Dinghy Propeller Pitch - Who's experimented?

[Cowpoos]

I am looking to get a second prop for my dinghy for higher loads. I haven't been able to find a decent online tool to help with the decision, so I was curious what other people's experiences are.

I have a 2.9m (9.5 ft) Aluminium RIB - Approximately 380 - 400kgs (830 - 880 pounds) all up with a heavy load - 15hp Suzuki Two Stroke outboard. I currently have a 9 1/4 x 10 3-blade prop, which easily gets on the plane with two adults with a total boat weight of 270kgs (595 Pounds).

I am thinking of going to a 4 blade prop, but I can't decide which pitch. So I am after actual experience with a similar set-up in the real world.

[rslifkin]

Put a tachometer on the engine and see what you get for RPM at WOT with a few different amounts of load. Knowing how the engine is loaded with the current prop will give you a better idea of what's appropriate for a new prop.

[Tin Tin]

Sorry I don't have exact same set up but in the absence of other replies my experience is:



We have a 10ft alloy Rib with 15HP Yamaha 2 stroke alloy 3 blade 9" pitch which is one down from standard I believe. It planes with 3, struggles with 4 but pulls a wake-boarder.



The things that will make a difference will be hydro fins that help get the stern up, not sure if I would go lower than 9" pitch as top end revs may be too high. Four blade will give more thrust. On a previous dinghy with 20HP tohatsu 4 stroke, dropping the pitch by an inch to a four blade stainless + hydrofins made a huge difference to load carrying.

[SailingHarmonie]

I have, and I'll tell you it makes a HUGE difference. You can get a simple and inexpensive tack driven by induction off the sparkplug wire. With the right pitch prop the boat will climb on plane faster, run faster, and last longer than with the standard prop that shipped with the engine.

For most engines mounted on lightweight inflatables the standard prop that comes with the engine is WAY too flat allowing the engine to overspeed and wear faster than it should.

[Cowpoos]

I have a tachometer, but I don't want to pay for a pop and find it doesn't work for my purpose as I won't be able to return it once it is used.

So this is why I want some strong opinions based on actual real world examples...to confirm the hunch I already have.

[CaptainRivet]

Originally Posted by Cowpoos

I have a tachometer, but I don't want to pay for a pop and find it doesn't work for my purpose as I won't be able to return it once it is used.

So this is why I want some strong opinions based on actual real world examples...to confirm the hunch I already have.

Are there no prop building companies in NZ.
In Europe I would call Gröver in Germany and they would calculate for a little fee the different possibilities and match to props they can supply or make for you.
Rslifkin told you what you need to do RPM on different loads because that gives you already a very good hint where you stand.
Also what do you actually wanna achieve with the 2nd prop? Are you switching forth and back when heavy or lightly loaded?

[rslifkin]

To give some more info, our dinghy is bordering on underpowered with a 6hp on it. The original 8 pitch prop is perfect with 1 person, does about 13 kts at WOT and hits just about 6000 RPM (upper end of the recommended range). It'll reach the recommended RPM range with 2 people, but it struggles slightly to get on plane (which it barely does with 2 of us). A 7 pitch runs a little faster with 2 people and gets closer to the upper end of the RPM range. Definitely better if we load it any heavier. With 1 person, however, top speed with the 7 pitch is only about 12 kts as you have to back off slightly to stay off the rev limiter.

Personally, I keep the 7 pitch on the outboard and carry the 8 pitch on the big boat as a spare, being that I already have it and it's usable enough.

[MicHughV]

Generally speaking, outboard manufacturers have got their engines and props pretty much dialed in correctly.
There is a mathematical science behind it all.
Changing props might give you more power at the cost of less speed, but you can't get both power and speed by changing a prop.
It's a complicated science, but if interested, read " The propellor handbook" authored by Dave Gerr.

[rslifkin]

Originally Posted by MicHughV

Generally speaking, outboard manufacturers have got their engines and props pretty much dialed in correctly.
There is a mathematical science behind it all.
Changing props might give you more power at the cost of less speed, but you can't get both power and speed by changing a prop.
It's a complicated science, but if interested, read " The propellor handbook" authored by Dave Gerr.

Assuming the boat you've attached the outboard to is close to what the manufacturer imagined, yes. But they do offer alternate props for a reason. If you put the outboard on a bigger boat (with a lower top speed) than the manufacturer expected you'll generally need a lower pitch. And if you put it on a light, fast boat you may need a higher pitch.

