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Old 10 May 2024, 05:35   #1
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Achilles LSI310-E w/ Tohatsu 6HP, advice to plane!

Hello all!

Just joined the community, I've just gotten my SIB setup this year and I've taken it out 3 times - unable to plane still, checked speed on 2 of the trips. Looking for advice on getting this thing going! Hoping for at least 12 MPH because it's really a snail at 5-7 MPH.

All of the speed information I've got comes from my Garmin fish finder, weights are approximate.

1st time, 8mph tops: solo 86kg, free-diving gear (no more than 15kg), 12kg (4 gals?) of gas.
2nd time, 8 mph tops: 2 adults 150kg, free-diving gear (no more than 30kg), 12kg of gas.

Now, with the nature of diving the SIB does take on a bit of water, but even fresh into the water dry I've never been able to get on plane.

The Tohatsu 6HP I've tried on the bottom-most trim level and the 3rd setting from the bottom. Factory propeller. I do not have trim tabs and would prefer not to have to add them if possible, as space is very limited already on the transom (I've got wheels).

The SIB I pump up to ~3 PSI for main tubes and keel, ~8 PSI for air floor.

If there's any more information I can provide do let me know. Thank you.
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Old 10 May 2024, 07:19   #2
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You really need to know what rpm the engines running at when your giving it full beans and loaded. Buy an rpm meter from Amazon. If its running low or high revs then you can re prop.
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Old 10 May 2024, 07:22   #3
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Agree with tinker knowing if your using the right prop will help but probably not by 4mph.
This won't be what you want to hear, but 8mph is about what I'd expect from a 6hp, especially 2 up with gear.
You need a bigger outboard.
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Old 10 May 2024, 08:45   #4
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We used to run a 2.9 excell sib as a tender & a 5hp Yamaha would get it planing with an adult & small child maybe try the boat with just 1 person & no gear just to prove the point moving weight about makes a big difference too.
I'd agree its on the limit but I'd expect it to get up & plane
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Old 10 May 2024, 10:09   #5
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I had an old c.craft with a wooden floor that planed with a 6hp, one up. Not sure how heavy it was but considerably heavier than modern SIBs.
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Old 10 May 2024, 10:21   #6
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A 6hp is so marginal to get to plane with a SIB 5/6kts is all you can rely on when passage planning.

The boat needs to be absolutely upon to max pressure and rechecked on the water, even more crucial with an air floor. You need minimal weight, just you and a fuel tank frankly. Wave conditions and wind then affect trying to plane with a small motor in a way unnoticed with one of larger size.

Down pitching the prop might just make a marginal improvement but as said above you need to see your revs with a small battery tach.
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Old 10 May 2024, 14:09   #7
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Quote:
Originally Posted by tinker View Post
You really need to know what rpm the engines running at when your giving it full beans and loaded. Buy an rpm meter from Amazon. If its running low or high revs then you can re prop.
Thanks all, will purchase a tach to throw on for the next outing. What annoys me is that I hit just about the same speed whether 1 up or 2 up. I will also try moving the gas tank to under the front seat.
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Old 10 May 2024, 14:16   #8
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A 6hp is so marginal to get to plane with a SIB 5/6kts is all you can rely on when passage planning.

The boat needs to be absolutely upon to max pressure and rechecked on the water, even more crucial with an air floor. You need minimal weight, just you and a fuel tank frankly. Wave conditions and wind then affect trying to plane with a small motor in a way unnoticed with one of larger size.

