sizing new prop

I have a new to me 50' Willard Navy utility vessel that came with a dinged up prop of 26x17.
I drove the boat home on the bad prop, couldnt exceed 1300 rpm / 7 knots, due to vibration, but still managed to get just under 2 gal/hr!
The boat is rated for a huge load of 150 persons, and I think the 26x17 prop is likely designed for this load, roughly 40,000lb, which I will never experience, so I would like to choose a new prop targeting about 24000 lb.
There is plenty of room between the hull and prop to go to a larger diameter, but I suspect that pitch may be more effective towards achieving the highest possible cruise speed, which I am hoping may come in at near 10 knots.
Anyway, I want to buy a used prop of a near appropriate dimension to test before I pull the trigger on a new one, any suggestions on size?
The motor is a Cummins 6bt, 185hp@2500 with 2:1 reduction gear, 1 3/4" shafting.
Thanks, Don

 

I don't think that you want to go any bigger. You are right about at optimal diameter and the tip speed is already very high if get to rated RPM. If you made 7 kts at 1300, you were probably only using about 35-40 HP. Extrapolating, it should only take about 120 HP to make 10 kts. That would match the current prop demand at about 2100 RPM (assuming it is a good prop). That is probably a pretty good point to operate the engine, while still leaving you the ability to get more power out if you needed. Obviously, resistance is going up dramatically with speed so you will need to experiment to find the optimal point for fuel consumption.

So I would say stick with the basic parameters you have currently.

 

Johneck, thanks for your input.
The 7 kts @ 1300 produced very little wake, I think that speed is probably very near the sweet spot for this boat.
That was with a pretty bad propeller, though, I could see lots of cavitation bubbles in the propwash, so am assuming that prop efficiency was poor.
I have located a 26x18 locally, will probably go with that and tweak if necessary.
As you said, it is good to have a little more power in your back pocket!

 

I have the identical 50ft Utility , tho it has the 6-71 DD with a 3-1 reduction.

The 3-1 was installed as the PO could not find a 1.5 as she was built with.

The boat has a 32x32 prop which can not eat the power the engine produces , even with the huge Navy injectors replaced wirg H50 for economy.

7 k is indeed the sweat spot and 2-3 GPH is the norm.

10-12K is easy to obtain but the burn is then 10+ GPH.

The USN specks figure 1 nm/gal as the norm, with kids operating on the pin full time.

The boat is well built and with the Fire Retardant resin and collision bulkhead can be USCG licenses for 45 or 50 Pax.

If converting to a cruiser the built in foam flotateion can be removed for a huge increase in interior volume.

Enjoy , they are great boats.

 

Thanks for the comeback, Fred.
Mine came with the 2:1 gear, seems kind of low on first impression (just seat-of-the-pants), 3:1 should put it in the tugboat category.
Have you done any modifications to your boat?
I have a ton of questions I need answered before I start hacking on it!
Got it hauled out and the bottom clean, now for the fun part of making a cool fishboat/cruiser out of her.