Trimaran Motoring Speeds with Larger Outboard, 24-28ft

[scape]

I hate to create a new thread on this, but I cannot find (and sorry if it exists plainly) much on different motor options for small trimarans. Most seem to have 10hp outboards and I've seen quoted a few times 6-8 knots speed under power and up to 10 wot.

I'm curious how fast something like a 20-25hp would net as an outboard on a medium sized trimaran like a ferrier/corsair 27 or so. I used a few sites and read calculations and came up rather short on what to expect, mostly nothing from uprating the motor size. Ideally I'd like to motor at 15 knots, if that's even possible. The extra weight from even a 25hp seems negligible. Appreciate the help!

[a64pilot]

To exceed hull speed I think you pretty much have to plane, unless you have a planing hull that means it's either not possible or requires an enormous amount of power. Or said another way its possible that doubling the power may only yield another knot and double fuel consumption to get that knot.
Although if they are getting 10 kts and if their waterline length is only 20' or so, thats well beyond hull speed, are they planing?

[sestina]

Recently, in my local boatyard, there was a 40' Dragonfly in for some work. Her engine? a 110hp Yanmar which can push her along at 15kts. The range isn't much...

Chatting to her owners, it seems that the grossly oversized engine was specified by her original owner, who used to have a MoBo. It seems that the sails were never used either. They are thinking of replacing the engine with something more modest and selling the big one for cruising funds.

[barnakiel]

A mono would go 7 knots so a tri may go faster as it may have finer hulls.

You will burn huge amount of fuel going very fast, unless your hulls can plane.

b.

[valhalla360]

The newer telestars had a 40hp as an option and reports of 13-15kts top speed.

[tridownunder]

my F25a has a 15hp outboard and will cruise at 7.5 knots happily but any more requires a lort more throttle.


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[tomfl]

As others have posted you may be adding a lot of weight and suffer increased fuel consumption for a rather small increase in top speed by adding a big outboard.

Another consideration is that what ever top speed you can reach may decrease when motoring into wind, waves, and current. The upside is smaller outboards will not push your boat against unfavorable conditions as well as a larger outboard (assuming the right prop and gearing).

[cattar]

Corsair F27 has a motor well and lid, larger on later boats. Seems like a 25hp would require a lot of cutting. Google for pdf rewrite of the original owner's manual. Says an old 8hp will push the boat at 7 knots, and a heavier, gas-guzzling 15hp will push the boat at 10 knots.

Corsairs sail so well, I wonder why you'd want a bigger motor. The hull is thin so the vibration through the boat becomes numbing when motoring all day. (Done that on a 31. Once.) And the extra weight of a bigger motor wouldn't be good for racing.

Other tris will have other specs. Generally on lighter tris with 25 hp, more weight, more gas use, more speed.

[ejlindahl]

I've heard there once was an F27 modified to have a 40 hp outboard. It was in Bellingham, Washington. Some thought it was a boondoggle. It was for a physician who needed to be able to respond on call. If you go on the Yahoo F-boats group and inquire/search there a people there who will know more about it.

[thinwater]

I had a Stiletto 27 (1600 pound cruising load on 2 hulls) that would go 12 knots with 18 hp.; . However, she gulped fuel (6.5 mpg) at that and was much happier at 8 knots (10 mpg). A light Stiletto with 25 hp will go very close to 15 knots.

The fact that there were 2 hulls is important; only 1/2 the weight on each one, and she did stat to plane a bit, which the tri will not (more weight, one hull). Thus, typically the Stiletto is faster than an F27 off the wind, and the F27 is generally faster up wind (better boards, more righting moment).

My PDQ goes ~ 7.5 knots on 18.8 hp with 8-9000 pounds (4500 pounds/hull). Inboard versions with 2x the hp (5000 pounds per hull, 18 hp per hull) only go a few tenths faster.

I've got a feeling 25 hp is going to dig a big hole, burn 2.5 gph, and go about 9 knots (3.6 mpg).

My PDQ goes ~ 7.5 knots on 18.8 hp with 8-9000 pounds (4500 pounds/hull). Inboard versions with 2x the hp only go a few tenths faster.