1986 Sea Nymph Fish n ski SS155 prop selection

[Piece715]

Ok I currently just got my 4 year restoration on the water... I bought a prop long before I knew what I was doing. I feel now that I'm not getting all the performance I can and feel the RPMs @ WOT are a little low. Really not looking to run into a lean problem running at the low end soooo here is all the information I have to the best of my knowledge. Please ask if ya have any questions! Thanks in advance for the help!

Oh and pic of the restoration on the water cuz im a proud poppy
photo(7).jpg


1) Performance issue you are trying to correct. - None per-say but feel my WOT RPMs are a little low

2) Current prop manufacturer, model, aluminum or stainless as a minimum. - Current is SS.. Pics attached with info... photo.jpg photo(2).jpg photo(3).jpg model SRG3R15P

Now the prop that was on the boat when I received it was an aluminum 3 blade 12.25" x 15 pitch; It is beat to heck and the rubber is shot; I believe the original motor manual recommended aluminum 11.75" x 17.

Took a hot minute to get it off the prop shaft which had to be scrapped and cleaned as the SS wouldn't even slide on. That is all fixed and again pictured stainless prop is on boat.

3) Current prop diameter and pitch (required). 11.75" x 15

4) Wide open throttle RPM and speed with an average load (very helpful)
Aluminum 3 blade 12.25" x 15 at WOT ~4400-4500 no GPS speed
Power tech stainless 3 blade 11.75" x 15 at WOT ~4300 GPS topped at 29MPH (rough water but was able to get directly behind another boat's wake... at a safe distance of course)
Load was exactly the same both times out: 4 peeps at about 650-700lbs total ish; 1 cooler (2 bags ice, 30 tasty bevs); 12 gallons gas; 20lb anchor and 6ft chain; and other basics (stereo 4 speakers, amp, 10in sub, fire extinguisher), cedar deck instead of plywood, PT130 power TnT weighs 40 lbs and sets motor back 4 inches, Motor is about 1-2 inches above bottom of hull as stated in TnT directions

5) Engine/drive make, model, year, and HP - According to the original motor manual the specs are as follows: 1985, 2 cylinder, 50 hp, WOT 4500-5500, Gear ratio 0.414 (12:29);

Now I went through a prop sizing website and the info they sent back after I answered the same type of questions for them was :

Manufacturer: Evinrude
Year: 1985
Horsepower: 50.0
Cylinders: 2
Type: 2 stroke
Gear Ratio: 2.42:1
Min WOT RPM: 4500
Max WOT RPM: 5500
Spline Count: 13

6) Boat make model, year, length and weight - 1986 Sea Nymph Fish n Ski SS155; When i contacted Lowe for information on my boat this is what they sent:

Unfortunately we do not have a 1986 Sea Nymph catalog.
We received very little documentation when we bought them out.
However from the 1984 catalog, it lists a SS-155 Fish & Ski.
The capacities should be similar but are not exact, they should be used as a reference only.
Max. Persons- 825lbs or 5 people
Max. Weight- 1250lbs
Max. H.P.- 80
Hull Weight- 615lbs
Remember these are not the specs for your particular boat, however they should be fairly close.

Iboats has this info:
This outboard fish and ski is 15 feet long and weighs 700 (pounds dry). This weight does not account for passengers, fuel, or gear. The beam of this craft is 82 inches. The max horsepower for this boat on our records is 80 hp .

Ok i feel thats it... let me know if I forgot anything

[steelespike]

What a beautiful job!
I think your tach is not accurate,reading low.Based on your numbers you can't go 29 at 4300.By the way 29 is very good with 4 aboard.
If you throw out your rpm and use the numbers with 10% slip you get 5400 at 29 mph.
At 6% slip would be about 5250. It may actually over rev with a light load.

[Piece715]

What a beautiful job!
I think your tach is not accurate,reading low.Based on your numbers you can't go 29 at 4300.By the way 29 is very good with 4 aboard.
If you throw out your rpm and use the numbers with 10% slip you get 5400 at 29 mph.
At 6% slip would be about 5250. It may actually over rev with a light load.

Thank you steele! It was a job to say the least but the finished product was well worth the 4 year wait.

I was figuring 30 to 35 is was all i was gonna get from her but the RPMs is what had me worried. Now the tach is plugged into the throttle control and has brand new wiring harness and is a brand new tach. Is there a way to recalibrate or is this a motor thread question.

So the current stainless is the prop i need... good to know! Now the GPS did come from an old car garmin i have suctioned to the windshield but it should still be accurate all the same... Now whats up with the tach?!

On a side note... is there a thread that points to how to run all the calculations for slip and so forth... I'm one of those weirdos that likes to learn new stuff and would like to see if I can figure it out as well... I trust ur numbers whole heartedly you can just call it my OCD side shining through... just another tool to throw in the old toolbox Thanks again for your input! Much appreciated!

[steelespike]

OK here goes.I'm computer challenged. I used google and just put in prop calculator. I clicked on the first result. Calculator is on a
aqua green page by Polson Enterprises. I'll type the address [url]www.rbbi.com/folders/prop/propcalc.htn
If it comes up in blue were golden. came up in blue but didn't work, typical of my efforts to use an address.
As far as your tach goes It should have a switch on the back to set it for the number of poles on the stator.
And the numbers don't necessarily match the number of poles.
I've never installed a tach but have seen plenty on here that were set wrong.

[Piece715]

Well we were out and about Sunday on the water and switched the tach over to "2" setting on the back and it started reading as it should.... 5 people, cooler, etc. Pushing 28-29 at 5300-5400 RPMs WOT. But the thing I'm not sure this is the right setting for my tach either. Back when i was at that point of my restore, there was a debate whether or not my motor was 6 pulse or 5 pulse.... Which doesn't make sense for the 2 setting giving the right reading.... or that could just be reading high for 2....

[steelespike]

The numbers add up. 5400,2.42 ratio,15" prop , 29 mph works out to 8% slip a reasonable number.
and an excellent speed for the load.

[Piece715]

The numbers add up. 5400,2.42 ratio,15" prop , 29 mph works out to 8% slip a reasonable number.
and an excellent speed for the load.

Thanks again steelespike... I'll have to check the tach and make sure its reading correctly. We were messing with it underway the other day. My buddy was adjusting while i was captaining. I have to crawl under the dash and see what's up with it. What he thought was a "2" could very well be a "5"... Those numbers do make much more sense.

I looked at teleflex's guide and according to it I have 10 pole 5 pulse engine soooo "5" should be the setting

oh and here is that link u were tryin to post Prop Calc

Here is my tach application guide
Tach.jpg