70 on 20 MA
 

I’ve recently repowered my 77 Seafari 20’ with a 115 Etec
I feel it’s borderline acceptable
If I had to do it over I’d go with a 150

Malcolm Tucker : He knows about motors?
Gomer Pyle : He hopped up an old V8 engine, put it on his rowboat. That thing'll do eighty. Now that's fast on water.

I mentioned the Aquasport experience because it was a bigger boat. From what I looked up just now, it was about 600 pounds heavier than the 20’ SeaCraft and it was almost there with a dead engine weight on the rear. Obviously, it’s not going to be a speed machine, but maybe doable for what one wants to do.

Bumpdraft-



Drat,I just lost a half hour of typing. Thank you for confirming and clarifying your observation of the Aquasport I was trying to determine the weight of that hull. Based on your data, I am still considering this.

I'm on the skeptical side myself on this, but how about keeping both motors for now, getting the boat fixed up and trying it out with the 70? If you like it, THEN sell the 100. If it's too much of a dog, put the 100 back on and sell the 70. I know it's a bit of a pain noodling with rigging, but the peace of mind of "knowing for sure" would be hard to resist for me. Eventually resale with a 70 would also suffer, as most folks are gonna look at that and say "Needs a bigger motor."

I have thought about doing just this. I expect that the F100 with new tilt/trim, water pump and clean carbs is worth something. Even with a largely brken tilt/trim, it was fine for 2 weeks of vacation.



At present, I run the F100 with a hydroshield (under skeg wing). Which lifts the stern but must cost something in drag. With a lighter motor, I may not need this. I run a 15P prop at present with a 2.33:1 gearcase (same ratio as F70).


It's an expensive experiment. But I am interested in the challenge.

The Yamaha 115 SHO is the lightest per HP you MAY/MIGHT/MAYBE have a shot at. 386 in 25". I compared weights of the big 3 recently in four stroke. The only caveat is Merc only advertises their short shaft which would be 20". In my opinion, the Merc 150 and Suzuki 140 shine pretty bright at this time. I have never owned either.

I would not want anything smaller than a 140 on an 18, 150 on a 20. My 18 will do 39 mph wot. I'm sure I could tweak it, prop pitch, etc and squeak a bit more out, but 39 in an 18 is flying to me!

A 70 hp would get a SC 20 on plane if it is lightly loaded - pretty sure that the boat performance would be limited with say 4 guys with gear on board . I think the Mercury 115 hp 4stroke is a great fit for the SC 20 - I have 150 Etec on mine which can push the boat faster than I need to go.

Ok, hold the presses FishStretcher. I called my friend to verify the story. He was the one that owned the Aquasport, with a 200 on it, sometime in between owning a 18 and a 20 SeaCraft. Our other friend that broke down with the twin 70s was actually in a 20' Robalo, still a heavy boat.
We were reminiscing about the old days and the marine science classes we took at Miami Dade college. Our class used two 22' Aquasports, powered by twin 55 hp Johnsons, borrowed from UM. There was about 12 or 13 of us on the two boats and we visited Fowey rocks lighthouse a little ways offshore. The keepers of the lighthouse showed us lenses and other gear. Twin 55s on heavily loaded boats and no problems. Nowadays nobody would think to power a 22 with 110 hp.

I have an ETEC 90 (avg actual 101 prop hp) on my 1987 (CSY-built, not Tracker) 18', but with the large 4.75" V6 gearcase. Planes in about 4.5 seconds, top speed running from a thunderstorm with 45 gals fuel, 4 adults and gear is about 31-32 mph.

It runs 36 mph at WOT lightly loaded and trimmed out. Cruises best at about 4000-4200 rpms at 24-25 mph. Holds plane down to 11-12 mph without trim tabs but with Hydro-Shield under the prop for stern lift.

Unless you really feel the need to travel at 40 mph+, the 90 Etec is fine for the 18' and light weight.
(And yet my 1987 '18 by CSY is actually placarded for 200 hp max instead of the later Tracker models and earlier SCI models which are rated at either 140 or 150 hp, although I have seen an '86 CSY 18' rated for 175 hp)