20' cc with 150 Yamaha- ? mph

Magothy1 · #1 08-19-2009, 10:37 PM

I have an '86 20' cc with a 150 Yamaha, fuel injected 2 stroke, it runs 40 mph with just me and full fuel, seems as tho it ought to do a bit better, running a 14x17 prop now.
Tach has quit, I think it was turning around 5500 rpm. Anyone do lots better with a similar setup ?

Bigshrimpin · #2 08-20-2009, 09:37 AM

You motor might be able to come up a few holes . . . which minimizes drag (increases RPM/speed).

Caymanboy · #3 08-20-2009, 11:26 PM

Need more info, T Top? Bimini top? Etc? I run a 1970 (light boat) with a 200hp 2 strk Yami, 50 kts, but I have to work at it, trim, water, etc.

eggsuckindog · #4 08-21-2009, 02:15 AM

Thats about right I would think, I have a 200 with a T Top and right at 50

#5 08-22-2009, 08:41 PM

I run a 1970 20 with a 175 2 stroke Yamaha on a bracket. I get about 49 loaded to fish with 2 people on board.

Magothy1 · #6 08-22-2009, 10:48 PM

Thanks for all that, I'll look to see about raising it, I can't recall right now where the cavitation plate is relative to the bottom. From the other posts it looks as tho it's in the ballpark. When I get the tach back to working I might try another prop with a little more pitch. It did well with five big adults, all 200 lb plus, jumped right out and ran well. It does have a T-top.
Thanks again,
Steve

jammingator · #7 08-24-2009, 02:01 PM

My '75 with a 2003 Yamaha VMax and T-Top will run 48 mph with me, my wife, 2 young kids and half a tank of fuel. Fun, fun!

3rdday · #8 08-26-2009, 11:06 PM

I was going to mention an 19P. You are doing ok though with the 17, if you fish offshore you will need the torque of the 17P. If you like to haul A$$ on a flat lake or the flat intercoastal then the 19 is PROBABLY your prop. And for sh*ts and giggles try a 21P.

Blue197320 · #9 08-26-2009, 11:32 PM

wow something must be wrong with my prop. ive got a 19P (i think) and at 5500 im gettin 44mph gps.

ive got a 2.4L 200 merc too. it should be hitting 50 with that motor.

Fr. Frank · #10 08-27-2009, 07:38 AM

Blue,
There may be nothing wrong with the prop, it just may be the wrong prop.

My 20' Seafari, used to have a 150 Merc remanufactured to produce about 190 hp, with a Bob's nose cone.

Started with a 17" pitch Laser prop, with which I got 5750 rpm and about 43-44 mph. It took about 8 seconds to plane, and fell off plane below 23 mph. It blew out in any kind of turn, and cavitated badly upon acceleration from a stop. That's WITH the hub vents blocked.

Changed to a 15"x17"P Mirage, planed in 4-5 seconds, and got 5800 rpms and about 46-47 mph WOT. I could trim all the way without blowing out in a turn, slight cavitation accelerating.

I tried several other props, but nothing near the performance of the Mirage. But I knew the boat could do better.

In 2002 I ordered a custom made prop from Power Tech. We spent a couple weeks going over the hull size/shape, weight, CG, etcetera, before I ordered a 15.25"D x 16.5"P polished prop from them. I planed in 3-4 seconds, and achieved a max of 6050 RPM at 53 mph lightly loaded. I could trim completely, and had moderate cavitation accelerating.

As you can see, just a little bit of propeller design change can make significant differences in performance.

If you go the route of ordering a custom prop, expect to pay for the engineer's time. My PT prop cost me right at $700, and was just a modification of an existing prop line. Of the shelf PT props in '02 were about $400 to $450.

I think the extra money spent was well worth it. I can't spend the money today I could then, but if I could, I would order another custom design now that my Seafari is running a 90 hp Optimax instead of a V6 Merc. I'm not happy with my current prop as it ventilates easily. Acceleration, cruise, and top-end performance are good, however.