Down pitching the prop might just make a marginal improvement but as said above you need to see your revs with a small battery tach.
Will double check the air pressure on the next outing, thanks for the tips.
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Old 10 May 2024, 16:12   #9
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I have a tohatsu 6hp on my honwave t32 and 2 up I can get 10.5 knots (12mph).
I've just bought a hydro force Caspian pro 280 to use as a tender and with the tohatsu I've managed 14mph 2up
It's all in the weight distribution. The 280 is extremely sensitive to weight distribution. The honwave not as sensitive.
I had the tohatsu on a t27 and it was poor 2 up, probably just displacement speed but planed solo (just)
I've not run the t32 or HF280 solo yet
The prop is 7.8 x 8
I've just bought a 7.8 x 7 to use on the sea but not tried it yet
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Old 10 May 2024, 16:27   #10
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This is my trim position
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Old 10 May 2024, 19:32   #11
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Originally Posted by Xpedience View Post
I will also try moving the gas tank to under the front seat.
This makes me think that perhaps you don't use a tiller extension either?
  • Keep the trim in the bottom hole
  • Use a tiller extension and sit forward
  • Check your pressures - when afloat do they match spec? IMO 8psi for a deck seems light. Pressure is absolutely critical
  • Absolutely use a tach and reprop if not making recommended revs
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Old 13 May 2024, 02:20   #12
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This is my trim position
Thanks for the trim position, that's what I'm currently running also.
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Old 13 May 2024, 02:23   #13
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This makes me think that perhaps you don't use a tiller extension either?
  • Keep the trim in the bottom hole
  • Use a tiller extension and sit forward
  • Check your pressures - when afloat do they match spec? IMO 8psi for a deck seems light. Pressure is absolutely critical
  • Absolutely use a tach and reprop if not making recommended revs
I have a short tiller extension but am generally seated in the very middle/front of the boat.

Here are the PSI's I try to match with my pressure gauge.

1. MAIN TUBE & INFLATABLE KEEL : 0.19 bar 2.8 PSI
2. INFLATABLE "AIRFIRM" FLOOR : 0.6 bar 8.7 PSI

I was running light for the deck at 8 PSI but not too far off... I will definitely try to roll closer to 9 PSI next run.
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Old 17 May 2024, 16:23   #14
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Headed out to sea tomorrow morning all!

1. Got a tachometer attached to the outboard now.
2. Extended external tank fuel line so it'll be placed more towards the front seat.
3. Will check pressure to be as close to max as I can.
4. Trim level set to 2nd from lowest.
5. Light tackle with 1 up tomorrow, also have a buddy that will arrive later in the day for 2 up testing.

Anything I'm forgetting? Will report results Saturday evening, my time.
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Old 17 May 2024, 17:32   #15
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Sounds like you have covered it all, see what revs you get at wide open throttle, have a great day.
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Old 17 May 2024, 17:50   #16
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Slightly overpressure the tubes and airdeck, as soon as it hits the water it will "loose" a little pressure.
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Old 17 May 2024, 19:44   #17
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Anything I'm forgetting? Will report results Saturday evening, my time.
IMHO you should commence testing with trim in the bottom position - if you achieve planing then up a notch and repeat.

Also pressure - you need to achieve full pressure while afloat. If your pump doesn't cut it, replace it.
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Old 18 May 2024, 19:37   #18
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Thanks for the well wishes, all.

There was some issue with the tachometer settings or maybe it was user error. I am using the RunLeader small engine tachometer and had it set to '1P2R' for 4 stroke, 1 cyl., but the numbers were definitely off... I was in the 10000-11000 RPM range at WOT. 2400 RPM at neutral or idle. Maybe some users can give insight into why this was? 4 wraps around the spark plug cable.

Disregarding that, here's today results, slightly better solo but ultimately still disappointed.

1 up, 95kg approx. load - topped out at 12 knots, much better and I believe I did get on plane. Tried both bottom-most trim and 1 level up from that, about the same results.

2 up, 160 kg approx. load - topped out at 6-7 knots, consistent with my previous outings. There's definitely something wrong with my setup and I don't know what... I guess I need to get my tachometer working correctly first.
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Old 18 May 2024, 20:10   #19
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Seems like you have to higher pitch on the.prop, setting on the rev meter.
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Old 18 May 2024, 20:17   #20
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What model 6hp Tohatsu do you have? The tacho readings you've given and the fact that you say you can't get on the plane, is it possible that you have a 2 cylinder 6hp Tohatsu that, as far as I can find out, is only available as a long shaft?
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