Hi Thrust prop for outboard (twin) Power Trimaran

[mike w. schultz]

Thanks in advance for any feed back, I will be mounting a pair of Yamaha 115 4 stroke Outboards in a 64ft power trimaran(displacement hull) in the next month or so... I am looking on feed back or real world experience with high thrust outboard prop(s) hoping to propel vessel to 12 to 19 knots. Are these props off the shelf or custom made? I have yet to purchase engines and might consider Suzuki as a alternative... Thanks Mike S.

 

[bajansailor]

Hello Mike - the catamaran in my avatar photo used to have a pair of Yamaha 115 hp 4 stroke O/B motors - she is 49' x 16', with slender hulls (L/B ratio about 15 on the waterline) . We built her 19 years ago; I cannot remember the gearbox reduction ratio, but the engines were not 'high thrust' as such - they would have had the highest reduction gearbox available at the time for this motor.
She achieved a maximum speed of 16 knots with about 10 people on board.
These engines worked well for a number of years until the owner decided to go up to a pair of 150 hp motors.
The 'standard' high thrust range from Yamaha appears to only go up to 60 hp -
Yamaha Outboards https://yamahaoutboards.com/en-us/home/outboards/jet-drive-high-thrust/high-thrust

We also built another power cat (but heavy displacement steel this time, 39' x 16', about 8 or 9 tonnes displacement) and she was fitted with a pair of the Yamaha 50 hp high thrust motors which suited her very well.

With your very slender main hull you might well find that a pair of 'standard' 115 hp O/B motors will work well for your application -
Yamaha Outboards https://yamahaoutboards.com/en-us/home/outboards/115-50-hp/f115
I had a play with the propeller selector link, and for load carrying it suggested a propeller with diameter 13.25" and pitch 14".
But there was no option for selecting a slender stabilised monohull.

 

[bajansailor]

"Thanks for this info. your research say anything about 4 blades vs 3 blades by chance..."

Sorry Mike, but I only looked at the propeller guide on the Yamaha website.

I have dealt in the past with Frank & Jimmy's Prop Shop in Fort Lauderdale, and found them to be very accurate in their predictions - this was for inboard engines, but I would hope that they would have a lot of experience with outboards as well (?).
It might be worthwhile contacting them for an opinion?
Home: Frank & Jimmie's Propeller https://www.fjprop.com/
It might be worthwhile to first find out what is the highest reduction gearbox which you can use with a Yamaha 115 hp?

 

[Mr Efficiency]

"high thrust" in outboards comes from larger diameter props, than the standard engine will take. High thrust variants usually use the gearcase of larger engines in the range, but with a greater reduction gear ratio. Mercury's "Command Thrust" engines are probably worth looking at.