over-size propellers

[Bill O]

Originally Posted by kmacdonald

If the present gear ratio is really 2.25:1 you will need a new gear. The prop you have is light years off and won't be of any use unless you get 3.5:1 gear and that may be your best option.

Not a transmission expert at all, so I'll ask how easy would it be to change out the gear vs. purchasing a different prop?

[more]

Originally Posted by kmacdonald

If the present gear ratio is really 2.25:1 you will need a new gear. The prop you have is light years off and won't be of any use unless you get 3.5:1 gear and that may be your best option.

he have sailboat not tug boat.
gear is picked by engine and purpose of a boat.
for him max 2700 rpm shaft 1200 rpm.
2400 rpm shaft rpm 1066 rpm.
crusing 2000 rpm

[kmacdonald]

Originally Posted by more

he have sailboat not tug boat.
gear is picked by engine and purpose of a boat.
for him max 2700 rpm shaft 1200 rpm.
2400 rpm shaft rpm 1066 rpm.
crusing 2000 rpm

Yep. I don't think you could find a 3.5:1 gear for that low HP. At any rate, the 2.25:1 gear isn't appropriate for that boat engine combination.

[Cheechako]

Originally Posted by Bill O

Not a transmission expert at all, so I'll ask how easy would it be to change out the gear vs. purchasing a different prop?

[Bobby Bluegum]

Thanks Bill and thanks kmacdonald. First, Bill, I hear you, but even if I could 'only' get to 2000rpm with the current prop modified as much as possible, that would be a fair way ahead of where I am right now and worthy of a few months trial (if I go that option.)
kmacdonald, to be perfectly honest, that 1:2.25 shaft revolution seems to be a bit suss to me now (I did originally measure it by turning the flywheel once rev and having a friend count the prop shaft revs!) but others here mention they have 1:1.8 so I'm going to measure that shaft revolution again. Regardless, I will discuss with (Porters, the prop manufacturer here in Sydney) and see what they have to say about remedies with existing prop. Wish me luck! I'll report back in a few days.

[kmacdonald]

The gear is usually labeled with the gear ratio. Being able to find and read it in a cramped engine room is another issue. Good luck.
I'm guessing the gear ratio you stated is correct since it doesn't come up to anywhere near 3600 rpm. If it won't get close to that damage to the engine could occur. I know of no engine manufacturers that will warranty an over propped engine. IT NEEDS TO TURN FULL RATED RPM!

[Cheechako]

Originally Posted by Bill O

For the prop calculator may want to increase the 13T weight. That was the dry weight, then add for fuel, water and all the other "stuff" you have onboard.

I don't remember the amount of pitch change one can adjust a fixed prop, but only expect a few inches. This may only get you to 2K or so on your motor.

You may need to purchase a different prop (used/reconditioned?) to get the right size/desired performance.

I think it's something like 15%.... IIRC

[deblen]

Some tech info here:
https://shop.tnorrismarine.co.uk/col...propeller-prop

[donradcliffe]

Originally Posted by kmacdonald

The gear is usually labeled with the gear ratio. Being able to find and read it in a cramped engine room is another issue. Good luck.
I'm guessing the gear ratio you stated is correct since it doesn't come up to anywhere near 3600 rpm. If it won't get close to that damage to the engine could occur. I know of no engine manufacturers that will warranty an over propped engine. IT NEEDS TO TURN FULL RATED RPM!

I'm going back about 15 years, but I think my Hurth 125 was about 2.1:1 and I swung a 19 inch 3 bladed maxprop with the 4108 to move a 15 ton boat.

First, your engine is more than old enough to legally drink, so don't worry about any warranty issues.

Second, the 4108 is very noisy compared to the Beta 50 I replaced, especially above 2500 rpm. You can cut the noise some with a Waker Airsep.

Third, the 4108 is notorious for leaking oil, and the leakage increases rapidly with higher RPM

Fourth, the 4108 is rated at 51 hp at a screaming 4000 rpm. In reality, they all overheat at anything over 30 hp. Most users cruise at 0.6-0.7 gph and less than 2500 rpm, which equates to 12-14 hp.

The answer: NEVER EVER RUN YOUR 4108 AT RATED RPM!