[MicHughV]

yes, you are getting to crux of the matter....the "pitch"...less pitch equates to more power (thrust) , but off course, also less speed.
you can't have it both ways....

But engine manufacturers also play the "horsepower" war game, which is a combination of displacement and rpm. ie, as an example, many 6hp engines are merely de-tuned 8 hp engines, when I say detuned, they are the same engine, they are merely jetted to run 500 rpm less.
At the dealership the difference is more pronounced as you'll end up paying $1,000's more for the 8 over the 6, which is the end game for the manufacturer and dealer.

It's a balance. I used to have a 15 hp engine on my dink, which could put the dink and 3 people plus dive gear on plane without a problem. Running it at WOT throttle was a bit of gambit, as the dink became almost airborne. But it was heavy and sucked fuel.
Now I use an 8, not as powerful, off course, and not as fast, but does the trick. Can't put three people on plane, but will do two, but it takes some doing at times.

Then off course, your choice in a dink can make a big difference. Clearly, a hard bottom dink is required, but these come in all shapes, sizes and weights.

At the end of the day, most people simply go to a marine store and buy a dink and motor off the floor, without giving the matter too much though. If anything, the $$$ purchase price is usually more a driving force than performance and then you are stuck with your purchase where changing to another prop is not likely to provide any significant improvements.

[Captain Bill]

I recently replaced my stock prop on my 15hp Tohatsu with 7 inch pitch prop. I wanted an 8 but couldn't find one. The original would only plane three people with difficulty and that took some weight shifting to get it to plane. It's no problem at all for the new prop. It does create an issue in that at speed and WOT it hits the rev limiter. Backing off slightly does not make a noticeable change in speed but does cause the rev limiter to turn off. It will stay on a plane even with a significant reduction in throttle. With a load the old prop would fall off a plane unless I kept the throttle pretty open. It's pretty clear that the original prop had too much pitch and this one probably has too little. Though with three people and dive gear it is really nice to be able to cruise at a reasonable throttle setting. The old prop could not get the boat on a plane at all with three people and dive gear. The other issue it created is that one doesn't want to give it WOT from a standstill with one person in the boat. It may not be a problem for a remote model with weight forward but on my tiller steering model the bow really goes up. The op's Suzuki has a rev limiter so it will not over-rev.

[MicHughV]

if I cranked my 15 hp wide open from a standing start, with just me in it, the bow would flip up near vertical, way to much power..

[Jammer]

My advice would be to try a four-blade prop with the same pitch as your three-blade prop. And do get a tach, they're inexpensive and useful. I have one with a built-in hourmeter.


If you shop around you should be able to find aftermarket props for under $100 if you want to experiment. There's not really any substitute for trying different props if you want to get the perfect one. Although, my experience is, that it doesn't make that much difference if you're off by a size.

[MicHughV]

a 9.5' dink is a tad on the small size for a 15 hp imo...

I think going to an 11' size dink will solve most all your problems..

This is me and my dink, plus 15 hp Suzuki. Dink is 11'3" I believe.....never needed more than about 1/2 throttle to go anywhere..

[SailingHarmonie]

Originally Posted by MicHughV

Generally speaking, outboard manufacturers have got their engines and props pretty much dialed in correctly.
There is a mathematical science behind it all.
Changing props might give you more power at the cost of less speed, but you can't get both power and speed by changing a prop.
It's a complicated science, but if interested, read " The propellor handbook" authored by Dave Gerr.

This is just not a correct interpretation of the "mathematical science" behind propellers. There is no single correct prop for an engine. Every boat and engine combination is different and requires its own calculations to pick a propeller. Small outboard engines are sold with a standard prop that the manufacturer figures will be "close enough" for most users in most cases. It is most certainly not "dialed in" to your boat and the way you drive it.

The whole text of Gerr's book (which is excellent, BTW) is all about solving this problem of matching a prop to a given engine and boat combination.

You most certainly CAN get more power and more speed by changing the prop. If a prop is significantly too highly pitched, the engine will never get up to its rated speed, and will not produce full torque or power. Shigting to a lower pitched prop will let the engine actually run up to the RPM where it generates its rated power, and you will go faster exactly BECAUSE you have more available power.