A prop proplem.

[Jason Edwards]

I am running a 13.5 x 20" prop on a 200 Johnson Oceanrunner pushing a 23' Promaster Striper boat. My problem is I believe I am over-propped. I can only milk about 4850-5000 rpm out of the engine and 40-43 mph at WOT. I know the rule of thumb is 1" of pitch/200 rpm but what about the diameter of the prop? If I decrease pitch can I increase diameter? What effect will this have on speed and rpm?

 

[Silvertip]

Re: A prop proplem.

Diameter has far less impact on rpm than pitch. Diameter is largely a function of lower unit design. Small engines use 9 - 10 inch diameter. Mid range = 12-1/4 - 13-3/4 and high power = 14 - 15 inch. If you look at the specifications for a prop of a given pitch from several manufacturers you'll see that diameter can vary by as much as 3/4 inch for the same engine. It's simply a design factor to achieve a certain blade area and to allow that design to fit a certain series of engines without hitting anything. I certainly don't claim to be a prop expert but I believe that engine should be running a 14-1/4 to 14-3/4 diameter prop and probably 17 - 18P for your particular boat/load (provided your tach and speed readings are accurate).

 

[Jason Edwards]

Re: A prop proplem.

I know if I decrease pitch the rpm's will raise. If I go from a 20P to a 17P and this raises my rpm's from 5000 to 5700 what do you think it will do to top end speed?

If I go from a 13 1/2 to a 14 1/2 diameter prop how will this affect performance? Less slippage? Quicker holeshot?

I know these are tough questions to answer but any insight would be greatly appreciated.

 

[Jason Edwards]

Re: A prop proplem.

bumping it up

 

[Dhadley]

Re: A prop proplem.

Diameter is related to gearcase size. Your V6 case has a larger diameter than a V4 case so a larger diameter prop keeps more blade area in contact with the water.

Diameter also has a lot to do with how high a given prop can run. The higher you raise the motor, the less drag you'll have.

Generally speaking a V6 style gearcase will run a prop diameter of at least 14 1/4" on a 3 blade. You didn't mention if you're running a 3 or 4 blade but we'll guess it's a 3 blade. That's a very small diameter so we'll further assume the motor is pretty deep.

All this is leading up to efficiency. At 5000 rpm and 43 mph, you're over 15% slip. Very inefficient and kinda expected. With that small a diameter we'd expect a lot of venting if we move the motor up to try and reduce the slip.

 

[Jason Edwards]

Re: A prop proplem.

What do you think would be an adequate prop size for my boat? I was thinking about going to an 14 1/2 x 17P. Would you have any suggestions.

 

[Dhadley]

Re: A prop proplem.

In the end that may be a good choice. I'm running a 1990 200 (basically the same output as your newer 200) on a 22 Hydra Sports and we run a 14 1/4 x 17 3 blade Turbo. It really depends on what brand and style prop you choose.

Let's look at the baseline of what you have. What brand & style prop are you running? Where is the AV plate when parallel to the keel? Even, up a 1/2", below the keel 1/2" etc?

 

[Jason Edwards]

Re: A prop proplem.

Dhadley -

I just checked and I am running a 13 1/2 x 20P "Raker" but I am unsure of what style of prop this is. I just bought this boat a few months ago.

The bottom of the cavitation plate is 1.5" above the keel and I do not have a hyrofoil installed.

 

[Jason Edwards]

Re: A prop proplem.

BTW - My prop is stainless.

 

[walleyehed]

Re: A prop proplem.

13.5X20 Raker is a 13 spline for V4's....If it is indeed a Raker, it "should" be 14.5X20....for the V6

 

[Jason Edwards]

Re: A prop proplem.

I will look again but I am almost certain it says 13.5 x 20P on the hub but I am certain it has Raker stamped in the outside on the prop.

 

[Jason Edwards]

Re: A prop proplem.

Walleyehed, you are not only a gentleman, but also a scholar. I looked at my prop again and it is indeed a 14 1/2 x 20P. I don't know where I came up with 13 1/2 but anyhow I was wrong. Thank you very much for checking up on me.

What do you think about the height of my motor in the earlier post. The bottom of the plate is 1.5" above the keel. This seems too high and I do have a jackplate installed. Any suggestions or comments?

 

[Dhadley]

Re: A prop proplem.

Here's a rule of thumb -- if you can use 100% of the trim range without venting the prop, try going up (increase the X dimension). The Raker likes height, I wouldn't be surprised if it'll run higher. 1.5" up isn't that much.

 

[Jason Edwards]

Re: A prop proplem.

I have tried that but when I get towards the top of the trim range the engine alarm goes off nearly ever time. I think I am lifting the engine out of the water so much that the water pump is sucking air and overheating the engine. When I do this the engine does not seem to vent and only revs to maybe 5200 rpm. Any ideas? Should I screw the motor into or out of the water a 1/2 of an inch at a time and see how it responds or should I begin with trying a new prop? The nearest lake isn't in my backyard, its an hour away so I want to be prepared to make the needed changes at the lake.

Thank you so much for helping me out.

 

[walleyehed]

Re: A prop proplem.

Jason, where are you located in KS, if you don't mind telling?

 

[Jason Edwards]

Re: A prop proplem.

Meade Ks, which is 45 miles southwest of Dodge City.

How about you?

 

[Jason Edwards]

Re: A prop proplem.

I just bought a Rapture 14 1/4 x 17P from a friend of a friend for $80. It is like new and he said it only has maybe 10 hours on it at the most. What do you think of this prop for my boat? For $80 I thought it was worth the try, I'll spend that much just in fuel just to get to and from the lake.

 

[walleyehed]

Re: A prop proplem.

Jason, email me
ktsander@st-tel.net

 

[Jason Edwards]

Re: A prop proplem.


I remeasured my av plate in comparison to the keel and it is actually 2.5" above the keel. I know the Raker likes height but my engine does not. When I trim up it seems like the Raker is really beginning to work but I lose water pressure and engine alarm sounds. Should I try dropping the engine into the water more so I get get more of a trim angle without losing water pressure?

 

[Crownie2]

Re: A prop proplem.

You might want to try closing off the top one or even two water intake holes on the lower unit...

Bob