20' Sea Ray - Prop Suggestions

[Struc]

1) Performance issue you are trying to correct.

First time out with the new (to me) boat, with a new prop (which will now be a good spare I guess), my RPM's were getting too high. The prop that came with the boat was pretty badly chewed up, and I didn't want to use it.

2) Current prop manufacturer, model, aluminum or stainless as a minimum.
3) Current prop diameter and pitch (required).

Solas Amita 14.5"x19 aluminum 3-blade, Prop #1511-145-19

4) Wide open throttle RPM and speed with an average load (very helpful)

5200RPM and climbing (I backed it down around here - probably had another 3-400 RPM in it). Was going 40mph (GPS) at the time.

5) Engine/drive make, model, year, and HP

1988 4.3l Mercruiser w/Alpha 1, 185HP

6) Boat make model, year, length and weight

1988 Sea Ray Seville, 20'. Dry Weight 2360#. Probably 3300 loaded with people/gas/etc.


So... Looking for suggestions. The easy answer is to go to a 21 pitch prop, but thought I would leave it up to the experts. My main goals with the boat are cruising (enjoyed it at 25-30mph when I was out) and tubing / skiing with the kids. Top speed is not a big deal to me at all, and I would expect to rarely even be at WOT. However, when I do want WOT, I don't want to be over-revving the engine. The boat REALLY got up out of the hole effortlessly, so I would guess that even a 21 pitch would still be pretty good in that area. But again, I'm going to ask for opinions from the experts...

 

[craze1cars]

Re: 20' Sea Ray - Prop Suggestions

Hard to offer suggestions when your baseline prop is "pretty badly chewed up". Chewed props will always slip too much and will therefore over-rev. How much over-rev? Impossible to know until you put a good one back on...

My best suggestion would be to test-drive a prop of a known pitch, with no damage, and then use that for a new baseline so you can make a more educated decision. I agree a 21 sounds like a good place to start....

Keep in mind that chewed up props are pretty cheap to have fixed. Take it to a local prop shop, have it repaired, keep it as a spare if it's still over-revving, and chat with the guy to see if he has a few good used ones for you to test-drive...

 

[John_S]

Re: 20' Sea Ray - Prop Suggestions

I read it as he did use a new 19" pitch prop, so a 21" pitch would be good choice. Keep the 19" P around, you may decide you like better acceleration.

 

[Struc]

Re: 20' Sea Ray - Prop Suggestions

I read it as he did use a new 19" pitch prop, so a 21" pitch would be good choice. Keep the 19" P around, you may decide you like better acceleration.

Correct, it was a brand new Solas prop. I didn't even want to take the boat out with the prop that came with it.

 

[craze1cars]

Re: 20' Sea Ray - Prop Suggestions

My misinterpretation, sorry...

With that knowledge, it still is a little difficult to predict what you need. You claim it was running 5,200 and still had another 300 to 400 in it. That brings you to potentially 5,500 or 5,600 with a known good 19 pitch prop.

Your motor's operating range is 4,400 to 4,800, yes or no???

If I am correct, then it sounds like you need to drop 700 to 800 rpm to get it to the top of this range. And 2" of pitch won't drop you that far. You'd need a 23 to drop that much.

But if your guestimate of it maybe having another 300 to 400 in it is wrong, then all bets are off....

Of course I'd be pretty surprised if your 20' cuddy with a 4.3 could push a 23 pitch prop. But then again we don't know your outdrive ratio...so it's entirely possible if you have a low ratio.

 

[bnicov]

Re: 20' Sea Ray - Prop Suggestions

Before you go spending money trying out different props, get your tachometer checked against a shop tach. You'd be surprised how far off they can be. You may need to get yours rebuilt or replaced, check with the manufacturer, I know that Faria guages rebuilds or replaces your existing guage quite cheaply as they have a lifetime warranty (pretty much just the shipping charges).

 

[Struc]

Re: 20' Sea Ray - Prop Suggestions

Thought I would just update and finish off this thread.

I took your advice and got a multimeter with tach built in, and sure enough, the dash tach is way off. I didn't test out on the water to see what it was at full speed, but just in my driveway with the muffs on, it was a significant amount different - Like 3000 actual RPM's was reading 3400 on the dash.

I doubt the boat is going to hit the water again this year, but next year I'll do some further testing to make sure I'm in the correct range. However, I'm pretty confident now that my prop is where it should be.

Thanks for the help!

 

[steelespike]

Re: 20' Sea Ray - Prop Suggestions

My very basic calculation iwith 1.65 gear ratio, 19" prop, 9% slip @40 mph = about 4000 rpm.
4800 = about 47. I think your slip will be lower. I think that setup should be closer to 4800 but we'll see in the spring.

 

[Struc]

Re: 20' Sea Ray - Prop Suggestions

My very basic calculation iwith 1.65 gear ratio

If I'm not mistaken, the 4.3l is 1.84 gear ratio?

 

[steelespike]

Re: 20' Sea Ray - Prop Suggestions

My info puts the 185 4.3 in 87 with a 1.84 ratio.

40 mph at 5200 indicates over 20% slip. At 9% 5200 would be about 46 mph.
1.84 @ 4500 = about 40 mph.@9% slip. likely your slip is less.

 

[Struc]

Re: 20' Sea Ray - Prop Suggestions

1.84 @ 4500 = about 40 mph.@9% slip. likely your slip is less.

This sounds exactly right, based on what I feel the dash tachometer is off by. Thanks for the math.

We'll see what it does exactly when I get her out next season.