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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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  • 8 months later...

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




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






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




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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