Repower could add too much weight?

Garrison · #1 01-05-2011, 06:59 PM

I have '74 20SF that came with a 200hp 1989 yamaha. It is time to repower. The old 2 stroke weighs roughly 391 lbs. A newer 150 4-stroke is 466lbs. I dont really want to add more weight to transom but cant drop to a 115hp to compensate. I have moved the batteries to the console from stern position. Not much else I can do to balance weight.
Any ideas or suggestions?

SteveH · #2 01-05-2011, 08:55 PM

I have a lot of experience with a seacraft 20. Mine was a master angler. anyway, the 150 etec (2 stroke) and 150 optimax (2 stroke) each weigh about 425(approx.) lbs. That is only slightly more than you had and you moved the battery forward so it is a wash in my book. Those would be my choices. You can also buy a 135 opti, but if memory serves, it weighs same as 150..... Etec makes a 130...... not sure what it weighs but you can visit evinrude website to figure it out....... good luck!!!

seacraftks · #3 01-05-2011, 09:24 PM

I have a sf 20 (1972) with a complete new transom with a Yamaha 130 the boat runs around 38 kts depending on weight, I believe the recommended engine was a 90 hp I cant remember who said it but someone will know that info on the greatest site on the web.

You can look at a Suzuki 140 thats a pretty good engine dont know the weight offhand

Bushwacker · #4 01-05-2011, 09:54 PM

You're wise to be paying attention to weight. Remember these boats were designed in the late 60's when the heaviest outboards were the V-4 OMC's and the I-6 Mercs, weighing 300 lbs or less! They aren't designed for the weight of modern 4-strokes, so they won't perform or ride as well as designed if you hang nearly 500 lbs on the transom! (Check those 4-strokes weights carefully - they're usually dry weights, so don't include the weight of 5-7 qts of oil plus the gear oil, and they often don't include the weight of the cowling, so the actual wet weight is often closer to 500 lbs!)

On my boat, I had a significant CG shift when I repowered and went from a 300 lb V-4 on the transom to a 429 lb motor moounted 30" back on a bracket! It raised min planing speed from 12-13 mph to about 20 mph and it hurt the ride, and remember that the Seafari is less stern heavy than the cc's to begin with! I had to go to a stern lifting 4-blade prop and a Doelfin to get back to where I originally was.

Although most folks today seem to think that a 20' boat with less than 150-200 hp is underpowered, I believe that they're biased by experience with the relatively inefficient conventional deep-V hulls. The SeaCraft VDH hull has been proven to be at least 10% faster than any other deep V of comparable weight and HP, and they will easily plane at low speeds if properly balanced. Power selection should be a function of how you normally use the boat. If you run offshore in rough water a lot, you should know that the relatively light 20' hull will start to go airborne at about 20 kts in seas of about 3' or more. If you often run in thosse conditions, you won't need (or be able to use!) more than about 115 hp! In my own case, I ran a old 115 (about 90 hp at the prop) for over 30 years, mostly offshore including a 1/2 dozen trips to the Abaco's, and I always felt that I had adequate power. When I repowered, I initially considered the 115 E-Tec, but they were not in production at the time, so I went with the 150. In hindsight, I have subsequently found myself taking extended cruises with as much or more weight than I have ever carried in the past, so I'm glad I went with the 150, although it took me about 6 months of experimenting to get it to ride and handle right. Even with a heavy load, I can easily cruise at 25 kts at 3700-3800 rpm and over 4 mpg avg, vs. 20 kts at 4500 and 2.8 mpg with the old motor! Plus I can easily cruise at 30 kts at ~ 4100-4200 if I have to, which is about as fast as the old motor would run at WOT!

On the other hand, if you only run in flat water in the ICW, then fine, hang 200 hp on it and let 'er rip!

Fr. Frank · #5 01-05-2011, 10:38 PM

Well said, Denny.

But then there's folks like me, who go from a 370lb Merc XR4 to a 375lb 90hp Optimax. Still, I've got all the power I need. I still plane easily with 6 adults on board, or 4 adults and all the gear for fishing. Okay, I can't blast along at 50+ mph anymore, but I did that only very rarely, anyway. With the V6, my average speed was about 25 mph. Guess what? With the 90hp my average speed is the same. And I use about one-third the fuel as before. And today, that's a huge issue.

uncleboo · #6 01-06-2011, 10:14 AM

According to the website, the Suzuki 140 4s weighs 410 dry. Although I have an 18, I put my batteries under the console which helped alot. I love the 140. It sips the gas, (sorry I don't have any numbers yet), and will run around 37 knots @ 5800-5900 rpm. The rear drains in the splashwell sit partially under water with nobody in the boat. When I get to the point of replacing the transom, I'll be raising it to 25" and the floor about 2".

eggsuckindog · #7 01-07-2011, 01:07 AM

THe EFI Mercs come in at about 400 also, that really should be your max - if going new the 130 Etec would be a nice fit

KenB · #8 01-07-2011, 03:20 PM

How does the tohatsu 90 or 115 compare to the opti? Good dealer network up here, and I think the 140 (or 135) whenever tohatsu releases it may be a good option for a 20SF repower. Thoughts? Father Frank wrote a great reply on another thread about the tohatsu, but since tohatsu and Opti share the same orbital technology, which one is better? For SF20?

Also, how much does a bracket help/hurt the ride of the 20? I know it is basically a no brainer on the 23, but it sounds like it might be less clearcut with the 20. (i.e. old carb'd 2 stroke on transom versus 4 stroke/DI on bracket). On the 23, most people are looking at v6 4 strokes, so the bracket is basically required...

LAST QUESTION: why not just get a rebuilt fuel injected 2 stroke? (Ox66, etc.) I know shrimp also had some great old merc options.

Bushwacker · #9 01-07-2011, 08:54 PM

Quote:

. . . Also, how much does a bracket help/hurt the ride of the 20? I know it is basically a no brainer on the 23, but it sounds like it might be less clearcut with the 20. (i.e. old carb'd 2 stroke on transom versus 4 stroke/DI on bracket). . .

The biggest difference I noticed was the significant increase in min planing speed. I was able to correct that problem with a 4-blade stern lifting prop and Doelfin. On a positive note, the geometry change with the 25" shaft and 30" setback evidently gives the motor more "leverage" on the boat, because power trim now has much more effect on running angle than it did with the 20" motor on transom, particularly at higher speeds. I believe this effect largely offsets the negative aspects of shifting the CG aft. If you have a large flotation tank on the bracket like the Hermco, that minimizes the effect on static trim, and seems to make the boat less sensitive to folks walkin around in back of boat or on swim platform. The waterline at the transom is within about an inch of what it was with original configuration; it's still self bailing at the dock, by about 1"!

Overall, if done right (big flotation tank and ~ 400 lb instead of 500 lb motor) I think a bracket is a great addition to the 20' hull. Elimination of the splashwell opens up a lot of usable storage volume and seating space in back of boat, and the big platform is great for diving and anchoring close to the beach!

Garrison · #10 01-14-2011, 07:24 PM

Thanks guys, plenty of ideas and advice to chew on. the weight is my biggest factor, not to jeopardise the ride. And will now consider putting a 115 on. Speed is not my main goal, but a quick plane is important. Fuel economy is obviously a big factor. I dont want to get into adding a bracket right now although the benefits are tempting. Just dont think it is absolutely necessary on the 20'. If anyone knows of a 115-150 good running motor give me a holler.