[Baby tug]

Running the prop calculator on boatdiesel.com , what I’m reading here doesn’t even come close to what they recommend , first off 4108 engine is under powered for your boat However for that engine they recommend 16.2×8.6 pitch , Or you’re 21 inch would have a pitch of 2.8 yes you read that right 21×2.8 , what I have read in this thread makes absolutely no sense to me

[kmacdonald]

Originally Posted by Baby tug

Running the prop calculator on boatdiesel.com , what I’m reading here doesn’t even come close to what they recommend , first off 4108 engine is under powered for your boat However for that engine they recommend 16.2×8.6 pitch , Or you’re 21 inch would have a pitch of 2.8 yes you read that right 21×2.8 , what I have read in this thread makes absolutely no sense to me

You might want to reread the posts. We are real close in what we recommend. With a 2.25:1 gear you said 16.2 X8.6. I posted 16.5 X 8.5. You are certainly right that the present 21" prop isn't in the ballpark. The engine would be considered underpowered by today's standard but it should be fine with the right gear. (3:1 gear)

[Bill O]

Originally Posted by Bobby Bluegum

First, Bill, I hear you, but even if I could 'only' get to 2000rpm with the current prop modified as much as possible, that would be a fair way ahead of where I am right now and worthy of a few months trial (if I go that option.)

Bobby,
Would do a few things before you go to the prop shop.

1. Find the plate on the trans that provides you w/the actual model and gear ratios.
2. Do the calculations for the correct prop size to get your to at least 2800-3000 rpm. (the max torque for your motor was somewhere around 2200. This rpm will give you the best power to fuel consumption, but some additional hp if needed can't hurt).

Really can't see them being able to do anything w/a 21" prop to make it work properly and you will most likely need a different prop.

If budget is a consideration, buy a used reconditioned prop of the correct size/pitch needed. Continued "experimentation" to find the right size is usually costly if you need to do a haul out each time to make changes. Less of a cost issue if you can dive and change the prop in the water.

[kmacdonald]

Prop it get the full 3600 rated rpm out of the engine like ALL manufacturers require for warranty. I know your warranty expired decades ago but am stating it to show the importance of proper prop selection. Over propping will cause plugged exhaust elbows, engine lugging, and shortened engine life.

[Bobby Bluegum]

Originally Posted by donradcliffe

I'm going back about 15 years, but I think my Hurth 125 was about 2.1:1 and I swung a 19 inch 3 bladed maxprop with the 4108 to move a 15 ton boat.

First, your engine is more than old enough to legally drink, so don't worry about any warranty issues.

Second, the 4108 is very noisy compared to the Beta 50 I replaced, especially above 2500 rpm. You can cut the noise some with a Waker Airsep.

Third, the 4108 is notorious for leaking oil, and the leakage increases rapidly with higher RPM

Fourth, the 4108 is rated at 51 hp at a screaming 4000 rpm. In reality, they all overheat at anything over 30 hp. Most users cruise at 0.6-0.7 gph and less than 2500 rpm, which equates to 12-14 hp.

The answer: NEVER EVER RUN YOUR 4108 AT RATED RPM!

Thanks for your post - seems most helpful to what I'm experiencing. In fact, the presumably 'safe' rpm range for my 4108 is marked in green on the Perkins tacho as starting at 1500rpm to 2500rpm. I've cruised on the engine either on its own or motor sailing for nearly two thousand miles now at around 1400rpm which gives around 5knots in calm to moderate conditions with the motor sounding sweet and no black smoke and have done sometimes up to three days continuous. Noise is bearable and oil weeping fairly minimal and consumption around 2.5l per hour. All good. My problem has been going into a breeze of 10-12k where speed drops to 3.5 and down to 2.5 in say 15k. I had the engine re-built in 2008 to factory specs and she's still running beautifully. I've read many times of 4108 owners rarely going above 1500 rpm for years without apparent damage...but anyway and regardless, have now spoken to the prop manufacturer and he is confident he can re-pitch the prop to allow rpm to go to 2000 at a cost of a couple hundred or so. As I have a haul-out planned for annual anti-foul in mid April, I will whip it off then and drive it straight over to the prop guy who will go to work on it immediately and I can collect it next day, by which time the anti-foul will be done, so no real extra expense in a specific haul-out just to remove and work on prop and refit. If that doesn't produce a practical resolution, then a new prop @ around A$2000 and 'perfection!' However, everything about my beautiful old lady has always been a compromise - this may be just another. (Btw: note to Bill O, I had a crane haul my Formosa out a few years ago onto the hard. She had about the normal amount of fuel and water aboard, plus all the usual crap I have in all the lockers, and the guy leaned out the window and yelled, "according to my weight monitor, she's exactly 13 tonnes.")

[Compass790]

Ya cant have both cobber. i.e. quiet cruising & max revs with a fixed prop.
Need variable pitch if you want both.
Given yr experience at the heads you are definitely in dangerous mode.
I'd vote for a used prop too, you can get good ones for not much $$ if you aren't in a hurry.