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  25hp four stroke vs 40 hp 2 stroke

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Author Topic:   25hp four stroke vs 40 hp 2 stroke
2manyboats posted 07-16-2004 11:58 AM ET (US)   Profile for 2manyboats   Send Email to 2manyboats  
Before I send one of the 13s off to college , my wife and I plan to run both 13s this weekend in side by side tests.
Her boat is a 1973 sport with a 2000 25 yamaha 4 stroke.
My boat will be the 1976 sport with 1992 40hp evinrude . I will compare top speed, fuel consumption and crusie speeds . No tach on either boat so some things will be guess work. I will post results.
2manyboats posted 07-18-2004 10:22 PM ET (US)     Profile for 2manyboats  Send Email to 2manyboats     
First let me say none of this is scientific , no tachs , unknown size prop on the 40 hp, just me and my wife boat riding and drinking beer.
We ran about 25 nautical miles over a 4 hour period in smooth to 1ft chop. About about 20 miles were a cruising speed (18 to 20 kts) the rest were idle speed or wot.

1973 13 sport with 25hp yamaha 4 stroke bimini top was up , one person on board started with 6 gal. gas and about 30 lbs. gear.
wot : 24kts
best cruise speed : 18kts
fuel used: 1.75 gals
nautical miles per gal. : 14.29

1976 13 sport 40hp evinrude 2 stroke , no top , one person on board , started with 6 gal gas and about 30 lbs. gear
wot : 31 kts.
best cruise: 20 kts.
fuel used : 4.3 gal.
nautical miles per gal. : 5.81

We did switch boats during the day and while the 4 stroke is smoother and quieter idleing after you get up to speed there was very little difference.
What did surprise me was how little difference the was in top speed, the 40 got there sooner , but I was thinking the 40 would run closer to 35 kts. Without a tach or knowing the prop size the may be more there.

Clark Roberts posted 07-19-2004 06:55 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Trim and tilt makes a whale (pun intended) of a difference in top speed and if you didn't have it on that 40hp then that's most likely why only 31knots top speed. With T&T you can find the "sweet spot" just before porpoising and the spot where wetted surface (read friction) is minimal. Clark... Spruce Creek Navy
DaveNJ posted 07-19-2004 07:47 AM ET (US)     Profile for DaveNJ  Send Email to DaveNJ     
Have you tried the 4 stroke pulling a tube or kneeboard?
How is it in calm waters pulling < 200 lbs. ?

My OMC 2 stroke 25hp is OK with this. I was wondering about power differences (between a 2 stroke and 4 stroke) if I got a 4 stroke in a few years.

thanks,
Dave

2manyboats posted 07-19-2004 08:17 AM ET (US)     Profile for 2manyboats  Send Email to 2manyboats     
Yes the 40 does have tilt and trim. I plan to move the gas tank to forward of the front seat. I will probable try to get a tach from my outboard mechanic just to see where we are rpm wise.

We have not pulled skiers or tubes with the 25 but the 4 stroke seems to have the same or better power than the 2 stroke it replaced.

Royboy posted 07-21-2004 01:33 PM ET (US)     Profile for Royboy  Send Email to Royboy     
Interesting test. There was a thread going on the GAM where I freaked a guy out by saying I got 33 MPH with my 25 hp 4 stroke, which turns out to be 28.7 Knots. I imagine the difference could be partially in the device used to measure speed (in my case a NAVMAN fish finder). I've found, from racing karts, that there can be wide differences in small gas engines, even stock ones with sequential serial numbers. The 24 kts you got at WOT might have even been closer to my 28.7 without the bimini.

How does the weight of the 40 hp compare to the 25? Apparently, there are people that don't comprehend that the engine, regardless of it's hp rating, must propel its own weight through the water too, so that when they add what seems like a lot more hp, in this case 38% more hp, they expect 38% more speed and it just don't work that way. In your experiment you got 23% more speed for a 38% increase in hp, which is quite good, except that it really cost in terms of range.

I believe the current 130 Sports are motored with either the 25 4 stroke or the 40 2 stroke. One is for those wishing to fish in style all weekend without stopping for gas, and the other is for those wishing to pull a skier.

Roy

2manyboats posted 07-21-2004 01:52 PM ET (US)     Profile for 2manyboats  Send Email to 2manyboats     
The speeds on these two boats were measured with garmin hand held gps.

The 25 came with a 9 7/8 x 9 aluminium prop even without a tach I could tell we were over reving . I changed to a
9 7/8 x 10 1/2 alum. We added 2.5 kts ,but still are not sure where the rpms are. The boat still jumps up on plane , so if I changed props again and ran with the top down we may be close to your number.

When we bought the 25 we thought we were giving up alot but that may not be the case.

Royboy posted 07-21-2004 02:12 PM ET (US)     Profile for Royboy  Send Email to Royboy     
My 25 has a 10 X 11 pitch prop on it, which might account for some more of the difference. It jumps up on plane with just me in the boat, but with another person it takes a bit longer.

Roy

Mumbo Jumbo posted 07-21-2004 09:09 PM ET (US)     Profile for Mumbo Jumbo  Send Email to Mumbo Jumbo     

I have clocked my 13 Whaler powered by a 25 Yamaha four stroke with power trim and tilt, using a GPS and I averaged, wot, 27.3mph (two way run) with one person on board, 6 gallons of gas, my usual gear and a cooler full of ice. I think it runs about the same with three people. The prop is a PowerTech Marine stainless 3 blade, 10” dia, x 11” pitch with cup (Model # YM30SRA3R11s-2) which is much superior to the aluminum prop that comes with the motor. I have no complaints regarding the acceleration. The power trim and tilt make this motor a fine match for the 13'. I would not recommend this engine without the trim and tilt feature.
jimh posted 07-22-2004 08:31 AM ET (US)     Profile for jimh  Send Email to jimh     
Part of that fuel economy difference (almost three times better with the four-stroke) has to be the difference in horsepower, too. Even the manufacturers claim only about a 40-percent improvement in fuel economy for a four-stroke engine.
pglein posted 06-22-2005 02:27 AM ET (US)     Profile for pglein  Send Email to pglein     
Sorry to drag up an old thread, but after I saw this linked in another thread, I thought I'd simply comment and say that I pretty much concur with the results based on when I repowered from an '88 Evinrude 40hp to an '01 Suzuki 25hp. The primary difference is in the weight of the engine. The smaller, lighter engines simply take less power to push around, and thus, there are definitely diminishing returns as you go up in HP.

But I wouldn't discount that 7kt top end difference. That's serious speed right there. But unless you're just a speed demon (like me) I don't imagine it matters much so long as you've got the power to cruise at the speed you're comfortable with while carrying the gear and people you wish to.

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