Jumping in with Both Feet on a 20 Seafari

[SailorChlud]

sorry folks.

[Bushwacker]

Hey John, sounds like you had a pretty successful first outing! FYI, I made a series of checklists on 3x5 cards that I keep in my truck for hookup, launching, and retrieving. They help to insure I don't forget anything, especially if I haven't used boat for a month or two!

Your speed/rpm numbers sound about right for a 15"P prop, assuming your gear ratio is 2.0:1. I used to run a 13 3/4 x 15P 3B SS OMC SST prop on my 115, and I used to cruise all day long on Bahama runs at 4500 rpm and about 20 kts/23 mph with a very heavy load. WOT was 5500/32-33 mph, again with a very heavy load and top raised. However in 1975, motors were rated at the powerhead instead of the prop, so the fact that you can turn a 3B 15x15P 3B prop, assuming you have a 2.0 ratio, says that your motor is putting out a fair amount more power, probably 120-125 hp at the prop. I suspect my old motor probably wasn't more than about 100-105 hp at the prop. Optimum rpm on most of the E-TECs is around 5500-5750, and I think your powerhead is very similar, so I think you'd be fine if you could wind it up to 5500. The crank on those V-4's is only about 6" long and they're good for about 9000 rpm, so 5500-6000 sure won't hurt it. But I'd raise the motor and try it again with the same prop to see if you can pick up a little more speed. Experiment with the trim and do some handling checks (sudden sharp turns) to see how high you can trim it it without prop ventilating or blowing out.

[Dogbomb]

Congratulations!
I'm so happy she performed well, and that you guys had a good day on the
water. Working out a few bugs should be easy now that the hard work
is done. I hope you have many more adventures this year!

[SailorChlud]

Thanks for the kinds words and input.

After checking the manual again, I discovered the WOT rpm range is really 4500-5500 rpms, so I am not that much over, but I have a 17 pitch prop and may just substitute that on the next run to see how that does.

I believe the Blue Seas battery switch is faulty, and will be checking that component carefully tonight or tomorrow - It worked as it should on Saturday night, and again on Sunday morning, but somehow lost the connection to the starting battery at the dock on launching.

I tested this on the water by switching the batteries back from combined to run position while the motor was running, and though the motor remained running, lost my tach readout, indicating that the start battery was disconnected in that position, and maybe not feeding the ignition/motor circuit. Back at the dock, the motor trim switch would not work on the run position, further confirming that the connection in the switch had been lost. But at the combined position the trim switch worked fine, so the house battery was powering it. Could just be a loose connection, but I am inclined to suspect an internal switch problem, as the battery cables are tight at the batteries and the switch, the cable terminals are bright/newly installed, and were working the night before.

Another small event of note on the test run - while running at 18 knots in the channel, with boats ahead and astern, I swiveled my helm chair just enough to catch the oversized keychain float I had stupidly attached to the key, and it turned off the motor! Startling is not the word for this! I am glad I had raised the transom, so the stern wave did not swamp us, but was I annoyed! So were the boats behind me!

[Bushwacker]

Originally Posted by SailorChlud

. . . I tested this on the water by switching the batteries back from combined to run position while the motor was running, and though the motor remained running, lost my tach readout, indicating that the start battery was disconnected in that position, and maybe not feeding the ignition/motor circuit. . .

Be careful switching batteries with the motor running, as that can blow the diodes in the alternator unless your battery switch is specifically designed for that! I think your tach signal comes from the alternator, so that might be why your tach quit working!

[SailorChlud]

Found the culprit on the switch question - a simple blown fuse. Apparently I undersized the fuse on the Blue Seas MRBF fuse on the starting battery. Will order new ones of the proper capacity and try again. Lesson learned.

[SailorChlud]

Denny:

In your response you mentioned: "your speed/rpm numbers sound about right for a 15"P prop, assuming your gear ratio is 2.0:1."

I looked it up and I have the large gearcase of M type, with 2.25:1 ratio.

[Bushwacker]

Originally Posted by SailorChlud

Denny:

In your response you mentioned: "your speed/rpm numbers sound about right for a 15"P prop, assuming your gear ratio is 2.0:1."

I looked it up and I have the large gearcase of M type, with 2.25:1 ratio.

With that much gear reduction you can probably spin a 17"P prop up to 5500.

Originally Posted by SailorChlud

. . . With a load of 3 people, about 30 gallons of fuel, the bimini up, and a BRP 3 blade prop 15x15, I observed a top speed at WOT of about 28.5 knots at 5600-5700 rpms . . .

With a 2.25:1 ratio and a 15"P prop, 28.5 kts works out to a prop slip of 8.9% at 5700 rpm and 7.3 % at 5600, if the rpm numbers are accurate. Slip numbers below 10% are very good, especially for a 3B prop. Do you have a digital tach, and are you running a stainless or aluminum prop?

[SailorChlud]

The prop I was running for the initial test is a 15x15 3 blade aluminum. This weekend if the weather forecast is ok, I will switch to a 3 blade 15x17 aluminum and record some numbers. I will not raise the motor just yet, because I did not feel I had optimum trim for my initial observation, and I also did not do any turning to assess blowout. I will try to do both on the next outing and then make a determination on the height after I feel I have the correct trim setting. I don't want to raise the motor, only to have to lower it again.

[SailorChlud]

It is the typical analog tach, not digital, unfortunately, but it is fresh from the dealer last month. Hope it is close to accurate.