Just bought first boat but has low top speed

[diybrad]

Re: Just bought first boat but has low top speed

Bottom condition, prop condition. Does it have a whale tail on it? Ignition timing. Here's a link that may help: http://forums.iboats.com/showthread.php?t=295070

Your situation is both low speed and low WOT RPM which tells me something is amiss, Can you get us your gear ratio? We also need the WOT RPM confirmed, and accurate (tach).

I want to thank everyone for being so willing to help here. I knew I was getting a project to begin with but I love learning new things.
Bottom is great other than a few 1-2 ft paint scratches. The prop I pretty dinged. I know it is a 21p but I do not know the size. I am going to try to find one to put on it. I am not sure of the gear size. How do I check that? Also the fin at the bottom of the lower unit is totally gone/broken off. I read that doesn't affect speed but can affect steering.

I am going to get a timing light tomorrow and check ignition timing. RPMs I don't think are very accurate. It doesnt sound to be working to hard. It also says that at idle it is at 1200 RPMs and that can't be right. Way to quiet for that rpm. I have an autometer tack that I am going to wire up tomorrow to get a realistic rpm.

I had 5 people on it yesterday and beer and ice and tubes. One of the guys is 320 lbs and it went the same speed when I had only 2 people. That's what had me confused.

Thanks

Brad

 

[QC]

Re: Just bought first boat but has low top speed

I had 5 people on it yesterday and beer and ice and tubes. One of the guys is 320 lbs and it went the same speed when I had only 2 people. That's what had me confused.

That would confuse me too

 

[diybrad]

Re: Just bought first boat but has low top speed

That would confuse me too

I know. That's what got me wondering. Those speeds were verified by gps so I know it is the one accurate number I have. Makes no sense. The RPMs registered a little lower but speed was the same.

 

[diybrad]

Re: Just bought first boat but has low top speed

i just searched the part number on the prop and it is a 13.75" with a 21p. i am still trying to figure out the gear ratio. Is this a proper size prop for 17.7 ft boat with a 4.3 v6?

 

[diybrad]

Re: Just bought first boat but has low top speed

i just noticed the Alpha one housing is stamped with a 184. Does this mean it is a 1.84 gear ratio?

 

[Levinz11]

Re: Just bought first boat but has low top speed

I know. That's what got me wondering. Those speeds were verified by gps so I know it is the one accurate number I have. Makes no sense. The RPMs registered a little lower but speed was the same.

On the back of your tach is a knob to adjust for 4-6-8 cylinder motors. Take a small flat-head and turn it back and forth like 20 times. Your tach may start working better.

 

[QC]

Re: Just bought first boat but has low top speed

Yes, a 1.84 is what should've been shipped with a 4.3 back then. This assumes no one has replaced individual gears. It is much more common that a whole drive would be replaced, and is why we always ask for confirmation.

Edit: What he said ^^^^^^^^

 

[Levinz11]

Re: Just bought first boat but has low top speed

i just searched the part number on the prop and it is a 13.75" with a 21p. i am still trying to figure out the gear ratio. Is this a proper size prop for 17.7 ft boat with a 4.3 v6?

I have the same size engine as well as the same exact size prop. My 18' boat will do 42-43 (GPS) at 4200 RPMS. Check your timing. Maybe advance it a few degress. My manual calls for 6* but I have mine set at 8*. A full tune up won't hurt either.

 

[rbh]

Re: Just bought first boat but has low top speed

Before you start digging into the foam, how dinged up is the prop?

 

[QC]

Re: Just bought first boat but has low top speed

I have the same size engine as well as the same exact size prop. My 18' boat will do 42-43 (GPS) at 4200 RPMS.

Just want to remind all that we need the gear ratio for this to be the most helpful. I am seeing a 1.68 ratio for the OMC so comparing prop and speed numbers to a 1.84 may confuse things

 

[diybrad]

Re: Just bought first boat but has low top speed

Some thoughts on how "I" would preceed.

With another person driving check the timing advance at WOT.

Pick up a cheap mecanical guage at the local auto store and plumb it into the fuel line at the carb. and again check it while at WOT. (I can't count how many times I've found poor fuel delivery to be the problem with mechanical fuel pumps and dirty pick up tubes in the tank.)

You've eliminated compression so timing and fuel delivery are the other two requirements for a well running engine. If both of those check out you can begin looking at the boat itself, IE: water logged and rot.

I will grab a cheap guage today when i pick up the timing light. What should i be looking for on a mechanical carb set up for pressure at WOT? What should i expect for timing advance on WOT?

 

[diybrad]

Re: Just bought first boat but has low top speed

Before you start digging into the foam, how dinged up is the prop?

The prop is pretty dinged and the leading edges are a little bent/mushroomed. I am looking for a prop locally right now to put on. What size should i look for? Same size or something with a little less pitch?

 

[QC]

Re: Just bought first boat but has low top speed

Your prop may be blowing out at the same speed, but I would expect RPM variations. I am getting 21% slip as is (4200 RPM, 1.84 and 36 MPH). This assumes those numbers are accurate. This indicates that prop is part of the problem. It does not confirm what the right pitch and design is. That's a long process. For now, I would think same/same. Aluminum 3 blade, 21 inch pitch.

 

[diybrad]

Re: Just bought first boat but has low top speed

Your prop may be blowing out at the same speed, but I would expect RPM variations. I am getting 21% slip as is (4200 RPM, 1.84 and 36 MPH). This assumes those numbers are accurate. This indicates that prop is part of the problem. It does not confirm what the right pitch and design is. That's a long process. For now, I would think same/same. Aluminum 3 blade, 21 inch pitch.

when running at 35-36 the rpms are bouncing on the guage between 4000-4400 but i am not sure the reliabilty of that guage.

 

[rbh]

Re: Just bought first boat but has low top speed

when running at 35-36 the rpms are bouncing on the guage between 4000-4400 but i am not sure the reliabilty of that guage.

Could be an issue between both the prop and your trimming out.
As you trim out both your speed and rpms go up, go up to much you get blow out, rpms shoot up and speed drops.
So when you add in the bent up prop that just adds to the issue.

BTW, did you change the fuel filter, and clean the air filter/flame retarder?