1988 evinrude 88spl prop selection

[07whitendirty]

Hi guys, I've been looking around and can't seam to find a good answer for this. I just got 1988 alumacraft v170 and with a 17 pitch 3 blade I'm running 38 mph down river at about 4800 rpm. I'm using a omc digital tach and a gps app on my ph which is accurate. I can only hit over 5000 of I trim up and it starts sucking air but I obviously lose speed. This was today with me and 1 other person in no gear besides life jackets. I was hoping to get closer to 45mph. This is my 2nd boat so i dont know too much. One guy said to drop to a 15 pitch, that'll get my rpms up which will give me more speed. But that dont seam right. I see a lot of guys running 5500 rpms on a boat similar to mine with this motor. What am I missing here? Thanks in advance.

 

[Sea Rider]

Welcome to Iboats,

Triming up it's not the way to go, trim should be set at a point the makes combo to run parallel to water surface once on plane, that's with weight evenly distributed on deck. Test same with OB trimmed at 90?, are there back or over water splashes at transom, prop aeration at close turns? report your findings. Need to determine if OB sits at a correct height on transom. Can work from there.

Happy Boating

 

[07whitendirty]

Had it out today and the cav plate is right below the water line at full bore and 90* from the transom.

 

[07whitendirty]

I also used a yard stick, rested on top of the cav plate and its level with the bottom of the hull.

 

[StingRay_90V4]

If it helps any. I have a 1992 90 Hp crossflow v4. Same engine I believe. With a 20 year old okc 13 1/4 x 17p prop on it. It spins it to 5600-5700 rpm at 44 to 45 mph. The boat is a 16' Stingray 501zp. I raised the motor up one hole and it seemed to help my setup a fair amount .

 

[07whitendirty]

Thank you for the reply. I can't get over 39 mph or 5000 rpm.

 

[StingRay_90V4]

Maybe try a 15 pitch . it will bring your rpm up about 200 to 400 rpm. You may not gain any extra top speed. But the motor will be much closer to its ideal 5500 to 5800 rpm optimal range at wot. And you may gain some speed. Hard to say until you try it. How heavy is your boat ? And Hull type ?

 

[jimmbo]

While the 88/90 hp power engine does have a 4500 - 5500 Full throttle RPM range, it does make its highest HP below 5000, actually about 4700 - 4800 rpm. It is a torquey engine with lots of bottom end, but is asthmatic at speeds over 5000 rpms(small ports and mild port timing) and will deliver less horsepower at 5500 than is does at 5000. Your rpms and mph are good for a 17 ft boat and 17 pitch.

 

[StingRay_90V4]

While the 88/90 hp power engine does have a 4500 - 5500 Full throttle RPM range, it does make its highest HP below 5000, actually about 4700 - 4800 rpm. It is a torquey engine with lots of bottom end, but is asthmatic at speeds over 5000 rpms(small ports and mild port timing) and will deliver less horsepower at 5500 than is does at 5000. Your rpms and mph are good for a 17 ft boat and 17 pitch.

If his speed and rpms good for a 17 foot boat. Then why would a boat like mine. A 16 foot Stingray with a 90 and a 17p prop rev to 5600 ish rpm and hit 45 mph all day long. With enough bottom end to pull a person arms out of there sockets at wot from a stop skiing. I know not all boats are exact and equal. But there is a big difference between his boat and what mine does both rpm wise and top speed. Just curious...

 

[Fed]

Make sure the hull doesn't have a hook in it.

I agree with jimmbo, they definitely run out of puff above 5000 or so but based on my own 90HP crossflow 17' glass boat (17" prop) it should go a little better than what it does. (But not much)

Compression test, check the carbs are opening fully, maybe a carb cleanout?