prop diameter and rpm correlation?

[Nextelbuddy]

my boat originally came wit ha 13.75 x 21p propeller with my 1:62 alpha 1 gen 2 drive.

currently it has a 14x19p quick silver prop.

right now i can only get a consistent 4200 rpms and 35-36mph confirmed by the onboard speedo and GPS speed app on my phone.

my 5.7 mercruiser engine has a max RPM of 4600 rpms according to the sea ray prop guide PDF and also othe rwebsites confirmed this as well.

would going back to a 13.75x21p help get me back to 4600 rpms and hopefully into the 40mph range?


i bought my boat second hand and previous owner said they replaced the engine and it onyl had 2 hours on it but now after going through it i can tell it has not been replaced but instead they most likely just added an edlebrock intake manifold and 4bbl carb and thats it.

the engine runs ok, it feels a bit tired if anything but it doesnt stutter or misfire it seems.

if i go from idle to full throttle there is a definite LAG you hear and feel before it starts moving but its not a stutter, almost like a pause

i checked the throttle cable and at full throttle, the throttle plates in the carb appear to be opening fully

i plan on ordering and replacing:

8 MR43T plugs
quicksilver spark plug wire kit
quick silver cap and rotor
new ignition coil
quick silver fuel filter


i wondering if doing the above maintenance items PLUS the original spec 13.75x21p SS prop I can get back to 4600 rpms and into the 40+ mph range.

I consistently see other owners if 1996 sea ray 210 with 5.7 engines getting into the 50mph range.

i'm also hoping that getting more speed on plane back will help with the annoying porpoising the boat does at full speed on the water

 

[dingbat]

Nothing has changed since I posted this on your last thread....

In reality, the diameter has little affect on RPM. Generally speaking, assuming your comparing props of the same make and design, the two main drivers of RPM are Pitch (~100 rpm per inch) and number of blades. (~ 200 rpm per blade).

If your changing make and model of prop, all beats are off. It becomes a crap shoot

Porpoising is not a speed or prop problem.....its maladjusted trim or Operator error

 

[Nextelbuddy]

Nothing has changed since I posted this on your last thread....


Porpoising is not a speed or prop problem.....it's maladjusted trim or Operator error

4200 rpms nets me 35-36mph trimmed down. in this scenario in lake lanier where i boat, the waters are choppy from heavy boat activity and even trimmed all the way down, my boat always porpoises which is very annoying and frustrating and makes boating almost not fun. getting to 35-36mph is a struggle with the busy lake

if i go out on a day that is not very windy or busy such as a week day, i can get to 35-36mph 4200 rpms faster with less or minimal porpoising.

if i try to trim UP to get more speed, the front end starts bouncing up and down and i hate it and trim back down, meanwhile i see other boats (other comparable bow rider sized boats) having fun driving fast smooth on the water with minimal/or 0 porpoising.

i've spoke to several people about this locally and they have told me that i need more speed that at 35 mph in my boat needs more speed to help keep the nose down a little more.

since it appears my boat and engine setup are not getting to the appropriate top speed for my particular hull (this boat should be doing 45+almost 50MPH according to other users) i want to fix my performance and top speed issues first.

Once i get get the performance and speed issues worked out better then i can focus on the porpoising issue and start from scratch and look into trim tabs/smart tabs, whale tail etc...

my good friend lets me drive his 1996 baja 272 and obviously apples to oranges but when i drive that, the boat does exactly what i want it to do, no porpoising or annoying hopping. it just goes and is smooth albeit 60+ mph


maybe my little 1996 21 ft sea ray hull just isnt designed for top speed runs on a busy choppy lake and im expecting more from it than it will give.


and thanks very much for the help and lending your expertise and ear. I appreciate it!

 

[Scott Danforth]

first, diameter has very little outcome on a prop

second, is your motor healthy. what are the compression numbers?

have you weighed your boat and compared it to stock. if you have the original shoreland'r trailer, the trailer weighs 1250# your boat should weigh 3300# dry and with full fuel and gear on the trailer, you shouild be 4850-5000

a healthy 5.7 in a bone dry 1996 210 signature with a1.62:1 ratio should be turning 4900 RPM and hitting 50mph with a 15x19p prop. max trim will be about 1/4 of the gauge

if you have a bunch of gear and "stuff" in the aft compartments, move it to the front. if you are arse heavy it will porpoise like crazy

if you have a bow lifting prop, it will porpoise like crazy

if your trim gauge is off, you will porpoise like crazy

 

[Nextelbuddy]

first, diameter has very little outcome on a prop

second, is your motor healthy. what are the compression numbers?

have you weighed your boat and compared it to stock. if you have the original shoreland'r trailer, the trailer weighs 1250# your boat should weigh 3300# dry and with full fuel and gear on the trailer, you shouild be 4850-5000

a healthy 5.7 in a bone dry 1996 210 signature with a1.62:1 ratio should be turning 4900 RPM and hitting 50mph with a 15x19p prop. max trim will be about 1/4 of the gauge

if you have a bunch of gear and "stuff" in the aft compartments, move it to the front. if you are arse heavy it will porpoise like crazy

if you have a bow lifting prop, it will porpoise like crazy

if your trim gauge is off, you will porpoise like crazy

i have not weighed my boat, i will look into trying to get that done.

i should go ahead and do a compression test, i have a tester so when i'm changing the spark plugs i will go ahead and do that.

my boat only ever has me and my wife with a full tank or half tank and nothing on the boat except life jackets, fire extinguishers and the 14 lb anchor and thats it. all the storage compartments are empty


thats interesting you mention 4900rpm and a 15x19p prop.

i have this PDF spreadsheet from Sea ray for their sport boats and mine is a base model 5.7 NON LX and originally came with a 2BBL carb and a 1.62:1 outdrive. the previous owner swapped out the intake manifold with an edelbrock and a 4BBL carb (i need to inspect it and see if probably is an automotive grade one and not marine grade)

according to the PDF my boat with a 210hp 2bbl 5.7, 1.62:1 gearing and a 13.75x21p has a max rpm of 4600 rpm

there are other variants of the boat that had 5.7 LX and EFI and 1.47:1 gearing and they had 14x19p props with max RPMs of 4800rpms


when i looked at other 5.7 engines from newer boats such as the MPI variants, they had max rpms of 5000 or more.



it sounds like you have some good experience with the 1996 sea ray 210 5.7 setup or at least some knowledge on it, where did you figure out a 15x19p and 4900 rpms?


thanks so much for the reply!