Another prop/RPM question

[Bru]

I have an Alumacraft Magnum 165 CS with a Johnson 70HP 2-stroke. The boat came with a 14x17 aluminum prop (part #176214). I am getting RPM's of 5600 but only 31-32 mph GPS with me and a half tank gas. By reading past posts, I think I should be able to get into the 36-37 mph range. My thoughts are that the prop diameter is too large for this motor. Would I pick up top end speed by decreasing diameter to 13 or 13 1/4 and increasing pitch to 19"? I know REW is the resident expert in this field. I won't spend another $150 if I will only get a 2 mph increase in speed. Thanks.
Lefty

[VMS.]

Hello!!

If memory serves me correctly, That motor has a top end RPM of 6000, so the first issue is that you are probably over-pitched a bit.

Going down in Diameter will help some...Something like for every 1/4 inch = 100 RPM. So...by going down to a 13-1/2 diameter, you should potentially pick up about 200 RPM.

Next, I would look at the bolt hole position of the motor, where it is mounted to the transom. If the motor is resting on the transom (i.e. the motor is in it's lowest bolt hole on the transom), move it up 2 holes. This should give you another 200 or so RPMs and put you in the range of where RPMs should be.

Now...if you are running a stock OMC prop, I'd suggest purchasing a different Prop. My personal favorite on my Johnson 50hp (on a Lunker V16 LTD console) is a Solas Alcup 3. It has a larger exhaust hub opening, which means the motor is not restricted as much, the hub is easilly replaced by yourself if you hit something and need the prop to get back. The prop is also cupped well enough to grab like a steel prop, and flex less than aluminum. You might find that with the Alcup, you can maintain the 14 inch diameter hub with the 17 pitch and be right on with 100 rpms to spare. If you have a full tank of gas in that rig, you should be right on the money for RPMs...

With this, you should pick up some speed and RPM's should be in a much better range than they are now. You will definitely like the changes.

I'd start by raising the motor up two holes and see what that does for you. Some boats gain more RPM with movements, and you might get enough just doing that.

Have fun tinkering :-)

Steve

[REW]

That prop is really too much for that motor.
That particular OMC prop, works very well on the 115 hp motos on a relatively heavy boat. With the larger diameter (14 as opposed to the nominal 13 1/4 or 13 1/2) it does a nice job of sucking up the extra power in the larger engines without overrevving the engines.

However, in your case, with less power you want to go to a smaller prop.

You are indicating that you have the 16 1/2 foot alumacraft. This is about a 800 - 900 lb boat.

With the weight of this boat - and the power available -it is likely that the very best speed under optimum conditions will be about 36.
Most of the time, I would expect that a speed of around 30-32 will be normal with typical loading conditions.

However, as the other post suggests, you can try going to a smaller diameter prop, and or possibly even a heavier pitch.

You do need to get the engine rpms to about 6000 rpm, with a light load - and running clean. You might find that the best performance for your rig will be with a Raker 18 pitch - and a diameter of 13 or even 12 3/4 inches. The lesser diameter will allow the engine to rev up to the required rpms.

The smaller diameter will likely cause you a loss of hole shot, but if the pitch is up, you should gain top speed, if your engine has enough power to get to max rpm.

As with any rig, correct propping, motor height, loading etc. is a bit of a trial and error situation.

If a person has lots of power - the prop isn't so important, because a "nominal" prop will take care of the boaters needs.
But, when a boater is a bit marginal on power, and wants more - then every tweak, motor height, prop setting, washout, rake and cup will all make tiny differences.

In the world of boating, and power, speed changes of 1-2 mph at costs of $500 - $1000 per mph are quite common.

Water is very unforgiving of "drag". Thus large changes in power and or propping etc. are required for rather small changes in top speed.

It is relatively easy to get a boat on plane, at a "nominal" planing speed. However, to continue to lift the boat, and get even more speed out of a boat, when it has already achieved its nominal planing speed, requires a great deal of extra power.

---
Summary -
Work with a dealer or prop shop to get the optimum rig for your boat for a few different conditions.
With the power that you are working with, it is quite likely that you will want to have at least two different props. i.e. one prop - that will work well for a heavier load, at a slower speed, and one prop that will work well for a lighter load at a higher top speed.

Props in the 15,17,18,19 or possibly the 20 pitch ranges should be considered. Prop diamters ranging from 12 1/2 to 13 3/4 should be considered as well.

A typical motor height of at least three holes above the transom should be considered. This hull is a relatively clean hull, and your top speed will be improved if you lift more of the motor out of the water, to reduce lower unit drag. A good starting point is three holes up - and try a half dozen props, using a gps, and tach, to calibrate boat speed, and engine rpm.
Check for planing characteristics, and possible cavitation or blow out on turns - when the motor is properly trimmed. If you are not getting excessive blow out on turns, then lift the motor further - i.e. to the 4th hole.
If you find that you get too much blow out or cavitation to get the boat on plane, or too much blow out on turns, then drop the motor - one hole at a time, until you find the best compromise.

Typically, for best performance, you want to continue to lift the motor until
a. you can't get the boat on plane, or
b. you get so much blow out on turns, that you have to slow down to get a new bite.

When you get the motor to this height, then drop it back one hole to have the best balance of performance and stability.

Take care and enjoy your rig.

REW

[Dealer]

RPMs are good.
Prop is the correct size aluminum prop, as your rpms demonstrate. Your reported speed is pretty "normal" for your rig.
You might try an Evinrude/Johnson (stainless) SST-13-7/8 X 17 "S" prop. --->P/N #176572
With which, we have had acceptable results on boat/engine combinations similar to yours..
This is a more agressive prop. than your aluminum and allows you to mount your engine higher. Doing so may get you another couple of miles per hour, max.
We experienced the expected rpm drop, but also porpoising when we tried an 18 Raker on your particular type hull and engine combination.

[Bru]

Thanks guys for all your suggestions. This is what makes this the best fishing site on the internet. I appreciate this. I used to use the name "lefty" but I think someone else is using that name now. During the winter, I have time to think about these things, we don't have enough ice yet in Ohio, but too much to fish in the boat.
I think I'll start by raising the motor, it costs nothing. If it still bothers me, I'll find a prop shop and start messing with diameter and pitch. Thanks again.
Bru

[T-Mac]

With an aluminum prop, raising the engine doesn't do much. If you get a stainless prop, raising the motor can often help your top end some. With a 70, no matter what you try, the increases in speed are not going to be huge.
Fun to try this and that and find out 'tho...right? :)