19'6 with a 115 yamaha 2-stroke - how fast should she go?

[fishgutts]

I have a 1972 19'6 with a 1999 115 yamaha 2-stroke.   It has a t-top and bottom paint.  I realize both could slow her down a little.  

I am only getting 5200 on the tach and 31 mph as my max speed.  

Is this right?  I have talked to a couple of guys running a 115 without a t-top who say they can get around 40 mph.

Any suggestions?

[bumpster]

A T-top will slow you down but only a couple of MPH. You could probably squeeze anothe few hundred RPMs out of the engine as well

[saltfly]

You can get your prop. repitched. For every inch of pitch you either add or subtruct 300 rpm depending on which way you go. But their are a few other thing you could also try.

[fishgutts]

The prop is stiletto 13.25 diameter, 15 pitch.  I am running with trim tabs and a jack plate.

Are there any guys out there running this same motor?  

What rpms are you getting?  what size is your prop.  What kind of speed can you get?

I would like to try another prop?  Should I go bigger or smaller?

Any other suggestions?

[MarshMarlowe196]

A 17" prop should get you around 5600-5700 RPM's which is where you want to be with that motor.

My guess is trimmed out, given your drag from the bottom paint and T-Top, you will top out around 37mph.

[fitz73222]

Hi Fishguts,
Are you measuring speed with GPS? What is the max RPM the factory allows? 5200 rpm sounds a little enemic for a 15 pitch an a 19-6 with a 115. Stilleto props have great bite < 10% slippage. I`m running the exact same prop on my flats boat with a 60 Bigfoot Merc so I know about the prop on a different application. I suggest running a compression test on the motor to make sure your getting the power. You should be running a 17 and mustering at least 5500 rpm; assuming a 2:1 gear ratio. Read Skoot`s posts with his 90 Tohatsu on his 19-6 with propellers and engine height. How does she come out of the hole? Has this boat always been kept in the water? Water logged stringers can add addtional weight and hinder performance. We will figure this out!!

[Aswaff400]

fastest i got with the 112 johnson, was 43mph on gps with a 10 knt breeze on my stern, 1 person, and only 6 gal of gas. i ran a 12.5x17 3 blade stainless and spun 58-6000 rpm trimmed out on a slight chop. also ran 12x19 4-blade composite prop, spun 5500 trimmed out... averaged about 38 mph with 2 people, full livewell and 12 gal of gas

much happier with the 200 evinrude though, 40 mph at 4000 rpm with 3 people, 40 gal of gas. :)
now running a 13.5x23 4-blade stainless and reach low to mid 50's 8)

[gran398]

Agree. You want her running 5750 @ WOT. Should put her in the upper 30's.

[fishgutts]

The speed is from my GPS.  I doubt the stringers are wet.  The boat is very clean and solid.  

I think the issue is from the motor.  It should be getting more than 5200.  I have not checked compression and will try that.  

My hole shot is great.  She jumps up fast.  

I will test my compression first and then start messing with the pitch of the prop.  Wouldn't a 17 drop my rpms?

[fitz73222]

Hey Fishguts,
Thats the point, you should be way over redline with a 15 and your not. The 17 would drop your RPM. Also double check that your tach is reading correctly by using a borrowed one if you can. Something isnt right here.

[gran398]

Agree, something isn't right. If she's running good, it's a prop/setup issue. Check previous posts per Skoot's same concerns. Sounds like she's running well. That Yammie...she's a keeper on that hull. Keep us posted.

[fishgutts]

Checked compression and all 4 were consistent between 103-107.  Not sure how accurate my compression gauge is, but at least they were consistent.

I guess I could try a different tach... I don't know... I would like to try a 17 prop.  But, if my RPM's are truly at 5200, then I have an issue that I am just not finding.

[gran398]

The compression is fine. You need to get her near blow-out at top end, and we aren't there yet.

No worries, we'll get her right.

 

[MarshMarlowe196]

Hey Fishguts,
Thats the point, you should be way over redline with a 15 and your not. The 17 would drop your RPM. Also double check that your tach is reading correctly by using a borrowed one if you can. Something isnt right here.

Agreed.

As mentioned your target RPM's at WOT should be around 5600-5700 and your speed should be in the upper 30's/very low 40's.  

If you were to drop down to say a 13" prop, your motor would be screaming along at around 26-28mph.  

As Fitz said, you should be over-revving and you're not- The issue is more than likely with the engine.

[fishgutts]

Well,  I just put on an old aluminum "spare" prop that came with the boat.   Took the boat for a spin.... results have me more confused than ever.

RPMs were up where they should be:  around 5700-5800.  

Speed was still at 30 mph on the GPS.

I would still like to try a stainless 17 prop and see how she performs.  

Fitz has me scared to death about the stringers possibly being water logged...  So, I figure the best way to test that theory out is take it to a weigh station.

What should the weight of this boat be?  I will factor in trailer, motor, and t-top and try to figure out if this thing is way too heavy or not.