Cavitation

[dozerII]

Last weekend I got out with my brother in his Starcraft 18' SS with a 115 Evenrude. We had some trouble with motor not putting out and found a broken wire to one of the coils, once we fixed it, the motor ran great. The boat got on plane easy and topped out at 40mph on GPS with a 13 x 19 aluminum 3 blade, and jumped up on plane and topped out at 38mph with a 13.25 x 17. Sorry I don't have rpm numbers the tach was not working properly.The problem we had was when he would start a turn at wot the motor would unload and rev up for a second then drop back down, it did the same thing once or twice when we would cross wake. Is this cavatation? The motor is mounted as low as it can go on the transome and the anticavitation plate is about .5 inchs above the bottom of the keel. Any one have suggestions?
Thanks Glen

 

[bob johnson]

Re: Cavitation

if the hull rides high, then sure it runs great in a straight line, but once you turn it the dynamics of the water flow changes...bottom line you need the prop underwater at a minimum realtionship to the waters surface..you broke the realtionship...

the solutions might be, motor mount adjustements...propeller style changes, and or driving technique changes...ie , no fast turns at WOT

outboard geometry in the lower unit, does not change much over the years...

but boat hull specifics are constantly changing....

that is why many performance boats have hydraulic jack plates...because one motor hieght is a compromise against totaly possible performance

good luck

bob

 

[Huron Angler]

Re: Cavitation

http://www.davisnet.com/MARINE/products/marine_product.asp?pnum=00448

Maybe one of these would help during turns?

I've heard that top speed will suffer but handling and ventilation may be reduced.

 

[trendsetter240]

Re: Cavitation

What you are describing is ventilation. The prop is breaking the surface of the water and losing grip on the water. Air is much less dense than water so the prop spins much faster and the boat loses speed.

Try adjusting the trim. Trim the motor in a few degrees before turning at high speeds. You only need the motor trimmed right out for maximum cruise and speed efficiency. Tight turns and water sports the trim so be adjusted in to reduce ventilation.

 

[BaileysBoat]

Re: Cavitation

What you are describing is ventilation.

Agree. Also agree about trimming in.

You could see about a cupped prop.

 

[bob johnson]

Re: Cavitation

I know when on plane in my boat, i pick up speed by triming OUT a little which raises the bow....

now does the bow JUST come up off the pivot of the transom, or does it rock and the transom actually goes down while the bow comes up?????


if the later, then the LU will be deeper in the water.....right?

I have seen the pictures of the bass boats at speed..and it seems that 18 feet of a 20 foot boat has air under it!!!! and the boat barely touches the water...

seems like the LU wouldnt be to deep in the water......SEEMS...but that is what they are striving for...less friction...high speed...can they turn that rig in that condition???

or do they have to trim and or slow to make a turn??


bob

 

[thompy]

Re: Cavitation

sounds like the SS props and 4 blade props help with that condition

 

[dozerII]

Re: Cavitation

Thanks everyone for the replies and ideas. I will let him know what you all have said. I think the problem will be worse for him one he has the normal load in the boat, his wife and him combined weight 333# and when we were testing we had three lard ***' in there 660#. Like I said in the last post the motor is all the way down on the transom now, so if trimming does't work he may have to try a Whale tale.

Thanks again
Glen