Prop Size

[Joe Vukan]

I have a 1977 Bertram 31 Bahia Mar with twin 454 Crusaders 330 hp 1:52 reduction. The props on the boat are 4 blade 18x18, the max rpm at wot is about 3500 rpm. I think the boat is over propped. I was reading that most 4 blade props the pitch should be .8 of the dia., this would leave me with a 18x14 prop. What was the prop size when these boats were built? What size 4 blade prop would work with the power train to give me a wot of 4400rpm?

[Bob H.]

Joe, rule of thumb is 1" pitch per 100 rpms, you may have a combination of bottom growth and tired engines. Looks like your chasing 900 rpms, I will let the brothers with gassers spec the prop size. I run 21 x 22 with cummins 355hp. BH

[mike ohlstein]

I think that the prop size is right but the gears are too tall. When you say 1:52 I assume you mean 2:1.52

Don't most of the gas boats use 2:1 gears?

[CaptPatrick]

Gas and low rpm diesels are in the 1.5:1 gear ratio for our style boats, high rpm diesels are 2:1 ratio, again for our style boats...

[Tony Meola]

My 75 FBC with 454 350 HP mercx had 18x19's and the engines turned up to 4200.

[John F.]

As I remember, my FBC with 454s and 1.5/1 gears had 18x18 3-blades, and 17x15 w/5 cup 4-blades and turned 4200+ when healthy. Are the wheels or boat new to you? Did she used to turn up to 4200-4400?

[Carl]

Not 1st hand info...but have noticed 454 owners having multiple WOT ratings.

My first thought is to make sure you "know" exactly were shes' supposed to rev up to and go from there.


I usually figure an inch of pitch= 200rpm change. But lots of factors can come into play.

[STraenkle]

I had about the same as Tony, a 1975 FBC with 1.5 BW gears and carb 454's, ran 18x19 3 blade almost no cup. When the engines were running well 4100 and just about 30 Knot WOT (and an ungodly amount of fuel).

[Carl]

I think that the prop size is right but the gears are too tall. When you say 1:52 I assume you mean 2:1.52

Don't most of the gas boats use 2:1 gears?

Mine had 1:1

[mike ohlstein]

Pat Panetta ran 1:1 and 50 mph.

[Joe Vukan]

The gear ratio is 1.52:1, there was a small amount of growth on the props and shafts. I'll get the props cleaned up and try them again in the spring.
I had the boat for about 5 years, 2 years on the hard for rework, gas tank,deck,rub rail, paint and etc. Put it in the water for 1 season and blew a motor ,bent valves. Replaced the motor with a long block on starboard, and risers,manifolds on both. Also replace the rudders with Capt Pats oversize ones.

[PeterPalmieri]

Just as a point of reference I have Tonys props on my boat and only hit 3900 RPMs WOT. I have a top and pretty beefy teak deck. I'm not sure if that's enough to shave 300 RPMs or not.

[Carl]

Joe-

Your original statement of going with 18x14 is correct using rule of thumb, in theory that should bring you to 4300.

But you need such a drastic change I wonder if there is another issue...or maybe boat was just overproped to begin with.
Mayb prop is out and just robbing power...but I assume it's both engines cranking up to 3500... Not one pulling the other?
Do wheels have a cup...a heavy cup? That could easily take 200 rpm or more of your WOT.

With a clean bottom and running gear, shaft turning freely by hand at dock, engines checked out and good tune up 3500 is the WOT? Can you verify your tacos...chances are both are not off equally. Any chance you have a spare set you can try with less pitch? Borrow a pair? Even 3 blade...just expect to gain 200 rpm by losing a blade.

My concern is your looking at a very big change from what you had and others seem to have...maybe engines are just getting tired....

Whatever the case, better to be under wheeled then over wheeled...plus you get to enjoy the ride a bit longer.

[Pete Fallon]

Joe Vukan,
On my 1961 express cruiser I had 454 CID 310 Hp Mercruisers with Hurth 1:5:1: 8 degree down angle trannies all new in 1997, turned up 4200 PMs at 41.8 mph(gps) the props were 3 blade Nibral Dyna Quad with 18" diameter with 17 degree pitch on 1-1/4" shafts. I got just about one mile per hour for each 100 of RPMS, 4200 RPMs= 41.8 MPH, 4000 RPMs = 40 MPH 3500RPMs=35 MPH 3000RPMs =30MPH etc, She would get on plane at 2200 RPMs and stay up until 1600RPMs before dropping off plane. Best results were running at 2600RPMs =26MPH with 1.5 miles to the gallon fuel burn. The engines were throttle bodied fuel injected engines with low profile struts clean bottom and 1/2 fuel (80 gallons).

