Noticeable Speed Diff - OMC 25 thru hub exhaust vs. Above Prop

**Stoker1** · 01-07-2017, 11:48 AM

Any speed difference between a Johnrude 25 with thru exhaust vs. above prop/sheer pin?

Kinda sorta in the market for a 25hp for the jon boat. As a teenager, I had an old 20hp above prop and it was ‘fast enough’ but these days of cell phones speedo apps, its a question I thought I’d ask.

Quick standouts are:
Thru hub = new technology - larger selection - and larger diameter. But for kicks and giggles, lets keep the conversation stock for comparison.

Would the above prop be able to get raised any higher?

**mrpenguin** · 01-07-2017, 04:03 PM

Much better prop section, better midsection and the ability to get the engine to breath better with the thru hub exhaust system.

You dont want the shear pin unless you are on a budget.

Great reading.

http://www.tinboats.net/forum/viewtopic.php?f=4&t=26650

**Instigator** · 01-08-2017, 07:33 PM

I ran a '73 25hp on a 10' inflatable all last season.
Great motor.
Pros's are its lighter than a through hub motor.

Cons's are, **** prop selection, broken shear pins and its loud if you run the motor high.

If you decide on that direction, mine is for sale.
Only due to finding a second one.

**FMP** · 01-08-2017, 07:40 PM

Love the sound of those wide open.

**johnboy 88 vegas** · 01-08-2017, 08:08 PM

When I was a kid me and my brother had an above prop 25 on a 14' aluminum boat....what a pos it was. It ran 1 mph faster than the 15 Gamefisher we pulled off the boat. Bought a brand new 25 Merc in 2003 and it was 9 mph faster than the Johnson....35 vs 26 mph.

**olboatman** · 01-08-2017, 08:10 PM

The shear pin motors have a better top end gear of 1.77 where the thru hub motors before 1982 have a 1.89 gear and newer fishing models are 2 to 1. Beleive it or not the snout (shear pin) LUs breath a lot better if you keep the cav. plate above the water(as you should) and the water pick gets water off the back of the prop so they can be run a lot higher without overheating. But you guys are correct about the prop selection as the thru hub models have a much larger amount out there. Another big problem with the shear pin LUs is the water pickup-exhaust snout limits the amount of rake the props can have and as you know this limits bow lift. I have been modding these motors for years as there are no end to parts world wide. Gary

**olboatman** · 01-08-2017, 08:12 PM

BTW The 25 can be made into a 35 with a carb and timing change!

Gary

**FMP** · 01-08-2017, 08:33 PM

Originally Posted by johnboy 88 vegas

When I was a kid me and my brother had an above prop 25 on a 14' aluminum boat....what a pos it was. It ran 1 mph faster than the 15 Gamefisher we pulled off the boat. Bought a brand new 25 Merc in 2003 and it was 9 mph faster than the Johnson....35 vs 26 mph.

For the record the Chrysler Force GameFisher 15 was no slug, out runs the omc 15, the 25 merc is just great.

**powerabout** · 01-09-2017, 12:42 AM

Can run the old omc higher than the thru hub assuming stock water pickups
Back in the day 25hp stock mono racing was big in oz the old gearbox was quicker
They were doing over 50mph
( OMC was king of the class right up till the Japs dropped propshaft rated engines on the market and the americans were asleep)

**Instigator** · 01-09-2017, 06:21 AM

25 OMC a slug?

When I raced kneel down the Merc 25 SS was a new motor/class.
One top ranked racer blew his up just before our Nationals. He borrowed the powerhead off his dads 25 OMC fish boat, adapted it to his Merc race motor/mid and won!

OMC owned every record in that class for yrs and still does.

I learned to do a deep water start on a slalom ski w/a 18 hp version of that motor in a 13' Whaler.
Id call that impressive.

**FMP** · 01-09-2017, 08:52 AM

I didn't say that the 25 OMC 's a slug.

**WillySteve** · 01-09-2017, 11:15 AM

Back in the day, my dad pulled 4 skiers regular with an old Tiller Control 35hp Sea Horse on a 14' fiberglass Lone Star!!!
He always had someone to sit in the front to hold down the bow!
Gaud.....that was back in the early 1950's!

**Stoker1** · 01-09-2017, 02:40 PM

Originally Posted by WillySteve

Back in the day, my dad pulled 4 skiers regular with an old Tiller Control 35hp Sea Horse on a 14' fiberglass Lone Star!!!
He always had someone to sit in the front to hold down the bow!
Gaud.....that was back in the early 1950's!

I'm amazed how they did stuff like that back then with such little HP. My folks did the same with an old Elgin and the way they talk about it, you'd think they were running 70.

**Instigator** · 01-09-2017, 05:49 PM

Remember too, 35 hp was as big as it got in the early/mid '50's.
Hence, twins being so popular back them. My dads/our first "big boat" was a plywood 18' cuddy w/twin '1954 25 hp rudes.