Speed of 20ft Crestliner w/70hp Yamaha

[branman1971]

I recently bought a 1997 Crestliner 20ft pontoon that came with a 1993 Force 70hp. With the Force, I could only get 12-13mph (gps) at about 4100rpm (WOT). I swapped the pontoon motor with the motor that was on my 16ft MFG runabout (which is a 2003 Yamaha 70hp 2 stroke,) and only got up to 14-15mph. The Force 70 runs my 16ft runabout like a raped ape so I am happy there, but I would like a few more mph on the pontoon for pulling tubers on the way out to the island and back. I have not hooked up the tachometer to the Yamaha 70 yet but I plan to tonight. I also plan on looking to see what the pitch of the prop is. The Yamaha's 3 blade prop is considerably bigger diameter than the Force's prop but I was told by a local repair shop that if my prop is wrong, and it is preventing the motor from spooling up, and it is creating a lag, it could blow the motor up. Ok, so now i am concerned. I was told normal operating rpm's should be 5000 to 5500 WOT. Is this correct? If my prop is not letting me get 5000rpm's, what do I do then to determine the correct prop to go with? Thanks

 

[SoulWinner]

Re: Speed of 20ft Crestliner w/70hp Yamaha

I used the Prop Wizard at Dans Discount Propellers online and input all the pertinent data about the boat and motor. You will need the weight of the boat all loaded up, or at least a close guess, and the normal operating WOT RPM of the motor. 5500 RPM seems to be the industry norm for outboard motors. Anyway, after you put all the info in the Prop Wizard it will give you are range of props with various pitch and diameter specs. I am guessing, but from my experience on dozens of toons with motors ranging from 40Hp to 115 hp, you should get between 18 and 21 MPH from your set-up. You gotta get a tach to see what RPM your turning with the current motor, but regardless, using a good prop wizard and plug in 5500 as WOT you can at least see the difference between what it recommends and what you have on the motor.

 

[DJ]

Re: Speed of 20ft Crestliner w/70hp Yamaha

"Propping" a pontoon is completely different than propping a runabout/fishing boat.

Pontoons absorb enormous amounts of power. Running an engine at 4100 RPM when it's operating range is 5500 will kill it FAST.

Go down in pitch to about 11" to 15". Let the engine churn along at its rated RPM and everyone will be happy. It will use a lot less fuel and the engine will LIVE.

Keep doing what you are doing and you will KILL two engines.

Prop, down, in pitch.

As an example. My family owns two "toons". Both Starcrafts.

The earliest one is a 2000 model (20') running a 75 Merc. 2-stroke. She is propped to turn 5600 at WOT (wide opoen throttle), about 26 MPH. That prop. is perfert for her. She loafs along at 3000-3500 and sips fuel.

That 75 Merc. now has over 1200 hours and has not hiccupped-once. She's propped right and can loaf at 3000 to 3500. But, she'll go to 5600, if need be-UNDERSTAND?

Prop down! Your two strokes do not like to lug.

 

[steelespike]

Re: Speed of 20ft Crestliner w/70hp Yamaha

To keep it simple get us the prop size,and the present max rpm and speed.
with that info we can get you setup.You may want to check the Force as well.If it was propped for the toon it will surely be under propped now.

 

[branman1971]

Re: Speed of 20ft Crestliner w/70hp Yamaha

Looked up the prop that Prop Wizard says I need. It called for a 13-1/4 to 13-3/4 with a pitch ranging from 15, 17, and 19, depending on my passengers and load out. Went out and looked at my prop and it is actually a 13-3/4 x 15. I got my tach hooked up and will take it out tomorrow to see what rpm's i am turning. I can't believe my prop was so close to what the wizard is recommending. The higher the number of pitch, the greater the degree of pitch correct?

 

[DJ]

Re: Speed of 20ft Crestliner w/70hp Yamaha

The higher the number of pitch, the greater the degree of pitch correct?

Correct. I can tell you now that 19 and 17 are way too high. 15" probably is too.

Be your own prop wizard. That engine needs to be able to turn toward the top end of its wide open throttle rating at WOT.

 

[Silvertip]

Re: Speed of 20ft Crestliner w/70hp Yamaha

Since you have Yamaha power, go to the Yamaha web site and look at performance bulletins. There is a good chance they have tested your exact pontoon (or at least another 20 footer in that category) with a 70 HP engine. The top speed, noise readings, and fuel consumption data are all listed in easy to understand terms. They also indicate which prop they had and a Yamaha part number for it.

 

[SoulWinner]

Re: Speed of 20ft Crestliner w/70hp Yamaha

The site Silvertip recommended is a great source of info. Here is a link:
http://www.yamaha-motor.com/outboard/products/perfbull.aspx

 

[Rancherlee]

Re: Speed of 20ft Crestliner w/70hp Yamaha

you should be in the mid 20's for speed with a light load I would think. My parents had a 20' Ercoa pontoon with a livewell and 3 couches with a 50hp 2 stroke yamaha and with 2 people on board she would run 23mph GPS. My 20' foot I just picked up is a bare deck model but alone I get 16mph with a little Johnson 28SPL and 12mph with 6 people on board both GPS speed. With your gear ratio (2.33:1) you should be around a 13 pitch prop to get you into your rpm range.

