21' Starcraft SS

[tigger3650]

Looking for a new prop. 1985 Starcraft SS with a 140 Merceuiser Alpha1. Prior to refurbish of hull and interior I was running about 28mph at 4500 rpm WOT. I am pretty sure I shaved about 800#s plus new laity during rework. Took it out on the lake. Running 33mph at 4500 rpm with more throttle to go. Boat will be used mainly for cruising and tubing with occasional Atlantic or Gulf fishing......What do you all think? Current prop is an aluminum 15" 14? pitch.

 

[steelespike]

Re: 21' Starcraft SS

My info indicates 4800 max rpm. The boat isn't excessively heavy at about 2400.It seems that a 14" prop should over rev.
A 14" prop is about the size you would put on a hard push pontoon.
Run her up briefly to wide open throttle A healthy 140 can stand 5000 enough to get a reading.
You may be able to check the tach against a shop tach.
Note the position of the switch on the back and operate it a few times to clean the contacts.
A 19" pitch is closer to what a healthy 140 would need.,

 

[tigger3650]

Re: 21' Starcraft SS

The tack is on, I replaced it about a year or two ago. The boat was real heavy when I got it. A lot of saturated wood and foam, especially in the bow. I tried a 15 deg 4 blade composit on it once (prior to the rework) and I got a lot of slip out of it and hard to get on plane. Normally I would swap around props from some of the shops around here but I am moving at the end of the month and just don't have the time take it out right now. Maybe after the 1st of the year I can get it out and open it up to see what WOT is. I will also throw the 4 blade 15 back on it and see what I get out of that one too. The more info the better.

 

[steelespike]

Re: 21' Starcraft SS

Is this an aluminum boat?
It is worrisome that the motor doesn't wind up more with a 14" pitch.
By the way prop size is measured in inches. yours is 15" X 14" 14" is the theoretical distance the prop moves forward in one revolution.
There is slip involved so actual movement is slightly less.

 

[Maclin]

Re: 21' Starcraft SS

Using prop calculator and assuming 1.98 drive ratio and 11% slip I get a 17" PITCHed prop. Your first post shows a 14 DEGREE which is not a pitch measurement but can be a blade rake measurement.

I think a 19 pitch would be best place to start based on some of the numbers you gave.

 

[jestor68]

Re: 21' Starcraft SS

The Mercury prop selector comes up with two choices in aluminum; a 14-1/2 X 19 pitch, or a 15-1/2 X 18 pitch. Based on your boat being under 3000 lbs, I'd go with the 14-1/2 X 19.

In aluminum, I recommend Solas props.

It's an aluminum boat with rivets.

 

[tigger3650]

Re: 21' Starcraft SS

What are your thoughts on a 14*19 stainless?

 

[jestor68]

Re: 21' Starcraft SS

What are your thoughts on a 14*19 stainless?

Sounds good.

 

[tigger3650]

Ok, here is an update. With deployments and moving, twice, and a new motor, I am finally trying to set this thing up with the right prop.

I went from a 1985 Mercruiser 140 to a mid 90s 3.0. Through other forums I figure I went from about 115 prop horse to 135 at the prop.

I was told the new prop that came with the motor I bought was a 17" stainless. Here are the numbers on the prop: Stiletto 11321 and M1321. It crosses as a 13" prop with 21" pitch. All the motors listed here on iBoats for that prop are outboards.

Not knowing the numbers at the time, I took the boat out to test it, I got 35mph (GPS) at Max rpm of 4800. I would like to keep my max rpm down to 4400 as not to beat the valves up.

I went back to the shop and told them my issue and asked if they had a 19" pitch I could trade the "17" for. He dug through a pile of props and handed me another stainless prop and told me I could keep it. I didn't have time to put the prop on prior to leaving the state but made an attempt at it today. It doesn't fit. The shaft fits into the hub, the splines match but the inner part of the prop rubs on the outdrive and the locking spline washer doesn't reach the splines. Apparently the wrong prop. I can't tell what the prop is. Here are the numbers: 35627; 27414; 19 35627. There is no manufacturer listed.

Now that I am in Maryland, my use of the boat has shifted. It will be used more for fishing and cruising than pulling a tube. I think the Stiletto will work for the tubing but probably not the best for fishing and cruising.

What do you all think I should look for with regards to diameter/ pitch?

I have a brand new 15" diameter 17" pitch Solas aluminum still in the box. I originally bought this prop for when I still had the 140. Should I try this one, or keep it in the box and put it towards a different prop.

I am not set on having a stainless prop. I normally prefer the aluminum, cheaper to replace than a shaft if I hit something.

Thoughts?

 

[steelespike]

You should really start a new post. Give us the details of your boat .
I can tell you a 13" prop isn't intended for an I/O.So your numbers are suspect.
A 3.0 at 4800 with a 21" prop would be at about 42 mph.
The fault is most likely the prop. Turn the 17" in a box for the same model in a 19"
Consider it a starting point. Your boat should be propped close to the rated wot rpm 4800,and if
you want to control the rpm do it with the throttle.

 

[tigger3650]

Thanks Steelespike. I reposted under a new topic. How much of a difference (from an rpm perspective) do you think the 13" diameter is making. Looking at that Solas 14.8x17 next to the 2 stainless props, the Solas "looks" like a lot more prop than the other 2.