prop sizing help

[get reel]

I have just purchased a 1989 28ft baha weekender and the boat is lugging along and I'm have to run at 3700rpm to keep at a steady speed 16kts and when I put the boat at full speed I can only reach 3900rpm at 18kts . Prop on this boat is a 18x20 that has been repitch to a 18x18 according to the report I found with the boat and looking at the data for this motor it should be able to run at 4400rpm at full speed . Would hope someone can help me determan what prop would fit better 3 blade or 4 blade and size and pitch. Motor size 454 at 340hp dryweight 8600lbs 10'6" beam 28'5" long .

[Jango]

I assume this unit is an 1/O not a pure inboard? Need to know Drive Ratio?
Max speed with 340HP and 10000 lb (fuel and min passenger load) is abt 24 K

[get reel]

It is a inboard with a velvet drive . I will have to get the gear ratio .I have the transmission model number 1018106 if this helps ?

[get reel]

Ok I have the gear numbers 1.88.1

[Jango]

According to PropCalc, ideal Prop is a 19 X 14, 3 blade (55% DAR), which should provide a top speed of 24 Knts @ 4500 rpm @ 15% slip with a total displacement of 10000 lb.
If space does not permit a 19 dia. a 18 x 14, 4 blade (63% DAR) can be substituted. Fewer Blades 3 vrs 4 are slightly more efficient and are more desireable
Your current problem. is TOO much pitch - overloading engine

[get reel]

The total weight I gave you was an estimate both tanks full of gas and gear along with 6 people on board .The total dry weight of the boat is listed at 8600 lbs . Would a nibral be better or standard brass ? I use the boat for fishing a do a lot of trolling with it . I should have plenty of room for the 19" prop .

[jonr]

Your current prop is good if you like getting good gas mileage (slower speed, less pumping losses, less friction) and reducing engine wear (due to lower HP output). But if you really need the maximum speed.....

[get reel]

Not after speed but looking to be able to cruise at 3200 to 3500 rpm and stay on plane . Also would like to have some throdel left to be able to move fast if needed . Trying to find the right prop before I spend the cash on a new one . I know this boat is a tank and wasn't built for speed just trying to the best performance I can out of it .

[Ad Hoc]

Not knowing much about these Baha boats, but, looking at the data you provided.

The 340hp, will be more like 300hp now, the engine is old. Your max displacement, being around 10,000lbs, or roughly 4.5 tonne.

With these figures, the hp/ton is around 66 so your max speed should be around the 21 knots-ish!...can't change that.

With your prop whizzing around at 2340, at max rpm of 4400, the diamater and prop should be in the range, 17~18 diameter, so that is fine. The pitch being around 11~13". As Jango says...too much pitch.

PS..if you have the time and money..i would start with a 17x12" and go from there.

[Jango]

Get Reel, Props are normally sized at MIN. load. If Max Load were used, motor could Over rev if full throttle were used during a Light load. I estimated 10000 Lb would be close to your min displacement - Two people and gear and min fuel.

If you size your prop for max displacement, Six people, gear and 100+ gal. of fuel, your displacement is probably closer to 11,000 lb.

Using 11000 lb with a " worn " engine at 300 HP, PropCalc shows a 19 x 13 Prop @ 4400 engine RPM, Providing 21 Knts. If your Prop is too small, it will cause excessive Slip with lost efficiency [ speed ]

Nibral is stronger than Bronze, but probably not necessary since Prop rpm,s are not high. If at all posible, see if you can try different Props before purchasing, since these larger sizes are expensive