---

1996 Dauntless 15 with a 2007 Yamaha 90HP 2 Stroke 20 inch (WOT rpm) 4500-5500

Outboard is mounted all the way up / cavitation plate is nearly perfect with keel (+ -) 1/2 inch
Prop is a Solas (older Saturn model) S.S. 3 blade D13-1/4 X 17P / O/B RPM (WOT) 5200 41.5 MPH (GPS)

I am considering a 4 blade prop for my Whaler. I have researched 2 makes and styles, Solas Titan HR 4 (D13-1/4 X 15P) S.S. and Stilletto Bay Pro 1 (4 Blade D13-1/4 X 16P) S.S..

From my calculations the Solas will take me very close to the 5500 RPM and the Stilletto will near 5350 - 5400 RPM. I am not interested in top end speed as I am in better fuel consumption at midrange operation. Any thoughts will be greatly appreciated.

Angel M.

---

I would use a 14" x 15" Stiletto Star 4.25 with its large blade area and low rake before I choose a four blade Bay Pro for that little boat.

---

Thank you Mr. Clark for responding. I hope you are not upset that my thread title had your name. Yes the Dauntless is only 15 feet in length but compared to most of B/W 15-teens its a bit more boat.

The Stilletto you suggest what (WOT) RPM's would I be looking at?

Why the increase in diameter?

You feel a 4 blade will not work for my application, please explain the draw backs so I can better understand?

My whaler came with the Solas prop when I purchased the boat used. The previous owner had tested various props and makes before deciding on the Solas.
He provided me with a detailed test log that he compiled detailing RPM's vs. MPH (GPS based) through various operating ranges ending with WOT and final top speed for each prop.

I am looking at the 4 blade on the thought that I could improve fuel consumption at low to mid-range operation, get on plane quicker with out burying the stern and pull a tubber or skier with less effort. I sometimes navigate the "Skinny" waters that the flats boats do and getting on plane in these ares can be tricky. Thus the 4 blade theory of better stern lift.

---

We don't have a choice in diameter....

We can only purchase a prop by PITCH from one brand or style of prop.
The engineers choose the diameter and blade design...

I have said this hundreds of times....
We by the brand of prop by Pitch ONLY....

Take a look at this chart for the Stiletto Star that Tom suggests....
Do you see where you can choose the diameter....? No.... Only the Pitch...
http://boatpropellers.iboats.com/Stil...ropellers/

So, forget about diameter..... Leave that up to the engineers....
In anyone brand, you would choose 13, 15, 17, 19, etc., pitch.....

---

Thanks Joe for your input. I didn't make any reference to a specific diameter, Mr. Clark suggested a brand and a size. This will be a lengthy write-up I can feel it...

My inquiry is for a 4 blade over a 3 blade. I have been in contact with an engineer from Solas regarding the topic but, I would be interested in understanding why Mr. Clark feels a 4 blade may not work for me.

I am aware of the small impact that diameter has (+ or -) over rpm as pitch will (+ or -) 150-200 rpm per every 1 inch (+ or -). To my understanding the D14 X 15P Stilletto could exceed my 5200 rpm by 400 more, yes / no? I am at 5200 rpm with a 17 pitch.

If not, please explain the advantage of this particular Stilletto model. From what I researched Yamaha typically suggested a D13 X 17P prop for this outboard.

I was originally just looking for a back-up / spare prop like my current Solas which is working great, then I got curious.

The prop Mr. Clark suggested has not come up during prop searches that I have done for my Yamaha 90 HP 2 Stroke on sites like iboat, Boats.net, and the likes. I have looked at 15, 16 and 17 inch pitches (3 blade) but the diameters have stayed in the 13 to 13-1/2 inch range and possibly one in 13-5/8.

Both Solas and iboats infered that the 4 blade props I was inquiring about are adequate for my motor with the exception of the RPM difference between the Solas 15 and Stilletto 16 inch
pitches. Keep in mind that my whaler is a bit over powered (not really if you consider 4 adults and dive gear).

I am curious to why the length of my whaler or small boats for that matter should stick to 3 blade and not a 4 blade. From what I've reviewed Mr. Clark appears to be quite versed in prop tech, so inquiring minds would like to know the draw backs.

The manufactures along with the retailers just want to sell so their input might leave some of the issues unmentioned for the consumer, thus my reason for the thread. There is quite a bit of confussion on the few forums I visited with regards to 3 blade vs. 4 blade purchase. So again any and all input will be greatly appreciated.

---

I was just going by what you said in post#3


I am not that up on 4 blade versus 3 blade but I know I had a 4 blade on my Outrage 22' Cuddy with a Johnson 175...
It was an Offshore prop.

