Opinions on if I should repower 18

[Bandit91]

Originally Posted by uncleboo

Yes, yes and yes. Check out my thread 'deck replacement'.

Just read through your thread, very informative. That’s a shame about your motor. But you did an amazing job on your boat. It looks great. Going to be a really nice setup with your new power.. Looks like your motor was a 25” shaft on a jack plate, is that correct? Or was it a 20” on the jack plate?

[otterhound]

I have an 18 SF with a 20" transom and '09 Mercury 90 hp 2 stroke. Weight wise at 303 lbs it seems perfect for our 18. It will cruise all day at 25 -28 mph, plane down to 14 -16 with out tabs and top out at 36 -37 with one aboard. It has a 4 blade aluminum Apollo prop, a Doel Fin and the ventilation plate is 1 1/2" above the bottom of the boat. It previously had a '92 Evinrude 120 hp. When we got the weight out of the back of the boat and the motor height set correctly the boat absolutely came alive. Although it's not a rocket ship it's nicely balanced, great in a head sea and economical to run. Best of luck!
Rod

[DonV]

"BTW, the F70 and F90 are exactly the same in terms of weight, dimensions, etc. Only difference, I believe, are the stickers and ECU...and the price"

Not true, the 70 is 1000cc and the 90 is 1800cc, the 90 is much heavier, 100# more. The 70 is only available in a 20" shaft and the 90 comes in 20 or 25". The 70 is basically a motorcycle engine with great HP/weight ratio. It was designed for the 16 to 18' technical skiff market.

https://yamahaoutboards.com/en-us/ho...15-50-hp/70-50

https://yamahaoutboards.com/en-us/ho...115-50-hp/f115

[DonV]

Real quick, back on post #7, the Optimax I mentioned is a 1.5L 3 cylinder for both the 115 and 90, they weigh the same. If they are still available the 115 Opti would work nice on a 18' SeaCraft.

[bmajvi]

My '75 18 footer had the 20'" transom. Back in '02 the Suzuki four strokes hadn't built up a reputation yet, and I found a 20" DF 140 that the dealer wanted to move. This was in the West Palm Beach area, where there's a good deal of flat water on the Intracoastal and sometimes outside, so you have an opportunity to run fast - but also occasionally pretty rough inlet running and ocean seas. My boat didn't have trim tabs, and don't remember checking low speed plane, but I don't recall feeling that it was an issue. I did however scare the living heck out of myself the first time I tried backing down on a fish out in the open ocean with a little sea running (the boat didn't come with washboards). In short order I made up some hinged boards out of Starboard, which allowed the motor to tilt when folded, and they helped a lot in keeping water out of the boat. I knew the motor put a bunch of weight aft, so I moved the batteries under the console and tried not to load heavy gear in the stern. As Don said a few clicks back, it's a good idea to first try to figure out how you're probably going to be using the boat, and go from there.

[mikeyk]

Originally Posted by DonV

"BTW, the F70 and F90 are exactly the same in terms of weight, dimensions, etc. Only difference, I believe, are the stickers and ECU...and the price"

Not true, the 70 is 1000cc and the 90 is 1800cc, the 90 is much heavier, 100# more. The 70 is only available in a 20" shaft and the 90 comes in 20 or 25". The 70 is basically a motorcycle engine with great HP/weight ratio. It was designed for the 16 to 18' technical skiff market.

https://yamahaoutboards.com/en-us/ho...15-50-hp/70-50

https://yamahaoutboards.com/en-us/ho...115-50-hp/f115

I stand humbly corrected. I was using the F75 as a comparison. It appears that the F70 and it's weight difference between the F75 is better suited for lighter boats where 70 hp is the upper end spec for mfgrs. power and engine weight, i.e., skiffs. Pushing a heavier boat with a 70 at the 5800 Rpm powerband range would probably require smaller diameter and/or lower pitch prop to utilize full horsepower. I noticed that Yamaha does not divulge torque specs. but using the formula
Torque = Hp x 5252 / Rpm (5252 is a constant)
the F70 yields 63.4 ft/lbs. @ 5800 rpm, the F75 71.6 ftllbs. @ 5500 rpm

[kmoose]

I think Terry has a 90 Etec on his 18 and it does fine.

[uncleboo]

Originally Posted by Bandit91

Just read through your thread, very informative. That’s a shame about your motor. But you did an amazing job on your boat. It looks great. Going to be a really nice setup with your new power.. Looks like your motor was a 25” shaft on a jack plate, is that correct? Or was it a 20” on the jack plate?

20" transom, 20" shaft and I raised the transom 3". 2-1/4" would have been better due to hydraulic steering head, but, I have it worked out. I went with the 3" jack plate solely for the purpose of adjusting the height of the motor without removing bolts.

[Beaver]

Some more info for the 90...

Bocaseacraft sent me a PM that said his 90 Suzuki goes 34 to 36 mph and has been averaging 6 mpg. In my book those are great numbers for an 18.

[Bandit91]

Originally Posted by Beaver

Some more info for the 90...

Bocaseacraft sent me a PM that said his 90 Suzuki goes 34 to 36 mph and has been averaging 6 mpg. In my book those are great numbers for an 18.

That’s good to know, I might send him a pm and talk to him about his setup. Thanks for the info.