What degree prop would be best....

[Bassnwhaler]

First, I hope all are well and staying sane. Second, I am going to try act like I know what I am asking but will probably mess this up:

I have a 1990 SeaNymph 195 GLS. Hull weight is @1300. Loaded I figure I am close to 1800 lbs. 19’ long.

engine: recently rebuilt 1990 Johnson 120 HP Looper

prop: 13.5 x 20 stainless (slightly cupped I am told and I can’t tell the manufacturer.)

WOT is 6300 rpm at 46 mph

I feel like I could go up to higher pitch prop for much better performance. While top end speed is nice I am really wanting to get lower rpms at a higher speed in the midrange. Basically I’d like to run at a good gas mileage rpm at a good speed. Currently 4000 rpm gets me 26 mph. 5000 gets me 32mph. Speed really jumps quickly after that but I fear running at much higher rpms (especially over 5500) due to possible engine damage. And I assume my gas is being used quickly over 4000 rpm.

so if I jump to a 22 or 23 would that get me:
much more speed at a more efficient rpm? Say 3700-4000 rpm?

This is an 8 month new to me boat and it came with a 1990 Johnson 90 hp with a 13.5x17 and I experienced almost the same performance. (Before it had a bad day and needed to be rebuilt) I expected much more from the 30hp jump and now figure I know so little about how hp, props and boat angles works that I better ask before wasting money on a different prop.

Thanks All!

Bassnwhaler

 

[Sea Rider]

What's the max wot rpm range factory stated for said motor ? 6300 K seem is over revving. If so, going one-two less pitch from current one will lessen the wot rpm currently achieved, by how much impossible to know. Other alternative, send current prop to a prop shop and have it repitched to a 18-19 one.

Happy Boating

 

[WesNewell]

You want to go up in pitch to decrease rpm's. I think you have it figured out pretty good. A 22-23P prop should bring your rpms down under 6K, increase top speed a bit and provide better fuel economy at cruise speed. BTW, the prop pitch is not measured in degrees. Search prop pitch for full explanation.

 

[JimS123]

You've got about 800 rpm to go, so that's a pitch change of about 3-4 inches.

If you try to re-pitch a prop you might be able to go 1 - 1- 1/2 inches but that's about all and that's less than what you need. You can't change the pitch of the blades as they come off the hub, so a re-pitch will only effect part of the prop.

Nevertheless, before you do anything, verify your RPM data. For your boat my guess would be you need a 21" prop, but your data doesn't compute. I question the accuracy of your tach.

 

[achris]

Spec maximum revs for that engine is 5000-6000, so 6300 isn't going to hurt anything. Also, the rev limiter in the power pack is 6700, and you're still way below that. So prop wise for that engine, I'd say you're fine. I prefer to under-prop slightly as it gives the engine a lighter load, causes less wear on the engine and the gearbox, and reduces (quite dramatically) the risk of detonation in the cylinders.

As for fuel consumption... He's my take, backed by my own controller experimentation.

I ran 3 different size props on the same boat, under the same conditions. A 17" (under propped), and 19" ('correctly' propped) and 21" (slightly over-propped). With each prop I made 2 runs on a river (flat calm water), one into the wind, the other with the wind. After each prop trial I put fuel in to account for fuel used in the previous test. I was alone on the boat, and recorded revs on a tacho that I had checked against a shop tacho. I recorded speed with a GPS and fuel consumption was with a fuel flow meter. Even if the tacho and the flow meter weren't calibrated, as I was using the same for all tests, the recordings were repeatable, so calibration would have been 'nulled' for this experiment...

So, the results. For a given boat speed, regardless of propeller, the fuel used per unit distance travel was the same. Yes, each prop for a given speed was running the engine at a different number of rpm, but for the lower rpm, the throttle was opened slightly more to produce the power required for the engine to turn that prop to get to that speed. And slightly less for the lower pitch prop. It's like running in a lower gear. You're making more revs, with less throttle input, due to a lower load.

Conclusion. Unless you are running at WOT all day every day, leave your prop as is. Changing to a higher pitch propeller (measured in 'inches', not degrees) to bring your engine speed down will not change, at all, the fuel consumed at that speed. All it will achieve is the engine working harder, accelerating slower, and wearing more...

Chris.....

 

[Faztbullet]

Just take your existing prop to a good prop shop and have some extra cup added..if they are a good shop they will know what to do exactly. This usually drops RPM by about 300RPM and will add to WOT speed.

 

[Bassnwhaler]

Great replies all. I appreciate the time y’all have taken to help me figure this out. I feel a ton better knowing this engine is ok at 6000 rpms. I rarely ever push it to that but I know my boys will. And if the gas mileage isn’t going to change AND the wear and tear on the engine is the same or less I will just keep this prop and be happy.

Sincere Thanks,

Bassnwhaler