Other 4 bladed props are also used for High Thrust... Like trolling motors and pontoon boats, etc...

Good question and maybe someone will have more info.

---

First of all, let me straighten a few things out.

I never said a four blade propeller may not work for your application. Please do not put words in my mouth. What I said is I would choose a 14" x 15" Star 4.25 before I choose the 13-1/4" x 16" Bay Pro. You ask about the drawbacks of choosing the Bay Pro over the Star? Answer: about $100 and *maybe* some loss of performance. My thinking is that if you can get a propeller that offers better performance and costs less, that is the prop to select.

Propellers are not generic pieces of hardware that can simply be described by number of blades and diameter and pitch. There is far more to propeller design than just the number of blades and the pitch and you cannot have any meaningful discussion of propellers without also discussing the particular model of propeller being considered. There is no way to generalize the differences between three blade and four blade propellers, though manufacturers (and ill-informed boaters) still try to do just that. Drives me nuts.

If anybody offers you advice about the right pitch or choosing a four blade over a three blade propeller, and they are NOT telling you what particular model of propeller they are talking about, they do not know what they are talking about.

Every different boat/motor combination is unique. A propeller that works well on one boat may not work well at all on another. Complicating things even further is the motor mounting height for a given boat which can vary anywhere from 1-1/2" to 3" for motors simply bolted onto the transom to much more for motors mounted on jackplates. Boaters also use their boats in different conditions. Two guys with identical rigs might select very different propellers if one boats on calm lakes and wants top speed and the other boats in rough ocean conditions and wants better acceleration and grip.

Selecting pitch and predicting WOT RPM cannot be done without considering the particular model of propeller because different models of propellers will yield very different calculated slip figures. The Stiletto Bay Pro will probably yield negative calculated slip whereas the Solas you have now is yielding about 1 percent calculated slip. Other propellers can have calculated slip of 10 percent or more. [ And please folks, let's leave the calculated slip vs. actual slip discussion for another thread. ]

Joe meant to say that we select a model of propeller in a given pitch, not diameter. I belabor this fact out not to criticize Joe for misstating it, but because I do not want anybody reading this now, or in the future, to misunderstand the point Joe was actually trying to make.

Within any given model of propeller, there will be various pitches offered and the diameter will be a given for that model and pitch. There are actually some exceptions to this rule, where a particular model of propeller in a given pitch is offered in two diameters, but they are very few.

We describe propellers by diameter x pitch, in that order, expressed in inches, because that is the conventional shorthand and it helps us to understand and remember how different models of propellers are designed and to compare them, though again, a particular model of propeller in a given pitch with have the diameter the propeller designer gave it, period.

---

OK, now let's talk about this particular application, a 1996 Dauntless 15 with a two stroke Yamaha 90 with its 5500 ROM reline and 2:1 gear ratio.

The boat is significantly overpowered, by 20 HP, so it has a high power to weight ratio making it somewhat unique and presumably fast.

Currently the motor is mounted as high as it can go (three holes up) using the fourth set of bolt holes (the lowest in the mounting bracket).

41.5 MPH at 5200 RPM is about 1 percent slip with the 13-1/4" x 17" Solas Saturn. I do not have personal experience with the Solas Saturn so I am unclear if 1 percent slip is typical or not. I will assume it is.

The stated goals for selecting a new propeller for this boat are to "...improve fuel consumption at low to mid-range operation, get on plane quicker without burying the stern and pull a tubber or skier with less effort. I sometimes navigate the "Skinny" waters that the flats boats do and getting on plane in these ares can be tricky."

OK, first of all, you will not have an easy time optimizing your fuel economy if you do not have a reliable and accurate way of measuring fuel flow. Do you have a fuel flow meter on your boat? If not, you will have to use a measured amount of fuel in a portable tank and use a GPS to log your distance and compare the two. That can be difficult to do accurately but if you are very strict and precise in your measurements, it can be done.

Getting on plane quickly without burying the stern requires a propeller with good stern lifting qualities. Propellers with larger diameters, more blade area and lower rake will offer more stern lift than small diameter, high rake propellers. The Stiletto Star fits that description with its 14" diameter, large blade area and reduced blade rake.

Selecting a propeller that gets the motor to the top of its recommended WOT engine speed range will also be a propeller that offers better acceleration than a prop that does not allow the motor to get close to it redline at WOT. The 17" Solas Saturn is only allowing 5200 RPM now on a motor with a 5500 RPM reline. If the Stiletto Star is chosen, there is no way the 17" pitch model will allow more RPM than the Saturn, therefore the 15" pitch is the one to use.

