92 Chris craft Concept 258 prop sizing

[Mnormand55]

hey guys just jumpd back into the world of boating picked up the above mentiond boat, had it a week love the lay out and the room for the size, just looking to get alitle more speed out of her, it has a reinstalled power plant in 2016 with a 454 mag, not sure what hp mag, but is a mag carb, know its a 4 bolt main out of a 94 Mariaha, anyway runs good no issues there, guy had the wrong prop on it and was a 4 blade aluminum 18p, which was way over reving, so i switch it to a the spare 20p, but didnt really gain anything at all, bit of rpm drop at WOT, so it within range i guess from 4600 to 5000, i had tried a 21p cleaver stainless but that ended up ion thebottom of lake but put me at 4600 @ WOT which i though was ntb , but is more of a top water prop not sure how good it is didnt get much testing time the hub sperated and i didnt have the right parts i geuss to keep it on the shaft apparently lol. so i wondering if i should go to a 21P regular like a mirage or laser 2 or something, or up it to a 23 and sacrifice and use the tabs a bit ?

 

[Mnormand55]

just a side note, boat is 5000lbs, usually max 4 adults usually only 2 so maybe 500lbs in gear, and been keeping around the same amount of fuel 20 gallons bout half a tank or so, im thinking boat should do 50 mph not sure how much of a strech that is. but cruising speed is usually at 3500 rpm at 31-32 mph. let me know what you guys think thanks

 

[porscheguy]

A 25’ single screw means you need some size on the prop. The laser2 imo doesn’t have enough diameter for a boat your size and weight. I would opt for a mirage plus. It was made for heavier boats. As for the pitch, go with a 19. As soon as your rpm falls below the max redline, adding more pitch seldom improves speed and only brings the rpm down. You already said a 20P aluminum 4 blade reduced your rpm noticeably but gave no improvement in speed over the 18. I’m going to assume you’ve got a bravo drive with a 1.65 ratio? A19p mirage plus (if you buy used, make sure you get a plus model, preferably with vents) should get you 48mph @4800rpm with 8% slip.

 

[Mnormand55]

ok, not sure of the drive thinking around there or 1.5:1, yes bravo 1, the 18p 4 blade that was on there reved to the mooon it seemd, over 5 prob like 5200, the 20p seems a little more in spec, prob around 4800 or 4900 depending on trim angle i guess, still trying to get use to boat, and what optimum trim level is. i will keep and eye out for a mirage plus then thank you.

 

[Owennine]

"A 25’ single screw means you need some size on the prop. The laser2 imo doesn’t have enough diameter for a boat your size and weight. I would opt for a mirage plus. It was made for heavier boats."

I didn't even know that. Thank you very much!

 

[Mnormand55]

porscheguy what diameter ? do i stay around like 14.5?

 

[Mnormand55]

turns out i have a chance to borrow a quicksilver 48 16548 a40 23p which is a laser 2. so i will test ur theory out and let u know how i do thursday

 

[porscheguy]

You don’t get to choose the diameter. On most performance oriented props, the diameter changes with the pitch by the manufacturer’s discretion. A 19P mirage plus is going to be 15.25 in diameter. A 21p is 14.75 in diameter. 23p and up are 14.375 diameter. Below 19p and the diameter goes up as those from 13p-17p are mostly being used on heavier cruisers that need to generate more thrust.

Other props like the revolution4 are 14.625 diameter regardless of the pitch.

Your boat will be fine with a 15.25x19p mirage plus. I routinely use a 14.75x21p mirage plus on the boat in my sig.

I’m not sure of your location, but if you’re in an area with an active boating community, you might be able to find one on Craigslist. I’ve got a standard rubber hub mirage I grabbed for $25 + $100 more to have it rehubbed and polished. I bought my 21p mirage plus used from a marina. It had been polished, placed on the shelf, and ignored. That was $250. I also got a 23p mirage+ from Craigslist for $100+ $125 to have it cupped a bit and polished. These props all look brand new after a trip through the prop shop. As I mentioned previously, if you buy used, make sure you get a “plus” model and make sure it has the PVS vents. The vents may or may not be of use, but the reason for getting them is for hub kit fitment. Early mirage plus props without vents use a slightly longer hub kit which isn’t easy to find and is a bit pricey. Later ones with vents can use the standard hub kit, although I prefer the rev4 hub kit as it has an extra bushing to support the prop on the propshaft.

if you can’t find one on the used market and opt for new, the mirage plus is also sold under the Quicksilver brand as the thunderbolt. Quicksilver stainless props are in short supply this year due to high demand for mercury branded props. But since the mirage plus is considered a bit old school, Some retailers still have the thunderbolt available. I’ll send a pm.