Every 31 is different but the best is try and get close to 1.5 miles per gallon total as you can,17 x17 3 blade or 18 x 17 seems the best for gas engines. The 4 blade props seemed to be best at 18 x18.

Most of the new diesel conversions have 21 x23 4 blade props on 1-1/2" shafts.

The original boats(1961 to 1966) had Chrysler 413 carbureted engines with 72 series BW trannies 1-3/8" shafts and 170 gallon fuel tanks until 1972 then 222 after 1972.They changed over to Mercuiser big blocks in the early 70's. The carb engines ate gas at a rate of 1 gallon per mile an hour. Real gas hogs, but back when gas was under $1.00 a gallon nobody cared. Hell I used to burn 60 gallons a day for a 40 mile round trip to the fishing grounds and it was under a $100.00 now its almost $700.00 for the same trip everyday.
Pete Fallon

[Craig G]

I have a Bahia Mar with Merc 454EFI MPs. It had 3 blade 18 x 20s on it when I got it but would only turn up about 3800 WOT. The boat accelerated very quickly, so I didn't rush to change anything. Recently, I brought the props to Frank and Jimmies in Fort Lauderdale and based on all the information I gave them, they tweaked them to 18 x 18. It seems to accelerate as well as before, but now I can get 4400 WOT and 30 kts. The sound of all that gas being sucked in is awesome, albeit at just over 26 GPH/each! I cruise at 20 Kts at 2800 rpm burning about 20 GPH total equating to 1 nautical mpg.

[dcasey10]

While most of you that have the gassers are using 454s, I have a prop question for my 440 chryslers.

1- previous 440 owners, what max rpms did your engines turn
2- what size wheels did you use.

My boat is doing 3800rpms and around 27knts with 3 blade 16.13 I believe. My BW are 2:1, not the ideal set for the gas engines but I think that there is a great deal of room for improving my speed by changing the props (no chance in changing gears for me just props) 16 seems very small to me.

DJ

[dcasey10]

I found a pair of 19x17 3 blade bronze props for around 600 locally and they are the right 1 3/8 size. worth trying? will the 19in props fit ok with the 1962 style down angle shaft?

[Capt Dick Dean]

The 31 I just sold had the 4 blades, 18x18 with 454's and here's what happened when Har and I went to Atlantic City. We went three times. I ran from Bay Shore, NY ... 210 NM and Harv left from Freeport. It' about an hour and fifteen minutes more for me to get to Freeport.
As we arrived in AC the first thing was the gas dock. Harv took on 125 gal and I took on the SAME... 125 gals. Both boats are Bahia Mar's but he had a 3 bladed prop.
I found 18x18 quads get more mileage but the top end suffers. I too had 3500 rpm. I new that with "3500" I was over proped. How come? Because I didn't get to 4400. That simple.
Capt Patrick said take of an inch off and make them 18x17. Then we got to 4000 and that was good for me.

Some time ago I thought I had 427 engines. And so did some others too. The engine casting # on the engine converts to 454.

And was Harv a little upset? You beta' cha!

[John F.]

Dick-

Good to hear from you. If you got 210 nm and only burned 120 gallons, you did great. Or was it 105 nm and 210 gallons? Either way, good to hear from you. I'm putting a set of 4-blades on Crows Nest this winter. Doug had them set up awhile ago. He said he lost a couple of knots at WOT, but otherwise they were nice.

John

[Capt Dick Dean]

Yeah, John ..... gas mileage to AC.
Sometimes it's too good to be true. And things are not as they seem to be. Let's do the numbers again.

210 over 125 = 1.68 and 110 over 125 = .88. The true # is 110. That's the distance traveled.

So I missed it a little. What the hell do you want from an 80 yr old man???

A week after my 31 was sold I picked up a Mako 23 straight inboard. I don't think the
trip to AC is in the cards with this boat unless the trailer goes with it.

[Charlie J]

and dick i took 50 gals

[John F.]

Good times. Really good times.