DRAGONFLY Trimarans User Forum

My DF800SW came with a Yamaha 6hp 2-stroke outboard with a 9x5 dual thrust propeller. Based upon the description of the propeller I would think it's the right choice for the boat/motor combination, but the performance is just ridiculous.

In any condition the motor needs a lot of revs to move the boat and the thrust is anything but "dual". Motoring speed would be 4-5 knots max in calm weather and only 1-2 knots against high winds. The motor has absolutely no problem picking up revs.

Is this normal for a "dual thrust" propeller and do I really need a "standard" propeller like a 9x7 or maybe an 8½x8 (or ?x?) instead?

I used to have a long shaft 8hp on my old 25. If I remember correctly, I would rarely break 7 knots in flat water.
Is the hull clean? I know that would rob a couple of knots from your top speed.
When was the last time the engined was tuned up? Could also be a cooling issue.

tpaliwoda wrote:I used to have a long shaft 8hp on my old 25. If I remember correctly, I would rarely break 7 knots in flat water.
Is the hull clean? I know that would rob a couple of knots from your top speed.
When was the last time the engined was tuned up? Could also be a cooling issue.

The hull is perfectly clean. The boat was just launched.

I don't understand your comments regarding tune-up and cooling. The motor itself runs perfectly and revs without any problem as I said. The "only" issue is converting all those revs to forward thrust.

It's almost like the motor revs freely without a propeller (the propeller do rotate).

Hi

you may have a prop with a rubber bush instead of a shear pin , in which case it is possible the bush has sheard, if so this will give thrust at low revs but will slip when aplying more power, if you have thrust at low revs and as you put on more revs and you dont get any speed this could be the problem

I second the last opinion.

I third that opinion. Same happened to me and is likely to be the problem if you've run engine aground or had a rope caught around prop etc. or possibly just wear and tear. Removing the rubber bush is straightforward with a vice to hold prop whilst gently bashing out the bush and replacing.