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Engine for 1981 Sport 15
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| pensmith |
Posted on 03/11/14 - 12:22 PM
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Just bought a 1981 Sport 15 with a 1981 Johnson 50. I'm thinking about re-powering with a new Yamaha or Tohatsu 4 stroke. There's quite a difference in price between the 60 and 70 hp. Do you guys have a recommendation? I want to tube/ski the Grandkids.
Thanks!
Edited by Tom W Clark on 03/11/14 - 12:56 PM
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| Tom W Clark |
Posted on 03/11/14 - 1:07 PM
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I don't think you can go wrong with any modern outboard. Have you seen the Engine Choices article for the 15 footers?
http://www.whalercentral.com/articles...icle_id=77
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| Joe Kriz |
Posted on 03/11/14 - 1:09 PM
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The largest Tohatsu for the 15' without exceeding the HP rating is 50hp and the 75hp weighs too much @337 pounds.
http://www.whalercentral.com/articles...icle_id=77
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| wlagarde |
Posted on 03/11/14 - 2:21 PM
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I have a 1976 sport 15 with a Nissan (Tohatsu) 50hp 2 stroke. Plenty of power for the boat. Top speed is 39mph. The motor is beautifully made and runs smooth with lots of low end torque and it's light at 160lbs. Mine is carbureted. The TLDI is the same basic engine with the low pressure direct injection system added (MUCH cleaner and better on fuel consumption than the carbureted version). The TDLI version of this engine also runs even smoother than the carbureted version. If I were to re-power my preference would be the Tohatsu TLDI 2 stroke 50hp or the Honda 4 stroke 50hp both at just over 200lbs.
Edited by wlagarde on 03/11/14 - 4:57 PM
1976 Sport 15 w/ 2005 50hp Nissan 2 stroke |
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| pensmith |
Posted on 03/14/14 - 5:33 PM
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Thanks for the advice guys! My local dealer will install a 2014 Yamaha F-70 for $7500 out the door. The F-60 is about 8 lbs. lighter and about $500 less money. My dealer says that the difference between the 60 and 70 is air intake configuration, and ECM. I had a Sport 15 with a Johnson 70 (no trim and tilt), and it would "chine walk at WOT. I believe that was due to improper trim.
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| Tom W Clark |
Posted on 03/14/14 - 5:55 PM
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$7500 is a very good price for the F70.
Be sure the dealer mounts the motor all the way up (three holes up)
You should use the 13-1/4" x 15" Stiletto Advantage/Turbo 1 propeller.
If you use the F60 you want a 13-1/4" x 14" Yamaha Performance Series 3-Blade (same as the Advantage/Turbo 1, but offered in even pitches)
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| FlyAU98 |
Posted on 03/14/14 - 6:15 PM
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pensmith wrote:
Thanks for the advice guys! My local dealer will install a 2014 Yamaha F-70 for $7500 out the door. The F-60 is about 8 lbs. lighter and about $500 less money. My dealer says that the difference between the 60 and 70 is air intake configuration, and ECM. I had a Sport 15 with a Johnson 70 (no trim and tilt), and it would "chine walk at WOT. I believe that was due to improper trim.
That price is a steal. Jump on it.
Get the 70..thats what the boat is rated for...you don't see many people wishing they had SMALLER engines.
If it starts chine walking, trim it down a little...
Follow Tom's advise and have your dealer mount that engine as high as it will go. I made the change from my old 70 mounted with the blind holes, to the new one mounted correctly (high) and the handling is noticeably better.
I'm still not 100% on the 13-1/4" x 15" Stiletto Advantage/Turbo 1 propeller, but its working well. I'd like to see some more RPM, its been a long winter, but I think I was only getting about 5700 out of the Stiletto.
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| Tom W Clark |
Posted on 03/14/14 - 6:23 PM
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5700 RPM? How fast is the boat going at that engine speed?
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| FlyAU98 |
Posted on 03/14/14 - 7:53 PM
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Tom W Clark wrote:
5700 RPM? How fast is the boat going at that engine speed?
You and I have talked about it...around 41mph if I remember correctly. When I get to Florida I'm going to try and find a 14" Yamaha prop to try out.
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| Tom W Clark |
Posted on 03/14/14 - 8:11 PM
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We may have talked about it but I talk to people about propellers every single day and I can't remember every discussion.
41 MPH with a 15" pitch prop on an F70 with its 2.33:1 gear ratio is -18 percent calculated propeller slip. That ain't happenin'
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| wlagarde |
Posted on 03/15/14 - 5:35 AM
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That new yamaha F-70 looks like a really nice engine. From my perspective though the additional 20hp comes an additional 100lbs. Does anyone know what the top speed of the boat is with this engine?
Also, achieving the max RPM for the rated operating range of an engine isn't always the ideal goal. One wants to achieve the rpm where the engine reaches maximum HP output and the two aren't necessarily the same. For example the new Yamaha F-70 operating range is 5300-6300rpm while it achieves the full 70hp at 5800rpm: http://www.yamaha-motor.com/outboard/...20(v2).pdf
Edited by wlagarde on 03/15/14 - 6:17 AM
1976 Sport 15 w/ 2005 50hp Nissan 2 stroke |
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| Tom W Clark |
Posted on 03/15/14 - 7:26 AM
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That's a common misconception. Motors are always rated for the HP smack dab in the middle of their recommended WOT engine speed range. The F70's recommended WOT range is 5300 to 6300 RPM and 5800 RPM is the middle of that range. That does not necessarily mean that is where peak horsepower is reached.
