What props sould I have?

I have a 28 sea bird, power is twin 225 chryslers, 1-1 velvet drives, 1.21 v-drives.It wieghs aprox 10000 lbs, It has 14/14 props.It will cruise around 20 kts at 3000rpm, its redline is 3800-4000rpm.From 3000rpm to 4000rpm it dos'nt go any faster, it just makes more noise.It seems underproped to me,I was told that there needs to be at least 1/2" of clearance between the prop and the hull, I have about 3".I would like to try different props, looking for sugestions as to weather a different diameter or different pitch would be the way to go.Any info would be helpfull.

Glenn Burgess

 

If you are having high Prop slippage an increase in Dia is required. Normally pitch is determined once adequate dia is found and is a product of HP.
Michigan wheel recommends 10% - 15% of prop dia as min. clearance, which should allow going to 15"or even 16" dia props. If you have a total of 450HP at the props with a 1.21 gear, pitch calculates to be 14.
14 P. @ 4000 motor rpm w. 10% prop slip provides top speed of about 39 mph or about 34 knots.
This assumes your HP at 450 which is probably high. I would try 16x13 props, although 15" dia may work. Each 1" of pitch will change max rpm by about 300.

 

i had a similar problem with my 25 foot Luhrs. i have a Chrysler 318 220-225 hp and 1 1/2 to 1 tranny. i found that going to a larger diameter only slowed me down even more. by going from a 14 inch to a 15 inch diameter prop i lost @800 rps. and for every degree of pitch i lost another 200 r.p.m's. my boat runs optimally with 14x 10 props.i get 4300-4400 r.p.m.'s wot and the engine does'nt load up.with the single prop i get about 15 m.p.h. at @3200 r.p.m.'s depending on wind and tide. i tried using the prop calculators on the web but they don't take into account prop diameter,hull weight and weight with full tanks of gas and gear.it all adds up. as far as clearance between the top of the prop blade and the hull you should have about 20%. that space allows for less water pressure between the prop and the hull and thereby reducing vibration. another thing to consider is idle speed. larger diameter and more pitch will give you faster idle speed.docking could be a problem if you have to maneuver between slip fingers with other boats around. i hope you find the right combination of diameter and pitch.

 

Take them to the prop shop and have them drop an inch of pitch and add a #3 cup. The cupping will allow the prop to grab the water at higher RPM's and the reduction of pitch will compensate.

UNLESS: the trannys are slipping. Get a mechanic to check the ouput speed of the tranny's to be sure they are not slipping at high speed. If not, do the cupping.

 

I just scraped the barnicles off the prop on my newly purchased old boat 1980 allweather double ender 6000 lbs 5000 dry weight. had a "larger" 29hp 1987Perkins M30 perama installed in 1987. thought they were using a 17" bronze michigan prop for this size engine existing prop is a 14 rh 9 owner said 5 knots was aout what he got out of it....Is the 14" prop undersized for this boat?
Geoh

 

3600 rpm max would like to see 8 knots but would settly for 7

Geoh

 

calculating propeller

Dear Sır
If you send your boat plans (below,side vıews) I would calculate to find
correct size of Propeller.
Same Details Lpx,Bpx,Lcg,Deadrise,Engine Names,Horsepower,Rpm, etc.
Oktay Çemberci
İstanbul/turkey

 

Prop Size

Hello Oktay

www.allweatherboats.com has boat description.
1987 Perkins Perama diesel engine 29 hp@3600rpm
Any help on prop diameter and pitch would be appreciated.

Thank you
George

 

Properler Size

Dear George
Your vessel data
Lwl = 21.70 feet
Bwl = 7.6 feet
Twl = 2.32 feet midship
Dep = 5000 Lb
Engine = Perama 29 bhp 3600rpm
Hurth 2.05 :1 (http://www.engine-cemberci.com/engine/PERKINS/PERKINS.HTM)
Results are
* Wageningen B type airfoil Propeller (Not Common in USA)
Diameter 14.93 inç
Pitch 9.00 inç
Dar % 53.25
Speed 8.30 knot
* Michigan USN type Segmental Blade
Diameter 12.76 inç
Pitch 10.00 inç
Dar % 71.47 ( Stock blade % 50 )
Speed 8.00 knot
USA Stock Propeller DAR is % 50 may be change the size - Diameter-Pitch
Oktay Çemberci
İstanbul/Turkey

 

Prop Size

Thank you Oktay for the help

I was hoping that 8 knots would be possable...
14 rh 9 is existing propeller. I just finished repainting the boat including new bottom paint...First time in the water will be next week...will let you know what speed i get... Definatly will re size to 12.76 rh 10 if not close to 7.5 knots.
My thinking was completly wrong, thought i needed a 17" diameter prop to increase my speed.

George

 

propeller size

Dear George
Your Propeller 14 *9 inç %50 Dar will be rearched 8 knots.
Our size for % 50 Dar is 12.73 * 10.50 inç Back Cavitation % 12 .
If you use 17 inç propeller Transmission Ratio must be 2.74:1
No problem for us to send propeller TO USA.
Oktay Çemberci
İstanbul/TUrkey