Older vs, Newer 25 outboard

[hovermn]

I have a '74 Johnson 25 hp short shaft on a 14' lund D/W. Speed is around 16-20 mph with 2 people, gear and a deck. This motor is not easy on gas, but it runs pretty decent.

How much of a difference would a newer motor have over mine. I think I remember hearing something about older outboards measured hp at the motor, while newer outboards are measured at the prop. Is this right? What year did they switch?

I guess I'd be happier with a more fuel efficient motor, a little better speed, and the peace of mind that goes with a newer motor. I was on the St. Croix between Hudson and just North of Stillwater yesterday, 16 miles round trip. I went through about 4 gals of gas. That can't be good, right?

Think I'd gain anything from a '90's outboard over the 70's?

[Jack Stasney]

I had a 70's 18hp Evinrude on my old 14ft boat. I could do about 17-20mph when light, and about 13-16mph when loaded up. If you went 16 miles and used 4 gals of gas compared to what I know mine use to do I'd say that sounds about right and maybe even good. As far as the peace of mind goes well... I used to love my old motor cause the compression checked out good and it always ran. There is a lot of room for opinion there.

[boatfixer]

They started to switch from crank shaft to prop shaft rating in 1983.

[hovermn]

I guess I really didn't have a basis to go by with the fuel, just thought it was a bit much. I have to be honest, I'm used to flying a hovercraft, which got much better mileage.

I know I lost some speed when I put a deck in the boat. The addition of 2 adults, gear, 2 batteries and trolling motors. The boat is weighted down some.

Any idea what the 70's 25 hp measures up in HP to a modern engine? Are we talking 2-3 hp, or isn't it a huge difference?

[polarsusd81]

I think you are probably fine with the motor you have, though a newer fuel injected motor or 4 stroke will get better mileage. I have a '76 14' Lund, pretty much the same boat with a flat floor, no deck up front though. I also run a transom mount t-motor and the battery is in the back. I have a 82-83 Mariner 15 horse longshaft. I have clocked with GPS at 31 with just me and gear, 25.5 with two guys and gear, and 17-18 with three guys and gear. My question to you is related to your prop. What do you have for a prop on that motor? I used to have a 12p prop on mine, broke a blade off and decided to replace it with a 10.5p prop. I decided to do that because I could just feel that the motor wasn't working to its full potential. I put that 10.5p prop on and she roars across the lake now. I would guess you probably have a 14p or 15p prop, and stepping down to a 12p or 13p prop might get her going for you. But that is only if the motor isn't revving as high as it should.

[polarsusd81]

I should have added...does the boat pop right up on plane or does it plow a bit before she goes up on plane? When you are in glass calm water how much faster can you go than when you are in a windy/choppy situation?

[boatfixer]

Any idea what the 70's 25 hp measures up in HP to a modern engine? Are we talking 2-3 hp, or isn't it a huge difference?

Approximately 10% of HP is lost in the gearcase. So yea, about 2 1/2 hp

[hovermn]

I can't read the pitch on my prop now, but I've been told it's a power prop, light on the pitch. I've tried 2 props with larger pitch, but it's pretty boggy and gave a great reduction of speed.

As far as planing, it doesn't pop right up, but takes a little bit. It also seems to take the motor a while to reach full RPM.

[Hula_Grub]

I bought an '07 Lund 1425 Classic tiller last year w/ a 25 HP Merc 4 stroke. Heavy boat for the size. Long story short, for performance and safety reasons I went from the factory 9.25" X 12.5" aluminum prop to a 10" X 9" composite prop.

I can now get 22 mph (gps) at 95% WOT with the livewell full and my 225 lb rear end, all in the back. It pops up on plane in about 5 to 7 seconds. I'd say I burn about 1 gallon per hour.

[DTro]

I'd say I burn about 1 gallon per hour.

wow, that seems pretty good. At 22mph, that's 22 mpg which would equal almost 150 miles per 6 gallons.

I have a larger 14ft with a late 80's 25hp merc. I'd estimate that I can go about 30 miles at WOT per 6 gallon tank. That's with 2 guys/gear/etc

I don't think the mileage in the original post is all that bad.