Most outboards will perform best and last longest if the boat is propped so the motor is at or near the upper limit of the WOT range when lightly loaded. This way the motor will still be in the range when heavily loaded.
Some outboards do better reaching something less than the redline. Some Evinrude E-TECs are like this The E-TEC 90, for example, has a 5500 RPM redline but does best if propped to hit 5000-5200 RPM at WOT.
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| wlagarde |
Posted on 03/15/14 - 8:23 AM
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Tom let me rephrase. Assuming propeller efficiency does not change substantially as a function of rpm (at least in the general 5000-6000rpm range where a motor typically operates at WOT), one wants to achieve the rpm where the engine reaches maximum horsepower under the most common operating conditions the boat is used while at the same time not exceeding the maximum rated rpm under the lightest load conditions the boat will be used.
Any reciprocating engine has a power curve and power generally rises as rpm rises until a peak is reached after which hp declines with increasing rpm. The curve is usually somewhat broad surrounding the horsepower beak. Whether Yamaha is providing us with accurate data is up for debate. If one doubts the data then contact the manufacturer to get the horsepower curve.
Now if propeller efficiency improves as engine rpm increases from peak hp rpm to maximum rpm then perhaps it might offset the decline in hp observed as engine rpm rises above the hp peak to the max operating range - resulting in improved performance with an engine rpm above the hp peak.
Now I will agree how all of this plays out in the real world depends and there will be no substitute for real world experimental data. However, I would argue in the absence of said data to start with a prop that achieves an engine rpm at or very near engine max hp and not allow the engine to over-rev with the lightest loaded expected. From there one can experiment with propping down or up until optimal performance is reached.
1976 Sport 15 w/ 2005 50hp Nissan 2 stroke |
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| Tom W Clark |
Posted on 03/15/14 - 8:51 AM
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Whether Yamaha is providing us with accurate data is up for debate.
What data? All outboard motor manufactures say something to the effect of:
"rated HP @ exact middle of RPM range"
That is not data. It is a stretch to interpret it to mean that is necessarily where peak horsepower is achieved.
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| Tom W Clark |
Posted on 03/15/14 - 9:12 AM
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That new yamaha F-70 looks like a really nice engine. From my perspective though the additional 20hp comes an additional 100lbs.
No, that 20 extra HP comes at cost of an additional 8 pounds.
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| wlagarde |
Posted on 03/15/14 - 9:25 AM
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Perhaps they have set the middle of the operating range to the peak horsepower of the engine? i.e. maybe the operating range is defined by a function of the peak horsepower rating rpm and redline rpm?
Here is an article that contains a representative hp curve that defines the concept: http://www.floridamarinesurveyors.com...props.html
Specific data relevant to any given engine are usually available from the engine manufacturer (i.e. they have placed the engine on a dynamometer and acquired the data). You may have to pry a little on the phone but if you preserver you can get it.
Yes, from my perspective (i.e. for my 50hp engine that weighs 160lbs) the extra 20hp comes at a cost of roughly an extra 100lbs.
Edited by wlagarde on 03/15/14 - 9:37 AM
1976 Sport 15 w/ 2005 50hp Nissan 2 stroke |
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| Tom W Clark |
Posted on 03/15/14 - 5:15 PM
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The Honda BF50 is the lightest 50 HP motor available today. It's a great little motor too but is 30 percent less powerful and only saves 50 pounds, not 100.
I had a 225 pound Johnson 70 HP two stroke on my 15 and it never occurred to me it was too heavy.
Pick the motor that suits your needs. Most people probably do not *need* 70 HP but it sure is fun.
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| FlyAU98 |
Posted on 03/15/14 - 7:35 PM
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Tom W Clark wrote:Most people probably do not *need* 70 HP but it sure is fun.
YES!!!!
Although I'd argue that although a Classic 15' doesn't NEED 70HP, it sure does like it!
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| wlagarde |
Posted on 03/15/14 - 8:10 PM
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No no I'm not saying it is too heavy and I'm sure those who put a 70 on their 15 are happy. Just that it's 100 lbs heavier than the setup I have so the gain in speed I would see if I went up to the F-70 would be offset some by the extra 100lbs. I'm topping out at 39mph. Tom and and FlyAU98 what top speed did/do you see with the 70?.
1976 Sport 15 w/ 2005 50hp Nissan 2 stroke |
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| Joe Kriz |
Posted on 03/15/14 - 10:45 PM
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The Nissan site states the 40 and 50hp weigh in at 206 pounds.
http://www.nissanmarine.com/products/...specs.html
The Yamaha 70 weighs in at 257 pounds.
http://www.whalercentral.com/articles...icle_id=77
That's only a difference of 51 pounds.
Were are you coming up with 100 pounds difference?
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