Propeller Pitch

[Larry H]

Greg,

That 17 pitch will only work if you have the HP to turn it!
You have 25 to 45 more hp than most 22's!

[Redƒox]

I heard that ... If the companies would just put in reduction-gears (a does Suzy) the smaller motors could turn a nice steep prop
My Yami will sling a 19 no problemo, but I get overloaded, and like the snap of the 17 pitch

[TyBoo]

Crap, I'm gonna be sorry I dredged this back up!

I was looking back through this discussion because I just put on a different prop to try (haven't got it wet yet), and I came across this very good question that I must have somehow missed previously:

[Sea Wolf]

Mike-

Sounds like you had a bad day, and are still mad about missing Blakely!

Good discussion of gearing in outboards!!! Like you say everything in gear and prop selection involves a series of trade-offs in trying to find the optimum solution for a specific application, as well as the fact that different people have different preferences that are also major factors in the decision making process.

Not much point in re-hashing most of what you've said, as it's all true except for the sideways pot-shots at Ford, the Nerd, other outboard brands, Red Fox, and Mother Nature.

You do have a very good point about the higher geared drive ratios have a greater range of rpm operation at the prop. This is very good, but also has its downside, in that the higher prop rpms lead to greater frictional losses, but this is relatively minor.

Gear ratio is like pitch, diameter, number of blades, blade design, and several other factors: each element in the interactive whole has to have its point of greatest return traded off away from its optimum to accommodate all the other factors which in turn must do the same as they cannot all be optimized absolutely at the same time. What we're seeking, then, is to find the combination of factors that give us the best possible performance we can get that suits our needs and style preferences.

To get the best efficiency in terms of energy and momentum exchange, we'd use a low gear ratio (2.3-2.6:1), turning the prop slowly to reduce frictional losses, use a large diameter prop, a higher pitch, and more blades. This would lead to less friction, less slip, and a more efficient push on the water with less losses to blade surface friction and push the boat through the water with less energy loss and better momentum exchange, just like the long oar, long slow stoke analogy. The problem here is that there isn't much room for error or accommodation. It's just like your truck analogy with the gears set up for highway freewheeling. Very little accommodation for various speeds and loads, etc. This might work if we had lots of extra torque and power.


We could go the other way and go for maximum accommodation:
High gear ratio (2-2.3:1), thereby speeding up the prop, using a smaller prop, with fewer blades, and less pitch. We would be able to turn the prop far easier, have more slip, acceleration, and more accommodation, but would suffer higher frictional losses on the blade surfaces and not as efficiently push the boat forward through the water just like with the short oar and fast, furious stroke analogy.

This is a bit over-simplified, but it does point out that we can go too far in either direction. The problem is to find the optimum middle ground on a whole bunch of variables, and we haven't even considered the boat, the loading, and sea condition variables. Add in elevation, temperature, and humidity changes, and the search for the universal prop becomes increasingly difficult!!

Your point is to have a gear ratio that works like a good middle range accomodating transmission in a truck, and it's a very good one. Mr. Fox has lots of extra power and torque and is more concerned with limiting prop ventilation in choppy water. Please don't point out to him he has a 2.07:1 ratio in that big Yammy!!!

Got to go to other fun things, including a dental appt.! Joe.

[rogerbum]

Sorry to be the know-nothing amateur here but here's my take on it....

I bought my boat with twin Honda's and whatever props they put on them at the factory - don't know what they are, don't care, have a spare in a box.

Boat goes fast enough (27kts under ideal conditions), boat goes slow enough (1.8 kts on a single engine) .

Roger on the SeaDNA

[TyBoo]

[gljjr]

Amen to that Roger!

Another thing that you have to take into consideration is the engine driving the prop. It could be that Suzuki chose the grear ratio they did because they wanted to put the power curve of the motor in a different place than say Honda or Yamaha chose.

Anyway this has certainly been an interesting thread!

[dogon dory]

Sorry, Folks - Post Deleted By Author

[TyBoo]

First thing out of the box, let me qualify my stake in this discussion. I really thought my simplistic comment earlier put the common boat owner's needs in the proper perspective. That comment being:

[dogon dory]

Sorry, Folks - Post Deleted By Author

[Sea Wolf]

Dan-

Sounds like you're "swearing off" on propeller discussions! Is there a group you can join like AA? I think I'll join you! Maybe we can get Mike to join too! Then we can sit around and find something else to divert our attention to like discussing whether twins or a single with a kicker is better.

Too bad this whole thing is so complex that it's hard to come up with solid, simple, easy to follow answers. But it's just that problem, exactly, that leads to these endless discussions. I'll try my best to bow out here.

Mike-

As far a friction with the water is concerned, it might seem that the larger propeller's greater surface area turning at a slower speed would be an equal trade off with the smaller one turning at a faster speed, but unfortunately friction in water goes up exponentially with the speed as opposed to arithmetically. For example, to put it in easier terms, if you double the speed, the friction goes up to four times* as great, not two times as great.

So the smaller, faster turning propeller winds up wasting more energy through friction to try to move the same amount of water. Since the smaller, faster turning one wastes a little more energy in friction, it can't move as much water, and the thrust forward to the boat is a little less, and the boat is a little slower, but not much.

In addition, my somewhat cavalier comments about the "potshots" were just another attempt to humor the "dry" discussion along, just as yours were!

* I don't remember if the exponent here is 2 or 3 or some other number, but this illustrates the idea.

Peace on the Propeller Front.

Joe.

[TyBoo]

What?? You mean that's it? Oh well, I'm going over to the whale thread.

[B~C]

yo Mike, if your looking for more action you could wander over to Ifish and ask about taking your ThunderJet out tuna fishing

[El and Bill]

Dan, and all the others in the pub --

This has been a most useful discussion, for those in the pub and those joining. We can each, individually, select those comments that best pertain to our boat's needs -- Thanks from us, and, we think, from new and future owners for the thoughtful and experienced responses -- El and Bill

[dogon dory]

Sorry, Folks - Post Deleted By Author