I've picked up a used 4.55m tinny which is equipped with a Honda 50hp four stroke outboard.

Quite sure the outboard is mounted too low on the transom as the anti-ventilation plate - which sometimes referred to as a cav or cavitation plate - is 1/2" below the keel. When on the plane the anti-ventilation is well buried below the surface of the water. I feel this is slowing the boat down with extra drag from the leg. Normally I would setup outboards to have the anti-ventilation plate to be just skimming the surface when at cruising speed on the plane.

However, the Honda BF50D has a third cooling water inlet. There are the usual two inlets either side of the skeg and an additional one on the lower side of the ventilation plate. I'm concerned about sucking air into that third pickup if the anti-ventilation plate is skimming the surface.

Is anyone running a Honda four stroke and can reassure me they run fine at this height?

Do a Google and YouTube search plenty of info there including a full manual.

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warwickl said..
Do a Google and YouTube search plenty of info there including a full manual.

Great reply Warwick....... maybe you can use that advice next time you post a question

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Craig66 said..

warwickl said..
Do a Google and YouTube search plenty of info there including a full manual.

Great reply Warwick....... maybe you can use that advice next time you post a question

Never
People need to have their say and this site would close.
I feel clever that I said that to you.
Most of us benefit.

I have a mercury 30 well it's actually made by tohatsu with the mercury branding.
same as the surf boats

the anti cav plate is at this level because your third intake would aerate if it was higher, and than your motor runs warmer and you chew more impellers as well.
you can change the height but nothing sucks more than burning out an impeller and overheating the motor, be interesting to see an actual photo.

do you have a long leg shaft or short leg???

Try tilting the motor a bit for less drag but a motor that size on a 4.5 tinny should fly like 25-30 knots fly.

what prop is on there??!

a good trick I run in my tinny when going out by myself is to put the esky right up the front ( about 30kg of ice/beer/food etc etc) also have the pvc tube on the outboard so I can sit on the middle seat and give it gas.

Cavitation plate should be level with base of hull when in operational position, some hulls its critical, my cat had to be spot on level, the bolt plate let's you slide motor up and down to get that perfect spot,

Sounds way to low. Actually the Cav plate would normally first start inline with keel but normally end up slightly above. Even as much as an inch.

Thanks guys - (except for Mr 'just google it' )

I have raised the outboard +40mm with good results on performance. Holeshot, Top speed and slow planning speed have all improved. The ventilation plate is now level with the hull which is still not as high as I'd like. Ideally I'd like the ventilation plate just skimming on the surface - this is recommended in the manual. Unfortunately the motor was originally installed so poorly I will need to redrill the transom to go higher and increase the transom height with an extra piece of RHS.

Prop is an aluminium Solas 3 blade 11.1 x 14 with evidence of cavitation burns mid blade. I plan to replace it with stainless once outboard height is finalised and WOT RPM can be accurately determined. At the moment I'm only hitting 5300 WOT so dropping back to a 13" propeller is looking like a better option.

Good result, now just but a decent s/s prop and you might get a wake like this

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knot board said..
Thanks guys - (except for Mr 'just google it' )

I have raised the outboard +40mm with good results on performance. Holeshot, Top speed and slow planning speed have all improved. The ventilation plate is now level with the hull which is still not as high as I'd like. Ideally I'd like the ventilation plate just skimming on the surface - this is recommended in the manual. Unfortunately the motor was originally installed so poorly I will need to redrill the transom to go higher and increase the transom height with an extra piece of RHS.

Prop is an aluminium Solas 3 blade 11.1 x 14 with evidence of cavitation burns mid blade. I plan to replace it with stainless once outboard height is finalised and WOT RPM can be accurately determined. At the moment I'm only hitting 5300 WOT so dropping back to a 13" propeller is looking like a better option.

Now my feelings are hurt, I apologise.
What I did do for you however was to do a Google search, found the full manual for your O/B plus all the info you needed.
Instead of fully explaining everything and that I did this for you I just posted my short advice.
Then thought we were just have a bit of fun.
Glad you got the required end result.

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Craig66 said..
Good result, now just but a decent s/s prop and you might get a wake like this

What only one engine?

Crap fitting job with those holes drilled wrong. How hard is it..FFS Bad installation is a real annoying thing for me. The amount of second hand boats that come through my boat yard with a poor or incorrect fittup, affecting the overall performance is staggering.

As for height, if you think it can go up, i often will mount up until it cavitates and lower from their. But if your not getting a v style spray up then it may be okay. Also depending on hull you may not get to much better performance.