Going to a smaller OB?

ashedd

Petty Officer 2nd Class
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I have a 20’ plywood cabin cruiser that weighs 1100lbs empty from the factory. It was made in 1966 and is “rated” at 110hp. It came new with twin Merc 500’s. I’m assuming that the Boat was designed with twin OB’s in mind in ‘66. It has a very wide transom notch and splash well.

I yanked the Mercs when I got it and installed my old trusty ‘83 115hp evinrude. I feel it’s a tad over powered. With a very flat prop I get 5800 WOT(was anticipating maybe pulling a skier) and top out around 27 knots gps. Got 35-36 knots with old course pitch prop but struggled to hit 5500 rpm.

I think I’d do just fine with 90hp from the same vintage 2 stroke. Would love an e-tec but I’m not made of money. I feel I could plane with as little as 50hp. Minimum planing speed is roughly 13-15 knots gps.

So the question is..
How do you think an ‘85 70hp rude with flat prop would do? Not interested in trying to pull a skier anymore. And I’m content to cruise at 18 knots and wot just over 20. The weight savings alone would be about 100lbs. Currently I squat a little in the rear. The fuel savings is also a plus.

The goal is really a smaller hood footprint so I can make the splash and transom notch smaller. I need deck space! These are pretty rare boats and I love it otherwise.

There is a good deal on an ?85(crank or prop rated?) 70hp locally.
 

Scott Danforth

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I personally would change props
 

Stumpalump

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I'd put the 70 on it. those old OMC 70's put out as much as some 90's Save the max powered boats for the weekend warriors and save the fuel for the endless miles you will do at cruise speed. There are two cruise speeds. One the motor likes and one the hull likes. that 70 will be right in the sweet spot at the speed yours likes to go.
 

jimmbo

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" Got 35-36 knots with old course pitch prop but struggled to hit 5500 rpm.
"The "coarse" pitch prop was putting the engine at the top of the recommended rpm range(4500 -5500) for your engine. However you said "With a very flat prop I get 5800 WOT(was anticipating maybe pulling a skier) and top out around 27 knots gps."

Something's not adding up... you only gained 300 rpm and dropped 8-9 knots
What props are you referring to? Pitches please
 

Sea Rider

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When going from a max rated engine to a much smaller one while on same boat need to dial a less pitch prop to pull wot rvs higher towards its max range. Won't have the same benefits as before but will have fun running combo at much less speed with less fuel consumption and without the need to pull up a skier behind.

Happy Boating
 

ashedd

Petty Officer 2nd Class
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Jun 16, 2015
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14X11 on there now, the ?flat? one. I don?t remember the other prop sizes. Would have to go hunt down my spares. There is one a little more course I liked in stainless steel but it?s got a slight vibration I don?t like, even after a trip to the prop shop. I think I have three props for that motor. All aluminum but the one that vibrates a little is the black stainless.

I wanted around 5800 rpm with the anticipation of hauling four people, plus gear, and with the ability to pull something in the water. I never run at WOT. Maybe run at 2800/3000. But that was before my generic tiny tach quit working. I only used that Boat once last year. I also have a 30? twin mercruiser Boat. Only run that sucker at 7 knots due too fuel costs.

The first prop on there was for a runabout and it went fast, but only got barely into the low wot rpm with the motor trimmed all the way up just shy of slipping. I think it hit 4K rpm. So all in all I?ve tried four different props on there.

" Got 35-36 knots with old course pitch prop but struggled to hit 5500 rpm.
"The "coarse" pitch prop was putting the engine at the top of the recommended rpm range(4500 -5500) for your engine. However you said "With a very flat prop I get 5800 WOT(was anticipating maybe pulling a skier) and top out around 27 knots gps."

Something's not adding up... you only gained 300 rpm and dropped 8-9 knots
What props are you referring to? Pitches please
 

ashedd

Petty Officer 2nd Class
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Here's a vid of normal cruise for me with that boat.
 

