New Boat - Bad Prop... What Should I Do?

[Taylor_T]

Hey Everyone,

I could really use some guidance.

I decided to abandon my restoration project and got myself a 1985 Chris Craft instead. I brought it home yesterday evening.

It runs great and everything, but the prop is in REALLY bad shape. I feel like it is unusable and I would feel nervous to take it out like this.

So... I need a new prop. I know pretty little about props and could use some help.

I have 2 questions: 1) Can I use this prop to take the boat out and test the RPMs? and 2) What would you recommend for a new prop in regards to the info below?

Using the sticky as a guide, here is what I am looking for and some information about my prop:

1) Performance issue you are trying to correct.

I need a prop that performs well for Wakeboarding. Wakeboarding/Watersports is most of what we do when we go boating. Good hole shot, hold speed well, etc.

2) Current prop manufacturer, model, aluminum or stainless as a minimum.

It has a Mercury, 78116A45P15, Aluminum prop right now. I'm not sure if this is the original prop or not. It seems like it is has a low pitch for others I've seen on similar set ups...

3) Current prop diameter and pitch (required).

15.25 X 15 P

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

I'm not sure. I just got the boat, so I am not certain of performance yet. I did Lake-Test the boat, but I'm still learning about all this boat stuff and didn't do a WOT test.

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

Mercruiser Alpha One (1.65:1), 1985, Chevy 305 5.0L, 200HP

6) Boat make model, year, length and weight

1985 Chris Craft, Scorpion, 1985, 18'6, not sure of weight

 

[fhhuber]

I would hesitate too run that prop in gear with the engine idling.....
Might be passable as an emergency prop to get you back to the ramp if its cut down and re-balanced. Otherwise its junk.
Cost at least 2/3 of the prop's value to get it restored and then you have a welded aluminum prop...

Best is to get to a boat dealer who has "demonstrator" props... start withthe OEM and try + 1 step pitch, - 1 step pitch, + 1 step dia, - 1 step dia... and see if you can get improvement. (probably not much)
Then buy the size determined to be best. SS preferred... but they cost more than the aluminum.

 

[Sea Rider]

Give that old prop his last chance, go for a wot run lightly loaded and check max achieved wot rpm, check if inside min-max wot rpm for that engine. Ideal is to run near max wot rpm with current weight, old prop water test will show actual rpm for that engine, can go from there buying same new if found to be an excellent water performer with current load or to a less pitch if in need to pull wot revs up.

Do you plan running that combo lightly, medium or fully loaded (3). Depending on load cam maximize a prop pitch that suits best that particular (3) boating requirement while running at max wot rpm.

Happy Boating

 

[steelespike]

Maybe you can find a marina that will let you test some props with the understanding you are buying.
If you can't buy to beg a prop loan then::
A prop shop should be able to rebuild your prop. then you can get some real test numbers. Lightly loaded,wot rpm and gps speed.
A 15" prop is suspect; 5.00, 200 hp should run in the 50 mph area at wot. with a 19 or 21" prop.

 

[Taylor_T]

Thanks for the advice, guys! Very helpful. There's not a lot of boat shops around here (northern utah) so I decided to take the boat out this morning and do a WOT test. That should be the last time that prop sees water (at least in it's condition).

Here's the results:

top speed: about 34/35 mph (GPS phone app)
RPMs: sat pretty steady at just under 4,300 rpm.
Prop: 15.25 X 15p
Motor/Drive: Mercruiser Alpha One (1.65:1), 1985, Chevy 305 5.0L, 200HP

This was with me, my dad, and a full gas tank. It got up on plane pretty quick as well. Does that seem reasonably with a 15.25 X 15p prop? Is it possible the damaged prop is effecting the performance?

I do have access to a 17p prop that I could test, it;s in OK shape (no major damage) although it's off of a 1976 boat so I'm not sure it would fit the alpha one...

I plan on using the boat primarily for wakeboarding and other water sports. So, I need a prop that will perform well for water sports (hole shot, holding speed, etc). Most of the time the boat will be ran under 25 mph. Every once in a while might want to go faster though... but not my priority. I plan on having the boat medium-heavily loaded most of the time, and will probably put some ballast bags in when it's just a handful of people.

I want to get a 4 blade prop, because I heard it is better for hole shot and maintaining speed... Although the 3 blade from this morning actually seemed to perform fairly well. Is there a prop you all would recommend? Has to be aluminum (budget dictates). Also what pitch/diameter would I need in a 4 blade?

 

[Maclin]

According to prop calculator you are only at 5% slip which is phenom territory. So if those numbers are with that damaged prop then not sure what to believe.

 

[Taylor_T]

According to prop calculator you are only at 5% slip which is phenom territory. So if those numbers are with that damaged prop then not sure what to believe.

Well... those are the numbers. Just curious, what calculator did you use? I punched the numbers in on this site and got 8% slip. Which I guess is pretty close...

And you know it's definitely possible that the tach is not 100% accurate.

: http://www.mercuryracing.com/prop-slip-calculator/

 

[Taylor_T]

Also, I took the boat out a second time yesterday with the 17p prop I had around. With an almost full tank of gas and 6 people (3 are 200+ lbs, 3 are 150-180 lbs) the boat was doing 3900 rpms and 30-31 mph WOT.. Probably not the best comparison since there were only 2 people in the boat when I tested the 15p prop.

It seems like a lot of people with similar drive set ups are able to use somewhere around 18-19 pitch props. Which I guess makes me wonder if I'm not getting as many horses out of the motor as I should... Or maybe it's normal for my boat?

Any recommendations on the prop and motor?

 

[Sea Rider]

As you have experienced the more weight you add to any boat, the less revs engine will produce. It's very difficult to dial a single best prop for medium light to full loaded applications. If dialing a pitch that achieves max wot revs factory stated for that enine while heavily loaded, will over rev when medium loaded, same when correctly pitched for medium loads and then adding heavy loads, will be lugging the engine.

Why is everybody wanting to dial best 4 blades prop & pitch, is it something fashionable Kardashian issue LOL!! , when correct 3 blade pitch is dialed, will achieve excellent results when running at top rpm of your given load. Good hole shot along fuel saving, once there can throttle throughout the whole throttle range and particular boating speed.


Happy Boating