Optimax Performance (or lack of it!!)

[Mike Brook]

Would welcome some guidance on Optimax performance.

Have just spent some 5 years with a 115yam 4s on the back of a 5.8 Rib Eye and this year opted to trade up and bought a 2002 7.5m Cobra with a 2s 200HP Optimax on the back with a 21p S/S screw. The Cobra jumps out of the water on acceleration, much more responsive than the Ribeye / Yam but appears to be relatively gutless at top end giving no more than 35knots which was not far off what we achieved in the Ribeye. Also it drinks fuel even if you treat it with respect averaging some 1.6 - 1.8+ l per Nm (I'm lookin into taking shares out in Fawley!!) We absolutely love the Cobra but are my expections too great on the Opti? Is a prop change needed - happy to loose some acceleration for top end / better consupmtion. Is it too drastic to consider a change to a 4s unit and if so how much and where?

Mike

[Hugh Jardon]

Originally Posted by Mike Brook

Would welcome some guidance on Optimax performance.

Have just spent some 5 years with a 115yam 4s on the back of a 5.8 Rib Eye and this year opted to trade up and bought a 2002 7.5m Cobra with a 2s 200HP Optimax on the back with a 21p S/S screw. The Cobra jumps out of the water on acceleration, much more responsive than the Ribeye / Yam but appears to be relatively gutless at top end giving no more than 35knots which was not far off what we achieved in the Ribeye. Also it drinks fuel even if you treat it with respect averaging some 1.6 - 1.8+ l per Nm (I'm lookin into taking shares out in Fawley!!) We absolutely love the Cobra but are my expections too great on the Opti? Is a prop change needed - happy to loose some acceleration for top end / better consupmtion. Is it too drastic to consider a change to a 4s unit and if so how much and where?

Mike

mike, what rpm are you seeing at 35knots? it sounds as if you are probably only seeing 4-4.5k revs at that speed?

fuel filters can affect fuel flow which would give top end performance issues

have you had it checked or serviced?

i would say that you probably need to try a 19 prop on that combination as the cobras are quite heavy, it would give you even more acceleration but may allow you to get further up the rev range

[codprawn]

It could be the engine is too low in the water.

[Mike Brook]

there hangs another issue - the rev counter is dodgy and in to be fixed but it certaintly isnt revving hard

Its was fully serviced the week before last - in time for a weeks ribbing in Torbay so don't think its fuel filters

Intereseting comment on the prop - might see if I can find one to try out - wouldnt this burn even more fuel though?

[codprawn]

the deeper the prop is in the water the better the acceleration but the more drag at higher speeds.

[Hugh Jardon]

mike, you are not that far from me and i have a 200 opti on a vipermax, would be happy to meet up and compare notes some time or meet up on the water some where.

i am not pulling top revs on mine so i can guarantee you are getting nowhere near top revs with yours.

if the engine has been well serviced they should have checked the throttle linkages etc for full travel.

i have driven cobras with 200 and 225 optis and i would say you should be getting more from her, i can not remember what props they had on them but maybe some cobra 7.5 owners with Opti 200s could reply on here.

i would give dave picton a call as ask him what speed etc pitch prop he recommends as he has sold a lot of cobras with optis as he loves em

[Mike Brook]

thanks Hugh

ribbing mainly done in the Solent - what about you

will be out on a jolly on Saturday so will look to collect a bit more information to clarify some of the above comments

[Hugh Jardon]

i wont be out these weekend as i will be fitting keel guard and windscreen and prepping the boat for cornwall trip next week, but if you gather some info on the revs etc etc then we can compare notes, i dont unfortunatley have a 19 inch prop otherwise i would have lent that to you to try

[ribit]

I have not been on a 7.5 on the move but back in the 90's a few people I knew had them with 200hp 2 stroke engines having run with them in Poole bay a few times I would say they ran a fraction over 50mph...I would assume but maybe wrong it's the same or a very simular hull to the Picton 21ft hard boats there was an extensive test once many years back using all the manufactures 200hp engines if I recall they did between 53-55mph on a 21 hard hulled boat so perhaps a few mph loss may be expected by tube drag and engine set up

[Cookee]

Originally Posted by Hugh Jardon

fuel filters can affect fuel flow which would give top end performance issues

i would say that you probably need to try a 19 prop on that combination as the cobras are quite heavy, it would give you even more acceleration but may allow you to get further up the rev range

I would have to question the fuel filter remark unless it's clogged up, and the Opti technology would definitely get the hump if it couldn't get enough fuel and tell you so! The Smartcraft instruments will tell you if you have a specific problem (that it is aware of!).

Water in the fuel and Opti injectors don't mix at all well, a 5 year old Opti might have injector issues which would cause all of the above symptoms. I think that it would have to be very heavy to need a 19" prop.

My guess would be setup - move some weight towards the back of the boat (do you have a big anchor?), and measure the height difference between the cavitation plate and the keel - raising the engine always helps speed .......... up to a point!