 

[branman1971]

Re: Speed of 20ft Crestliner w/70hp Yamaha

Ok, took the boat out tonight for a run with a tachometer this time. Here's what I found. With only me in the boat, with 2 batteries and about 15gallons of fuel, and the 13.75 X 15 prop, it ran:

1000rpm = 6.5mph
1500rpm = 10mph
2000rpm = 17mph

The motor would not rev above 2000 rpm unless I went to nuetral and pulled the high idle lever. It runs really good out of the hole, and gets to 15mph pretty quick, but no top end.

The last motor and prop I had on this boat was a 70hp Force with a Quicksilver prop with a part number of 2078R13. Not sure what that breaks out to because I can't find it online, but I think it's about a 10inch with a 13 pitch. Anyways, when I ran this other Force motor with that prop, I could only get 4000rpm at about 12mph, loaded. I found a Yamaha prop (13 5/8 x 13) for about 100.00. Sound like it would work?

So, thanks yall for all your help, and I hope someone has an idea for a new prop.

 

[Rancherlee]

Re: Speed of 20ft Crestliner w/70hp Yamaha

Ok, took the boat out tonight for a run with a tachometer this time. Here's what I found. With only me in the boat, with 2 batteries and about 15gallons of fuel, and the 13.75 X 15 prop, it ran:

1000rpm = 6.5mph
1500rpm = 10mph
2000rpm = 17mph

The motor would not rev above 2000 rpm unless I went to nuetral and pulled the high idle lever. It runs really good out of the hole, and gets to 15mph pretty quick, but no top end.

The last motor and prop I had on this boat was a 70hp Force with a Quicksilver prop with a part number of 2078R13. Not sure what that breaks out to because I can't find it online, but I think it's about a 10inch with a 13 pitch. Anyways, when I ran this other Force motor with that prop, I could only get 4000rpm at about 12mph, loaded. I found a Yamaha prop (13 5/8 x 13) for about 100.00. Sound like it would work?

So, thanks yall for all your help, and I hope someone has an idea for a new prop.

Sounds like you have a bad tach or the tach is not on the proper setting for the yamaha. 17mph is WAY too fast for 2000rpm, should be reading 3500+ rpm @ 17mph with that 15 pitch prop. Also it doesn't seem like your engine it making full power either, is the throttle cable adjusted properly?

 

[branman1971]

Re: Speed of 20ft Crestliner w/70hp Yamaha

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[branman1971]

Re: Speed of 20ft Crestliner w/70hp Yamaha

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[branman1971]

Re: Speed of 20ft Crestliner w/70hp Yamaha

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[branman1971]

Re: Speed of 20ft Crestliner w/70hp Yamaha

I deleted some of my previous posts because the info I gave was while using a faulty tachometer, and don't want to confuse people with what is going on.

I got a timing light today and took the boat out for some test runs with the GPS also. Turns out the tach I have is not reading accurately. Had only me, some gear, a couple batteries, and about 10gallons of fuel onboard. With my 13.75 x 15 prop on 20ft pontoon I got:

2000rpm @6.5mph
3000rpm @11mph
4000rpm @16mph
4500rpm @18mph

This sounds more like it to me. Couldn't get over 4500rpm. Think I will go to a 13pitch prop and see what it does. Sound right to you guys?

 

[Rancherlee]

Re: Speed of 20ft Crestliner w/70hp Yamaha

You definately need a 13 pitch prop then. Using an online calculator going from a 15 to a 13 should get you at least another 600-700rpm. How heavy is your pontoon? I know when I looked at new toon's a few years ago the crestliners looked like a heavy/well built pontoon. An 11 pitch might be in order for heavy loads/water sports but would probibly over rev with a light load.

 

[branman1971]

Re: Speed of 20ft Crestliner w/70hp Yamaha

Yeah, I will go with a 13 and see how it does. Found a new 13.75 x 13 for 89.99 with free shipping. WOOHOO

 

[SoulWinner]

Re: Speed of 20ft Crestliner w/70hp Yamaha

Good. Sounds like you have it figured out. Please let us know how it runs. Thanks.

 

[branman1971]

Re: Speed of 20ft Crestliner w/70hp Yamaha

Ok, took off the 15pitch and put the new 13 3/4 x 13 prop on the pontoon boat and got 5000rpm at WOT, which is better than the 4200rpm that i was getting, but not 5500. Top MPH was 18 with good amount of gear and 4 adults. Should I just go with it?

 

[quantico]

Re: Speed of 20ft Crestliner w/70hp Yamaha

I have ben really surprised with the fairly tall pitch props being suggested. I have dealt with older pontoon boats 21 - 24 feet with 40 - 50 hp engines and I have run 9 -10 pitch props on both boats to get the engines hitting 5500 rpm at full throttle with on person on board. I use a xhs prop with four blades and really like the setup on my 21 foot boat.

I would not keep that prop setup. I would go down one inch in pitch and measure rpm again. That prop can be used as a spare when the other one is being fixed or replaced.

My neighbor had carbon chunks coming out of his 50 hp 4cyl merc with a 11 - 12 pitch prop and now has no issues with a 10 pitch that is setup correctly.

You are running a stronger engine than I am, so you might get away with a bit higher pitch prop. Your boat might also be lighter ( mine has a stereo and amp and two batteries and a top and power tilt added on ) . The newer larger pontoon chambers also skim along better than the older smaller tunes on my older boat, so again what is great for me might be different than your machine.

I would get that setup correct and stay with it and get that wide open throttle correct with maybe two people and full fuel, which is a typical lite load on a pontoon.