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

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.

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

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

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

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.

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

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.

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

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.

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

Need more HP. :D :D

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

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...

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

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.

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

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.

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

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

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

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

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

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!

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

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.

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

Thanks again Boys for the great Data. I will try the 13.75 X 17 since I bought it. ( 15 bucks on ebay), and Ill get a 13.75 X 15 ( new). We will see what the prop and lifting engine will produce. Thanks again

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

I took her out and ran the 13.75 X 17, and got 4900 RPMS at WOT and 31 MPH. Looking at the clearance between the bottom of Cav plate and the tips of the prop we are looking at 1/4" ,I also have another 13.25 X 17, and a bid on a 13.75 X 15. I'm 1" above keel with motor now. I'll try the smaller diameter 17 P first, then the 15 P 13.75. trying to get above 5200. Im 400 short now. I haven't checked compression yet, I am running with a Stingray Hydrofoil. Sounds like the 15 P is how I'll have to go being at minimum HP for that boat and keep the 17 P for spare. I run shallow alot in this Bay system and seem to nick the props often. Thanks in advance again!

http://i358.photobucket.com/albums/o...g?t=1231862144

http://i358.photobucket.com/albums/o...g?t=1231862304

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

I second what everyone is saying. but if you want call Mike @ Mike Hood propellers 713-649-7508. He is on Telephone rd., just north of Airport. There is not another better around here, I feel. It's pretty impressive to watch him in action.

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

Update. I should have checked compression from the get-go.
After not getting above 4900 RPMS with still a 17P prop and motor 1" above keel, I checked compression and I found bottom portside cylinder 70 PSI all the rest had 120. Pulled head and found no visual damage, bore seemed smooth , no gouging, I did find a very small piece of aluminum in the head dome. Decarbed Cylinder 2X with Seafoam, no change. Put some 30 wt oil in cylinder and compression jumped to 90. Bad Ring(s). In stead of pulling powerhead and rebuilding 1 or all cylinders I opted for a little more HP and got a totally rebuilt, bored over and fully warrantied 1995 112 SPL from a local marine motor shop (cash is king these days, bargains abound) Its now more like a 120 HP due to .020 boring and bubbleback exhaust Vs. Flatback exhaust. Had it rigged yesterday and I'm ready for the break in procedure. I will hopefully be in the 5500 RPM range by Sunday :) I stuck with the 17P prop for the higher HP. I now have 90 Johnson on Craigslist and if I cant sell it for 750 bucks I'll rebuild and put it on a skinny water skiff like an Explorer or something real shallow. Never can have TOO many boats. ( in the end maybe 1 less wife if this keeps up !)
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