can any one help me to understand

[trendsetter240]

Re: can any one help me to understand

Nice looking boat!

You need 4 accurate numbers in order to determine if your boat is setup and working correctly.

1. RPM
2. Speed
3. Drive gear ratio
4. Propeller pitch.

The only thing we know for certain is that your speed was 38 MPH since you used an accurate GPS. When you can accurately identify the other 3 factors then the answers will be easy.

 

[mannydvk11]

Re: can any one help me to understand

Nice looking boat!

You need 4 accurate numbers in order to determine if your boat is setup and working correctly.

1. RPM
2. Speed
3. Drive gear ratio
4. Propeller pitch.

The only thing we know for certain is that your speed was 38 MPH since you used an accurate GPS. When you can accurately identify the other 3 factors then the answers will be easy.

OK here is what i got i purchase a shop tachometer according to the reading i have multiply by the number of cylinders to obtain RPM value here the reading

tachometer

boat tach

this is on idle

 

[mannydvk11]

Re: can any one help me to understand

here is my prop

here is whats stamp

 

[trendsetter240]

Re: can any one help me to understand

Awesome pics!

So your RPM is accurate at least at the lower RPMs. You will need to test it again while under load on a plane to be certain it's working properly throughout the range.

The drive is 1.81:1 and your prop pitch is 17". If you look closely you can see the 17 stamped after the model number on the prop.

So 38 MPH, 1.81:1 ratio, 17" pitch prop and 6000 RPM means your slip is at 28% which is terrible.

The prop you have on there looks quite worn and appears to be a "big ear" pusher prop for applications like pontoon boats. With a stern heavy boat like yours I'd go with a high rake 4 blade prop in the 21" pitch range. Other I'm sure will have other recommendations.

The Solas Amita 4-blade 21" pitch props here on iboats.com are relatively inexpensive and provide consistent results.

 

[steelespike]

Re: can any one help me to understand

To go another way 1.81 ratio,17" prop,10% slip 38.9 mph = 4860.
Of course I'm guessing on the slip so rpm could be off high or low.

 

[trendsetter240]

Re: can any one help me to understand

To go another way 1.81 ratio,17" prop,10% slip 38.9 mph = 4860.
Of course I'm guessing on the slip so rpm could be off high or low.

Yeah, I agree but a 5.7 litre motor, 1.81:1 ratio should have no problem spinning that prop to 6000 RPM on his boat. It looks worn out like it's been run in silt and sandy water over the years. I'm fairly confident that the tach is reasonably close but he should test on the water to be sure.

 

[mannydvk11]

Re: can any one help me to understand

ok let me see if i get this i will need a prop 14 X 21p o what will be the magic number my current prop is 15 X 19p i will change it this Saturday o next week i don't know if i order online o i pick one up at a marine store

 

[trendsetter240]

Re: can any one help me to understand

ok let me see if i get this i will need a prop 14 X 21p o what will be the magic number my current prop is 15 X 19p i will change it this Saturday o next week i don't know if i order online o i pick one up at a marine store

The stamp on the prop indicates that it is 17p, does it say 15x 19p somewhere else on the prop?

If I were you, now that you have that shop tach, take the boat out and run it up to full throttle and check that the RPM is correct first. If the RPM is correct then yes I would recommend a high-rake, 4-blade aluminum prop like the Solas Amita. They are like 125$ from iboats.com but they are sold elsewhere.

If you test the RPM and it's not correct then a 21p prop may not be the right one so it would be best to test. Unless of course the place you buy it from will allow you to return it if it's not right.

 

[Tail_Gunner]

Re: can any one help me to understand

To go another way 1.81 ratio,17" prop,10% slip 38.9 mph = 4860.
Of course I'm guessing on the slip so rpm could be off high or low.

Bingo we have a winner...Tbolt v is rev limited to 5000

 

[trendsetter240]

Re: can any one help me to understand

Bingo we have a winner...Tbolt v is rev limited to 5000

Now that's something I didn't think of. But actually wouldn't it be set at 5250 RPM on this motor 50 rpm above the maximum recommended RPM?

If he actually is hitting the rev limiter then the tach should actually drop out because the spark is being dropped on several cylinders. No matter what he needs a higher pitched prop. 1.81:1 on a 350 should be a 21p or even 23p on a 4500lbs boat.

 

[mannydvk11]

Re: can any one help me to understand

Now that's something I didn't think of. But actually wouldn't it be set at 5250 RPM on this motor 50 rpm above the maximum recommended RPM?

If he actually is hitting the rev limiter then the tach should actually drop out because the spark is being dropped on several cylinders. No matter what he needs a higher pitched prop. 1.81:1 on a 350 should be a 21p or even 23p on a 4500lbs boat.

the number stamp are in the stem drive the numbers on the prop i dont have a pic but they are 15X19p

 

[trendsetter240]

Re: can any one help me to understand

the number stamp are in the stem drive the numbers on the prop i dont have a pic but they are 15X19p

I was talking about this pic you posted. It looks to me like it reads 1817 17 which would indicate a 17p prop.

 

[mannydvk11]

Re: can any one help me to understand

i just when out side i am really really sorry my prop is 1912 17 and is 15 X 17

i truly apologize for the confusion

 

[mannydvk11]

Re: can any one help me to understand

trendsetter240 good i never know this that number and i was looking at the number on the prop i clean the white rust out and it show 17p

 

[mannydvk11]

Re: can any one help me to understand

here is a pic

 

[trendsetter240]

Re: can any one help me to understand

Yeah no worries so it's definitely 17p. I think it's clear you do need a new prop and from the information provided I'd say it's going to be the 14x 21p 4-blade aluminum. If you live in the usa then iboats ships pretty quickly.

There is still some question as to what is happening with the 6000 RPM reading at full throttle. So it's up to you if you want to test with the shop tach first or go ahead with the purchase.

Cheers

 

[mannydvk11]

Re: can any one help me to understand

ok ill try to tested out Saturday now i have one more Q? before i order and is what is the mayor differences in 3 and 4 blades regarding performance will my boat travel more smooth in idle speed with the 4 prop

 

[trendsetter240]

Re: can any one help me to understand

ok ill try to tested out Saturday now i have one more Q? before i order and is what is the mayor differences in 3 and 4 blades regarding performance will my boat travel more smooth in idle speed with the 4 prop

The biggest difference I find is that the 4-blades perform much better at cruising speed (around 3500rpm). You can stay on a plane at lower speeds and get slightly better fuel economy. They also perform better at docking speeds and seem to grip more in reverse at low speeds.

And yes, they tend to be smoother than 3 blades but the difference isn't huge.

 

[jestor68]

Re: can any one help me to understand

Don't do anything until you verify the accuracy of the tach at higher rpm. You only checked it at idle and just off idle. A tach can be accurate at low speed and off the wall at higher speeds. That motor cannot hit 6000 rpm if the rpm limiter is working.

I ran the numbers and came up with about the same answer as steelspike. The tach is wrong.

 

[mannydvk11]

Re: can any one help me to understand

The biggest difference I find is that the 4-blades perform much better at cruising speed (around 3500rpm). You can stay on a plane at lower speeds and get slightly better fuel economy. They also perform better at docking speeds and seem to grip more in reverse at low speeds.

And yes, they tend to be smoother than 3 blades but the difference isn't huge.

that is what am looking for in a prop and my don't have any of those thanks i will let you guys know asap if i get home from work early any of this days i will take it out and do the tack test at hi rpms