[Dunaruna]

Happy new year guys and gals!!I've got a small problem, everytime I push the merc over 3500RPM the prop starts to suck air. I can't lower the motor without major structural changes to the transom, nor can I lengthen the leg (currently 20") because of other structual problems. The water pickup is not effected, it pee's fine at any RPM.The motor is a 2005 50HP EFI 4stroke bigfoot with an uncupped 14 x 10 (48-8543-42-445-10P).It is not a major problem as most of the time we are cruising around 2000RPM but every now and again I would like the extra ponies in high wind etc.Will a 9" pitch aleviate the ventilation? A 14 x 9 is within the reccommended size range for my boat (8 ton).Will cupping help?I would like to drop the motor down a few inches but until the boat gets slipped in 5 years or so that is not an option.Aldo

[walleyehed]

some cup would help greatly...If your RPM is close at WOT (which you may not know if it's venting) a 9P cupped should do wonders.It may be a case where a custom 8P is required.

[Dunaruna]

I was hoping you would say that.I think I will buy the 9" and get it cupped, even if it doesn't help the venting I can keep it for a spare.What's the skinny on reverse cupping?Thanks for the info.Aldo

[walleyehed]

Reverse hook props are a work of art on a houseboat application...great fwd AND reverse control....I haven't seen any under 10P though....

[Dunaruna]

Last 2 questions - I promise, how will reverse cupping affect the ventilation problem I am having and is it feasible for me to buy a new (non cupped) 9" and have the local prop shop reverse cup it?

[walleyehed]

The reverse hook props are still cupped in the fwd motion side as well..it's a fairly unique prop. The proplem with running a tripple cup as is used on some houseboats and pontoons, is they don't reverse well.If you had a 9P and added cup, it would be like a factory 11P un-cupped. I wish I had a pic of the reverse hook, cupped prop...the blade profile is different as well as the cup technique. Your 9P with a bit of cup may fix your main problem though.

[Dunaruna]

Thank you, the information you have shared is brilliant.

[Texasmark]

Another thought. Maybe a "Whales Tail" would help. Since your prop seems to be sucking air from above, at the higher rpm's. You know that's the purpose of the "anti cavitation plate" just above the prop. And nowadays the engine mfgr's make them bigger than they used to so there must be something to it So if some is good, lots (whalestale) ought to be great.Don't know what it would hurt nor what the cost differential is.Last thought is that I'd bet the anticavitation plate on the reg 50 and the bigfoot are the same size even though the lower unit for the bigfoot made room to swing the big prop. So in turning the big prop, and pushing hard on the houseboat makes for increased opportunity to ventilate from the normal 50.Different approach worth considering. Might ask someone that uses a whales tale what it did for them....or just ask the mfgr.Mark

[Texasmark]

Just realized that I can and can't spell Whales Tail.....sometimes I am telling a "tale" about the whale.But I really do think it has merit to help in reducing the sucking air in from above (cavitation) problem and probably 1/3 the cost of a new prop.....and you can keep the mph you got or thought you were getting with your current prop at 3500 rpm's......I think.Mark

[ufm82]

Texas- he may not be sucking air from above. The pressure behind the prop is getting so low that the water actually boils, resulting in the ventilation. (Actually cavitation in this case.) They are two different issues. Wouldn't you agree walleye? I've heard of the props you mentioned but haven't seen one. Can you find a picture? UFM82

[Dunaruna]

No, it's not cavitation. It is definetly venting from the surface.The boat has a partially submerged 200 gallon fuel tank right in front of the motor, this tank also acts as a leg protector (don't panic, it's made from 1/4 inch plate steel and has very substantial bash plates). This tank causes a void around the leg when travelling above 10MPH. The problem is, I fitted new alloy pontoons that have allowed the boat to float 5" higher than it originally did, I was able to lower the motor only 2 inches. If I could have lowered the motor 5", I would not have a problem.BTW, the anti-vent plate on the bigfoot is 33% than the standard 50HP.Just a thought, are leg extensions available for the bigfoot? Maybe I'll ask in the merc forum.

[paulie0735]

Hi Aldo, can you not add a simple bracket to the transom to get the engine back into the correct position?

[Dunaruna]

Maybe, but I need to slip the boat first (about $2500 out & in) and then remove the swim platform and have a closer look before I can start doing structural changes.This was taken in 2004 just after the motor was fitted, unfortunetly there was no way to anticipate the current problem.

[walleyehed]

Looking at the reflection off of 1 blade of the prop tells me YOU NEED SOME CUP!!!!!! You need some grab, bite, get-hold added to that prop....I feel it's going to continue venting like it is...Can you look down through the deck under power and see if the AV plate is quite a ways above the water???

[Dunaruna]

You're correct yoda, there ain't no cupping.

Originally posted by walleyehed: Can you look down through the deck under power and see if the AV plate is quite a ways above the water???

Yes and yes, sort of.There are two plates on the bigfoot, the upper plate is out of the water at speeds above 10mph, the main anti-vent plate is just under or riding the surface.I'll get the 9" and get it cupped and next time the tub gets slipped I'll do some re-engineering to the mounting frame.As I said in the first post, it ain't a life and death problem, most of the time we are cruising around 5 mph.