Propeller sizing question

[John 40]

Hi, I am finishing a 30 ft. downeast build. The boat weighs about 10000 lbs. semi displacement hull with hard chine, Qsb 440 h.p. cummins with 1.96 to one gear ,24" maximum propeller diameter and max. rpm 3450. Three diffent propeller shops have different sizing on a prop. One was 22"x22", one 23"x24" and one was 24"x23". Don't know which one to try. Seems like a big difference in sizing, any insight would be appreciated. Thanks

 

[eyschulman]

I would go with the largest diameter prop that fits with adequate clearence. I don't know about your gear ratio you should check with similar boats. The calculator I used is calling for higher gear ratio.

 

[WoundUpMarine]

What hull are you finishing? how wide is your 30'? sorry not much help about the propeller, good luck with the build though.

 

[captainlarry84]

You will definitely over turn the 22 x 22 Propeller & by a lot.

The 23 X 24 is cheaper because you only pay for diameter. I feel that you will over turn this wheel by 50-75 RPMs

The 24 X 23 is where I would start. I also think that you may over turn this wheel by about 50 to 75 RPMs which is good, because once you are fully loaded you will lose some RP Keep in mind that the 23 X 24 & 24 X 23 both carry the same amount of load only one will be more efficient than the other. Meaning the 24 may be 80 % efficient while the 23 may be 70 % efficient. Also remember that pitch is what moves you forward.

 

[plowin]

First of all I am not a prop or shaft specialist but there are many many boats running wheels of that size and reduction with far smaller shafts than 2". For instance there are a lot of hulls like a 32' hollands and 35' brunos that 2'" shafting is not even an option. Why would you say that 2" is a must? Again I am not a specialist.I would say that the only "must" is that the shafting be Aquamet 22. An 1.75" Aquamet 22 shaft will take a bejesusly amount of abuse!

 

[plowin]

Your honor, based on the edit made on the post, please strike the question from the record!!

 

[John 40]

Thank you all for your help. The hull is a pettegrow and the beam is 10'10".
Capt. Larry thanks for the response, I don't know much about propping a boat, however the varying sizes didn't make sense. I did go with the 2" shaft because I was told it would be on the safe side.

 

[eyschulman]

You are fine with the 2" shaft. Your motor has plenty of toque so 2" is a good safety margin. Yes you can get away with less shaft but the key wording there is you are getting away with it. better to be safe. A 24" prop almost square is a good starting point 3 or 4 blade. Do check with other similar boats on gear ratio. I am thinking you may need more than 2:1

 

[atlanticblue]

Just wondering if I have the ideal prop for my boat? It's a 35 Bruno Stillman 320 hp 1 3/4 shaft? Its a four blade,,It says on prop 21-15. Not sure if the 15 is 1.5! Thanks

 

[captainlarry84]

Performance

John,

Breakdown your power train. 2 shafting is always wise. A 24 propeller has a min bore size of 1.5 & a max of 2. You could get away with 1.75 no problems. If it was my build I would go with the 2 shaft.

You mention a max of 24 diameter as per the builder. Is that the max diameter a 30 Pettegrow can swing? If so then a 2:0 gear is correct. As your 24 wheel will be close to square which is something we like to see.

If, however you can swing a 26 wheel than the 2.5 gear as mentioned by Eyschulman is something you may want to research.

2.5 gears are excellent on DE boats everything turns nice and slow which extends bearing, shaft life and machinery.

2:0 gear which is always the safest as you the most flexibility in performance.

Back to the 24 X 23 4 bladed wheel with a 2:0 gear I like the package. I think the boat will have a great turn of speed @ an est of 80% these are my projected speeds.

WOT @ 3450 RPMs speed 26 knots
3000 RPMs speed 23.73 knots
2600 RPMs speed 19.70 knots

Lastly I would inquire if the builders are going to use a wet tube which is the better way to go.

Can you post some pictures of the hull bottom? Keep us posted on this very exciding new build. So far I think you are making all the correct choices. With the 24 propeller even plus or minus 1 of pitch puts you well within a great performance range.

 

[John 40]

Larry,
Thanks for taking the time to run these numbers. This build is already 80 percent complete. I bought it as a kit with motor,shaft,rudder installed. I have been finishing it the last year and am sending it back up to have it awl gripped, electrical and all the finishing touches. I will try posting some pics. The max. wheel is 24" and that gives me 3" of clearance between the hull and the prop tips. I looked at the 2.5 to 1 but the prop would have been like a 24x30 from what i was told. There was no option of 2.25 to 1 in my gear which i think would have been optimum. Thanks again

 

[captainlarry84]

All Good... The home stretch is comng!

Ok then 24 is the number. A 24 X 30 would have been a cavitation nightmare. So you are where you want to be. Trying to feed 30 of pitch on a 24 blade with a keel in front would burn up your propeller in a season. Make sure you put in a good bonding system to protect that nice new 24: Nibral wheel. Also make sure that you have enough exposed shaft for a shaft zinc. Will the rudder be SS or bronze? I prefer bronze rudders with nibral they seem to get along better together.