75 HP 1979 Johnson 14ft modified tri hull bass boat

[sjoefl01]

I picked up this $500 boat and believe it or not the engine seems to be running pretty good. The prop is chewed up really bad and I see a stainless on Craigs List with a 21 pitch. The price is right. Do you think this is too much pitch for this engine hull combination? I'm not racing or anything. I just want to fish and have fun.

 

[Sea Rider]

In real world boating you should tach current delivered prop on a wot run as usually loaded and check if OB runs middle to max wot rpm range factory stated. Check current one with a tach, no matter if chewed as long hub is not spun. Go from there maximizing other prop. Bad investment buying blindly any prop sold on the net as won't know if will peform top.

Happy Boating

 

[Texasmark]

+1.........

21P on that sized boat, assumed to be light weight, with a couple of folks and gear ought to scream with a 75. I ran a 70 on a 15' Kingfisher with a 21, light boat somewhat similar hull, fished alone and it screamed. But I had no speedometer and no tach so it was seat of the pants feeling I had bouncing across the chop. But for the long haul....life of your engine, best to do as Rider said. I sold the boat before I knew if I was doing any damage to the engine, but it sounded like it was running around top end of recommended band. Hole shot was great. Back then tachs were mostly for inboards and I/Os. No power trim, seat of the pants boating.

 

[ondarvr]

Get the numbers off your current prop to at least know where you're at right now.

The tach is required to prop it correctly.

The correct aluminum prop is far better than the wrong SS prop.

 

[David Young]

My 1977 15 foot tri-hull with a 70 horsepower Evinrude suggest a 17 pitch prop with a possibility of going with a 15 inch pitch as a starting point in the OMC repair manual. I'm running the 17 pitch with good results. You really need to see what your wide open throttle RPM is

 

[Texasmark]

My 1977 15 foot tri-hull with a 70 horsepower Evinrude suggest a 17 pitch prop with a possibility of going with a 15 inch pitch as a starting point in the OMC repair manual. I'm running the 17 pitch with good results. You really need to see what your wide open throttle RPM is

Yours is a lot more boat than the one I mentioned and I would expect your pitch to be much less than mine where my boat was light, no windshield or top, one person, pretty much flat bottom, little wind resistance.

 

[jimmbo]

I picked up this $500 boat and believe it or not the engine seems to be running pretty good. The prop is chewed up really bad and I see a stainless on Craigs List with a 21 pitch. The price is right. Do you think this is too much pitch for this engine hull combination? I'm not racing or anything. I just want to fish and have fun.

If the prop is not cupped you might get away with a 21, if it is a cupped 21 then it is too much pitch for that boat and motor combo. In reality, you need to prop that engine as close to 5500 to avoid it dying a horrible twitching death

 

[flyingscott]

That motor needs to run close to 5800 rpm. The block porting in those is different than the 70 hp models. The different porting sacrifices low end power for the extra 5 hp over the 70. 21" pitch is going to be a tough prop for that motor to turn.

 

[jimmbo]

That motor needs to run close to 5800 rpm. The block porting in those is different than the 70 hp models. The different porting sacrifices low end power for the extra 5 hp over the 70. 21" pitch is going to be a tough prop for that motor to turn.

You're right, 5800 is the number to shoot for. At about 1.5 hp/cubic inch it will have a soggy low end

 

[gm280]

sjoef101, if you don't presently have a tach and want to read those RPMs cheaply, buy a Tiny Tach and run your WOT and see what is going on. Those Tiny Tachs are about $30 dollars all over EBay. And they simply wrap the wire coming out of the tach unit around one of your spark plug wires and have at it. They also work for most any yard equipment like lawn mowers and such as well.

 

[flyingscott]

You also will want to find the service bulletin for ventilating the prop it will help with the holeshot.

 

[Texasmark]

No Title

You also will want to find the service bulletin for ventilating the prop it will help with the holeshot.

Ventilating/porting is a deliberate effort to get the rpms of the engine up at a faster rate with a high pitched prop by allowing exhaust gasses to flow across the blades which reduce the density of the water, reducing the load on the engine. It also allows you to pop up a slalom skier with a higher pitched prop so you don't need 2 props for normal running and playing with toys.

Getting the rpms up gets the horsepower up faster which gets you out of the hole faster and 2 stroke engines love to run, not lug. Once on top, when the drag between the boat hull and the water are reduced at higher speeds, it allows you to get the benefit of the higher pitch for more mph.....as long as you have enough hp to keep the rpms (for my engines) at the top of the rpm recommended range or a bit higher.
It's here in the archives and being there you can probably google the question as it was an OMC prop where the notice was applied. I used it to get the idea for porting my Ballistic 13 ? x 24XL I only drilled a 5/16 hole and on this prop that was plenty. The idea on porting is to get about 1000 rpm enhancement when firewalling the throttle in the hole and having a 3-5 second shot. Immediately after the prop should lock up and the rpms should drop back to where they would be had you not ported (all 3 blades identically where shown in my picture). If the holes are too small no effect. If too large the engine will rev too high and the boat will be sluggish in the hole with the engine revving much higher than 1000 give or take. So start small (5/16) and enlarge 1/16" till you get your sweet spot.

You can read about porting if you go to the Boating parts and accessories, props, and look around there for the instruction tutorial....tells you all about props and what kind of performance you can get from what.

The prop shown was not from the OMC I mentioned above. It was used on a stock 90 hp Merc I used on a 17 ? alum boat running right at 50 and 5700 +/- rpm, engine cowl and wake in my Avatar.

 

[sjoefl01]

Okay. I am finally back to the prop. Ive been working on other things. The prop says 13 1/4 17. Below that it has DM 19 stamped in it. What does that mean?

 

[Texasmark]

May be a mfgr part number. 13 ? is the diameter, 17 is the average pitch....that's the number you use when selecting a prop or comparing. That general diameter runs 60-125 hp small gear box Mercs and a 150 small, close geared, gearbox for light, fast boats. The diameter moves around and is usually larger for pusher props (which also have shallower pitches) and smaller for higher speed props that run light fast boats, all in the same general class of props to fit a particular brand and general class of engine. Just the way prop design works. Fancier the prop the more the pitch of the prop moves around as you work from the root around the outer dia to the tip. While you are looking at your prop check the maierial and feel the trailing edge of a blade. If it has a little lip on it (the blade, not the barrel) it's cupped and at WOT, on a boat where it can make a difference, it ups your pitch another inch.

 

[David Young]

Yours is a lot more boat than the one I mentioned and I would expect your pitch to be much less than mine where my boat was light, no windshield or top, one person, pretty much flat bottom, little wind resistance.

Thank you. This made me 'smile'. First time I have ever been told, "I have a lot more boat"