20SC w/ 115 #'s and opinions needed

[mrobertson]

Today i splashed my 1989 Tracker 20 with a brand new 2005 Yamaha 115 V-4 2 stroke. I bought the engine leftover, still in the crate, so its brand new not rebuilt.

My restoration pics are in the repairs section. Boat has a new deck, new 50 gallon fuel tank, no T-Top, and a 25 inch transom. It's a very basic fly fishing and light tackle setup, so it's probably pretty light compared to others.


Bare in mind, i have 3 hours on the engine. After i got through the first hour of break in, i could push to WOT and trim it back to 4000 RPM's.

4000 RPM's - 25 mph
4200 RPM's - 27 mph
4600 RPM's - 30 mph

I don't have alot of time yet opening it up but the few times i have i only get to 5000 - 5100 RPM's and about 31 - 32 mph.

I almost feel like the prop loses it's "bite" if that makes sense. Running 30 mph, i have plenty left on the throttle but the boat doesn't go anywhere when i open it up.

The prop i am running is a 13.25 x 17p ALUMINUM prop. I'm 100% certain i could do better with a different prop.

So - i'm rolling over to you, prop gods.................what kind of prop should i look for?!?!?!


-Mike

[McGillicuddy]

Mike,
Congrats! Where the heck did you find a new 2005? Per the other thread, you boat looks great!
Not a prop god but have tried a few on a 20 w a 115.
Need more info. What brand and model prop? How high above the keel is your anti cavitation plate at level trim.
Are you achieving max rpms at WOT? That's the first step. 4600 rpm does not sound like WOT. But 30 mph at 4600 ain't bad. If thats all you're getting you need to check motor height and prop ID. If you are running an aluminum Yamaha semi-cleaver, I think you can do better.

I think the 20 with a 115 really runs best with a 15 pitch. The exception would be an etec 115 that probably has the balls to get a 17p up to speed. I actually gained top end by dropping in pitch because the motor found its sweet spot.

That said if you regularly fish skinny water you might be able to raise your motor and that will gain you rpm thus more pitch is possible.

I keep preachin' the same thing: the Merc Black Max/Quicksilver Black Diamond are tremendous stern lifters that are easy to turn and grip pretty well. 13-3/4 x 15 p. More blade area less cup and rake than a "hot prop." Out of the hole instantly. All of these props are avilable with a insert-able hub so testing is fairly cheap and simplified.

If you're fishing inshore near structure I see no reason to go stainless. If you do choose ss, Power Tech has a MQF series that is the same shape and will perform like a champ. Between 30 to 40 mph I don't think you'll find a better over all prop shape.PT Props also has a 4 blade semicleaver "SCD" that mimics the Yammie shape, could prove be very economical as well as offering great hole-shot, grip and top end. These props (shape) are designed for stern heavy rigs under 50 mph. Many props simply excel at higher speeds. If you had a 150 etec you could look to more aggressive cups and rakes.

Powertech Props has some nice info viddys that teach about prop selection.

[mrobertson]

The engine popped up on craigslist a couple of months back. Here local in Maryland a yamaha dealer had 3 of them. I'm not sure why they were never sold, but the dealer put them for sale and i got the last of the 3. New in crate, never run............it's a really sweet engine for sure. Yamaha would even honor a 2 year warranty if i wanted to pay to have a dealer install it. I chose to install myself as i hope to not have any major issues with it inside 2 years.

The safe operating range for the engine is 5500 RPM.

Right now my anti-cavitation plate is slightly above the keel but not a full inch. I only have 1 more hole to go up.

The prop is a Mercury Black Diamond 13.25 x 17p aluminum prop.

The boat comes right out of the hole with very little bow rise. I can stay on plane down to about 3500 RPM's and probably 13 - 14 mph.

I'm very satisfied with the cruise speed. I was expecting a little more top end but thats not terribly important. I fish the chesapeake bay waters mostly. 95% of the time an alum prop is fine. Where the stainless comes in handy is when i fish skinny water around the barrier islands where you are running in sand. The sand will file down an aluminum prop in no time at all.

Let me ask this...............if i had the exact same prop as i do now, just in stainless steel.....how would it affect the performance? Would i typically more or less RPM's, what would it change?

[McGillicuddy]

I think your planing numbers sound fine. But motor height sounds low. That motor is light by modern standards and is a real nice match for the 20. Definitely raise the motor a notch at least -too bad you can't go more. Get the 13-3/4 by 15 p Quicksilver Black Diamond/Black Max. Quicksilver number is QA2034X. Not sure, but I think Black Max is 48-77342 A45. If your prop is new, see if your prop guy will swap you. Offer him a small token of your esteem like a 20 spot or a case of beer for the guys. Let him keep the hub since you already have one. Take your prop box with if you still have it.

A stainless will typically be harder to turn so you would have to go to a prop of less pitch. Rule of thumb for same prop shape, drop 1"-2" of pitch for stainless.

You definitely want toget as close to that 5500 as possible. Otherwise the motor is bogging and will suffer in the long run. If you cant turn the 15p to at least 5300 you may have to go to the Yamaha semi cleaver series. Probably K (alum I think)or M15 series (SS). They come in stainless too. The Merc product otherwise would be better all around but the semi cleaver might be easier to turn having less blade area, and might get you better top end.

