went from 3 blade to a four blade prop

[mbaker812]

so I went from a 3 blade 14.5 x 21 prop to a 4 blade 14 x 21. handles low speed better and maneuvers excellent but seems like it works harder to get on plane.
motor is 5.7 efi rebuilt to be 300 hp or close old motor replaced was 230 hp 1999 efi
1.47 alpha 1 gen 2 lower unit
boat is a sea ray sundeck 210 1999
top speed at 4600 rpms was 42
I dont really go for top speed so that is not an issue but should I go down in pitch, to take some work off the motor and running gear?

 

[alldodge]

Went from 3B 21 to 4B 21 - the 4B is equal to 3B 23P
You increased pitch and it would make it slower coming up

 

[mbaker812]

I hit a log this weekend with that new four blade 14 x 21 and snapped a blade off so I ordered a new 14 x 19 to see if it helps. The old 3 blade 14.25 x 21 gets on plane in around 12-13 seconds the four blade takes a bit longer to plane and it seem like the motor works a bit harder, like it doesnt rev as quick or maybe lugging a bit. once I rev to say 2500 it starts to come up pretty quick. I didnt think going lower than a 19 pitch would be good and like the control the 4 blade gives at low rpm.

ok could I go to a 4b 17 pitch would that be close to a 19 pitch and is there any benefits?

 

[alldodge]

A 4B 19 is equal to your original 3B 21.
I bites better and will probably give you more lift. If there is to much lift you will porpoise, but that's only if you get to much

 

[mbaker812]

Sorry Mr Dodge I see you have answered the question. Thank you

 

[mbaker812]

thank you again for your answers and advice. Have a great day.

 

[jimmbo]

Check that you didn't bend the Propshaft

 

[mbaker812]

Check that you didn't bend the Propshaft

I have already checked that. thank goodness I had an aluminum prop on there. I grew up on the ohio river and only had one boat I put a stainless prop on due all the underwater debris. Thanks for the input.

 

[QBhoy]

Hi. I haven’t checked…but I’m almost positive that those numbers sound a little off or crazy bad for prop slip. I’d think that with a 1.47 ratio…and a 21” at 4600…to only be doing in the 40’s…will show an almost ridiculous prop slip. Could be wrong. But that’s the ratio I have and when running a 21”…I’m sure I would be at a significantly quicker speed. Maybe worth checking your numbers, ratio, instruments or pitch again perhaps. See what you think.

 

[QBhoy]

You’re at 33% prop slip there. I think something isn’t right perhaps.

 

[mbaker812]

Hi. I haven’t checked…but I’m almost positive that those numbers sound a little off or crazy bad for prop slip. I’d think that with a 1.47 ratio…and a 21” at 4600…to only be doing in the 40’s…will show an almost ridiculous prop slip. Could be wrong. But that’s the ratio I have and when running a 21”…I’m sure I would be at a significantly quicker speed. Maybe worth checking your numbers, ratio, instruments or pitch again perhaps. See what you think.

I would agree but I have to start somewhere. the boat is heavy I dont think this boat brand new did better than 46-48. the speed was on GPS and the rpms on the tach. the boat weighs 3500 lbs dry. brand new motor runs great. guess I could check the tach with my tach lead on my timing light to see if they match.

 

[QBhoy]

I would agree but I have to start somewhere. the boat is heavy I dont think this boat brand new did better than 46-48. the speed was on GPS and the rpms on the tach. the boat weighs 3500 lbs dry. brand new motor runs great. guess I could check the tach with my tach lead on my timing light to see if they match.

No worries. I’d say you definitely need to check one of the values for sure. How sure are you about the gear ratio ?
You mentioned it’s an efi. What was changed on the engine to now have 40hp more than standard?

 

[mbaker812]

You’re at 33% prop slip there. I think something isn’t right perhaps.

so do you think a 4b 17 would help it? I used a prop calculator to see a better slip number.

 

[alldodge]

so do you think a 4b 17 would help it? I used a prop calculator to see a better slip number.

Don't get ahead of your self. Your calculated numbers are coming from the 4B 21 props which are not very efficient and your lugging the motor. Get some numbers from the 3B 21 to use as a bench mark

 

[QBhoy]

so do you think a 4b 17 would help it? I used a prop calculator to see a better slip number.

Don’t hold me to it…but just from knowing what I know about my boat which definitely has a 1.47 ratio and about 300hp…I know that if I run about 4600 rpm with a 21”…it is going pretty bloody quick. And on its best day…not far off 70 at its max rpms.
I’d suspect either your gear ratio is much lower than 1.47 or your tachometer is way way off.
All that aside…I’d also naturally think that that set up just couldn’t turn a 21” to 4600 rpm with that ratio. And if it did…it would be significantly quicker than 42 mph. If it does have that ratio…the natural guess at a starting point for a prop would be something like a 17”.

 

[mbaker812]

Don't get ahead of your self. Your calculated numbers are coming from the 4B 21 props which are not very efficient and your lugging the motor. Get some numbers from the 3B 21 to use as a bench mark

thanks Mr Dodge that is what I am thinking also. i ordered the 4b 19 but put the 3b 14,25 x 21 back on and will see what that looks like. I can always return the 4b if not used and get the correct 4b afterwards

 

[QBhoy]

thanks Mr Dodge that is what I am thinking also. i ordered the 4b 19 but put the 3b 14,25 x 21 back on and will see what that looks like. I can always return the 4b if not used and get the correct 4b afterwards

Let us know how you get on with that. But fundamentally, there is no way that set up is turning a 21” prop well…if it’s a 1.47. Can’t urge you enough to double check the variables. All the best.

 

[mbaker812]

No worries. I’d say you definitely need to check one of the values for sure. How sure are you about the gear ratio ?
You mentioned it’s an efi. What was changed on the engine to now have 40hp more than standard?

bought a 300hp long block, that being said not figuring I would get the max out of it. spoe at length with OBD diagnostics about the tune and he said it could run lean at top end but my plugs dont show that. going to take ALLDoges advice and get numbers with the 3B back on there and see how that computes and the get the 4B that will give me a decent slip number. also will check the tach. appreciate all.

 

[QBhoy]

bought a 300hp long block, that being said not figuring I would get the max out of it. spoe at length with OBD diagnostics about the tune and he said it could run lean at top end but my plugs dont show that. going to take ALLDoges advice and get numbers with the 3B back on there and see how that computes and the get the 4B that will give me a decent slip number. also will check the tach. appreciate all.

No bother. Best of luck and let us know how she goes. Always keen and interested in these things.