Evin 115 on Starcraft 18

[Tacklewasher]

So I'm getting close to getting the speed I think I chould out of this combination, but I'm thinking I should be able to get a bit more. But it was only recently I was running on 4 cylinders.

Was out yesterday and, as the fish weren't biting, I went for a couple of speed runs.

I've got a 13X19 Stainless prop on right now. The boat (on the trailer) weighs out at 2400 lbs. Single axle trailer but a heavy sucker so I honestly think the boat is under 2000. Add me (260ish) and the kicker (80ish) and I'm still under 2500 for the boat.

Max speed was 36.5 mph (by GPS) at about 5200 RPM. That's giving me a slip of 22%

Cruising at 3200 RPM, I got about 26 mph. That's about 10% slip.

Raising the motor doesn't seem to be an option as I ventilate if I make even a slow turn.

My other prop (which does not have a pitch marked on it) gets me 32.5 at about 5800. If I assume the same 22% slip, then that prop must be a 15.

I've run a bunch of seafoam through. In fact, when I was down under a 1/4 tank yeaterday, I added a 1/2 can of seafoam and ran till I could tell the pickup was sucking air. Filled the tank back up and continued running. Didn't change performance.

So, should I be happy or can I get an extra 3-4 mph by tweaking the engine some? Is the 22% slip at wot an issue?

 

[ezeke]

Re: Evin 115 on Starcraft 18

I have a 1976 18' Starcraft with a 1976 Johnson 115. I have the a/v plate dead on line with the bottom of the boat. The boat will do 40MPH at WOT. Boat and motor are not modified.

It is running on an OMC 14X17 aluminum prop #176214.

It is very quick out of the hole. The speed at 3200 RPM is about the same as yours. If I were to go to stainless, I would use a 14X19.

[Edit] The stainless prop that I have is 13 7/8 X 19 - OMC #176573, not 14X19

 

[Tacklewasher]

Re: Evin 115 on Starcraft 18

What rpm is your WOT?

And I really need to take a picture of my motor down and compare the cav plate. I think I'm a little above the hull.

As for modified, this boat started it's life with an OMC Stringer drive. But I'm thinking I'm leaving the back of the boat as is. Have thought about a jack plate but they seem to be for moving the motor up, not down. Will be replacing the floor and will likely put in a under-floor fuel tank in the process

 

[ezeke]

Re: Evin 115 on Starcraft 18

WOT is 5500+. I think that the gear ratio is the same as yours.

I run mine with the thrust rod in the second hole off the transom.

Starcraft boats don't usually hook, but take a look anyway. Try to keep the hull as clean as you can, especially on the planing surface - it helps.

If you get around to that tank, I sure would like to see the photos.

 

[Tacklewasher]

Re: Evin 115 on Starcraft 18

Well. Measured the motor before coming to work and it is right where I though it was.

Using a carpenters square, my a/v plate is about 1 inch above the bottom of the boat. I have the hyd trim and I can trim out enough that the boat slows down before ventilating so I'm not going to worry any more about my motor height.

Now I'm thinking I should try a bigger prop. I wonder if I can find a 14X17 around here for it to try. I'm thinking that would pick up my rpm and my speed.

And I don't think there is much difference between our motors. Should be almost identical. So thank you for responding as you have the setup closest to mine that I have seen.

 

[emdsapmgr]

Re: Evin 115 on Starcraft 18

The 5200 rpm figure is on the low side of normal. I would look to go down to a 17. Should pickup 300 rpm or so. What stainless prop are you running? My experience with the OMC stainless SST is that it will loose bite at high trim settings when mounted higher on a heavy boat. Similar to your concern about prop slip at high rpm. Just not enough cup to lift and hold the boat. For instance, a raker holds better.

 

[Tacklewasher]

Re: Evin 115 on Starcraft 18

So, as this has seemed to turn into a prop issue, more can be read here