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Old 11 August 2011, 17:52   #1
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Technique advice

Hi guys, many of you will be aware of my faux pas (fitting a long shaft motor when an XL was required). By fitting a planing plate I have managed to get to a point where I am able to get my DR750 with Merc 150 up to 28 knots in favourable conditions. This is one up with 300ltrs fuel in the tanks.

My issue now is this:

I struggle to get the rig onto the plane at the moment. If I put a handful of throttle on the prop ventilates and boat goes no faster than 7 knots. I am really having to tickle the rig to get above this 7 knots. I am fumbling here but so far the best method I have found is to set around 3k rpm with the engine trimmed out a good way. Then I trim down and finally find a point where the boat starts to accelerate beyond 7 knots. Then a touch more trim down and eventually she starts to develop some speed. Once over about 12 knots I can pretty much get her to WOT at 5800rpm and she'll settle out at between 22 to 25 knots.

I am sure there must be a more scientific way of setting trim but am quite honestly at a loss now. Open to advice please.

Thanx - Ian (by the way I have the extension kit on the way but trapped in Madrid by customs for now).
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Old 11 August 2011, 18:40   #2
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Start trimming the other way round: trim half way down - start engine - accelerate - then gradually trim out. If it ventilates trim back in until it stops. You'll know when.
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Old 12 August 2011, 03:03   #3
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You can get converters for some engines to make them longer - they do one for the 115 - don't ask me how I know!
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Old 12 August 2011, 03:36   #4
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Quote:
Originally Posted by The Grocer View Post
Start trimming the other way round: trim half way down - start engine - accelerate - then gradually trim out. If it ventilates trim back in until it stops. You'll know when.
What he said

Quote:
Originally Posted by Cookee View Post
You can get converters for some engines to make them longer - they do one for the 115 - don't ask me how I know!
Blooming expensive though!

Welcome

A four blade prop would probably help a lot
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Old 12 August 2011, 04:10   #5
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What prop are you running? Is it in good condition?

If you are using a three blade prop try a 4 blade. For example the rev 4 has very high grip levels and give a great hole shot, it can be mounted high and still grip. You wouldn't lose much top end if any.

Try the Propgods forum, Ken may be able to help out with some better advice.
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Old 12 August 2011, 04:28   #6
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In my opinion squire, you're about a hundred gee gees short of what's required to get that lot moving. This is your major issue, moreso than shaft length.

You could propping down to tug pitch regions, say 15"P.

I have a 7m Vipermax with a 220Ltr tank. With 9 POB on weds eve, 250hp didn't exactly throw us onto the plane.
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Old 12 August 2011, 09:40   #7
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Hundred geegees too short Mollers? Different hulls of course but the previous owner had her on the plane with the XL motor and 12 divers and kit!

Oscar tried a 17 found the 19 better. I'd play more with props if they were here and free to return if no improvement otherwise an expensive gamble eh?

Coverter thingy? Wot like the one I have waiting for me in customs in Madrid?

I'll give the trim technique a go thanx guys. As I said I was fumbling without any proper guidance. I obviously have it wrong and will check your idea tommoz!

Thanx all!
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Old 12 August 2011, 10:01   #8
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It was more about the number of blades and design rather than pitch. From what you say it seems that you are getting loads of slip and 4 blades are known for having great grip and can be mounted relitively high without ventilating.

If you are running a 19 pitch and getting 28 knots at 5800 rpm you are getting between 39 and 46% slip at WOT depending on your gear ratio. A good set up should run at about 10%.

I realise you have an L and need an XL (about 5" difference) but the prop must be half out of the water! Where is the (anti) cavitation plate when you are cruising? With a good set up it should be visible above the water. Is yours way above the water?
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Old 12 August 2011, 11:57   #9
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I would suggest lowering the outboard more. The cheapest way of doing this and keeping your original engine is to lower the transom if you can. chop the required amount off the top being mindfull about water level (you don't want to flood the inside).

I would guess what is happening is that as you approach planing speed the displacment hump that is being pushed back from the bow is causing the stern to lift and creating a dip right where the prop is. This causes the ventilation and prevents the boat from gaining any extra speed.

How does the prop grip during turns?
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Old 12 August 2011, 17:53   #10
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Thanx for coming in guys - won't be chopping the transom and don't really need to now the extension is coming. Anti cav plate is visible but seems to be on the water just about. I have a large whale tail planing plate which gave me a huge gain.

She's good in the turns. Yes I understand about the prop but am reluctant to pay without being 100 percent sure and of course now that the extension is paid for and hopefully coming it wouldn't be worthwhile as a solution to this.

It's in the meantime (before extension) and also an after technique I was looking for really. I'll try the trim half way down and see how that goes.

Thanx again.
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