Prop Selection Yamaha F20 4-stroke

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STMU
Posts: 148
Joined: October 4th, 2017, 5:56 am

Prop Selection Yamaha F20 4-stroke

Post by STMU »

Hey everyone, I need some help propping a 20hp yamaha 4 stroke on a tiller 16' Alweld jon boat. I currently have a 9 1/4 x 10 - 3blade prop. Solo boat runs great, hoping right on plane and cruising 24-25mph at WOT. With me, the wife and a loaded down cooler it still does well, taking a little bit longer to get on plane and pushing 18-20mph. With 2 large guys and a big cooler, it starts to lose more overall top speed (15/16 mph).

I suspect the 2 large guy scenario has more to do with overloading the boat and not enough overall engine power. But never the less...my question...will changing the pitch help with top speed at all? Could I go with an 11 or 12? Or, would any performance change be negligible and I should stick with what I have?

I have no major complaints about the boat performance, but am needing a new prop and wondering if its worth the effort trying a different pitch. Thank you!
Salty Gator
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Re: Prop Selection Yamaha F20 4-stroke

Post by Salty Gator »

STMU wrote:Hey everyone, I need some help propping a 20hp yamaha 4 stroke on a tiller 16' Alweld jon boat. I currently have a 9 1/4 x 10 - 3blade prop. Solo boat runs great, hoping right on plane and cruising 24-25mph at WOT. With me, the wife and a loaded down cooler it still does well, taking a little bit longer to get on plane and pushing 18-20mph. With 2 large guys and a big cooler, it starts to lose more overall top speed (15/16 mph).

I suspect the 2 large guy scenario has more to do with overloading the boat and not enough overall engine power. But never the less...my question...will changing the pitch help with top speed at all? Could I go with an 11 or 12? Or, would any performance change be negligible and I should stick with what I have?

I have no major complaints about the boat performance, but am needing a new prop and wondering if its worth the effort trying a different pitch. Thank you!

You may loose a lot of hole shot if you go with that much pitch. You can call power tech and they will walk you through it. Good luck
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tailwaters
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Re: Prop Selection Yamaha F20 4-stroke

Post by tailwaters »

I have a G3 1548 with a Yamaha 25hp 2 stroke. It has an aluminum 10x11 4 blade on it and gets 22mph fully loaded (salty gator, myself, Yeti 45, 2 batteries and a trolling motor). Prior to that it just had the stock prop on it that came from Yamaha. My hole shot wasn't very good and I kept getting blow outs in turns. The 4 blade fixed those issues and my top end stayed about the same. I could achieve better top end by going with a 3 blade but that wasn't my concern. Before buying a new prop let me make a few suggestions of things to check first.
- Make sure you motor is mounted at the correct height on the transum. The running plate should be level with the bottom of the boat
- Make sure the weight in you boat is evenly dispersed. Too much foward or back plays a big roll in hole shot and top end.
- Make sure your motor is trimed properly. By moving up two notches on mine I was able to get and extra 2 mph out of mine.

Going from 24-25 mph to 15-16 mph just shows you how much a differance weight plays in the performance of a boat. Try not to bring less stuff next time. Assuming you are flats fishing a "Big" cooler is not needed and that adds alot of weight.
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Re: Prop Selection Yamaha F20 4-stroke

Post by silverking »

Good tips already. Another critical factor is the rpms. Do you have a tachometer installed? Check with the engine manual to see what you should be turning at wide open throttle. To maximize performance and keep the outboard from laboring or bogging down, you should be within approx. 500 rpms of the recommended WOT performance. In other words, if the recommended top end is 5800 rpms, ideally you should be 5600-5800 rpms wide open with a normal load. Adjusting the pitch up or down will impact the performance.

Of course, that is for normal operating conditions. If you overload the boat with two big anglers, a heavy cooler, full tank of fuel, gear and other stuff, your numbers are not going to be as good. You'll get a better hole shot with a four-blade prop but lose top end. Stainless steel doesn't flex as much as aluminum, but it's also less forgiving if you hit something hard.

If you don't have a tach, a Yamaha service center like Shields or Sunshine Boats could hook one up temporarily to see what kind of engine load you're getting.

SG's suggestion of calling PowerTech is a good one. Their technicians have a ton of experience and can probably recommend the best prop based on the boat and load.

Good luck and post up the results. Might save another forum member some trial and tribulations. :thumbup:
"Sun rise and sun sets. Since the beginning, it hasn't changed yet." Little Feat
STMU
Posts: 148
Joined: October 4th, 2017, 5:56 am

Re: Prop Selection Yamaha F20 4-stroke

Post by STMU »

tailwaters wrote:I have a G3 1548 with a Yamaha 25hp 2 stroke. It has an aluminum 10x11 4 blade on it and gets 22mph fully loaded (salty gator, myself, Yeti 45, 2 batteries and a trolling motor). Prior to that it just had the stock prop on it that came from Yamaha. My hole shot wasn't very good and I kept getting blow outs in turns. The 4 blade fixed those issues and my top end stayed about the same. I could achieve better top end by going with a 3 blade but that wasn't my concern. Before buying a new prop let me make a few suggestions of things to check first.
- Make sure you motor is mounted at the correct height on the transum. The running plate should be level with the bottom of the boat
- Make sure the weight in you boat is evenly dispersed. Too much foward or back plays a big roll in hole shot and top end.
- Make sure your motor is trimed properly. By moving up two notches on mine I was able to get and extra 2 mph out of mine.

