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Author Topic:   Prop: 100-HP Yamaha 4-stroke
Ogden Dunes posted 05-31-2002 02:37 AM ET (US)   Profile for Ogden Dunes   Send Email to Ogden Dunes  
I have decided to repower with a 100 HP Yamaha 4 stroke. The salespeople at Hastings Marine told me there are two prop sizes(21 or 23 inch) but would not commit to what size to use on my Whaler. I am putting this motor on a 1984 18 foot supersport. Does anyone have prop experience with this type of application?


Clark Roberts posted 05-31-2002 08:12 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Dunes, there are props for that engine from about 9" to 26" pitch and Merc props will fit so you have many, many choices! You need to select a prop that will allow full throttle running at 6000 rpm... this will give you the best performance and economy! Happy Whalin'... clark... Spruce Creek Navy
Don Fisher posted 05-31-2002 08:55 AM ET (US)     Profile for Don Fisher  Send Email to Don Fisher     
Scott - That's a tough one to answer without knowing more, such as what altitude you will be operating at, etc. Assuming you are at sea level, that sounds like a lot of pitch to turn for the 100hp motor on an 18 foot boat. Most places will let you take different props out to try, and so long as you don't damage them, will let you return the ones you don't need. For me, since I operate at altitudes from sea level to 3700 feet (Lake Powell) to 7200 feet (Eastern Sierra lakes), I have 3 different props with different pitches. I know this may not answer your question, but perhaps it will give you some direction. Good luck!
jimh posted 05-31-2002 10:16 AM ET (US)     Profile for jimh  Send Email to jimh     

There are two reasons why it will be difficult to predict the prop pitch for your application:

--The Boat: I am not sure what boat you are talking about with an "18 foot Supersport". As far as I know there is no Whaler with this designation. So we don't have any idea of your boat.

--The Engine: often the gear ratio on newer four-stroke engines is quite different than two-stroke engines of similar horsepower. It may be your engine is geared down quite a bit and can handle larger prop pitches. It is hard to compare your engine to other "100-HP" engines because we don't have a feel for the lower unit gear reduction and the engine's power band or "guts".

If you get the prop as part of the total engine price, I'd probably go for the lower pitch. However, I would try to get the dealer to permit an exchange if that turned out to be unsatisfactory.

I picked the lower pitch because I think it is better to have the engine under-propped than over-propped. An under-propped engine can be used without harm as long as your keep the maximum RPM within limits. If you over-prop the engine you will constantly be lugging the engine, and that can be harmful.

If the prop is an extra item, you might consider sourcing it from a prop shop or some dealer that will let you have a try-out period. Typically, so long as you don't damage the prop, most dealers selling props will permit you to test different props to determine the correct one for your application.

And, as others have mentioned, it is not unusual to have several props for a boat. On my little 15-Sport I have props in 14, 15, and 16 inch pitch. (These are rather inexpensive aluminum $100 props so it is not a bank breaker to buy a couple.)

jimh posted 05-31-2002 10:23 AM ET (US)     Profile for jimh  Send Email to jimh     
[Changed topic; was "repower"--jimh.]
lhg posted 05-31-2002 11:29 AM ET (US)     Profile for lhg    
I assume you're talking about a 17 Super Sport. If so, and considering Yamaha's low gear reduction of 2.31, you're going to want a prop like a 20" Merc Laser II (for speed) or a High Five (for smoooooth pulling power).
Get the engine up about an inch or so, and you should get about 47 MPH out of either. if you think the boat will run faster than that, go up to a 22" pitch. Don't even think about using an aluminum prop on that engine. These SS props cost about $380 bought right, and as Clark mentioned, have choice of hub to fit Yamaha, or any other brand.
Tom W Clark posted 05-31-2002 11:36 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Ogden Dunes,

As jimh points out, there is no 18' Super Sport. I think we can safely assume the boat in question is a 17' (16'-7") Super Sport because it is the largest of the Super Sports and the only one with a 100 hp rating and weight capacity that would allow a Yamaha F100. The "8" key also just happens to be next to the "7" key on our keyboards, so a typo is understandable.

If the two choices are a 21" prop and a 23" prop, I'd say go with the 21". I say this for two reasons:

1) My own two 17' Montauks had 90's which used either 17" or 19" props. While the F100 may have lower gearing I still imagine a 23" prop as being VERY tall for your boat.

2) Again as jimh points out, It is better to be under propped than over propped.

But either way if the prop you get is not the right one just exchange it for the other. If the dealer is unwilling to commit to deciding which is best now then it seems it is their responsibility to adjust the boat’s performance via a different prop if the one they provide is unsatisfactory just as they are obliged to make adjustments if some other aspect of the motor proves unsatisfactory. It is the dealer's job to fgure out which prop is right for you.

tbyrne posted 05-31-2002 12:12 PM ET (US)     Profile for tbyrne    
There are many more Yamaha props available - see the yamaha-motor(s).com website (under accessories). I think you would be well served by a 19" pitch prop. I have a 2002 Yamaha 80 on an 18', 1400 lb boat and it turns the 17" pitch prop at 6,000 rpm with a light load. The dealer should experiment until you get the right pitch.
Ogden Dunes posted 06-28-2002 06:14 AM ET (US)     Profile for Ogden Dunes  Send Email to Ogden Dunes     
I just finished about 20 hrs of running this Yamaha 4 stroke 100 On a 17 ft Super Sport and decided to do a speed analysis with a Garmin 3 GPS.

With 450 lbs of weight, (driver and passengers) a Turning Point Express 13.25 dia, 19 inch pitch prop, running at 5900 rpm, 18 gallons fuel. The speed was between 44 and 47 mph.


Bigshot posted 07-01-2002 10:20 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Scott....I would step up to a 20 or 21" if I were you. With the weight you are running at about redline so with just you I assume you will exceed it. Try a 21 and see what gives. I found overpropping a 4 stroke is better than under propping because you get a better cruise and higher top-end. As long as you are within it's WOT range (ie: 5300-6000) you are good.

My montauk is rated at 52-5800 and with a 17" I hit 38.5mph @ 5800 and cruise at 4k was 22.5mph. With a 19" I hit 39.5 at 5450 and at 4k I am running 29mph.

Ogden Dunes posted 07-02-2002 11:21 AM ET (US)     Profile for Ogden Dunes  Send Email to Ogden Dunes     

Thanks for the advise. After our vacation I will try a 21 pitch and let you know how it performs.


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