I'd like to know also.

Any info suggestions/info would be helpful, Steelspike.

Thanks

 

[steelespike]

Re: How to determine prop size?

Unless someone has a similar setup you may not get much specific info.
If you check the nada used boat site you will find info on the weight and
motor size for the hydra sport. then if you search the net for performance
reports on the 175 opti you may find reports on the opti on boats of similar size/weight to yours.You may get a feel for the typical prop range.You might see if there is a hydra sport discussion site.
The ss prop is generally more efficient than al.Sometimes a little less pitch is suggested.Sometimes the ss prop will lift the boat producing more rpm so the pitch drop may not work.Changing material can throw out the usual 200 rpm per inch rpm change.
Props can be expensive especially ss.the Hustler turning point is said to respond similar to a stainless at aluminum prices.In your position I would try to find a dealer that will lt you exchange props.If I had to flat out by a prop with no exchange I would do al and lean toward a slightly smaller pitch.If your under propped you can at least use the prop if you stay within rpm range of the opti.Then based on your specs you could upgrade and save the prop as a spare.If you end up overpropped you shouldn't run it.There is an exchange program here at iboats I don't know if your setup applies.

 

[Prop MD]

Re: How to determine prop size?

Simply put...

The stainless propeller will flex less. So with the 17 pitch you are getting a true 17 pitch.

The Aluminum propeller is not as strong of material so the 18 pitch propeller will flex, typically about 1" and will preform like a 17 pitch propeller.

If you are going to stainless, and have as many "what ifs" as you do, start with a 3 blade. 17 is a very common pitch. Find a place that has a pitch exchange program.

We offer a exchange program on all our stainless props. Yopu can change size, pitch, or even brands untill you find the best fit for you.

I wont put a link (I dont think I can).

No links or emails particularly for the sale of a product we have here.

 

[Btuvi]

Re: How to determine prop size?

Thanks all.

I may take you up on that Propmd.