'89 Starcraft 18SS w/ 115 Optimax 4500rpm

[garycinn]

This is for a friend's '89 Starcraft 18SS re-powered with 2007 Merc Optimax 115. It had an Evenrude 70. The 'rude would top out at 30mph gps at unknown rpm with (I think) a 17P 3-blade aluminum prop.

Goal is to improve the boat as a ski, tube, and all around family boat. In that order.

I think this aluminum boat weighs something like 1200 lbs plus motor and gear. The dealer repowered it with the Opti 115 with a Merc Laser II 19P 3-blade SS prop. After break-in, the package is a total dog. Goose it and it takes ~20 seconds to get on plane. It maxes out at WOT @ 4500 rpm at 38 mph with 600 lbs of bodies and a 10 gals of fuel, a long run, and trimmed up to get max speed. Max rpm on this engine should be in the 5300 to 5700 range, if I'm not mistaken.

It seems like this motor should be able to pull a 19P. We are going to try a loaned 16P 3-blade SS tonight to see what happens -- weather permitting.

The owner is very frustrated that this new toy performs worse than the original 70 HP 'rude. It would deep start a 200 lb slalom skiier. I wouldn't even want to try on this new engine. It would completely re-define the medical procedure known as a nasal flush.

I'll report back.

If anyone has any suggestions, I'd like to hear them.

 

[walleyehed]

Re: '89 Starcraft 18SS w/ 115 Optimax 4500rpm

I have the same boat...I have a bit more HP now, but I had 2 different V4 crossflows on this boat and it runs best with 4" set back, 3-1/4" high(AV plate), or with no set-back, the engine should be in the third hole. Engine on the transom, I could only get 5200-5300 with a 17P Nauticpro SS (Same as an SST by OMC). I had to drop to a 15P to get my R's up where they should have been...but, I'm running above 3000ft elevation so you should be able to reach max R's with a 17P at sea-level, or close, if properly set-up and not over-loaded.

 

[garycinn]

Re: '89 Starcraft 18SS w/ 115 Optimax 4500rpm

Thanks for the reply, walleyehed. I appreciate it. It will really help me to hone this down to where it should be.

Right now the Optimax is set to the second hole from the top. It seemed too low to me. The bottom cavitation plate was very much submerged while we were on plane. Are you saying that it should be in the third hole, or up one more hole on the transom?

Tonight we put on a Vengeance 16P 3-blade SS prop. The performance was much better, but still not there.

Here are the new stats with the Veng 16P: WOT rpm 5000 @ 35 mph.

The Vengeance isn't ported like the Laser II, but the diam and blades are smaller.

The bottom end is just not there. We were able to deep water start a slalom skiier, but we needed more.

Any suggestions on where to go now -- 15P Laser II, a 13P of some sort, a 4 blade?

Thanks

 

[Silvertip]

Re: '89 Starcraft 18SS w/ 115 Optimax 4500rpm

I run an Alumacraft Navigator 165CS with a 75HP Merc triple and it swings a 19P prop no sweat. Either that engine is not delivering power or the boat is a ton heavy. The 115 should have no trouble with a 19P.

 

[garycinn]

Re: '89 Starcraft 18SS w/ 115 Optimax 4500rpm

Thanks for the post Silvertip. I will suggest to him the idea of weighing the boat and subtracting 800ish for the trailer. I also think it would be prudent to verify the fuel delivery regarding the Merc approved primer bulb.

The throttle linkage doesn't seem right either. It appears that the major action of the throttle happens in the last 2 inches of throw. We are wondering if the throttle linkage at the console or engine is correct.

When we gave it full throttle to pull the skiier, it kind of bogged down, then built up some rpms, then finally behaved somewhat normal.

 

[rndn]

Re: '89 Starcraft 18SS w/ 115 Optimax 4500rpm

A 115 Optimax should launch that boat out of the water. Maybe as you observed the dealer didn't hook up the throttle properly.

 

[garycinn]

Re: '89 Starcraft 18SS w/ 115 Optimax 4500rpm

The owner left the boat w/ the dealer to see what is going on. Based on prop calculators, this boat should go about 40mph w/ a 19P. I'll post regarding the results.

 

[garycinn]

Re: '89 Starcraft 18SS w/ 115 Optimax 4500rpm

Dealer discovered that new Merc throttle console was installed improperly causing the cable throw to be completely off. We were only getting 60% of the power.

The thing planes instantly now. It has so much torque that when you punch it while cruising you have to hold on while you are in the rear facing seats.

It looks like now that the engine/throttle situation is fixed, we will have to go back to the prop issue. Tossing the 16P and will probably try the Laser II 19P again. With the 16P it hit 5750 and wanted more. Didn't have a GPS with us, but it flies now. May settle in for a 4-blade 18P.

Owner is very happy.

 

[walleyehed]

Re: '89 Starcraft 18SS w/ 115 Optimax 4500rpm

Do as you wish with the prop, but after testing just under 450 props on my 18SS with 3 different engines, I can tell you first-hand, if the engine is mounted to the transom and not on a jack-plate you are going to hurt your over-all performance with the 4-bld. It has too much stern lift for this boat...and..you can't get the engine high enough to utilize the 4-bld technology..if it were a heavy deep-V fiberglass that may be a different story.
Also, even though you are running a Merc, the Laser does better on this rig with the 150hp class engines. With a 115, I honestly believe you would be happier with a stiletto or if set up correctly, a Turbo1.

 

[garycinn]

Re: '89 Starcraft 18SS w/ 115 Optimax 4500rpm

walleyehed, thank you for the very valuable feedback. I really appreciate it. I will have him see if his dealer carries the Precision Propeller Turbo I or the Stiletto.

What pitch would you recommend he starts with for this configuration?

 

[walleyehed]

Re: '89 Starcraft 18SS w/ 115 Optimax 4500rpm

If you can get some solid info from the laser in a given pitch, the stiletto will show very simular rpm numbers. If you go with a T1, 2" LESS pitch will be simular to a stiletto, but more bow-lift.