How to Estimate Optimal Propeller Pitch?

[Silverhawk24]

All,

Turning to the experts with a boat problem I'm trying to solve. Specifically, I'm in the process of changing the gear reduction of the transmission on my boat and will need to get a new set of propellers with a different pitch. The old set up worked just ok... I tested different propellers, but ended up settling on the one shown below. Testing different sets on the new transmission is possible, but expensive.

As such, I've compiled some data based on the old transmission and propeller (pdf below). My question is, based on the data I have, can I run scenarios to estimate the theoretical optimal propeller pitch using known variables? If so, are there parameters that I should try to solve for / match to known parameters (e.g., prop torque should equal some ratio of engine torque or something like that). I realize this is totally theoretical, but I can keep costs down if I can get the first test close.

One thing to note, the "Slip" column describes the difference between theoretical speed vs. observed speed... basically, it denotes the frictional loss = 100% Efficient - Observed Efficiency. The major factors here are i) frictional loss from hull drag, ii) propeller drag and iii) props ability to "hold" the water. I also suspect it relates to how efficient the propeller and drive are at transferring engine torque into thrust. The blue "Act Slip" figs are observed values, while the green "Est Slip" figures are plugged. The new transmission (sterndrive, actually) is a dual prop design that will decrease the single propeller slip figures significantly. The 12% I show for top RPM is based on observed data from others with a similar engine, hull and transmission combo.

Any help greatly appreciated!
View attachment Sterndrive Calcs_Updated_v2.pdf

 

[Silverhawk24]

I'm happy to share the spreadsheet, if helpful. The calcs are all audited and confirmed.

 

[SteamKing]

You can run various simulations, you just need to know something about the new propeller's performance characteristics. Most of this information is generally proprietary. The usual approach in these situations is to get the manufacturer of your transmission or the dealer to help you with a propeller recommendation, given the output characteristics of your engine and the size and type of boat you have.

If this approach doesn't work, I can recommend a third-party to go to for advice.

 

[Silverhawk24]

Last thing... not spamming this...

If there's some optimal relationship to shoot for between engine torque and some other metric... say thrust which I didn't calculate here... then I can introduce pitch as a variable with the book ends being min of 0" pitch and max of 25" pitch (from observed data). If I set thrust at zero for 0" pitch, which I realize is an imperfect assumption, then I can at least get a linear relationship. If that's over simplified, if there's a pitch vs thrust curve out there that is non-linear, I can take the estimate one step further.

 

[rcgldr]

The issue with pitch versus thrust, in addition to prop diameter, you also need to know the speed of the boat (divided by the effective "slip" to determine the corresponding "speed" of the prop in terms of prop pitch times prop angular velocity (rpm)).

 

[Silverhawk24]

Thanks to both... I'm no physicist so might be barking up wrong tree... I am, however, an economist so always looking for an equilibrium, fake or otherwise. If I'm getting carried away here, just let me know...

Steamking: I tried this angle, but the engine is out of production and couldn't get a Yanmar rep on the line that was willing to wager a guess... best I got was to recommend trial and error. The boat is a fast planing deep-vee hull with lifting strakes to help it get out of its way while getting on plane. When up on plane, it rides on the strakes reducing friction caused by the wetted hull surface area

rcgldr: I have this data. I guess what I'm struggling with is the following:

Since I've increased the gear reduction, I assume that I would increase the pitch of the prop to generate the same theoretical speed / thrust at a given RPM. The RPM the engine can generate, assuming normal operation, largely depends on the opposing force the prop puts on the engine. Since the force the same prop generates with a higher reduction is less, I need more force at the prop to make sure RPM's don't climb beyond the recommended range (in this case, 3550 RPMs). With the observed data I have and assuming all things equal to the observation conditions, are there two opposing forces I should look to match or a constant to solve for?

It makes intuitive sense to me that the torque at the engine and/or prop should relate somehow to to the force generated by the prop. There are a whole bunch of prop variables, but the material ones are the effective pitch (all adjustments accounted for - cup, rake, etc.) and diameter. In my example, weight won't factor in as it's minimal and won't change in the new set up.

Prob time I give this experiment up and go the empirical route

 

[SteamKing]

No, you misunderstood my post. In order to optimize the propeller, you need to know the performance characteristics of the propeller itself. This is the information which the manufacturer sometimes keeps proprietary. From your spreadsheet, it looks like you know the torque output of your engine v. RPM. Further, in order for your propeller to be loaded properly, it must be able to absorb the torque being furnished to it by the engine-gear combination installed in your boat. It is reasonable to assume that since the torque at the propeller has increased with the new gears, some increase in pitch will be required to allow the propeller to absorb the additional torque.