Scott Danforth

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to dial in the prop, you need to be at WOT. you size the prop for WOT load and hitting the upper end of the RPM limit.

need actual RPM and GPS speed along with the exact prop you have on the boat at the time. specifically the part number, pitch and diameter. you can have 5 different props at 14 x 11p and they will all perform different because some may be bow lifting, some stern lifting
 

ashedd

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I?m not worried about the current setup.

I?d just like to know if an ?85 70hp Evinrude would be enough to push my boat at the speed I like to travel. I was only giving the current prop setup because someone asked.

Thanks for all the input. If I keep the current 115hp on there then I may get back to the props for that motor. But just to clarify, I have three different size/pitch props for the 115 Evinrude. The 14X11 being the one I use.
 

Scott Danforth

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your traveling upstream in a barbed wire canoe and your chain breaks, how many pancakes does it take to wall-paper the floor?

the prop on the 115 would have nothing to add to the conversation if your hell-bent on swapping motors.

as I stated, optimize what you have and you would be happier than dealing with swapping motors.
 

Sea Rider

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If plan keep using the 115 HP-OB and if having 3 spare props dial one that makes OB to run towards its max wot rpm range factory stated. Doesn't mean you need to run OB at full throttle all the time, once at max wot range as currently loaded can throtthe at the speed you like most with excellent fuel consumption idle to around 3/4 throttle.

If Tiny Tach installed on the 115 HP has had its Waterloo Time get a new original one, cheap ones doesn't last and may not read accurate. Need one to wot test your spare props including the one currently installed.

Happy Boating
 

jimmbo

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If you have to run the 70 at WOT to attain the speed you are cruising with the 115 at part throttle, I doubt very much you will be happy. Odds are the same prop used on the 115 will fit and be close on the 70. The lower units have different gear ratios(2:1 for the 115, and 29:12 for the 70), allowing a narrower range of prop pitches to fit various engine models. In your case the two engines mentioned use the same props.
 

Sea Rider

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OP will need to mentally dial that with a 70 HP although running at middle to max wot range, combo will achieve less top speed along less passengers transporting capacity compared to when he had a larger 115 HP. The nice issue is that a 70 HP consumes less fuel than a 115 HP if both say are 2 or 4 strokes OB's.

Happy Boating
 

jimmbo

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There won?t be much fuel saving, if any, if the 70 has to run full throttle to maintain the part throttle cruise of the 115.
 

Scott Danforth

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outboards burn fuel at 0.5# of fuel per hp per hour..... so depending on what you are commanding, that is your fuel rate.
 

jimmbo

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outboards burn fuel at 0.5# of fuel per hp per hour..... so depending on what you are commanding, that is your fuel rate.

Is that a WOT consumption? The last 20% of throttle raises the fuel consumption far more than 20%
 

Scott Danforth

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Its a typical 2-stroke BSFC (Brake-Specific Fuel Consumption) thruouth the RPM band there will be minor fluctuations, however it will get you close enough.

4-stroke I/O is 0.4#/hp/hr and diesels are 0.33#/hp/hr
 

Sea Rider

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OP won't possibly duplicate the benefits achieved with a 115 HP with a much less 70 HP. To have fun will need to carry way less passengers, prop OB right to rev middle to max wot rpm range and expect less top end speed. In reality it's not how fast you reach your destination point, for me enjoying the stress free ride in contact with mother nature while sipping some cold ones during boating is all...

Happy Boating
 

Stumpalump

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Here's a vid of normal cruise for me with that boat.

That was a great vid! I was wishing I was there. That motor sounded like it was hardly workin yet you were planing and cruising perfect. 70 will be perfect and probably better than loafin alone with a big engine that may carbon up. Flat light hulls are wonderful! Can you believe what the market has turned into? Shiny plastic and vinyl bling with hulls that perform so poorly that a boat of your size would need one or two big V8's. Ha ha y'all know I'm right again but since this post does not conform to the clique it will get deleted. Sent you a PM.
 
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