Correct props?

[jarhead]

I just purchased a '73 SRV 240 Weekender with twin 165s and was wondering if the props should be the same as 1979. SeaRay's prop chart only goes back to 1979. I believe the chart calls for 15 1/2" with a 15 pitch. The boat now has 21 pitch on it so I'm sure a change is in order. I will be using it for salmon trolling on Lake Michigan so a speed of 1.8 mph on the bottom is what I want to achieve without sacrificing too much on the top end. Any input would be greatly appreciated. Thank you.

 

[Cap10Pat]

You can't draw that conclusion from those facts. The engines could be different and, more importantly, the sterndrive gear ratios could be different, (plus other factors).

Assuming the engines are in good shape and making full power and rpms in neutral, you can run the boat at full power in open water. Your engines should be able to make the rated rpms, and be essentially limited to that by the correct props. If that boat should have 15" props and you have 21" props on there, one would expect your engines NOT to be able to make the rated rpms - indicating incorrect props.

I take published "specs" with a grain of salt anyway. Most good propping is by trial and error. The simple props of 1973 have been improved upon, and there are various models now. So diameter and pitch fail to describe a prop fully.

My gut tells me 21" is too big, but you'll have to try something and adjust from there.

 

[jarhead]

Thanks Cap10Pat. I won't have a chance to test it for about a month yet, but when I do is there a guideline to go by? For example: The max rpm for my engines is 4300, so if I reach 4100 do I go so much pitch less for each 100 rpm or something similar? I really don't want to buy several different props to get the right ones. Thanks.

jarhead

 

[Cap10Pat]

If you reach 4100 then consider yourself done. You can't fine-tune it too much. That's why max rpm is usually given in a range of about 400 revs.
People talk about 100-200 revs per inch of pitch, but that's different for 100HP engines than for 400HP engines. And two engines impact that too. Then, some props are only available in 2 inch increments. (Gives you something to think about all winter).

Some boatyards participate in the Mercury prop demo program, where you can try a prop and see how it works. Others may have plenty of used props they would like to sell and will let you try. The catch is you have to haul your boat to change the props in most cases - so be sure you know the costs.

I would start with a 15" prop - just my gut feel. Either aluminum (Mercury Black Max) or stainless (Mercury Vengence is nice). (I actually have a 15" Vengence that was on my boat when I got it, but I only have one). If you find a pair of used props, just be sure they turn in the direction you need (you'll have to check that), that they're the same model, and that they're in good shape. You may have to get them maintained.

 

[jarhead]

Thanks again Cap10Pat. I'll start with 15s. We just got another foot of snow in the last 2 nights, so before I plow today I have to sweep the wet heavy stuff off of the mooring cover before I get to anything else. I hate winter! The way it sounds I should get it on at least the second try, so it shouldn't be too expensive.

jarhead

 

[jarhead]

Cap10Pat, I believe you are right about the 15s. I can only hit 3000 rpm with the 21s. I'm not looking for high top speed, but it should do pretty good since it hit 33.7mph in a 3' chop at 300 rpm. Thanks for the help.

 

[Cap10Pat]

It's important that you use props that allow your engines to turn the full rpm that is specified. Often a bigger prop will provide better speed at lessor rpms, but will not allow the engines to reach full rpm. The result is that your engine is working too hard and will actually die sooner. You will also have louder sound as the engine works harder. So get the right props. You will be much better off in mileage, decibles, and engine life.

 

[old_navy]

I've talked with my marina and they are willing to allow me to test different props to find the best suit for my boat. You might talk with your marina. They have a couple of suggestions and we will start with them.

 

[tgilmore99]

Hey guys I am also new to this boating thing. Did I read an earlier statement correctly?
If my rmps are only at 3600 to 3800. I my have the wrong size props on it? I have a 1985 270 w 5.7 omc. I have been told they should be around 4600. Any suggestions other than above?

 

[Cap10Pat]

I'm not entirely sure about the rpms for the OMC, but I think you're about right. You may have the wrong props. But first you have to get the engine checked and tuned. You also need a clean bottom and outdrive. Then the correct prop will let you reach the full specified rpm. And, it's important to do that.