70 hp evinrude prop question

randyray41

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I have a 78, 70 HP on a 13' boat. I'm running a 13X19 aluminum prop. The cavitation plate is even with bottom of boat. Motor is as high as it will go. My problem is with the 19 prop i'm turning 62-6300 RPM wot with motor trimmed all the way down. 43.9 mph GPS. It looks like a 21" prop is the highest pitch available for my 13 spline shaft. I don't believe that will be enough, especially if I want to get it trimmed up. Any suggestions, would a 19" SS make any huge difference?
 

Sea Rider

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Cav plate even with bottom is no indication that OB will perform top as it's just a refference. Trim OB for combo to ride parallel to water level when on plane, report max wot achieved.

Other alternative would be repitching same prop going one more pitch to 20 to reduce over revving. Was combo light, medium light, fully loaded when tested ?.

Can you post a lateral pic of lower leg showing both upper and lower plates ?

Happy Boating
 

randyray41

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These pics are not exactly what you want and all I have at the moment. Its light, just me 170lbs, 9 gallons of gas and a battery, plus safety equipment.
 

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Sea Rider

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First pic is OK, will work on that one. What's the min-max wot rpm factory stated ? You plan to go boating as currently loaded most of the times or maybe adding 1-2 more passengers ? Bear in mind that if adding more passsengers wot rpm will drop accordingly.

Happy Boating
 

randyray41

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Min/max is 4500-5500. Max persons would be one more. Even then it still runs about 6k. Trim all the way down. I know once it's trimmed up to get bow on up it will get on pad and really run. But the rpm just to high. I trimmed and it was almost 6500 but boat was riding like it supposed to for speed. Only did it for 20 secs. Old motor don't want to have the insides on the outside.
 

jimmbo

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Turning Point still makes a 23 in prop for your motor

prop part number 21432311

Hub Kit part number 11500300

It is Al. their SS only go up to 21

Myself, I would be looking at Higher Performance props allowing for higher engine heights
 

Sea Rider

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To check correct OB/transom height, throttle as usual and when boat is on plane trim OB at 90? to have hull and anticav plate riding paralell to water level, for that combo must be well deck balanced.

70 HP.JPG

Under those suggested conditions, try to adjust OB height on transom for water flow at speed to pass right under the yellow line, there's an edge there that will cut flow nicely. For that will need to have somebody riding next to you and checking water flow is passing undisturbed under small upper plate. Personally would try a 21 pitch, should lower wot revs to around or near 5.5K wot revs. If need slight less revs could re pitch that 21 to a 20, for that will need a wot test on flat calm no wind water cond.

That boat is max HP transom rated for ? 70 seems too much for that size boat, besides OB mount is offsetting OB weight aft.

Happy Boating
 

randyray41

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The HP placard is under the control box. I have to take it off tomorrow to see if I can figure out what is wrong with kill switch. Im curious to know what its rated as well. The previous owner had a 115 HP hung on it at one time.

I believe I'm going to order the Turning point 21P prop and see how it does.
 

randyray41

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Tach is brand new and correct. Idles at 700 rpm and its spot on up to 3k rpm. I would assume it would be correct further up as well. Had boat mechanic check it with his.
 

phillnjack2

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Apr 30, 2011
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first of all you can buy a 13.5x 18 or 13.5 x 20 or 13.5 x 22 and even a 24 pitch in the omc raker series for this engine.

you need to raise engine up so centre of the propeller shaft is about 3 inches under the bottom of the boats transom.
forget about the cav plate, you dont need to worry about that for now. and at wide open throttle the cav plate should be nowhere near the water to get a bit of speed.
get a long tube and fit to the tell tale so you can have that up the front to watch to make sure you get good water flow from it, or better still get a water pressure guage.
The 13 x 19 alloy is rubbish compared to the 13x19 sst from omc, they also make a 21 sst and a 23 sst for your 13 splines.
also you can get a 13 3/8x 21 ballistic that has huge amount of rake.
Now dont worry about over revving as that is not going to happen, these engines can rev to 6500 all day long and not harm them.
your max power is at around 5800 to 6200, forget what it said about the 4500 to 5500 in the brochures. that is for the cruiser brigade and those who dont understand these engines at all.

forget about ordering a brand new prop, go on ebay there is thousands that will fit your motor.
a 13 x 21 omc sst can be found cheap on ebay, and the best for you would be a 20 inch or 22 inch omc raker, that will give you 13.5 x 22 pitch and go like a rocket.
there is plenty of them around and the older ones are better than the new ones made under license by solas, the newer raker is weaker.
I have 8 different props and ranging from 15 pitch to 23 pitch, and I would like to get my revs up to around 6500 with a 19 pitch.

your prop slip figures are not to bad but could be better, get the tank and battery as far back as possible to let the bow come up and get boat moving quicker.
if your boat rides level your losing tons of speed, it needs to be bow up and engine level with water if possible, less wetted area and good thrust from prop.
this is where the raker props come into play, they get the bow up and give real good top speed.
with a 22 raker youl be going pretty fast and being a vented prop your holeshot wont be too bad neither, but if you want fast pull away then a 13x19 omc sst and vent holes will be the right prop. top end wont improve much more than 2 mph, but getting there will be better.
 

phillnjack2

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oh and any prop under 14 inch diameter for omc 13 spline will fit, so also keep eye out for cheap 13 spline v4 props as they also fit.
also take the fins off the engine and get the tabs up a bit, they are causing drag big time.
they might help you get on plane but are killing your speed by keeping the front down too much.
 
Last edited:

phillnjack2

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here is some of my props, all fit the omc 13 spline apart from the ballistic that will when I get new hub.
best for my 70 is a 13x 19 and 12 3/4 x 21, and running with propshaft 3 inches under transom and still getting very good water pressure.
 

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randyray41

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Phillnjack2. You added a whole
New twist to my dilemma. The trim tabs are 30lb water adjustable. They self adjust. The whale
Tail I just put on to try to keep
Boat from porposing so much. Had no effect so it will be coming off. Boat will beat you to death unless it's wide open and the trim is all the way down. I think
I need weight in front of boat.

You are the first I've heard that going over manufacturers suggested rpm range was ok.

I will look on eBay and see if I can buy a suitable prop at a reasonable price.

I will measure center prop shaft and report where it sitting.
Thanks for your input.
 

randyray41

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randyray41

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I'm about 7" from center of prop shaft to bottom of boat. But I can't raise motor any higher. Motor is as high mounted it will go.
 

Sea Rider

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Water flow at speed must pas necessarily under small watwer deflector plate (yellow line) for water to fully cover both middle side holes you see on pic, right where the doel fin begins. Those oldie OB's have very limited plates heights between them to play than newer OB's.

If you raise too much OB towards cav plate, prop will lose best thrust as there's less water height around prop to grip as intended, good for straight water courses, at tight close turns, on choppy and windy waters OB will perform awfull, ecessive prop aereation.

Some issues that don't add for combo to work properly :

-13 footer boat
- 70 HP OB
-OB riser (offsets OB's weight aft)
-Solo boating (poor como balance)

Happy Boating
 
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