Happy with my prop, but what will a 4 blade get me?

[rothfm]

1993 Johnson 115 90deg crossflow.

I'm happy with about 5400 rpm wot..I currently run a 13 3/4 x 15 and am happy with it. I run smaller carbs vs the originals and I know I've dropped horsepower, to be remedied when I put on the original 5/16 throat carbs back on after rebuilding.

Wondering if I could get a bit more speed while keeping up the RPM's by using a similar 4 blade?

Generically what does the 4th blade do given similar sized 3 blades as comparison? Lower rpm? more thrust? slower speed, etc??

Thanks

 

[WaterDR]

Re: Happy with my prop, but what will a 4 blade get me?

The four blade will have more surface area in the water. You will need to drop pitch. Four blades are generally more manueverable but lose top end.

 

[MikDee]

Re: Happy with my prop, but what will a 4 blade get me?

Ditto!

 

[KD4UPL]

Re: Happy with my prop, but what will a 4 blade get me?

A 4 blade is not for gaining speed. You can gain some hole shot. You might drop your minimum planing speed. You might gains some slow speed bite for maneuvering.

 

[steelespike]

Re: Happy with my prop, but what will a 4 blade get me?

The 4 blade should improve hole shot,may stay on plane easier at reduced throttle,seem smoother and respond better around the dock.
Usually suggested to reduce pitch 1" to maintain rpm.The pitch reduction alone may lose a couple mph at wot.
If you get us your present gps speed at 5400 rpm.We can tell better how your doing.
Tell us more about your boat.

 

[dazk14]

Re: Happy with my prop, but what will a 4 blade get me?

What is the specific prop you are using? Aluminum, stainless, part #, or model. Is it dinged up?

 

[rothfm]

Re: Happy with my prop, but what will a 4 blade get me?

What is the specific prop you are using? Aluminum, stainless, part #, or model. Is it dinged up?

Thanks for the replys...the existing prop is alum, OMC branded 13 3/4 x 15...I dont have gps info. I fish and havent been particular about speed etc...It runs very good with acceptable hole shot, mid range and to me the top speed is ok at about 30mph, and WOT rpm is 5400.

Its a 20 walk around ,about 2100 without the engine (I believe). Motor is 310lbs.

I just might concentrate on getting the orig carbs back on after they are rebuilt. The small throat carbs basically made HP drop to that of appx a 90.

I've tried a 13.5 x 17 then a 19 pitch....each is progressively worse, sluggish and lose alot of RPM.

When the correct carbs are on I will again try at least the 17p, or maybe try the same 15p but in a 4 blade.

 

[steelespike]

Re: Happy with my prop, but what will a 4 blade get me?

If we use 31 mph as your "about 30" we gat a slip of about 19% A more typical slip is about 12%.
Slip is the difference between your pitch and the actual movement through the water. 15" theoretically moves 15" in one revolution
imagining the water as a solid.
reasons for high slip indication:: heavy boat, poor prop,Tach reading high,Speed reading low.
The prop calculator results are only as accurate as we put into it.
I would say your prop size is reasonable considering about 90 hp actual, I think a 115 typical prop sizes are around 17"or 19"
Your happy with your numbers. When you get your power back; once you test with the 15 you probably will end up with a 17.

 

[dazk14]

Re: Happy with my prop, but what will a 4 blade get me?

If your going out into the outer harbor, you'll likely enjoy the characteristic of a 4 blade, minus losing a mph. Much slower planing speed when rough, tracks straighter, better mid-range economy, quicker acceleration (after you slow down for Tug Boat waves).

We've put on a lot of 4 blades for guys running in Boston Harbor, and they all said they'd never go back to the 3 blade. It's a windy city and on weekends you add big boat chop as well.

You haven't been out this year, have you?

 

[rothfm]

Re: Happy with my prop, but what will a 4 blade get me?

Thanks all,

DAZK14....yes, I'm out in Boston harbor. But not quite yet. Tuff spring! The slower planing, and better handling sounds good. I can sacrafice some MPH I guess as there are very few days I can go fast unfortunately.

When I put the orig carbs back on and get back to the correct HP, I just might try a 4 blade.

I trailer and also go to the cape, Vinyard, Nomans and to block Island a few time.

 

[dazk14]

Re: Happy with my prop, but what will a 4 blade get me?

Thanks all,

DAZK14....yes, I'm out in Boston harbor. But not quite yet. Tuff spring! The slower planing, and better handling sounds good. I can sacrafice some MPH I guess as there are very few days I can go fast unfortunately.

When I put the orig carbs back on and get back to the correct HP, I just might try a 4 blade.

I trailer and also go to the cape, Vinyard, Nomans and to block Island a few time.

You will definitely appreciate the 4 blade. A $117 Solas will work great.

 

[Georgesalmon]

Re: Happy with my prop, but what will a 4 blade get me?

When I changed to a four blade it felt like I added 4 wheel drive. Big help when I have to run in large following seas.