Only 4500 -4800 RPMS- 90 HP Johnson on 20'SF

[eggsuckindog]

If have a 15 already you don't have much going down, thats about it I think. The best idea was to raise the motor and I will bet its too low and maybe way to low. In any case, it should be 1 to 1 1/2" ABOVE the the hull bottom, you don't want to drag the cavatation plate around, that will help I bet.

[lost2a6]

Need more HP.

[Texascoastalfish]

This could be true about the HP I admit , but I recall these 70's 20' CC's were sold pretty much with 90's to 115 HP's, being the hull was designed for a 300-325 lb motor. I guess If I can only muster say 4800-4900 RPM is this going to put too much wear on the motor ? I also need to recheck my pitch. I think its 15, Its up on a sling down at the water so Ill verify this weekend, raising a hole or two is definitely an option.
Thanks again Boys, I learn something everytime.
Regards...

[Fr. Frank]

Raise it only 1 hole at a time. Bottom of the cavitation plate 1" to 1 1/2" above the keel line is right.

4800 max RPM is too little, and puts extra torque stress on rods and bearings. You end up using more fuel. too.

Start with raising the engine. All that costs is time and sealant for the bolt holes.

[Bushwacker]

I ran a 1975 "115" V-4 (really about 90 hp at prop)on my Seafari with a 13 3/4 x 15" Pitch OMC SST prop that would turn 5400-5500 with a heavy load. Cav plate was 1" above bottom. That same prop should work for you unless motor is mounted too low or is way down on compression/power.

[Texascoastalfish]

Correction......................
The prop I have is a 13.25 X 17. I measured the bottom of Cav plate and it was below the bottom of the keel by about 1/4 ".
I have raised the motor 1 hole and it looks to be a little over 1" above keel now. I am at the next to the last highest hole. The prop is really torn up from a close encounter with a oyster reef and I have a 13.75 X 17 off eBay I'm putting on it. Its a used aluminum so it may be worn down to near 13.50. Will the slightly larger diameter slow the RPM'S ? I know raising the motor will increase it some. If this doesn't help Ill do down to a 13.25 X 15

[Fr. Frank]

Smaller diameter will give you more RPMs. I think you'll need to go to a 15" pitch, though.

[Bushwacker]

In addition to a 13.75 x 15P prop, I also tried 13.5 x 17P and 13.25 x 19P aluminum props on my old 90hp-at-the-prop "115", (1975 vintage) on my Seafari. It turned the 17P up to about 5200; 19P only turned about 4900-5000. Best prop was 13.75 x 15P, which turned 5500-5600 in either stainless or aluminum versions. If the blades are badly chewed up on the props your testing, you've lost a bunch of efficiency and may be getting misleading results. I know several folks running 115's on 19-20' hulls and they're all running 15" pitch props! There's probably a pretty strong message somewhere in all that data!

[Fr. Frank]

The gear ratio makes a huge difference. I used to run a 15"P on my current Seafari when I had the 120 Chrysler. HP at the prop was about 95-100, and I turned 5200 rpms. The gear ratio on my Chrysler was 1.98, and I had a top speed of about 28-29 mph, and cruised at 23-24 mph.

Now that I have an '06 Mercury 90 hp Optimax, I have about 100-105 hp at the prop, my gear ratio is 2.33, I turn that same 5200 rpms, running a 12.75"x20"P SS 4-blade, with a top end of 32 mph, and cruise at 4500 rpms at 25-26 mph.

I was running a 3 blade Apollo SS, in 13"x21", with a top end of 34 mph at 5100 rpms, but it would fall off plane below 21 mph.