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

[Texascoastalfish]

Hey All,
Have had my 76 20'-SF out and about all around Galveston Bay,and West Galveston Bay in 3'+ chop and I've got to say..What a ride!! Put her quartering into oncoming seas and it was high and dry at 18-20 knts.No pounding. The North wind was howling at 20 mph. With a following sea 3+ chop it was even smoother although I kept quartering and close eye on the swells, they were close enough together to not have too much worry on plowing into the backs. My Brother kept exclaiming. Man this boat rides great ! I was smugly thinking. Those boys on the Forum were 100 % correct. Imitated,,,,never duplicated....OK enough romancing..

In the sheltered bay areas wide open GPS showed 30-31 MPH , but only 4800 or so RPM. Is this a prop issue I wonder ? I'm running 3.75" X 15 aluminum, may motor too low. The Motor was propped for a 17' Magnum trihull before I got it.. But 31 MPH is nice at WOT. What would be the recommended procedure to increase RPM to 5200-5400 ? I believe thats the recommended WOT for a 84 90 HP Johnson.
As always thanks in advance and a Merry Christmas and Happy New Year.

[bridgeman]

Your going to pick up about 200-300 rpms per inch pitch of prop you go down providing its the same type..my experience

[3rdday]

right on, right on! I just experienced the same, I was only winding up to 5100 with a 21 pitch, dropped down to a 19 pitch and getting 5500-5600 trimmed properly.

[McGillicuddy]

As you reduce your pitch to increase your rpms, you may also want to consider the height of the motor (anti-cav plate in relation to hull bottom), and the shape of the prop. If you're going to change it out rather than have it re-pitched, you might want to check around for a prop that is more efficient with your hull. Especially, since the prop was previously fitted for a tri-hull. Some designs are certainly easier to turn than others and will have varying characteristics.

[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.