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Old 01 December 2006, 15:04   #1
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auxiliary engine trim & speed

When running on the auxiliary engine, would you/should you run it trimmed right in or trimmed right out, and why?

Bit of a run around with the aux last weekend (still running it in) and I tried both but it didn't seem to make much difference to speed, about 4.2-4.5kt whatever.

The other interesting thing was, there was also almost no difference in speed (about 0.2 kt) between half throttle and full throttle, the engine was obviously working much harder at full throttle by the noise (and probably much higher fuel consumption too) but made no difference to the boat's forward speed - a bit odd? anyway I settled for toddling around at half throttle which was much more relaxed, until I lost something over the side and by the time I had turned the damn thing around to look for it, it had sunk....
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Old 01 December 2006, 15:10   #2
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You have probably got it to the max displacement speed by half throttle, I would think!
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Old 01 December 2006, 16:38   #3
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Originally Posted by BogMonster View Post
When running on the auxiliary engine, would you/should you run it trimmed right in or trimmed right out, and why?
Definately trimmed out Stephen, apparently if you run it trimmed in you can do awful damage on the underside of your tubes!
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Old 01 December 2006, 17:03   #4
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Definately trimmed out Stephen, apparently if you run it trimmed in you can do awful damage on the underside of your tubes!
NOISE!! This member needs suspending. Or, at least suspending by his member!
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Old 01 December 2006, 17:31   #5
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The other interesting thing was, there was also almost no difference in speed (about 0.2 kt) between half throttle and full throttle, the engine was obviously working much harder at full throttle by the noise (and probably much higher fuel consumption too) but made no difference to the boat's forward speed - a bit odd? anyway I settled for toddling around at half throttle which was much more relaxed, until I lost something over the side and by the time I had turned the damn thing around to look for it, it had sunk....
The main difference in the power will be when you are punching through a sea - extra power is really noticeable then. True displacement characteristics - will get to theroretical displacement hull speed (square root of LWL in feet as a rough guide), relatively easily in flat water (as Nick has already mentioned) but requires much more power to get over that spd.
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Old 01 December 2006, 18:16   #6
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When running on the auxiliary engine, would you/should you run it trimmed right in or trimmed right out, and why?
Personally, I'd run it trimmed so the aux was thrusting directly astern. You don't have enough power to get the bow up so you might as well use what you've got as efficiently as possible.
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Old 02 December 2006, 05:27   #7
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Originally Posted by BogMonster View Post
When running on the auxiliary engine, would you/should you run it trimmed right in or trimmed right out, and why?

Bit of a run around with the aux last weekend (still running it in) and I tried both but it didn't seem to make much difference to speed, about 4.2-4.5kt whatever.

The other interesting thing was, there was also almost no difference in speed (about 0.2 kt) between half throttle and full throttle, the engine was obviously working much harder at full throttle by the noise (and probably much higher fuel consumption too) but made no difference to the boat's forward speed - a bit odd? anyway I settled for toddling around at half throttle which was much more relaxed, until I lost something over the side and by the time I had turned the damn thing around to look for it, it had sunk....
Steven,

I have the same engine; 6hp suz, on basically the same boat, encountered the same issue with it (although ran it in before full throt).

There are two props for this engine, possibly johnson have more? 6" and 7" Ribcraft are very good with swopping props, so sent the 7" back and now have 6" ..engine a little happier- 5 1/2 knots .still no where near max revs. In hind site the 4hp would have done just as well as the extra 2 hp are up where we can't rev too.
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Old 02 December 2006, 09:19   #8
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You can also get high thrust 4 blade props for some of these engines - also saildrives but maybe they have different ratio gearcases?
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Old 02 December 2006, 16:08   #9
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Steven,

I have the same engine; 6hp suz, on basically the same boat, encountered the same issue with it (although ran it in before full throt).

There are two props for this engine, possibly johnson have more? 6" and 7" Ribcraft are very good with swopping props, so sent the 7" back and now have 6" ..engine a little happier- 5 1/2 knots .still no where near max revs. In hind site the 4hp would have done just as well as the extra 2 hp are up where we can't rev too.
I don't know what the prop is that's fitted to it but I'll see - it is certainly nowhere near max revs. I will get hold of Oakley and see if there is a finer prop but the displacement speed may be the answer - 18.85 feet of boat (5.8m) square rooted gives 4.3kt....

I've changed the locating pin so the engine is thrusting straight ahead (ooer) so I'll see what happens tomorrow if the 'kin wind dies down.

The fuel consumption seemed good - I was toddling around on half throttle for a good half an hour last weekend and it only used about half a litre of fuel (1/3 of the internal tank)

Also saw a Rigid Raider today with two DF140s on - those are lovely engines - so quiet at idle you can hardly tell they are running. Definitely four stroke for me on the next boat
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