The 14" x 15" Stiletto Star will probably yield a calculated slip of about -8 percent based on the last test data I have of it on a Dauntless powered by a Yamaha. This suggests the WOT engine speed will end up being about 5400 RPM. If the boat gains a little top speed the RPM will rise even more.

---

One of these days we need to write an article on this subject....

If you look at all the various brands and models of props, we really don't have a choice in the diameter.

As Tom mentions above with the 14" x 15" pitch...
In that model and for the particular motor, you will only have choices in pitch. Again, the diameter is a given that the engineers have figured for that particular motor.

We do not have these choices in that model prop above:
13 1/2 x15
13 3/4 x15
13 7/8 x15
etc., etc......

Here is a statement on props from Evinrude.

Here is the complete article from Evinrude.
http://www.evinrude.com/Content/Pdf/e...766315.pdf

If you choose your motor in the list (this happens to be for Evinrude), then choose the model of prop. Here is what you will see if you choose the Viper model for 40hp to 130hp E-Tec....
763929 ViperTM 3 blade 13 7/8" x 15 pitch
763930 ViperTM 3 blade 13 7/8" x 17 pitch
763931 ViperTM 3 blade 13 7/8" x 19 pitch
763932 ViperTM 3 blade 13 7/8" x 21 pitch

You will notice the diameters are the same. This is not always the case but it is here.
And again, we have no choice in choosing the diameter.

We cannot say we want a 14" x 15" pitch in this Viper model propellor for our motor... Not possible....

That is the bottom line on diameters... The engineers figure this out along with rake etc...

Here are selections of props for different applications and shows 3 blade and 4 blade information.

---

Thank you Mr. Clark and Mr. Kriz for the input. Mr. Clark I would like to apologize if you felt I was misquoting your suggestion, was not my intention.

Many good points have been presented and I can better appreciate that making a good prop selection will require some serious homework on my and any other boaters part.

Mr. Clark if I understood correctly on the brief topic of stern lift, you mentioned a prop with a bit more diameter or surface area correct? That was why reason for considering a 4 blade over 3.

No, I do not have a flow meter. What I typically do which is a somewhat crude method is calculate or average actual fuel consumed by tank readings. My Moeller tank is an 18 Gal. model.

My bay trip destinations (unless doing some fishing) are pretty consistant. I will typically retrace my GPS route to and from the destination and surf conditions are typically light to moderate chop. I try to maintain a conservative rpm of (3800-4200) while in route (to and from) and when conditions allow, I also try to factor the motor op. hours.

Where I notice the elevated fuel usage is when I find that I have done more "No-Wake Zone" idle speed travel than usual. More and more waterway areas down here in So. Fla. and the Fla. Keyes are now being deemed "No-Wake" or idle speed zones.

I came to the conclusion that my prop was not efficient at these lesser speeds for the boat thus the 4 blade decision.

I will research your suggestion and attempt to contact the Stilletto tech's for further input. Mr. Clark, can you offer a contact at Stilletto that I could speak with? Thank you both again for your patience and time regarding this topic.

---

Angel -- I do not follow that logic. The boat gets reduced fuel mileage at slow speed because two stroke outboard are very inefficient at slow speeds. It has little or nothing to do with the propeller, but even if it did, I can think of no reason going to a four blade propellor would help.

For an intermediate gearcase motor like your 90 on a Dauntless 15, there really are not that many propeller choices. Yamaha offers a number of propeller models that might work including their Performance Series 3 Blade, Performance Series 4 blade and Pontoon Performance Series. These propellers are all made by Precision Propeller Industries, Inc. (PPI) which is owned my Yamaha. PPI also makes Stiletto and Turbo brand propellers. In fact, the Stiletto Star I have recommended is the same as the Yamaha Pontoon Performance Series except it will cost quite a bit less.

There is nobody in Stiletto tech support or their sales network that knows more about fitting Stiletto propellers to Whalers than I do.

---

Here is one small article Yamaha wrote about their Pontoon Performance Series propeller. The topic was unusual propeller applications (they do an article like this almost every month to highlight the versatility of their propellers and to help folks think "outside the box")

---

Just got a chance to read your last post Mr. Clark and thank you for all your valuable input on the subject.

I am aware that 2 strokes have issues a low idle speeds, I was hoping that I could somewhat improve on this by re-proping the Dauntless a bit more efficiently.

I will follow up with your Stilletto model suggestion and I do agree, you are the "Whaler Proping" guru as far as I'm concerned.

Thanks once again...

---

Actually, it is the four strokes that suffer prop rattle not the two strokes.