Performance numbers

[Fr. Frank]

Since it came up again in two more PM's this week, here are my performance numbers and info.

'72 Seafari 20' w/ 2006 Mercury 90 ELPT Optimax that weighs 376 lbs, transom mounted. Boat has Trinidad anti-fouling bottom paint, and is unrestored condition.

Currently turning a Quicksilver Trophy Plus 4 blade 13x21 SS prop, with a skeg-mounted hydro-shield. (I would be better off with 19" pitch)

WOT with 3 adults and full fuel is 5350 rpms at 34 mph in a light chop, burning 10.8 gph or 3.1 mpg. (I need to turn 5700 RPMS at WOT to be just right on the prop)

Cruise is 3900-4100 rpms at 23-25 mph, burning about 4.9 gph or about 5.1 mpg.

Minimum planing speed is 14 mph, but requires throttling up to 17 mph, and then falling back to 14.

Absolute Best Economy is 3800 RPMS at 22 mph and 5.3 mpg

[cdavisdb]

Those numbers look a lot like my old seafari 20, 150 merc from the 1970s, just a lot better fuel economy and slightly slower wot.

Mine had the same issue with low speed planing, like many boats. I've had two that did not, a 24 Seabird (24'deepV, 350 chevy, volvo duoprop) and my 25 Seafari with essentially the same power package. Both would plane solidly at about 12 knots. I always ascribed the low speed planing in these boats to the duoprop's low end torque, but maybe it was something else. Any opinion?

[Bushwacker]

Connor,

I think low speed planing is all about weight distribution. My Seafari would plane at about 12 kts (with a Bahamas type load, including about 80 gallons of gas) with the 300 lb 115 hp V-4, a 13 3/4 x 15 SST prop, a Doelfin and trim tabs. On the Bahama trips I always had a lot of gear in the cabin including a couple of dive tanks in a rack I built aft of the head, the gas jugs were carried amidships, and I put a 60 qt cooler in the stepdown. I mounted pad eyes in the 4 corners of both the cockpit AND the cabin. I ran a web of lines between them to keep everything from shifting around in rough conditions, and I didn't put any heavy stuff in the aft area of the cockpit.

Current rig with a much heavier motor (+ 130 lbs) sitting 30" further aft on a bracket would not plane below about 18-19 kts, UNTIL I went to a 4 blade prop! A stern lifting prop makes a world of difference. It now planes easily at about 12 kts. I bet that Duoprop, with 2X the number of blades, creates a lot of stern lift. Both the Seabird and 25 Seafari have so much deadrise that I would think they would struggle at low speed without a good stern-lifting prop. Denny

[cdavisdb]

Must be the stern lifting prop. We dove very heavy in the 20 seafari, 10 tanks in the boat, 7-8 of them forward, and the boat still did not want to plane slow. In the other boats, weight distribution did not seem to matter much.

[Blue_Heron]

I think low speed planing is all about weight distribution..

Denny, I think you also have to take into account the engine's power curve and the propping. Granted, a tail heavy boat won't like planing at low speeds, but if you're running a big two stroke that needs to run at low rpms at 12-15 kts, the engine may bog and drop you off plane if it's not propped right. I reduced the minimum planing speed on my 20 SF considerably by going to a 17P Mirage. The combination of lower pitch and stern lift made the difference. I suspect Connor's Duoprop, with very low slip, is making the difference for him at low speeds. I should be able to offer a comparison when I launch my 25 with a Bravo I drive.
Dave

[Bushwacker]

. . . if you're running a big two stroke that needs to run at low rpms at 12-15 kts, the engine may bog . . .

Dave, That's a good point about the engine's torque curve and where you're operating on it. Probably makes a good argument for running a smaller engine also, at least with a 2 stroke outboard! With my old 115 I was running a 15" pitch SST prop and it was turning about 3200-3500 at min planing speed, so it was making fairly decent torque. I normally cruised at 20 kts @ 4500, but on one trip with following seas, it started falling off plane when going "uphill", so I punched it up to 4600-4700 and then it would stay on plane with no problem! I later put some marks on the throttle quadrant and then had someone move the throttle while I watched the linkage on the motor. What I discovered was that up to about 4500 rpm, the linkage went to full spark advance, but the throttle butterflies only moved about 10-15 degrees! During the last 30 degrees of "power lever" movement, the throttle butterflies opened almost 90 degrees! The motor also got a lot louder and was obviously developing a lot more power. (Visualize the sound of a toilet flushing with the sound of the gas pouring thru all 4 wide open barrels of the carbs! ) Denny

[DonV]

with the sound of the gas pouring thru all 4 wide open barrels of the carbs

Yeah Denny!! We all know what that sounds like and it's now at a smooth $4+ per gallon!!

[FishStretcher]

Fr. Frank

Can you suggest a prop for my 20 foot M.A.? I just bought a 2000 Yamaha F100 (F90) which I read is the same as a 4 stroke Merc 90. I think the prop splines are interchangeable. It is ~356 lb dry. I have been looking at 4 blade Solas props, or Craigslist, but I am not sure on pitch. I have a lightish kicker (58 lb) on the stern and am more concerned with being able to play in the rough stuff than go fast, so response to throttle is key. Is a 4 blade 17" prop too little pitch?

[Fr. Frank]

2000 Yamadog F100: 2.31:1 gear ratio, Max of max rpm is 6000.

Assuming you're getting a true 100 hp, then you probably need a 13.75"D x 19"P, or a 13.5"D x 17"P to be able to turn 6000 rpms fully loaded (or close to it). You should have a top speed in the 32-35 mph range. I would try to start with a 19"P prop.

Rremember: More pitch = less rpms. You could get by with a 13.25"x20"P or maybe even a 13.25"x21"P, but the Yamaha F100 is NOT known to be a torquey motor.

[FishStretcher]

Props confuse me
A 13.75 x 19 or a 13.5 x17? The second is less RPM and less pitch than the first. Maybe this is the range you think I will be in?

There is a 4 blase SS Yamaha 13.75 x 18 for sale locally. $275 asking price. Not sure if it will fit my gearcase. It fits the F150.

Also, how do you like the hydroshield?

2000 Yamadog F100: 2.31:1 gear ratio, Max of max rpm is 6000.

Assuming you're getting a true 100 hp, then you probably need a 13.75"D x 19"P, or a 13.5"D x 17"P to be able to turn 6000 rpms fully loaded (or close to it). You should have a top speed in the 32-35 mph range. I would try to start with a 19"P prop.

Rremember: More pitch = less rpms. You could get by with a 13.25"x20"P or maybe even a 13.25"x21"P, but the Yamaha F100 is NOT known to be a torquey motor.