Prop Shop Talk about diameter

[jrttoday]

Good evening/day/night/morning.... whatever! I have a 1972 Seahorse that came with a 21p Aluminum, and gonna guess it was 12 3/4" originally? Found this 4 blade ss on a 2002 90hp Evinrude while picking a mechanics brain for an issue I have with my 100esl72r.
New prop is only 17p and slightly cupped. I've got 1/2" clearance to the hydrofoil, an inch with it off. Seems like I read something somewhere about clearance, but can't remember or find it. As long as it doesn't hit and performs well - any concerns/considerations etc??????http://

 

[jrttoday]

I do know a bit about props; diameter affects WOT among other things. The old prop has been reworked a few times; last time at least 1/4" was lost per blade. She turned up easier, but little/no rpm gain - makes me think carb linkages need adjusting or timing@WOT?
Lots of things to be considered when repropping. That old four cylinder has tons of torque. Without two people and gear, my boat weighs in at 1900 pounds. Add 12 or 15gals of gas, a cooler or two, and I always run with 20gals of water in the 41gal livewell. She's only 15'4" head to toe (an old Terry Style hull I built).

Loaded or empty, seems to run about the same; 4600r's is max. Going from 21p to 17p should net + or - 800rpm. Cupping might lose a few from the 800. Going up to 14" in diameter and adding another blade will take some more. Am shooting for a 400rpm gain; the operating range is 4500/5500. Will never know until I get her out there. I just feel like I read something about some math in relation to a minimal distance for a prop to the cavitation plate. Tried googling it but no luck. I know I saw it or maybe I'm dreaming! It's late............:sleeping:

 

[emdsapmgr]

I'm thinking that the hydrofoil is mounted on the wrong side of the anti-ventilation plate-the underside. Anyone I've seen is mounted to the top. The water streaming from under the transom on a planning boat needs to flow just under that antivent plate. You have the hydrofoil mounted there. So it will drag thru the water and probably cause some excess water spray. That can be a reason for poor performance, poor fuel economy and lower speeds. You need to prop that engine to run right at the 5500 rpm mark. At 4600 rpm's you are lugging that engine. Try a 17 pitch prop.

 

[jrttoday]

No Title

Think it was a POS Sting Ray Hydrofoil on it when I bought it. In '72, Johnson did not offer power t/t on this model, so I left it on. Mounted motor to a CMC t/t unit with the cavitation plate 1" above bottom of hull; took the chance, boat hadn't been out yet and would have to drill more holes to remount. Worked out great!! From 3800 on, cavitation plate is completely out of the water and old prop doesn't slip even when cornering. but......

That Sting Ray had wings like a Manta Ray: loved the lift, but treacherous cornering - unpredictable. Years ago, I had "The Edge" stainless steel hydro on a 150 Merc - it was mounted from underneath. They say it's a stronger mount and having an understanding of force on structure, I see it is. After hitting a submerged log in 40 feet of water, lost the portside fin and made this one. Not as much lift but don't need that much (OK w/o) and no longer fight it while turning at speeds.

This motor has so much torque, lugging is unrecognizable with any load or at any rpm. However, you're right and my goal is to increase WOT rpm's even though I seldom run flat out. A motor can be strained at lower revs. New prop (and it isn't new) is a 14x17p. With the extra blade, any hydrofoil might be just for show???
My question, at this point, is about diameter and clearance to the cavitation plate.

 

[oldboat1]

don't recall seeing or hearing any performance notes about minimal clearance between the blade tip and the antivent plate, but think you have a good chance of getting some cavitation because of the bolts and cutout in that hydrofoil. At least I think it would effectively be the same as cavitation -- blade tip catching a lot of disturbance under there. Not sure offhand how commercial foils are mounted (top of the plate, but not sure about the mounting hardware -- thinking carriage bolt style from the bottom, so flow is minimally disturbed.)

got to believe that rascal would withstand a deadhead, btw -- might lose a lower unit, but that hydrofoil would survive!

 

[jrttoday]

sometimes, I'm mechanically declined lol actually not, just working on a tight budget. In the past, I've taken my stuff to shops for work only to feel raped! NO MORE!! Been reading and posting on here lately trying figure out what the issue is with this old motor. I have just received an old shop manual and have been going over everything on this motor. It has an electrical problem because I can't fix it with a hammer!!! :lol:

Anyway, was adjusting/synchronizing the carburetor linkages with timing linkages today and discovered possibly/probably "why" she won't turn over 4600. I spun the yoke on the throttle cam out 5 full turns!!! Now, the linkages bottom out on the stops (timing advance stop and throttle stop), the butterflies lay flat, and I am wary of putting that 21p back on!!
Don't know if was set up at the factory that way or if the previous owner had it dialed back?

 

[jrttoday]

BTW, I cleaned that prop with Drano and a Scotchbrite pad, Then waxed it with some dried up Turtle wax I found hiding in a cabinet

 

[oldboat1]

carrying quite a load -- heavy boat. would think a cupped 4-blade with a 17 pitch would be preferable to the 21 pitch for sure. If you have it retuned from the PO's settings, think I would go with the 4-blade (JMO).

points for creativity.....

 

[jrttoday]

carrying quite a load -- heavy boat. would think a cupped 4-blade with a 17 pitch would be preferable to the 21 pitch for sure. If you have it retuned from the PO's settings, think I would go with the 4-blade (JMO).

points for creativity.....

flattery will get you nowhere! lol but thanks!! Creativity sometimes costs more..... and more than just $$$"s. Have you looked at my thread "EXPERT"? I am not one, just an adult child with too much time on his hands!! lol
I agree, that four looks promising and I have enough torque for it.

 

[jrttoday]

that'll make any difference, replaced bolts w/pan head ones. Foil is out of the water, blade tips break surface around 3800revs.

 

[oldboat1]

need some input from outboard racers, maybe. I try to keep the top dry, the bottom wet, and the prop in the water. Think hydros probably run with props half out of the water, but don't understand the physics of that -- cupped prop probably helpful.

 

[jrttoday]

need some input from outboard racers, maybe. I try to keep the top dry, the bottom wet, and the prop in the water. Think hydros probably run with props half out of the water, but don't understand the physics of that -- cupped prop probably helpful.

:joyous:, not entering the race, just going to finish. Just as the automotive industry incorporates developments from racing for safety and efficiency, so has the boating industry. The physics of a prop surfacing results in less friction; this allows the prop to spin faster. The "sweet spot" is where the prop slips enough to increase rpm's and maintains enough thrust to keep the boat moving forward without losing speed. A delicate balance, but easily attainable.

Easier done with a hydraulic jack plate, easy enough with a manual one, or you end up drilling a bunch of holes in the transom - not advisable.
Starting point "Rule of Thumb" - with a jack plate that offsets the motor 6" from the transom, set cavitation plate 1" higher than bottom of hull. Depending on the boat and how it's loaded, you can go higher, but might need to go lower to keep from cavitating or have any kind of hole shot.

Bass Boats employ these techniques more so than your average boater.

 

[jrttoday]

poor word choice - "friction" better describes to solid surfaces interaction; "drag" is what's created in aerodynamics or hydrodynamics.

 

[racerone]

This stuff is getting a bit complicated for me.--------I had to repeat grade 5 and was never in grade 6 .

 

[jrttoday]

This stuff is getting a bit complicated for me.--------I had to repeat grade 5 and was never in grade 6 .

my head got so hard by the 8th, I drooped out and they named Grade 8 bolts after me :lol: