DF25A propeller experience

[2014DF25ARS]

I have a DF25ARS with about 45 hrs on it. Have a 4" SMIS gauge which is now working great, plus paddlewheel attached. I carry a Garmin Nuvi GPS with me most of the time to use for GPS speed.

My boat is a 1955 Feathercraft Deluxe Runabout, a 13-ft V-hull bare-bones tinny with remote (front) steering and 250 lb factory weight. It's rated for a 40 HP motor, but I was concerned about motor weight (and cost) when choosing a 4-stroke motor, so I chose a 25.

My motor came with an aluminum 10 1/4 diameter 11-pitch prop. With that prop, I hit max RPM of 5850 at GPS speed approx. 28 MPH (on my home lake with no significant current). DF25A has advertised gear ratio of 2.09. When I do these numbers in a prop calculator, it tells me I'm getting 3% slip, which shocked me at first - seems too good to be true. But it's hard to argue with the numbers. Honestly, this default prop seems nearly perfect for my application. It gets me close to the top of my RPM range and seems extremely efficient with only 3% slip. As I see it, it would basically take more than 25 HP to push my boat any faster.

A few weeks ago I was at a gathering with several other similar boats. I ended up in a bit of a race with one of them. He was running a 1959 Evinrude or some such. He was slightly faster than me, pulling away at WOT by perhaps 1 MPH. He told me he clocked himself at 29.7 MPH during that run. I sorely want to beat him next time we meet, using my shiny new plastic 4-stroke motor vs his smoking, leaky, bulletproof 55-year-old motor, and it seems to me that one of the few things for me to improve is to try to squeeze a few more % out of my prop. I reckoned that a stainless prop might have less resistance than the aluminum prop, allowing me to rev up to 6000 rather than 5850, thereby gaining about 1 MPH.

With that in mind, I bought a Solas New Saturn 11-pitch prop, which has a diameter of 10.5 inches (vs the rated 10.25 inches for my motor). I put the prop on my boat and found several things:

(1) The engine is much louder than before when I'm idling out of the harbor in gear. Sound from the exhaust is much more noticeable.

(2) The engine seems to vibrate more in those idling conditions, though my perception may be clouded by the increased noise.

(3) I can readily hit the 6200 rev limit with the stainless prop.

(4) Unfortunately, at the rev limit with the stainless prop, I'm not going any faster than I was at 5850 with the aluminum prop! I'm not sure how to explain this... unless the Suzuki factory prop is actually more than 11 pitch, or the Solas is actually less...

So I've gone to a local prop shop and had a discussion with the magician who owns the place. We agreed he's going to cup my stainless prop, which appears to have little/no noticeable cup from the factory. I'm going to take the prop to him Monday.

In another thread, Moonlighter said that when he's evaluating props, he likes to get a table of RPM vs speed at 500 RPM intervals, and then a holeshot test of time to reach 5,000 RPM. Therefore I went out this morning and collected said data in my boat with the stainless prop. I'm going to give the data to the prop man with my prop Monday.

Unfortunately, the only convenient place for me to do my test this morning was the local Tennessee River, which has a current of 2-4 MPH at various times of year. So I did four runs up the river: 2 downstream and 2 upstream. I was noting both the paddlewheel water speed and the GPS speed, but I'm using only the GPS speed, averaged between an equal number of upstream and downstream runs. My boat begins to plane around 3500 RPM with this prop, so that's where I began the data collection; at 3500 I'm not quite reaching plane.

Here's my data before cupping the prop. I will follow with post-cup data in a week or so:

11-pitch Solas New Saturn stainless prop

RPM Speed(MPH) Slip?
3500 13 25%
4000 15.25 23%
4500 19.5 12.5%
5000 22 12%
5500 24.5 10%
6000 27 9.7%

I hit 5000 RPM from an in-gear idle in about 2 secs ("one-one thousand, two-one thousand). In my perfect world, I want to hit 30 MPH at 6,000 RPM...

I'll keep you in suspense for a week or so till I have a chance to test with the cupped prop. Unless someone convinces me to NOT have the stainless prop cupped. I'm leaving the Suzuki prop alone, because it seems nearly ideal to me.

[Moonlighter]

The mystery and magic of props!

The only real way to be certain about the pitch of any prop is to have it propscanned and measured by a spe******t shop, then you will know.

I will bet that your alloy Suzuki prop isn't the 11" pitch it is claimed to be, as 3% slip is pretty well impossible as you know.

Will cupping the stainless 11" Solas get you the extra speed? Maybe. I always understood, though I am happy to stand corrected, that cupping will help the prop grip the water better and reduce things like cavitation in turns, and when trimmed out, but not sure if it will get you more speed?

If Solas in the USA operates the same as they do here, they will swap props with you until you find the one you are happy with.

I'd be more inclined to go up an inch or even two inches in pitch, should reduce revs by 150-200rpm per inch and give you a bit more top end speed.

But maybe your local prop expert can assure you that you'll get the extra speed you're after when they've finished with it, they may be repitching it a bit as well?? Although once the prop has been worked on, you will then likely be stuck with it as Solas wont want it returned.

