I just joined the site because I needed to ask the experts. I know alot about engines and fabrication but, I do not have the slightest clue on how to chose the size and pitch of a prop. Here's my scenario: I'm in the process of building my longtail motor. I'm using a yamaha XVZ12TK 90hp motorcycle engine. I'm using this motor because 1. It was free. 2. The bike was shaft driven. There is a output shaft coming out of the back of the tranny that I can fab a coupling for. I'm not planning on switching gears or anything while running. I'm going to choose a gear to run on and a prop to match it. So I am looking for opinions on what gear to use and what size/pitch prop to use. The motor red lines at 7500rpm so, I'm assuming a safe wide open throttle speed is 3/4 of that(5600rpm). I found the gear ratios of the transmission and at wide open throttle the output shaft speed should be (1st gear):2154rpm (2):3164. (3):4148. (4):5233. (5):6154.

UPDATE: 10/12/15 I'm finally through with the engine restoration.

-- idling to show you what I'm dealing with.
I'm to the point of having the drive shaft cut for a prop. I know there is not a standard. But what is the most popular way and size? Threaded, keyed, pinned?

Dude...you're going where no man has gone before! (As far as I know anyway) Good luck with this endeavor. When it comes to what prop, alot of that will also depend on what kind of load you intend to tote. No-one has ever proped a multigeared engine as far as I know.

What size boat is it going on?

You might just wanna start with a hammer prop. I feel like you are gonna need to go even bigger than that because it's just gonna blow out

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I'd give a shout to beaver tail and see what they were using on the old predator longtails.. If I remember correctly the folks at backh20 had something to do with that motor so you could try them as well.

I was waiting for someone to comment about the hammer prop

It's really going to be hard to find a long tail prop that will have enough pitch to it to justify 90 hp. I honestly don't even think I would want to drive a 90 hp long tail though.. It's bad enough when the little 27 hp ones bite and throw the handle against you. Problem is the rpms are there once you get up to third gear, but will it have enough on the low end to be able to dig it down in the mud and it turn fast enough to get you on plane in a short distance? I don't believe the gear ratio for a motorcycle transmission would be very low once in the higher gears. But I could be wrong

Always wanted to do that. I'd start with a 12 3/4 X 11p. The cool thing would be you can select a lower gear for loads and a high gear for light loads. If it red lines at 7500 then I would try to put my rpms at 6000 loaded that way you could drop down to a lower gear for heavy mud/ load. Keep us informed of your build. Good luck. Btw 2 blade for heavy mud. 3 blade for veggies and open water.


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You may not be able to hold on due to the torque. Them long tails can be tough.


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There are some ignent mother fuckers up in this thread.. It was just
Over 90 hp.... Those fancy 35 hp fuel injected motors your about to put on your barge are 993 cc (pretty sure that's right, if it's not right it's close)

Buncha morons runnin' around this thread. If you combined your brain with jakehawk's and then put it in the skull of a cat that poor bastard would keel over and die from a lack of nervous system.

was sure it was 90 hp bud I was being a smart ass. I almost purchased one. With the exception of the predator, I just wanted to go a lil faster then a long tail will allow me to go. There great for mud ! If I just duck hunted with my boat I'd have a long tail. Iv owned a long tail as well just not what I care to use for fishing. There is nothing wrong with them but they are what they are . Will get you almost anywhere you need to go just not in a hurry.

He isn't asking for a frame, drive system, a motor, or a prop. He is simply asking for propeller dimensions/pitch. Which beaver tail or backwater could help him with.....

If you don't mind post a video running this monster when you get it all set up the way you want it.

I don't think anyone said anything about anything in the drivetrain or frame... He wanted to know if he should just chose a gear and based off of what gear he picked how it should be propped. If you want to just get that same prop call Jake at backwater.

Which is why I said call Jake at backwater... This was done not too long ago on here by someone else and he said Jake is the one you need to talk to about finding a prop viewtopic.php?f=6&t=61276

Sorry guys. Forgot you were experts at everything mud motor and have a built a 90hp motor before... to the original poster. Everyone is throwing what they think is their best idea at you because no one has done it. Good luck

It has ben done before !

Thank you for all the responses. I'll give that backwater guy a shout. I originally chose the longtail because of the simplicity and ease of fabrication. But if it's going to be a pain in the dick to drive, I'll suck it up and fork out the money to make a surface drive. The good thing is I'll be able to adjust the prop speed through the sheave dimensions. I know there has to be an algorithm somewhere for finding optimal prop size and speed for a given power output. And yes TooTall your summary of my question was correct.

**updated**

AJ. I wish you the best of luck on your build! One question??? Please ??? Can I be there when you test drive this thing. 90 hp! 12" or bigger prop! In the mud! Your talking supper godevil shit on a LONGTAIL!! I gotta see it. Once again. Good luck!

Haha Thanks. I'm on my way to pickup a used hydraulic steering arm off a normal outboard. I'm hoping this will solve the potential issue of my motor jerking if it's snags something. I'm going to connect the two hoses of the cylinder together with an adjustable flow control valve in the middle. I have the needle valve already. This will limit the amount the motor can swivel if it encounters a sudden shock. But allow it to twist freely if you move it at at a slower rate. You can almost think of it like a steering stabilizer on a motorcycle. I just hope the piston seals inside the cylinder can withstand the high pressure shock.