What would you do.

[David Young]

I messed up my prop at the end of last summer and bought a new one. The old prop didn't have any numbers except for the diameter of 13.25, i looked at my factory repair manual where it said most motors were shipped out with a pitch of 17. I am now using a Solas Amita this is what i had with the old prop and now with the new prop.

Both props plane in 4 seconds.

Old prop, 13.25 don't know the pitch GPS top speed 35.8 mph at 5,500 rpm.

New prop. 13.25X17 GPS top speed 34.3 at 5,000 rpm.

Motor sounds good but is it in your opinion "Lugging"? (if this is the right word.)

I'm okay with the 1 1/2 mph top speed loss with 500 rpm less, but would it be worth it to buy the 13.50X15 (this is the next size offered from Solas) to bring the rpm up some?

1977 MFG 15 foot with a 1977 70 horsepower Evinrude motor.


Thanks for your help

 

[Sea Rider]

Check factory wot rpm range for that OB to know if boating inside its wot parameters, lugging an engine is revving at or less than MIN wot rpm range.

Happy Boating

 

[David Young]

The factory manual says "recommend full power operating range is from 4500 to 5500 rpm. In order to get the best performance from your outboard the upper range, from 5000-5500 rpm, is the engine speed to use in selecting the proper propeller".

I guess 5000 rpm is okay. I should have read my manual

Do you think i would get my 1 1/2 mph back with the 13.50X15 propeller?

 

[bruceb58]

You can have the old prop repaired and the repair place can measure the pitch of it.

 

[David Young]

You can have the old prop repaired and the repair place can measure the pitch of it.

A new one just cost $100. I'm sure it would cost more than that to repair a bronze prop. It 'was' a beautiful bronze prop

 

[Sea Rider]

Assume you have a tach in good working order, was 5 K wot rpm achieved with you alone on flat calm no wind water cond ? going one less pitch from old one should bump your OB 300-500 + wot rpm which will be great. Should achieve beter hole shot, carry extra load than as before, top speed should drop a bit, not that wow though.

Engine will work much nicer throughout the whole throttle range with better furl economy. All props should be tached, see if possible to borrow one fron a prop shop and test, nobody likes investing on trial props to find after wot test that doesn't work as expected..

Happy Boating

 

[David Young]

All of my test were done with my wife and i and two 6 gallon gas tanks full. My 'tach' appears to be working right. I rebuilt my carburetors with all new parts, even new floats, at the same time i ordered my new 13.25X17 prop.

Is it 'possible" i lost the 500 rpm with me doing something wrong when i rebuilt the carbs?

When it warms up i was going to put the old prop back on just to see if i get back the 500 rpm. Then i would know if i messed something up with the carbs.

I checked to see if i was getting 'full throttle', i am.

Thanks for your help. I'm trying to learn all i can at my 'old' age. I'm changing the water pump in the next couple weeks. I have the factory kit that came with everything needed

 

[Sea Rider]

Try both props with you alone and check both wot runs, having already both props, stay with the one that gives near or top 5500 wot rpm. 1 +.- pitch can account for as much as 500 +.- revs gain or loss. When boating with wife, max rpm will de reduced accordingly. Is that a 2 strokes OB ? have you tried carbon guard products squirted through carbs, works fantastic, can restore power loss if powerhead has not been ever decarboned inside.

If you regularly go boating with wife, try to match a prop that achieves 5500 wot revs with both of you and fuel, will achieve a better hole shot, carry more load and engine to run nicely throughout the whole throttle range including nice fuel consumption.

Happy Boating

 

[steelespike]

Rpm range is 4500 to 5500. So your right in the middle.
That Bronze prop will very likely out perform the Solas al. prop.
The design is more sophisticated ,thinner blades etc.
I'm thinking your tach is off slow to make 34.3 with the 17" Solas prop you would need about 5800 rpm.
At 35.8 mph and a 17" bronze prop you would be doing about 6100 rpm.
Your numbers calculate to about 0 slip,not possible. Typical runabout slip is about 12%.
Slip is the calculation to determine the actual forward movement through the water.
​

 

[David Young]

Rpm range is 4500 to 5500. So your right in the middle.
That Bronze prop will very likely out perform the Solas al. prop.
The design is more sophisticated ,thinner blades etc.
I'm thinking your tach is off slow to make 34.3 with the 17" Solas prop you would need about 5800 rpm.
At 35.8 mph and a 17" bronze prop you would be doing about 6100 rpm.
Your numbers calculate to about 0 slip,not possible. Typical runabout slip is about 12%.
Slip is the calculation to determine the actual forward movement through the water.
​

I was hoping you would come across my Thread and do your 'Math Magic' with what i have going. This will give me something to work on when it warms up. I was worried more about maybe my motor was 'lugging', which it isn't. Thank you all for your help.