 

[Silverhawk24]

Thanks, SteamKing. I misread your post, indeed.

In fact, you've answered the question. Despite all the figures I have at my disposal, the most important ones related to the prop are out of reach.

I'll try the prop manufacturer or, if that fails, go with tried and true trial and error.

Thanks and best,

 

[Silverhawk24]

Try them for a recommendation, that is... not for the data.

Cheers

 

[rcgldr]

Since I've increased the gear reduction, I assume that I would increase the pitch of the prop to generate the same theoretical speed / thrust at a given RPM.

Assuming the same "slip ratio", to end up with the same maximum speed, it would seem that the pitch of the prop should by multiplied by (old gear reduction) / (new gear reduction). Say the old gear reduction was 1/4 (1 prop revolution for every 4 engine revolutions), and the new gear reduction is 1/5. Then the prop pitch should be multiplied by (1/4) / (1/5) = 5/4 = 1.25. If the old pitch was 12", the new pitch should be 15", but all of this is just theory. The "slip ratio" may change for a high pitch prop unless the diameter is also increased (this makes the prop more "efficient").

Since this is a speed boat, you would probably better off asking this question at a "speed boat" or "hot boat" type forum.

 

[Silverhawk24]

Thanks, rcgldr.

That was where I originally took my question, however, I wasn't satisfied with the "rules of thumb" they relied on as they've failed me in the past. Was hoping to basically average the results of the theoretical and heuristic approach as my starting point.

With respect to quick and dirty approach you outlined above, matching the theoretical advance ratio of two different reduction ratios by varying the pitch is exactly where I've ended up. ~27" of pitch with the new gear.

Thanks!

 

[rcgldr]

With respect to quick and dirty approach you outlined above, matching the theoretical advance ratio of two different reduction ratios by varying the pitch is exactly where I've ended up. ~27" of pitch with the new gear.

The next problem is diameter. My guess is that increasing the pitch is also going to affect the slip ratio (making the prop less efficient). You'll need to compensate for this with increased diameter.

Since you're concerned about the torque load that the prop places on the engine, perhaps there is some formula for appoximating the prop torque versus diameter and pitch (and boat speed), so you can get an approximate idea of what the prop diameter should be.

Again, a boat oriented forum would probably be the place to ask before you purchase a propeller.

 

[sophiecentaur]

This is very specialist stuff. Afaik, you can get the optimum pitch for a given prop at only one speed with a particular hull, with a particular load. So you would have to find out details about your hull first, I think. My interest is more in slow speed, displacement sailing cruiser propeller spec but the same principles apply - even moreso, in your case because Fuel Costs. Personally, I am always aiming to turn off my engine as soon as possible and would only use it for a long run 'in extremis'. I think a replacement engine for mine (25hp, diesel) would be about the same cost as your transmission system, alone.
You can probably expect to have to spend money and put yourself in the hands of a good prop specialist. But you are right to go to them after having done your homework. A power boat forum will probably give you more practical knowledge than you'll get from PF. There may even be someone with the same hull as yours. Having said that, I have heard good reports of the care with which some companies take of their customers - they tend to be enthusiasts, unlike the sellers of New Boats and flashy kit.
BTW, why did you change the gear reduction in the first place?

 

[CWatters]

Not really my field but..

The pitch of an aircraft prop is a bit like the gear ration of a car. A fine pitch allows it to accelerate quickly but gives a lower top speed. A coarse pitch gives better top speed but worse acceleration. The original schneider trophy racers had a coarse fixed pitch prop optimised for top speed and that made it difficult for them to accelerate to take off speed, particularly in a swell.

So perhaps it makes a difference if this is a short coarse or long coarse racing boat?

 

[CWatters]

Google suggests that around 5-10% slip is about optimum for top speed so the 22.5 might not be far off (if estimated = actual). Some references suggest there is quite a difference between makes/models with the same nominal pitch and diameter. That's certainly what I've seen for competition model aircraft.

Perhaps see some of the comments here..
http://www.thehulltruth.com/boating-forum/377725-how-much-normal-prop-slip-2.html

 

[sophiecentaur]

Airscrews and water screws are very different in detail. I don't think things translate well between the two.
Economy is pretty important for anyone who is not just out for speed at any expense. Running a big engine for 50 hours a year is a significant expense. I've heard at least 1l/hr/10 hp as a rule of thumb - (mixing the units, I'm afraid). For a 100hp engine, that's £10+ per hour or £500 a year for very light use. I recently heard of a guy who changed is Marina to another one, nearer the shops in Portsmouth because his wife likes to visit the shops by boat so often. He claims to have saved the difference in Marina fees! My point is that arm waving may not be the best approach to this. Get some proper advice and save money.