Proper sized props 36 Mark II 4-154's

I have a Gulfstar 36' 1976 Mark II trawler she is powered by a pair of Perkins 4-154 engines that appear to be healthy, strong and running well. Can anyone help me determine the original size of the 3 blade props used on this vessel? The boat appears to be slightly under propped this is actually good not bad in my opinion. I would like to know the size and pitch of the original wheels?
I'm also thinking about putting a pair of 4 blade wheels on her. Has anyone done this and if so what size and pitch were the wheels? The motors with the current props were run at 3300 rpms for about 5 minutes no smoke was observed and the boat ran without vibration. Now, I have not photo Tach'ed the engines to see if the current Tachs are accurate. This needs to be done.
Another question what is the original size of the shafts? The surveyor said they are 1 and 3/8ths. The transmissions are tagged Borgs with a 2.96 to 1 gear ratio. Any input would be greatly appreciated.
The hull ID# is GFS3623211 75.
Thanks,
Bill Lentz
1976 Gulstar 36 trawler
Soon to be renamed Wireless Too.

Three blade props are fine.

Eric of coarse 3 blade props are okay. I'm not starting a debate about that. I am interested in knowing the size and pitch a Gulfstar equipped like mine came with. There are advantages and disadvantages to 4 blade props.
I am curious if others have tried 4 blade wheels and also the standard size and pitch of the 3 blade wheels. Also knowing the shaft size is important to me.
Thanks,
Bill

Billy I'm with ya.

I wouldn't be very concerned about what Gulfstar did. Many new boats weren't propped correctly in the 70s mine included.

To prop your boat yourself with the aid of engineering data and prop shop experience you can prop your boat correctly yourself.

The amount of power available, the reduction ratio and the longest span between bearings will determine the minimum dia of propeller shaft.

Then the power and the prop shaft speed determined by your reduction ratio and your engine rpm rated for maximum power should determine the best #of blades (almost certainly 3 blade) the blade area and the best dia for the approx speed. Then it's best to have a prop that's not too far from square .. as in 15X15. Dia X 1/2 the dia as in 15 X 7 1/2". Like 15 X 12. Or 15 X 13 but not 15 X 8 or 9" of pitch. Too little pitch requires too much dia and as a result too much blade area. The engine's power is wasted just turning the blades through the water. Like having no pitch at all. The blade will be large but you won't be pushing water. With a square prop too much water will spill around the tips because of too much slippage and thrust will be lost. There are rules of thumb. Ask your prop guy.

So when you get the ideal prop nailed a bit it should be considered how much prop room you have. There are rules of thumb for that too. Then if need be make an adjustment to the # of blades, dia and pitch and then you'll have a prop that is so close to what your boat needs only a slight correction need be made.

It's best to do both. Find out from GS owners what original was and run your numbers. Compare and adjust to suit you and your own boat.

A good source of information is Michigan Wheel .. Call them.

Bill- I would not be too interested in what props came with the boat, better to focus on what props are best for the boat in the condition it is now.

Until you get the phototach data, it is all guesswork. Those engines are rated at 3000, so if you are getting 3300 you are seriously underpropped. In fact at 3300 the engine governors are likely reducing fuel rate so it makes it hard to calculate optimal prop.

Get the phototach and check helm tach calibration. Can be done dockside in neutral, rev to 3300 on helm tach and go shoot the engine.

What props are on there now, dia and pitch?

Shaft size, measure it behind the tranny.

Boatdiesel has a good prop calculator, enter your boat, engine and gear data and it comes pretty close.

For a boat like yours, three blades are recommended as long as optimal 3b can fit. 3b more efficient than 4b, all else equal.

Actually depending on which rating used these engines are capable of 4000 rpms. With that being said in marine service 3000 should be the top. I have 18" wheels on the boat I don't know the prop pitch. The surveyor says the shafts are 1" & 3/8ths. My transmissions are 2.91 to 1 that's whats on the plate. Also I did the prop calc on boat diesel it looks like 18" is the right wheel but the pitch looks to be 18 as well. I am not familiar with using their prop calculator.
I'll add one thing I had less than 50 gallons of fuel when we sea trialed. I believe the tank can hold another 200 gallons so perhaps I'm not so underpropped especially if the Tach's are off?
Bill

Yes prop to 3000
The 107 & 8 Perkins are the same.
I use an 18" prop w a 107 cu in engine and 2.57-1 reduction.
Looks like you should have a 19 or 20" wheel. 18 X 18" indicates a need for more dia and less pitch. 19 X 14 or 20 X 15.
1 3/8ths shaft sounds right.

I don't know what the pitch is I got that number using the BD prop calculator.
I do know it's 18" though.
Bill