longtail power to wheel matching

I may elect to build a longtail drive for my river boat. I have available a Cat engine, 85 HP and will use a velvet drive with the 12 degree down angle.

I have no clue as how to match the gearbox ratio and prop size.

Any ideas?

Ryland

 

You need to figure out the displacement of the boat, the target speed and the resistance you will have at that target speed. Those parameters determine the propeller type.

 

resistance?!!

I'd like to see six or seven knots. Its a twin hull which slips easily through the water, but I have no clue as to the resistance. I have weight, waterline and wetted area data.

Ryland

 

I did just that. The prop manufacturer has a calculator that figures all factors and specs a prop (wheel) . I ordered two of them, to arrive late August. I received the engine, a Catapillar 3054C and a Velvet drive transmission with a 15 degree down angle to facilitate the longtail drive arrangement.

Ryland

 

Since the prop will only be required at the end , perhaps launching the boat and towing it at speed and recording the drag will allow the prop calculations to be closer.

Usually its a case of guesstimates and "Third Time Lucky" for props.

 

I ordered the props from a reputable source that advised that they could tweak them for best performance. With so many variables, this may be just a good place to start.

Ryland

 

The Thai smaller longtails I have seen can be run without a gearbox, by using the correct prop, but they are gasoline engines, not diesel... May require a large, high pitch prop, though. One of the James Bond movies had a longtail boat that used a large car engine to do some impressive stunts.

Porta

 

drivin'

I am using an 82 hp Cat engine with a velvet drive transmission 2:1 ratio and a 22inch 14 pitch prop four blade. This project marches forward perhaps rudderless, (pun intended) . We are decking it this week. This is no dream, it is happening, better or worse. The landing ramp is finished and installed, still pondering the options of winch-lifted or hydraulic. We are committed to having live hydraulics onboard anyway, (anchor yanker, etc.) so the ramp is destined to use hydraulics,below with cylinders, or above with winch and cables.

I'm not sure where this will go, but I am certain to go there.

I have backed up from a laminate (3") deck to a single layer of wood. This will save 16K of weight. It feels very sturdy with one layer placed. I may elect to reinforce in places where vehicles may cross and be stowed.

My too much fun alarm is sounding.

Want photos?

Ryland

 

Stop being a tease...hell yes!

 

Please....

 

photos

I posted these in another thread, but will do so here, too.

The vessel comes along well. deck and landing ramp almost complete.Pilot house and engine module are next. Will install bow and stern thrusters on one side.

This thing is getting scary big. It will disassemble in the middle for transport to the river.

I only have a few sketches and calculations, no real drawings or blueprints. The pontoons were drawn on a napkin in a bar, (NAPCAD). I'm fairly well connected in the metalworking business in this region, so with a word, I can get just about anything I need promptly. Heat and beat technology works, but it makes a big difference to have a couple of lasers and plasma cutters in the 'brotherhood'.

The engine is a Caterpillar 3054C 85 HP. It turns at 2400 rpm a Velvet Drive transmission and a 22" 4 blade prop. These will be constructed in a 'longtail' configuration. The vessel will operate in rivers and semi-protected waters where logs, snags and debris are a certainty. The longtail suits those conditions nicely.

Ryland