 

[Mnormand55]

Hey so that 23p laser 2 has a solid hub what style brass washer with the locking groves do I need ?

 

[Mnormand55]

Think it's called a tab washer ? porscheguy

 

[porscheguy]

The tab washer is Quicksilver p/n 14-816629q.

The bronze castle washer is p/n 12-31211Q03

Those are Quicksilver part numbers, but you can at least see what they look like. I believe I bought Sierra brand.

sierra castle washer: 18-3714
tab washer: 18-3200

 

[Mnormand55]

awesome grabbed a quicksilver one today will try this prop out tomorrow and let u know what my results are,and adjust as needed

thanks alot

 

[Mnormand55]

23p didn't work was to much lost 3mph on top and cruising speed sufferd a bit too, tried a 19p also and was over reving didn't really gain anything cruis speed was little lower too thinking a 21p

 

[porscheguy]

What kind of 19p did you try? Some random aluminum, or a laser2?

what was your rpm with the 23p?

you had a 21p cleaver that only gave you 4600rpm. The mirage+ is a very grippy prop. 4600 is within the operating range for the engine but it’s still leaving 200 rpm on the table. If you want to wring 50mph out of it, you need to use every available rpm, with minimal slip. I’d opt for the 19p mirage plus. You can always add some cup if it over revs too much.

 

[Mnormand55]

the 19P was a vengeance, the 23p was boaderline i feel, was around 4400 - 4500 but wasnt any good, was to much prop but also think it was an outbord prop with square venting holes. i can try a 22p 4 blade stainless but think its gonna be to much... what do u mean by add some cup? im thinking u might be right with the mirage plus, either that or a 21p 3 blade i duno

 

[porscheguy]

1. A vengeance prop is a stainless casting of a standard mercury 3 blade aluminum prop. They’re not good or bad, they’re just a basic prop in stainless.

2. I agree that a 22p stainless 4 blade is still way too much pitch. You’ll probably be at the same rpm you had with the laser.

3. A 21p 3 blade is still going to keep your rpm too low. You’ve already tried this with a cleaver and it didn’t work out.

4. If you run your finger over the blade of a prop, you’ll notice at the trailing edge, the blade curls upward. That is cup. If you lay a straight edge on the blade you’ll note that it only makes contact at the leading edge and the trailing edge. The center of the blade will be slightly lower. What cup does is prevent water from spilling off the sides of prop and directs it rearward. It increases grip, reduces slip, and raises the bow. In addition cup is like having a bit of extra pitch. In fact, if you take a prop to a prop shop and request they increase pitch, what they’ll really do is increase the cup.

5. The reason I said to look for a mirage plus versus the standard mirage is because the difference between the two is primarily due to the amount of cup from the factory. The plus has more cup than the original. I’ve got a 23p mirage and a 21p mirage+. Both props turn about the same rpm. Guess which one pushes my boat faster? The 21p. I’ve also got a 23p mirage plus. That’s the slowest one of all on my boat because that cup reduces me to 4500rpm. It’s just way too much on my boat.

6. Prop slip is the difference, stated as a percentage between the theoretical performance of your prop and the actual observed performance. It is influenced by the prop, drive, hull shape, boat weight, and probably some other things. The pitch stamped on the prop is theoretical because it’s based on 0% slip. So on the 21p mirage+ above, my slip is just under 7% (which is phenomenal btw). The slip on the standard 23p mirage is 15% (nothing to brag about).

7. There’s a lot of info to digest, but if your boat is going to hit 50mph, go with a mirage+ 19p. It should give you use of your entire rpm range(which you’ll need to use all of it) while keeping your slip as low as possible.

 

[Mnormand55]

lol funny that were saying the 22p is to much prop llol, it actually is the best prop ive tried an i left it on form yesterday, works better then my 20p 4 blade actually gained 3mph on top and droped my cruise rpm to about 3250 from 3500 @30 mph i think i may run with this for now, see how much buddy wants for it the marina lent it to me to try said bring it back when ur done. i get what ur saying about the mirage plus, and the cupping, but i dont have access to one and am not about to buy one and have it not work. my WOT with this 4 blade could be a bit better bit does pick up 4500 to boarder line 4600 @45-46mph so it works would be curious to try a mirage plus tho still, i think being that this boat is a litter heavier the 4 blades work well with it but im no rocket scientist lol.

 

[Mnormand55]

actually if i could squeeze a little more hp outa this motor i think that it would be awesome and could probably squeeze a few more outa it, but thats another game, not sure where to start with that , or reduce some weight

 

[porscheguy]

Out of curiosity, where are you located?

 

[Mnormand55]

im up in Ontario canada you ?