Sunseeker Cobra 39 sluggish
 

Hi,

New to this forum but it looks great with lots of competent people.
I'm from Sweden and bought a Sunseeker Cobra 39 with 2 newly built 454. The boat weighs around 10k lbs.
The outdrives says TRS 330 and are newly built also.
By newly I mean 0 hours on them. The engines are also improved with better performing intakes, carburators and cam, should be around 400hp. (From 330 i guess)

Engines run great and shifting is very smooth. The problem is that it is very slow.
At 2000 rpm it's around 8 knots (~10 mph) and at 4000-4500 it seems to max out at ~20 mph.
Doesn't feel like slippage because it's equally slow from low rpm to high rpm.
The propellers are stainless "Ballistic" 14,5x19. The propellerblades are the cut kind, don't know the name for it, last piece in the end are cut.
We have asked the previous owner that ran the boat with original engines and he said he used those and accived at least 45-50mph.
The engines rev up very fast, seems like it doesn't have any resistance at all. Feels under geared (or over geared? )

Do you guys have any idea what this could be? The only thing that is not rebuilt is the trans thingy between the engine and the drive, could that one cause this kind of behavior?

Thank you in advance!

Around those number, yes.

If the props are Michigan Wheel's Ballistic XHS A series prop stamped 933519 (RH) / 937519 (LH) you may have 2 issues with them:
http://www.miwheel.com/propellers/brands/ballistic/

1) they are an economy prop designed for O/B that has been used by volvo penta on smaller boats
2) if equipped with plastic hubs or pressed in rubber hubs - you may very well have blown the hubs out under the engine torque/ weight of the boat so the prop shaft is spinning at a different rate than the props (and only getting worse)

What ignitions are you running

Those are the props, they are no good?

What does ignitions mean? Sparkplugs?

distributors, timing spark box, timing modules ?

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.

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.

Have to ask so don't takequestinos personal:
Are you trimming the outdrives to best speed after you get on plane ?

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)?

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.

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 ) :poopoo:

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 :)

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

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?

i like these ...https://www.ebay.com/itm/Mercury-Mir...YAAOSwCMFaBGzL

Yes, it can affect it a lot. Clean the hull. Does the hull have bottom paint also?

cleaning the bottom may make a big difference but if it is not enough the first question is do you know what the ratio in the drives are? with that info we can figure out what the slip is and what it should run. also did you ever ride in the boat with the previous owner and really see it run those numbers? not to say it did not run that with this setup but it would not be the first a seller embellished the performance numbers.

I find it odd if it was slip that both engines are slipping the same? Both couplers are slipping the same? The slip isn't getting worse?

Sorry just trying to help.

It seems like both engines are turning 4500rpms. How are the props?

Is the throttle all the way forward when your at 4000-4500rpm?
IMO if the gears changed you Would be over revving and most likely on a rev limiter.

TRS are only 1.5:1 ratio

yes but there is a TR drive that was 2:1. not sure if the gears interchange in the housing but I think the housings look the same outside and the decals could be wrong or the wrong gears put in if they interchange in the housings. just trying to look at every possibility.
at the speeds and rpm he his showing it is something like 60% slip with a 1.5:1 and 40% with a 2:1 +/-