First Time Prop Purchase

[AguaSki]

Last spring I purchased my first boat. While a summer of boating has been great I know I still have a lot to learn. Prop selection is new for me.<br /><br />The boat I purchased is a 1978 Glastron SSV-188 with a 1978 Johnson 140 hp v4. The motor was completely rebuilt by a local marine shop and came with a 1 year warranty. Normal passenger load is a second adult, and two small children under the age of six. All foam under the floor is completely dry so I am not carrying extra weight. My maximum rpm at W.O.T. is 5,100 and my maximum speed is about 38 mph (I have not purchased GPS yet so I can only measure from my speedo). The recommended rpm range is 4,500 – 5,500. While I am safely within the recommended rpm range, I would like to get to operate near the upper limit of the range. I am also hoping to increase my maximum speed while still maintaining my acceptable hole shot. The boat is mostly used for cruising and to water ski Arizona’s warm freshwater lakes. <br /><br />The current prop is stainless steel painted black, but I can’t find any identification numbers. Yesterday I completely removed and cleaned the prop in hopes of finding some numbers but found nothing. Assuming I acquired this boat without a prop, what would be the recommended prop to start with? Also, am I missing anything when I am looking for numbers on the prop? I removed the prop and looked inside and out at all angles and could not find anything.<br /><br />The iboats prop finder is asking me to select a pitch between 10-24. Given that I am unsure of my current pitch, I am looking for suggestions on a starting pitch, size and brand on a stainless prop. I am planning to retire my current prop as a spare.

 

[shep70057]

Re: First Time Prop Purchase

Best thing for you to do is take your current prop to a prop doc. A reputable shop should be able to measure your current prop. That way you are not just guessing and wind up buying the wrong prop. Most shops measure props for nothing.

 

[Silvertip]

Re: First Time Prop Purchase

You know what your current RPM is, you also know what your "approximate maximum speed is, if you know what the lower unit gear ratio is (probably 2.0:1) you can plug those numbers into the prop calculator below and determine what the approximate pitch is on the current prop. Use about 10 - 12% slippage. I just ran those numbers and pitch came up as 17.48. I suspect the actual number should be 19. If there is a local prop shop in your area they can measure the pitch for you. Before you spend money on a prop, experiment with trim settings and engine height first and make sure the engine is performing as it should. The best prop in the world cannot make badly tuned engine run like it should. Get that right and then think about props. You also need to realize that hole shot and top speed are mutually exclusive. Like your car, you can't get maximum speed in low gear. Your prop works like the transmission in your car but it has only one gear. You either prop for power, cruise, or top end. Most people tend to prop for cruise unless they have a high performance hull like a bass boat and must get from "A" to "B" in a hurry or unless they do mostly water sports. My thoughts are for your boat a 19 inch prop for water sports and cruising is in the ballpark. "Maybe" a 21 will provide the top end.<br /><br />Here's the calculator: http://www.rbbi.com/folders/prop/propcalc.htm