1985 Evinrude 115 (TL) on 1985 Larson ?Senza

[Jachin]

I may be asking this question prematurely, because I'm in the midst of a gearcase overhaul but would like to get opinions on prop selection if possible. The boat is 16.5' (I think) NOT a bowrider, it has a 20gal under bow gas tank, and typically is run around 1/2 tank with less than 300# gear/persons on board. It has been running a 4 blade 13x21 stainless, with (if I understand prior posts) a lightly cupped aft pitch. Runs just shy of 50mph (by speedo), but will overrev if allowed so I have to keep my eye on the tach. I also have a 3 blade 13 3/4x23 stainless, aft pitch, heavier cupping, and a 3 blade 14x21 aluminum, aft pitch, no cup. Would the 14x21 be a better prop to help limit overrev, I can't do WOT presently; or do I need to try something else entirely? I don't ski the boat, I just use it to jet across the lake and fish and have no problem coming on plane. Thanks in advance.

 

[Jachin]

...the 13x21 is a Solas; I think the 13 3/4x23 is OMC; I have no idea who makes the 14x21 it has a #390750 on the hub and 02352 stamped on the body. I am presuming an rpm limit of 5,000 rpm. I could likely hit 6000 as I am now.

 

[Jachin]

If there is something wrong with the question I would appreciate an education. Apparently the boat is listed at 750#, the motor at 301#; so total weight tare 1051#. Add 400# for gas, gear, battery, and occupants - 1450 (<1500 anyway).

 

[WesNewell]

I had a very similar 1983 Kingfisher 15V that weighed 750# with a 115 rude on it and used a 23p ss 3 blade on it. Iirc it came with a 19p alum prop which was way under pitched.

 

[Jachin]

Thanks for that. I'm guessing my best foundation will come by swapping each one of the three props in and seeing how they affect the performance/handling. Since it's running a 21 pitch and over reving I'm guessing I need 23 pitch or better; I'm not sure what advantage/disadvantage I'll get from going 3 blade vs 4 blade.

 

[WesNewell]

Back in the early 80's it was all about more hp for more speed. I've since learned that's just one element for a setup to get the max out of an engine. Having the proper mounting height means a lot more than you may think. Mounted too low can account for up to a 20% performance loss, so that's the very forst thing you should take into consideration. If it's at the proper heigth, then and only then should you start find in the proper pitch for maximum performance. Here's a good example of that.
https://www.thehulltruth.com/boating-forum/820886-prop-selection-advice-2.html

 

[Jachin]

Really appreciate that. I had just watched a video the other day about engine height suggesting that the water inlet should be just below the bottom line of the boat. My water inlet is on the bottom lower housing, behind the prop torpedo, so that's not a factor, but as it is now it appears as though the cavitation plate would be about 8" above the bottom of the boat - dependent on which "step" in the hull you measured from (it's tiered). When I get my camera charged up I'll take some pictures because even the mounting is kind of "different". The motor is as high as it will go on a mount that has slides on it, and the slides are in the full up position.

 

[racerone]

Post a picture of the " water inlet on the bottom of the tail shaft "----would like to see that.

 

[Jachin]

As soon as the camera's up. I guess it would be better stated "at the bottom of the lower housing, aft of the prop torpedo". It also has an exhaust discharge in the midsection, which I think is out of the norm. It is not a quiet boat.

 

[Jachin]

Can't get photos to upload into an ideabook, can't even get it to let me create an ideabook. (I did read the tutorial).
I'll try from here.

 

[Jachin]

As far as mounting goes (since the above worked) Here's the plate, and current position (without gear box). The cavitation plate is about 3" below the mounting flange of the lower unit.

 

[Jachin]

And although unrelated, here's the exhaust discharge.

 

[racerone]

The " low water pick-up " nose cone is a modification.-----That exhaust outlet is also a modification.------Makes one think about what was done to the engine too.----Any pictures of the powerhead ?

 

[Jachin]

Glad to oblige; happy for the education. What do I know?

 

[Jachin]

laterals:

 

[racerone]

The exhaust cover shown ( between cylinders ) was commonly used on the 140 HP models.-------Your motor has had much work done to it.

 

[Jachin]

Interesting. I'll have to keep that in mind if I wind up in the power head at all.

 

[jimmbo]

A 1985 would be a Prop Rated 115, so it would have the Bubbleback Exhaust. However, as mentioned, there have been other differences from a 115

 

[racerone]

What bore size are the carburetors.-------Agreed in 1985 the 140 was the small bore V-4 looper style engine.----The 115 did have the tuned exhaust and prop rated.-----Is engine block serial # the same as the number on the transom bracket ?

 

[Jachin]

I'll have to see if I can locate the block number. I'll report what I find.