Question about Prop pitch and size.
 

Hey Guys, I Just got my boat in the water a few times this year and I had a question about the prop I am or should be running. My boat is a 21' wellcraft Elite with a 300hp 350. I am running a 19p 15-1/2 Mirage prop. The boat comes out of the water strong and fast and plans much faster than my Frineds 271 Fastech, and she tops out at about 54mph. My question is what would make the top end go up a little more? Which way should I go with pitch and size? Thanks.

Try a 21 picth prop.

So a Higher pitch like a 21 would get my top speed up some?

350 mags should run in the 46-4800 range wot by my mercury paperwork ( mpi's a little higher like48-5000). If you are running in that range your prop selection is right. If you are over reving or hitting the rev limiter on the mpi go up in pitch. I run 23p laser2s on my carb mags , my bud runs the 23p mirage pluses on tha mpi mags.

I am running a 350 with a comp marine cam (218/218 with a 110LSA) and Vortec heads with a 750 Carb. It turns about 4500-4700 with the 19P. The power range on the cam says to 5500.

Does the size of the prop (15-1/2inch) have anything to do with it. I know most 19 are about 14-1/2.

Yes it could. A larger diameter prop also has more blade area. I run a 21" LaserII. While the LaserII prop is the same pitch as the Mirage I had before the blades are angled back more ( more rake), has smaller blades, and it has a smaller diameter. The increase in rake helps to carry the bow better at speed with less drive trim. The smaller blades have less area and the smaller diameter allows the engine to spin up faster which gets me out of the hole quicker without losing top end speed like a lower pitch would.

If you do swap props an increase in pitch from 19" to 21" will drop engine RPM by 200.
General rule of thumb....every 1" change in pitch equals a 100 rpm change.

And props for out drives are listed in odd pitch numbers (17", 19", 21", etc). Props for outboards are in even pitch increments. The two are not interchangeable.

Thanks Guys. Anyone know what the rev limiter is on the Thunder Volt 4 ignition?

Does the size of the prop have anything to do with it? My prop is a 15 1/2, where most 19p I have seen are around 14.

It might.
The bigger the diameter, the more blade area, the bigger bite the blades have on water, the more strain that's on the engine, the slower the boat gets out of the hole.

Switching from a 21" to a 19" pitch of the same model prop (Mirage to Mirage) you will see a difference in time to plane but you will drop top end speed. If you went to a Mirage Plus with cupped blades then the prop will act like it's 1" larger in pitch.

Selecting a prop is a tough decision. I spent lots of time reading eveything I could on props. First thing I had to decide is what I needed the prop to do. I needed to get on plane quicker and I felt the boat needed too much drive trim to carry the bow. I knew a drop in pitch would speed up the time to plane but it would drop my top speed. I also knew a change in blade diameter would make a differance and an increase in blade rake would help carry the bow better. I searched and found a prop in the same pitch as I was running but with steeper rake, smaller blades, and a slightly smaller diameter...the Laser II.

How much should I trim her up. I stop trim just before i get a little roster tail.

Hey Iggy, he is not even close to what that small block should be running RPM wise, you really think he can pull a 21?

From what I have read, Mirage props are mostly made for big block boats. This one is 15 1/2" big. Most 19 or 21p props are around 14". So maybe I just need to go with a smaller diameter prop like a 19p 14" Laser. Does this sound right?

CALL BBLADES they have a prop swap you give them your info they send you a prop for testing its a little extra say 25 bucks I would start with a laser 21 and if you pick up speed but are a little low on rpms you can have it labbed to get them back if the 21 is right at 4800 I would go to a 23 and lab it that should get you back to 4800 and you might pick up 5mph. a friend of mine is putting in a 350 325hp he had a 220hp merc before its a 17ft runabout we are hoping to turn a 24 3 blade turbo prop I have which will have it running in the 60plus range:D . if not he will buy a 21. art

who is BBlades???? you have there contact info?

Bret at BBlades will take care of you.
BBLADES Professional Propellers, LLC
N5007 Highway 73
Princeton WI, 54968
Phone: 920-295-4435
Fax: 920-295-4436

I never said he could run a 21" pitch. I used that size as an example.
A 21" isn't all that big. My 1988 F-206 LS Formula has the original 260hp 350 MAG and it came from the factory with a 21" Mirage (which I still have) on a Alpha drive. Top end is 58 mph @ 4800.


I trim till the engine increases rpm without an increase in speed. At this point the prop is slipping. Then I lower the drive till it stops slipping. Some day I'll get the trim indicator fixed and I'll know exactly where it's at.

My trim indicator is broke too. Its just kinda frustrating because I see all these people with stock 5.7 260hp hitting 55-60 and here I am with a Vortec 350, comp cam and a 750 carb and I can't get anything decent. My boat is only 20.6' so its not like I am pushing a 24' with a 350.

does anyone Know of any "lend a prop" dealers on the easter shore near Kent Island?

Ok, I tried a 19pitch cleaver prop. yesterday and got the rpms at 5200 but still only got 53mph. Do you think I should go to a 21 or 23? I was thinking a 21p laser would get the rpm down and maybe a few more mPH.

oh , I know I used the same picth but the Mirage was 15 1/2 where the cleaver was 14 1/4