Trim tabs also help every thing.

[Bushwacker]

Originally Posted by McGillicuddy

. . . A stainless will typically be harder to turn so you would have to go to a prop of less pitch. Rule of thumb for same prop shape, drop 1"-2" of pitch for stainless. . .

Gillie,

My experience, at least with Michigan props, is that they're able to make the blades thinner on the SS props, so they're actually EASIER to turn than the same size aluminum. When I started trying 4 blade props, the local shop recommended a 14" dia x 20" pitch aluminum Vortex prop. The equivalent SS prop was the 4B Apollo, but they made it slightly bigger at 14.125" diameter x 20" pitch, I guess to compensate for the thinner blades. I could turn both props to the same rpm with same load, but the SS prop was about 3 mph faster!

When I was running the old 115 on my boat, the 13 3/4 x 15p OMC SST prop turned about the same rpm as the same size aluminum prop, but the SST had a lot more cup in the trailing edge. I was able to raise the motor 1 hole and also picked up about 3 mph with the SST. It was an excellent prop and the 15x15 4B SS PowerTech prop I'm now running supposedly has the same blade profile as the old SST. The 15x15 4B prop seems to be very efficient and runs about 4-5% slip at optimum cruise! Denny

[McGillicuddy]

Originally Posted by Bushwacker

Gillie,

My experience, at least with Michigan props, is that they're able to make the blades thinner on the SS props, so they're actually EASIER to turn than the same size aluminum. When I started trying 4 blade props, the local shop recommended a 14" dia x 20" pitch aluminum Vortex prop. The equivalent SS prop was the 4B Apollo, but they made it slightly bigger at 14.125" diameter x 20" pitch, I guess to compensate for the thinner blades. I could turn both props to the same rpm with same load, but the SS prop was about 3 mph faster!

When I was running the old 115 on my boat, the 13 3/4 x 15p OMC SST prop turned about the same rpm as the same size aluminum prop, but the SST had a lot more cup in the trailing edge. I was able to raise the motor 1 hole and also picked up about 3 mph with the SST. It was an excellent prop and the 15x15 4B SS PowerTech prop I'm now running supposedly has the same blade profile as the old SST. The 15x15 4B prop seems to be very efficient and runs about 4-5% slip at optimum cruise! Denny

Denny,
Thanks for sharing that experience. Duly noted. Theoretically, the ability to make blades thinner, should make them easier to turn. I think where maybe it gets tricky is that the finer casting makes more complexity available to the shape thus they can use more cup or rake in a similar geometry to improve the desired performance characteristics. Between that and the added weight varying models may or may not prove harder to turn.

Interesting that the Vortex/Apollo actually increases the diameter in stainless.

When I asked PowerTech about replacing my Black Max with their stainless equivalent (MQF series) they suggested dropping an inch of pitch even though they share the same diameter and approximate blade profile. That was the basis for my suggestion to Mike. A have not tried the stainless MQF version yet. And again they are not "identical" - my guess is that PowerTech has tweaked the design a bit.

Cheers,
Gillie

[Bushwacker]

Originally Posted by McGillicuddy

. . . I think where maybe it gets tricky is that the finer casting makes more complexity available to the shape thus they can use more cup or rake in a similar geometry to improve the desired performance characteristics. Between that and the added weight varying models may or may not prove harder to turn. . .

Gillie, that's a good observation. Whenever we improved our blade casting technology at Pratt, I never knew any engineer that didn't want to introduce more complex cooling schemes and 3-D aero designs that made the parts even harder to make! I'm sure prop designers are no different!

One other thing the could make the stainless props harder to turn is that the stronger material will hold it's shape better under load, where the aluminum props will tend to flatten out and lose pitch or cup. I think the Comprop folks that make the composite props actually claim that as an advantage, i.e. the prop flattens out to provide less pitch and allow engine to rev further up into power band when getting on plane. However that might be a good example of the old saying about fixing a problem, "If you can't FIX it, FEATURE it!" Bottom line is that prop design is so complex that you can't generalize . . . the reduced drag of thinner blades can easily be offset by subtle tweaks in blade shape, profile, rake, etc.! Denny

[mrobertson]

you guys have gone and gotten all scientific on me. now my head hurts!!!

i'm going to start with a black diamond 15p and see how the boat runs with that first. Then i will repost my numbers and we can go from there.

thanks for all the info!!!!

-Mike

[McGillicuddy]

Mike,
That 15p will be a big difference but even if you get 300 or 400 rpms out of the new prop you'll still have a ways to go. If you were only getting 4600 rpm wide open, if its not too big of a PITA you're gonna seriously want to consider raising that motor a notch, too. You'll likely get another 300 rpm and get real close to where you want to be (5500 rpm).

Also confirm that your tach is set to 4 cylinders so you know its not mis-reading.

If you want to post a shot of the keels relation to the anti-cav plate you might get better opinions.

Good luck. Please keep the gang posted on you results.

[eggsuckindog]

My honest opinion is you are kind far away from 6000 as you pushing the low end of HP for the hull. I would get ahold of Ken at Prop Gods-google. He will have some ideas and handles alot of prop lines and is pretty damn good on advice, he will be worth his weight in gold on this application