[seajewel]

I am in the process of trying to have extra props and in need of replacement. I received great advice from senior members about a mishap I experienced as a new boater. I have been reading Chapmans like a bible!! Advice I get from the book about different props is basically left to personal preference.
I have a Carver 33SS (38 LOA) with twin 6.0 mpi crusaders that comes with standard 22X 22 nibrals. I found 2 different deals for brand new 21X22 and 21 X 24 nibrals ( obvious difference is diameter and pitch). It would make sense to go for the 21X24 to reduce rpm load on my engines ( with regular gas andf less heating that way?) and maybe enhance gas mileage?? Also -- having a higher pitch when I put the boat in gear without using the throttles, what does that do to my idle speed ( go higher I would guess) ( but a smaller diameter might make it negligible)..
I am just trying to process all this wonderful new boating language!
Thanks for all your help.
Hisham

[Capt Bill11]

For the time being, and if your boat is running up to spec with your original props, I would stick with spare props that are the same as your originals.

Props can be tweeked later after you get more time under your belt running the boat. You might try talking with some folks on a Carver owners forum and see what changes they have made to their props. As well as talking with a prop shop.

And quite frankly, unless you find a killer deal on a used set of props, I wouldn't be in a big hurry to get a spare set.

To learn more about props, take a look at:

Propeller Handbook : The Complete Reference for Choosing, Installing,
and Understanding Boat Propellers

by David Gerr

International Marine Publications Systems

[seajewel]

Thank you Capt Bill -- good advice!
I received another message from Nicholas on another thread --basically - I just need to speak to a prop shop and get familiar with my boat's personality as I am sure even two of the exact same boats may have different traits on the water! I will for sure try to get that book by this week-end!

[CODOG]

Sorry to state the obvious but double-check the number of blades, shaft diameter and taper / keyway spec before you buy replacement props. Decent suppliers will ask these questions at point of order, others will just send you what you order. There is a difference, as a few have found out.

[TedZ]

Gerr is excellent but his info is dated. Four was rare and 5 bladed technology was not available in small (our size) props when his book was written. If money is no object talk to the prop manufacturers. you should be able to improve both mid-range and top speed with increased efficiency (MPG) at all speeds (i know not possible but, in fact, it is being done all of the time) with modern props. Drawback is they are expensive so the payback is protracted.

My $02.

Ted

[Capt Bill11]

Good point Ted. But it is the best book I've seen on props.

And if you do go with new props, look very closely at Veem's.

[hookumup]

my experience is to check with the boat manufacturer and compare to what you have. An objective of propping is to control engine load so that the max mfg engine rpms are obtained. regardless of what is accomplished by tweeking propellers it all comes back to the engine.

tweeking has only to with increaseing the propeller efficiency while it is doing it's job. that said you can spend a ton of money chaseing margines particularly if your boat is near its hull speed. Truly, you can tweek to your crusing rpm which for a gas engine is 65 yo 70% of max rpm, very generally speaking. At least on diesels the fuel burn curve is predictable by the engine maker

i would grease the shafts where the prop slips on and make sure the shaft zincs are near a strut. on our stuff, we have the propeller hub taped so that a piece of metal can be placed over the end of the shaft with bolts passing thru which screw into the propeller hub as avery neat way to pull the propller off

I would suggest you read up on the relationship among diameter, pitch and cup