Throttle vs boat speed

[Zach Johnson]

We have lots of posts here about assorted engines being employed on different boats, but we have little relative data to help an owner get the proper engine for their boat, in the conditions they're likely to sail in. For example, a Tiki26 that navigates coastal rivers and associated tides is likely to have more engine than required for navigating an inland lake. 

So let's hear it: What engines are you using on what boat? What speeds do you achieve at what throttle and in what conditions (with/against current and wind, etc)?

Hopefully this will help some people as they shop engines over the winter.

[Ian Bamsey]

Interesting question.

On my Tiki 21, I'm using a Honda BF5. I use the boat in coastal waters where tide is a factor. Up and down the coast, making a passage, a 1 to 2knt tide makes all the difference and around headlands it can create over falls with some 3-5knt races. Getting in and out of the rivers (e.g. back to my mooring) I might need to push against 3 or 4knts. Here a little extra power can get me home. It's a long shaft and my platform transom is deeper than the design, so I don't suffer cavitation. 

At 1/2 throttle, in smooth condition the boat makes 5knts comfortably and with 3/4 throttle, 6knt. The optima is therefore less than 1/2 throttle. The local prevailing wind is SW and I often wanting to head south into the chop. The motor work well to help with the beat, but I get trouble with the engine cutting out when water splashes up under the engine hood, probably via the vent underneath and to the back of the engine. The engine would normally not get so wet with a normal full transom down into the water.

One interesting observation is that in light airs, whilst motoring home (due to a lack of wind), we made a steady 5knts. We raised the sails and because the wind was on the beam, and the sea state was mirror like, even the light breeze gave another knot and a half (nearly a 30% increase). Had it been from behind, it would have added little or no speed, but perhaps a little extra fuel economy. 

Here's that light airs day ...

Ian

[Bart]

For my tiki 30,  i use a yamaha 9.9 high trust longshaft.

On a lake it give's me 6 knots 3/4 throttle, 7 knots full throttle. with 25 knots wind from the front i go 4,5 knots full throttle.

on a big lake (shallow with short, steep waves) i get caviation with wind 20 knots upwards.

I might change the engine for a extra long shaft version. I prefer a 9.9 but i might try a 8hp...

Bart

[Zach Johnson]

Good info. Seems like these even with lower SOGs, these little outboards are nearly maxed out.

Bart, your info is especially relevant for me as I'll be building a Tanenui which isn't all too dissimilar from your T30. I'll be dealing with tidal currents averaging 2-3 knots and pushing up to 4 knots in places, so I'll look into how I can get some more power to the water.