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Author Topic:   Prop 50-HP Yamaha High Thrust 4-stroke
elaelap posted 02-12-2003 11:13 PM ET (US)   Profile for elaelap   Send Email to elaelap  
In an earlier post, I requested and received opinions and advice regarding my possible acquisition of a 50 hp four stroke Yamaha "High Thrust" (like a "Big Foot", I understand) motor. I'm repowering a '71 Katama 16/17 which I've been working on since last October. This motor is not, by any means, my first choice; however, it was my only choice, in exchange for work.

Well, today the deal was finalized and the motor is mine, in a box, waiting to be picked up along with controls and a tach. I have a limited choice of props, which is why I'm posting this request for advice. This will bore some of you, but let me tell you quickly what I will be using the boat for: I primarily fish for salmon, albacore, and rockfish out of Bodega Bay, 60 miles north of San Francisco. It is a very rare day here when the swells are less than four feet, with 2-3 ft wind waves on top. It's a beautiful, rocky lee coast with lumpy seas and strong afternoon northwesterlies as a constant. I will also be using my boat infrequently in the SF Bay and the Sacramento River Delta. I never waterski or wakeboard, so "hole shot" speed is not of great importance to me. I will either be trolling at 2 knots, give or take a tad, or cruising at the most at 20 mph. Top end is of no great concern; fuel economy and cruising range is (I will have only a 17.5 gallon fuel tank under the seat). This motor was engineered to push big pontoons and sailboats, according to Yamaha literature. Its gear ratio is 12:28 (2.33), whatever the hell that means (and please excuse my abysmal ignorance of these things), as opposed to the Yamaha four stroke normal 50 hp motor's 13:24 (1.85).

My question to you nice and knowledgable folks is (keeping in mind I don't know pitch from petunias) what sort of prop would best serve my needs?

My apologies for this long post. I'm jazzed about getting a new motor, even though it's not the one I would have chosen, especially after earlier helpful opinions and advice from several of you -- but it's the only show in town, it's mine now, and with your help and the right prop I'm going to learn to love it.

Thanks in advance,


elaelap posted 02-12-2003 11:40 PM ET (US)     Profile for elaelap  Send Email to elaelap     
Re-reading my post, I realized, 1) that I sure as hell won't get many albacore trolling at 2 knots...make that 2 knots for salmon and six or seven for tuna; and 2) if I dare ask for more advice...when the motor is mounted, should the cavitation plate be below, right at, or above the "keel"?

Again, thanks,


Clark Roberts posted 02-13-2003 08:48 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
You need a prop that will allow engine to rev to approx 6000 rpm at wide open throttle (WOT) trimmed out with a light load! Testing is the only way to tell for sure and of course you need a tachometer. The HT 50 Yamaha uses a Merc lower unit from the 75/90/100/115/125 hp series engines so there is a wide range of prop styles and pitches available. BTW, a 2.33:1 ratio is ratio of engine crank revolutions to one prop revolution... like rear axle ratio on auto... so engine turns 2.33 times for each prop turn... etc..etc...etc.. note, final ratio of engine turns:distance through water includes the prop pitch, percentage slip, etc. so testing is only satisfactory method to determine for sure! Happy Whalin'... Clark
Bigshot posted 02-13-2003 09:51 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Well my 70 suzuki 4 strooke is like a 2.30 or something so maybe I can help. I would think a 17" would be where you want to start. My Montauk is heavier than yours and you have less HP and I need like an 18". I would try a 16-17" and you should be in the park.
Over the LINE posted 02-13-2003 04:14 PM ET (US)     Profile for Over the LINE  Send Email to Over the LINE     
In the same kind of situation a few years ago, I found a local prop shop that handled used props. The owner was willing to let me take a prop to test and then return it for a different diameter/pitch until I found the right prop. When I was finnished testing I bought a NEW prop from him. I paid a little more for the service and his advice, but got what I wanted in the end.
Bigshot posted 02-13-2003 04:59 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
PS....OMC V4 props work on the Zukis, just have to shim them a bit sometimes.
elaelap posted 02-13-2003 06:51 PM ET (US)     Profile for elaelap  Send Email to elaelap     
Thanks for the info and advice, guys. What about the mounting level?


jimh posted 02-13-2003 09:39 PM ET (US)     Profile for jimh  Send Email to jimh     
[Changed TOPIC; was "Advice re Prop for a New Motor"--jimh]
elaelap posted 02-13-2003 11:58 PM ET (US)     Profile for elaelap  Send Email to elaelap     
Ooops...sorry, Jim.


Bigshot posted 02-15-2003 12:32 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
As high as she will go and still perform well. That is a big gearcase so I think you will need to jack it up some.

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