TLAR* Anarchy

[Gabe_nyc]

I’m converting a 20 ft sailing catamaran to a powered fast dinghy.

(It’s a test — it might well end up being a stupid project, but that’s not at issue here.)

I would like to power it with a couple of big trolling motors, but I don’t know how to size them.

I have a big pull gauge and a friend who can tow me around to see how much pull the boat (name will be “TLAR 609”) requires, but I don’t know how to interpret the results:

If the boat requires a towing force of 100 lbs to go 5 kts/h, does that mean that 2 trolling motors with 50 lbs pull each will drive the boat at approximately** that same speed?

_____

* That Looks About Right

** In theory there’s no difference between practice and theory, but in practice there is.

 

[Ncik]

If the boat requires a towing force of 100 lbs to go 5 kts/h, does that mean that 2 trolling motors with 50 lbs pull each will drive the boat at approximately** that same speed?

Basically, but it gets complicated. If the trolling motors pull 50lbs each in a stationary flow (ie a static pull/bollard test) then their pull will be a bit different at speed. But for a low cost project it shouldn't be far off.

 

[Zonker]

Exactly. Trolling motors are rated to give their thrust at zero knots speed. Downhill, with a brisk tailwind. 

At 5 knots (not knots/h, the unit of knots has it built in = nautical miles/hr) you might get 20 or 25% less thrust depending on the prop design. 

 

[LionessRacing]

And there is the question of drag due to trim, if your towing lifts or buries hulls, as well as the efficiency of motor either ahead or behind hull. Make motors 30% larger, not likely to regret extra power. 

 

[Marcjsmith]

also take into account how much weight you'll be carrying.  the more people and items you put on the boat,  the bigger the motors and more batteries you'll need, which in turn adds more weight and more drag.

 

[casc27]

Why am I suddenly hearing Tim the Tool Man Taylor's voice grunting "More Power!"?

 

[Rasputin22]

Why am I suddenly hearing Tim the Tool Man Taylor's voice grunting "More Power!"?

 

[Gabe_nyc]

And there is the question of drag due to trim, if your towing lifts or buries hulls, as well as the efficiency of motor either ahead or behind hull. 

Thank you. That is a valid point. I realize that this would be a very crude procedure, but it would be only in very flat water to start with.

My marina is literally next door to the US Merchant Marine Academy and ordinarily I might chat up some of their folks, but COVID makes casual approaches like that unwelcome ...

 

[Gabe_nyc]

also take into account how much weight you'll be carrying.  the more people and items you put on the boat,  the bigger the motors and more batteries you'll need, which in turn adds more weight and more drag.

Why am I suddenly hearing Tim the Tool Man Taylor's voice grunting "More Power!"?

Both of you are correct. I think that for this season I will look for a cheap 6-10 hp outboard and sort out payload / handling etc.

If it appears promising I will work on converting to electric for next spring.

A fellow Anarchist in U.K. converted the same cat to power and gets 20kts from 25hp but I will be happy w 10(ish) kts.