79 Evinrude 100hp, 20ft pontoon

[SoArkSi]

I have a 20ft sweetwater pontoon boat with a 79 evinrude 100 hp outboard on it. The dry weight on the boat is 1541lbs and wet is around 2154lbs. The motor weighs 301lbs, I dont know if either one of those weights includes the motor. The motor gear ratio is 13:26, it currently has a 13 dia X 19p stainless steel prop that looks to be cupped. with the boat modderately loaded down the boat runs 4400 rpms and 23.4 mph. My service manual says the ideal rpm for the boat is between 4500 and 5500 rpm with peak rpm being 5000, which is where I woule like to be or no more than 5200 rpm. I have looked through all the calculators online but cannot find one that will go from my cupped stainless prop to a non cupped alumuinum, because I am goin to get a Solas hub and a 3 blade prop and a 4 blade prob to try using the single hub to save money. I will be mostly crusing and maybe pulling a one man tube every once in a while. If anyone can help me find a size of a 3 and 4 blade aluminum for the 5000 rpm area or guide me to a good calculator that would help go from cupped stainless to non cupped aluminum would be great. I read through the "read first" post and believe I got all the info needed. Thank you for any help.

 

[SoArkSi]

Re: 79 Evinrude 100hp, 20ft pontoon

I was thinking either a 13-3/10 X 17 3 blade solas rubex and a 13 X 15 4 blade solas rubex that i can use the same hub for and save some money. The 3 blade should get me close to the 5000 rpm goal and i think the 4 blade should be close to since they say you should move to a smaller pitch when going to a 4 blade prop. Any suggestions would be great.

 

[steelespike]

Re: 79 Evinrude 100hp, 20ft pontoon

I think you have way too much prop.I think you need a prop around 15" and cup is a good thing especially on a pontoon where ventilation can be a problem.Also If I was replacing a
pontoon prop I would go to a 4 blade which also resist venting well.
When you change materials prop selection becomes even more unpredictable.
The solas props have a good performance record. I would get one 4 blade 15" pitch and evaluate its performance then select a second prop as necessary. A 4" pitch change should get about 600-800 rpm. Your motor is better off being able to rev freely within its rpm range.
The lower the rpm the harder it will be working especially on a pontoon.

 

[SoArkSi]

Re: 79 Evinrude 100hp, 20ft pontoon

That sounds good, they have a 15 4 blade in the hub adapter style I wanted. I guess I just need to call solas and see what kind of cup it has or if any because I would like to have 2 props to try out when I take the boat back out