What prop do I need?

[tygray6]

This is my first post and I will try to provide as much info as possible. I have a 1986 Thompson Cutlass 185 with an mcm 120 engine and Alpha One outdrive. Here are my results from the props I have:

14.25 x 19 - 3 blade (Prop Wizard recommended). This prop would only get me to 12 mph and 2800 - 3000 rpm with a heavy loaded boat and pulling kids on a tube. 3000 rpm and 21 mph with 2 adults.

13.5 x 17 - 3 blade. This prop will get me to about 3500 rpm and 21 mph with just a couple adults in the boat.

I would like to know what are my ideal props for pulling and top end speed and I realize that I'm limited with only 120 hp but my kids are still young and small.

Thank You in advance for any help.

 

[WesNewell]

At wot, with a normal load, you want the rpm's to be in the middle of the rpm range specified for your engine, What that is I don't know. Maybe 4000 to 4800. if your above test were at wot, you are way over pitched. A wag would be you need a 13-14P prop

 

[tygray6]

4200 - 4600 is ideal wot rpms for that engine and I had a boat mechanic suggest a 15.25 x 15. Would that still be too much prop from your experience?

 

[Texasmark]

Not getting in on the exact prop answer but generally speaking, 120 HP with the load you listed is asking a lot, even without actively employing water toys, especially if folks love to sit in the jump seats adjacent to the engine cover and don't want to get up and move forward till you get on plane and go back and sit down. BTDT

 

[WesNewell]

4200 - 4600 is ideal wot rpms for that engine and I had a boat mechanic suggest a 15.25 x 15. Would that still be too much prop from your experience?

I would think so, but the only way to know for sure is try it. But if I had to buy it to test it, I'd go with a 14P. And that may still be too much. But it will surely get you closer to the wot range.

 

[tygray6]

I'm going to assume I should stick with 3 blade but what would I be gaining or losing by going to a 4 blade?

 

[WesNewell]

Probably losing rpms and top speed, but maybe a better hole shot. To compensate for rpm lose, you would likely need to lower the pitch an inch from a 3 blade.

 

[Texasmark]

4200 - 4600 is ideal wot rpms for that engine and I had a boat mechanic suggest a 15.25 x 15. Would that still be too much prop from your experience?

You are running 1000 rpms low with your 17P and light load. With an Alpha 1 Drive, your gear ratio could be anywhere from 1.47 to 2.4. This has a tremendous effect on selecting the right prop and maximizing performance. Driving your boat forward in the water is your "drive train" consisting of engine HP (torquexRPM/5252), gear ratio of your outdrive, prop pitch, and prop slip.

Torque curves usually peak around the mid RPMs and depending on the engine design fall off at some rate on out to max rated HP RPMs. Once torque peaks, it's up to RPMs to get the HP out of the engine and HP is what spins the prop shaft....the prop spinning develops the thrust that propels you through the water. If the resistance of the water is such that the engine can't move on up in RPMs, what HP your engine is capable of developing isn't realized.

You have to get your RPMs up. Hull drag is an RPM killer. Boats attempting to get out of the water (the hole shot) need to get RPMs up fast to overcome this high load condition. Trim tabs (attached to the transom) help to get the bow down and improve the hole shot and boat attitude while running at reduced speed....help to keep the stern up and aid in maintaining constant speed with varying water conditions.

Shallow pitch or deliberately venting the prop with port holes under the leading edge of the blade are ways to help in getting the boat out of the hole and up on the water, reducing drag and improving performance. Shallow pitch means that your RPMs will top out much quicker at WOT but will limit your speed while a ported prop can be higher in pitch, still get the hole shot but have the pitch to get the speed at higher RPMs.

If we knew your gear ratio, we could calculate your current prop slip and see if your lack of performance is a prop problem or an engine lack of performance problem....again 120 ponies on that boat aren't all that much even if you are realizing (attempting to realize) all of them....again BTDT.

 

[tygray6]

1.98 is my ratio, I'm really starting to think it's something more then my prop and I would love to hear any suggestions on what it could be. (Engine compression, outdrive slip, fuel problems)

 

[Texasmark]

Your slip is showing 25% which is pretty high, even with a loaded boat and water toys. If you had zero slip you would be doing 28. If you installed a 15 and it did allow the engine to even get up another 500 rpm (4000), you would still be running 21 mph and therefore the slip would still be 25%

4600 rpm (your max rated for that engine) at a reasonable 10% slip (since you would be going faster and the drag of the hull would be reduced, reducing slip) for heavy load on that boat, you'd be around 33 mph with that 17P prop. 25% slip at 21 mph is telling me that you are at/just over breakover point from your hole shot and there is a lot of drag for the prop to fight, even for a general purpose alum. prop. I am back to your 120 doesn't seem to be putting out 120 ponies.

 

[tygray6]

So I took some replies seriously and changed the top end of the outdrive to a 1.62/1 and changed the engine to an mcm898 305 V8. I went out to test the 14.5x19p prop I have just to guage what I needed and I can't judge because I can't open it all the way up. At about 3/4 throttle or a little more I'm pegged at 6 grand rpms and 35 mph. I was wondering if anyone could give me a couple prop sizes to try so I can narrow it down more. I was thinking a 23p but I'm guessing that I could have hit 7k with full throttle.