Keep us posted, this promises to be interesting, as I'm doing a similar project to yours.

A mate and I have just gone partners in a similar tiller steer tinny, to build up as a river/estuary weapon. We have plans to head up to Cape York Peninsula in northern Australia, to chase Barra.

It's a 4.3m (14ft) V nose punt made of welded aluminium. 1990 vintage but looks maybe 5 years old, it's hardly been used by the look of it, came with a 1995 Johnson 25hp, also looks hardly used. Must have sat in someone's garage for most of the time, we figure.

It had 3 cross-thwart seats, we've ripped the centre one out and had a flat floor put in as well as front and rear casting decks, bow-mount electric trolling motor is installed and electronics bracket made up. We are debating how well the old 25 Johnno will push it along with 2 fat old bastards like us on board, so I've got my local Zuke guy on the lookout for a good, used, 30-40hp 4 stroke. But once it's ready for the water we will try to Johnno and see how it goes.

[2014DF25ARS]

I dropped off my two props at the wizard's lair over the weekend. He has a nice set of lockers outside the door where you can leave a prop locked up with a work order, and he finds them Monday morning. I left my factory prop: Suzuki 10.25 x 11 aluminum; and the Solas 10.5 x 11 SS. I left him the performance numbers I showed earlier, and asked him to make the SS prop perform better than the factory aluminum prop.

Spoke with him this morning. He said he measured both props and found that the Suzuki prop has Pitch progressing from 11 to 12, while the Solas prop has pitch progressing from 9 to 11 on two of the blades, and 9 to 10 or 10.5 on the third blade. Says the Solas prop has a slight Cup on it, but not much. He feels that the biggest difference between the two props is the Rake. Suzuki Rake is 11-deg, while the Solas prop has only 7-deg Rake. He feels that the additional Rake is allowing the Suzuki prop to lift my light hull further out of the water and hence the additional speed. Frankly, this concept as I understood it from him makes no sense to me. He's planning to add Rake to the Solas prop and add a bit of additional Cup as well as try to tweak the Pitch on the weak blade (though he said that's really not a problem.) Promises I'll have it back by the weekend...

It's starting to feel to me like the prop industry has made all this sh*t up and somehow reached agreement - probably in a smoke-filled room behind a boat show somewhere - to stick to these terms when describing their products, in order to add some credibility to what they sell. But this guy has me convinced he knows what he's doing, and his reputation is sound in these parts.

Fingers crossed

[Moonlighter]

Hahahaha, had a good laugh at that!

It is serious voodoo magic that the really good prop guys can do. I can't explain it either!

We do have a prop company locally here that uses a bit of gear called a propscan, they can measure the props using this and give you a computer printout that shows the rake and variation in pitch and also, I think, balance.

They do a before and after measurement so you can see what has changed.

My local Suzuki guy now sends off all brand new props that are going to be used in a twin engine installation to be scanned and tuned so they are identical, as he has found this avoids issues like engine resonance. He reckons the differences he has found in brand new props that are supposed to be identifical have been quite marked - pretty much as you have found.

Keep us posted!

[BD1976]

This last weekend I put a powertech ss prop on my DF25A and the prop chatter
in gear at idle is terrible. It levels out with a little throttle but I need to figure
out if there is another ss prop that will be better or if I need to stick with aluminum. At idle this thing sounds like a tractor!

[Solarman]

Copied the following from Hoosem.. who shared it on "DF300 prop woes" page.

Interesting read, a good short course on props.


Interesting to read for all boat owners who wants to know a little about props:
prop-school-part-1-introduction
prop-school-part-2-terminology
prop-school-part-3-blade-rake
prop-school-part-4-blade-cup
prop-school-part-5-blade-efficiency
prop-school-part-6-slip

Click on pics to see better.

Enjoy.

[2014DF25ARS]

I agree with BD’s experience re: in-gear idle noise. Both the stainless props I put on my DF25A are much louder and vibrate more than the factory aluminum prop at no-wake speed. I just live with it... I don’t notice much difference when at cruising speed.

[Hoosmatroos]

Keep in mind that a SS prop also gives you a much louder "clunck" when you put it in gear. The SS prop is much havier so more strain is put on the clutch. When you hit an object with a SS prop then there can be more damage than with an aluminium one. But, a SS can be more efficient. With that in mind, when you do only some fishing and occasionally put the throttle down, can you justify the costs for a SS prop? When you do a lot of hard runs, then a SS makes sense in my opinion.

Says the guy with a DF200 with 85% trolling and a SS prop

[Moonlighter]

To be honest, on a 25hp engine I think you woulD be hard pressed to notice much difference with a SS prop over a good alloy one.

That, plus the typical use of smaller motors in rivers and dams and the increased likelihood of hitting the bottom, rocks, logs or something else, and I dont think a SS prop makes much sense or value. I would stay with an aluminium prop myself. Have one on my DF40 that is used in similar places.

[BD1976]

Thanks for the info guys! I am sending my ss prop back and going to stick with
aluminum.