Prop rpm, speed ?

Guys
Here's the question
I have a Honda 75 with a prop turning 6000 rpm, ( the correct top rpm speed), boat running 38, prop is 13.25x15 , 4 blade aluminum
I think the boat should run faster. It's a 17 G3 bass boat aluminum, total weight 1600 lbs with me in it and motor weight, gear.
My question is, if i try the same prop with a 17 pitch, will the boat actually run faster or will the drop in rpm, approximately 400, negate the assumed higher pitch speed?

Also, if I add weight up front, (say a 50lb sand bag) , will that let me trim higher and thus perhaps run the 17 pitch prop at 6000 rpms? Currently boat porpoises a lot when trimming higher.

Thanks



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Is the speed from a GPS or dash? the numbers look good; a 17 might get you up to 42 and run around 5500 which is where I would be comfortable at. There's others around on the board with a lot of knowledge on propping.

Call Power Tech Props

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Originally Posted by brettw

Guys
Here's the question
I have a Honda 75 with a prop turning 6000 rpm, ( the correct top rpm speed), boat running 38, prop is 13.25x15 , 4 blade aluminum
I think the boat should run faster. It's a 17 G3 bass boat aluminum, total weight 1600 lbs with me in it and motor weight, gear.
My question is, if i try the same prop with a 17 pitch, will the boat actually run faster or will the drop in rpm, approximately 400, negate the assumed higher pitch speed?

Also, if I add weight up front, (say a 50lb sand bag) , will that let me trim higher and thus perhaps run the 17 pitch prop at 6000 rpms? Currently boat porpoises a lot when trimming higher.

Thanks



Sent from my iPhone using Tapatalk

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Your speed/numbers look about right. USUALLY a 4 blade will produce lower top end speed, but take off and handling are better. I don't think adding a sand bag will help. The extra weight would probably negate the possible small speed increase from trimming up more.
Have you considered a stainless 3 blade? There's online prop calculators that will get you really close on pitch based on what boat/motor you have. I would definitely prop it where you're on high end of rpm range with a light load.
To answer your question, generally top end will be more going up in pitch regardless of the rpm drop. Unless you increase pitch so much that you're lower than rpm range. So many variables with propping.

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Originally Posted by skintback

Your speed/numbers look about right. USUALLY a 4 blade will produce lower top end speed, but take off and handling are better. I don't think adding a sand bag will help. The extra weight would probably negate the possible small speed increase from trimming up more.
Have you considered a stainless 3 blade? There's online prop calculators that will get you really close on pitch based on what boat/motor you have. I would definitely prop it where you're on high end of rpm range with a light load.
To answer your question, generally top end will be more going up in pitch regardless of the rpm drop. Unless you increase pitch so much that you're lower than rpm range. So many variables with propping.

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Thanks
I have a 3 blade 13.25x19 SS prop and can trade it in to the dealer. It drops the motor down to 5000 rpm, which I fear could eventually damage the motor.
Trade it for a 17 pitch?


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Quote:

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Originally Posted by brettw

Thanks
I have a 3 blade 13.25x19 SS prop and can trade it in to the dealer. It drops the motor down to 5000 rpm, which I fear could eventually damage the motor.
Trade it for a 17 pitch?


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Generally to stay at the same rpms, you would increase pitch going from 4 blade to 3 blade, so the 17 pitch would be the logical option. Would probably get you around 5500-5800ish rpms and a few mph. If 6000 is your max rpms, you'll want at least 5500 with a light load. That way when you're loaded with people and gear you'll still be in rpm range.
BUT, stainless and aluminum can be 2 different animals. Some stainless can be very aggressive with lots of cupping and rake making the prop have more 'traction'. Most SS props have cupping except for the cheaper general purpose SS which are more for durability than performance.

For example assuming you had a plain jane 3 blade 17 pitch aluminum getting 6000 rpms, an aggressive cupped stainless 17 pitch could be 5500 rpms because it's more 'efficient'. Generally speaking you would drop pitch an inch or two when swapping from AL to SS to stay at same rpms. Lots and lots of variables.

In a perfect world the 17" prop running at 5500 RPM would be faster and easier on the engine. However I'm not sure this old world is perfect.

I sent the 19 pitch back, for a 17, 3 blade stainless, will see


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you never said if that speed was from a GPS or the wishometer on the dash. Assuming you have a current model Honda 75 with 2.33:1 gear ratios then the numbers you gave says you are running -3.89% slip on your prop, so either your tach is wrong or your speedometer or both. With that said, the 17 3 blade will still be a better option than the 4 blade aluminum. The other thing to look at is motor position on the transom, you could raise it one hole to increase the RPM, but this might let the prop blow out to easy and you not be able to generate enough lift, or you could lower it one hole, this would lower the RPM slightly, but could give you more working trim and give you enough lift to carry the front and stop the porpoising. I used to have a stick steer crappie magnum with a 40 E-rude and I ran a 17 pitch stainless prop on it, I had the motor set one hole up from as low as it would go on the transom and it ran great at about 5600 RPM. I tried raising the motor one hole, and started blowing out the prop when I trimmed it all the way up and the boat had a tendency to porpoise. Back to the second hole it went and I was happy with it there.