Trimaran speed under outboard

[macsnail]

Hi, about 30 ft sailing trimarans (like Corsair or Dragonfly) usually have about 10 PS outboard.
Does anyone has experience what's the speed in calm water they reach with such or similar engine?

Thank for sharing your experience

 

[Wing Mark]

My guess is you generally wouldn't be choosing the prop for best top speed in flat water.

 

[macsnail]

Yes, right but my concern is not the prop. Simply I plan to change my boat to a tri. A pointed advantage is a higer range. .... but sometimes you have to come back under no wind.... U wanted to know with what speed I can estimate.....

 

[LONG_KEELER]

Yes, right but my concern is not the prop. Simply I plan to change my boat to a tri. A pointed advantage is a higer range. .... but sometimes you have to come back under no wind.... U wanted to know with what speed I can estimate.....

Interesting question.
Like cats, Tri's don't actually plane , and overloaded, could possibly be limited to waterline speed. I had a tri belt pass me this season under outboard so I would be interested to know.

I would think that Laminar Flow and Tranona will come up with the answers.

 

[dunedin]

Perhaps try a PM to AngusMcDoon who I believe has/had a tri around that size for many years, and covered some substantial distances

 

[Laminar Flow]

Based on its D/L ratio the 28' Dragonfly could do about 10-11kts in flat water with a 10hp motor (at 85% of output) and at the boat's "nominal" displacement (sailboat data). Any loading will bring down the D/L and with it the speed.

 

[AngusMcDoon]

Hi, about 30 ft sailing trimarans (like Corsair or Dragonfly) usually have about 10 PS outboard.
Does anyone has experience what's the speed in calm water they reach with such or similar engine?

18 years experience with a DF920 with 10hp engine.
5.5 knots easily doable and a good all day speed. 6.5 knots if you're in a hurry but fuel consumption goes up sharply, and then refuelling becomes an issue. Plan on 5.5 knots.

 

[AngusMcDoon]

Based on its D/L ratio the 28' Dragonfly could do about 10-11kts in flat water with a 10hp motor (at 85% of output) and at the boat's "nominal" displacement (sailboat data). Any loading will bring down the D/L and with it the speed.

That's not achievable whatever the theoretical calculations say.

 

[AngusMcDoon]

My guess is you generally wouldn't be choosing the prop for best top speed in flat water.

On outboard engines the prop comes from the manufacturer with the engine. It's not a separate purchase like inboards. The few manufacturers that do outboard engines designed to power sailing boats at low speed generally don't give you a lot of prop choices - maybe only one, but it's better than the high speed choice.

 

[AngusMcDoon]

Interesting question.
Like cats, Tri's don't actually plane , and overloaded, could possibly be limited to waterline speed.

The lwl to bwl ration for modern production tris is typically about 10:1 for main hull and 18:1 for the floats. At these ratios the empirical waterline length/speed calculations break down completely. An overloaded 30' tri when powered up on a good beam reach will still scoot around at speeds way beyond what the empirical calculation suggests - maybe only 15 knots rather than 18 knots. It's noticeable how having a few extra salad dodging biffas on board along with their extensive pie supplies slows things down a bit, but not a lot.