River Boat prop

We launched the 'Sebastian Marie' a few months ago and have cruised a hundred hours or so, shaking her down, refining and tweaking systems, and now, armed with a little data we are ready to optimize the prop. The first prop was ordered based on the Vic prop calculator. The result is somewhat sub-standard but among the input items, my hull type was not considered. I'm going to drop a little data here in hopes that someone will have some input. This is my first try at building a boat and I am armed only with ignorance.

The vessel is a tri-hull pontoon, hulls 65' in length, 60" in diameter.
She weighs about 80,000 lbs and has a 26" waterline
I have fitted her with a John Deere 85 hp diesel, 2400 rpm max with a 2:1 reduction
This drives a 20' longtail
The current prop is 24" dia with a 12 pitch, four blades. It's driven by a 1.5" shaft
This works, but not as well as I would like. The prop cavitates above 1800 engine RPM and the vessel only makes 5 MPH
The prop shop says they can increase the existing pitch a little, but don't expect a miracle.

Please advise,

Ryland

 

I’m curious as to how you would do the math for a propeller worksheet on a hull like this.
Do you add the length/beam/draft of all three hulls?
I tried it that way, got 18.7 x 17.8 for 4 blades, likely wrong, see below.
Also does a standard propeller calculator even apply to a long Tail?
I’ve seem some tailers that appeared to run more as a surface piercing prop than fully submerged.
I would think complete submersion would be absolutely necessary in this case, for maximum efficiency from low power.

 

Thanks for your interest. The prop is fully submerged at least 20" below the surface. For calculating purposes, ( out of ignorance) I added the wetted width of the three pontoons for a beam specification.

 

You have several factors working against you. You have a steep drop angle on the shaft. Flat props don't like steep drop angles. Adding a double Cardan joint to take about 20 degrees of drop off the prop would help considerably. Increasing gear reduction and running a bit higher P/D and a bit more diameter would also help. This means a bigger diameter shaft, which kinda sucks when you need 20' of it. But given you are a tinkerer, and a tractor PTO cardon joint isn't too pricey and might even be lying around, I'd get that part done and test it again with the same prop and shaft. Try to improve the inflow to the prop while you are at it. You will want that prop to have no more than 10 degrees of drop.

Check to make sure you can manage the torque from the cocked prop. It will want to dig. I guestimate about 25,000 ftlbs of torque over 20' at a 20 degree bend. But maybe the pivot is closer than 20 feet. It will need braced pretty good fwd and rev.

 

Ran the numbers through the prop calculator as one hull 9’ wide, came up with some different numbers.
Now I’m getting prop size of 18.7 x 11.
The bad is that it will take 126 hp to make 8.6 knots, and 175 hp to top out at hull speed at 18.8.
So until someone can clarify correct method of how to run the numbers for a multihull vessel, I’ve gotta say it’s both overpropped and underpowered.

 

The down angle is 14 degrees. The only problem to solve is speed. As it is, she pokes along at 5 MPH but only burns 2 gallons per hour. I don't see redesigning this drive at this point. What I'm looking for is a prop solution that will optimize the existing configuration.

 

There isn't anything you can do simply, the Cardan shaft is the simplist and cheapest. The alternative is to change to a 3 or 3.5:1 gearing, 2 or 2.25" shaft, increase the D to about 27 or 28 inches, and increase the pitch to about 18 0r 20 inches.

If the prop is 20" submerged, then it looks like the shaft drops about 7 feet from tailshaft to prop - 32" below the surface and about 4.5' above the surface. That's 20 degrees. If you meant the shaft is 20" below, that isn't deep enough given your loading. You are ventilating. And you really need to improve the flow in front of the shaft. Cut the truss away and work in a streamlined thin shaft cowl no more than 15% thickness to chord. This should get you a little, but I doubt it will get you enough to make the difference between being happy and not being happy.

 

Yes, there is good news/bad news. The good news is that you've got things just about right as far as the prop is concerned. The bad news is that the performance is about as good as it is going to get with 85 HP pushing that thing. Phil is correct that streamlining the claptrap in front of the prop will help, but not enough to really see a difference in speed. Perhaps a couple of 250 HP outboards would help?