100hp on a 20 sf?

Yz2009 · #1 08-09-2010, 07:42 PM

I've been reading a little on the 20 sf's. I'm considering buying a early 80's 20sf and have a few questions. I've seen some people saying 90-130 hp can be good for these hulls and was wondering is there any truth to that. I've currently got a seaox 2300 with a yamaha f100 and cruise at 22mph at 4700rpm and wot 28mph at 5400rpm. The hulls of the seaox and seacraft weigh about that same but the seacraft has a deeper v and is 3ft shorter. If I was to put the f100 on the seacraft would I get similar results? Also I've read 4s engines are not a good combination on these hulls but the f100 only weighs 366lbs and I could get rid of the oil tank and move the battery to the center console. Any insight would be greatly appreciated. Thanks.

McGillicuddy · #2 08-09-2010, 08:16 PM

Some would say you'll be underpowered, but you'll be fine. If you're satisfied with your Sea Ox numbers you'll be happy with that motor on the SeaCraft 20. Move everything forward like you mentioned and make sure the prop has some stern lifting qualities. Probably top out at 33mph and get excellent economy at low 20s mph cruise. Get some trim tabs or at least an SE foil to get on plane quicker.

I have a Seafari 20 with an '83 115 Merc and get 36 mph WOT and get out of the hole very quickly.

Fr. Frank · #3 08-10-2010, 12:49 AM

I have a '72 Seafari 20', '06 Mercury 90 hp Optimax with 13.25"X21P 4 blade and skeg-mounted Hydro-shield. WOT 33-34 mph, cruise 24 mph at 5.1 mpg. Planes quickly.

Remember, the most common original equipment motors on these 20' SeaCrafts were the 85 hp and 115 hp V4 Evinrude/Johnsons, or the 90 hp or 115 hp inline 6 cylinder Mercury motors. As a SeaCraft dealer, I have even rigged a 20 SeaCraft with a single 70 hp motor, and it did fine. There is also another member with a 20' Seafari rigged with a Mercury 80/85 hp two-stroke 4 cylinder. These hulls don't need all that extra horsepower.

But if you want to put the extra power on, they'll handle it just fine. Locally, there's a 20' 1971 SeaCraft Superfisherman with a Porta-Bracket and a 250 Yamaha HPDI that runs like a raped ape. (Of course, he only averages 2.5 miles per gallon.)

intergalactic · #4 11-03-2010, 10:45 PM

This is a good thread to read. I was wondering about a Honda BF90 on a bracket on a 1971 SF20. The bracket is aluminum, but i will foam fill it. A honda BF90 is 370lbs more or less. I don't need more than 30 knots, and a bit less is fine.

McGillicuddy · #5 11-04-2010, 02:34 AM

No need to foam fill the bracket if its already a closed hollow chamber. If you don't already have the Honda you might consider an Etec 90 for your application. You will cut the weight by 50 lbs and get every bit the of the 4 stroke economy.

intergalactic · #6 11-04-2010, 10:24 AM

I don't know anything about the etecs. I know I love my other Honda because it ALWAYS starts. The economy is nice as is the lack of two stroke fog.

I was even thinking dual 50s and no kicker with a flotation bracket. I have no motor yet. Heavy, but lighter than a motor and kicker.

Fr. Frank · #7 11-04-2010, 11:38 AM

If you're thinking twin 50's, think twin 60's by Honda or Evinrude, instead. The weight and fuel consumption is similar, and the power is 20% greater.

seacraftks · #8 11-07-2010, 07:37 PM

a 130 yamaha is all you need top speed with no trim tabs is 42 mph

everybody · #9 11-10-2010, 09:18 AM

What kind of #'s are people getting from the 140 suzuki's

pair of jacks · #10 11-10-2010, 02:16 PM

Interesting thread- I posted some months ago here that I was considering repowering with a 150 Yami 4stroke from my current 200 Opti- I received a lot of "don't do it" comments even though I think 150 hp is plenty for a 21 ft. SeaCraft- but the concensus here is that less is ok- why is that?