Top speed question part 2

[zach103]

Ok i know i posted before but i didnt know the rpms of the motor to find top speed.. i have a 17ft open bow weighs about 1700 with all the junk and such. this last trip we had there was 3 of us. about 400lbs.. boat was running at about 4800 rpms and only hit 28.. now i was told here it should be able to hit at least 35 and maybe 40 so i was just wondering if its the prop or if there something in the engine that needs to be adjusted..
this weekend it also died alot when it was idling and was very jumpy if that helps any.

 

[SparkieBoat]

Re: Top speed question part 2

we need to know what motor you have. also if your engine is not running properly you should fix it first before trying to reprop.

 

[zach103]

Re: Top speed question part 2

Sorry 1988 johnson spl 90 hp.

 

[SparkieBoat]

Re: Top speed question part 2

we also need to know what your current prop is (pitch + diameter + alum or SS + how many blades). you need to get your motor into the mid 5K RPM range, so if your motor is in good running condition you need to go down about 4 pitchs. also make sure your motor is not too low, sometimes raising a motor 2 or 3 holes can make a big difference. a pic of your motor trimmed down so we can see where it is on the boat especially how it lines up with the bottom of the boat would be helpful.

 

[zach103]

Re: Top speed question part 2

Ok prop i believe is a 15x19 aluminum 3 blade. heres a picture of the boat all the way down.. it can only go higher up.

 

[SparkieBoat]

Re: Top speed question part 2

raise your motor up as high as it will go, then take it for a test spin, be sure to trim it all the way down when you first start out, usually I have it where my fin is just out of the water at WOT and trimmed out. if your prop starts catching air lower it 1 hole and test again. but from what I see, you should be able to go up to the bottom holes with out ventilating. you will probably not be able to trim up as much, but thats ok. confirm your speed at WOT and RPMs, also confirm the pitch on your prop. it should have some numbers on the inside hub

 

[craze1cars]

Re: Top speed question part 2

this weekend it also died alot when it was idling and was very jumpy if that helps any.

This is a bit of a red flag to me. Your RPMS are low, but we have no idea if it is because you have too much prop pitch, or becuase you are down 20% on horsepower due to a mechanical problem. So I highly recommend you get the engine tuned properly first, and only then consider investing money in different props...

It's 23 years old...could be anything. Spark plugs? Carb clean & rebuild? General tune-up? Air leak causing lean condition? Ignition timing? Compression check?

I agree a 90 hp on a 17 ft runabout should be able to get at least into the low to mid 30's without a problem.

 

[SparkieBoat]

Re: Top speed question part 2

i did mention that same thing in post #4, the engine must be in good running condition before you proceed, but the motor can defiantly go up 1.5 to 2.25 inches.

 

[zach103]

Re: Top speed question part 2

I deff agree.. i posted in the johnson forum about the problems.. new sparks and just cleaned the carbs.. ill have the boat back on friday to confirm the prop.. im going to have a buddy go thru the entire engine but he was just here and had the engine running on muffs and said he may need to adjust it when its under load.... ill have him take a look and well lift it up to the top.. now only question is will the 4 bolts holding it on the transom be sufficient enough to keep it from moving around when its that high

 

[SparkieBoat]

Re: Top speed question part 2

yes

 

[zach103]

Re: Top speed question part 2

Ok. just double checking.. im paranoid about about certain stuff.. cousin called me a nut about my fish tank lol.

 

[zach103]

Re: Top speed question part 2

Ok so i took a look at the prop and its 13 3/4 x 15 ... I dont know where i got the original numbers from lol

 

[SparkieBoat]

Re: Top speed question part 2

you need to raise the motor as high as you can, also do compression and spark check and timing and linkage.

 

[zach103]

Re: Top speed question part 2

ok so i lifted up the engine. does this look any beter?
View attachment 109748

 

[SparkieBoat]

Re: Top speed question part 2

now test run it and see if it is too high or not..if it is too high you will hear the motor reving high with out speed increase...if it cavitates drop it 1 hole at the time until it stops. make sure you trim the motor all the way down to start out with.

 

[zach103]

Re: Top speed question part 2

ok will do.. Gotta put the carbs back on first