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Old 11 August 2011, 16:52   #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, 17:40   #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, 02:03   #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, 02:36   #4
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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.
What he said

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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!
Blooming expensive though!

Welcome

A four blade prop would probably help a lot
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Old 12 August 2011, 03:10   #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, 03:28   #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, 08:40   #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, 09:01   #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, 10:57   #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, 16:53   #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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Old 13 August 2011, 01:33   #11
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One further question - is 3k about the right revs to set before I try to trim to find the grip or would you use a different figure? I suppose it's dependant on individual circumstances is it?
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Old 13 August 2011, 04:23   #12
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I'm not sure about your 3k rpm to "find Grip" Does your prop have vent holes in it? these are holes drilled into the hub around the circumference at the gearbox end (as opposed to the prop nut end) From astanding start you would normally trim all the way down, apply the throttle & get the boat moving & planing. Once she is planing, start to trim up until the prop cavitates & trim back in slightly. Trim in on tight turns. I had similar problems to you but for different reasons, 4 blade props (rev 4's) made a huge difference. Even now, if I try & set off with the engines trimmed out, I just sit there & "wheelspin"
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Old 13 August 2011, 04:37   #13
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One further question - is 3k about the right revs to set before I try to trim to find the grip or would you use a different figure? I suppose it's dependant on individual circumstances is it?
How do I do it? I can feel (and see) the bow rising as I accelerate, leaving the trim alone; then the boat levels as it gets "over the hump" and onto the plane. This when I start to trim out; a little at a time to let the boat settle; the boat should accelerate more without additional throttle as the drag is reduced. Eventually no more benefit is gained or the boat slows - trim in a touch and enjoy the ride!

(Turning: big wide turns - leave the trim alone, probably; tight, fast turns - trim in to prevent loss of hull grip and/or ventilation).

There are people on here who know far more than I do; and if you haven't already I can recommend the RYA powerboat courses.

Each boat will vary, and each journey will be different; and as loads change through the day so the settings will change. The dials and numbers on the console are only guides - as you get to know your boat you will feel what the right settings are for each occasion. This is part of the fun of driving a fast boat...
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Old 13 August 2011, 04:54   #14
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Thanx Russ that's great advice -These are great fun but I'm still a yottie at heart........

Ducks for cover!
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Old 13 August 2011, 07:05   #15
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If your cav plate is just visible you don't not need the extention. It there anything mounted on the transom near the prop? Transducer or something? Even a small obstruction can disrupt the flow over the prop especially when getting onto the plane. If you have anything in the way it can make a big difference. You said the previous owner had an XL but that may have been to overcome a problem that could be solved more efficiently.

Adding the extention will drop your prop another 5" that is going to create massive drag, reduce speed and increase consumption. You really should try a different prop before you bolt that thing on. Try and borrow one from someone.

Technique wise, I have always trimmed right in, throttle up and once on the plane trim to suit.
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Old 13 August 2011, 20:24   #16
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Thanx Oscar. Valiant state that a single motor must be XL. Twins can be L. I just didn't check that before buying the replacement.........

The little thing in front of the prop is the skeg! 25 inch transom and 20 inch leg doesn't quite work does it? You mentioned before that I am experiencing 46% slip or thereabouts. The boat is a standard DR750, the prop is a standard 3 bladed lazer in good nick - surely that only leaves leg length no?
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Old 14 August 2011, 05:06   #17
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It's the leg length not the prop, although a higher pitch might reduce the ventilation.

I have a gut feeling (no facts, sry) that having a 150hp may be making the problem worse - it's at the lower end of acceptable power for this size of boat. Valiant show it as the recommended HP, but list 115-300 as the useable range. That's a bit of a crowd pleaser. Most users on here would be starting at 200hp or more for that RIB.

In any case, you have committed yourself to action (leg extension) and until then you're going to have to keep your trim down.

Anyhoo, there's my last call for boarding - gotta dash - just time for a last quick one at the bar...

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Old 14 August 2011, 06:03   #18
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Rib Magazine tested this combo (Merc 150) and stated 50mph was attainable - sounds very optimistic to me - no mention of shaft length tho!
RIB Magazine - Rigid Inflatable Boats, RIB Boats - RIB International Ltd - Valiant DR 750

Also interesting that the Valiant brochure shows shots of a DR750 with Merc 175 & 200's and definitely states XL for a single motor installation

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Old 14 August 2011, 06:24   #19
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Ya right Will. I am looking for a replacement as we think it's pertinent for a business to have a spare. I was interested in going down to a 135 but with your advice I'll edge towards the 200-225 I think.

Thanx for the linky to the article John, will download it now and take a gander when I'm at work this avvy!

Cheers guys!
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Old 14 August 2011, 06:26   #20
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Rib Magazine tested this combo (Merc 150) and stated 50mph was attainable - sounds very optimistic to me
HMS's statistics tend to be, um, a little hazy
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