Yamaha F70 on a 20 MA?

FishStretcher · #1 09-18-2021, 10:15 AM

Toying with the idea of a rehab- cleaning the carbs and fixing the tilt/ trim on my Yamaha F100 and selling it.

Then finally fixing the transom on my 20 MA with coosa and divinycel.

A Yamaha F70 with EFI is 253 lbs. Versus 363lb or so for the F100.That and a foam transom, and needing and carrying less fuel has me wondering.

I think this is a question for Fr Frank.

This boat is too small and light to go fast in a Buzzards Bay chop. I always try with the F100 and never can air it out. This may astonish some of you, but it gets airborne too often with just a F100. I can't get to 30 statute MPH on any sort of regular basis.

This would never work for a Florida boat, I know. But a new, light EFI engine is appealing. And kind of fits with the Mosely vibe of "light is right".

No raising decks, no worrying about flooded scuppers, no worry about center of gravity...

DoozleD · #2 09-20-2021, 07:44 AM

i had a f115, the thing was a dog. had no balls.

bumpdraft · #3 09-20-2021, 03:15 PM

A long time ago, I was fishing/camping with several friend in the Keys. One of my friends had a 22Aquasport with twin 70s. One of his motors quit on him and he couldn’t quite get the boat to plane going home. We threw him a long rope and we cruised the 25 miles or so back to dock. I think if he could have ditched the dead motor and maybe put a different prop on it, he could have made it without any help.

jorgeinmiami · #4 09-20-2021, 05:56 PM

Better have more and not need it than need it and not have it

77SceptreOB · #5 09-20-2021, 06:58 PM

Quote:

Originally Posted by bumpdraft

A long time ago, I was fishing/camping with several friend in the Keys. One of my friends had a 22Aquasport with twin 70s. One of his motors quit on him and he couldn’t quite get the boat to plane going home. We threw him a long rope and we cruised the 25 miles or so back to dock. I think if he could have ditched the dead motor and maybe put a different prop on it, he could have made it without any help.

So you pulled him up on a plane and then retrieved the rope and and both boats cruised home on a plane?

bumpdraft · #6 09-20-2021, 07:29 PM

No, we did try that, but he could not keep up, so we hooked up again and just stayed that way. There were no waves to worry about. Maybe if we transferred all weight to another boat, it would have been better and I think that could be an issue on the 20. Keep a light load and put the right prop on and I think it would work.

Fr. Frank · #7 09-21-2021, 06:41 PM

I actually did rig 1 (one) 20' with an Evinrude 70 back in the early 80's. It could get the boat on plane but was clearly inadequate. But customer is always right, eh?

I think 85-115 hp is the right HP range for the 20' if you're worried about weight. Your F100 is a good motor . If you're getting airborne in the heavy chop trying running with the motor trimmed all the way down. Adjust your propeller torque tab for that configuration. Alternatively, and better yet, let me suggest adjustable trim tabs. Keep the nose down and it's much harder to get airborne.

The 20' was designed for a single 275-315 lb outboard, or in the case of some of the wide transom cut-out models of '68-73, up to twin 50-65 hp motors at 190 lbs each.

The very first Seafari I rigged back in '83 was a '69 re-rigged from its original twin 190 lb '69 Johnson 55 hp 3-cylinders to a single Mercury 140 ELPT inline 6 weighing in a bit over 300 lbs

DonV · #8 09-21-2021, 07:35 PM

It's a 1000cc motorcycle engine that works great on the 16', to maybe an 18' "technical skiff" that weigh nothing......on a 20' SeaCraft get real!!

Too blunt? Sorry, Yamaha is known for pushing the low end of the 10% variance on their engines' HP ratings!

bumpdraft · #9 09-21-2021, 08:24 PM

Quote:

Originally Posted by Fr. Frank

I actually did rig 1 (one) 20' with an Evinrude 70 back in the early 80's. It could get the boat on plane but was clearly inadequate. But customer is always right, eh?

Just curious, was that motor rated at the crankshaft?

FishStretcher · #10 09-21-2021, 08:36 PM

Fr. Frank- For some reason, I thought you rigged a 75 HP once, but I couldn't find a reference.

I noticed one of the recommended power options on the Mosely site was 80 HP for a 20CC and stated 35MPH top speed. But a 1400lb hull weight.

I don't run a heavy boat and was estimating the mass benefit of a foam transom and a 110 lb lighter outboard. I wish the 90 was a bit lighter, but it's not.

Quote:

Originally Posted by Fr. Frank

I actually did rig 1 (one) 20' with an Evinrude 70 back in the early 80's. It could get the boat on plane but was clearly inadequate. But customer is always right, eh?

I think 85-115 hp is the right HP range for the 20' if you're worried about weight. Your F100 is a good motor . If you're getting airborne in the heavy chop trying running with the motor trimmed all the way down. Adjust your propeller torque tab for that configuration. Alternatively, and better yet, let me suggest adjustable trim tabs. Keep the nose down and it's much harder to get airborne.

The 20' was designed for a single 275-315 lb outboard, or in the case of some of the wide transom cut-out models of '68-73, up to twin 50-65 hp motors at 190 lbs each.

The very first Seafari I rigged back in '83 was a '69 re-rigged from its original twin 190 lb '69 Johnson 55 hp 3-cylinders to a single Mercury 140 ELPT inline 6 weighing in a bit over 300 lbs