Wha prop do I need? all numbers included

[divo]

Hello to all. I am in need of some prop advice. I am looking for a 3 blade alum prop tuned for top end. Dont care much for low end, its a fishing boat, wont be pulling anything. just want as much speed as I can get out of this rig. Here are the numbers:

Boat...alum fishing. 16' length, 77"beam. fully rigged weight with 20 gals fuel. 1430 pounds. current prop. 13 x 19 3 blade alum(looks to be oem).

engine: 1995 Johnson 85 horse v4 (j85ttleo) 2/1 gear ratio ( I think)


Performance with two aboard ( bout 350#) and 20 gallons fuel:

39.2 mph (gps) @ 4275 rpm.

Cav plate 3/4" above bottom of hull.
I am going to raise the motor up 1 hole which should put me about 1 1/2" up

My service manual recomends 4500-5500 rpms. Whats a good rpm to be at? What prop should I try? Thanks in advance.

Dave

http://shareaproject.com/pages/projectTut,p,234,00.html

 

[jimmbo]

Re: Wha prop do I need? all numbers included

At 4275 rpm you are definitely over propped. Does the engine have power trim, and where are you trimmed at for that reading? However, 4275 rpm with 10% slip works out 34.5 mph, is the tach set correctly for your engine? 39.2 with 10% slip would be approx 4860 rpm, still low. In that case a 17 in. pitch would bring you up to 5250- 5300 rpm and raising the engine 1 hole would bring it close to 5400 rpm

 

[divo]

Re: Wha prop do I need? all numbers included

Thanks for the reply. The tach appears accurate. I seems fine at idle and goes up with motor. Its new and the dial on back is set to "6P". It does have power trim and these readings are when its bout 75% to 85% trimmed in. Thats where I got the most speed and rpms.

 

[divo]

Re: Wha prop do I need? all numbers included

OK, I took mit back out to the lake. I was reading the tach wrong.... The real numbers are: 39 mph @ 4575 rpm

Thanks

 

[kenmyfam]

Re: Wha prop do I need? all numbers included

See if you can go with a 17" pitch by getting one to "try" from a dealer and go from there. Just make sure you actually buy your replacement from the dealer for their goodwill. Then down the road when you need to do it again the dealer will be more than happy to help out.

 

[Tredeb]

Re: Wha prop do I need? all numbers included

the dial on back is set to "6P".

Wouldn't that be 6 plugs? you said you only have 4. Or am I off base here?

 

[evin268fs]

Re: Wha prop do I need? all numbers included

I would try a 17P Viper or a 18 P Raker, they will lift the boat a lot better than the SST 19, droping pitch should help get the r's up and the better lift of these props should raise the r's a bit and allow you to trim out more, also let you raise the motor a hole if your water pressure is ok.

 

[divo]

Re: Wha prop do I need? all numbers included

I would try a 17P Viper or a 18 P Raker, they will lift the boat a lot better than the SST 19, droping pitch should help get the r's up and the better lift of these props should raise the r's a bit and allow you to trim out more, also let you raise the motor a hole if your water pressure is ok.

Thanks for the reply. I went and borrowed a 13 1/4" x 17" alum 3 blade from the dealer today. I will try it out tomorrow and see what I get. He didnt have a Viper there and that is what I wanted to try. The prop I borrowed is used and they said they would sell it to me for 30 bucks if it works out and I like it. Now comes the decision.....if this prop works, and its only 30 bucks......what would I gain?...and would it be worth spending several hundred on a ss viper?

Thanks

 

[jimmbo]

Re: Wha prop do I need? all numbers included

Wouldn't that be 6 plugs? you said you only have 4. Or am I off base here?

That 6 pulses, not plugs. These tachs run off the altnator

 

[Tredeb]

Re: Wha prop do I need? all numbers included

That 6 pulses, not plugs. These tachs run off the altnator

Sorry for my misinformation. I was thinking it was the same as a car engine.

My bad.

 

[evin268fs]

Re: Wha prop do I need? all numbers included

Thanks for the reply. I went and borrowed a 13 1/4" x 17" alum 3 blade from the dealer today. I will try it out tomorrow and see what I get. He didnt have a Viper there and that is what I wanted to try. The prop I borrowed is used and they said they would sell it to me for 30 bucks if it works out and I like it. Now comes the decision.....if this prop works, and its only 30 bucks......what would I gain?...and would it be worth spending several hundred on a ss viper?

Thanks

You said you wanted the most speed, you need a larger diameter, 13.5 or 13 7/8. The cupping and larger diameter in the stainless props allow you to trim the motor out more. Lifting the bow out of the water, less drag. I tried getteing a rebuilt 13 x 19 prop off ebay, ran worse than my 17 Viper, anything past 1/3 trim no increase in speed, just more rpms Slipping. Bow never felt like it lifted. Got 42/43 out of a stock 17P Viper, 45/46 after having it worked, getting an 19 Viper worked right now. Just hope I will be able to slalom with it.

CHECK EBAY can get them in fair to new out of box condition just have to be patient, Can get them in the 100-200 range.

 

[divo]

Re: Wha prop do I need? all numbers included

k...went out with a 13 1/4 x 17 3 blade alum. Went out alone with a half tank of gas so about 180 pounds lighter than other tests. 5600 @ 37 mph. With both props, I had to keep it trimmed in. 75% with the 19" prop, and 85% with the 17" prop or it would start to porpoise. I couldnt trim out any farther. I am going to order a set of smart tabs to help with the porpoising. Being able to trim out more with the tabs, do you think I could go back to a 19" pitch and try a viper? or would the 18" raker work better? I can also raise the motor up 1 more hole. the water pressure guage is showing good pressure. Thank You

 

[jimmbo]

Re: Wha prop do I need? all numbers included

The numbers do not add up. 39mph @ 4575 rpm with a 19" prop and 37mph @ 5600 rpm. 95% of the time a 2 inch change yields a 400 rpm change. A change of more than 1100 rpm is not normal. Even if the 19 was cupped and the 17 was not.

5600 with 17 with no slipage would be 66ft/sec which would be 45mph. With 10% slip... 40.5 mph

4600 with 19 with no slip would be 61ft/sec which would be approx 40 mph. With 10% slip... 36 mph

 

[divo]

Re: Wha prop do I need? all numbers included

the test with the 17" pitch prop was when the boat was bout 180 pounds lighter. I dont know if that makes that much difference or not....tonight I will go out with both props, change them at the lake, and get accurate numbers for both under the same conditions. Thanks

 

[hitace]

Re: Wha prop do I need? all numbers included

you are looking for a prop that will give you more speed 4 good ones are

"viper,raker,raker2,rebel and mercs x7 alloy props but only testing will tell the best for your boat