What is good speed prop for light boat?

[Higgins79]

I have a 40hp Nissan motor on a 1648 mod v. boat. The dry boat weight is about 250lbs. The previous owner had it on a heavy duty 700+ pound duck boat and he has a high torque prop on the motor but the prop has no serial number so I don't know what it is. The props available for this motor range from pitch 15" to 7". Which would be the best if I'm looking for top end speed as opposed to high torque? This boat is so light that I don't need the torque.

 

[jestor68]

Re: What is good speed prop for light boat?

There's more to it than just the weight of the boat.

To figure your operating weight you add in the motor weight, fuel, people and gear to see how much total weight you are asking the motor to push. It adds up quickly, and you would be purprised how much you might end up with.

When you finish the calculations, come back and let us know how much total weight you have. Just the dry weight of the boat alone doesn't mean much.

Props don't develop "torque"; motors develop torque. Props develop "thrust". The term you're looking for is "high thrust" when referring to propellers designed for pontoon or work boats.

Also, from your other thread; determine for certain that you have the correct length (shaft) motor on your boat. It was sounding like you had a long shaft motor on a short transom boat.

Until you get your set up straightened out, messing with prop pitch is a waste of time.

 

[Higgins79]

Re: What is good speed prop for light boat?

My boat's transom is 21.5 inches from top to bottom exactly. I don't know why it's not 20. I weigh 205, motor weighs 145, battery about 40-50 I'd guess. Fuel tank around 10 and anchor around 25. The anchor is the only thing at the front. All in all I'm lookin at 680 pounds with one person and 880 with 2.

 

[Silvertip]

Re: What is good speed prop for light boat?

All of this data is worth nothing unless you know what the wide open throttle rpm is with the current setup and what the manufacturer specifies. There is only one correct prop. That prop will allow the engine to operate at or very near the top of the manufacturers recommended WOT rpm band. If the prop is wrong at that rpm, it is wrong across the entire rpm band. Without a tach and GPS or speedometer numbers you can only guess what is happening.

 

[jestor68]

Re: What is good speed prop for light boat?

The transom height sounds fine. It's not unusual to see transom height of 22 inches on some boats.

Before we go into prop selection(your 14" doesn't sound out of line), I am going to suggest that you try something that might well cure your handling issues. It has worked for thousands others on all types of boats, so there is a high probability it will work on your boat. It is called a Doel Fin, and it will only cost about $ 40 to find out. It has solved the planing and porpoising issues on thousands of other boats in your size and category(tiller). And yes; I have personally used it on a couple boats. It worked.

I believe they're sold right here.

 

[steelespike]

Re: What is good speed prop for light boat?

The prop size should be near the base of the blades or under the prop nut.
To determine what you need we need your present prop size and your wot rpm and gps speed with a light load.

 

[smokeonthewater]

Re: What is good speed prop for light boat?

the absolute BEST thing you can do and for free or nearly free is move the fuel and battery as far forward as possible... every pound moved from rear to front has double the effect of only adding weight PLUS you increase your moment of inertia thus further fighting the porpoising.

I also recommend a tinytach or equivalent... available on ebay for as low as $20.... determining rpm is mandatory in prop selection... mph while good to know, is not as important as rpm....

As I asked before, is the antivent plate at or above the keel with the motor vertical (IE is the exact length of the motor as it currently sits on the boat, from the top of the transom to the plate less than 21.5")?... possibly, even with the motor being correct for the boat, you might still be able to raise it a couple inches and get an improvement...

 

[Mi duckdown]

Re: What is good speed prop for light boat?

What silvertip Posted. Plan and simple.

 

[Higgins79]

Re: What is good speed prop for light boat?

The keel on the boat is actually quite deep. I actually don't have the boat with me right now so I can't look at where the antivent plate is in relation to the keel. I'm taking it out this Monday so I'll be able to check it out. Thanks for the advice yall.

 

[smokeonthewater]

Re: What is good speed prop for light boat?

in that case, monday, also look at where the anti vent plate is compared to water level while on plane... best scenario is you are able to see it just above the surface at speed.

 

[smokeonthewater]

Re: What is good speed prop for light boat?

Well? Did you not do your homework???