Going from 24-25 mph to 15-16 mph just shows you how much a differance weight plays in the performance of a boat. Try not to bring less stuff next time. Assuming you are flats fishing a "Big" cooler is not needed and that adds alot of weight.
Great advice. Thank you. When I was building the boat and mounting the motor I spent a lot of time to make sure the motor was mounted at the correct height and trimmed properly before bolting it to the transom. The weight, on the other hand, is probably the problem. I'm always conscious of how much weight I am putting in, but sometimes I like to chase cobia, which requires a bit of extra weight in the cooler. I also play with the distribution a good bit, and end up having the second person move up to the front or middle when taking off. Once on plane, I'm usually good to go. My hole shot is pretty good overall, so I don't think 4-blade will help (especially given the top end performance issues). Did you ever try a 3-blade with an 11 pitch?

The description of your boat actually sounds similar to mine, and I wonder if the extra 5 horse and 2-stroke performance have something to do with the extra speed your getting?
Salty Gator wrote:You may loose a lot of hole shot if you go with that much pitch. You can call power tech and they will walk you through it. Good luck
I looked up their website and will definitely be giving them a call.
silverking wrote:If you don't have a tach, a Yamaha service center like Shields or Sunshine Boats could hook one up temporarily to see what kind of engine load you're getting.
I don't have a tach and have been wondering about how to figure that information out. I will give a shields a call and see if they can help.

Thanks for all the help everyone.
rockyg
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Re: Prop Selection Yamaha F20 4-stroke

Post by rockyg »

Excellent advice above from guys who know what they are talking about.

I will mention another factor. Your boat is simply underpowered. That little 20 is doing all it can. You can throw props at it all you want but your two big guys and a big cooler and 10 rods and 3 tackle boxes and bait, and the dog and some lunch and don't forget beer scenario just ain't going to happen with a 20 horse motor.

A 16 welded Jon boat is a heavy boat......and needs a 40hp motor. Even a 25 would be better. (Anything bigger would be better)

So pat that little 20 on the head as you walk by and thank it for pushing it's guts out for you.
Last edited by rockyg on July 19th, 2018, 11:51 am, edited 1 time in total.
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rockyg
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Re: Prop Selection Yamaha F20 4-stroke

Post by rockyg »

On a happier note I see that Tailwaters new to him bigger Jon is way slower than my Gheenoe so I will have no problems beating him (and Salty Gator) to all his secret spots. :lol:
“It's hard to measure almost.....because almost doesn't matter”
― John Dutton
STMU
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Re: Prop Selection Yamaha F20 4-stroke

Post by STMU »

Thanks Rocky, I've got no major complaints with the motor, just wondering if a little extra 'get-up' is possible. Plus, I knew the 20hp was a little underpowered, but I got a heck of a deal and 90% of the time I'm going just above idle anyways!

At the time I got it, the Yamaha 25hps that were coming out were 50lbs heavier than the 20hp. Yamaha was using the same block for the 40's on the 25's (they stopped that nonsense soon after). I couldn't find a good used one, and the price difference between the 20 and 25, and the added weight led me to the decision to go with the 20hp. The only time I regret it is when somebody passes me to my spot!!! Maybe I could pack less beer, but I'd rather just go slow!!!
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Re: Prop Selection Yamaha F20 4-stroke

Post by Salty Gator »

STMU wrote:Thanks Rocky, I've got no major complaints with the motor, just wondering if a little extra 'get-up' is possible. Plus, I knew the 20hp was a little underpowered, but I got a heck of a deal and 90% of the time I'm going just above idle anyways!

At the time I got it, the Yamaha 25hps that were coming out were 50lbs heavier than the 20hp. Yamaha was using the same block for the 40's on the 25's (they stopped that nonsense soon after). I couldn't find a good used one, and the price difference between the 20 and 25, and the added weight led me to the decision to go with the 20hp. The only time I regret it is when somebody passes me to my spot!!! Maybe I could pack less beer, but I'd rather just go slow!!!
Don’t pack less beer, just leave a little earlier :beer:
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tailwaters
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Re: Prop Selection Yamaha F20 4-stroke

Post by tailwaters »

rockyg wrote:Excellent advice above from guys who know what they are talking about.

I will mention another factor. Your boat is simply underpowered. That little 20 is doing all it can. You can throw props at it all you want but your two big guys and a big cooler and 10 rods and 3 tackle boxes and bait, and the dog and some lunch and don't forget beer scenario just ain't going to happen with a 20 horse motor.

A 16 welded Jon boat is a heavy boat......and needs a 40hp motor. Even a 25 would be better. (Anything bigger would be better)

So pat that little 20 on the head as you walk by and thank it for pushing it's guts out for you.

I thought the same thing as Rocky regarding needing a bigger motor. Assumed this was a new to you boat and didn't want to go there. I have read about others with the same setup as mine running 28 mph. Those guys generally ran SS 3 blade. Where I spend most of my time fishing there are lots of rocks around and it's very shallow. I would much rather replace the prop than a drive shaft which is why I stayed with aluminum.

Power Tec props are some of the best you can buy and they have great CS. Just be prepared for the price tag. The one on my 250hp cost around $700

You mentioned needing the big cooler for Cobia. You might want to look at a fish bag. They are easy to stow away and don't weight anything. When you need to use it just take the ice out of the smaller cooler.
rockyg wrote:On a happier note I see that Tailwaters new to him bigger Jon is way slower than my Gheenoe so I will have no problems beating him (and Salty Gator) to all his secret spots. :lol:
But I know all the short cuts :)
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Re: Prop Selection Yamaha F20 4-stroke

Post by silverking »

I've got a Mercury Trophy Plus four-blade 12P stainless prop on my 60-hp Mercury four-stroke. Great hole shot and top end. Spins up to 5950 rpms, per factory recommendations.

I also know all the shortcuts. So I can beat tailwaters and rockyg both to the secret spots. :lol:
"Sun rise and sun sets. Since the beginning, it hasn't changed yet." Little Feat
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