Prop suggestions

About inboard or outboard motors.

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rbrandenstein
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Prop suggestions

Postby rbrandenstein » Sat Feb 07, 2015 8:43 pm

I just launched my Malahini with my Yamaha F70. Runs well and is very quiet. However, I don't think the prop is correct for this boat and would like some thoughts and suggestions.

The prop is a SS 13x13" with high rake. It was on the engine when I bought it. However, when running at 6000 RPM, the boat is only doing 29 MPH. It seems the throttle has more to go, but I do not want to over rev it. The book recommends 5800-6200 as WOT. My old Mercury 650 could do about 33 MPH, but I don't know the exact pitch. I would think the new motor would push the boat to higher speeds than the Merc 650.

So, I am thinking the pitch needs to increase. Many of the performance briefs on the Yamaha outboard site use 13.5x15 pitch props.

So, should I go to 15 pitch or higher, like 17 or 19? I am not sure how to determine the best approach without buying a bunch or props and trying each one, which is too expensive.
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vupilot
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Re: Prop suggestions

Postby vupilot » Sat Feb 07, 2015 9:18 pm

Enjoying the warm up we are having this weekend eh? 53F here today!

Typically an inch more pitch would result in a 200 rpm drop. So If you are at 6000 rpm and might even have more throttle I think going to a 15 at a minimum is going to be needed and if you have a lot more throttle left over a 17 could be needed but I think the half inch in diameter and two inches in pitch is gonna get you pretty close. The half inch in larger diameter will also drop your max rpm slightly but not much. The added diameter will help more in preserving your acceleration. Diameter for push, twist for speed.

Were you running with heavy load of people today? If you had a lot of passengers today and still the motor was that under-loaded pushing all that weight I might be inclined to go right at a 17 but that would be in theory an almost 800 rpm drop from whatever your full throttle rpm would have been had you gotten to full throttle. So I tend to not believe you would need to go that high. If you were just a couple people in the boat today I think the 13.5x15 is gonna get you pretty close to where you need to be especially when you have a full load. If you buy the prop that is within an inch of perfect a prop shop can add or subtract an inch of pitch to get it spot on for you.

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Bill Edmundson
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Re: Prop suggestions

Postby Bill Edmundson » Sun Feb 08, 2015 5:59 am

Bob

I think Chris is about right 15 pitch should get you back to 33 mph. At 6200rpm you might get to 35mph.

Do you know what the gear ratio is?

Bill
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gdcarpenter
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Re: Prop suggestions

Postby gdcarpenter » Sun Feb 08, 2015 7:52 am

Prop selection seems to be as much art as science, hit gold on the 5th prop for my ZIP, 30 MPH @ 5,600 RPM. A 15" pitch would likely be a good starting point, but at some point you will likely have to find what your 'true' WOT RPM is to really 'dial it in'.
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rbrandenstein
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Re: Prop suggestions

Postby rbrandenstein » Sun Feb 08, 2015 8:43 am

Thanks for the information. It was 70+ here on Saturday, so I was able to launch the boat with the new motor and go out for a spin. Pretty nice for Feb. 7. It is now back in our slip at the lake.

There were two adults in the boat for our testing.

Bill, the gear ratio is 2.33.
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chugalug
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Re: Prop suggestions

Postby chugalug » Sun Feb 08, 2015 8:50 am

:D When I used to have my old blue boat-(24 ft Fibreform with 188 hp) ,the prop it came with was 17 inch pitch ,it got up on plane with 2 persons ok more people wouldn't get up on plane.The spare was 19 inch that was worse. I got 15 power prop.Turned more rpm's burnt more fuel, got up on plane easy with 6 adults, and actually went faster.was actually underpowered(should have had at least a 260 )But went 45 any way.I sure miss that old gas guzzler.That hull was made using balsa core. :D
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Bill Edmundson
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Re: Prop suggestions

Postby Bill Edmundson » Sun Feb 08, 2015 10:42 am

Bob

You're getting about as much speed as your going to get with a 13 pitch. At 6000 engine rpm the prop. is turning 2575 rpm. That translates to 31.7 mph. If you are getting to 29 mph, that is only 10% slip. That's pretty good.

The prop. is turning 2660 rpm at 6200 rpm. A 15 pitch would be 37 mph, with slip, about 33 mph. At 17 pitch that's 43 mph and 38 mph. The 38 might be possible. I think you would have to drop in diameter.

The question now what you want to do. You want a larger dia. and flatter pitch for hole shot and pulling skiers. You want a smaller diameter and more pitch for speed.

As I remember, from an earlier life, tandem skiing is about 24 mph and slalom skiing is about 27 mph. We just cruise. I usually run 24 to 30 mph. WOT I can get 38 or 39 mph.

Bill
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rbrandenstein
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Re: Prop suggestions

Postby rbrandenstein » Mon Feb 09, 2015 11:46 am

Bill,
Thanks for all the details on props. I never spent much time investigating them, but it does boil down to basic physics. The most common props for this engine are 13.5x15 and 13.25x17. So, I'll have to consider my usage and pick one. I am leaning toward the 15 pitch.
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Bill Edmundson
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Re: Prop suggestions

Postby Bill Edmundson » Mon Feb 09, 2015 12:05 pm

Bob

I think 15 is a good idea. You could have a prop. shop re-pitch it up or down. I think you would only be able to go down with the 17.

Bill
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
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rbrandenstein
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Re: Prop suggestions

Postby rbrandenstein » Mon Mar 23, 2015 9:12 am

Just an update.

I bought a Solas Rubex 13"d x 15"p aluminum prop and mounted it this past weekend and gave it a spin. It definitely made a difference and is probably where I want to be. I could go WOT and the RPMs were around 5800 - 6000. The speed was around 33-34 mph. I was alone at the time.

It still accelerated nicely and got on plane quickly. We'll see what happens with a little more load.

Thanks to all for comments and suggestions.
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