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Old 10 July 2013, 16:37   #21
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Interestingly we were out on the water tonight and at WOT we were down about 2mph from the other day.
I checked the pressure when we got back, and although it felt fine the main tubes were down from 3psi to 1.5psi, and the keel had gone from 3psi down to 1psi (the valve cap was a bit loose)
Although l have not tried it since re-inflating it, l bet it is back to normal now.
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Old 10 July 2013, 17:03   #22
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All inflatables looses pressure at the end of the day no matter how well they have been topped to at the beginning of the day.

That's why it's good to carry along a nice hand or foot pump and a gauge to top it back again to at least 3.0 psi once sib is resting on water for some minutes for pressure to stabilize to keep performing as expected.

Happy Boating
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Old 11 July 2013, 02:53   #23
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Thanks Loco - I'm pretty confident the prop is fine as turning a breaker bar and drive into the splines will not shift it. I run the common SIB set-up; 3.4m air floor with this 9.8 short shaft and the standard 8.5 prop with a light load (1.5 adults) - do you see an advantage in going for a 7.5 ever in this set-up? Where do I buy a simple/cheap tach to use with this engine?
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Old 11 July 2013, 07:46   #24
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You have almost the same set up as me. Except my boat is a 2.85m. I have an 8.5 pitch prop and at WOT the engine is doing 5500rpm, the speed is 18-19mph two up and about 20mph with just me on board.
I suggest you get a tachometer before replacing the prop. Something like a TinyTach, just google/ebay the name.

I just checked and there is an ebay seller in the USA offering them for $38.99 with free postage. Although l assume this means local. To the UK is usually about £7.

There are other similar devices, you could post on the electronics section of this forum for other suggestions.
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Old 11 July 2013, 08:06   #25
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Thanks Lightning, will do.

I'll do some speed runs Saturday and try out some variables too.

I've always run mine fully trimmed in (lowest hole) - others do the same?

I also don't use a raiser block under the engine as many do (the old Twats do splashback a bit) - I have a 10mm and 18mm piece ready to try this too so will record the results and see it it affects top whack.
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Old 11 July 2013, 10:06   #26
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I did mess about with trim positions and found that the third hole out seemed best. This equates to the engine leg being at a right angle to the water when the boat is at rest.

My Mercury 6hp only just got on the plane two up so l spent a bit of time trying trim positions.
But the Tohatsu has no problem planing the boat, even two up with a 9.5 pitch prop (8.5 currently fitted) so l left it on position 3.

I did get a fair bit of splashback when l fitted the Tohatsu, so l put a block of wood under the clamp to raise it a couple of cm and this stopped water splashing back into the boat.

Although the boat will do 20mph it is better with the engine throttled back to about 5,000 rpm where it sits at about 17mph.

The specification of the Zodiac 285s actually states that it is rated to a maximum speed of 17mph. Which probably explains it.

With your boat being larger this won't apply.

TBH l think l am doing pretty well out of this Zodiac as it is really designed as a tender and not a high performance craft.
I think the fact that it has a solid floor and V-keel helps with regard to performance.

The max engine hp is given as 8 with 4-6 as a recommendation, although the max transom weight is 40kg. The Tohatsu 9.8 l have fitted weighs 26kg.
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Old 14 July 2013, 05:02   #27
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Awesome day out yesterday. Speeds were well up.

Not sure then what was the issue before but with the same set-up as before we were back to 16kts+

Then did some runs to test various.

Timmed right in (hole 1) 16.2 kts
hole 2 16.3 kts
hole3 16.8 kts

Then with 18mm wooden raiser under engine:

hole 1 16.5 kts
hole 2 16.8 kts
hole 3 17.2 kts*

*briefly hit 17.8 as absolute max recorded.

Not bad for a little 3.4m rubber boat and 9.8 horses.

It drove best on hole 2 with as usual some weight in the front (Pelicase with gear and anchor).

But with the wooden raiser the set-up was so much better, quicker take off, splashing eliminated but still no cavitations or issues in sharp turns.

Loco - where's that post/pic you posted of water flow over the cav plate etc - I am sure mine was just as it should be but cannot find the pic now to compare?
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Old 16 July 2013, 04:42   #28
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I need to be on hole 4 from fully in with my setup to avoid splashback. Also I have an 18mm block of wood to raise the engine.

I tried it without the block of wood and the splashback was terrible.

But my transom is angled, so the engine needs to be on hole 3 (from fully in) just for the prop to be horizontal at rest.
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Old 16 July 2013, 12:58   #29
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I have the 2.9 Sunsport with 1 inch block to raise the engine and on hole 4. With 2 people the prop just pops out of the water when it settles on the plane. Fine on tight corners with just me. Splashback has been reduced. I do run with the 7.5 prop to hit the WOT rpm range and find it is quicker to plane than the 8.5
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Old 16 July 2013, 17:11   #30
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Just ordered a tiny tach thingy so I'll see how the revs go with the new set-up. Took the kids to the beach this evening and the sea was glass like, unbelievably mill pond like, sadly no boat with us...
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