Engine height

[kiwiguy]

Could someone please tell me what height my outboard should be on the transom. boat shoots out of the hole but over heats due to to much of the intake out of the water. The cavitation plate seems to be only 1/2 inch below the hull at idle and virtualy sits on top of the water at wot. Motor is a suzuki DT 85

 

[garyN]

Re: Engine height

Kiwiguy, you may have a short shaft on a boat that requires a long or extra long. <br /><br />Most outboards today need 20in shafts - yours is definate not deep enough - I believe the rule is at least 2.5 inches from bottom of hull to cavitation plate.<br /><br />Also, <br />Whoever answers this question can you also tell me what you think about this,<br /><br />I have a 12 ft aquopro rib with a 15 in mercury 15 on it. Run great but tends to bow down and throws up a bow spray that comes into the boat.<br /><br />Do you think that by raising the motor hight on the transoms (say one-half to one inc) that it would change?<br /><br />Thanks,<br /><br />Gary

 

[ob]

Re: Engine height

kiwiguy,According to your explanation of the engine height ,it doesn't sound to me as if it is too high.Typically the anti-ventilation plate is located roughly even to slightly above the "mean' bottom of the boat surface when trimmed level and viewed out of water.Did you recently make a height change or has the engine performed well before at its present height setting?Are you experiencing excessive prop ventilation when coming out of the hole or executing tight turns at 2/3 throttle while 'on plane'?If not I'd say your engine height is not your overheating issue.<br />Is your rig eqipped with a water pressure guage?My feeling is that your cooling system needs attention.

 

[walleyehed]

Re: Engine height

Ditto ob's statements......anti-ventilation plate on the surface on plane is as low as should ever be needed...in most cases, we lift them a hole or 2 up.

 

[mjbrueck]

Re: Engine height

The cavitation plate should be sitting "on top of the water at wot". Have you checked the impeller. My guess is it's working well enough to cool the engine at idle, but not pushing through enough water to cool the engine at higher rpms.<br /><br />GaryN,<br /><br />Have you tried moving the trim pin out (tilted up more)? Assuming it's not a boat design issue, either the motor is trimmed in too much or you have too much weight in the bow.

 

[kiwiguy]

Re: Engine height

Water pump has just been done,(thought that might be the problem), but no change. The motor was rebuilt by me about 3 months ago and I made sure all the water jackets and feed tubes were spotless. I had this same problem before the re-build. The boat has power trim and I have tried all positions.After about 3 minutes at 5500rpm's the overheat alarm sounds. If I back off to around 3000rpm's the alarm stops. I have spoken to a friend who had a similar problem and he made a pair of very slimline scoops and mounted them facing forward over the intake grills.... said it worled a treat. I would think this would affect the overall performance... It is really pissing me off cause everytime the alarm goes off i normally have a cold bear in hand... gives me the biggest fright and I spill the damn thing

 

[mjbrueck]

Re: Engine height

I don't think it's related to the depth below the hull (unless you have something in front of it inhibiting or aerating flow - skeg, barnacles, etc.). Even on a flat-bottomed hull you should be able to operate with the cav plate slightly above the hull bottom (and higher the more deadrise).<br /><br />You said it happened before the rebuild (and new impeller). Has it done this since day 1 or was it a recent problem? Have you felt the tell-tale to see if it's actual temperature problem or bad gauge/sender?

 

[ob]

Re: Engine height

Another component to consider that I didn't see mention of are the T/stat assys..Are they known to be in good working order?Maybe the pressure bypass feature isn't allowing full flow at higher rpms.Have you actually felt or used a thermo melt stick to confirm that the engine is indeed overheating?I'm thinking in the lines of a possible defective overtemp sensor.

 

[Qld_Kev]

Re: Engine height

I think ob is more targetting the problem. By the sounds the height is ok, you may actually go up a bolt. I would replace the thermostats, also try the melt stick. The only other things I'm thinking is the timing too far advanced or carb(s) running too lean... this will cause overheating aswell.

 

[walleyehed]

Re: Engine height

Did you replace the rubber cylinder water passage inserts at rebuild????

 

[kiwiguy]

Re: Engine height

Thanks so much for your input. I checked the overheat alarm and found it was faulty. replaced it yesterday and took the old girl for a fang..... no problems at all. I didn't realise that the cavitation plate rode as high as that on the plane. Cheers.