Changing prop pitch vs changing gear raios

[coffeeguy]

What are the pros and the cons of going to a larger pitch prop ei a 30p with a 2.00:1 lower unit vssay a 28 p and a 1.87:1 lower? Does a larger pitch prop have drawbacks? I am thing in an automotive way with the relationship between tire height and gear ratio

[lilmule]

Im not a racer but the bigger wheel and gear ratio would be hole shot and low end ,and the 1.87 would do better for top end ,just my opinion.
A lot depends upon what your goal is and what size weight and length or type boat one has.A very heavy barge requires the 2/1 to move it around,and a very light Allison wants a 1.62 or a stratos maybe that 1.87.
!.75 and 1.86 were standard for most mercs and omc,not withstanding sportmaster type addons with low gear raios available.
A 26 ft cruiser with a 250 hp weighing 4000 lbs would want a 2/1 to get it moving
A 1200-1800 lb bass boat does nicely with standard gearing.
And if your goal is to fly then 1.71,1.68,1.62 providing one has the motor and boat that can run it.
Any prop calculator will show you the difference in speed between wheels and gear ratios

[vnemous]

Its much more cost effective (cheaper) to change props than gear set.

[gfinch]

The larger the pitch the more paddle wheel effect. Smaller pitch are easier to come by and more efficent.

[coffeeguy]

What do you mean by paddle wheel effect|?

[dorton]

when there is less resistance at the top of the prop than there is on the bottom(ex: a surfacing prop) the prop tends to try to pull the motor to the right, Does like a paddle wheel on an old steam boat.

[astro@ptd.net]

Coffeeguy, I think what you need to tell everyone is what you are running now and trying to achieve. Be specific. My own experience with the exact same situation ended up swapping out my original 1.78:1 lower for a 1.87:1 lower. I replaced my 150XR4 powerhead for a 200HP EFI and that change in HP needed different gears to match the power curve. Now I am happy. Here is a great article.....http://www.land-and-sea.com/articles...ve_article.htm

Good Luck, Craig

[whipper]

The larger the pitch the more paddle wheel effect. Smaller pitch are easier to come by and more efficent.

Depending on the hull you want to try and have as small a pitch as per performance requirements. On a light hull with good power to tall of pitch like Gary said will have to much stern lift to be safe and efficient. A little higher rpm and smaller pitch is more desirable to a point of course. Most combos have been tried and tested. Find out who has a set up like yours and talk to them. What I did was increase my limitter so I could run smaller props. I still have the same power and gearcase but I also can achieve the same top ends i did before only with a smaller wheel. The benifit of this better hole shots and midrange with the same top end. I can run a 27/28 pitch and get the same top end as when I used a 30 pitch now going from a stock 6750rpm limitter to a 7500rpm limiter. Thats most lickly your best bet than changing gears. I mean you can do both and a lot more if you want to? Gets expensive though. If you get taller gears and increase the limitter you might need tighter heads to get the torque to spin a good sized wheel above 7000? Its endless as you know what you can do. 32 pitch seems to be the tallest prop most ever want to go with a 21fter or less with 300hp or less. Theres a reason for this. The chances of a blow out are greater with a bigger wheel if they start to paddle. Prop hight and other variables also come into play but you get the just of it now. For realy high speeds 120-150 some will use smaller diamiter props with the incrased pitch sizes for a lot of other reasons also to detailed to get into right now. Soooo If you have a 2.0 gears and are turning a 30 to the limitter you might just try increasing your Limitter. That would enable you to turn a 28 and get the same top end or in some cases even higher depending on if you can turn the 30 that much higher. 30 pitch isnt that bad for 90% of the hulls around hear. Like mentioned on what hull? That makes a differance also.

[rock]

Good post. I have also wondered about the effects of final ratio and the different ways to get there. With the stock cars it was always tall tire/low gear vs short tire/tall gear. Still the same final ratio but what are the advantages of one over the other. What has been posted makes sense. Smaller prop/taller gear would equal less torque and better handling. My ModVp had a 1.76 gear and a 28 et and would still top end pretty decent even with the small blade. Good information and good explanations. Thanks

Rock

[pyro]

Taller props kind of push the water sideways as they work, so the bottom end and midrange is relatively very inefficient, and the midrange will have more severe steering torque caused by this paddlewheel effect in a surface-piercing application.

Once speed picks up, and the water is entering the blade at close the speed it's slicing through it at, things work a bit better for the taller props.

2:1 gearcases really only belong on 2.0 liter 150's, horse-duty deckboat and ski show motors, and the occasional drag racer. But, in a pinch, use what you've got...

[coffeeguy]

Boat is a Charger DL (Cdn built ski boat) with an 8.5 inch pad. The lower unit ratio was about 10 turns of the motor for 5.25 turns of the prop. May be a little more so I guess I can assume a 1.87 ratio. I found an 28 big ear a4 reved as high as I was willing to go(6200) so I now have a 30 chopper II to try and will hopefully gps it this weekend.

[dubber]

What about the mounting height of the motor vs. prop sizes? If it is mounted too high will it not rev higher than you want it to before you want it to so to speak? Do you have a jack plate that can allow you to play with the height?

A wise sage schooled in the way of the Charger once told me that my motor was mounted too high. Lowered it a bit and presto. It helped a lot. Maybe he will chime in if he sees this thread...

[baja200merk]

pyro pretty much summed it up...

since its hard to supply enough water for a 34 to push at low speed it tends to slip more. The bigger wheels also tend to be less efficient because they tend to cause the previously mentioned paddle wheel effect.

I run a 34 but it slips at 12% my 30 slips at 7-8% (more efficient). the difference between 1.86 and 1.62 is about 1000rpm for a given speed. this is why i want to change to 1.62 as i can keep the old maytag in the rpm range where it makes power and run a more efficient prop that will slip less everywhere. and yes it will turn the 1.62s

[Wizard]

My promax started as a 150 so it has 2.0/1's in my coned torquemaster. Thought about trading them out for 1.87's but so far am happy with the performance on my lightweight hull. My main prop is a 29p DAH SRX that pitches over 30 overall. With the 2.0's it still holeshots like a deamon and accellerates hard all the way to 90. Since I can rarely go that fast on my lake I like the ripping holeshot. With my 26 chopper it's downright scarry!

[coffeeguy]

Gps speed today was 59.6 mph @5000 and 64.6 @5300using a 30 p chopper II. I could tell the motor was working copared to the 28P A-4.Prop shaft is basicly even with the pad. Does this seem to high? The boat does have a power jack plate.