A new prop, Where should I start?

[tigger3650]

I have 21ft StarCraft with now a 3.0 Mercruiser and an Alpha 1 outdrive. The boat has recently (but slowly) been completely redone. All the old heavy marine ply and saturated pour in foam was removed. The plywood (including the transom) was replaced with 3/4 Arauco Exterior ply, all triple sealed (6 coats on the edges) with Spar Urethane. After the rework, the boat is roughly 800lbs lighter. It originally had a 1985 Mercruiser 140 with a 14" diameter 14" pitched prop. I would get roughly 28mph wide open at 4500 rpm. After the rework with the same 140, I was getting 33mph at 4500 but still had some throttle to go. The boat then sat with very little use for about 3 years due to transferring (twice) and a long deployment. On the second move (to Maryland) in December, being from Florida I never even though about draining the block and manifold. Now you can guess why it currently has a mid 90s 3.0. Through other forums I figure I went from about 115 at the prop 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?

 

[tigger3650]

So last night in the garage, I got out the new Solas 14.8 x 17 and set it next to the Unknown Stainless 19. They are roughly the same diameter both outside and hub but the distance from the back of the prop to the front of the hub is much shorter on the Solas. It will definitely fit.

Looking at the 2 props side by side, the Solas just looks a lot more aggressive. Should I use this at least maybe for tubing if the rpms stay a little higher?

Should I just try this first and see where I am at since I already have it and don't have much else to choose from as a starting point?

How would this 14.8 x 17 aluminum perform compared to the 13 x 21 stainless mentioned above?

 

[alldodge]

Trying to catch the main points of your thread
Your WOT should be between 4400 to 4800 and you want to keep it around 4400
Installed 17 pitch and reached 35 mph at 4800 rpm

Have a 15 inch diameter 17 pitch aluminum and should you try it?

Your WOT with the stainless 17 was 4800, if the 15 diameter will fit, sure give it run. My guess will be you will drop 400 to 600 rpm at WOT.

Should the 17 Solas be used be used for tubing?

See where your at and determine if changes needs to be done

How would this 14.8 x 17 aluminum perform compared to the 13 x 21 stainless mentioned above?

The 21 will probably be slower taking off even with a smaller diameter, but the WOT will change

To put this in perspective to my view point. The original 17 pitch prop which allows the motor to reach 4800 rpm. This is what I would stick with unless the boat does not meet all your needs for running and tubing, etc. The only way to find out what the others will do is to install and run

 

[tigger3650]

AllDodge,

My initial post was a little misleading. I was "told" the prop that came with the new motor was a 17". It wasn't until I ran it, tried to track down a 19"and got it back to Maryland and tried to swap it out that I discovered it was actually a 13 x 21". I also noticed the inner diameter is a lot smaller than the other 2 props.

All said, the numbers from above are with a 13 x 21 stainless.

Does this change your suggestions any?

Do you still thing the 15 x 17 Aluminum will drop the rpm? How do you think it will affect top end?

 

[alldodge]

If at WOT the 13 x 21 turned 4800 then the 15 x 17 may turn about the same but probably a bit more IMO. The larger diameter has an impact but I'm not as much as the pitch. Either way your not going to hurt anything by trying so long as you continue to watch the rpm's.

I'm thinking you will wind up in the 14.5 x 19 prop for a all around, but do check what you have first

 

[tigger3650]

AllDodge,

I agree, especially after looking at them all side by side, I think I am going to try the 15x17 just to see. It won't be for a few days at least. Winds are gusting around 45-55 today!!

 

[steelespike]

The difference with your 13" diameter prop is more than just diameter. You said 35 at 4800 . That "could" be as much as 42 mph.
If the tach is accurate it indicates about 20% slip, normal is about 12%.On a typical 18 to 21 ft boat.
If you look up props for your 3.0 you may note that basically as pitch goes up diameter goes down.
The I/O usually won't go below 13 3/4 diameter and could be as much as 16" in diameter.

 

[Bondo]

The difference with your 13" diameter prop is more than just diameter.

Ayuh,.... Due to the smaller barrel to match the smaller gear case it's built for,....
I Gotta believe it's ventilatin' to beat 'ell,....

I've tried it before, 'n it would just Blow out under much power,....

Somewhere in these threads of the Op, it's mentioned that 2 different props were tried, but one rubbed the gear case,...
That's because not all props take the same hardware kit,...
The prop that rubbed, had the wrong thrust washer ahead of it,....

 

[tigger3650]

Bondo,

The prop that rubbed is the one I can't seem to cross reference with the numbers that are stamped on it. I think the problem with that one is the hub. The shaft is not long enough to protrude past the end of the hub.


My previous props had the, thrust washer, locking spacer and nut as all separate pieces of hardware.

The 13 x 21 that came with the new motor, had the locking spacer built into the hub.

The 19 unknown bottoms out on the gearcase and only the threads of the shaft are exposed beyond the hub.

I wonder if it is possible to have the hub swapped out? Not sure how that would affect the reliability of it after that.

 

[steelespike]

13" diameter won't work on the Alpha 1.The barrel of the prop is too small.