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As I understand it ventilation and cavitation are different. Ventilation is when air comes down from the surface, causing a "vent". Cavitation is when the low pressure side of a prop or foil drops so low that effectively the water boils, causing an air cavity in the water. Related, but different, and very often confused,even by many who should know better.

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maybe it's a bad translation?

 

perhaps the japanese term covers both?

 

certainly yamaha + honda manuals seem to call the plate above the prop that stops air being drawn down the "anti-cavitation" plate in their manuals

 

anyone bored enough to see what mercury + johnson call it? 

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anyone bored enough to see what mercury + johnson call it? 

 

I started to search the web, but outboards bore me so instead of just being bored, I quickly became really really bored!

 

Thank heaven for websites like this one!

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an old thread

 

but have been researching prop specs

 

as previously stated

 

the yamaha 8hp 2-stroke is a high power, low weight,  popular choice for 8.5mt multis

 

it comes standard with a large pitch 10'?  all white prop designed to get light dinghies planning

 

there are also 2 lower "geared"  "dual thrust" options for boats heavier than a dinghy that will never reach those speeds

 

the black hubed  9" x 7"N, for light weight multis (9 inch diameter with 7 inch pitch)

 

 (cast into the prop alloy)

 

and the black hubed 9" x 5"N for heavier boats with heavy keels

 

the black hub is "dual-thrust" in yamaha speak, which means in-hub exhaust so the blades are less likely to cavitate in reverse and lose bite.............most light tinnys/inflatables etc don't reverse much so don't need this exhaust gas diversion

 

solas also make 5 props for the yami 8hp 2 stroke, the 4 bladed versions with in-hub exhaust probably the best choice for multis

 

http://www.rubexprops.com/boat-propellers/Yamaha/?motor=H0080C&ver=a&model=8HP%202%20Stroke%20(7%20Spline)

 

yachts heavier than 900kg probably want more power AND lower gearing

 

for them the 9.9hp was made with lower reduction gearing to prop (planing speeds not needed)

 

AND a range of dual thrust props for differing boat weights

 

..............

 

cavitation protection in swells

 

small multis with a single outboard mounted centrally seem better with the long shaft engines as the hulls can lift the prop out in swells + waves

 

yamaha make just the 1 long shaft option but tohatsu make a long and an extra long

 

as Greg has on Mysterex

 

but even the standard long shaft yamaha can still get lifted clear enough for the engine to scream as the prop half spins in air.......one way to help prevent this is to bolt an aftermarket hydrofoil to the engine's small anti-cavitation plate just above the prop

 

194156799.jpg

 

http://www.trademe.co.nz/motors/boats-marine/parts-accessories/outboards/auction-1168791372.htm

 

the barnys fitted 1 to capricorn's 8hp 2-stroke yami

 

gallery_1988_37_413454.jpg

 

and it seems to be working well

 

they are just a cut + bent bit of alloy sheet so am going to knock up another out of some spare ally checker-plate for the spare engine

 

also outboard mounts that have multiple down positions help to keep the prop from spinning free

 

capricorn is set so the 2nd from bottom hole for motoring off the mooring, with sails already raised

 

but need the max bottom setting for motoring in, as when head to wind to drop sails, my weight moving from stern to bow, can lift the prop clear from the 2nd from bottom position

I need a new prop for my boat and this post pretty much summed up the information I needed thanks. Also all the other contributions to this thread, excellent thanks. I will post my results soon

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