Sunseeker Cobra 39 sluggish

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09-24-2018, 10:21 AM

#11

CobraGuy

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I’m a noob so you can’t offend me

The boat never plane, it’s to slow. I heard from others with the same model and it plane around 30mph, mine only reach 20.

09-24-2018, 10:45 AM

#12

speicher lane

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Quote:

Originally Posted by CobraGuy

What does O/B stand for?
We thought about the hubs and tried different props, unfortunately it was the same dimensions so speed was exactly the same but at least we could rule out that the hubs was slipping. The prop we used had a fixed one.

O/B indicates outboard - the weight of your boat and the loading can basically flatten the pitch out of the prop loosing efficiency and effectiveness ( Aluminum would have broken blades long ago)... a prop that offers a lot more blade density/thickness is generally used on heaver express cruisers like a Mercury Mirage Plus to combat blade deflection

Is the transmission fluid burnt (stinks and dark colour)?

09-24-2018, 01:21 PM

#13

Griff

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Quote:

Originally Posted by CobraGuy

Sorry, dont really know. I know some mechanic talked about some Merc lightning or some flashy name like that.

Engines runs well though, I can rev up to 5k easy and probably above that but with newly built engines I would like to stay below that. The boat should work anyway I belive.

Probably Merc Thunderbolt ignitions which are standard.
Are you raising the trim tabs???

09-24-2018, 01:44 PM

#14

CobraGuy

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Quote:

Originally Posted by Griff

Probably Merc Thunderbolt ignitions which are standard.
Are you raising the trim tabs???

Correct, that was the name

Yes, tried working with the trimtabs, no real difference. Front rise up and down but only a small increase in speed, 1-2 mph but it's doesn't plane.

09-24-2018, 02:34 PM

#15

F-2 Speedy

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you said you can rev them to 5k ?? is that under load, in gear trying to get on plane, The reason I asked about ignitions is if their TB4 vs TB5 there is different timing sequence to set base timing to get full advance, but maybe your engine builder already knows this. I have to ask ( are the props on the right drives )

09-24-2018, 02:54 PM

#16

CobraGuy

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Quote:

Originally Posted by F-2 Speedy

Yes, we have done a short rev to about 5k trying to see if anything happens and they rev fine but it doesn't pull at all. Never had this heavy/big boat before but 28-30 feets are really pulling when at WOT, with this one nothing really happens. Engines rev away and I go from my comfortable 8(10mph) knots to about 18(21mph) (2k rev-4,5k rev), no real resistance, it revs to that in 1 second.

I really hope and think they are on the correct place

I tried to just go 1 engine at a time and achive about the same result on each engine, a few mph slower like expected.

Everything feels kind of normal if I my top speed were supposed to be 20mph. But with 8-900hp I was hoping for more

09-24-2018, 02:56 PM

#17

CobraGuy

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I'm more into cars and it feels like I have the LOW engaged in a 4x4 car. Stuff is happening but no speed.

09-24-2018, 03:00 PM

#18

CobraGuy

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Question: The props seem/look kind of small on this kind of boat. In the combination with 10-20% barnicles(the small shells) on the hull, could this affect the performance this much?

09-24-2018, 04:11 PM

#19

hogie roll

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Quote:

Originally Posted by CobraGuy

Question: The props seem/look kind of small on this kind of boat. In the combination with 10-20% barnicles(the small shells) on the hull, could this affect the performance this much?

Pics man.

You did try different props? sounds like hubs to me.

also the drives are trimmable, not just the tabs.